Compaction, paving and milling handbook. Theory and practice by Dynapac Compaction Equipment AB

COMPACTION, PAVING AND MILLING Theory and practice

ATLAS COPCO | COMPACTION, PAVING AND MILLING

COMPACTION, PAVING AND MILLING Theory and practice

Copyright Atlas Copco Road Construction Equipment, Sweden 2014 Production Happiend Reklambyr책, Sweden Photo Atlas Copco, iStockphoto, Dreamstime, Fotolia We reserve the right to change specifications without notice. Photos and illustrations do not always show standard versions of machines. The information is a general description only, all information is supplied without liability.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

COMPACTION, PAVING AND MILLING THEORY AND PRACTICE This handbook presents a general overview of soil and asphalt materials, and suitable methods and equipment for their compaction. It also deals with asphalt paving as well as cold milling techniques and equipment. The principle purpose of the book is to assist that important group of authority employees, contractors and consultants who are concerned with compaction, paving and milling. It should also be useful to students and others looking for an introduction to these subjects.

Dynapac has been at the forefront of vibratory compaction and paving technology for many years. Its growth as an international organization has been based on the solid foundation of its research and technical expertise. This experience, now gathered under the banner of its Technology and Application Center (TAC), has provided the company with the knowledge and tools to design and manufacture compaction equipment, paving machines and milling machines that not only ensure that a job is done satisfactorily, but also,

significantly, that the equipment remains on the job. Through the TAC, Atlas Copco has developed CompBase, a unique tool to predict the most suitable choice of equipment for a given job with given specifications. In effect, it is a bank of compaction and equipment-related data, based on full-scale tests carried out under controlled conditions on Dynapac compaction equipment working on various soil types. The test material comprises hundreds of thousands of measurements. For given conditions, CompBase suggests

ATLAS COPCO | COMPACTION, PAVING AND MILLING

the optimum type of equipment and the suitable number of machines required. In practice, the CompBase predictions have proved to be very useful with a high degree of accuracy. The TAC has developed an equivalent program for asphalt paving applications, PaveComp, which helps asphalt contractors and others involved in the surfacing business to select not only the right machines for a given lay-down rate and given type of asphalt bitumen mix but also the best combination of paver and roller train to achieve

the specified density cost efficiently and using the best asphalt surfacing practices. Dynapac offers the market a complete range of vibratory rollers from the largest asphalt tandem rollers in the world to smaller repair work rollers. Vibratory single drum rollers are available for all soil applications. The roller range also comprises static smooth drum rollers and pneumatic tyred rollers. Atlas Copco compaction equipment is supplemented by a range of tracked and wheeled asphalt pavers, material feeders and planers. The

pavers are available with a full range of screeds designed to handle all paving applications. Atlas Copco has compaction, paving and milling equipment manufacturing facilities in Sweden, Germany, Brazil, China and India. The Atlas Copco products are sold through Customer Centers and distributors in all major areas of the world. The Atlas Copco world-wide network runs a global parts and service back-up to maintain product integrity over a long productive life.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

CONTENT Applications for compaction and paving techniques

Soil compaction Type of soil

Compaction methods

Compaction equipment

Compaction properties of different soils

Special applications

Specifications and filed control methods

Field control methods

Asphalt paving, compaction and milling Asphalt paving and compaction

Quality and functional requirements for asphalt pavements

Type of surfacing

Mixed asphalt components

Mix design proportioning

Properties of asphalt mixes

Manufacturing process and transportation

Asphalt pavers

Paving operations

Asphalt compaction

Rolling procedures

Choice of asphalt compactors

Specifications and field control

Cold milling applications

What to look for in ... ... a vibratory roller

... a static smooth drum roller

... a pneumatic tyred roller (PTR)

... cold milling equipment

... asphalt paving equipment

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APPLICATONS

APPLICATIONS FOR COMPACTION AND PAVING TECHNIQUES Compaction is defined as the process of increasing the density and load-bearing properties of a material through the application of either static or dynamic external forces. It is required in many areas of the construction industry. The following pages describe in brief the most common applications – roads, streets, motorways, airfields, earth dams, railway embankments and foundations for buildings. Other applications include parking areas, storage yards, sports areas, industrial and residential areas, harbour constructions, reservoirs and canal linings.

In the construction field, the load bearing properties and stability of rock fill, soil, asphalt and concrete, their impermeability and their ability to withstand loads are all correlated to the adequacy of the compaction of the material. To illustrate the importance of compaction, a one-percent increase in density normally corresponds to at least a 10–15% increase in bearing capacity. Although compaction may only account for some 1–4% of the total construction costs, its role in the quality and life span of a finished project is immeasurable. If compaction is inadequate or incorrectly performed, settlement and other

failures are likely to occur with resultant high rehabilitation and/or maintenance costs. In a number of the above applications, principally roads, airfields and parking and storage areas, the life span of the construction is also dependent on the quality of the surfacing. For asphalt concrete the degree of compaction is decisive to strength, wearing resistance, impermeability and durability. In addition, correct surface evenness, uniform layer thickness and the correct grades and crossslopes are all necessary for a long, low-maintenance service life. As a consequence, the perfor mance of the paving equipment

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is in many aspects crucial to the quality of the finished surfacing.

Soil and asphalt structures

The design of a soil structure has to take into account a number of factors such as loading, environmental conditions, material available as well as climate. The loads may vary depending on the type of structure, but the main aim is to distribute them down through the structure. The most common types of load are traffic, buildings and water pressure. In a road, for example, the load is distributed through the different layers. The greatest load distri-

APPLICATONS

bution is to be found in the upper layer and this diminishes the deeper you get into the road. The various layers have to bear the weight of the layer above as well as the traffic load. Any structure has an effect on and is affected by its environment, and this must be taken into consi deration during the construction phase. Today, contractors try to recycle material which is on-site to reduce the pressure on quarries as well as the need to exploit virgin sites. As far as possible, they will choose local material as fill material and to produce the asphalt. Bringing in material from outside not only has repercussions on costs but also on the environment. Sometimes it is unavoidable to transport material when, for example, the asphalt mix needs to have special properties. The effect of the climate must also be taken into consideration during the actual construction period and the service life of the structure. In cold climates, conside ration must be given to frost and the risk of low temperature cracking in asphalt. In hot climates, due

consideration should be given to the stability of the asphalt layers to minimize the risk of deformation. In all these conditions, compaction has a major significance on the function of the structure, its service life and the maintenance costs. Roads There are many types of road from small secondary country roads to large multi-lane motorways. The main criterion placed on a road is that it should be able to transport people and goods in a safe, rapid, economic and comfortable manner. In order to fulfil this, certain demands are placed on where the roads are built, their surface evenness and surface friction. A road is built on an embankment or in a cut and is made up of a number of layers – embankment, base course, binder course and wearing course. (See diagram.) Sometimes there is a need for cement-reinforced base course to enhance the load bearing capacity of the road.

Roads

Cut

Material description, see p. 11.

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APPLICATONS

Railways In many parts of the world, rail足 ways are still the major form of transport for goods and passengers. The transport of heavy materials such as ore, coal and other minerals places great stress on the railway embankments. Railways are built according to the same principles as a road except for the bound upper layers. The ballast bed on top serves to keep the sleepers in place. Bitumen bound top layer ar also used to keep the sleepers in place. For the construction of high speed railways, considerably stricter requirements are being imposed on embankments and ballast beds. Airfields Runways, taxiing areas and aprons are all exposed to heavy loads in airport complexes. They are built up in the same way as roads but the specifications are more stringent. In addition to this, under no circum足 stances may the surface break up so that loose stones can end up in the airplane engines.

Cut

Railways

Foundations for buildings and or/bridges Foundations are essentially built in the same way as roads up to the base course. Layer thickness can differ depending on the nature of the load the structure is expected to carry. Airfields

Foundations for buildings, bridges etc.

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APPLICATONS

Earth dams Canals

Canals Canals have to be designed and built to withstand enormous water pressure. In addition, they have to be water tight so that they don’t leak and they have to be able offer protection from the risk of erosion caused by the flowing water. The bottom of the canal is levelled off and compacted. A filter layer comprising sand and gravel is then put in place and, on top of that, a sealing layer of fine soil, cement or asphalt. There is always a certain amount of leakage through the seal ant and the filter ensures that the sealant does not get washed away. If fine soil is used as the sealing layer, it must be covered by an erosion resistant layer. It is highly important that the various layers are correctly compacted to avoid cracks in the sealing layer. Earth dams There are certain similarities between the functional properties of canals and earth dams but they are built in different ways. An earth dam has a core of impermeable material, for example, fine soil or asphalt. On either side of the core there is a filter and, outside

that, a shoulder. The impermeable core and filter fulfil the same function as the sealing layer and filter in a canal, while the shoulder keeps the various layers in place. The dam wall surface is exposed to enormous water pressure – some dams are more than 100 m high. The shoulder also offers protection against erosion. An alternative construction allows for a layer of concrete or asphalt upstream instead of using an impermeable core. The core comprises an imperm eable soil (silt and clay) or asphalt. It is important to use soil with

similar properties and that no lamination (layering) takes place during compaction. The filter consists of sand and gravel and serves to keep the core material in place as the water forces its way through the core. It is unavoidable that water will seep through the core but it is important to keep the rate low. The shoulder can consist of practically any type of fill material but rock fill is the most common. It is important that the surfaces up and downstream are protected against erosion. Asphalt wearing course Asphalt binder course Asphalt base course Base course Sub-base Embankment Shoulder/Erosion protection Filter Core/lining Ballast Natural ground

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SOIL COMPACTION

TYPE OF SOIL Soils may be divided into a number of different categories depending on their composition, geological history and physical properties.

Grain-size distribution Grain-size distribution is of great importance for the mechanical properties for a soil and for the selection of compaction equipment. The grain-size distribution is determined by a sieve test and a sedimentation test if necessary. Ocular analysis can also be used to categorize coarse-grained soil. Sieve test The dried soil sample is passed through a number of standard sieves which differ in mesh size. The amount of material remaining on each sieve is calculated as a percentage of the total weight of the sample. The figures are plotted on a graph in a cumulative curve showing the grain-size distribution of the material.

Sedimentation test A sedimentation test should be performed if the amount of fines exceeds a certain level, for example, 15%. In a sedimentation test the soil sample (approximately 40â&#x20AC;&#x201C;60 grams) is mixed with water and chemicals. After careful mixing, the density of the solution is measured using a hydrometer after 1, 2, 4, etc. minutes. Afterwards the grain-size distribution can be calculated and plotted. Soil is categorized into different fractions according to grain size as follows (from the smallest to the largest): clay, silt, sand, gravel, cobbles and boulders. The different fractions rarely occur individually in nature. They usually occur in combinations of two or more different fractions, for example sandy gravel, silty sand, silty clay, sandy-silty clay, etc.

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Sieve test

SOIL COMPACTION

Gradation of Sand and Gravel Gradation is an important factor for load bearing properties and for the compaction and is determined from the grain-size distribution curve.

Cu =

d60 d10

Cu=d60/d10 where d60 and d10 are the particle diameters corresponding to values of 60 and 10 percent

on the grain-size distribution curve. If Cu is less than 6 the soil is considered uniformly graded and if Cu is greater than 15 the soil is considered well-graded. In between these two, the soil is medium-graded. The limits differ from one classification system to another. In well-graded material, repre足 sented by a curve covering a full range of grain-sizes, the voids left by the large particles are filled by

the smaller ones. This results in a dense structure and good load bearing properties. A curve showing grains of more or less the same size indicates a uniformly graded material. In this case, there are no smaller particles to fill the voids. Consequently, it is harder to achieve high density and load bearing properties in uniformly graded material than in well-graded material.

Well-graded material

Uniformly graded material

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SOIL COMPACTION

Consistency Consistency is important in a fine-grained soil. The consistency of any fine-grained (plastic) soil may be soft, firm, or hard depending on the amount of water. As the soil changes consistency, so do its mechanical properties. There are certain limits of soil consistency which are the basis for differentiation among highly plastic, slightly plastic and non-plastic material. These limits are designated Liquid Limit (LL), Plastic Limit (PL) and Shrinkage Limit (SL). Plasticity Index (PI) is defined as the difference between liquid and plastic limit. The liquid limit is defined as the water content at which the soil just begins to flow when lightly jarred 25 times in a standard cup. The plastic limit is defined as the water content at which soil can be rolled into a strand without breaking until it is only 3 mm in diameter. The shrinkage limit is defined as the water content at which the soil does not shrink any longer when being dried. The soil also changes colour and becomes lighter as the water content decreases. A soil with a low plasticity index is very sensiÂ tive to changes in the water content. If the water content increases, the load bearing properties of the soil decreases.

Boulders

Sand

Cobbles

Silt

Gravel

Clay

Origin of soils

The composition of a soil and the way that it was formed affect its suitability for use as a construction material. Soils can be split into two main categories: mineral and organic. Soil structures use only mineral soils. Organic soils such as earth and peat are not suitable or even allowed as they are constantly decomposing and their load bearing properties is low and unpredictable. Mineral soils are formed through weathering and natural mechanical effect. They can also be formed artificially by blasting and crushing. Their durability depends on the mineral composition and the way in which the rock was formed. There are three types of formation: igneous, sedimentary and metamorphic. Igneous rock Igneous rock types are formed from the cooling process of magma, a natural solution of high-temperature, rockforming constituents under high pressure. Magma contains a large amount of water vapor and other gases and is always underground. Liquid rock that reaches the surface and loses its water and gases becomes lava. In general magma that is formed about 10 km below the earthâ&#x20AC;&#x2122;s surface contains large amounts of silica and is rich in sodium, potassium and aluminium and tends to form granitic rocks. Magma originating between 10 and 40 km below the surface tends to form gabbroid rocks while, deeper down, it tends to form peridotitic rocks.

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SOIL COMPACTION

Sedimentary rock In time, rock, when exposed to the atmosphere, will be broken up or dissolved by weathering and erosion. The material is re-deposited by wind, water and glaciers and builds into sedimentary rocks. The fragmented material, moving as loose particles, settles out by weight, smaller particles travel longer distances. The most distinctive characteristic of sedimentary rock is its layering or stratification. The most abundant types of sedimentary rock are shale, sandstone and limestone. The material ranges from very soft to being as hard as some of the igneous rock types.

or sedimentary rock caused by heat and pressure. The transition from one stage to another is gradual. As a result of this, all intermediate stages are represented. Eventually the metamorphism may be thorough enough to destroy all evidence of the original state. Metamorphic rock is usually harder than the original rock type. Gneiss is a typical example.

Metamorphic rock Metamorphic rock is formed by the changes in texture of igneous

Grain shape The shape of the grain has a certain influence on the compactability and load bearing capacity of the soil in question. The grain shape is related to the way in which the rock was formed and how it has been affected over the years. Grain shape can be divided into six categories ranging from well rounded to very angular.

Classification of soil types Mineral soil types are generally classified by grain-size fractions. The determination of the range of grain-sizes in the material is the basis for the classification of the soil. Grain-size classification systems vary from country to country. The classification of cohesive soils also involves determining their consistency. One of the most common grain-size classification systems is the Unified Soil Classification

System (USCS) established in USA, which categorizes soils in 15 groups identified by name and letter symbols. The AASHTO Classification System (American Association of Highway and Transportation Officials) intended for road con struction was also developed in the USA. The grain-size classification systems used in different European countries are the same except when it comes to the classification of larger particles. Soils can also be generally

Well-rounded grains are found in soils that have been formed by the affect of wind and weather. The particles grind against each other under the influence of water and wind. This type of soil is most commonly found in river deposits, lake sediment, dunes, loess and glacifluvial deposits. Angular grains are formed by mechanical influence on the rock by glaciers. Moraine is typical example of soils with this grain type, although the whole range of grain shapes can be present. Very angular grains are artifici ally manufactured by the blasting and special crushing processes.

classified in larger groups, for example, as coarse-grained or finegrained, granular or non-granular and cohesive or non-cohesive soils. There are no general rules that govern the permitted maximum content of fines in coarse-grained and granular types of soils. Values vary between 15–50% depending on the classification system. A coarse grained soil is generally regarded as free-draining if it contains a maximum of 5–10% fines (silt and clay).

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SOIL COMPACTION

Resistance to compaction There are four types of resistance to compaction in soil and rockfill: friction, cohesion, apparent cohesion and particle mass. Friction is caused by the interaction between the particles and is the main resistance in a coarse-grained soil. Internal friction in a soil is a result of the forces acting at the contact points between the individual particles.

Cohesion is caused by molecular forces between the smallest particles and constitutes the main source of resistance in a fine-grained soil. Apparent cohesion is caused by the capillary forces of the water in the soil and occurs more or less in all soils. If water is added, the water will finally also act as a lubricant between the soil particles. Particle Mass. Heavy particles require compaction of heavy equipment in order to be able to relocate to a denser state.

Cohesion appears in clay as a result of the molecular forces acting between the miniscule particles. The stronger the cohesion, the greater the compaction effort required.

Apparent cohesion is caused by the capillary forces created in the water that partially fills the void in the soil. The apparent cohesion holds the particles together with “elastic” ties. The smaller the particles – the greater the apparent cohesion.

Most soils attain their highest dry density at a certain optimum water content¹ for a given compaction effort. In simple terms, a soil with water content below the optimum requires more compaction effort to reach the same density as soil at optimum water content, whereas a wet soil is soft and easier to compact. The highest dry density is obtained at the optimum water content, between the wet and dry states. The most common method for determining this state is the Proctor test. Clean sand and gravel, as well as other free-draining coarse materials, are less sensitive to variations in water content, and can attain maximum density in a completely dry or in a water-saturated state as long as the internal resistance to compaction is overcome during the compaction process. 1

In some literature water content is expressed as moisture content. This book has chosen to use water content.

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SOIL COMPACTION

Laboratory compaction tests The optimum water content can be determined in a laboratory compaction test. There are two basic types of laboratory compaction test. One employs a standard weight falling onto a soil sample in a mold; the other is a standardized vibro-compaction test. The most common method is the Proctor test which relies on a falling weight. The Proctor test is recognized as the most common laboratory method for determining the relationship between density and water content. The test establi shes the optimum water content for a soil as well as the reference density. The density is expressed as dry density, which is the ratio between the weight of the dried soil particles and the volume of the sample. Proctor test A sample of the soil to be tested is placed in a cylindrical mould and compacted with a falling weight. Maximum particle size is limited to one-tenth of the diameter of the mould. If there is a low percentage of large particles, the maximum particle size is limited to one-fifth of the diameter of the mould. The size of the mould is 4” (102 mm), and 6” (153 mm) for larger particles. The Proctor test can be carried out in one of the variants known as Standard and Modified Proctor. The compaction effort is 4,5 times greater for Modified Proctor than Standard.

The Standard Proctor test uses a 5,5 lb (approx. 2,5 kg) rammer with a fall height of 12” (305 mm). The soil sample is compacted in three separate layers.

The Modified Proctor uses a 10 lb (approx. 4,5 kg) rammer with a fall height of 18” (457 mm). The soil sample is compacted in five separate layers.

