What is Crazing in Concrete? Causes and Prevention of Crazing

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What is Crazing in Concrete?

Crazing in concrete 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 instances (Fig. 1(a) & (b)). Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first weak. Often they are not readily visible until the surface has been wetted and it is beginning to dry. They do not affect the structural integrity of concrete and rarely do they affect durability. However crazed surfaces can be unsightly.

Causes of Crazing in Concrete

Crazing in concrete usually occurs because of wrong construction practices like:

• Poor or inadequate curing - Curing of concrete is required to maintain the moisture content when concrete starts to set and gain strength. When the evaporation rate from the concrete surface is higher than the moisture gain from curing, the crazing cracks occurs in concrete. This occurs due to direct sunlight, low humidity, or drying winds.
• Intermittent wet curing and drying - intermittent curing allows the concrete surface to dry for sometime and this leads to concrete crazing.
• Excessive floating is the accumulation of cement paste on the top of concrete while the coarse aggregate settles down. This causes the moisture accumulation at top which when dries up causes crazing.
• 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. This will create a weak surface on the concrete due to concentration of fines on surface.
• Over vibration loading extra bleed & laitance on surface.

Preventive Measures for Crazing in Concrete

• Proper and early start of curing prevents the loss of moisture in concrete and helps in hydration process of concrete. The maintenance of continuous supply of moisture in concrete prevent the appearance of crazing on concrete surface.
• Use of curing compound on the surface prevents the rapid evaporation of moisture from concrete surface and crazing is prevented.
• Never sprinkle dry cement or a mixture of cement and fine sand on the surface of the plastic concrete to prevent the appearance of crazing.
• Use low water-cement ratio as possible, consistent with adequate compaction.
• Use workability enhancing air-entrained concrete with a moderate slump. Air-entrainment reduces rate of bleeding in fresh concrete and reduces the likelihood of crazing.
• Use low slump concrete, Higher slump allows the concrete mixture to segregate, resulting in a weak surface layer.
• Avoid steep moisture difference between concrete 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 finishing mixes, not richer than 1:3.
• Avoid over vibration which results in bringing too much slurry to the top or side.
• Avoid grouting processes or rubbing the surface with neat cement paste.

References: 

1. ACI 302.1R Guide for concrete floor and slab construction
2. IS177T Concrete slab surface defects - Causes, prevention and repair by Portland Cement Association
3. Avoiding Common Outdoor flatwork problems by Ward Malisch, in Concrete Construction, July 1990.