Concrete is hard and stiff material which may crack due to several reasons. Cracking concrete might result into a huge issue on site. So in every step of the way from batching to final casting on site or even the preparation of the area where the concrete to be poured must be thoroughly inspected.
Cracking in concrete is unavoidable, but there is certain limits define by codes for the crack width. In order to take any preventive measure, first, it is necessary to understand the reasons of crack development.
1. Prevent Permeability of Water in Concrete
Concrete is a porous material. Water can be entrained through capillary action into the tiny pores. In cold region, this entrained water freeze inside the concrete and on freezing its volume increase. This may cause cracking in concrete. For underground and water retaining structures, the pores, water will seep and lead to corrosion of steel reinforcement and thus more cracks with develop.
In order to prevent this problem, impermeable layer on the surface can be provided. Surface Finishing, Paints and Water Proofing agents can help resisting permeability. Coating the bars of water retaining structure avoid corrosion. Air Entrain and Sulphate Resisting Cement can also help to prevent cracking.
2. Adequate Reinforcement Detailing
Concrete itself is a brittle material. Addition of reinforcement makes it capable of resisting tensile stresses. A properly detailed member prevents brittle failure even in the case of crack in the concrete. Codes specify the criteria for maximum spacing of bars at different locations. Lesser spacing is provided at joints and end of the span to avoid shear cracking.
Here compaction is in two ways:
Compaction of concrete
Compaction of the surface.
A properly compacted concrete mix has lesser pores and ultimately more strength. Reinforcement spacing should be provided in a way that concrete can reach all the corners. The concrete cover should also be adequate. Compaction of concrete can be done either manually or using vibrators. Choice of vibrator depends on the type of construction.
Compaction of the ground surface is necessary to avoid settlement. Removal of vegetation from site and properly compacted backfill can avoid uneven foundation settlement which may ultimately lead to cracking throughout the structure. Roller drum can be used to compact the soil surface.
4. Proper Curing
Curing significantly helps to prevent cracks. During hydration process, water in concrete evaporates so quickly leaving pores on the surface. To avoid this situation, the concrete surface should be kept moist. Wet bags should be wrapped around vertical concrete surfaces and water pond on horizontal smooth areas, so that crack will not form on concrete. Curing is critical in hot and dry weather when the rate of evaporation is very high.
5. Proper Design Mix
The strength of concrete depends on proper design mix. Different design mix ratios are used for different items of construction. Such as we need more strength for columns and shear walls as compare to beams, slabs, and foundation. Selection of concrete strength also depends on loading and stresses applied to it.
A proper design mix contains accurate water quantity. This percentage of water in mix decreases with the increase of strength requirement. The excess amount of water in the mix will not give correct strength required.
Also, the excess water in concrete evaporates at a high rate, cause shrinkage in concrete. This shrinkage may cause the development of crack. If the span of concrete surface is large enough, this shrinkage force may break it to pieces. In order to avoid cracks due to shrinkage, span should be provided as per code specification. Intended Control joints at regular spacing also help the development of crack on the preferred location. This intended cracks can be made using grooving tools with specific thickness and depth.
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