Designing Mixtures to Reduce Shrinkage Potential

Figure 1. A crack that follows the rounded shape where the slab lost contact with the base.

Producing a concrete mixture with low shrinkage potential for flatwork applications is of high importance. Early-age volume changes can cause significant shrinkage and cracking (seeEarly-Age Cracking), but the focus of this article will be the shrinkage caused by the drying of concrete after the slab has been cured (see Curing vs. Drying Concrete). As concrete shrinks, tensile forces develop and may lead to warping and cracking of the slab (Figure 1).

Common factors for drying shrinkage potential include coarse aggregate type, aggregate size and volume, water content of the mixture, mixture temperature, cement type, and chemical admixtures. Tremper and Spellman (1963) studied the shrinkage of concrete and found the factors to be multiplicative. Three factors are discussed below with recommendations to reduce shrinkage potential of the concrete mixture.

Coarse Aggregate

Figure 2. Comparison of coarse aggregates; 3/4 in. maximum size (left) and 1 1/4 maximum size (right). The largest appropriate aggregate should be used to reduce shrinkage and warping of slabs.

High absorption, high shrinkage aggregates should be avoided when shrinkage potential is of concern. Additionally, the aggregate should be properly washed and protected from contamination during storage and handling. The largest size, well-graded coarse aggregate consistent with placement, workability, finishability, and durability requirements appropriate to the specific application should be used (Figure 2). As the maximum size increases in a well-graded aggregate, the void volume that must be filled by cement paste (cement and water) decreases. The shrinkage potential of paste is quite high when compared to the aggregates; therefore as the paste volume is reduced, so is the potential for shrinkage.

Water Content

The water content of a concrete mixture has a large effect on the shrinkage potential of the mixture (Figure 3). In addition to optimizing the aggregate volume fraction, water content decreases may also be realized by avoiding high mixture temperatures, utilizing moderate water-to-cementitious ratios (between 0.40 and 0.50), and using plasticizers to achieve adequate workability.

Figure 3. Relationship between total water content and drying shrinkage. A large number of mixtures with various proportions is represented within the shaded area of the curves. Drying shrinkage increases with increasing water contents.

Chemical Admixtures

Chemical admixtures that contribute significantly to shrinkage, such as calcium chloride accelerators, should be avoided when possible. While some water-reducing admixtures may increase shrinkage potential, the water content reductions realized by their usage may offset these effects. In some cases, locally available materials, project conditions, or project performance requirements may dictate the use of shrinkage-reducing admixtures.

References 
ACI Committee 360, Guide to Design of Slabs-on-Ground, ACI 360R-10, American Concrete Institute, Farmington Hills, Michigan, 2010, 72 pages.

Kosmatka, Steven H.; Kerkhoff, Beatrix; and Panarese, William C.; Design and Control of Concrete Mixtures, EB001, 14th edition, Portland Cement Association, Skokie, Illinois, USA, 2002, 358 pages.

Tarr, Scott M., and Farny, James A.; Concrete Floors on Ground, EB075, Fourth Edition, Portland Cement Association, Skokie, Illinois, USA, 2008, 252 pages.

Tremper, Bailey, and Spellman, D. L., "Shrinkage of Concrete—Comparison of Laboratory and Field Performance," Report No. 63-13, Highway Research Record,No. 3, pages 30–61, State of California Department of Public Works Division of Highways, 1963.