Crazing Concrete Surfaces

Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer that is rich in paste or mortar. These cracks are rarely more than ⅛ inch [3 mm] deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas between cracks are typically no more than 1½ inch [40 mm] across and may be as small as ½ or ⅜ inch [12 or 20 mm] in unusual instances. Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first week. Craze cracking is more visible when the surface is drying after it has been wet.

Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete. Craze cracks are generally not a precursor to future deterioration, durability, or wear resistance, especially in interior slabs. Crazed surfaces can be unsightly and cracks can become more obvious as dirt gets embedded in them.

Concrete surface crazing usually occurs if one or more good concrete practices are not followed during installation.

• Place concrete at a moderate slump (3 to 5 inches [75 to 125 mm]). Higher slump (up to 6 or 7 inches [150 to 175 mm]) can be used if it is achieved by using mid-range or high-range water-reducing admixtures. These mixtures will be less susceptible to segregation and will bleed less
• Place concrete on 3 to 4 inches [75 to 100 mm] compactible granular fill to absorb some water from concrete and reduce bleeding. Moisten the subgrade only if conditions exist for a high evaporation rate. For interior slabs placed on a vapor retarder, avoid adding water to the concrete to increase slump
• AVOID the use of jitterbugs and vibrating screeds, especially with concrete with slump greater than 3 inches [75 mm]. DO NOT overwork or over-finish concrete with bull floats or other finishing tools while concrete is still plastic. This results in excessive mortar at the surface

• Start curing the concrete as soon finishing is completed, particularly after hard troweling. At this stage the concrete surface should not be allowed to rapidly dry. Keep the surface wet by either ponding with water, covering it with damp burlap and keeping it continuously moist for a minimum of 3 days, or spraying the surface with a liquid-membrane curing compound. Avoid alternate wetting and drying of concrete surfaces at an early age. Avoid curing with water that is more than 20°F [10°C] cooler than concrete
• If concrete has low bleeding characteristics or if evaporation rate is high, protect the surface from drying by using evaporation retarders or other means to reduce the drying of the surface. These precautions will be necessary with slower setting concrete or if concrete is placed on cold subgrade that can cause differential setting