Controlling Concrete Cracks

This is primarily for slabs on grade, but some of it applies to other concrete applications:

  1. Cracks in slabs are mostly caused by drying shrinkage, including plastic shrinkage cracks which are another form of drying shrinkage.
  2. Drying shrinkage is a function of the water-cement ratio of the concrete and the hydration process, coupled with environmental factors of temperature and humidity.

That’s a lot of variables to try to control, so the simplistic approach is to let the concrete crack but control where those cracks occur. We do that by control joints. Here are a few rules to use for control joints:

  1. Make sure the thickness of the concrete is maintained within reasonable tolerances. As an example, for a 6" thick slab, the tolerance should be -1/4", +3/8". This gives a 5/8" variation in thickness of the slab. While this might seem tight for large slab construction, it is imperative to prevent random cracking.
  • Random cracking is not really random. Theres a cause for them and they are usually a result of not controlling thickness and restraint conditions.
  • To keep cracks to a minimum, they should be spaced appropriately. The spacing each way should create square sections of concrete delineated by the joints. If rectangular sections other than a square must be used, the ratio of length to width should be maintained at no greater than 1.25:1.
  1. The spacing of the control joints should be no greater than 24 to 36 times the thickness in inches.
  2. The depth of saw cuts should be at least 20 percent of the thickness, preferably 25 percent.
  3. The saw cuts should be made as soon as the concrete has set sufficiently to prevent ravelling of the coarse aggregate by the saw cutting operation, and in no case later than the same day of placement. Schedule the placement accordingly.

Feel free to add to this…