THE INNOVATORS IN SURFACE PREPARATION
HIGHWAYS Improving skid resistance on road surfaces
Introduction Road surface condition generally and skid resistance in particular is now in the forefront of all highway authority engineers and highway maintenance contractors. There is a heightened international interest in the relationship between skid resistance and accident levels and there is commitment to reduce all road accidents, especially fatalities. Recent developments in design of asphalt surfacing materials has produced many benefits including, noise reduction, spray reduction, faster laying with reduced cost, but it is still early days for these materials compared to over 100 years of traditional HRA and chips. The new thin surfacing materials require high levels of quality control, both during production and laying to avoid surfaces with excess bitumen on the surface. Skid resistance of these materials is lower when first laid and may not reach its optimum for up to 2 years.
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Blasting to remove the binder from the surface of the a gregate particles will give the road surface its maximum skid resistance in a uniform way across the whole ca riageway within just a few weeks of the surface course being laid. In addition when in 3 or 4 years the aggregate has become polished and the skid resistance dropped below an acceptable level blasting could regenerate it by exposing fresh aggregate and removing the polished surface. This may extend the life of the road by several years.
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In addition to this safety aspect of asphalt there is also a modern trend towards using materials with different colours and textures both for aesthetic reasons to enhance town centres, commercial projects and urban regeneration schemes, and for safety reasons of lane identification, hazard warning and demarcation. Blasting in conjunction with asphalt mix design can achieve a whole new range of decorative finishes for asphalt including blends of aggregates of several colours and textures in one surface. Professional blasting of asphalt can offer solutions with significant benefits in all these areas.
THE INNOVATORS IN SURFACE PREPARATION
Biggest benefits • All BLASTRAC techniques are purely mechanical, and therefore very clean • None of our technologies are using chemical substances of are wasting valuable drinking water • Perfectly clean (closed circuit precess, no waste to dispose) • Safe (no risk of underground deterioration by the process itself or by frozen water, no damage on built-in lightning devices • Very fast evacuation of the highway if necessary • Permanent control can be made through grip testing devices / CFME (Continuous Friction Measuring Equipment) • Can prolong the life by a number of years without the need to immediately invest in new top coat
texture of a surface course There are principally 3 types of texture which are important Megatexture factors in the performance of a runway surface, Megatexture, is a component of surface profile, how flat the surface is. Macrotexture and Microtexture. In addition to these the terms Low megatexture reduces the demands on the vehicles suspension, especially during braking and steering, it Negative texture and Positive texture are often referred to, these reduces water ponding. Poor megatexture can be a result are types of Macrotexture and more to do with the method used of surface deformation, rutting or poor laying technique, and can affect water drainage with increased risk of or type of material laid. aquaplaning.
Positive Macrotexture is created by the voids or gaps between the raised aggregate particles which protrude from a flat surface Positive Texture typically occurs in HRA + Chips
Negative Macrotexture is created by the voids or gaps which indent or go down between aggregate particles from a flat surface. Typically occurs in thin surfacing materials such as SMA’s
Macrotexture
is effectively the average depth of the gaps or voids between the coarse aggregate particles in a surface, it is relatively stable, and can be measured by laser profilometers traveling at normal speeds. This is recorded as SMTD sensor measured texture depth.
Macrotexture is the depth of the spaces or voids between the aggregate particels. The texture depth / macro texture of an asphalt road surface will normally be between 1.0mm to 1.5mm.
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It is this macrotexture which allows the water to drain away from the surface and greatly contributes to skid resistance in wet conditions.
Aggregates used in asphalt surface courses will typically have PSV figures from 50 to 68, the higher the number the greater the friction or skid resistance.
Macrotexture is an important characteristic of road surfaces which affects stopping distances at high speed. There are two mechanisms involved:
Microtexture is the fine component of surface texture formed by the tiny interstices on the surface of the aggregate particles. It is the main contributor to providing grip or skid resistance with the tyre, particularly at low speeds.
1. Drainage Good texture depth assists water drainage, preventing the formation of a patch or sheet of water across the surface which could cause aquaplaning.
2. Hysteresis Good texture depth is necessary to allow mechanical deformation of the tyre (hysteresis) which dissipates energy in the form of heat. Macrotexture has traditionally been measured by the Sand Patch Test which involves spreading a known volume of sand across the road surface in a circular pattern until it has all fallen into the voids between the aggregate particles. The diameter of the sand patch is then measured and a formula used to calculate texture depth. This method is suitable for small areas or trial work but is slow and labour intensive.Current methods are based on laser measuring devices. Infra-red laser pulses are reflected from the road surface onto a diode array. The position of the returned pulses is used to estimate the vertical distance between sensor and road. The root-meansquare distance, a measure of the variation in the vertical distance, is used to calculate the average texture depth, referred to as Sensor Measured Texture Depth (SMTD). Microtexture of a road surface is provided by the roughness or texture of the surface of the individual aggregate particles.
