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Crazing concrete surfaces
Fine random surface cracking caused by poor curing, wet mixes, or incorrect finishing practices.
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. These cracks are rarely more than 1/2 inch deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas enclosed are typically no more than 1½ inches across.
Craze cracks generally develop at an early age, becoming apparent the day after placement or by the end of the first week. They are often not visible until the surface has been wetted and is beginning to dry out. They do not affect structural integrity or durability, but crazed surfaces can be unsightly.
Concrete surface crazing usually occurs because one or more rules of good concrete practice were not followed. The most frequent causes are:
- Poor or inadequate curing: intermittent wetting and drying or delayed application of curing permits rapid drying and provokes crazing.
- Too wet a mix, excessive floating, or use of a jitterbug that depresses coarse aggregate and produces excessive cement paste and fines at the surface.
- Finishing while there is bleed water on the surface, or using a steel trowel when the smooth surface brings up too much water and cement fines.
- Sprinkling cement on the surface to dry up bleed water: this concentrates fines at the surface.
- Carbonation: a chemical reaction between cement and carbon dioxide from unvented heaters, which softens the surface.
Start curing the concrete as soon as possible. Keep the surface wet by flooding, covering with damp burlap kept continuously moist for a minimum of 3 days, or spraying with a liquid membrane curing compound.
Use moderate slump (3–5 inches) of air-entrained concrete. Higher slump (6–7 inches) can be used only if the mixture is designed to produce the required strength without excessive bleeding.
Never sprinkle or trowel dry cement into the surface to absorb bleed water; remove bleed water by dragging a garden hose across the surface. Do not perform any finishing operation while bleed water is present. Dampen the subgrade prior to concrete placement to prevent it absorbing too much water.