SB COX

Before You Pour


Minimize Random Cracking in Concrete Slabs

Everyone desires a crack-free concrete slab, whether it is a basement floor, garage slab, carport, patio, or driveway. Random cracks do not generally affect the structural performance of a slab, but they are unsightly and generally lead to complaints regarding the construction of the slab. Fortunately, there are steps to take to minimize, if not eliminate, these unsightly cracks.

Concrete is a mixture of cement, stone, sand, water, and added chemicals to improve the workability or durability of the mix. After it is placed, or "poured", the concrete begins to cure and as it cures it begins to shrink. Shrinkage is inherent to concrete as it hardens, therefore random cracks will occur if shrinkage is not addressed. Shrinkage is the reduction in concrete volume due to the loss of water that takes place physically and chemically. Most of the shrinkage occurs within a few days, but continues for many years.

To allow for shrinkage and temperature movement, it is necessary to include control joints in the concrete slabs. Control joints can be made by sawing, forming, or tooling a groove about 1/4 the thickness of the slab. The joints are placed no further apart than 30 times the thickness of the slab. To avoid long thin sections, the length should be no more than 1.5 times the width, even if the joint spacing is much less than 30 times the thickness.

Proper design of concrete slabs and joint spacing is essential to minimize random cracks. Concrete mixes should not be poured "too wet" and the slabs must be cured appropriately. Contact S. B. Cox Ready Mix, Inc. for more information regarding mix design and curing. Proper planning can prevent many unwanted headaches.

Summary

The majority of cracks occur within 2-3 days after concrete has been placed. These are preventive measures that will minimize cracking in that period:

  • Make sure sub-grade is well-compacted
  • Check that form work is secure
  • Ensure that sub-grade and form work are moist before pouring
  • Do not add water to ready-mixed concrete in placing
  • Compact low-slump mixes well
  • Cut sufficient contraction joints to allow for shrinkage
  • Provide expansion joints where necessary
  • Start curing as soon as possible
  • Maintain proper curing for an adequate period


  phone number    (804) 364-0500

  fax    Fax (804) 364-1753

email  sales@coxreadymix.com

Copyright 2017
S.B. Cox Ready Mix Inc.
2160 Lanier Lane
Rockville, VA 23146