Low Drying Shrinkage

Random cracks in concrete surfaces are the most visible indications of poor quality in concrete. When the homeowner or man in the street see cracks, they see some sort of failure. With little or no knowledge of the concrete strength or whether project specifications were met, the layman feels sure that something has gone wrong. To a warehouse owner, whose forklift runs over a concrete floor, severe cracking can mean major problems. To people in the concrete industry, all too often cracks are considered an inevitable part of the business, and in the absence of any failure of project specifications, passed off as “tough luck” to the owner. Concrete is rarely removed because it has cracked, and repairs are rarely satisfactory. If you don’t think the problem is widespread, walk down the street where you live and look at the driveways-most of them will have random cracking.

It is a fact that concrete shrinks upon drying and that this shrinkage is the underlying cause of cracking. Concrete placers take steps to control cracks with site preparation, placement of control joints, and curing, but these don’t always do the job. Less often, architects or contractors consider specifying low drying shrinkage concrete mixes at the onset.

Concrete mixes are available which exhibit less than half the shrinkage of conventional mixes. To attain the lowest shrinkage, every facet of mix design must be given special attention. Cement selection, coarse and fine aggregate selection and proportions, void structure, admixtures considerations, and low water content are all important. Especially important is the selection of the coarse aggregate—hard, 100% crushed aggregate is vital. Low shrinkage mixes are not necessarily the easiest mixes to place, as the sand to rock ratio is kept low. Also, they are not necessarily the least expensive, although they are quite often no more expensive than ordinary mixes.

The Structural Engineers Association of Northern California (SEAONC) has developed specifications and testing procedures for assuring concrete with lower drying shrinkage. These specifications usually appear on special projects such as sophisticated structures requiring earthquake precautions, public buildings, wastewater treatment plants, etc. On such projects, the specifications require the use of low shrinkage mixes and the concrete is tested for compliance. In most construction, however, there is no consideration of drying shrinkage. If architects, contractors and concrete suppliers would get together at bid time and select low shrinkage mixes whenever possible, especially for visible flat work and certainly for warehouse floors, it would go a long way toward improving customer satisfaction.