Question: We recently poured a slab with 3000 psi straight cement mix. The ambient temperature was 72° F and the high temperature for the day was 86° F. The mix temperature was 78° F with 5.5-inch slump and 5.8% air. We hand-screeded the slab and then used riding trowels with pans followed by finishing blades. When we sawed the joints there was no raveling or any surface problems. The general contractor used water curing with burlap and chose to start placing water on the slab less than 18 hours after the slab was poured, when hydration had heated the concrete up to about 104° F. The water used for curing was drawn from a deep well and was very cold.

After curing, the slab surface had become brittle and when sounded, appeared to be delaminated. I understand that the temperature of the water used for curing should not exceed 20° F difference from the in-place slab temperature. Could the shock of the water with the temperature gradient cause the surface problems?

Answer: You’re right that ACI 308, Guide to Curing, says, “Care needs to be taken to avoid thermal shock or excessively steep thermal gradients due to use of cold curing water. Curing water should not be more than 11° C (20° F) cooler than the internal concrete temperature to minimize stresses due to temperature gradients that could cause cracking.” So there could have been some thermal shock of the slab surface.

However, Scott Tarr comments: I’m not a fan of panning or troweling concrete with air content greater than 3%. Slightly above 3% may be okay but you had 5.8%. The higher the air content, the greater the risk of creating coalesced lenses of air just below the surface. Working the surface of air-entrained concrete with pans and finishing blades can create a weak zone of high air voids just beneath the surface. If this happened during finishing, applying cold water to the surface could easily cause this weak zone to fracture in shear as the surface contracts relative to the warmer slab interior. If the slab was specified to be air entrained and given a hard-troweled finish, this is a design risk.