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A crack in concrete could originate from faulty subgrade, incorrectly placed reinforcements, alkali-aggregate reaction, poor jointing, overloading or any of a score of other reasons. The causes are important because repair preparation might be substantially different for many types of cracking, and the proper choice of patching material and procedures could differ also. Sometimes it takes a chemist and a concrete petrographer with a roomful of laboratory equipment to definitively identify the type of concrete deterioration; and frequently, deterioration stems from two or more causes. For example, spalling may have occurred not only because of faulty finishing practices that have removed the entrained air from the surface of a slab but also because low-strength concrete was used.

First, consider whether repair work is warranted. If the deterioration is slight and neither continuing nor interfering with use of the concrete, the best choice may be to leave it as it is. For example, if slight overloading caused by moving heavy equipment into a new building results in floor cracks that remain tightly closed, repair might be unnecessary and undesirable provided subsequent traffic would not likely result in progressive spalling and enlargement of the cracks. For ease of reference, problems illustrated in this article are divided into three categories: Cracks ruptures in concrete that are comparatively long and narrow; voids holes in concrete that are comparatively deep and wide; and scaling/spalling/erosion surface spalls that are shallow.

Problems illustrated include: plastic cracking, random cracks, crazing, alkali-aggregate reaction cracks, honeycombing, sand streaking, scaling, spalling, dusting, popouts, water cavitation, and water erosion.