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SOIL COMPACTION

COMPACTION METHODS Compaction equipment for soil materials is based on three main principles: static load, vibration and impact. Different factors influence the selection of compaction method and the compaction result: • type of soil • water content • layer thickness • stiffness of underlying layer

Static compaction Static compactors were the first real mechanical compaction machines. Static compaction equipment uses the dead weight of the machine to apply pressure to a particular surface and compress the underlying particles. The only way to vary the pressure exerted on the surface is to alter the weight or the contact area of the equipment. Static equipment will normally achieve adequate compaction on thin layers. Time, a function of the speed of the static compactor and the number of passes, also affects the final result. Conventional types of static compactors include static three-wheel rollers, static tandem rollers and pneumatic tyred rollers (PTR). Vibratory compaction Vibratory compactors deliver a rapid succession of impacts against the underlying surface from

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SOIL COMPACTION

Static compaction

Vibratory compaction

where the vibrations, or pressure waves, are transmitted through the material to set the soil particles in motion. This reduces or practically eliminates the internal friction, and facilitates the rearrangement of the particles into positions which increase the density. The simul足 taneous increase in the number of contact points between the particles leads to increased load-bearing properties. Vibration is most effective friction soils. Even though it has less effect on cohesive soils, it still improves the efficiency when these types of soil are compacted. Vibratory compaction achieves higher densities and better depth effect than static compaction on all materials, and final density can be attained with fewer passes. All of which explains why vibratory equipment is more efficient and economical than heavy static equipment in almost all situations.

Vibratory compaction causes loosening of the uppermost surface of a layer. The depth of the surface loosening depends on the soil type, its gradation and the water content. On a coarse-grained, uniformly graded soil compacted on high amplitude; there will be a more pronounced loosening effect. The loose soil on the surface is compacted as the next layer is placed on top of it and compacted. Impact compaction Impact compaction relies on a high impact force. It generates a greater force on the surface than a static compactor. The force of the impact produces a pressure wave in the soil which generates high pressure at depth as well. Tampers and tamping rollers work on the impact principle. In certain cases, rollers with triangular, rectangular or pentagonal drums may be used with relative

Impact compaction

good depth effect. As this type of compactor will leave a non-compacted area between each impact, many passes are required to ensure uniform compaction. Impact rollers must be operated at significantly higher speeds than static or vibratory compactors to realize their full effect. They are most economic on large areas. The importance of the stiffness of the surface underneath The compaction effect is influenced by the stiffness of the underlying ground. Compaction cannot be fully achieved if the underlying surface yields. It is often impossible to achi足 eve a high degree of compaction in a fill resting on an underlying layer with low bearing capacity, for example, a fine-grained soil with high water content.

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SOIL COMPACTION

COMPACTION EQUIPMENT Choice of compaction equipment must take into consideration the type of soil, the layer thickness, compaction specifications and the size of the job. The most important consideration is the ability of the machine to fulfil the compaction specifications in a cost-effective manner. You do not select the largest roller for a small job such as a driveway. Conversely you wouldnâ&#x20AC;&#x2122;t choose a small single drum roller for a dam job, other than as a complement to other equipment. There are a number of machine types in current use for soil compaction. The most common ones, and their generally accepted designations, are presented below.

Vibratory tandem rollers p Normally with vibration and drive on both drums. Mainly designed for asphalt compaction but sometimes also used for compaction of base course, sand and gravel. Weight range: 1â&#x20AC;&#x201C;18 tons. The most important compaction parameters are the static linear load, amplitude, frequency and the speed. A higher static linear load gives a better compaction effect and the amplitude controls the compaction depth. The frequency should match the amplitude chosen for the current layer thickness. The speed should not exceed 6 km/h otherwise there will be a noticeable decrease in the compaction effect. Most suitable on thin to medium layer thicknesses on coarse-grained soils.

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SOIL COMPACTION

Self-propelled single-drum vibratory rollers p With one vibrating drum and pneumatic drive wheels. Used on rock fill and soil. Special pad-foot versions are very effective on clay. Weight range: 4–27 tons. The most important compaction parameters are static linear load, amplitude, frequency and speed. A high static linear load gives a better compaction effect. The amplitude helps determine the compaction depth. The frequency should correspond to the amplitude and the material to be compacted. The speed should not exceed 6 km/h otherwise there will be a noticeable decrease in the compaction effect. Suitable on all kind of soils in relatively thick layers. On rock fill, only the heaviest rollers are suitable.

Pneumatic tyred rollers p Normally with 7–11 pneumatic tyres. Front and rear tyres overlap. The compaction effort can be varied by ballasting with water, sand or special cast-iron weights. Weight range: 10–35 tons. The most important compaction parameters are the wheel load and speed. A higher wheel load gives a better compaction effect. The speed should not exceed 6 km/h otherwise there will be a noticeable decrease in the compaction effect. Most suitable on thin layers.

Static three-wheel rollers p Two driving steel drums and a steering drum with rigid frame, or three-wheel drive and an articulated frame. The compaction effort can be varied by ballasting. Weight range: 8–15 tons. The most important compaction parameter is the static linear load and the speed. A higher static linear load gives a better compaction effect. The speed should not exceed 6 km/h otherwise there will be a noticeable decrease in the compaction effect. Most suitable on thin layers of coarse-grained soils such and sand or gravel.

Static tamping rollers Four pad-foot drums. Articulated steering. Run at faster speeds than vibratory rollers. Used for impact compaction. Effective on cohesive soils. Weight range: 15–35 tons. The most important compaction parameters are the wheel load, width of wheel, shape of pads and the speed. A higher wheel load gives a better compaction effect. The speed should exceed 10 km/h otherwise there will be a noticeable decrease in the compaction effect. Most suitable on thin layers and large surfaces.

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SOIL COMPACTION

COMPACTION PROPERTIES OF DIFFERENT SOILS The choice of compaction equipment must take into account a number of factors. These factors include: • type of work and size of work-site • type of soil and water content • layer thickness • stiffness of underlying layer • compaction specifications • capacity requirements • climatic conditions The following section looks at different types of soils and their compaction properties.

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Rock fill (Boulders and Cobbles)

Rock fill includes boulders and cobbles which vary in size from a chicken’s egg up to around 1,5 meters (5 ft). Rock fill, blasted rock, crushed rock or natural material. Boulders and cobbles are the dominant fraction although small fractions may also occur. The maximum stone size and gradation of rock fill is determined by the type and quality of the rock and the rock blasting procedure. Primary rock such as basalt, gneiss and granite, have a high strength, and blasted rock fill with a size of up to 1.0–1,5 m has a small amount of fines. When rock fill consist of lime or sand stone, the maximum stone size is smaller and the amount of fines can be such that considerable settlement will occur if the fill is not properly compacted. The maximum particle size permitted is normally two-thirds of the layer thickness but, from a compaction point of view, it is advantageous if the maximum boulder size does not exceed one-third of the layer thickness. This reduces the risk of rock crushing during compaction.

SOIL COMPACTION

Boulders

Gravel

Silt

Cobbles

Sand

Clay

Vibratory equipment has proven to be the most suitable and costeffective method for compaction. Static and impact compaction are not really well suited to rock fill. Impact compaction can be used if a heavy falling weight is used. However, heavy falling weights increase the risk of crushing. Medium-heavy and heavy vibratory compaction equipment is required for rock fill in order to relocate the large boulders and achieve the necessary density and stability. The risk of crushing the rocks must be observed and may influence the choice of roller size, ampli tude and the number of passes. Rock fill compaction exerts extreme loads on the compaction equipment which is why it is important to select machines that are specially designed for this purpose.

Gravel and Sand

Gravel and sand range in size from a chicken’s egg down to 0,063 mm or in some cases 0,075 mm. They can include fractions of other soil types which will affect their compaction properties. The compaction properties of gravel and sand are influenced by the water content; compaction is most effective at the optimum water content. If the fines content is less than 5–10%, the soil is classified as free-draining. In free-draining gravel and sand, excess water is

drained out during compaction. This means that the compaction work can continue also when it is raining or then the surface is flooded. If the soil is not free-draining, problems are likely to occur if attempts are made to compact the material above the optimum water content. The soil will become elastic and springy. It may be impossible to achieve the compaction requirements as the soil becomes water saturated at a lower density than the one specified. When sand and gravel are uniformly graded it is difficult to obtain high density close to the surface (top 10–15 cm) owing to the low shear strength of the material. The material tends to get pressed up behind the roller drum and the surface therefore attains compara tively low density. This has no great significance in practice. When compacting in layers, the loose top surface is compacted as the next layer is rolled. The surface loosening should be considered when carrying out compaction tests. As a rule, all types and sizes of machines can be used for compaction of gravel and sand. The choice will depend on compaction and capacity requirements. Medium to heavy vibratory rollers will achieve compaction on thick layers whereas smaller roller is more suitable for limited layer thicknesses and volumes.

Silt

Silt varies in grain size from 0,063 mm down to 0,002 mm, although these limits may vary slightly according to soil classifi cation system. It can include fractions of other soil types which will affect its compaction properties. In pure silt or silt that is mixed with coarse-grained fractions, there is little cohesion. With higher clay content, cohesion will increase. As with all fine-grained soils, the compaction of silt is heavily dependent on water content. For good compaction effect the water content should not diverge too much from the optimum. At optimum water content, silt is relatively easy to compact. At high water content and under the influence of vibration or traffic, silt is transformed into a more or less fluid state. Vibratory equipment is the most effective for silt. Layer thickness can be almost the same as for gravel and sand if the clay content is not too high. If the clay content is higher than 5% (but less than 15%) large machines and thinner layers are required to overcome the cohesion in the material. In such cases, a pad-foot drum may give better results than a smooth drum. In addition, vibratory plates and smooth drum rollers may have traction problems especially when the water content is a little higher.

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SOIL COMPACTION

Soil volumes Soils have different densities depending on whether they are in situ, loose or compacted. The compacted layer thickness is always stated in the design of new structures. Soil volume can be defined under different conditions • in natural state (in situ) • loose state (un-compacted) • compacted

Compaction properties of soil A summary of the compaction properties of fine-grained and coarse-grained soils. Coarse-grained materials Relatively easy to compact, especially by vibration. High bearing capacity. Free-draining soils are not susceptible to soaking and frost.

Gravel

Sand

Fine-grained materials Water content, and thus weather conditions, are important to compaction results. To be compacted in relatively thin layers.

Silt

The table below gives the relative volumes of different soil types.

Clay consists of the smallest particles from around 0,002 mm and downward. The particles are so small that they cannot be discerned by the human eye. A clay content of 15% is sufficient for the soil to display the properties of clay where cohesion and apparent cohesion are the main resistance factors. The effect of cohesion depends on the clay content, the grain size and shape as well as the mineral composition of the clay. It can vary widely between two different clays with the same grain distribution curve but different grain shapes and mineral composition. The water content has a great effect on the compaction resistance of the material. Compaction is easiest at or above the optimum water content. Clay requires a relatively high compaction effort (compared with coarse-grained soils). Vibratory pad-foot rollers are very suitable for compaction of clay at water contents below the optimum. They can transmit the high pressures and shear forces needed to compact his material at or below the optimum water content. Here the compressive strength is the highest. Layer thicknesses are normally restricted to 15–40 cm depending on the machine size.

Boulders and Cobbles

(blasted)

(not blasted)

Gravel and Sand

Silt

Clay

1,0 m3

1,75 m3

1,2 m3

1,3 m3

1,5 m3

1,4 m3

0,9 m3

0,85 m3

Clay

Maximum permissible content of fines in free-draining soils: 5–10%

Clay

Rock fill

High speed tamping rollers are also suitable for compaction of clay. They are very economical on large clay fills. In such cases the clay is placed in 15–20 cm layers. Clay with a water content above the optimum has less compressive strength and can be compacted using vibratory smooth drums or with pneumatic tyred rollers.

Lime stabilization

Cohesive soils are not possible to compact when the water content is high. Stabilization of the material using lime is one way of improving the compactablity and the stability of the material. Lime is spread on the surface and mixed into the material using a rotary soil stabilizer. The lime binds part of the water and in time it also creates a chemical binding that substantially increases the strength of the clay. Vibratory pad-foot rollers are often the best choice for compaction of stabilized materials.

Sub-base and base course

Sub-base and base course are selected materials and should be within specified limits of a gradation curve. The main fraction consists of gravel. In certain countries relatively high amounts of fines are allowed in the sub-base, but it then loses its free-draining properties.

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Sub-bases and base courses normally have high compaction specifications and require a higher compaction effort than fill material for the same layer thickness. Vibratory equipment is the most effective on sub-base and base course. Impact compaction is not suitable. In some cases, where the base course is thin (less than 10–15 cm) static rollers can be used. They are especially suitable if material loosening is to be avoided. A base course should always be finished off with a couple of static passes before surfacing work can begin.

Stabilization

Sub-base and base courses can sometime consist of granular materials mixed with cement, fly-ash or even bitumen. This is done in order to increase the load bearing properties of the material. Stabilized base course material is often placed using a paver in order to achieve the best possible evenness.

Sub-base and base course

SOIL COMPACTION

SPECIAL APPLICATIONS There are a number of applications (see below) that require a special approach and methods and where general guidelines for compaction do not apply.

Slope compaction Slope compaction can be required for the construction of dams and canals. Dams with an impervious upstream surface of asphalt or cement concrete are one example where good slope compaction is especially desirable. A self-propelled single-drum vibratory roller is the most suitable type of machine for slope compaction. Whether the roller needs to be winch-aided or not depends on the incline. When compacting, the vibration should be switched on for the upward journey and off for

the downward one. If the roller is winched a strong mesh should be used to protect the operator and a safety wire should be attached to the machine. Always use a roll over protection system (ROPS). Prior to using machines on slopes check with the manufacturer that the machines can operate continuously on the incline in question. Dry compaction Normally all types of soil are compacted most efficiently at optimum water content. However, in some areas such as arid or semi-arid areas, it may be impractical or too costly to water the soil. In such cases gravel and sand can be compacted in a dry state (water content <1,5%). It is important to take into account the saline content of the soil since a high content may be detrimental to the load bearing properties of the material. Dry compaction has been applied

with good results, on relatively thick layers, in road and airfield constructions in desert areas. Soil stabilization Stabilization increases the strength of a structure. It can be used on very loose soils as well as on base courses to help withstand very heavy load situations. In loose soils, such as clay, stabilization may be chemical or mechanical. In chemical stabilization, lime, cement and/or fly ash is mixed with the soil. Other chemicals can also be used. In mechanical stabilization a coarse-grained soil is added. The stabilizing agent is mixed with the soil and compaction takes place as soon as possible and when there is no risk of the compaction equipment sinking into the soil. Cement is often used to stabilize base courses. Once the cement has been added, the material should be compacted within one hour as the cement and water start to react immediately.

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SOIL COMPACTION

Roller Compacted Concrete In contrast to cement stabilized base courses, roller compacted concrete (RCC) is earth-moist concrete with a 5–6% water content that is pre-mixed, transported to the site and laid using standard hauling and spreading equipment. It is then compacted with vibratory rollers. RCC is used in concrete dams as a fill and in industrial and port areas where heavy vehicles travel and maneuver at low speed. Asphalt is not suitable for this kind of applications. In dam construction the concrete has low cement content (4–7%) and is normally spread in layers of 20–30 cm. Other suitable applications include paving in tunnels and mines. Test areas At the start of a road construction project, test strips are often set up to establish suitable compaction procedures which meet compaction specifications. On large compaction jobs, for example the construction of a dam, a full scale compaction test may be carried out employing a number of

different types of roller to establish the best compaction practice. One way of setting up a test is to lay down a strip where the layer thickness increases from virtually zero to the thickest required. The specified measurements can be made on the different thicknesses as the compaction process proceeds. In this way, the maximum layer thickness can be determined for the job in question. Ground vibrations A vibratory roller in operation generates pressure and shear waves as well as surface waves. These surface waves are of primary concern for structures on or near the surface of the soil. An approximate and general rule has been established stating that ground vibrations that do not exceed 10mm/s do not cause any damage to building with foundations on soil. Considerably higher safety limits, around 50 mm/s, applies to blasting operations. It is also worth noting that a simultaneous measurement of ground vibrations in a building structure and the surrounding ground shows a significant difference in wave velocity. A velocity of 10 mm/s in the ground corresponds to 2–5 mm/s in the actual structure.

Practical research has led to the following recommended safety distances resulting in a wave velocity not exceeding 5 mm/s in the building foundation.

Please note that the below information is to be seen as a general recommendation and that Atlas Copco does not accept any responsibility to any actual damage that may occur even if these recommendations are followed. As all materials behave differently it is recommended that vibration monitoring equipment is installed in any building where structural damage must be avoided.

Safety distances for vibratory rollers (including a safety factor of 2) Self-propelled vibratory roller with pneumatic drive wheels (high amplitude) Safety distance in meters = 3 times the drum module weight (in tons) Vibratory tandem rollers (high amplitude setting) Safety distance in meters = 2 times the drum module weight.

Common setup for a test area on site. Maximum acceptable layer thickness and the suitable number of passes can be determined.

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SOIL COMPACTION

SPECIFICATIONS AND FIELD CONTROL METHODS There are three types of main specification, which sometimes can be combined – method, end result and function.

Irrespective of the type of specifi cation, there is a call from governments, governmental agencies, private owner-operators as well as contractors themselves for effective quality assurance methods.

leeway in the choice of equipment, and lend themselves to the most economical method of achieving specified densities. Often, vibratory equipment enables the contractor to work to the best margins.

Method specifications Method specifications stipulate detailed rules for type of equipment to be used, number of roller passes, roller speed, layer thickness, type of soil and water content of the soil. The contractor is required to follow these rules in the compaction work.

Functional specifications A third type of specification is known as the function specification where specified functions, for example the settlement, evenness and friction, have to be fulfilled for a certain contractual period. The contractor is free to use the materials, layer thicknesses and equipment of his choice as long as a specified quality can be achieved. This type of contract is often linked to a Build-Own/Operate-Transfer, BOT, contract where the contractor assumes operation of the highway or other structure for a certain time – including maintenance and other work – before transferring it back to the local road authority.

End-result specifications End-results are specified for the majority of the compaction work involved in the construction of roads, railways, dams and foundations world-wide. The specification may include minimum densities or minimum load bearing properties. The trend towards end-result specifications is universal. They offer more

METHOD SPECIFICATION

HOW COMPACTION WORK SHOULD BE CARRIED OUT • Material and layer thickness • Machine type and size • Machine settings and number of passes The contractor must sign off on that the stipulated method was followed. END-RESULT SPECIFICATION

WHAT COMPACTION RESULTS SHOULD BE ACHIEVED • Compaction result • Control method The compaction method is decided by the contractor. The end result must be tested and reported to the project owner for approval. FUNCTIONAL SPECIFICATION

FUNCTION OF THE COMPLETED STRUCTURE OVER TIME • Traffic volumes • Expected life time • Minimum acceptable road quality (for example evenness, friction, rut depth etc) The contractor designs and builds the road to meet the functional requirements, maintenance responsibility is also included in the contract.

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SOIL COMPACTION

FIELD CONTROL METHODS There are a number of methods of controlling specifications on soil in the field. These include density tests, load bearing tests, levelling tests, and others, all of which are spot measurements. Another method is the roller-mounted compaction meter linked to a documentation system that continuously controls the compaction process and the results.

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SOIL COMPACTION

Replacement method The sand replacement and water balloon methods are used as replacement methods. A small hole is dug in the soil. The contents are weighed and the volume of the hole is determined by filling it with calibrated dry sand or with the water balloon.

Tube sampling For fine-grained soils, especially clay, a tube is pressed down into the material to remove a core sample for density tests.

Nuclear gauge method A nuclear density gauge provides an immediate indication of the density of the compacted layer. It also measures the water content. It works on the principle that radi ation from a radioactive isotope through a material is attenuated in proportion to its density. Best results are obtained in homogeneous soils.

