CAUSES, EVALUATION AND REPAIR OF CRACKS IN CONCRETE

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1 3.0 Causes and control of cracking: 3.1 Plastic Shrinkage Cracking: It occurs within 1 to 8 hours after placing, when subjected to a very rapid loss of moisture caused by a combination of factors, which include air and concrete temperatures, relative humidity and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surface evaporation in either hot or cold weather. Fig.3a, Plastic Shrinkage Crack Pattern Fig. 3b, Plastic Shrinkage Cracks When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. A zero bleed concrete will shrink due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth which may form a random, polygonal pattern, or may appear as essentially parallel to one another as shown in fig. 3 a & b. These cracks are often fairly wide at the surface. They range from 5 cm to a few m in length and are spaced from 5 cm to as much as 3m apart. Plastic shrinkage cracks begin as shallow cracks but can become full depth cracks. Preventive measure: Dampening sub grade, early start of curing, sunshade, windbreaker, fog nozzle, plastic sheet to cover, evaporation retardant (e.g. aliphatic alcohol within one hour of concrete placement). Repairs: If closely spaced chipping the area and repairing as per para If farther apart, sealing and grouting. 3.2 Plastic Settlement Cracking: After initial placement, vibration and finishing concrete has tendency to continue to consolidate. During this period, the plastic concrete may be

2 locally restrained by reinforcing steel, earlier placed hardened concrete or formwork. This local restraint Fig. 4a, Settlement Crack at Surface Crack & Void Fig 4b, Settlement Causing may result in voids under the obstruction and cracks above the obstruction (Fig. 4a & 4b). When associated with reinforcing steel, settlement cracking increases with increasing bar size, increasing slump, and decreasing cover. The degree of settlement cracking maybe intensified by insufficient vibration or by the use of leaking or highly flexible forms. Preventive Measures: Avoiding congestion of rebars; rigid design of forms; proper vibrating needle penetration; provision of time interval between the placements of concrete in columns, deep beams, thick slabs and beams (with advance pour planning). The use of the lowest possible slump, and an increase in concrete cover will reduce settlement cracking. Repairs: If closely spaced chipping the area and repairing as per para If farther apart, sealing and grouting. 3.3 Drying Shrinkage Crack: When associated with reinforcing steel, settlement cracking increases with increasing bar size, increasing slump and decreasing cover. The degree of settlement may increase with insufficient vibration, lack of compaction at top layers of concrete, or by the use of leaking or highly flexible forms. This is more of a problem with high bleed concrete particularly in winter when the cooler temperatures provide longer time to initial set and therefore a higher amount of bleed (Fig. 5).

3 Fig. 5, Drying Shrinkage Cracks in slab Preventive Measures: Rigid formwork, leak proof formwork, use of screed vibrator and float for surface finishing. Designing a lower bleed concrete mix is recommended compared with the 5% maximum. Further use of a rich concrete mix (400 kg/m³ binder, W/C = 0.4) should be encouraged. 3.4 Long Term Shrinkage Cracking: Shrinkage is the inherent property of cement paste, which in pure form may shrink up to 1%. Aggregate provides internal restraint that reduces the magnitude of this volume change to about 0.06 percent. Shrinkage also occurs partly due to hydration of cement. On wetting concrete tends to expand but to a lower extent as compared to original shrinkage. Concrete would continue to shrink during its lifetime albeit at a very reduced rate. If the shrinkage of concrete could take place without restraint, the concrete would not crack. The combination of shrinkage and restraint cause tensile stresses to develop in the concrete, leading to cracking. In thicker section of concrete, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface causes cracks to develop that may, with time, penetrate deeper into the concrete. Long term measurements on some large reinforced concrete bridge structures have shown that the strain due to drying shrinkage after 5 years was about 30 x As the tensile strain capacity of hardened concrete is in the range 80 to 150 x 10-6, it is clear that long term drying shrinkage alone could not initiate the no-load-induced cracks. However it certainly plays an important role. The shrinkage of a particular concrete mix is also affected by additional factors such as temperature history, curing, relative humidity and ratio of volume to exposed surface. Sound aggregates for concrete have low shrinkage and the more quantity of it is present in concrete smaller would be the shrinkage. Fig. 6a & 6b shows shrinkage cracks in concrete tunnel and PSC girder respectively.

