Crazing is the development of fine shallow hair cracks on create surface. Due to crazing the surface appearance of concrete looks not so pleasing. This may appear within few day after curing or sometimes after a longer periods. These cracks do not penetrate deep onto the concrete and do not affect structural strength of concrete.
Why Crazing Occurs on Concrete Surface?
Crazing occurs due to drying shrinkage or carbonation or due to differential shrinkage between the surface of the concrete and the main body of the concrete. This differential shrinkage is accentuated if the skin is richer than the parent concrete. Drying shrinkage is greatest when concrete dries up fast after curing. So if care is not taken while curing, it will lead to crazing on concrete surface.
How to Prevent Crazing on Concrete Surface?
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.