Microtexture is measured by PSV using the British Pendulum Tester or by mobile methods such as the Griptester or SCRIM. Aggregates used in asphalt surface courses will typically have PSV figures from 50 to 68, the higher the number the greater the friction or skid resistance.
Skid Resistance Surface Grip, is necessary to enable drivers to accelerate, decelerate and change direction on the road surface. This grip is provided by the friction generated between the vehicle’s tyres and the road surface, and in turn this friction provides the force necessary to transmit the vehicles energy into the manoeuvre being attempted by the driver. Characteristics of the vehicle and actions of the driver define the magnitude of the friction force required to complete the manoeuvre successfully. If the friction generated is insufficient, grip is lost and control of the intended manoeuvre is lost. In the United States, it has been estimated that inadequate highway pavement conditions contribute to 10,000 of the 43,000 annual highway fatalities. Poor pavement friction or surface texture increases total crashes and also contributes to wet-weather crashes resulting in increased fatalities, more serious personal injuries, and significant traffic delays. World estimates are 1.2million highway related deaths and 50million serious highway related injuries annually. It is truly an international problem and FHWA and AASHTO representatives have conducted scanning tours to discuss and observe some of the best safety-related practices used around the world. The United Kingdom revised skid resistance policy based on 15 years experience is considered as best practice. Research has shown increasing the texture depth from 0.3mm to 1.5mm reduces the accident rate by about 50%, and increasing the skid resistance from 0.35 to 0.6 reduces the accident rate about 65%.
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THE INNOVATORS IN SURFACE PREPARATION
Polished Stone Value
Polished Stone Value (PSV) is a measure of how resistant an aggregate is to becoming polished or losing its skidfriction properties, usually referred to as its “microtexture”. The PSV test assesses the susceptibility of aggregate to polishing. The test is a 2 stage process, firstly accelerated polishing and secondly determination of the resulting friction value using the British pendulum tester. Aggregates used in asphalt surface courses will typically have results from 50 to 68, the higher the number the greater the friction or skid resistance. In addition to the aggregates skid resistance properties or Microtexture it is also important to know how durable these properties are, this is usually measured by AAV.
Accelerated Polishing Machine
Los Angeles Abrasion Value Test
Los Angeles Abrasion Value Test (LAAV) test measures degradation of the aggregate upon impact of a 25kg charge, this however does not adequately represent abrasion under tyred traffic as simulated in the AAV test, it is therefore less relevant in asphalt surface course evaluation.
SCRIM (Sideway-force Coefficient Routine Investigation Machine)
is used to measure the wet skidding resistance of a road surface. The measurements of skidding resistance recorded by SCRIM are used to identify lengths of road that are at or below Investigatory levels defined for a range of different Site Categories. A SCRIM survey is undertaken at a target test speed of 50 km/h. When the vehicle moves forward, the test wheel slides in a forward direction on a wet road surface. The force generated by the resistance to skidding is related to the wet skidding resistance of the road surface. Measurement of this sideways component allows the sideway force coefficient to be calculated as an average for each continuous 5,10 or 20m section.
British Pendulun Skid Friction Tester
Aggregate Abrasion Value (AAV) is a measure of the resistance of an aggregate to surface wear by dry abrasion. Aggregates with poor AAV will wear away quickly and this will have a significant effect on road surface texture depth “macrotexture”. AAV is measured by preparing two specimens which are pressed against the surface of a steel disc rotating in a horizontal plane, with a force of 0.365 newtons per sq. centimetre. Sand, fed by hoppers, is used as an abrasive. After 500 disc revolutions the amount of material abraded is measured by calculation of the weight loss of the aggregate. The percentage loss in mass of chippings is known as the ‘Aggregate Abrasion Value’ (AAV), and ranges from about 1 for hard flints to over 16. Aggregates used in asphalt surface courses will typically have results from 10 to 16, the lower the number the less wear. Note: Both PSV and AAV are important for skid resistance, but a high PSV aggregate does not necessarily have a good AAV and vice versa.
Test wheels are mounted midvehicle, in both the nearside and offside wheel paths at an angle of 20 degrees to the direction of travel. The test wheel which has a smooth pneumatic tyre of standard hardness is freely rotating and is applied to the road surface under a known load. A controlled flow of water wets the road surface immediately in front of the test wheel. When the vehicle moves forward, the test wheel slides in a forward direction on a wet road surface. The force generated by the resistance to skidding is related to the wet skidding resistance of the road surface. Measurement of this sideways component allows the sideway force coefficient to be calculated as an average for each continuous 5,10 or 20m section.