4 Fig. 6a, Lateral Cracks in Tunnel Ceiling Fig. 6b, Cracks in Girder Preventive Measures: Minimum water content, use of plasticizer for compensating workability due to lesser water, use of highest possible aggregate content and hence smaller quantity of cements, eliminate external restrains (e.g. smooth polythene sheet on the sub grade for base slab), sufficiently close spaced reinforcement (e.g. generally 15 cm in slabs & walls). Repairs: Sealing & grouting as necessary depending on the width of crack. 3.5 Concrete Crazing: Crazing is the development of a network of fine random cracks or fissures on the surface of concrete caused by shrinkage of the surface layer. These cracks are rarely more than 3mm deep, and are more noticeable on over floated or steel-troweled surfaces. The irregular hexagonal areas enclosed by the cracks are typically no more than 40mm wide and may be as small as 10mm in unusual instances (Fig. 7a & 7b). Fig. 7a, Crazing Fig. 7b, Crazing pattern Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first weak. Often they are not readily visible until the surface has been wetted and it is beginning to dry. They do not affect the structural integrity of concrete and rarely do they affect durability. However crazed surfaces can be unsightly. Crazing in concrete usually occurs because of wrong construction practices like: Poor or inadequate curing. Intermittent wet curing and drying. Excessive floating Excessive laitance on surface. Finishing with float when bleed water is on the surface. Sprinkling cement on the surface to dry up the bleed water. Over vibration loading extra bleed & laitance on surface.

5 Preventive Measure: Proper and early start of curing. Use of curing compound on the surface. Never sprinkle dry cement or a mixture of cement and fine sand on the surface of the plastic concrete. 3.6 Thermal Cracking: Temperature difference within a concrete structure may be caused by portions of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. These temperature differences result in differential volume change, leading to cracks. This is normally associated with mass concrete including large and thicker sections ( 500mm) of column, piers, beams, footings and slabs. Temperature differential due to changes in the ambient temperature can affect any structure. The temperature gradient may be caused by either the centre of the concrete heating up more than the outside due to the liberation of heat during cement hydration or more rapid cooling of the exterior relative to the interior. Both cases result in tensile stresses on the exterior and, if the tensile strength is exceeded, cracking will occur. The tensile stresses are proportional to the temperature differential, the coefficient of thermal expansion, the effective modulus of elasticity (which is reduced by creep), and the degree of restraint. The more massive is the structure, the greater is the potential for temperature differential and restraint. Hardened concrete has a coefficient of thermal expansion that may range from 4 to 9 x 10-6 per deg. F. When one portion of a structure is subjected to a temperature induced volume change, the potential for thermally induced cracking exists. Special consideration should be given to the design of structures in which some portions are exposed to temperature changes, while other portions of the structure are either partially or completely protected. A drop in temperature may result in cracking in the exposed element, while increases in temperature may cause cracking in the protected portion of the structure. Preventive Measures: Reducing maximum internal temperature. Delaying the onset of cooling. Controlling the rate at which the concrete cools by insulating the exposed concrete surface during first 5 days. This could be done by 50mm thick thermocol sheets encased with polythene sheet laid over concrete surfaces already covered with hessian cloth and water sprinkler keeping the hessian wet. The temperature gradient

6 between core of concrete and the surfaces should not be allowed to be more than 15 0 C. Increasing the tensile strength of concrete. Reducing the concrete temperature at placement up to say 32 0 C. Using low heat of hydration cement or using fly ash replacement of part of cement. Keeping steel formwork warm by air heating during winter. Use of thermally insulating material as formwork. Keeping insulating formwork for longer duration. Low grade of cement, OPC 33 grade is the best. Cement with high C 2 S content. Repairs: Sealing and grouting. 3.7 Cracking due to Chemical Reaction: Deleterious chemical reactions may cause cracking of concrete. These reactions may be due to materials used to make the concrete or materials that come into contact with the concrete after it has hardened. Concrete may crack with time as the result of slowly developing expansive reactions between aggregate containing active silica and alkalis derived from cement hydration, admixtures or external sources (e.g. curing water, ground water, alkaline solutions stored or used in the finished structure). The alkali silica reaction results in the formation of a swelling gel, which tends to draw water from other portions of the concrete. These causes local expansion and accompanying tensile stresses and may eventually result in the complete deterioration of the structure (Fig. 8) Fig. 8, Deterioration From Alkali-Silica Reaction Certain carbonate rocks participate in reactions with alkalis, which in some instances produce detrimental expansion & cracking. These are usually associated with argillaceous dolomite limestone. Preventive Measures: Proper selection of aggregate it should be innocuous to alkalinity.

