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 instance.

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. 

Preventive Measure for Concrete Crazing:

• Carefully cure the concrete surface, so that the initial drying period is extended over as long a time as possible so that shrinkage of the outer skin is kept in conformity with the shrinkage of the main body of the concrete.
• Don’t begin finishing operations until the concrete is free of surface water. Troweling while there is still bleed water on the surface will produce a high water-cement ratio and weaken the surface layer.
• Don’t sprinkle cement on the surface to dry up bleed water since this concentrates fines on the surface and may result in crazing.
• Use air-entrained concrete with a moderate slump. Air-entrainment helps reduce the rate of bleeding of fresh concrete and reduces the likelihood of crazing. Slump refers to the amount of water contained in the concrete. A higher slump concrete may allow the concrete mixture to segregate, resulting in a weak surface layer.
• Avoid steep moisture gradient between the surface and the interior of the concrete.
• Trowel the surface as little as possible and in particular avoid the use of steel float.
• Avoid the use of rich facing mixes, not richer than 1:3.
• Use as low water-cement ratio as possible, consistent with adequate compaction.
• Avoid grouting processes or rubbing the surface with neat cement paste.
• Avoid over vibration which results in bringing too much slurry to the top or side.