All concrete experiences some degree of shrinkage that can affect its long-term strength and durability as well as result in cracks. Since shrinkage is an inherent property of concrete, it is impossible to totally eliminate cracking but it can be controlled. Concrete engineers such as those at The Conco Companies work diligently to understand and address the issues associated with shrinkage.
We know that shrinkage is caused by a loss of volume that occurs in different stages of the process but it is still a challenge to prevent or alleviate shrinkage, and thereby the cracking. It is an especially important consideration on projects that involve large mat pours such as the ones that Conco is doing on the Salesforce Tower in San Francisco and recently completed on the Wilshire Grand in Los Angeles.
According to the Portland Cement Association, “The shortening of concrete slabs can be caused by temperature decreases or moisture loss. These two causes are also related to curling and warping of slabs, respectively. Curling is the deformation of the slab due to a difference in temperature between the surface and the bottom of the slab (temperature gradient). Slab “warping” is the deformation of the slab surface profile due to a difference in moisture between the surface and bottom of the slab (moisture gradient).”*
To ensure the placement of concrete moves quickly, it is mixed with more water than what is necessary to hydrate the cement materials. Once the concrete is placed, the excess water separates from the hardening concrete, which results in a loss of volume that causes the concrete to shrink. The concrete’s reinforcement or base friction works to restrain it, which in turn can create tensile stresses in the slab causing cracks.
In an effort to prevent the shrinkage, one of the things we do at Conco is to watch out for fluctuating temperatures during the first 24 hours after placement. We also take considerable care to use the correct concrete mixture design so that the least amount of extra water is added and the largest size of aggregate is used. All of this can work to help reduce shrinkage.
Conco is a leading supplier of concrete services for the Western United States. We got our start in the Bay Area in 1959 by offering clients the best value on a wide range of concrete services and products. Since that time, we have taken a leading role as one of the top concrete contractors in the region. We have four regional offices serving California, Washington, Oregon, Nevada and Colorado.