Static plate load test Static plate load test is performed on the surface of the compacted material. By measuring the defor mation under the plate (with a known area and load) it is possible to calculate the modulus of elasti city of the compacted soil. The load bearing properties of the underlying layers will have an influence on the measurement. The degree of influence depends on the thickness of the compacted layer.

Penetration test There are several types of pene tration tests which represent an attempt to quantify the behavior of a soil. One of the most common is the California Bearing Ratio (CBR) test. The CBR test is an arbitrary test. It does not attempt to measure directly any of the fundamental properties of the soil sample. In essence, it consists of driving a standard cylindrical plunger into the soil sample at a standard rate of penetration and measuring the resistance to penetration offered by the soil. This resistance is then compared with certain standard results. The ratio of the result for the soil to the standard result is reported as the CBR. The California Bearing Ratio test is mostly used on fine-grained soils.

Falling-weight test Falling-weight test units are an effective and rapid way of measuring the load bearing properties of the surface of the construction layers on site. The test can normally be handled by one operator. The unit measures the surface deflection caused by a falling weight and from that calculates a dynamic modulus of elasticity. There are both light and heavy falling weights.

Levelling of surface settlement This method is mostly used on rock fill, cobbles and boulders. The level of a number of reference points is checked with a levelling instrument before and after compaction. It does not provide a direct measurement of the density.

Proof rolling This is a test where a very heavy pneumatic-tyred roller is run over the compacted surface and the indentation may not exceed a certain depth.

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SOIL COMPACTION

Continuous compaction control (CCC) Many highway and other specifying agencies ask for documented proof that a contract has been completed as specified over the entire surface in question and not only at a number of random sampling spots. The demand for Quality Assurance has led to the development of sophisticated documentation (control and monitoring) systems that plot and record the result from a compaction meter using the roller as a measuring device. The instantaneous and continuous registration of the entire compacted surface provides major benefits compared with conventional control methods which may disrupt and delay the compaction work. Conventional testing methods for soil compaction may in some cases result in costs which are greater than for the compaction job itself. The compaction meter has proven to be a very cost-effective control method. The use of the compaction meter and documentation system, in combination with a limited number of density/load-bearing tests, is included in specifications in a number of countries mainly for coarse-grained soils. The documentation of the compaction results gives all stakeholders valuable information regarding the quality and uniformity obtained. Even if the use of a compaction meter not is included in the specifications, it will help operators to identify weak spots which need more roller passes, and, in general, to optimize the number of passes to avoid over-rolling.

Compaction meter and documentation systems Principle and function A roller-mounted compaction meter consists of an accelerometer mounted on the vibrating drum. The accelerometer readings are sent to processor and the readings are presented to the operator on the control panel of the roller. The signals from the accelerometer are converted to values that indicate a measure of the stiffness of the ground. The system records conditions at certain depths. The actual depth depends on the size of the roller and amplitude selected. A computer documents and presents the measured values on a screen which can be placed in view of the roller operator. The

documentation system enables the entire area that has been rolled to be presented on the screen. Use of colour and other graphics make it immediately apparent which areas require additional compaction. A GNSS receiver provides accurate positioning and speed information to the on-board system. The documented result can then be transferred to the office for final analysis and storage. Applications The compaction meter (with or with足 out the documentation system) is most suitable on coarse-grained soil and rock fills. A soft, un-compacted soil gives little response while a hard, well compacted soil will give a better response. The stiffness increases in proportion to the bearing capacity.

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CCC can be used and specified as one or more of the four different methods described below. Weak area analysis The CCC values are used to define one or more areas where the soil stiffness is the lowest. These areas are the subjected to directed testing. If the areas pass the test this means the rest of the area is okay.

SOIL COMPACTION

Calibrated target value The CCC values are calibrated to create a correlation to the accep足 tance control method, typically static plate load tests. Using this correlation a corresponding CCC value is determined and used as the target value for the area.

Pass count Using the CCC system to count and document the number of passes made helps the operator achieve even compaction results and provides documentation that the job was done according to the specification.

Progress The CCC values increase with every compaction pass, a higher increase for the first passes and a lower increase for the later passes. Once then increase from one pass to the next is below a certain level there is very little more compaction

to be achieved. This is one measure that can be used to determine when the compaction is finished. It is also possible to use several methods in combination, this is one example: Target number of passes: 6 Weak area analysis: 2 areas Progress: Maximum 5%

The operator runs six vibratory passes with the roller. When this is completed, change view to look at the compaction meter value and locate the two weakest areas. Check if the compaction increase from the previous pass is less than 5%. If it is more than five percent it is still possible to increase the stiffness on these areas, compaction is not finished here. Make more passes and review where the weak areas are to be found now. Check the increase percentage; if it less than five percent from the previous pass this means that the roller that is being used cannot achieve more compaction. It is then suitable to run acceptance testing on these areas. If the test fails it is most likely a material problem as the machine is not able to achieve a higher stiffness. A successful procedure used in road and airfield constructions has been to first register the compaction meter values over the compacted areas, and then perform static load bearing tests in a limited number of points, selected where the lowest compaction meter values were mea足sured. This procedure should give a good certainty that a prescribed load bearing capacity

is attained over the entire area of, for example a base course. On fine-grained soils the bearing capacity is, to a high degree, related to the water content. As the compaction meter indicates the load bearing properties, no direct relationship exists between the compaction meter value and the soil density. The compaction meter can therefore not be used to directly guide the compaction work as on coarse-grained soil. Information given by the compaction meter on the level and uniformity of the load bearing properties may, however be of great value. A useful application of the compaction meter is to detect soft and weak spots of fine-grained soils with high water content. Such spots are found in fill materials as well as in natural ground. Rollers equipped with compaction meters have therefore, with good results, been used to survey the ground surfaces on which road and railway embankments shall be built. Even if the use of a compaction meter is not included in the specifications, it will help operators to identify areas which need more roller passes, and, in general, to optimize the number of passes to avoid over-rolling.

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ASPHALT PAVING, COMPACTION AND MILLING

ASPHALT PAVING, COMPACTION AND MILLING A road traffic system is multifaceted. It comprises roads, the people who use those roads and vehicles. How well the system functions as a whole depends on the individual componentsâ&#x20AC;&#x2122; characteristics, how they interact, and the impact of outside factors such as climate, light conditions, etc.

1 Road surfacing has a decisive impact on traffic. The type of wearing course and the condition it is in affect the behaviour of the vehicles using the road and, thereby, road safety. They also affect the cost of travelling as well as the environment. The majority of all paved roads are surfaced with asphaltÂš. Concrete is also used but is, in general, less common, although there are some countries where concrete is the preferred material. Asphalt is used in the wearing, binder and base courses. 1

Asphalt refers to a mixture of bitumen binder with mineral aggregate and filler.

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2 3

ASPHALT PAVING, COMPACTION AND MILLING

3 The image has been manipulated to highlight the three separate layers.

The wearing course (1) provides an even, weather-resistant and high-friction running surface which can withstand abrasive forces. It makes the road safe and the ride comfortable. In combination with the other layers in the pavement, the wearing course helps to distribute the traffic loads to avoid excessive loading of the entire pavement. The binder course (2) fulfils the same load distributing function and provides an even, level surface to carry the wearing course.

The base course (3) is the main component which provides the strength and load distributing properties of the pavement. On roads with light traffic, it is usually made from graded crushed stone. On more heavily trafficked roads, a fully bituminous road base or cement stabilised granular base may be employed to achieve the required strength and durability.

material and the thickness of each layer in the pavement are critical to good service life. Correct design requires knowledge of the materialsâ&#x20AC;&#x2122; different properties and the expected load and intensity of the traffic. In addition, it must take into account local climatic conditions as well as the economic constraints.

In the design of the pavement (the part of a road above the embankment) the choice of

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ASPHALT PAVING, COMPACTION AND MILLING

QUALITY AND FUNCTIONAL REQUIREMENTS FOR ASPHALT PAVEMENTS An asphalt surfacing is normally built to last for a certain period of time, for example 20 years. Its durability will depend on the quality of the components, the mix design, and the manufacturing process from asphalt mixing to final compaction.

THERE ARE A NUMBER OF FUNCTIONAL REQUIREMENTS THAT A ROAD SURFACE HAS TO COMPLY WITH TO MAKE IT USABLE.

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The quality of an asphalt surfacing can be measured against a number of properties. The most important include: • Resistance to plastic deformation, which can be expressed as stability • Traffic and temperature related fatigue • Load distribution • Sensitivity to water • Ageing

1. Evenness If a road is to function satisfactorily over a given period of time, the surface has to be even. Unevenness reduces traffic speed and prolongs journeys. It reduces riding quality and increases vehicle and tyre wear. It also increases the impact effect of vehicles on the road, which in turn accelerates road wear and thus shortens the service life. Transversal unevenness refers to rutting as a result of wear on the wearing course or deformation in one or more of the underlying layers. A measure of this is often the depths of the ruts. Longitudinal unevenness refers to lengthways unevenness of the road or road section. Different methods are used to measure its occurrence, such as the International Roughness Index (IRI).

ASPHALT PAVING, COMPACTION AND MILLING

The choice of surfacing and its qualities depend on the weight and intensity of traffic as well as the climatic conditions that the road is expected to be subjected to within the given period.

TYPE OF SURFACING If the surfacing is to function as intended, the various ingredients – binder, aggregate and filler – need to be selected carefully with a view to optimising the final mix. Good quality material and good design are not enough to guarantee a long service life. Just as vital are the way the material is manufactured and the way it is laid down and compacted. The benefits of firstclass material can quickly disappear if the quality in one of the stages in the production chain fails to come up to standard.

The three main components in an asphalt mix are binder, aggregate and filler. In many cases, the binder will also contain additives. In practice, there are two main types of asphalt, mixed asphalt and surface treatments.

2a. Texture

improves road safety through better skid resistance and enhances the feeling of safety.

Texture refers to the surface roughness. Texture is broken down into varying degrees: macro- (0.5–50 mm) and microtexture (<0.5 mm). Micro texture indicates the roughness of the stone in the layer. Macro and micro textures both have an effect on tyre wear – the smoother the texture the less the wear. The macro texture has a significant influence on tyre noise and the friction between tyre and road surface.

2b. Friction Friction is measured as the quotient of a vertical and horizontal force of a wheel using a standardised formula. Good friction

Mixed asphalt

Although there is a wide variation of mixed asphalt there is no generally acknowledged classification system. The most common way to categorise them is by mix temperature: Hot Mix Asphalt (HMA),

Warm Mix Asphalt (WMA) and Cold Mix Asphalt (CMA). HMA as well as WMA is a mixture of heated aggregate, bitumen and filler. It is manufactured in batch or drum mix plants at high temperatures i.e. 130–180 °C (HMA). The penetration value of the bitumen is determined by outside factors such as climate and traffic. Hot climates and heavy traffic require a high penetration value, for example. The binder can be modified with different additives such as polymers.

as well as the risk of aqua planing.

Load bearing capacity

3. Retroreflection Retroreflection is a measure of the “brightness” of the surface. High retroreflection enhances visibility in darkness which permits higher speeds and shorter journey times. Better visibility should lead to better safety at night, but the ability to travel at higher speeds may counteract this.

4. Porosity Porosity is the ability of the surface to drain out water. A porous surface also reduces tyre noise. Less water on the road significantly reduces the dirt and water spray

The load bearing capacity of a road has a marked influence on the road’s service life. A reduced load bearing capability can force heavy traffic to choose other routes (often longer).

Abrasion resistance The road’s abrasion resistance is not simply interesting for its service life. If the wear off from the surface is large, the dust that is worn off may cause pollution in the vicinity of the road.

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ASPHALT PAVING, COMPACTION AND MILLING

Cold Mix Asphalt (CMA) CMA is produced using cold aggregates and a pre-heated binder (75–85°C). It is used as base, binder and/or wearing course for roads with a low traffic volume. Warm Mix Asphalt (WMA) WMA is produced at a lower temperature than HMA, 100–140 °C. A lower temperature means that less energy is consumed in producing the asphalt. Less energy used mean lower emissions from the production. The application for WMA is basically the same as for HMA. Hot Mix Asphalt (HMA) HMA is produced as Dense asphalt concrete for use as a base, binder or wearing course or as Stone Mastic asphalt (SMA) or Porous asphalt for use as wearing course. The aggregate in a dense mix or an asphalt base has a dense gradation while SMA and Porous Asphalt are both open graded. Dense Asphalt displays good ageing resistance thanks to its low air-void content. Dense asphalt is suitable for all asphalt pavement applications. It is less abrasion resistant and stable than SMA. The main characteristics of SMA are the gap in the fine end of the curve, a high content of coarse aggregate and the high filler con-

tent. The coarse material builds a skeleton of aggregate in the mix. The binder content in SMA is somewhat higher than in Dense Asphalt which in turn has higher binder content than Porous Asphalt. Fibres are sometimes used as a carrier for the binder to ensure sufficient high binder content in SMA and Porous Asphalt. The air void content in Dense Asphalt and SMA is usually 2–5% (Asphalt Base: 4–7%), while in Porous Asphalt it is considerably higher at around 15–20%. SMA is well suited for wearing courses on high volume roads owing to the high stone content which provides good resistance against abrasion as well as good stability. As the name implies, Porous (or drainage) asphalt has good draining properties. This reduces the risk of water spray and aquaplaning. It has good retro-reflective properties in darkness and rain and tyre noise is lower than on other types of surfacing. These benefits diminish relatively quickly as the pores in the surface get clogged with other particles and dirt. Owing to its structure, Porous Asphalt is more susceptible to climatic effects. This can have a negative effect on the water resistance and the ageing properties of the binder and shorten the service life.

Surface treatment

Surface treatment is the process of laying binder and aggregate separately. Examples of coating include surface sealing, penetration macadam and slurry seal. Surface sealing Surface sealing prevents water from penetrating the road. The amount of binder is important for a long service life of the surface treatment. The aggregate needs to be uniform in size and washed to remove the fines to ensure good adhesion. Penetration macadam Penetration macadam is sometimes used as a base and wearing course. It comprises a layer of aggregate over which a layer of bituminous binder is spread. If the binder is a bitumen emulsion, the viscosity and dispersion properties should be such that the binder does not penetrate more than 50% of the aggregate’s layer thickness. Slurry seal Slurry seal involves spreading a binder and then laying sand on top. Pre-manufactured emulsion slurry can also be used. Slurry seal is used to fill cracks and other cavities to prevent water penetrating the road surface.

Dense asphalt concrete

Porous asphalt

Stone mastic asphalt

Hot Mix Asphalt.

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Surface sealing.

ASPHALT PAVING, COMPACTION AND MILLING

MIXED ASPHALT COMPONENTS Asphalt normally consists of three material components, the binder, the aggregate and the filler. Some surfacing material includes additives such as adhesives, polymers, fibres and recycled material.

Binders

Cutback and emulsion Cutback is a mixture of bitumen and solvent, for example naphtha, while emulsion is a mix of bitumen, an emulsifier and water. They both enhance the fluid properties of a mix at low temperatures. When the

solvent or water evaporates, the bitumen retains its original properties. The properties of the binder in the road are mainly determined by the constituent bitumen. The use of cutback is on the decline, owing to environmental concerns, while the use of emulsion is increasing. The most common areas of application include, surface treatment, CMA, tack-coating, slurry sealing and penetration. Specifications and test methods for bitumen In most countries bitumen is classified according to viscosity or penetration. Ageing properties are determined by the measurement of one or several parameters before and after ageing in the laboratory according to stipulated methods.

Stiffness modulus (MPa)

Bitumen The binder in an asphalt mix is referred to as bituminous, i.e. it contains bitumen in some form. Bitumen is a thermo-plastic material which means that it becomes softer and more fluid when heated and hardens when cooled. The process is repeatable. It can also be described as a visco-elastic material which means that its stiffness is a function of temperature as well as loading time. From the figure below you can see that the stiffness at a given loading time decreases when the temperature increases. The figure also shows that at a given temperature stiffness decreases as the loading time increases. When the bitumen is mixed with

aggregate, it must be sufficiently viscous to cover the surface of the aggregate. However, it cannot be too fluid as the binder will drain off from the surface of the aggregate during storage or transportation. The viscosity must also facilitate the paving and compaction process. The binder should provide stability to avoid excessive deformation, but it must be flexible enough to avoid the risk of cracking. The adhesive qualities of the binder determine how much aggregate loosens from the surface (ravelling).

mm x 10

Loading time (S)

Bitumen stiffness as a function of temperature and loading time for a 100 Pen bitumen.

25째C Specifications and test methods for bitumen Determination of penetration.

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ASPHALT PAVING, COMPACTION AND MILLING

This modified Bitumen Test Data Chart shows the maximum and minimum mixing and compaction temperatures for a 100 pen bitumen with the softening point of 50 °C.

Bitumen Test Data Chart The Bitumen Test Data Chart (BTDC) is used to predict the temperature/viscosity relationship of the bitumen over a wide range of temperatures, and is very useful to ensure the appropriate viscosity for any grade of bitumen. The BTDC consists of a horizontal temperature scale and two vertical scales for penetration and viscosity. The temperature scale is linear while the penetration scale is logarithmic. The viscosity scale has been designed so that penetration classified bitumen with normal temperature susceptibility or penetration index will give straight-line relationships. There are optimum bitumen viscosities for the manufacturing and compaction of bituminous mixes. Excessive viscosity during mixing, results in the aggregate not being coated properly while if the viscosity is too low, the bitumen will easily coat the aggregate but may subsequently drain off the aggregate. If the viscosity is too low during compaction, the mix will be extremely soft and workable. This may result in shoving or transversal movement of the mix. High viscosity will significantly reduce the workability of the mix and consequently make it more difficult to compact. Performance Grade (PG) The United States uses Superpave to specify asphalt materials. Asphalt binders are specified according to a performance based specification. The temperature of the pavement in which the binder is going to be used determines the choice of binder. Performance graded bitumen is classified according to the highest and lowest pavement temperature at which the bitumen must have the ability to avoid rutting and low-temperature cracking. For example, a PG 64–22 (sixty-four minus twenty-two) is designed to prevent rutting on a hot summer day where the temperature is +64 °C 20 mm below the surface and to counteract low temperature cracking in the winter at -22 °C at the surface.

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Aggregate

Aggregate is a general term for all the mineral consti tuents of an asphalt mix. It includes crushed stone, gravel, sand, slag and fines. In asphalt, the weight of the aggregate accounts for about 85% of the total weight of the mix. The quality of the aggregate is dependent on both the origin of the aggregates as well as the production method (natural or crushed material). The properties of an aggregate that directly or indirectly influence the functional properties of the surface are grain-size distribution, porosity, grain shape, durability, abrasion resistance, polish resistance and resistance to weathering. A number of these are interrelated. Particle properties The most important physical properties of a mineral are strength and shape. The quality of a rock material can be partially improved in the production stage. In principle, each crushing stage can improve the materials’ mechanical properties. Shaping, for example, increases the abrasion resistance of the aggregate as well as the stability of the mix in a wearing course and therefore prolongs the service life of the road. Grain-size distribution Grain-size distribution is the basic property of an aggre gate. The grain-size distribution of a given sample is determined by a sieve test where the dried sample is passed through a number of standard sieves which

ASPHALT PAVING, COMPACTION AND MILLING

differ in screen size. The grain-size distribution is described graphically in the form of a gradation curve. The grain-size distribution determines the type of mix. Varying the grain-size distribution for a given mix type will influence the functional properties of the asphalt. Filler The filler is used to fill the voids between the coarser particles and to stiffen the binder. It thereby contributes to the stability of the asphalt mix. The filler (particles <0,074 mm) may be obtained from the dust collecting system at an asphalt plant or specially produced by crushing. Special fillers such as slaked lime and cement are sometimes used to reduce the risk of stripping. Additives The increase in traffic in many countries has led to the need for high quality roads. The development of newer and better materials is one solution to the problem, and a large number of additives have been proposed for asphalt mixes. They can be generally classified into two groups. The first comprises commercial products designed to improve the function of the asphalt. They include polymers, adhesives, ageing inhibitors, softeners, stability enhancers (natural asphalt, oxidation catalysts) and fibres to carry the binder. A number of polymers – elastomers and plastomers – have been used to modify bitumen to improve service life and the road surface function. They can be used to enhance stability at high temperatures or improve cracking resistance at low temperatures, for example. The second group comprises various types of recycled products such as granulated rubber or fly ash.