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SCRIMTEX
adds to the value of SCRIM wet road skidding resistance by also measuring the surface macro-texture in front of the SCRIM test wheel. This can be achieved in both wheeltracks of a double-sided machine and provides, in conjunction with air and surface temperature, the ultimate requirement for the assessment of road surface condition. Processing is undertaken using a suite of computer programs. The SCRIM Readings (SR) recorded by SCRIM are corrected for speed where necessary and are rejected if the test speed is below 30 km/h. Speed-corrected SCRIM readings are converted to SCRIM coefficients which in turn are compared with a range of investigatory levels for different Site Categories to identify deficient lengths of road. SCRIM is a completely self-contained system consisting of a commercial vehicle chassis fitted with a large water tank, one or two measuring wheel assemblies and data recording electronics.
Mean summer SCRIM coefficient (mssc)
Mean Summer SCRIM Coefficient (MSSC) Traffic levels, seasonal variations and surface temperature all have an effect on the SFC (surface friction coefficient) of a road surface. The SFC tends to rise in winter when wet and cold weather creates a gritty detritus which roughens the surface. In drier summer conditions, surface detritus is dusty and the surface becomes polished giving a reduction in SFC. In order to arrive at a standard base for assessment and comparison three SCRIM surveys are carried out between May and September to establish the mean summer SCRIM coefficient – MSSC.
griptester
can be a flexible, accurate and economical way to measure road network skid resistance. Simply towed behind any vehicle it works in the same basic way as SCRIM and can do 80kms of testing on one tank of water. Data can be integrated into computer for recording and analysis.
Surface Texture There has been growing concern internationally regarding the relationship between skidding accidents and road surface texture. Over the last few years there have been many conferences discussing the subject in infinite detail, at an international conference last year it was the main subject. Earlier in this paper the details and benefits of thin surfacing, SMA materials and HFS were explained, but as with all products there are usually some disadvantages or difficulties. In some situations these can be serious and different techniques or remedies have to be explored. Firstly the problem of;
“Early life skid resistance of thin asphalt surfacing” Quoting from a report by Dorset County Council about SMA it said “When laid the material at the surface of an SMA will be the binder mortar, a combination of bitumen binder, filler, stabilizing additives and some fine aggregate particles. At this stage it is unlikely there is any of the high PSV aggregate which is to provide the skid resistance exposed on the surface.
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THE INNOVATORS IN SURFACE PREPARATION
The MSSC values indicate a lower level of skid resistance in the initial year after re-surfacing. As the bitumen binder mortar is abraded from the surface and the aggregate is exposed skid resistance improves and continues to improve over the next two or three years. This will depend very much of the traffic intensity of the road. After this the polishing process of the aggregate by the vehicle tyres will gradually reduce the skid resistance of the surface.” Results from the survey by Dorset County Council showed that:
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There was a 30% chance of SMA having a skid resistance value lower than the investigatory level for the site category in the 12 months after surfacing. Skid resistance improved with time and in one year the MSSC values had increased approximately 11% and remained stable for the next two years before falling to 6% in the fifth year.
The Blastrac blasting process can be used to overcome this problem easily as has been proved by the trials carried out recently. The common practice to improve skid resistance at high risk locations in the UK is to apply HFS (Anti Skid), there are several difficulties with this, especially on heavily textured surfaces. Firstly HFS can not be applied for several weeks until the bitumen has started to oxidize and lost the oils, phenols etc from the surface. On heavily textured surfaces it requires large amounts of expensive resin material to fill the voids before achieving a satisfactory coverage. When applied it is predominantly only bonding to the bitumen which is still coating the aggregate, there fore the bond is only as good as the bitumen to the aggregate, which is relatively weak and flexible. The thermo-elastic properties of the HFS resin and the bitumen binder are likely to be different and in many cases early failure of the HFS occurs. Having applied HFS to the surface, even if fully successful, while it will have significantly improved the skid resistance (microtexture) it will have reduced texture depth (macrotexture) and may have reduced water drainage from the surface. The Blastrac blasting process can be used in two ways to overcome this problem, firstly by blasting to remove the bitumen binder and expose the high PSV aggregate sufficient skid resistance may be achieved without the need for HFS, saving a significant cost and retaining the macrotexture as per the design of the surface course. Or Secondly if HFS application is essential, then by blasting the surface first it will remove the bitumen from the surface and allow application of the HFS resin directly onto the aggregate, providing a better bond and improving durability.