7 Cement with low alkalinity (preferably less than 0.5, IS:456 limit is 0.6). Use of Pozzolanas (like fly ash and blast furnace slag) which themselves contain very fine highly active silicon. Repairs: Cracks should be sealed and grouted but only after 3 to 5 years. Until then these cracks may be temporarily surface sealed. 3.8 Steel Corrosion induced Cracking: Corrosion of the steel produces iron oxides and hydroxides, which have a volume much greater than the volume of the original metallic iron. This increase in volume causes high radial bursting stresses around reinforcing bars and results in local radial cracks. Fig. 9, Corrosion Induced Cracking These splitting cracks can propagate along the bar, resulting in the formation of longitudinal cracks or spalling of the concrete. A broad crack may also form at a plane of bars parallel to a concrete surface resulting in delamination, a well-known problem in bridge decks. Cracks provide easy access for oxygen, moisture and chlorides and then minor splitting cracks can create a condition in which corrosion and cracking are accelerated. The key to protect metal from corrosion is to stop or reverse the chemical reaction. Reinforcing steel does not corrode in concrete because a tightly adhering protective oxide coating forms in the highly alkaline environment. This is known as passive protection. Reinforcing steel may corrode, however, if the alkalinity of the concrete is reduced through carbonation or if the passivity of this steel is destroyed by aggressive icons (usually chlorides). Corrosion can continue if a longitudinal crack forms parallel to the reinforcement, because passivity is lost at many locations, and oxygen and moisture are readily available along the full length of the crack. Other causes of longitudinal cracking such as high bond stresses, transverse tension, shrinkage and settlement, can initiate corrosion. Preventive Measures: Dense concrete with low permeability.

8 Adequate cover to steel. In case of large bars and thick covers, it may be necessary to add small transverse reinforcement while maintaining the min. cover requirements to limit splitting and to reduce the surface crack width. For severe exposure conditions coated reinforcement, sealers and overlays on concrete surface, corrosion inhibiting admixtures and catholic protection. Concrete should be allowed to breathe, that is, any concrete surface treatment must allow water to evaporate from the concrete. Repairs: Entire loose concrete should be removed beyond the reinforcement bars by at least 30 mm. Corrosion on steel bars should be removed by scrapping /sand blasting so that shiny surface of metal appears. Repair should then be carried out as per para Errors in Design and Detailing: Errors that may result in unacceptable cracking include: Poorly detailed reentrant corners in walls, precast members and slabs. Improper selection and / or detailing of reinforcement. Restraint to members subjects to volume changes caused by variations in temperature and moisture. Lack of adequate contraction joints. Improper design of foundations resulting in differential movement within the structure. Reentrant corners provide a location for the concentration of stress and, therefore, are prime locations for the initiation of cracks. Well known examples are window and door openings in concrete walls (Fig.10a) and dapped end beams (Fig.10b). Fig. 10a,Typical Crack at Reentrant Corners of Beam Fig. 10b, Crack at End Additional properly anchored diagonal reinforcement is required to keep the inevitable cracks narrow and prevent them from propagating.

9 Lightly reinforced member tied structurally to a load-bearing member is another vulnerable location for cracking. When structural member deform under loading due to integral part of it, the non-structural member also deforms and takes load in proportion to its stiffness. As this may not be reinforced adequately for this load, it cracks with unsightly appearance. The restraint of members subjected to volume changes results frequently in cracks. Stresses that can occur in concrete due to restrained creep, temperature differential, and drying shrinkage can be many times the stresses that occur due to loading. A slab, wall, or a beam restrained against shortening, even if prestressed, can easily develop tensile stresses sufficient to cause cracking. Properly designed walls should have contraction joints spaced from one to three times the wall height unless they are provided with steel against such tensile stresses. Beams should be allowed to move. Cast in place post tensioned construction that does not permit shortening of the prestressed member in susceptible to cracking in both the member and the supporting structure. The problem with restraint of structural members is especially serious in pre-tensioned and precast members that may be welded to the supports at both ends. Repairs: Permanent repairs through sealing and grouting to such cracks should be undertaken once the cracks has become dormant with the passage of time. Temporarily cracks could be sealed with flexible sealant to avoid corrosion initiation. Para may be referred for this Improper Construction Joint: Construction joints (C/J) are the weak planes vulnerable for development of cracks unless it is properly prepared. Following precautions should be observed: Wire brush should not be used on green concrete for such purpose. Green cutting or washing of laitance with high-pressure water jet should be preferred. Coarse aggregates should be partly exposed during C/J preparation. Chisel should not be used for cutting concrete until concrete has attained full strength. Scabbler or pneumatic chisel with similar tool should be used on hardened concrete. Before concreting, the C/J surface should be cleaned of dust, soaked with water for hours and cement slurry may be applied on it. Repairs: Sealing and grouting as explained in para should be done.