Curing and curling of concrete slabs over vapor retarders

Q. Could you address the effect of curing methods on curling?

A. While one might think that curing would minimize curling of a hard-troweled floor by increasing the modulus of elasticity and preventing the development of a moisture gradient, commonly used curing methods provide neither benefit. This does not mean, however, that curing of hard-troweled floors is not important. Curing maintains favorable conditions at the surface so that the cementitious components can hydrate and the surface achieves the design strength, wear resistance, hardness, and density needed for performance, appearance, and cleanability.

The modulus of the slab is not significantly affected by curing because, regardless of the method of curing, the interior concrete below the densified hard-troweled surface remains adequately moist for a sufficient period of time to hydrate the cementitious components and achieve the design compressive strength and modulus. The development of a moisture gradient (the root source of curling) may be delayed by effective curing, but a gradient will develop shortly after the curing covering or membrane is removed and the surface begins to dry. Evidence that curling cannot be eliminated by curing was provided in a 1958 PCA study, where corner and edge curling of concrete slabs was measured after 2 weeks from removal of curing water. The slabs were initially cured under wet burlap for 7 days and then ponded with water for an additional 4 to 5 weeks.

Various types of curing have different impacts on slab curling:

• When a slab is water cured (flooded or kept continuously wet by the addition of water), water can penetrate the joints and collect in the base or between the slab and vapor retarder. This excess water can keep the bottom of the slab saturated for an extended period of time or even cause the slab bottom to expand, resulting in increased curling as well as causing other problems with base and subgrade materials. This method of curing is not advisable for slabs;
• Using water-retaining coverings without continual water addition can keep the slab surface moist. While the coverings delay curling while in place (usually for a week), curling begins shortly after removal; and
• Liquid-applied membrane-forming curing compounds retain moisture. Because they stay in place longer than coverings, they can delay curling for a longer period. However, many products that claim to be curing compounds or curing aids are not capable of retaining moisture and therefore cannot be considered curing compounds. Although the sales literature may indicate that the product complies with the moisture retaining requirements of ASTM C309 when used on hard-troweled surfaces, it’s important to note that a hard-troweled densified surface can retain sufficient moisture to meet the requirements of ASTM C309, even without any curing compound or aid.

As for moisture gradient, the application of a polymer coating on the exposed surface will reduce the moisture gradient in a slab over the long term. This effect, coupled with reduced carbonation and associated reductions in drying shrinkage, can reduce curling.

In summary, while some method of effective curing is recommended for all concrete slabs, the primary reason behind the recommendation is not to reduce curling potential but to produce a quality exposed surface. Concrete below the densified hard-troweled surface will cure regardless of any additional curing.

References: ACI 302.1R; CI: Why Slabs Curl?; CI: Why Slabs Curl II?; ASTM C309

Topics in Concrete: Slab, Concrete; Curing of Concrete; Shrinkage of Concrete