Curling, shrinkage, and w/cm

Q. Section 10.5.1 of ACI 302.2R states that concretes with a water-cementitious material ratio (w/cm) of 0.40 to 0.45 and compressive strengths of 4500 to 5000 psi (31 to 34 MPa) are likely to have an increased potential for shrinkage, curling, and cracking. Can you explain why? It was my understanding that using lower w/cm would produce concrete with lower shrinkage and curling.

A. Curling of slabs is directly related to drying shrinkage. Although drying shrinkage (and curling) can be reduced by decreasing the total water content in the concrete mixture, this is not equivalent to reduced w/cm. Shrinkage in concrete occurs in the cement paste, not the aggregate. The role of aggregate is to restrain shrinkage of the cement paste. An addition of cementitious material(s) to meet a specified w/cm of 0.45 or lower, will increase the paste content of the mixture, lower the aggregate volume, and increase the potential for drying shrinkage (and curling).

Other factors that influence drying shrinkage include aggregate size, shape, and elastic properties, cement composition, admixtures, and cementitious materials. An increase in the maximum aggregate size or rounder shape of aggregates will lower the paste content and decrease drying shrinkage. An aggregate with a high modulus of elasticity will also lower drying shrinkage of concrete. As for cement composition, increased shrinkage may be exhibited by concretes made with low sulfate cements, high alumina content cements, or finely ground cements. Some water-reducing and high-range water-reducing admixtures may increase concrete shrinkage. Ground slag may increase shrinkage with increased replacement, while silica fume at less than 7.5% replacement decreases shrinkage.

For more information on factors affecting drying shrinkage, refer to Section 14.4 of ACI 360R and Chapter 2 of ACI 209.1R.

References: ACI 209.1R-05; ACI 360R-10; ACI 302.1R-15; ACI 302.2R-22

Topics in Concrete: Slab; Shrinkage