TENDER MIX

HARSH MIX

Tendency to tender mix – unstable surfacings

Tendency to harsh mix – stable surfacings

Natural aggregate

Crushed aggregate

Low stone content

High stone content

Small maximum stone size

Large maximum stone size

Low filler content

High filler content

Tender mixes often contain natural (rounded) aggregate with a small amount of filler. They tend to be soft and require careful compaction to avoid lateral displacement and surface cracks. They tend to produce unstable surfacings.

Harsh mixes are a result of the use of crushed aggregate containing a high percentage of coarse material and a sufficient amount of filler. Resistance to compaction is strong, and they require a large compaction effort to reach specified density. They tend to produce stable surfacings.

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ASPHALT PAVING, COMPACTION AND MILLING

Gyratory compaction of asphalt sample.

Marshall compaction of asphalt sample.

MIX DESIGN PROPORTIONING Correct mix design is essential to a durable road.

Design involves the choice of material (binder, aggregate, filler and additives) with properties suited to the final required results and the mixing of these ingredients in the correct proportions. Outside factors such as climate and traffic intensity and volumes must also be taken into consideration. The temperature span determines the choice of bitumen. The types of aggregate and binder must relate to the traffic load. The greater the intensity, the higher these requirements are.

The type and volume of traffic has a strong bearing on choice of aggregate and binder as well as the design of the mix. Weight, axle configurations and tyre pressures should also be considered. Once the choice of ingredients has been made, the aggregate with the required gradation curve must be produced, A number of aggregate samples are mixed with different amounts of the selected binder to give a variation of binder content within given limits. One of

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the samples should be the binder content recommended in the high way authority’s own technical specifications. The binder content of the other samples should be in suitable intervals above and below this nominal content. The mixes are then compacted using a Gyratory compactor or Marshall apparatus. The compacted samples are then analysed for air void content, strength, etc. and the optimal mix is chosen.

ASPHALT PAVING, COMPACTION AND MILLING

Mix proportions of two different asphalt mixes with a maximum aggregate size of 16 mm. On the left is a dense asphalt concrete and on the right a stone mastic asphalt (SMA). Note the high content of large aggregate in the SMA. Dense asphalt concrete.

Stone mastic asphalt (SMA).

PROPERTIES OF ASPHALT MIXES

Internal friction

Adhesion

Asphalt and soil have a lot in common; however, a major distinction between them lies in the adhesive properties of the bitumen used to bind the particles in an asphalt mix. Asphalt mixes show wide variations in composition and properties. Their properties and compactability are primarily a function of: • Internal friction • Adhesion • Viscous resistance/temperature

Internal friction The first of these, internal friction, is determined mainly by the aggregate properties, and is more apparent in a well-graded mix than in an

open-graded one. A mix containing natural round aggregate, where the particles can move past each other relatively easily under compaction, has less internal friction than a mix with angular, crushed aggregate. The mix with crushed aggregate consequently needs a higher com paction effort and also gives an asphalt surfacing of higher strength and stability. High stone content and large maximum stone size are other factors that result in stable mixes.

Adhesion Adhesion is what makes the binder attach itself to the aggregate. Viscous resistance Viscous resistance is a function of the viscosity of the bitumen and the actual temperature of the mix. The viscous resistance works against the rearrangement of the particles under compaction, and the lower the temperature the greater this resistance is.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

ASPHALT PAVING, COMPACTION AND MILLING

MANUFACTURING PROCESS AND TRANSPORTATION Mixing Asphalt mixes are normally manufactured in either continuous or batch-type asphalt works. The asphalt plant can be mobile or stationary. Capacity normally varies between about 100 and 300 tonnes per hour in batch plants while continuous asphalt plants are used for the production of larger volumes of the same type of mix. Here capacity varies between 50 and 600 tonnes per hour. Naturally, the constituent components of an asphalt mix all have a decisive influence on the final quality of the mix. As more than 90% of the mix comprises aggregate, the quality of the mix is

highly dependent on the quality of the aggregate which is a function of the crushing process. It is also important to handle the aggregate in the correct manner to avoid deterioration of the gradation curve and exposure to moisture. A dry, well-graded aggregate is the foundation of a good asphalt mix. In modern plants, the proportioning of the aggregate is largely governed by automatic process controllers according to pre-programmed recipes. The aggregate is dried and heated in dryer drums. In the actual manufacturing process, bitumen and filler are added to the aggregate to form the mix. There are different types of filler

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according to the desired properties of the mix. Amines are added to improve adhesives qualities, fibres are used to allow higher volumes of bitumen, while polymers improve the binder properties. The constituents are mixed according to a set pattern in the mixer to achieve a homogenous asphalt mix. Mixing time will vary according to mix and type of mixer. It is important to final quality that the time is neither too short nor too long. Once ready the mix is transported to insulated and/or heated storage silos to reduce the cool off effect. Measures also have to be taken to ensure that the asphalt mix does not oxidise or segregate.

ASPHALT PAVING, COMPACTION AND MILLING

Using a material feeder increases the overall paving capacity while at the same time avoiding contact between truck and paver. More homogeneous mix temperature and better evenness can be expected. Constant speed and quality of the paved material also improve the compaction result.

Transportation Transportation of the mix from the asphalt plant to the site goes through three stages before it is laid down on the road surface: loading at the asphalt plant, transport to the site and tipping into the hopper of the paver. To avoid disruption the transportation must be well planned and carried out correctly. During loading it is important to minimise the risk for segregation. Loading must be quick and the load should be distributed as evenly as possible over the whole trailer. A steep-sided pile will cause the mix to segregate. The transport to the site must be well-planned. If a paver has to stop to wait for a new load, the quality of the surfaced road will suffer. It can lead to unevenness and reduced compaction both of which may shorten the service life of the road. On the other hand, a convoy of waiting lorries should be avoided at the site. The asphalt mix may cool off while waiting, which may lead to unsatisfactory compaction

results or having to discard the mix. The unloading of the asphalt mass requires skill to avoid segregation and to avoid stoppages. A large quantity of asphalt mix retains the heat for a longer period than a smaller amount. If it is placed on an insulated truck and covered properly, the chance of delivering at the correct temperature increases significantly. A rounded bed on the truck or trailer is also an advantage as the cold corners on a regular truck can be avoided. There are various mathematical formulae for working out the cost of transport of asphalt mix. The overriding aim of any such calculation must naturally be to achieve cost-effective transport and maintain the quality of the asphalt mix.

wearing course on a previously paved road. Tack coating is an important stage in road surfacing, and is often specified in road-building regulations. Correctly applied, tack coating prevents peeling and corrugation caused by traffic. It is also essential for load bearing properties that the layers bind well together. The improved adhesion afforded by tack coating means there will be fewer tendencies towards displacement of the mix or crack formation when rolling.

Tack coating Tack coating is the use of an asphalt emulsion or cutback to “glue” or bind together two surface layers – for instance when adding a new

ATLAS COPCO | COMPACTION, PAVING AND MILLING

ASPHALT PAVING, COMPACTION AND MILLING

ASPHALT PAVERS The task of an asphalt paver is to produce an even surface layer with homogeneous pre-compaction in order to give sufficient mix stability for the roller to start the compaction process. It also has to provide a homogeneous texture.

The performance of the paver is the most important factor when it comes to achieving these requirements. All modern asphalt pavers consist of two main units: the tractor and the floating screed. Tractor unit The tractor unit is driven by either pneumatic-tyred wheels or crawler tracks. Wheeled pavers are easy to transport. Their high travelling speeds allow them to move about the work site rapidly and to move easily between different sites on public roads. The good traction of tracked pavers makes them suitable for use on unbound surfaces and when laying unbound or cement stabilized base materials. Tracked pavers are also required when laying extra wide sections and on steep inclinations.

Material flow The asphalt mix is discharged into the receiving hopper of the paver as it pushes the rear tyres of the haul truck. The mix is carried from the hopper to the rear of the machine by twin or single slat conveyors, and then on to the auger (screw conveyor) which distributes the mix laterally over the entire working width of the screed. The height of the auger is adjustable to allow for different layer thicknesses. The material flow is regulated by the speed of the slat conveyor and the auger.

The conveyor speed must be correlated to the forward speed of the paver and the height of the material which is spread out ahead of the screed. This height has to be kept as constant as possible.

1 2

3 4

A smooth material flow throughout the paver â&#x20AC;&#x201C; from the hopper (1), through the conveyors (2), past the auger (3) and to the screed (4) â&#x20AC;&#x201C; is essential to good paving results.

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ASPHALT PAVING, COMPACTION AND MILLING

Screed unit The screed levels and pre-compacts the asphalt mix to a specified thickness, grade, cross-slope and crown profile. The self-levelling floating screed is attached to the tractor by side arms at tow-points located on either side of the tractor near its central point. Here, the vertical movements caused by any surface unevenness are at a minimum. This allows the screed to produce an even surface even if the underlying base is somewhat irregular. As each, successive asphalt layer is placed on top of the other; irregularities become less and less apparent. The tow points are set to give the required thickness of the mat. Their position may then be continuously finely adjusted by electronic systems. A grade controller automatically maintains the surface level against a reference surface such as a control ski or a string line, while a slope controller is used to maintain the transverse inclination of the screed. Angle of attack The angle between the bottom plate of the screed and the surface being paved is known as the angle of attack. This varies from screed to screed according to screed weight, the contact area of the bottom plate and the shape of the leading edge of the screed. The layer thickness and type of material also influences the required angle of attack The desired surface evenness is obtained if all the forces acting on the screed are in equilibrium. Only then will the screed maintain a constant angle of attack resulting in an even layer thickness.

The angle of attack may be increased or decreased by raising or lowering the tow point level. Any movement of the tow points upsets the equilibrium and results in a rise or fall of the screed. Once the screed has attained the new level, the angle of attack is restored, and the forces revert to a state of equilibrium.

1 Tow point on the tractor, adjustable in height. 2 Traction force pulling the screed forward. 3 Resistance from the head of material and the friction between the screed bottom plates and the paved material. 4 Screed mass acting on the material. 5 Lifting force generated by the angle of attack and the forward movement of the screed. 6 Angle of attack generating lifting force and pre-compaction of the paved material.

1 2

4 3

6 5

The angle between the bottom plate of the screed and the surface being paved is known as the angle of attack. Any change in the level of the tow points results in a corresponding adjustment of this angle. The desired surface evenness is obtained if all the forces acting on the screed are in equilibrium.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

ASPHALT PAVING, COMPACTION AND MILLING

Heating of the screed bottom plates Screeds are heated with diesel-fired or propane gas burners or electricity to prevent the bottom plate sticking up the hot mix which could cause drag marks on the surface. Fixed or variable paving width The most common type of screed is the telescopic screed which has a hydraulically variable working width. The telescopic screed allows the operator to vary the working width at the flick of a switch. This is very convenient in urban areas and other places where the paving width varies. A fixed screed is the most economical and suitable choice for applications where the paving width is constant for longer periods. Increasing or decreasing the paving width on a fixed screed requires a few hours of mechanical work to attach/detach and adjust the extension boxes. Compaction systems in screeds The main parameter affecting the screedâ&#x20AC;&#x2122;s ability to pre-compact the asphalt mix is its weight. A heavier screed will be able to reach a higher pre-compaction than a lighter one. Additional systems such as tampers and vibration generators are often attached to improve the flow of material below the screed.

The choice of a tamping and/or vibrating screed depends on the application as well as the mix type, maximum stone size, layer thickness as well as local preferences and specifications.

courses, special high-compaction screeds have been developed. These screeds are extra heavy and equipped with an additional compaction bar at the rear of the screed.

Tamper The tamping mechanism uses a vertical, high-amplitude tamper bar that moves at low frequencies. The main purpose of the tamper is to facilitate the flow of material below the screed plate. The tamping unit is followed by a static or vibrating bottom plate (screed plate). The width of the tamper and the tamping frequency limits the maximum paving speed. Drag marks can be the result of the paving speed is too high in relation to the tamper frequency.

Choice of screed and tractor unit

Vibrations The screed plate is equipped with a vibration generating system. The vibrations reduce the friction between screed plate and asphalt mix, letting the screed float more easily over the material. The vibrations will also cause some of the bitumen to rise to the asphalt surface, providing additional lubrication and enhancing the surface texture. High compaction screeds For special applications such as cement-stabilised layers and base

Fixed or variable paving width

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The choice of paving unit starts with the screed. It has to be able to lay at the desired width. Choice of screed also depends of type of asphalt mix and layer thickness. The choice of tractor will depend on what screed that was chosen. The tractor unit must be powerful enough to tow and support the screed at the specified widths. It must also be able to cope with the required lay down capacity or tonnage available per hour. Choice of wheels or tracks will depend on the foundation type as well as the paving width. Even if the underlying surface is somewhat uneven, modern electronic levelling systems automatically adjust the mat thickness to assure the correct cross-slope and grade, and maintain a level surface. The use of such systems does not take away the demand for proper preparation of the underlying surface. The better the evenness of the existing surface the better the evenness will be on the new layer as well.

Tamper bar

Bottom plate

Secondary compactor

ASPHALT PAVING, COMPACTION AND MILLING

PAVING OPERATIONS Any stoppages in the operation will result in a pavement of inferior quality and a shorter life-time. The paver speed should be kept constant and should correlate with the available mix tonnage (determined by the asphalt plant capacity and the number of available trucks). To achieve specified results, a number of points need to be taken into consideration. First the required paving width has to be set and the screed must be heated to prevent the mix sticking to the bottom plate. The tow points need to be set to the height that corresponds to the desired mat thickness. If necessary, the screed must be adjusted to allow for a crown profile. The height of the auger is also crucial to the outcome. If it is set too low it will interfere with the material flow under the screed which will result in an open texture and cause the mat to tear or create an uneven surface. If it is too high, the mix might not reach the outer edges of the screed, too much mix will sit in the auger channel and this will make it more difficulty to move the paver forward. Too much material also means it will move slowly and may cool down too much before going under the screed. Ideally, the distance between the mat surface and the lower edge of the auger flights should be equivalent to roughly five times the maximum stone size. There are a number of factors that need to be controlled during a paving operation. They include: • Head of material (in front of the screed) • Paving speed • Actual layer thickness

• • • • •

Careful planning of mix supply and transportation is crucial to maintaining a non-stop paving operation.

Surface evenness Paving width Joints Laying temperature Mix segregation

Head of material The head of material (the amount of material spread out in front of the screed) should be constant over the entire working width. It has a decisive influence on the vertical position of the screed. As mentioned earlier, the levelling action of a screed relies on a state of equilibrium between all the forces acting on it. Any change in these forces causes the screed

to move up or down accordingly. If the head of material gets too high the resistance to forward travel increases, and, in an attempt to overcome this resistance, the screed starts to rise. A ridge will then appear in the mat or the layer thickness will increase. Excessive material also accelerates the wear on the augers. If, on the other hand, the head of material is too low, the screed settles because there is not enough material to support it. An automatic system that monitors and controls the material flow through the conveyors and auger as well as the screed level will significantly reduce these effects.

How the head of material affects the height of the screed

Head of material too high. The screed rises.

Head of material too low. The screed settles.

Head of material correct. With a correct and constant head of material, the sum of all forces acting on the screed is in equilibrium, and the screed is able to maintain the desired level.

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ASPHALT PAVING, COMPACTION AND MILLING

Paving speed The paver speed should be as con stant as possible since variations in speed will result in an uneven surface. An automatic system to pre-set and maintain speeds under varying load conditions is recommended. Stoppages are also a problem. They may not only blemish the surface, they also result in temperature segregation. Every time the paver stops the screed tends to sink into the mat. The material in the auger box ahead of the screed and the mix just behind the paver, which are inaccessible to the rollers then cool down, while the mix below the screed remains hot. When the paver starts again the screed will lift slightly to overcome the cooler material ahead of the screed leaving a ridge in the mat. If the paver is forced to stop, the screed can be locked in position using a “screed stop system” which works off the hydraulic lift cylinders. This prevents the screed from sinking into the mat and reduces the problems associated with paver stoppages.

Common paving speeds range from 2 up to 20 m/min depending on mix type and equipment performance. There is a minimum speed to keep the screed floating. If the paving speed drops below this minimum level, the screed will settle. The layer will then be too thin. Speeds should be kept at around 2–4 m/min to achieve high densities when using high-compaction screeds. Layer thickness and surface evenness In order to achieve the specified evenness, normally expressed as a maximum permissible deviation in height measured over a certain distance, the layer thickness may vary to account for irregularities in the underlying surface. Where necessary, electronic levelling devices such as grade and/ or slope controllers should be used. These systems automatically adjust the mat thickness to maintain a level surface. If the paver is being operated manually, the crew must avoid frequent corrections of the height of the screed.

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Photo of high class asphalt surface.

ASPHALT PAVING, COMPACTION AND MILLING

Overlap (2–3 cm) paved by the paver

Joints The paving and compaction procedures employed for longitudinal and transversal joints are important to the overall quality and appearance of an asphalt surfacing. When laying an asphalt lane beside an existing lane the height of the screed above the surface must be carefully adjusted to allow for the compaction effect of the following roller, i.e. the un-compacted layer should be some 15–20% thicker. An automatic grade controller working off the adjacent lane is very useful for joint matching. The side overlap of the joint should be about 25 to 50 mm. There should be as little raking of

the joints as possible, so laying must be precise. To create a smooth transverse joint, the paver screed should be placed on top of the previously placed mat just in front of the joint. As the forces on the screed need to be in equilibrium when the paver resumes its work, only enough asphalt to cover the auger shaft is brought in before the paver moves forward. In order to ensure a good bond at joints, tack coating should be applied to the exposed surface. Type of mix and laying temperature Stiff mixes require heavy screeds, whereas less stable mixes require relatively light ones. Stiff mixes tend to lift the screed above the required level while tender mixes very often do not have the resistance to adequately support the weight of the screed. The load of the screed on tender mixes can be reduced with the help of a ”screed unload system” which transfers the weight of the screed to the tractor. This not only allows heavy screeds to be used on tender

Segregation of the mix can occur across the mat, at its edges and at the centre. It is one of the most common causes of damage to asphalt surfacings.

mixes: it also improves traction, and helps to obtain an even surface and uniform degree of compaction. Another factor that affects the outcome of a paving operation is the laying temperature of the mix. Variations in temperature cause variations in surface evenness and the compaction effect of the screed. As asphalt mix becomes more resistant to compaction as it cools down. The tractor unit must be able to provide the traction force to overcome this resistance. The laying of cold mixes therefore requires pavers with good traction and relatively heavy screeds. Furthermore, a cold mat may tear as the flowability of the asphalt decreases with temperature. Mix segregation Mix segregation is primarily the segregation of larger aggregates in an asphalt mix, and is one of the most common causes of damage to an asphalt surfacing. Segregation may occur early in the truck-loading stage at the asphalt plant, especially if the mix is poured too slowly into the truck. It is always difficult to avoid a certain stone concentration along the sides of the truck bed. Once the asphalt is segregated, it may remain so right through the paver and, at worst, result in a non-uniform surface. Segregation at the edges of the lanes may be caused by stone segregation along the sides on the truck and incorrect mix distribution in front of the screed. For example, if the material level is too high, it will slope towards the outer edges where stones can segregate. The height setting of the auger is another important factor in this respect. The segregated strip in the middle of the lane is caused by the auger drive unit located at the centre of the augers. Auger drives at the outer ends of the shafts will prevent this occurring. Transversal segregation zones normally arise from the segregation of materials at the front and back ends of the truck or from improper truck change procedures.

