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Why does concrete crack?

Category: Upkeep

Some of the most frequently asked questions we get from new homeowners are about cracks in concrete. They could be in the garage floor, driveway, sidewalk, basement floor or wall. But one thing is certain about concrete-it will crack. The American Concrete Institute addresses this issue in ACI 302.1-04. “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction (Ytterberg1987; Campbell et al. 1976)”.

So, given that concrete cracks, why does it happen? There are several common reasons:

  1. Shrinkage – Probably the single biggest reason for cracks is plastic shrinkage. When concrete is first placed it has a significant amount of water in it. As the concrete cures a chemical reaction occurs between the water and cement to cause the concrete to harden. But as the water dries the concrete slab gets a bit smaller. This produces tension in the concrete and can produce cracks. Contractors try to minimize these cracks by using the minimum amount of water necessary to install the concrete, place control joints in the new concrete to encourage the cracks to form in the control joint rather than randomly in the concrete and by controlling the release of water from the drying concrete.
  2. Restraint – Because we just learned that concrete shrinks as it cures, if there are immovable objects located in the concrete such as columns, cracks are likely to form radiating from these objects. Contractors try to minimize these cracks by using expansion material around immovable objects and by locating control joints near the objects.
  3. Settlement – If the ground under the concrete settles and there is nothing to support the concrete, cracks can result. Contractors minimize issues with settlement by properly preparing the ground underneath the concrete and not installing concrete on frozen ground.
  4. Loads – When concrete is subject to loads exceeding its capacity, cracks can result. For example, if a large delivery truck drives over the edge of a concrete driveway, cracks can occur.

~the Brennans