Shrinkage And W Cm.pdf

Concrete

Q&A

Shrinkage and w/cm

Q.

Section 9.5.1 of ACI 302.2R1 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.2 Drying shrinkage is governed by aggregate properties and the shrinkage of the cement paste. Because the aggregate restrains shrinkage of the cement paste: “The most important factor in affecting the potential shrinkage of concrete is the total volume of aggregate in a mixture…”3 Increasing the total water or cement content in the mixture will decrease the aggregate volume and increase drying shrinkage. Therefore, 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). Increasing the compressive strength of the concrete may also result in increased drying shrinkage. As Section 14.5 of ACI 360R2 states: “In the competitive concrete supply market, increases of 1-day, 3-day, and 28-day compressive strengths are often obtained at the expense of increased shrinkage. More cement and more water per cubic yard (cubic meter) (not necessarily a higher w/c), a higher shrinkage cement, or a water-reducing admixture that increases shrinkage, are the typical means used for increasing compressive strength.” Walker and Holland4 provide an extended discussion on the subject of low w/cm: “Most people in the concrete industry know that more water in a given mix will increase shrinkage, but many do not know that more cementitious material can have the same result (although to a lesser extent). On the other hand, if a poor mid- or high-range water reducer is used to keep the same cementitious material content and workability but reduce the water content, the resulting expected shrinkage decrease can actually end up being an increase instead. Furthermore, the increased compressive strength (and resulting higher modulus of elasticity) of either method of 64

july 2013 Concrete international

achieving the lower w/cm will increase curl. Thus, using this practice, the designer can unintentionally increase the chances of having significant shrinkage and curling.” Other factors that influence drying shrinkage include aggregate size, shape, and elastic properties; cement composition; admixtures; and cementitious materials.2,3 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 360R2 and Chapter 2 of ACI 209.1R.3 References 1. ACI Committee 302, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials (ACI 302.2R-06),” American Concrete Institute, Farmington Hills, MI, 2006, 42 pp. 2. ACI Committee 360, “Guide to Design of Slabs-on-Ground (ACI 360R-10),” American Concrete Institute, Farmington Hills, MI, 2010, 72 pp. 3. ACI Committee 209, “Report on Factors Affecting Shrinkage and Creep of Hardened Concrete (ACI 209.1R-05),” American Concrete Institute, Farmington Hills, MI, 2005, 12 pp. 4. Walker, W.W., and Holland, J.A., “The First Commandment for Floor Slabs: Thou Shalt Not Curl Nor Crack…(Hopefully),” Concrete International, V. 21, No. 1, 1999, pp. 47-53. Questions in this column were asked by users of ACI documents and have been answered by ACI staff or by a member or members of ACI technical committees. The answers do not represent the official position of an ACI committee. Only a published committee document represents the formal consensus of the committee and the Institute. We invite comment on any of the questions and answers published in this column. Write to the Editor, Concrete International, 38800 Country Club Drive, Farmington Hills, MI 48331; contact us by fax at (248) 848-3701; or e-mail [email protected].