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ASPHALT PAVING, COMPACTION AND MILLING

ASPHALT COMPACTION There are a number of types of roller for asphalt compaction in current use. These include vibratory, static and pneumatic tyred rollers. The actual choice of machine depends on the type and size of the job, and is often related to local preferences.

The compaction effort of a static steel wheel roller is primarily dependent on its static weight but is also influenced by the drum diameter. Pneumatic tyred rollers rely on static weight and tyre pressures for their compaction effort. They are often used in combination with static smooth-drum or vibratory rollers for finish rolling to remove drum marks, and for surface sea ling. These benefits are primarily related to finishing rolling rather than compaction. Vibratory rollers combine the static load of the drum with dynamic loads. The vibration largely reduces the internal friction in the mix and improves compaction effect even when used with com paratively low static linear loads.

A vibratory asphalt roller always has a higher capacity (expressed in tons of asphalt laid per hour) than a static roller of the same weight. On harsh mixes this difference is even more pronounced.

Static three-wheel rollers p Modern types of three-wheel rollers have three large driven drums and articulated steering, as opposed to conventional models which have two driving steel drums and a smaller steering drum. The compaction effect can be varied by ballasting with water. Weight range: 8–15 tons

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ASPHALT PAVING, COMPACTION AND MILLING

Vibratory tandem rollers p Normally with vibration and drive on both drums. Articulated or pivot steering. Weight range: 1–18tons

Combination (Combi) rollers p Pneumatic tyred rollers p Normally with 7–11 pneumatic tyres. The compaction effect can be varied by ballasting usually with water or sand. Changing the tyre pressure changes to contact area which also affects the compaction effect. Weight range: 10–35 tons

One vibrating drum and one axle with pneumatic tyres. Articulated or pivot steering. Weight range: 4–15 tons

Single drum asphalt compactors One vibrating drum and smooth pneumatic tyres on rear axle. Articulated frame. Weight :10 tones

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ASPHALT PAVING, COMPACTION AND MILLING

ROLLING PROCEDURES Roller compaction of asphalt mixes can be divided into three stages, Initial rolling, Main compaction and Finish rolling.

The compactability of a hot mix asphalt is dependent on its temperature. The normal lay-down temperature is 130 to 160°C. Within this range the mix is soft and plastic. As the temperature drops the viscosity and the resistance to compaction both increase. In general, compaction rolling should start as soon as possible after lay-down. With a vibratory roller, the compaction normally can start with vibrating passes. On tender and unstable mixes, it may be more suitable to start with two static passes which should be made at low rolling speed, 1–2 km/h. The roller should follow as close as possible behind the paver so that the compaction can take place as soon as possible after paving. The compaction must be finished before the mix has cooled down. However, if the roller is repeatedly run over the same area at very short intervals when the mix temperature is high,

the surface may crack and it may result in a drop in density. The main purpose of finish rolling (which is effective down to around 60°C) is to remove roller marks and other surface blemishes. It also improves the texture of the surface. Finish rolling may also increase density especially if the mat is comparatively hot. Many countries use pneumatic tyred rollers to seal the surface although traffic has a sealing effect on the asphalt surfacing on streets and roads. As there is no such traffic on runways, pneumatic tyred rollers are often specified for finish rolling. On thin layers, and in unfavourable conditions, the time available for compaction may be as little as five minutes. Under the same conditions, a thick layer will retain its temperature for up to several hours. The need for fast, effective compaction is therefore greater on thin layers than on thick ones.

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Thickness

Minutes

Temperature, °C

Thickness

Hours

Temperature, °C The cooling pattern of asphalt is determined by layer thickness, ambient temperature, ground temperature and weather conditions. Thin asphalt layers cool more rapidly than thick ones. They therefore may require fast and effective compaction.

ASPHALT PAVING, COMPACTION AND MILLING

A vibratory tandem roller will achieve uniform compaction over an entire paving area by following a correct rolling pattern. 3

To start with all joints must be compacted: first the transversal ones, then the longitudinal ones. The pattern is made up of parallel rolling lanes divided into rolling zones some 30â&#x20AC;&#x201C;50 m long. Actual zone length is determined by the speed of the paver and time available for rolling before the mix cools down.

The first lane is started at the lowest edge of the asphalt surface. Passes are made forward and backward in the same track. Track changes must always be made on a compacted area to avoid marking the mat.

As a rule, the roller must keep as close to the paver as possible. In all rolling patterns it is important to try to maintain a constant rolling zone length. Landmarks or cones are helpful in this respect.

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ASPHALT PAVING, COMPACTION AND MILLING

Roller input The number and size of rollers required on a job is determined by the lay down rate expressed in square metres per hour. To arrive at this figure a number of elements have to be taken into consideration. A number of elements must be taken into consideration to arrive at this figure. Every paving job can be mea sured by the tonnage of hot mix to be laid down per hour. On large jobs the tonnage figure is usually governed by the capacity of the asphalt plant. Mix tonnage, the paving width, and the layer thickness together determine the speed of the paver. The speed multiplied with the paving width will give you the lay down rate in square meters per hour. This then serves as a basis for the required roller input. Allowances should be made for temporary peaks in mix supply. Suitable rolling speeds range from 2 to 6 km/h. Low speeds are

used on thick layers and when high degrees of compaction are specified. The number of roller passes depends on a number of factors, primarily, the compaction properties of the mix and the specified degree of compaction. Static linear load and vibration characteristics also have a decisive influence. Thin layers with a high stone content are best compacted with a combination of high frequency and low amplitude to reduce the risk of aggregate crushing. Stable mixes and thick layers are best compacted at high amplitude. It is advisable to run a test strip to determine a suitable rolling procedure to reach the specified degree of compaction. A density gauge is a great asset as the density values can be read off immediately. A professional roller manufacturer should be able to supply you with recommendations on roller selection, settings and rolling patterns.

Rolling pattern The paved width is divided into roller lanes. The number of lanes depends on the drum width and paving width. The drum width should be related to the paving width so that, for example, three parallel roller lanes are sufficient to cover the paving width and that excessive overlap is avoided. Lane changes should be made on a previously compacted surface to avoid marks on the mat. In addition, the roller should never be allowed to stand still on a hot mix. Joint compaction Efficient joint compaction is im portant to pavement quality. As illustrated, there are two main alternatives to ensure adequate joint compaction.

Transversal joint. Start by rolling along the joint with about 10 cm (4”) of the drum on the hot asphalt. Move more of the drum over on the hot mix for every pass. If the space is limited due to obstructions or traffic, try rolling at an angle to the joint.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

ASPHALT PAVING, COMPACTION AND MILLING

Harsh mixes The increase in traffic loads has led to the need for more stable asphalt surfacings. As a consequence, harsh asphalt mixes, containing high viscosity bitumen and crushed aggregate with high stone content, are now becoming more and more common. Their high mechanical resistance to compaction requires efficient compaction methods. In this respect, vibratory rollers are the best choice to meet specified densities. Tender mixes Soft tender mixes are prone to lateral displacement during compaction which may result in small transversal surface cracks (3 to 5 mm deep). They can normally be closed by suitable finish rolling or by subsequent traffic action. If longitudinal cracks appear they are often deep and very difficult to close completely. The rolling of tender mixes requires special measures. Often they must be

allowed to cool down before rolling starts. This means that the roller has to operate relatively far behind the paver, and in many cases it may be best to work with long lanes (100 m or more). In order to stabilise the mix it is often suitable to start the compaction with two passes in a static mode or using a pneumatic tyred roller (PTR). A large drum diameter and a slow approach also help to prevent shoving or cracks. It is often a suitable to select a low amplitude and high frequency on these mixes. A PTR is suitable for finishing the surface. Thin layers Thin layers normally result in fast paving speeds and high surface capacities; but they may put a strain on the rolling capacity if allowances have not been made. The roller must never increase its speed in order to keep up with the paver; there is a risk that density will not be achieved. In order to achieve sufficient compaction, the number

of machines has to be increased. To avoid crushing the aggregate, low amplitude and a high frequency should be used. In addition, thin layers cool rapidly which is why the rollers must be able to attain specified densities fast and efficiently. Thick layers It is possible to achieve high denﾂｭ siﾂｭties on asphalt layers up to 15 cm thick. However, rolling on very thick surfaces may create surface undulations. On thick layers rolling should start at some distance from the edge of the lane. The roller passes should then be made successively closer to the edge to prevent the edge from being displaced. A large drum diameter together with a high amplitude setting is very suitable on these applications. The high amplitude will guarantee that good compaction is achieved throughout the layer.

The main difficulties working with harsh mixes are in overcoming the resistance to compaction, which is a result of the internal friction of the aggregate. Therefore, high compaction effort should be applied窶馬ormally using vibratory rollers.

Tender mixes are very plastic in their hot state and may be pressed out under the drum during rolling resulting in hairline cracks and the risk of lateral mix displacement. Adequate compaction may be reached if the mix is allowed to cool somewhat.

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ASPHALT PAVING, COMPACTION AND MILLING

CHOICE OF ASPHALT COMPACTORS When selecting a roller or combination of rollers consideration should be given not only to the ability of the machine to reach the specified density but also to the economics involved in doing so.

In general, it can be said that the probability of a vibratory roller reaching target density is usually better than that of a static machine. This probability increases as layers get thicker, as density requirements become more stringent and when mixes become harsher. On tender mixes and when relatively low degrees of compaction are specified, for example, conventional static steel wheel rollers alone or in combination with pneumatic tyred rollers have the same probability of reaching compaction as a vibratory roller. On the other hand, on harsh mixes requiring a high degree of compaction the probability of success will definitely favour vibratory rollers. A vibratory roller can handle both

compaction and finish rolling. During the compaction stage it should achieve final compaction. On small jobs the roller can then switch to static operation to finish the surface. On larger jobs, the finish rolling is performed by a static smooth drum roller or a pneumatic tyred one. As vibratory rollers have a higher production rate than their static counterparts, they are especially economical on large pavement constructions. Small vibratory tandem rollers have now cornered a large part of the market for small asphalt surfacings. Roller manufacturers should have the appropriate tools to support you in selecting the right roller for the job. He should also be able to give recommendations on roller settings and expected capacity.

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Vibratory roller

Static roller

Pneumatic roller

ASPHALT PAVING, COMPACTION AND MILLING

SPECIFICATIONS AND FIELD CONTROL In an end result specification, an authority sets a minimum degree of compaction which is then checked by laboratory and field tests. End result specifications are the most common for large projects.

A method specification lists the type and size of rollers to be used as well as the machine settings and number of passes required. End result specifications are often applied for asphalt pavement constructions. The density require ments normally fall in the range 97–100% degree of compaction (50 or 75 blows on the asphalt sample). Requirements can also be stated as a maximum permissible air void content. Asphalt contracts often include penalty clauses which stipulate fines to be deducted from payment if the contractor fails to meet specified densities. The normal method for field density control is to remove a core sample with a diamond drill.

Density and air void content are determined on the sample cores in a laboratory. Density gauges can be used for rapid density testing on site. As mentioned earlier, they are very practical when establishing suitable rolling procedures at the start of a job. The final approval of the density level is, however, generally based on core drilling. Function specifications normally involve the entire road design, not only the bituminous layers. However, special requirements can be connected to the function of these layers, for instance: maximum rut depth after a defined period of time, surface evenness requirements. Other quality controls of asphalt surfacings include checking the surface evenness, texture depth and skid resistance.

Continuous Compaction Control-quality and productivity improvement. A continuous compaction control system with GNSS positioning and infra-red temperature meters help make rolling operations more efficient. At the same time it is a tool that helps the operator achieve higher and more homogeneous compaction. By visualizing the actual number of passes, it is possible to reduce the risk of under-compaction due to operator errors. This also minimizes inefficiencies due to excessive passes that can lead to over-compaction.

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ASPHALT PAVING, COMPACTION AND MILLING

COLD MILLING APPLICATIONS Cold milling is an integrated part of the construction cycle of any road. Asphalt milling is used to remove an old and worn wearing course or the entire asphalt pavement. It can also be used to improve the surface friction or remove ruts on a wearing course that is otherwise in good condition.

Joints for a new overlay can be prepared and down driven manholes can be cut free so that they can be pulled up to the correct level again. A narrow trench for laying down, for instance, fibre-optic cables can also be cut. Different machines are used for the different applications depending on capacity requirements, size of the job site, manoeuvrability etc. The material that is removed is, to a large extent, recycled as unbound gravel base course. It can also be added as a part of virgin asphalt mixes produced in various types of asphalt plants.

A worn out pavement is removed using cold milling equipment, the material is transported to an asphalt plant where it is recycled and forms a part of the new asphalt. This new asphalt is paved and compacted to create a new wearing course.

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Cold milling process How it works The rotating milling drum is equipped with a large number of replaceable cutting bits. The milling drum breaks up and removes the material from the upper section of the pavement. The drum is pressed down by lowering the entire planer chassis until it reaches the desired cutting depth, the planer then starts it’s forward motion, effectively removing the asphalt pavement to the desired depth. The drum is configured to work against the direction of travel, so-called “up-cutting”. This is the safest way to operate as a down-cutting drum may “jump” onto the pavement and propel the machine forward in an un-controlled way. The working depth is controlled by rising or lowering the drum/ chassis assembly; this can be controlled manually or assisted by an electronic levelling system. The power to rotate the drum is transmitted from the engine either by v-belts (direct drive) or by a system of hydraulic pumps, hoses, valves and hydraulic motors. The direct drive has the best efficiency and an automatic load sensing belt tensioner helps protect the engine if the drum hits any hard objects, whereas hydrostatic drive system has the advantage of being almost maintenance free.

ASPHALT PAVING, COMPACTION AND MILLING

Why it is done Adding a new layer of asphalt on top of an old and distressed one is only a temporary fix. Cracks will migrate through the new material and destroy it in a short time. The adhesion between the layers will also suffer unless the cracked pave ment is removed. Curb height is another issue; adding new wearing courses will effectively leave no curb height after a while. On bridges there is a weight restriction, adding a new wearing course means that 100–150 kg/m² of asphalt would be added to the existing weight. This could jeopardize the stability of the bridge. The solution to these issues is to use a planer to remove the old asphalt and to replace it with a new layer. Asphalt paving requires an even surface to provide a uniform layer thickness. Old asphalt wearing courses can be rutted and have incorrect cross slope. This can all be corrected using a cold planer to remove the uneven layer. Compact cold planers are also used to prepare a surface for paving, cutting close to curb stones, around man-holes and gutters etc. Drums/holders/bits The drum is the true area of production for a cold planer, the width

and the tooth configuration decides which application is suitable. Normal spacing for the teeth is 15 mm for removal of material (Coarse, half lane milling) and drum widths vary from 350 mm to 2200 mm. For fine milling such as friction improvement or removal of traffic markings, a closer spacing is used. It is typically one half of the standard bit spacing. Drums with a spacing of 3–4 mm are named micro milling drums. In case of a fine spaced milling drum, the cutting depth is limited due to the reduced power per bit. Fine and micro milling drums are also available with two bits or more one line. This allows the operator to double the working speed without getting a fishbone structure on the milling surface. Lately, eco drums have become more and more popular. Here, the tool spacing ranges from 15–25 mm meaning the number of bits per drum is reduced. The reason for this is to reduce the cost of running the machine. The drawback is that this kind of drum can only be used in soft asphalt conditions; it also has the disadvantage that the surface is very rough. The bits or teeth are subjected to massive wear and the life time can vary from under one hour to a few days depending on the

application, production rate etc. A bit consists of a tungsten carbide tip, a steel body, a rotating washer for reduced wear and a pre-pressed retainer for fast and easy mounting. Road milling bits have a standardized shank size of 20 mm and the different brands are normally interchangeable. Special bits like mini-bits with different shank sizes are available. Easily replaceable teeth and, in some cases, holders are vital to obtain a high productivity as these can be damaged by objects in the ground. Street iron such as man-hole covers etc. are among the greatest dangers here. This is especially dangerous if they were paved over and are not noticed until hit by the milling drum. Bit holders are differentiated in two groups: The welded-on block, which is mainly used for compact planers or for fine spaced milling drums. The other type is the quick exchange holder system with a welded base and a replaceable sleeve that is used for larger cold planers. The helical pattern of holders and teeth help move the millings towards the discharge point. By means of the kicker paddles the reclaimed asphalt pavement is thrown out of the cutter box onto the conveyor.

Kicker

Cutting bit

Edge cutter

Tool holder

Components. Picture of a milling drum with named parts and arrows showing the build-up of the drum.

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ASPHALT PAVING, COMPACTION AND MILLING

High capacity/large jobs

Applications High capacity/large jobs The high capacity jobs require large production machines, always equip ped with an electronic levelling system. These jobs have a typical working width from 1,0–2,2 meters and a cutting depth of 0–320 mm. The planer is followed by a road sweeper and sometime high pressure cleaners to ensure a dust free surface. Tack coating and laying of the new wearing course follows on the cleaned surface. This means that the process must flow continuously and with good results. The method is described as “mill and fill” and is typically used during night-time work on otherwise busy roads. Proper levelling and control of the milling depth as well as the logistics on the site is critical on these jobs. 1 cm of excessive removal of material over 10 km means that an additional 490 tons of asphalt must be paved to reach the correct level. This would represent a significant amount of money.