The Blasting Process The basic blasting process has been around for over 100 years and Blastrac was the inventor of the first mobile shot blasting process in the early 80’s. Blastrac are now the undisputed global leader in this field. The Blastrac system is a mechanical process which is designed to remove surface contaminants, surface imperfections and coatings. The process is fully controlled, safe and environmentally sound. It uses no water, no chemicals or solvents, emits no pollutants or dust to the atmosphere and the removed material can often be fully recycled.Steel shot is fed by gravity through a control valve into an impellor. The impellor turning at high speed throws the steel shot through an adjustable opening at high velocity and at a specific angle on to the surface over which the self propelled machine is travelling. The steel shot impacts the surface and bounces off, as it does so material from the surface whether this is contaminants, coatings or the surface material itself is abraded and loose material together with the shot is drawn up into the machine by the airflow created by the vacuum unit.
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Brush seals enclose the blast head to the surface and air is drawn in through and under the brushes from the surface to ensure no shot or material from the blast head escapes to the atmosphere. The debris and shot is drawn up into the separation chamber and by a dual cyclone and magnetic separation process the debris is drawn off to the dust collector and the shot is recycled back to the blast head. Accurate control of the blast pattern and the degree of removal or texture created is controlled using various machine settings and operational practices. These include:
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Shot type Shot size Shot delivery control valve setting Impellor / Rotor speed Dust collector vacuum (airflow) setting Forward speed of machine Number and direction of passes
Other variables outside of the control of the operator include:
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Humidity Temperature Type of surface Type of aggregate in surface Type and depth of contaminant Required finish
It is therefore clear that while the required finish to the surface can 99% of the time be achieved, to initially set the machine correctly and find the best practice for each site requires time and test areas before work begins. Of course experienced operators are frequently able to identify all these things and set off almost immediately after arriving on site with the correct settings.
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Blastrac equipment is now regularly being used on roads and highways throughout the world. Blasting asphalt not only improves skid resistance, but also improves appearance. In some situations, especially in residential areas with low traffic levels, blasting can be used to identify parking areas and traffic management issues. This can eliminate the need for painting lines and markings, reducing cost and providing a more subtle approach to the requirement for area identification.
THE INNOVATORS IN SURFACE PREPARATION
blastrac machines There are several blast machines suitable for use on roads / highways including: Blastrac 2-20DT Working width
550mm
Blasting capacity
Up to 450m²/h on concrete
Motor
2 x 11kW / 400V - 50Hz or 60Hz / 63A - Three phase
Weight
575 kg
Dimensions L / W / H
1950 mm / 720mm / 1400mm
Drive system / speed
Electric / 0.5 – 33 m/min
Blastrac BMR-85D Working width
550mm
Blasting capacity
Up to 600 m2/hr on concrete
Abrasive consumption
100-200 g/m2 (depending on surface)
Weight
700 kg (without drive unit)
Dimensions L / W / H
1680mm / 720mm / 1050mm
Drive system / speed
Hydraulic / 24 km/h
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Blastrac 2-45DTM Working width
1150mm
Blasting capacity
Up to 3000m²/h on concrete or asphalt (depending on hardness, regulation requirements and pollution)
Weight
1750kg (without side shift and drive unit)
Dimensions L / W / H
1680mm / 720mm / 1050mm
Drive system / speed
Hydraulic / 0 to 4.7 km/hr
Working speed
0,5 to 40 mtr/min
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THE INNOVATORS IN SURFACE PREPARATION
Maintaining the correct level of skid resistance Reading all of the preceding information will clearly bring the reader to conclude that while a suitable skid resistance may be able to be achieved at the outset it will inevitably reduce as trafficking and ageing begin to take effect. How quickly the skid resistance level reduces to below what is acceptable varies significantly from site to site and will be affected by many things including, traffic levels and speeds, weather conditions, aggregate and binder properties etc. Highway authorities in most of the European countries routinely monitor the road network to assess the level of skid resistance in line with the standards set by the local Highways Agency. When the Characteristic SCRIM Coefficient (CSC) levels are at or below the assigned investigatory levels a site investigation is carried out to determine whether to improve the skid resistance or some other action is needed. BLASTRAC IS CONTINUALLY WORKING WITH CLIENTS AND CONTRACTORS TO DEVELOP AND IMPROVE ITS PRODUCTS IN ORDER TO ASIST THE HIGHWAY AUTHORITIES WITH THEIR OBJECTIVE TO REDUCE FATALITIES AND INJURY ACCIDENTS ON THE ROADS. References: 1. Design Manual for Roads and Bridges. Volume 7. Section 3. Part 1. HD 28/04 Skid Resistance. 2. P G Roe and R Lagarde-Forest TRL Limited. Report PPR060. The Early Life Skid Resistance of Asphalt Surfaces.
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Ride-on units
Hand-held Equipment
Scarifying
Shotblasting
Dust Collection Systems
STEEL BLASTING
Grinding / Polishing
Stripping
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