What is "pattern cracking" and how can it be prevented?
Sometimes called crazing, pattern cracking is simply the result of surface shrinkage occurring at a faster rate than shrinkage beneath the surface. The real culprit is probably inadequate curing which permits the surface of the concrete to dry out faster than the rest of it. Since the drying out process is accompanied by shrinking, there is a difference in the amount of movement between the surface concrete and the concrete immediately beneath the surface. Because the concrete has comparatively little strength when subjected to this strain, hairline pattern cracks form in the surface. Pattern cracking may also be the result of having too much cement in the wearing surface of the slab. This condition also tends to cause greater shrinkage on the surface. Slabs should never be dusted with dry cement to hasten drying or to facilitate finishing. Crazing, dusting, plastic cracking and other concreting problems are discussed in an excellent pamphlet published recently by Alpha Portland Cement Company. The company's engineering department conducted a series of tests under job conditions to determine the causes of such defects and the most effective preventive measures. Here are the steps they consider essential to laying quality concrete floors: Use a relatively dry mix, not over 4-inch slump, on a damp subgrade. Compact by tamping, rolling or vibrating. If vibration is used, it should be uniformly applied and slump should not exceed one inch. Strike off and wood float to grade immediately. If necessary, use steel trowel sparingly to remove float marks. Avoid excessive troweling! Where an extremely smooth surface is desired, an intermediate troweling may be used with great care immediately after the water sheen leaves the surface. Give final steel troweling when finger pressure just dents the surface. Use enough finishers to handle the concrete when ready. The trowel will then produce a ringing sound. Cure with waterproof paper, membrane curing compounds, wet burlap, or by pending with water. Start curing as soon as possible. Do not omit any of these steps. Often small imperfections in fresh concrete surfaces do not show when the concrete is cured; so excessive troweling is not needed. The pamphlet may be obtained free of charge by writing to James M. Martino, sales promotion manager, Alpha Portland Cement Company, 15 South Third Street, Easton, Pennsylvania.