About Concrete |
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Crazing |
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Crazing is the development of a fine network of random cracks on the surface of concrete caused by the shrinkage of the surface layer. Generally, these cracks develop at an early stage and are evident the day after placement or within the first week. Crazing does not affect the structural integrity of the concrete and rarely affect the durability or wear resistance. Common Causes Concrete surface crazing usually occurs because one or more of the rules of "good concrete practice" were not followed. The most frequent causes are : • Poor or inadequate curing. Intermittent wet curing or even the delayed application of curing will permit rapid drying of the surface and promote crazing. • An excessive concentration of cement paste and fines at the surface caused by an overly wet mix, which allows coarse aggregate to settle. • Bullfloating or finishing while there is bleed water on the surface or the use of a steel trowel sealing the surface and diluting the cement paste. • Sprinkling cement on the surface to dry up the bleed water is a frequent cause of crazing surfaces. This concentrates fines on the surface. • Concrete which contain a high percentage of sand fines will increase the water demand in the mix. • Crazing may occur when water sprinkling is employed as a curing method. Care must be taken to insure that a continuous spray rather than an intermittent spray are used. Curing water that is much colder (20 degrees F lower) than the concrete can also contribute to crazing. Prevention • Avoid high slump concrete - do not exceed ( 4 ) inch slump. • Start curing as soon as possible. Damp burlap must be kept moist for a minimum of 3 days. A liquid membrane-curing compound is most common. A high quality cure and seal is best. • Limit bleeding by increasing air entrainment in the mix. • Limit troweling and don't trowel too early. • On outside work use a broom finish whenever practical. • Use the right amount of finishing at the right time. Do not perform finishing operations while there is excess moisture or bleed water on the surface. • Dampen the subgrade just prior to concrete placement. If a vapor barrier, such as polyethylene, is required on the subgrade, cover it with 2 inches of damp compacted sand to reduce bleeding. |
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