Patch work/city work These applications are almost as demanding as the high production jobs. Good manoeuvrability, balance and weight is required to finalize the jobs quickly in order to minimize traffic disturbance. These machines are also often operated in confined areas requiring them to be manoeuvrable and operator friendly. Manhole covers and sewer grates create a challenging working environment for these machines. Good lighting, low noise level and good all-round visibility is highly important as a lot of the work is carried out at night. Typical cutting widths range from 350 mm to 1m and the cutting depth is normally in the range of 5–8 cm, although state of the art planer models can be utilized for deeper cut jobs as well. Joints/cleaning In preparation of a new overlay on top of the existing wearing course the joints must be prepared, both for the start and at the end of the paved lane. All intersections must also be prepared by milling

ATLAS COPCO | COMPACTION, PAVING AND MILLING

a “wedge” to create a strong joint. Cleaning up after a high capacity machine includes milling around man-hole covers, sewer grates etc. This is carried out mostly by machines with working widths ranging from 350–500 mm, but also a 1000 mm machine with rear conveyor is highly manoeuvrable and can almost turn on the spot to be effective in milling round man-hole covers etc. The most suitable machines for joints/cleaning and patch/city work are grouped as compact cold planers Special applications The removal of the top surface with a fine spaced cutting drum is referred to as fine milling. This includes removal of shallow ruts in an asphalt or concrete pavement, correction of a slope for better water drainage, the removal of painted road markings as well as friction improvement of otherwise good wearing courses. Since no material will be placed on the milled surface it is important that this is even and uniform enough to

ASPHALT PAVING, COMPACTION AND MILLING

traffic right away. A regular milling drum with 15 mm bit spacing will not be able to produce the desired surface quality. This application requires changing to a fine milling drum with narrower tooth spacing, typically around the half of the regular tool spacing or less depending on the job requirements. These jobs are not very demanding from a productivity stand point, however accurate levelling and grade/slope control is required to avoid cutting too deep. Levelling systems Due to its usage and the corresponding job requirements levelling of compact cold planers is most often done manually. This means that the operator is lowering or raising the rear legs hydraulically by activating a switch. Control of the cutting depth is done by monitoring the numbers on a scale. The operator is responsible for a sufficient levelling result. In case of higher demands for surface evenness or just for easier working, compact cold planers can be equipped with an electronic levelling system. This controls the actual cutting depth or slope value according to pre-set figures. Sometimes a non-contacting ultra-sonic ski is used for levelling off a curb or a string line. Most often larger cold planers are equipped with an advanced levelling system for accurate working results. Two grade sensors on either side is the minimum requirement. In addition to this, a slope sensor can be used if only a single sided grade reference is available. For better averaging, several sensors are connected per side to a long averaging beam. This system provides good accuracy for fine-milling operations. MillimeterGNSS or laser levelling systems are niches for special milling operation for e.g. on race courses, airports or large parking areas.

Special applications

Patch work/city work

Joints/cleaning

Levelling systems

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

WHAT TO LOOK FOR IN

COMPACTION, PAVING & MILLING EQUIPMENT It is difficult to evaluate the performance of rollers without field tests. These should preferably carried out on different materials and under different conditions. This section, however, defines and discusses the parameters and data which may be employed to evaluate and compare rollers from a specification. The ISO8811 standard establishes the guidelines as to which technical data and parameters are suitable for specifying vibratory rollers. Vibratory roller specifications are therefore structured and comparable if presented according to the standard.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

WHAT TO LOOK FOR IN

A VIBRATORY ROLLER Vibratory rollers evolved from the original static rollers, which generated compaction by static weight only. The comparison of two vibratory rollers is a difficult task; in fact, it is impossible to perform from a specification sheet only. However, different parameters have an impact on a roller´s effect. So it is important to have a thorough understanding of the parameters involved.

Compaction performance VITAL DATA • Static linear load • Frequency • Amplitude (fixed or variable)

Compaction performance is norm ally what sets different rollers apart. In this respect, compaction effort plays a major role: the higher the compaction effort, the greater depth effect and the fewer the number of passes required. Compaction effort is influence: • static linear load • amplitude • frequency • The ratio between static and vibrating mass • Drum diameter Other factors include rolling speed, and the number of vibrating drums. Centrifugal force is not a decisive factor for compaction performance. Static linear load For a smooth-drum vibratory roller, the static linear load is the static weight of the roller on the ground divided by the rolling width of the drum expressed in kg/cm or kN/m. The static load is the weight of the drum assembly plus the parts of the frame carried by the drum (drum module weight). The ISO8811 stan-

dard includes the weight of the operator as well as full fuel and half-full water tanks in the static operating weight. A significant increase in the static linear load increases the compaction effort, and reduces the number of passes required. The total weight of a self-propelled single-drum vibratory roller does not give a direct indication of the compaction effect. Comparisons based on total weight can therefore be misleading. A true picture emerges only by comparing the static linear loads of the vibrating drum modules. Frequency and amplitude Frequency is the number of drum impacts per time unit measured in Hz (vibrations per second) or vpm (vibrations per minute). Amplitude is the maximum movement of the drum from the axis, and is usually expressed in mm. This means that the total drum movement corresponds to twice the nominal amplitude. The influence of frequency and amplitude on the compaction effect has been the subject of discussion for many years. Laboratory and field tests indicate that frequencies between 25 and 40 Hz (1,500 and 2,400 vpm) have maximum compaction effect on soil. A change in frequency within this range will not significantly affect compaction effort.

However, a change in amplitude has a pronounced effect on compaction and depth effect. High amplitudes are especially important on materials which require a high compaction effort, such as coarse rock fill and dry clay soils. Vibratory rollers designed to compact large volumes of soil and rock fill, boulders and cobles in thick layers should have an high amplitude in the range of 1,5–2,1 mm.

The static linear load is the weight of the drum assembly plus the parts of the frame carried by the drum (drum module weight).

Frequency is the number of drum impacts per time unit measured in Hz or vibrations per minute. Amplitude is the maximum movement of the drum from the axis and is usually expressed in mm.

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Degree of compaction Low amplitude

High amplitude

Depth The normal relationships between compaction effect, frequency and amplitude on soil. Amplitude has a significant effect whereas frequency may vary within a given optimum range.

On asphalt, frequencies between 45 and 70 Hz (2,700 and 4200 vpm) have been found to produce the best results. Suitable amplitudes for asphalt should not exceed one mm. High frequencies result in small impact spacing (the distance between each drum impact) which helps to prevent surface rippling. Impact spacing is a function of frequency and speed: low frequency at high speed gives wide impact spacing while high frequency at low speeds gives close impact spacing. Amplitude settings It is a great advantage to be able to alter the vibratory force and depth effect of the roller. The best way of doing this is by altering the amplitude. With adjustable amplitude

settings, the compaction effort can be adjusted to suit different types of material and layer thickness. Some rollers have the ability to change the amplitude automatically during the compaction process and adapt it to the requirements of the underlying surface. Adjustable amplitude is essential in asphalt compaction. When working on a tender mix or with thin lifts, best results are achieved from a low amplitude setting. This also reduces the risk of crushing weak aggregate. Conversely, harsh mixes and thick layers require relatively high amplitudes. Adjustable amplitude settings result in a variable compaction effort. On soils, the operator can change the amplitude to suit different layer thickness. On asphalt, adjustable amplitude can be used to adapt the roller to the different compaction needs of harsh and tender mixes and changes in layer thickness. When compacting thick layers to a high density, it is best to start at a high amplitude. As the material increases in density the drum often starts to double-jump. This does not increase the density even if the number of passes are increased and, even worse, the material may be crushed and the machine may be damaged. The double-jumping can be prevented, and the density will increase by reverting to low amplitude.

Automatic vibration control (AVC) Modern asphalt rollers should be equipped with an automatic vibra tion control which cuts out vibra tions at speeds below a certain limit. This prevents vibrations acting on the surface when the roller is stationary or when it slows down to change direction of travel. Rolling speed Rolling speed has a definite influence on compaction effect. To a certain extent, high rolling speeds can be compensated for by an increase in the number of passes. However, optimum speeds for soil compaction lie in the 3–6 km/h range. The compaction of thick layers of soil and rock fill to high degrees of compaction requires speeds in the lower part of the range. Optimum speeds are somewhat higher for asphalt than for soil. Constant speed is important in obtaining a uniform degree of compaction, and a speedometer on the roller is a help in this respect. Speed control is especially important in asphalt compaction. Number of vibrating drums Two vibrating drums reduce the number of passes required, thereby increasing the roller capacity. With only one vibrating drum, the roller will require about 80% more passes than with a vibrating tandem roller of the same size. However, varia tions do exist depending on the type of material to be compacted Frame/Drum weight ratio The frame must be heavy enough to press the drum to the ground when vibrating, however If the frame is too heavy, it will reduce the effect of the vibrations. The drum weight should be one third to half the weight of the frame.

A vibratory tandem roller can have one or two vibrating drums. In general double vibrating drums increase capacity by about 80%, as the roller does not have to make so many passes.

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

A roller with a drum width of 1500 mm covers paving widths from 3,5 up to 4,2 m with three parallel lanes. Using a roller with 1700 mm drum width is, in this case, uneconomical as it will still require three parallel lanes. The overlap will increase and the efficiency will go down. Roller size must match both the compaction requirement s and the expected paving width in order to assure the compaction quality as well as the operational efficiency.

CENTRIFUGAL FORCE AND TOTAL APPLIED FORCE (TAF)

mum drum width to cover the asphalt paver width using a minimum number of parallel rolling lanes.

It is incorrect to assume that a higher centrifugal force results in a higher compaction effort. Total applied force (TAF) used to be regarded as a good yardstick for the measurement of compaction effort in the early days of vibratory compaction. It is the sum of the static weight and centrifugal force, and, as with centrifugal force, it is easy to draw the wrong conclusions.

Drum diameter A large diameter reduces rolling resistance. This can be especially important in preventing shoving of asphalt mats, and in minimising hair-line cracks when rolling tender, unstable asphalt mixes. A large drum diameter is preferable.

DRUM DESIGN VITAL DATA • Drum width • Drum diameter • Drum shell thickness

Drum width In soil compaction, the drum width generally determines surface area capacity. A wider drum results in a greater surface coverage per pass. However, the same does not apply to asphalt compaction, where the width of the asphalt paver also has to be taken into account. In asphalt surfacing work, the drum width of the roller needs to be correlated to the paving width. There is an opti-

Drum shell thickness The drum of a roller is subject to wear. The compaction of finegrained material causes less wear than the compaction of coarse rock fill. Very abrasive types of rocks may cause exceptional wear. The drum shell thickness and quality of the steel therefore determine the lifetime as well as the ability of the drum to withstand deformation. Drum shell finish is also decisive. For soil compaction, current drum shell bending technologies result in sufficiently round and even drums. For asphalt rollers the demands are higher. Therefore, these drums are normally machined. The result is a drum that will produce smooth and even asphalt surfaces.

Split drums A split drum design allows the two drum halves to operate at different speeds. This reduces scuffing of the asphalt mat when operating on sharp curves. If a roller does not have split drums, the operator should follow the standard accepted rolling procedures on curves to ensure the job is done properly. Be careful not to use split-drum rollers on soil applications. Rolling on stiff soils creates a forging effect on the drum shell. This will eventually widen the drum and bridge the gap between the two drum halves, destroying the intended purpose and benefits of a split drum.

Sharp turns along the curve may result in tearing the surface when compacting asphalt. This can be avoided by rolling in two or more directions.

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

TRACTION VITAL DATA Soil rollers • Operating weight • Tractor module weight • Drum module weight • Tyre size • Gradeability Asphalt rollers • Operating weight • Front drum module weight • Rear drum module weight • Gradeability

The ability of a roller to run up a slope is termed gradeability. Figures for grade ability of different machines should be related to comparable procedures and conditions. In leaflets and literature theoretical gradeability is stated in accordance with ISO8811.

Many factors influence traction, the following being especially applicable to soil compactors:

risk of the drum or wheels spinning and there by consuming all the hydraulic power.

Drum drive Drum drive improves traction because it permits the entire weight of the roller to be used to develop the tractive effort. It is particularly suitable on thick layers and difficult materials, for example uniformly graded dry sand (dry compaction). It may also help gradeability, i.e. the ability of the roller to work on inclines. Adding a flow-divider for the wheel and drum-drive hydraulics will also help improve traction. Traction control and anti-spin systems further improve the traction by reducing or eliminating the

Drum diameter and static linear load A large drum diameter and low static linear load results in a low angle of approach to the material being compacted and the larger the drum diameter and the lower the static linear load, the lower the angle of approach. Consequently there is less resistance to rolling. Weight distribution between the tractor and drum module Without drum drive, a fifty-fifty split between the weight of front and rear modules indicates satisfactory traction. The heavier the tractor module is in relation to

The angle of approach influences the resistance to rolling. If the drum is small and heavy it will exert a horizontal force which in turn produces a higher rolling resistance which may increase the need for engine power.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

the drum module, the better the traction. Traction can be improved by ballasting the tyres with water or by opting for a model with drum drive (a standard feature on a number of soil compactors with heavy drums). The size and tread of the tyres The section width, section depth and rim diameter of the tyres on rollers with pneumatic drive wheels are all decisive to the grip the tyres exert on the underlying surface. Diamond tread tyres provide sufficient grip for the majority of the applications at hand. Tractor type tread tyres are available when additional traction is required. Drive transmission The power and torque of the hydra ulic motor, choice of gear ratio and axle characteristics (planetary drive, no-spin differential) are correlated to the ability of a roller to cope with inclines. Anti-spin systems improve the traction by monitoring and avoiding spinning wheels or drums. Power is transferred to the drum or wheel with the best traction.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

MANOEUVRABILITY VITAL DATA • Turning radius inner • Turning radius outer • Wheel base • Lateral minimum overhang • Curb clearance • Speed range

With a small turning radius, the machine is relatively easy to handle in confined spaces. Minimal overhang improves the capability of the roller in tight quarter work. High ground and curb clearance allows the machine to steer clear of obstacles. A 0–10 km/h speed range is adequate to cover all normal applications. High transport speed can be an advantage when moving rollers on a large job-site. Low reverse speed is of no importance for transport but can affect compaction capacity as passes are normally made in forward and reverse. Good all-round visibility is essential to manoeuvrability as rollers work in both forward and reverse modes. It is vital that the driver has a clear view of the drum edges, even with the drum at maximum offset. Some tandem rollers have the possibility to offset the drums up to 1200 mm to facilitate rolling alongside kerb stones and around curves. This gives an option to shift the centre of gravity in order to be able to work on weak shoulders. It also increases the surface capacity when performing finish rolling.

also with offset drums, is one of the advantages with this concept Fixed frame with pivot steering

A fixed frame roller can steer the front and rear drum individually or in combination to re-create the tracking of an articulated roller. This concept allows for a large offset distance, however, the drum edge visibility is then sacrificed. Fixed frame roller are also shorter than the articulated ones making for better manoeuvrability and easier transportation.

SPRINKLING SYSTEM

An asphalt roller must have a satis factory water sprinkling system to prevent pick-up on the drums. Modern asphalt rollers have a

pump-driven system, as opposed to gravity fed systems which may malfunction especially when the roller is working on inclines. Plastic tanks and hoses prevent corrosion to the system. Due to the difficulty of finding clean sprinkler-water on some job-sites, it is very important to have a well working filtering system. The system should consist of at least two and preferably three filters: tank inlet, in-line filter at the pump and at each nozzle. It is also desirable to have a back-up system with an extra sprinkler pump and additional sprinkler bars. The tank volume should be large enough for a normal eight hour working day. Sprinkler timer is a very useful feature to help reducing the water usage.

Frame concepts Tandem rollers are typically built according to one of the two dominating frame concepts: Articulated steering, with or without optional pivot steering

The articulated roller provides good tracking of the two drums with a central articulation joint. With addi tional pivot steering it is possible to create an offset between the drums. Good drum edge visibility,

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

A well-sorted, comfortable operator station simplifies the operator’s job and thus contributes to better roller performance.

ENGINE VITAL DATA • Emission rating • Make and model • Rated power at ...... • Fuel tank capacity

Naturally, the engine has to have a large enough output to allow the roller to function properly. In addition, it must have adequate reserve power to counter any drop in power as the engine wears over the years, as well as a reserve for working at high altitudes. Fuel tank capacity should permit work for at least one working shift. The noise level should be low to give the operator and those close to the machine a better environment in which to work. Low emission engines reduce the environmental impact. Additional systems that constantly control the engine RPM in accordance with the power required by the machine and automatically reduces engine RPM to idle when the machine is standing can significantly reduce the fuel consumption. Emission regulations govern the level of emission control the engine must relate to. This depends on the market where the machine is to be used. After treatment systems for the higher emission control stages (EU/EPA) require low or ultra-low sulphur diesel (below 15 ppm sulphur). This requirement limits the markets where such machines can be used.

TRANSPORTATION VITAL DATA • Shipping weight • Overall width • Overall length • Overall height

Overall length, width and height plus shipping weight have a direct bearing on transportation. Local haulage restrictions also have to be taken into consideration. A total machine height below three meters significantly reduces transportation cost on many markets. The choice of trucks is far greater for lower machines.

OTHER IMPORTANT FACTORS

The following information is rarely mentioned in specification pamphlets, and will require a more detailed talk with a manufacturer’s representative.

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Operator comfort The more comfortable an operator is, the better the performance. The operator station must promote comfort. The platform should be well insulated against vibrations to prevent excess fatigue and the ambient noise levels should not disturb the well-being or concentration of the operator. Good allround visibility is essential to be able to perform the job safely. In addition, all controls should be positioned within easy reach of the seat, and the operator panel should be logically sorted and easy to read. Tachometers/hour meters, voltmeters, fuel and temperature gauges all contribute to making life easier. A movable swivel seat, integrated with the most vital controls improves driver ergonometric as it allows him/her to place the seat where it gives the best overview of the area to be rolled.

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Safety Safety is vital not only for the operator but for those working close to the roller as well. The brake system must be active on both drums in asphalt rollers and on both drum and drive-wheels on a single drum roller. This should also be backed up by an emergency system, both hand applied or automatically applied as the hydraulic pressure drops. A parking brake is also required. Most accidents happen when the operator is getting on or off the roller, so attention should be paid to non-slip platforms, safety rails around the operator area, and the provision of secure footholds up to the platform. ROPS (Roll-Over Protection Systems) or safety cabs with built in ROPS protection must be on the list of optional equipment to enhance operator safety. For many markets this is a requirement. Availability Stoppages in large earth-moving or asphalt surfacing operations are extremely costly. Machine availability is therefore crucial.

A roller that operates with very few stoppages due to breakdowns should prove to be an economical investment. The acquisition price is in no way decisive to the overall economy of the machine, a fact that should never be overlooked when buying a roller. Maintenance High machine availability is a decisive factor in determining the profitability of a roller. It depends not only on quality engineering but also on good serviceability, which itself is a function of easy accessibility to all vital components and an efficient spare parts service to make sure that the machine stays up and running as much as possible. Service and maintenance instructions, operatorâ&#x20AC;&#x2122;s instructions and workshop manuals should be available in major languages. Easy daily maintenance is essential on any machine: ready access to lubrication points, filters, etc. will make life easier for an operator and help ensure that the job gets done. It is an advantage if the roller has as few lubrication points as possible.

ROPS that include a seat-belt provide protection for the operator in the event of a roll-over. Combined with a Falling Object Protection System (FOPS) it also shields the operator from falling debris when operating in a trench. Reverse alarms help increase safety when reversing.

Adaptability It is a great benefit if the roller can operate over a wide range of field conditions, for example on different types of soil, terrain, and at high altitudes. A rollerâ&#x20AC;&#x2122;s ability to do so will be of great value to the user. The adaptability of the equipment may also be a decisive factor in the economy. For example, vibratory tandem rollers are attractive because they are suitable for both asphalt and base and sub-base compaction. Self-propelled vibratory rollers which can be switched from smooth to pad-foot drums are also at a premium. With an interchangeable drum, all the compaction work can be done by one unit. This cuts acquisition costs and will also help keep down maintenance and spare parts expenditure. Pad foot shells reduce the amplitude on smooth drum soil compactors, but can be an acceptable compromise for converting a smooth drum roller to a pad foot version for shorter times.

As a rule easy access means easy maintenance which in turn means less downtime.

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A STATIC SMOOTH DRUM ROLLER Static smooth drum rollers were the first type of mechanical equipment used for soil and asphalt compaction. They are often used together with pneumatic tired and/or vibratory rollers.

