CHAPTER 8 - CONCRETE
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COMPOSITION OF CONCRETE CONCRETE TEST TYPES OF CONCRETE CONCRETE DEFECTS
COMPOSITION of CONCRETE •
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A mixture of particles of stone (aggregate)bound together with cement. The active agents are cement and water. When they are mixed, a chemical reaction takes place. The cement and water mixture will fill the empty space between the particles of aggregate and will slowly harden to form or become concrete. Concrete ratio cement:sand:gravel 1 cement:2 sand:3 gravel
Materials Composed In Concrete 1.
Cement
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the substance which bind the particles of aggregate to form a mass of high compressive strength material.
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it is manufactured by heating a mixture of finely powdered clay and limestone with water to a temperature of 1300 Centigrade
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this process will form Clinker which will be grounded to a fine powder with the addition of gypsum (calcium sulphate)
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the result obtained is called Cement
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the most common cement is Portland Cement
Materials Composed In Concrete 2. Aggregate •
Gravels, crushed stone and sand
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2 important factors of good aggregates: • durability • cleanliness
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2 types of aggregates: fine aggregate – fine and natural sand coarse aggregate – natural gravel and crushed stone - for the usage of reinforced concrete - for foundation and mass
Concrete Mix • The objective of designing a concrete mix is to determine the proportions of cement, fine, coarse aggregate and water is appropriate to achieve the following requirements: • Compressive strength The maximum compressive stress which a material is capable of sustaining.
• Workability The relative ease or difficulty of placing and consolidating concrete the form.
• Durability Ability of the hardened mass to resist the effects of climate and exposure.
• Finish The final product
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Concrete Mix
Admixtures Defined as materials other than cement, aggregates and water which are added to the mixture. Only small quantities are required so great care has to be taken to ensure uniform distribution throughout the mix. Examples of Admixtures are: –
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accelerators water reducer air entrainment retarders
Admixtures
Accelerators increase the rate of setting / temperature rise in the setting. It is useful when working in low temperature Water reducing reduce the attractive forces between cement particles thus increasing the workability. Air entrainment promotes the formation of air bubbles during mixing and results in improved durability. The presence of air in the hardened concrete tends to reduce its compressive strength but workability is improved. Retarders delay the beginning of setting and hardening of concrete. Most useful in countries where concrete has to be transported for long distances and also for concreting in large quantities in hot weather.
Concrete Properties Concrete has relatively: • High compressive strength • Low tensile strength • constant in elasticity at low stress levels • Low coefficient of thermal expansion • Crack due to shrinkage and tension • Concrete tests- to ensure the properties of concrete correspond to specifications for the application
Cement Manufacturing
Types of Cement •
Portland Cement – Also known as ordinary cement. It is the most widely used cement. It is named due to the resemblance of its properties with a well known natural stone quarried in Portland, U.K. Lime , silica and alumina are the 3 main constituents of cement with traces of iron oxide, magnesia, sulphur trioxide and alkalis.
Pozzolonic Cement – General purpose cement which can be used in all situations. It is manufactured by grinding together Portland Cement and pozzolonic materials with addition of gypsum. The properties of this cement is similar to ordinary Portland Cement. It produces low heat on hydration. It offers great resistance to chemical attack. It suitable for construction in sea water, hydraulic works and mass concrete works.
Types of Cement •
High Alumina Cement – It is manufactured by fusing together 40% each of bauxite (aluminium ore) and lime, 15% iron oxide and 5% silica and magnesia, then grinding together to fine powder, much finer than Portland Cement. It’s initial setting time is about 3 ½ to 4 hours. It hardens rapidly, that at the end of 24 hours, it achieves the compressive strength that normally can be obtained from other cement after minimum period of 7 days. It has excellent resistance to action of sulphats and also to acid water and sea water. It also can resist high temperature / fire. The rapid hardening quality permits early removal of formwork, thus reducing cost where repeated use of formwork is possible. However due to the high cost of bauxite, this cement is costlier than Portland cement.
