Types of Concrete Cracks

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Cracks in concrete

Concrete will crack. Despite the best efforts of the contractor or homeowner who pours concrete, cracks do develop. Crack-free concrete is rare. In the day-to-day world of the average homeowner there will be cracks in driveways, concrete floors or even foundations. The cracks can appear soon after the concrete was poured or may not appear for a year or more. What kind of crack and what kind of concrete was used will help determine what kind of repairs can be made.

Causes of Cracks in Concrete

Puddle on concrete lot

Concrete will crack under a variety of circumstances. One of the main reasons cracks appear is the use of excessive amounts of water in the original mix. Water is added to make the concrete easier to pour when installing. Shrinkage that occurs when the concrete hardens and dries out causes cracks to appear. Cracks in concrete are given a variety of names but many have a similar appearance.

Random Crack

Weed growing through crack in concrete

The crack that spreads slowly in more than one direction is commonly called a random crack. It could be caused by normal shrinkage of the concrete when hardening and drying or by settlement of the undercourse on which the concrete was poured. This type of crack may expand as moisture seeps in and under the concrete and freezes. Flaking of the concrete around the crack may also appear. This crack may appear if adequate stress lines were not cut into the newly laid concrete.

Settlement Cracking

Settlement crack with stress line

Settlement cracks may appear when the underlying ground has not been compacted or prepared properly or if the subsoil was not of the proper consistency. The settlement crack may also appear as a random crack over areas where the soil of the subgrade was not level or where it settled unevenly after the concrete was poured. In some instances it may be necessary to excavate the area and prepare it for a course of sand to prevent settlement and future cracking.

Heaving Cracks

Heaving crack in concrete

Heaving cracks may appear during a hard freeze in cold northern climates. Frost heaves may be caused by pouring concrete on soil that holds water instead of draining it away. Alternating freezing and thawing through the winter months may also produce frost-heave cracks. These often appear similar to settlement cracks. The frost heave may settle down with the advent of warmer weather, but the resulting cracks may allow even more moisture to seep into the underlying soil which may cause future problems.

Evaluating Cracks in Concrete

Concrete floor crack

Evaluating the crack in your concrete driveway, garage floor, foundation or other concrete slab is important and will help determine what type of repair is needed. A shrinking crack in a driveway or concrete slab is not likely to cause a structural problem. However it may allow water to seep in under the slab which would cause future problems.

The settlement crack may indicate that preparation of the underlaying soil was not adequate. Soil may not have been compacted or the sub soil may have needed a course of sand over it before the concrete was laid.

Substantial damage may occur with frost-heave cracks. Care and attention to proper soil drainage prior to pouring the concrete floor or slab is very important in cold, damp climates.

references & resources

Lon Quist

Lon Quist wrote news and scripts for radio stations. His novel "Pennyman" was published by MtSky Press. He has a broadcasting degree from the American Institute of the Air and engineering degree from Brown Institute. Licensed by the FCC, he was a radio station chief engineer and has been a writer for 40 years.