There are two main types of static smooth drum roller: the threewheel version and the two-wheel tandem version. The conventional three-wheel model has two large driven rear drums and a smaller front drum that steers the roller. Modern three-wheel rollers have three large driven drums of equal diameter as well as articulated steering. This section defines the data used to compare static smooth drum rollers. It also serves as a basic guide to what to look for when selecting a roller of this type.

COMPACTION PERFORMANCE

drum width) The compaction effect also depends on the drum diameter and is further discussed later in this section. Static linear load On conventional 10–15 ton static three-wheel rollers, the static linear load of the rear drums varies between 50 and 80 kg/cm. For asphalt compaction, the static linear load should exceed 50 kg/ cm. The static linear load of the front drum is some 30% lower than that of the rear drums. Therefore, the rear drums must pass over the entire surface to achieve uniform compaction. With modern types of static three-wheel rollers (with equally large diameter drums and articulated steering), the three drums have the same static linear load when the roller is correctly ballasted. This enables the roller to achieve uniform compaction across the entire roller width and therefore follow simpler rolling patterns. With a rolling width of 2,1 m, the roller can cover a width up to 4 m (allowing for overlap) in two parallel passes. The three large driven drums ensure a smooth and efficient rolling action.

VITAL DATA • Static linear load • Drum diameter

The compaction effort of a static smooth drum roller is primarily a function of static linear load (i.e., the weight of the roller divided by

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The drum diameter of conventional static tandem rollers varies between 1,2 and 1,5 m while drum widths range from 1,1 to 1,4 m. Their static linear loads are somewhat lower than those of static three-wheel rollers of the same total weight.

Articulated static rollers with equal static linear load on all drums. This assures uniform compaction over the entire rolling width.

Rigid frame three-wheel static roller.

Rigid frame tandem roller (also known as a deadweight roller).

Drums of equal diameter provide uniform compaction effort across the entire machine width, which conventional three-wheel rollers do not.

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Ballast A 10–12 ton static roller normally requires 2–3 tons of ballast. Water is the most convenient type of ballast. The main ballast is normally placed in the drums and thereby lowers the centre of gravity. Drum overlap There should be an overlap of at least 50 mm, and drums should overlap when turning.

Articulated centrepivot steering ensures proper drum overlap and equal force over the entire rolling width even when turning or changing lanes.

Drum diameter The larger the diameter of the drum, the lower the rolling resistance and angle of approach to the material being compacted. In general, when the static linear load exceeds 50 kg/cm, it is desirable if the drum diameter is 1,500 mm or more. Drum arc and pressure The drum arc is the area in contact with the drum at a given penetration depth. This factor must be taken into account when determining the compaction effect and a roller’s suitability on, for example, a tender (unstable) mix. These mixes are prone to excessive movement and cracks during rolling. A small contact area gives a large contact pressure; however, if a roller with a small drum diameter causes a bow wave and surface cracks. A roller with larger drums will give better rolling performance and compaction effect. In general, the greater the drum diameter and contact area the more suitable the roller is on unstable mixes.

A conventional type of static roller is liable to rut the asphalt surface.

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Dual controls allow the operator to select the most confortable position for good all-round visibility.

The curves in the chart show that contact pressure is lower at deeper penetration and increases as the drum rides out of the material as compaction increases. Larger diameter drums have a lower contact pressure than small diameter drums at the same penetration, however, despite this; they may achieve better compaction effect than a smaller diameter drum. (See previous explanation in this section.)

Speed As a rule, static smooth drum rollers achieve best compaction within the speed range 3 to 6 km/h. Dual speed hydraulic motors will provide for extra speed when moving around the site. Drum design The edge of the drums should be chamfered to reduce the risk of drum marks on the asphalt mat, and the drums must be equipped with scrapers to allow the roller to work on a wide range of materials.

A chamfered drum edge will reduce drum-marks on the asphalt mat.

Some manufacturers of static three-wheel rollers with articulated steering offer such options as flexible front drums, and split rear drums. Flexible front drums allow the drums to tilt or flex 1â&#x20AC;&#x201C;2 degrees from the upright position which can be advantageous when compacting the road crown. Split drums are used to eliminate pushing the material when turning on sharp bends. However, the operator can modify his rolling pattern to eliminate the need for such an option. Sprinkler system The roller must have an effective sprinkler system to wet the drums to avoid pick up when compacting asphalt. Modern asphalt rollers have pump-driven sprinklers. An efficient system of filters (filling, pump and nozzle filters) will prevent stoppages due to blocked nozzles. Drive system Hydrostatic drive gives the operator full and easy control over speed, stopping and change of direction. With hydrostatic drive on all drums the roller has good traction. It makes the roller more versatile and allows it to be used on unstable asphalt mixes; it also eliminates tendencies to shoving and lateral displacement.

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Brake system The brakes must be sufficiently powerful to operate safely even when the roller is fully ballasted. Fail-safe systems backed up by an emergency system are essential to operator and worksite safety.

GENERAL FEATURES Operator comfort The design of the machine should allow for maximum operator safety, comfort and visibility. Dual controls or swivel seat allow the operator to select the most comfortable position for best safety, visibility and, ultimately, productivity. Maintenance It is essential that the roller is supported by a reliable maintenÂ ance service. It is best to check that a full back-up service is available where the roller is intended to be used. This will ensure minimum downtime should periodic servicing or repair needs arise. Regular checks of wear parts and substances (i.e. water, oil, etc.) should be conducted. Maintenance must be easy to perform. Easily accessible maintenance points and long service intervals are important.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

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A PNEUMATIC TYRED ROLLER (PTR) Towed pneumatic tyred rollers have been used for many years. In the early days, rollers up to 200 tons were not only used for compaction but also for identifying weak spots (proof rolling). These towed versions have almost disappeared with the evolution of vibratory rollers for soil compaction; therefore this section only deals with self-propelled PTRs. It by no means covers all there is to know, for example, about tyre technology; but rather it discusses fundamental compaction technology when using PTRs, and the basic points to consider when selecting such a roller.

Compaction performance VITAL DATA • Wheel load • Tyre ground contact pressure • Tyre contact area

The compaction effect of a PTR is primarily determined by two parameters: wheel load and the ground contact pressure of the tyre, which is correlated to the tyre inflation pressure. (See next page.) On thick layers large tyre with a large contact area has a better

compaction effect than a smaller tyre with the same ground contact pressure. This is especially important in soil compaction. Wheel load The number of wheels directly affects the wheel load. Pneumatic tyred rollers in the medium heavy class normally have seven or nine wheels and a maximum wheel load of over 3,000 kg which is sufficient for most types of compaction. Authorities often specify the number of wheels and the minimum wheel load.

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Wheel load can be calculated by using the simple formula: Total weight + ballast Number of wheels

= Wheel load

Ballast Large amounts of ballast are normally needed to reach the required operating weight. There are a number of different ways to ballast a PTR, for example iron bars, sand and water. Scrap iron is used as a permanent ballast. However, it is time-consuming to load and remove. In some cases iron bars are fitted under the roller, but this adds to the expense. Modern PTRs have modular ballast systems where weight can be conveniently added. This simplifies ballasting procedures and makes it easier to keep track of the actual ballast weight. Normally 5 to 8 m³ are required but as with iron bars loading and unloading can be time-consuming. On the other hand, sand is easier to dispose of when the machine is to be transported without ballast. Sand also tends to dry out, so it has to be checked from time to time to ensure it is still wet.

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General ground contact pressure pattern 1. Standard diagonal type tyre Particles are likely to move laterally. Pressure range 0,3–0,9 MPa. 2. Radial tyre More evenly distributed with variable pressure. 3. Wide-base tyre Wide-base tyres cause less lateral movement of the particles than standard tyres. Pressure 0,4 MPa.

Ground contact pressure chart (CP2100/2700)

The main parts of a tyre

kPa

Tire inflation pressure (kPa) 240 350 480 620 720 830

Wheel load (kg) 1125 1375 1825 2250 2750 3000

Ground contact pressure (kPa) 200 240 270 300 330 340 220 260 300 330 350 380 240 280 340 380 400 430 250 310 360 410 440 480 260 330 390 440 480 520 270 330 410 460 490 540

psi

Carcass

Tire inflation pressure (psi) 50 70 90 105 120

Wheel load (lbs) 2,500 3,000 4,000 5,000 6,000 6,500

Ground contact pressure (psi) 29 35 39 44 47 49 31 38 44 48 51 55 35 41 49 55 58 62 37 45 52 60 64 69 38 47 57 64 70 75 39 48 59 66 71 78

Water Although easy to handle, the drawback of water is its low volumetric weight. In addition, the ballast tanks must be watertight. In some cases a PTR equipped with a pump and nozzles may be used for water spreading. The pump, which is electrically driven, fills and discharges the water.

Tyres

Bead wire

This section deals with some of the geometry of a tyre and a tyre’s compaction characteristics. There are three main types of tyre:

diagonal tyres, radial tyres and low-profile, full flotation tyres (which also include widebase tyres). All major manufacturers of industrial tyres produce diagonal and radial types, whereas only a couple offer widebase versions. Diagonal and radial tyres are more versatile and can be used at different pressures from 0,3–0,9 MPa depending on the ply. They are suitable for both soil and asphalt compaction. However, a radial tyre has a more even pressure pattern than a diagonal tyre. This reduces the risk of tyre marks in

Tread

Ply (number of layers)

the asphalt surface. Wide-base tyres are used at a fixed pressure of 0,4 MPa. They are suitable for surface sealing and finish rolling on asphalt. They are also used on stabilised soil but are less suitable on soil as they do not have the same depth effect as diagonal and radial tyres. The contact pattern and pressure distribution for these types of tyres are shown above. As the pressure of a diagonal or radial tyre can be varied, there will be changes in the contact pattern.

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WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Tyre overlap is important on a PTR, the rear wheel cover the gaps left by the front wheel. The result in full area coverage with one pass of the roller.

GENERAL FEATURES

Ground contact area and ground contact pressure During compaction the contact area of the tyre changes constantly as the material is compacted, and as a result penetration decreases with each pass. Values for ground contact area are thus only comparable when measured on a flat hard surface, such as a steel plate. At present no gauges are available to indicate ground contact pressure so it is left to the operator to judge the pressure. If the tyres sink into the material, the tyre pressure control system, more commonly known as ”Air on the run” or ”Air on the go”, can be used to reduce the tyre pressure. An increase in pressure will also increase the pressure against the ground. The advantage of a central air pressure control system is that it

allows the operator to maintain a selected constant pressure on all tyres during all phases of rolling. In practice, it is impossible for the operator to continually adapt the tyre pressure to the prevailing surface stability of the mix. The table indicates the tyre contact area and ground contact pressure for different wheel loads and tyre inflation pressures. Overlap The front and rear tyres should overlap by at least 30–50 mm at normal pressure. In order to achieve uniform compaction effect, and to avoid tyre marks on asphalt, the overlap between pressure contact areas is more important. This overlap can be checked by running the roller on sand and checking the penetration of the front and rear tyres.

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Asphalt pick-up During the initial rolling, all PTRs will pick up asphalt unless special measures are taken. Special anti-sticking agents such as bio-degradable emulsion oils are commonly available and should be used. A common method to prevent pick-up is to preheat the tyres by running the roller on a surface that has already been compacted by steel wheel rollers and is still hot. Little or no pick-up will occur once the difference in temperature between the asphalt mat and the tyre is no more than 20 to 50 °C. Water from the sprinkler system is then sufficient to prevent any pickup. However, the amount of water must be reduced to an absolute minimum since it cools the tyres. Mats and scrapers also help minimise pick-up during the initial tyre warm-up period. Skirts around the rubber tyres help keep the tyres hot. Tyre skirts are especially useful in windy conditions.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Oscillating or pivoting wheels PTR’s should have oscillating or pivoting wheels on at least one axle. Oscillating wheels give better results on soil compaction, but pivoting wheels are sufficient for asphalt. Normally only the front axle oscillates or pivots. Drive system Mechanical transmission is laborious to operate while modern transmission systems, such as power shift, hydrostatic and torque converter allow for quick stopping and starting and in general easy operation. A differential action on the rear wheels will prevent shoving of the material when turning. The front wheels, which are non-driving, have an automatic differential action.

Operators station

Brakes The net weight of the PTR is approximately one third of the maximum ballasted weight. As the roller travels at relatively high

speeds between job sites and when refuelling, the brakes must give ample stopping capacity even when the PTR is fully ballasted.

The following formula is used to calculate surface capacity (A) during soil and asphalt compaction:

Here, ρ is the asphalt mix density in tons/m³. (Average value for ρ is 2.3 tons/m³.)

c x W x v x 1000

As drum width is a non-variable for a given roller, the capacity can only be affected by the number of passes, roller speed and layer thickness. Capacity is determined by drum width (W), roller speed (v), layer thickness after compaction (H) and the number of passes (n).

COMPACTION CAPACITY One parameter that determines the efficiency of a roller is its capacity. The main factors to be taken into consideration are:

A= • Drum width • Roller speed • Layer thickness (after compaction) • Number of passes

Efficiency factor, c, (i.e. the practical capacity divided by theoretical capacity. Depends on required overlap, the effective time of operation, etc. In practice the value of c can be set at 0.5–0.6 for asphalt and 0.75 for soil.)

m2/h

The corresponding volume capacity for soils is then: QS =

c x W x v x H x 1000 n

m³/h

Asphalt compaction is measured in tons per hour and is calculated by the following formula: QA =

c x W x v x H x 1000 x ρ n

t/h

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COLD MILLING EQUIPMENT The difference in size of cold milling jobs can be huge, they range from a small preparation for a new overlay on a city street to a huge multi-lane removal of the complete asphalt overlay on several kilometers of highway. Regardless of the size of the job it is vital that the selected machine performs flawlessly and with the best possible overall economy.

Operator ergonomics are important along with easy access to all frequent service points. Fuel efficiency and noise level are other parameters to consider before buying a piece of cold milling equipment.

Compact planers Compact planers (rear loading) Designed as either 3- or 4-wheeled units they allow quick transportation and the highest manoeuvrability. Compact planers are equipped with an easily removable rear conveyor system for working in confined areas, across a road or cutting around manhole covers. These types of machines are normally operated by one operator only. Typically several smaller jobs are finished during one day with compact planers. Loading of the reclaimed asphalt pavement is either by a short conveyor into an e.g. wheeled loader bucket or with a standard long conveyor, loading a dump truck. In practice most of the times â&#x20AC;&#x201C; due to the job circumstances (patch work jobs) or available space â&#x20AC;&#x201C; compact planers are used without a conveyor. In order to have limited left-overs, the drum is located at the rear end of the chassis for a short overhang.

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Additionally the right rear leg can be folded-in to ensure flush cutting alongside curbs or walls. Due to that skid steer loaders with drum attachments, sometimes used for small milling jobs, do not have the same productivity and accuracy as wheeled compact planers. Drum design The placement of tool holders and tool spacing determine the application for the milling drum. As bit changes are frequent it is important that this activity can be done with relative speed and ease.

Milling drum for a compact planer with cutting bits and kicker plates.

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Compact planer

Manoeuvrability As compact planers are used to cut around man-hole covers it is vital that they offer the best possible manoeuvrability. They, literally, need to turn on the spot in order to perform as required. Operatorâ&#x20AC;&#x2122;s station All frequent controls need to be placed logically and within easy reach of the operator. The platform must provide safe access means as well as proper guard rails etc. Good all-round visibility and a low noise level is important to provide a safe and productive working environment. A vibration isolated platform increases the operators comfort and a roof helps protects from both sun and rain.

A safe and well organized operatorâ&#x20AC;&#x2122;s station is a vital part in assuring efficient, high quality results.

Traction and drive system The propulsion system moves the machine and, together with the drum drive system, has a large influence on the productivity. Powerful hydraulics and an antispin system helps ensure that the machine performs as expected. Transportation Compact dimensions and a roof that can easily be lowered helps keep transportation efforts and cost to a minimum. Tie-down points should be clearly marked and easy to hook up to.

Engine The engine needs to deliver sufficient power for the milling drum as well as propulsion of the planers. Fuel efficiency is a key factor for an economical milling operation. Maintaining a low noise level is a vital working environment parameter, it is also important to reduce the disturbance of the general public as milling operations often take place at night. National and international agreements determine the level of emission control that the engine is required to meet.

Conveyor system An efficient, low maintenance conveyor system with sufficient capacity loads the material onto the waiting truck for transportation off site. The conveyor can be swung to either side, thereby enabling the truck to run on a parallel track. Sprinkling system Water is essential to the cold milling operation; a sufficiently large water tank helps the machine to stay productive and not have to stop for replenishment. The water is needed to cool and lubricate the bits as well as to help reduce the dust that is generated in the milling process. Optimizing the water usage helps operational efficiency. Easy to drain sprinkler systems and tanks decreases the risk of damage due freezing.

Good traction is a key element of a compact planer.

Water is required for cooling and cleaning of the milling drum during operation. A large water tank assures efficient operation with few stops for refilling.

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Large planers Large planers (front discharge) High capacity machines are equip ped with a front loading conveyor system as part of a two stage con veyor system for efficient and quick loading as well as with crawlers instead wheels for reduced ground pressure and better traction. The primary conveyor picks up the material from the cutter housing and conveys it to the front part of the machine, where it hands over the milled material onto the discharge conveyor. The discharge conveyor is slideable to either side to allow following a truck in a curve or unload the material to the side for later pick up. An efficient transport organization is required as the hourly production is quite high. Well prepared logistics with harmonized cutting speed, enough dump trucks as well as water and fuel tank trucks for refilling at any time makes the difference for a successful milling job. Sometimes “slowing down” in order to allow a constant milling process or cutting hard asphalt pavement in two layers is advisable. Sometimes parts of or all the RAP remains on the road as a base course for less demanding road applications.

Large planer

Drum design A high capacity drum is the key performance part on a large planer; it is in the interaction between the drum and the asphalt that the actual milling takes place. The wear on the bits, and sometimes the holders can be high. Fast and easy bit changes are important to ensure productivity on a large planer. The drum must be designed for easy replacement and maintenance. Operator station The large planer is a big piece of equipment. Cameras are used to help give the operator good overview of the working areas and all corners of the machine. Logically placed controls and a degree of automation further improve both

Quick change tool holders are important to speed up the exchange of the cutting bits.

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quality and productivity. Safe access and proper guard rails are important components in creating a safe working operators station. Traction and drive system The track drive need to be long lasting and should only require basic maintenance. Good weight distribution in the machine helps provide good traction. The hydraulic drive system should have proportional speed control and an anti-spin system for best performance.

A well-designed operator’s station with safe access and an ergonomical operator’s panel helps the operator stay comfortable and efficient.

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Conveyor system A front-loading conveyer that can be raised and lowered as well as swung sideways allows the operator full flexibility regarding adaptation to the truck that is being used as well as where the truck need to be; in the same track as the planer or on a parallel one. Joystick controls provide safe and accurate steering of the conveyor.

A swing conveyor improves the flexibility with regards to placing the trucks. This improves the operational efficiency.

Sprinkler system Large amounts of water are required to keep the bits cool and the dust under control. A high pressure sprinkler system uses minimal amounts of water while, at the same time keeping the cutting bits clean and cool. Water replenishment

on-the-run is a good feature to keep productivity high as even a larger water tank need to be topped up on a long shift. Engine The engine is the heart of a large planer. The drum drive is taken straight from the crank shaft and hydraulic pumps provide power to the propulsion system as well as the levelling and other functions. Fuel efficiency is important as well as low noise. Engine emission ratings are controlled be national and international laws and the engine should conform to these. Accessibility to frequent service points should be easy and safe.