Types of Cement •
Sulphate Resisting Cement – It is used in construction of foundation in soil which contains very high proportion of sulphates since it has excellent resistance to sulphate attacks.
Waterproof Cement – It is prepared by mixing ordinary cement and small percentage of some metal stearates at the time of grinding. It has more resistance to penetration by water and oils. This type of cement is used for the construction of tanks, reservoirs, retaining walls, swimming pools, dams, bridges, piers etc.
CONCRETE TEST
Concrete Test
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Concrete tests – tests which are performed to learn more about the properties of a specific sample of concrete. A number of different tests can be performed on concrete, both at site and in a laboratory. As concrete is an important structural element, therefore testing is mandatory and construction companies must provide documentation of their testing and results.
Concrete Test • The purpose is to test the quality and workability of concrete. • Workability means it can be easily mixed, handled, transported and placed in position and compacted. • Workability of concrete mix is affected by the following: • Reducing the proportion of coarse aggregates (the finer the grading, the greater the workability) • Shapes of aggregates (rounded ones are more workable) • Increasing the quantity of water to such limit that the water cement ratio is still maintained • Increasing the quantity of cement • Process of mixing • Basically there are 2 (two) type of tests: • Slump test • Cube test
Slump Test • • •
The slump test is universally adopted as a check of the consistency of concrete in the field. The test is made with a metal cone 300 mm high, having a bottom diameter of 200 mm and a top diameter of 100mm. Method: – Within 2 minutes, fill cone in 4 equal layers. – Tamp each layer 25 times with steel rod. – Strike off top level and clean off any leakage around base of cone. – Then without delay, raise mould carefully vertically. – Measure slump to nearest 6 mm. If shear or collapse slump occur, repeat test with another sample. – Record results – Clean and dry mould
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Slump Test
Slump Test
Slump Test
Cube Test • •
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Samples of concrete are poured into 150 x150 x 150 mm mould and allowed to harden. Specimen are to be kept free from vibration and under damp sacks for 24 hours before removing them from moulds. They should be marked and stored in water at temperature between 10 and 21 degree centigrade then taken to laboratory where they must be stored again for 24 hours before being tested. The cubes are then tested to the strength to resist compaction. An average of 3 cubes are used for final calculation of strength. Test can be done after 3, 7, 28 days and also after 3 months and a year.
Cube Test
Cube Test
Cube Test
TYPES OF CONCRETE
Types of Concrete • • • •
Reinforced Concrete Pre stressed Concrete Precast Concrete Lightweight Concrete
Reinforced Concrete • Concrete is strong in compression but weak in tension, therefore plain concrete can be used where the member is in pure compression. • Steel on the other hand is strong in compression and tension but long steel bar can develop full strength in tension but cannot resist equal amount of compressive force. • Thus combination of steel and concrete are used to take up the stress and are known as reinforced concrete • Reinforcement are referred to the steel bars or welded wire fabric.
Pre Stressed Concrete • Normally applied to concrete members that are subjected to loads that cause bending. • Members include beams, girders and slabs. • Concrete is compressed to counteract load by stretching wires. • The entire cross section of the member is effective in supporting load • There are 2 (two) types: – Pre tension concrete – Post tension concrete
Pre Stressed Concrete • Pre tension Concrete – Steel wires are stretched in the form prior to the placement of concrete – Concrete is then casted and hardened – Wires are then released from the outside anchorage – As wires contract, they transmit compressive stress to the concrete through the concrete wire bond – It has a very high strength – Stranded steel wires are used in pretensioning (between 2 – 5 mm diameter)
Pre Stressed Concrete • Pre tension Concrete
Pre Stressed Concrete • Post tension Concrete – Steel cables are located in ducts embedded in the concrete. – Steel rods or cables are mechanically anchored to the one end of the beam – After concrete hardened, hydraulic jacks are used to stretch the steel rods. – The free ends of the steel are then mechanically anchored. – Since the mechanical anchor hold the tension, the concrete need not be bonded to the steel.
