Shrinkage cracking and upward slab edge curling are common problems of enclosed industrial floor slabs on grade. The edges of these slabs curl upward because of differential shrinkage when the top of the slab dries to lower moisture content than the bottom of the slab. This can be caused by moist subgrades, low humidity air on the upper slab surface, and because in order to make it workable, concrete must be made with much more water than is needed to hydrate the cement. Evaporation of moisture from the upper surface of slabs is what causes drying shrinkage. Curling is caused by the difference in drying shrinkage between the top and bottom of the slab.

The effects of shrinkage and curling due to loss of moisture from the slab surface often are overlooked by designers because of the great emphasis placed on compressive strength and slump testing, and also because of the lack of information on curling. Owners expect floor slabs to be relatively free of shrinkage cracks and free of curled edges at control and construction joints. In my opinion, for enclosed slabs on grade made with Portland cement concrete, these problems are worse today than 25 years ago. This is true for a number of reasons:

1. There are two basic references for floors on ground: PCA’s “Concrete Floors on Ground”, I and ACI’s “Guide for Concrete Floor and Slab Construction” (ACI 302).2 Neither of these emphasizes the need for low-shrinkage concrete for floor slabs on grade. The words “warping” and “curling” do not appear in the table of contents of the PCA book. That book implies that if slump is low, then almost everything has been done to minimize shrinkage. Neither publication suggests that the designer specify shrinkage testing of both cement and concrete and choose the cement and aggregates that will provide the lowest shrinkage concrete. With foreign cements and clinker flooding the U. S. market in the 1980s, designers must check cements. The PCA and ACI documents should specify that shrinkage testing is every bit as important as compressive strength testing for enclosed slabs on grade.
2. The 1960 to 1980 market demand for high-early-strength cement to allow rapid form removal in multistory construction has resulted in cements that have high shrinkage. The use of these high-early cements is promoted for slabs on grade by ACI 302, which somewhat arbitrarily requires a minimum 3-day compressive strength of 1800 psi (12.4 MPa) for all floors. High-early- strength cements increase slab shrinkage. The ACI and PCA documents should specify that cements with limited shrinkage be used for slabs on grade.
3. ACI 302 requires 4000 and 4500 psi (27.6 and 31.0 MPa) 28-day compressive strengths for enclosed single-course industrial floor slabs on grade, up from 3500 psi (24.1 MPa) specified 25 years ago. Despite a lower water cement ratio, these higher 28-day strength concretes usually contain either more high-early strength cement or admixtures that increase the total water content and thereby increase shrinkage.
4. Clean, low shrinkage aggregates are less available today than 25 years ago because environmental considerations restrict quarry operations.
5. Floor slabs are being built on higher moisture content subgrades as the cost of good industrial land has risen. Moist subgrades increase the moisture gradient through the slab, which increases upward curling at free edges.
6. Excessively low slumps such as the 3 in. (75 mm) maximum slump currently allowed by ACI 302 for Class 4 and 5 floors has encouraged the use of high range water reducers (HRWR) in order to increase workability. There is some evidence that HRWRs actually may increase shrinkage even though the total water content is not increased. Fortunately ACI 302’s maximum allowable slump is proposed to be raised to 4 in. (l00 mm) in 1987. This change will provide workability without using HRWRs.

Drying shrinkage: definition and amount
The definition used in this paper for the phrase “drying shrinkage” is borrowed from Mather’s Highway Research Board Committee3 which defined drying shrinkage of concrete as “the reduction in concrete volume resulting from a loss of water from the concrete after hardening.” That committee quoted Washa4 as saying that “drying shrinkage of concrete is caused principally by the contraction of the calcium silicate gel . . . when the moisture content of the gel is decreased.”

All practical portland cement concrete shrinks about 400 to 800 millionths [0.0004 to 0.0008 in.lin. (0.004 to 0.0008 mm/mm)] due to drying, according to the PCA document, “Volume Changes of Concrete,” 5 but that document states “when drying shrinkage is restrained by reinforcing, shrinkage can be reduced by up to one-half.”

Aggregate and shrinkage
To provide the workability needed for placement, practical concrete mixes always contain more water than is needed to hydrate the cement. When this excess water evaporates, the cement paste shrinks. To fully restrain shrinkage of the cement paste, concrete would have to contain the maximum practical amount of an incompressible and clean aggregate.

If the dry-rodded volume of an incompressible and clean coarse aggregate were equal to the concrete volume, then the coarse aggregate would fully restrain cement paste shrinkage. That is never the case, though, for conventional floor slab concrete because such a stony concrete mix would be totally unworkable.

In actual practice, the dry-rodded volume of the coarse aggregate is only 50 to 60 percent of the concrete volume if Y2 in. (13 mm) maximum sized aggregate is used, but can be as high as 75 percent of the concrete volume if 1Y2 in. maximum sized aggregate is used, according to Table 5.3.6 of ACI 211, “Standard Practice For Selecting Proportions for Normal, Heavyweight, and Mass Concrete.”6 Therefore, a large maximum sized coarse aggregate, slightly less than \13 the thickness of the slab, should be specified in order to maximize the amount of aggregate in floor slab concrete. This aggregate must be low in shrinkage.