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Plastic shrinkage cracks are caused by a rapid loss of water from concrete surface before setting of concrete such that the rate of evaporation of surface water is higher than the rate of replacement of upward rising water. Tensile force is developed at concrete surface which forms plastic shrinkage cracks when the concrete starts to stiffen. Plastic shrinkage cracks appear to be parallel to each other with spacing of about 300mm to 1m. The cracks are shallow and generally do not intersect the perimeter of concrete slab.
Crazing is the formation of a network of fine cracks on concrete surface caused by early shrinkage of surface layer. The pattern of crazing cracks is in the form of irregular hexagon. The cause of crazing cracks is the shrinkage of concentrated dense cement paste at concrete surface. A wet mix tends to depress the coarse aggregates and form a highly concentrated cement paste and fines on surface. Hence, the difference between plastic shrinkage cracks and crazing cracks lies in the fact that crazing cracks arise from the shrinkage of weak material such as laitance in concrete surface while plastic shrinkage cracks appear even in normal concrete surface.