Pre Stressed Concrete • Post tension Concrete
Precast Concrete • Concrete elements which are made in factory and then transported to site. • Concrete are placed in moulds, under controlled factory conditions and transferred to site for final erection. • Concrete components such as columns, footings, beams, etc. made as per the required dimensions. • The precast concrete is transported to the construction site, lifted and positioned at the predetermined place.
Precast Concrete • Manufactured in a controlled casting environment - easier to control mix, placement and curing. • Quality can be controlled and maintained easily. Elements can be casted in advance therefore saves time. Precast concrete is cheaper in production if large structures are to be constructed in big amount. • This allows high quality concrete to be produced at relatively low cost.
Precast Concrete Products that are normally precasted are :
Paving slabs Kerbs Drains Bridge beams Walls Posts Culverts Railway sleepers
Precast Concrete
Lightweight Concrete • Are concrete which are light and produced with lightweight material or induced air to create void in concrete or induced foam agent into concrete to create small sealed voids. • There are 3 (three) types of lightweight concrete: – No fines concrete – Aerated concrete – Lightweight aggregate concrete (foamed/fibrereinforced)
Lightweight Concrete No fines concrete – Produced with lightweight only coarse aggregate which are mixed with cement and water. – Aggregate could be gravel or crushed bricks or blastfurnace slag – The aggregates should have uniform size to produce uniform voids in the concrete. – To ensure uniform coating of aggregates particles with cement and water paste, the aggregates must be wetted before mixing. – Constructionally, since the no fines concrete in nature, it needs protective covering externally such as cladding or plastering. – Used mainly in load bearing wall or non load bearing walls in framed structure.
Lightweight Concrete Aerated concrete – 1 part powdered zinc or aluminium is mixed to every 1000 part cement. – This zinc mixes with cement causing hydrogen to evolve when mixed with water. – As the concrete hardens, small sealed voids are produced in the concrete – Widely used as precast blocks and partitions – Extremely low density concrete may be produced if fine sand is used. However the concrete has high shrinkage and moisture movement.
Lightweight Concrete Lightweight aggregate concrete – Naturally occuring lightweight aggregate used e.g volcanic cinders and saw dust. – Now majority of aggregates are manufactured from denser materials e.g slate or clay – Used as structural concrete and wall blocks.
Lightweight Concrete Advantages • Lighter building load • Improve thermal performance – high thermal performance compared to normal concrete • Better resistance to fire • Lightweight aggregates are obtained from waste materials • Easier to cut, shape or carve • Easier to install – reduce construction period
Lightweight Concrete Disadvantages • Lower density therefore low in strength • Density of concrete is less thus effectiveness in soundproofing is less.
CONCRETE DEFECTS
Defects that appear on the surface of concrete during construction or within a relatively short time after completion, are usually caused by poor quality materials, improper mix design, lack of proper placing and curing procedures, or poor workmanship.
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Cracks Cracking can be the result of one or a combination of factors such as drying shrinkage, thermal contraction and restraint to shortening and applied loads. Cracks can also be a sign of corroded reinforcement. Cracks are divided into two groups which are structural cracks and non-structural cracks.
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Spalling Spalling is commonly known as concrete cancer. Rebars that are effected by moisture or salt from the environment cause the steel to rust and expand, thus pushing off surface that encases the rebars. Carbonation that triggers corrosion and insufficient concrete cover also cause spalling.
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Delamination Delamination can be a result from bleed water and bleed air being trapped below the prematurely densified mortar surface. Disruptive stresses from chloride – induced corrosion of steel reinforcement results delamination that are deeper called spalls. This defect can be detected by using sounding method by dragging a chain on the surface of tapping using a hammer. Hollow sounds produced indicate delaminated area.
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Honeycombing Formation of honeycombing is due to the presence of air and bubble at the surface of the formwork and results a separation between aggregates and cement mixture. Improper consolidation during construction process such as not properly vibrating the concrete mixture will cause honeycombing to occur.
