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Cracks formed by restraint to volume changes caused by loss of excess water. The time at which shrinkage cracks occur depends on the rate of drying but is usually several months to three or four years after casting. The loss of moisture from fresh concrete results in a reduction in volume. If the shrinkage movement is opposed by some external or internal restraint, stresses will develop. When the stress exceed the tensile capacity of the concrete, cracks develop.
Thin members with a large surface area such as slabs are particularly vulnerable. Drying out occurs from the surface and hence the surface layer is first affected. The surfaces of large cross-section members may crack because movement is restrained by the inner section of concrete.
Concrete near to corners and edges is particularly prone to cracking as loss of moisture takes place from the adjacent surfaces. There is no typical pattern of drying shrinkage cracking as the cracks form at any location where there is a restraint to shrinkage movement. The cracks are usually approx. at right angles to the direction of restraint.
Some aggregates are susceptible to a change of volume due to moisture fluctuations and must be used with caution. The occurrence of drying shrinkage cracks can be significantly reduced by appropriate Mix design and adequate Curing.
Acknowledgement: The Concrete Society
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