Crazing

Crazing, a network pattern of fine cracks that do not penetrate much below the surface, is caused by minor surface shrinkage.

Crazing cracks are very fine and barely visible except when the concrete is drying after the surface has been wet. The cracks encompass small concrete areas less than 50 mm (2 in.) in dimension, forming a chicken-wire pattern. The term “map cracking” is often used to refer to cracks that are similar to crazing cracks only more visible and surrounding larger areas of concrete. Although crazing cracks may be unsightly and can collect dirt, crazing is not structurally serious and does not ordinarily indicate the start of future deterioration.

When concrete is just beginning to gain strength, the climatic conditions, particularly the relative humidity during the drying period in a wetting and drying cycle, are an important cause of crazing. Low humidity, high air temperature, hot sun, or drying wind, either separately or in any combination, can cause rapid surface drying that encourages crazing. A surface into which dry cement has been cast to hasten drying and finishing will be more subject to crazing. The conditions that contribute to dusting, as described below, also will increase the tendency to craze.

To prevent crazing, curing procedures should begin early, within minutes after final finishing when weather conditions warrant. When the temperature is high and the sun is out, some method of curing with water should be used, since this will stop rapid drying and lower the surface temperature. The concrete should be protected against rapid changes in temperature and moisture wherever feasible.