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Scaling Scaling is quite similar to spalling but not as serious. Scaling is the local flaking or peeling of surface mortar, caused primarily by hydraulic pressures from freeze-thaw cycles affecting the concrete at the surface. Scaling happens when water is added to increase the workability of concrete. The rate of water cement ratio in the concrete will increase and further reduce the strength and durability of the concrete.
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Popouts Popouts is a conical fragment that breaks out the surface of concrete thus leaving a hole usually displaying fractured aggregate particle at the bottom of the hole. Contributory factors to popouts in concrete can be either due to pieces of porous rocks having a high rate of absorption and relatively low specific gravity or swelling of aggregates under moisture conditions. It forms a pressure that raptures the concrete surface.
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Blisters Blisters are hollow, low-profile bumps on the concrete surface typically 2-3 inches in diameter. Insufficient vibration during compaction that does not adequately release entrapped air, or overuse of vibration that leaves the surface with excessive fines, inviting crusting and early finishing.
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Crazing Crazing is a network pattern of fine cracks that do not penetrate much below the surface. It is caused by minor surface shrinkage. Crazing cracks may be unsightly and can collect dirt, crazing is not structurally serious and does not ordinarily indicate the start of future deterioration.
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Dusting surfaces Chalking and powdering at the surface of a concrete slab is called dusting. It powders under any kind of traffic and it can be easily scratched with nail or even sweeping.
CONCRETE DEFECTS AND DETERIORATION The reasons why concrete deteriorates can be attributed to either original construction defects and/or environmental effects.
Construction Defects Construction Defects include structural design errors, incorrect mix ratios, poor workmanship, insufficient curing and inadequate reinforcement cover. • Design Failure. Inadequate consideration at the design stage such as poor structural design, inadequate provision of expansion joints, incorrect load calculations, excessively slender designs, poor mix design (wrong grading and selection of aggregates, incorrect cement/water ratio etc) can all lead to failure of the concrete.
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Physical Damage. Impact damage from collision, explosion, fire damage, general wear from abrasion, overloading, damage from plants such as ivy and micro organism like algae can leave concrete surfaces damaged and open to attack from other aggressive forces.
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Poor Workmanship. Failure to observe design detail and poor quality control, incorrect placement of the concrete, inconsistent mixes, inadequate compaction or vibration, cement/water ratio not being adhered to, poor curing of the concrete, inadequate cover to steel reinforcement, allowing corrosion promoting chemicals to be included during mixing and placement are all factors which affect the durability of concrete.
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Structural Movement. Structural movement to the whole or elements of the structure by subsidence, shrinkage, incorrect or insufficient expansion/contraction tolerances or joints can cause undue strain and cracking to occur.
Environmental Effects Environmental Effects are a major contributor to the Concrete deterioration of structures some of the most common are : • Frost Attack. Concrete (unless protected) is porous and absorbs moisture. As water freezes it expands and can cause the surface of the concrete to spall and breakaway. This leaves the surface further exposed and reduces the cover to any steel reinforcement. • Chemical Attack. Aggressive chemicals, particularly acids will affect the concrete causing friable/crumbly surfaces. Chemical spillages and exposure to certain gases, acid rain and some industrial emissions can all have a detrimental effect
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Carbonation. Carbon dioxide and other gases combine with moisture and penetrate the concrete through micro pores, and react with the calcium hydroxide which gives the concrete its natural alkalinity. This alkalinity, formed during the hydration process, gives the steel reinforcement a passive surface. Once the alkalinity is reduced the steel starts to corrode.
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Alkali-Silica Reaction (ASR). Certain types of aggregate with poor alkali resistance interact with alkaline fluids in the pores of the concrete to form a silica gel around the surface. This gel absorbs moisture, causing it to expand, and ultimately leads to cracking and further deterioration of the concrete.
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Chloride Attack. Dissolved chlorides in the atmosphere, for example de-icing salts, combined with moisture, will penetrate concrete and attack the steel reinforcement, causing corrosion and eventual cracking and spalling of the concrete.
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