There are two adages about concrete: it will harden, and it will crack. Despite the best efforts of the contractor or homeowner who pours concrete, cracks do develop. Crack-free concrete is rare, and in the day-to-day world there will be cracks in driveways, concrete floors and building foundations.
While cracks can appear soon after the concrete is poured, they may not appear for a year or more. The type of crack and the kind of concrete that was used will help determine the repairs that will need to be made. Evaluating the crack in your concrete driveway, garage floor, foundation or other concrete slab is important. 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.
Concrete will crack under a variety of circumstances, but a few of the main reasons are; placing the concrete at too high of a slump, no contraction joints, and the use of excessive amounts of water in the original mix.
Cracks that occur before hardening usually are the result of settlement within the concrete mass, or shrinkage of the surface (plastic-shrinkage cracks) caused by loss of water while the concrete is still plastic.
Plastic-shrinkage cracks are most common in slabs and are relatively short cracks that may occur before final finishing on days when wind, a low humidity, and a high temperature occur. Surface moisture evaporates faster than it can be replaced by rising bleed water, causing the surface to shrink more than the interior concrete. As the interior concrete restrains shrinkage of the surface concrete, stresses can develop that exceed the concrete’s tensile strength, resulting in surface cracks. Plastic-shrinkage cracks are of varying lengths spaced from a few centimeters (inches) up to 3 m (10 ft) apart and often penetrate to mid-depth of a slab.
Settlement cracks may develop over embedded items, such as reinforcing steel, or adjacent to forms or hardened concrete as the concrete settles or subsides. Settlement cracking results from insufficient consolidation (vibration), high slumps (overly wet concrete), or a lack of adequate cover over embedded items.
Cracks that occur after hardening usually are the result of drying shrinkage, thermal contraction, or subgrade settlement. While drying, hardened concrete will shrink about 1/16 in. in 10 ft of length. One method to accommodate this shrinkage and control the location of cracks is to place construction joints at regular intervals. For example, joints can be constructed to force cracks to occur in places where they are inconspicuous or predictable. Horizontal reinforcement steel can be installed to reduce the number of cracks or prevent those that do occur from opening too wide.
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.
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. The best ‘formula’ for concrete is a water to cement ratio between .58 to .50 for most applications of 3,000 to 4,000 psi.
“Most cracks are not of a structural concern,” says Dave Mandel with Sequatchie Concrete. “Width and differential movement from one side to the other are used to evaluate severity of the crack. Most cracks are only of aesthetic concern, which must be considered when evaluating. However, what is acceptable to one person may not be acceptable to another.”
There are various ways to repair cracks in concrete including sawing out a section containing the crack and replacing the concrete. On interior slabs not subject to extreme temperature change, you can saw a V-cut can be made and then the crack can be filled with a non-shrink grout.
“It’s all about shrinkage and restraint of the concrete,” explains Mandel. “Cut down on shrinkage and you will cut down stresses created by the concrete contracting due to shrinkage. This movement of the slab creates stress which is released through development.”
Other procedures which can reduce cracking in concrete include the following practices:
– Minimize the mix water content by maximizing the size and amount of coarse aggregate and by using low-shrinkage aggregate.
– Use the lowest amount of mix water required for workability and placement; do not permit overly wet consistencies.
– Use calcium chloride admixtures only when necessary.
– Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks (more important in slabs).
– Provide contraction joints at reasonable intervals, 30 times the wall thickness is a good “rule-of-thumb”.
– Prevent extreme changes in temperature after placement and initial cure.
– Properly place and consolidate the concrete. (Source: Design and Control of Concrete Mixtures, EB001, and Diagnosis and Control of Alkali-Aggregate Reactions in Concrete, IS413