CAPACITY CALCULATIONS Achieving 100% productivity is virtually impossible regardless what kind of construction equipment it concerns; this is also true for a cold planer. This is due to an array of potential scheduled and non-scheduled interruptions (Water/fuel refill, tool change, waiting for a truck, traffic etc). For this reason the theoretical productivity must be multiplied with an efficiency factor Fe. Fe normally rates from 80% for motor way jobs down to 40% for city jobs. e=

Actual productivity Theoretical productivity

(Normally 0,4–0,8)

It must also be noticed that the “loose volume“ of the milled material is higher than the volume of the “box” that was milled, “bank volume “. This explains the “swell factor” fs. fs =

Loose volume Bank volume

(normally 1,2–1,5)

The bank volume has a density close to the pavement density (j). For asphalt, j averages around 2,3 tons/m³. The working speed is assumed to be an average hourly speed but expressed in m/min (v x fe) This means that high productivity is defined by a high and continuous working speed as well as few stops.

The working speed is assumed to be an average hourly speed expressed in m/min (v). This means that high productivity is defined by a high and continuous working speed as well as few stops and interruptions which improves the efficiency factor. The following formulas are useful in calculating the productivity and for evaluation of daily or hourly performance to establish experience based data on the swell and efficiency factors. Surface:

Qs = W x v x 60 x e (m2/h)

Bank volume:

Qbv = W x v x 60 x e x D (m3/h)

Loose volume:

Qlv = W x v x 60 x e x D x fs (m3/h)

Weight:

Qw = W x v x 60 x e D x ρ (ton/h)

Where

W is Working width (m)

v is

D is Cutting depth (m)

Working speed (m/min)

e is Efficiency factor

fs is Swell factor

ρ is Bank density (Typically around

(Typically between 0,4 and 0,8) (Typically between 1,2 and 1,5) 2,3 tons per cubic meter)

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ASPHALT PAVING EQUIPMENT Selecting the right asphalt paver is the cornerstone of a successful paving operation. The choice is primarily based on parameters such as working width, surface conditions, required mano euvrability, etc.; but the overall quality of the asphalt paver and its performance are equally important. The design and effectiveness of the major assemblies and systems, for example the screed and the material distribution and drive systems, should be carefully evaluated as it is these that will ultimately determine the asphalt paver’s capabilities and productivity. The following section provides a basic guide to what to look for when choosing paving equipment.

Tractor unit

Undercarriage

Traction

Tracked asphalt pavers Asphalt pavers have separate track drives for good manoeuvrability and require an electronic synchronising system to enable the machine to run straight. (The need will depend on track length.)

VITAL DATA Tracked Asphalt Pavers • Operating weight • Weight tractor • Weight screed • Track dimensions (length x width) Wheeled Asphalt Pavers • Operating weight • Rear axle weight • Weight tractor • Weight screed • Number of drive wheels • Tyre sizes

An asphalt paver must be able to cope with the conditions it is likely to encounter on site. Its ability to do so will largely depend on whether the tractor has crawler tracks or pneumatic rubber tyre wheels. The choice between these two rests on a number of factors:

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Wheeled asphalt pavers Two front axles on a common centre bearing increase the self-levelling properties of the tractor unit. The weight balance and ground pressure per wheel give better traction on soft ground. Additional front-wheel drive can increase the traction by up to 25%. Anti-spin Control systems can be either electronically or hydraulically controlled by load sensors. The system avoids spinning drive wheels regardless of surface conditions and varying load.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Surface If the asphalt paver is to work on un-bound materials, tracks are better than wheels as they provide greater traction. A wheeled asphalt paver is faster and easier to transport than a tracked one, and is often preferred for work on hard surfaces.

Tracked asphalt pavers are used for laying large widths, close to 10 m with a hydraulically extendable screed and 14 m with a fixed width screed.

Working width On wide working widths, traction is important owing to the large forces acting on the screed. In general, crawlers provide better traction than wheels. Assuming that the asphalt paver is working under normal conditions, machines with two drive wheels can be used for widths up to about 6â&#x20AC;&#x201C;7 m, machines with 4 drive wheels for widths up to 8 meters (depending on screed weight), and tracked machines for widths up to 10 m and more. Mix design The type of mix also has an effect on traction. While bituminous materials above 150Â°C have a comparatively high flow-ability, cold asphalt mixes and stabilised or unbound gravel bases have a higher internal resistance to movement, and will thus require greater traction.

The paving width is limited for wheeled asphalt pavers. Pavers with a single drive axle can pave widths of up to around 6,6 m, whereas models with four or six wheel drive can pave up to 7,5 or 9 m respectively.

Screed type As heavy screeds require more traction than light screeds, the screed (telescopic, fixed, tamper, vibration etc.) should be selected prior to choosing the tractor unit.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

A wheeled asphalt paver moves easily from site to site on public roads.

Transportation VITAL DATA • Shipping weight • Overall width • Overall length • Overall height • Travelling speed

Low-bed trailers are needed to move tracked asphalt pavers between job sites, whereas wheeled asphalt pavers can travel under their own power, provided that the site is within reasonable distance, and provided that they have relatively high transportation speeds. If a trailer transporter is not required costs can be kept down. Most tracked pavers travel at about 4 km/h whereas wheeled machines reach speeds of around 20 km/h, whereas tracked machines travel at significantly lower speeds. This means that wheeled asphalt pavers are faster around the site, and can move quickly from one end of the mat to the other to start laying again.

Engine VITAL DATA • Make and model • Rated effect at ...... • Fuel tank capacity • Cooling system • Optional engines

As with any piece of construction machinery, the engine should provide power with economy. It should be a recognized brand so spare parts are easy and fast to obtain. High tonnage work requires an engine big enough to withstand the pressures of long hours of paving. Water-cooled engines are less noisy owing to their water jacket and they meet environmental requirements for low emission engines. Low fuel consumption is an important parameter. Hydraulic systems that allow the machine full functionality even when the engine is not running at full speed are useful. The systems can be automatic and set the engine rpm according to the power required by the machine.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

They can also be manual and allow the operator to select the engine speed desired on the required power output

Material distribution system VITAL DATA • Hopper capacity • Number of conveyors • Auger diameter • Flow-through capacity

Material flow A smooth uninterrupted material flow through the asphalt paver is a prerequisite to a effective job. The material distribution system must provide a constant supply from the truck to the screed. Three major assemblies need to be considered: Hopper, Conveyor system and Auger system Hopper The size of the hopper is important. Large hoppers can accept large quantities of mix, helping to pro-

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

The material is transported from the hopper via the conveyors to the augers at the back of the paver. The augers distribute the material evenly in front of the screed. Reversible augers means that the material can be transported in any direction required.

long the cool-off time of the mix. In addition, a large hopper enables the asphalt paver to continue to work while the feeder trucks are being interchanged. The hopper should be carefully designed to allow an easy flow of material down to the conveyors. ‘‘Cold corners’’ must be avoided since the cold lumps of asphalt may destroy the asphalt mat if they leave the hopper and enter the conveyor system. The sides must fold upward and inward to help the exchange of material in the hopper. Independently folding sides must permit paving in restricted areas, along walls, etc. Conveyors The purpose of the conveyors (bar feeders) is to transfer the mix from the hopper to the rear of the machine ahead of the screed. An asphalt paver has one or two conveyors depending on its size. Large asphalt pavers working at high lay-down rates require two conveyors. The speed and size of the conveyors as well as the cross section of the channel govern the

capacity of an asphalt paver. The flow-through capacity is defined as the maximum quantity of material transferred from the hopper to the rear of the asphalt paver within a given time. A common measurement for this figure is the lay-down rate expressed in tons per hour. In modern material feed systems, the two conveyors work independently of each other. They also work independently of the auger system. There is therefore a separate drive system for each conveyor. As the conveying speed is monitored and proportionally controlled by an automated system, discharge gates (flow gates) are not necessary. Auger system The auger system (spiral screw conveyor) distributes the mix evenly and continuously in front of the screed. There are essentially two types of auger system: the conventional type driven by a central gear unit and a version that has separate drive units at the outer ends of the auger. Both systems have their advantages.

The centrally-driven auger is easier to maintain. In the end-driven auger, there is no obstacle to the free flow of material as the drives are placed at the end. Both systems give good results provided that the central auger drive box is kept very narrow. The speed of the auger should be able to be proportionally controlled and automatically adjusted by material limit switches or touch-free ultrasonic sensors, which are increasingly used for auger speed control. The capacity of the auger is governed by its diameter, the pitch of the flights and its maximum rpm. The auger system should be easy to raise and lower to allow for optimum material flow for different layer thickness. Correct height is a precondition of constant and stable screed position when it comes to evenness and height level. The lowest point of the auger should be at a distance above the screed plate equal to approximately five times the maximum particle size. Adjustment should be quick and simple. Ratchets or hydraulic systems are both suitable.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Capacity Capacity is ultimately a question of the supply of asphalt to the asphalt paver. A continuous supply of material means that a large amount of material can be laid down. The capacity of an asphalt paver is determined by the paving width, the layer thickness and the average speed of the asphalt paver.

Screeds VITAL DATA • Screed type – Tamping – Vibrating – Fixed/Telescopic • Screed weight • Heating system

The screed is the most important part of the paving machine. It is, in effect, the main tool of the asphalt paver. Wear parts All components that come into contact with the mix during the laying process are subject to wear,

and should therefore be made of high grade materials. The bottom plates and the tamper assembly, the conveyor chains and bars and the auger flights, are fast wearing as they are constantly exposed to the material. The speed with which they wear out depends on the quality of the steel and, of course, the type and amounts of material passing through the asphalt paver. Heating systems There are three principal heating systems for screeds: diesel burners, gas burners and electrical heaters. Gas is cleaner than diesel and requires less sophisticated techno logy. Gas systems incorporate a fail-safe device to protect the crew against the risk of explosion. Electrical heating systems require an extra generator, which entails an extra cost. However, electrical heating is convenient and nearly as fast as other heating systems. Gas heating needs 20–30 minutes

The rigidity of the extended screed is crucial to paving results. Without a robust telescopic system the screed sags, especially at the ends.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

to heat the screed and an electrically heated system requires 30–45 minutes. It is also easy to thermos tatically control the temperature. The choice of system may ultimately rest on the supply of fuel. Gas is often not readily available, whereas there will invariably be a supply of diesel to run the asphalt paver. The temperature settings are infinitely variable. However, to prevent the risk of overheating, the screed heater should be thermostatically controlled within a limited range. Fixed screeds As the name implies, fixed screeds cannot be hydraulically extended; they require bolt-on extension boxes for widths wider than the basic one. The attaching and aligning of the extensions takes time. However, a fixed screed is normally set up for one working width and kept there. If you work on a wide variety of widths, a telescopic screed is a better choice. Telescopic screeds Telescopic screeds allow the ope rator to change the paving width at the flick of a switch during the laying process thus avoiding timeconsuming ”boxing out”. Today, the majority of asphalt pavers are equipped with telescopic screeds. The extended screeds must be rigid to ensure correct levels over the entire paving width. Wide-width paving (up to 9,7 m) requires an effective guide and support system for the hydraulically operated extensions. The adjustment to align the telescopic extensions with the main screed should be quick and simple, require no special tools, and should be able to be performed while laying.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Tamper bar

Bottom plate

Secondary compactor

Major quality aspects in a screed are rigidity and high quality wear parts, for example, tamper bar (1), bottom plate (2) and secondary compactor (3). The latter is only found on high-compaction screeds.

Paving Performance VITAL DATA a) Tamping screed • Screed weight • Contact area of tamping elements • Amplitude (tamper stroke) • Frequency b) Vibrating screed • Screed weight • Contact area of bottom plate • Amplitude • Frequency

Screeds can be equipped with a tamping mechanism, a vibratory system or a combination of the two. The compaction effort of a screed is primarily determined by: • Screed weight and contact area of tamping or vibrating elements • Paving speed • Frequency and amplitude of tamping or vibrating elements • The State of the compacting elements (tamper contour and dimensions) • The state of the screed plates

Screed weight and contact area Harsh mixes and stabilised or unbound gravel require relatively heavy screeds to obtain the desired pre-compaction and a uniform, even surface. Conversely, relatively light screeds should be used on unstable, tender asphalt mixes where there is a risk of the screed sinking into the mat. Paving speed The paving speed has an influence on the paver pre-compaction. The higher the speed, the lower the density achieved. The narrow contact area of tampers precludes fast paving speeds. As the speed of the asphalt paver increases, the number of tamper impacts per unit area decreases, and consequently the material feeding effect of the tamper also decreases. If the speed is too high compared with the tamper activity, it may result in an uneven surface with poor texture. The actual speed limit depends on the width of the

tampers and the frequency at which they operate. Vibrating screeds with a large contact area allow the fastest paving speeds. High compaction screeds require speeds to be kept between 2–4 m/min to achieve the desired high compaction effect. Frequency and amplitude Tamping units work with a vertical stroke (double amplitude) of 4–6 mm, and a vibration speed in the range of 500–1500 rpm (8–25 Hz). Vibrating screeds have lower amplitude but a higher frequency (up to 3000 rpm/50 Hz). State of compacting elements New tampers or vibratory mechanisms together with new screed plates will naturally produce the best overall pre-compaction and surface evenness. For instance, the material feed will vary over the working width and might result in drag marks or an open textured mat if the tamper blades are worn.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

Screed load system A screed load system can also be useful in some cases. The system transfers part of the weight of the tractor unit via the screed lift rams to the screed. This provides additional load to the screed and enables it to maintain the desired level after a long stoppage. Loss of mat temperature would otherwise decrease the compactability of the mix and raise the screed. The result would be an un-even asphalt surface.

Screed levelling systems Levelling systems The screed side arms are attached to hydraulic levelling cylinders which control the level of the screed. The cylinders can be extended or retracted at the flick of a switch. The tow-point position determines the layer thickness. Screed stop system The screed should always be able to be locked in position to stop it sinking into the mat when the asphalt paver stops. Modern machines have a hydraulic screedstop system which automatically locks the screed lift rams when the asphalt paver stops during a paving operation. When the asphalt paver restarts, it automatically releases the pressure. Screed unload system A screed unload system should be provided to prevent the screed sinking when laying extremely soft materials. The system transfers part of the screed weight via the screed lift rams to the tractor unit. The rams give the screed additional support and allow it to maintain the desired level. The unload system is also useful to improve traction as it transfers weight from the screed to the drive wheels.

Integrated tack coating An asphalt paver with an integrated bitumen tank can provide tack coating and asphalt paving in one step. These types of asphalt pavers are suitable for road maintenance applications on thin layers (1,5 x maximum stone size) for top layers (wearing courses). Automatic levelling system Precision laying requires modern electronic systems that automatically control the material thickness. There are two main systems: the grade controller, which helps to maintain laying thickness, i.e. the surface evenness; and the slope controller which checks the cross-slope of a layer. String line systems are used in conjunction with a grade controller to ensure the longitudinal evenness of the mat where there is no other appropriate surface to work off. Electronic grade controller A grade controller, working off a reference surface, automatically maintains the height of the screed and the layer thickness of the material. For best results the reference surface needs to be as level as possible. Touch-free ultrasonic sensors that scan the reference surface are the most common type. Where there is a level surface, such as a kerbstone, a short control ski (approximately 30 cm) can be used to sense the variations in height. Short skis are also used as a joint

ATLAS COPCO | COMPACTION, PAVING AND MILLING

matcher when laying a new lane parallel to the one that is already in place. Long control skis (between 6â&#x20AC;&#x201C;9 m) are used when the existing surface is not fully even. They ride over bumps and dips, averaging out the longitudinal errors of the surface being paved. That is why they are also known as averaging beams. An ultra-sonic Big Ski can be up to 13 meters long. The grade controller can also work off string lines. These are rigged up when no accurate surface is available to work off, such as on new road constructions. Laser systems may be used to control the screed level in open areas, such as parking lots, playgrounds or runways. Electronic slope controller The slope controller maintains the specified left and right-hand cross-slope of the mat during the laying procedure. It detects any deviation of the screed from a pre-set cross-slope and generates the necessary signals to restore the original setting. The slope controller works off the screed itself, and is attached by a mechanical link system connected to the left and right side of the screed. Computerised levelling systems Computerised levelling systems are also available under various brand names. Their use requires qualified people on the paving team and good understanding between jobsite management and the surveyor personnel. Computerised systems use the ground surface as the refeÂ rence for the grade sensor. The layer thickness is calculated from specified height reference points on the ground and the planned top surface of the wearing course. Before starting agreement has to be reached where the height points have to be surveyed relative to the centre of the road.

WHAT TO LOOK FOR IN COMPACTION, PAVING AND MILLING EQUIPMENT

3D control systems 3D control systems that work in relation to digital design files can be used to control the paving where the quality demands are high. The systems are controlled and positioned from a satellite navigation system (GNSS) and a laser. Using a robotized total station is another option to provide positioning information to the control system

General features Safety Rails should be provided in all exposed areas to ensure that an operator cannot fall off. Safety guards should also be provided over the auger to stop anything from falling down into the system. The screed covers and platform should have good anti-skid protection. Fail-safe heating systems will prevent the risk of explosion and injury to the crew. On wheeled asphalt pavers, the main hydrostatic braking system must be backed up by a hand brake and an emergency foot brake. Operator comfort Paving can be arduous work. The more comfortable and relaxed the operator, the better the asphalt paver will perform. All controls must be within easy reach.

The control console should slide easily across the platform to give the operator good all-round visibility. A clear view of the supply truck, augers and screed is essential to effective paving. Automatic feed control takes pressure off the operator and allows the operator to concentrate on steer足ing and pushing the truck. It also provides for trouble-free truck changes. However, it must be possible to switch to manual control, if required. Seats should be comfortable and easily adjusted to the height and build of the operator.

Versatility Versatility depends on the type of screed used and the ability of the asphalt paver to cope with different materials. A telescopic screed for example, is far more versatile than a fixed screed as it can pave around obstacles. A machine that can lay sub-base material one day, and a smooth wearing course the next, and that can pave on a four-metre width on one site and a seven-metre one on another, will always be more benficial.

Availability The availability of an asphalt paver is a function of the integral quality of the machine. A high integral quality results from the wear resistance of the screed plate, auger and conveyor chains, the ability of the engine and hydraulics to withstand heavy tonnage as well as the proximity of good service back-up and spare parts. Availability is enhanced if the manufacturer uses well-known, components throughout the asphalt paver. That is because easier access to replacement parts heightens availability.

Maintenance and Service Daily maintenance is essential to keep an asphalt paver up and working. Greasing, checking of hydraulic fluid and oil levels as well as spraying with a cleaning agent must all be as easy as possible to help ensure that the work is done. In this respect, central lubrication systems, easily removable side panels and deck plates, easy access to oil drains and clearly visible level gauges are a great help.

Modern command units with LC displays and PLC control provide up to the minute status control, fail-safe protection and less down time.

Fold up covers and removable side plates mean easy maintenance and short downtimes.

ATLAS COPCO | COMPACTION, PAVING AND MILLING

COMPACTION, PAVING AND MILLING

ATLAS COPCO | COMPACTION, PAVING AND MILLING

PMI 3492 0270 01

ATLAS COPCO | COMPACTION, PAVING AND MILLING