Q.: What causes delayed setting of superplasticized concrete? We placed a 2-foot-thick slab on grade for a water treatment plant. Maximum air temperature during the pour was 65 degrees F. We placed the concrete directly onto a polyethylene vapor barrier. At midnight, 10 hours after placing, the concrete was still liquid so the crew went home. Next morning the concrete had set. I'm investigating the problem but don't know exactly what to look for.
A.: Several factors probably combined to cause the problem. The first question to ask is whether there were any other admixtures in the concrete: water reducers, retarders, or fly ash. Combining a super with another admixture that retards setting can sometimes cause abnormally long setting times. For instance, concrete with only a water reducer might set in 4 hours. Concrete with only a superplasticizer might set in 4 hours. But concrete with both the water reducer and superplasticizer might take 6 hours or more to set.
What type of cement was used? For a water treatment plant, especially with the thick section, Type II or Type V cement may have been used. These types set slower than Type I or III cements.
Excessive bleeding may have slowed setting. Part of the stiffening process that permits finishing is caused by drying, part by cement hydration. Because the section was so thick and there was a vapor barrier beneath it, there was probably a lot of bleed water on the surface that prevented any drying.
Finally, temperature could be a factor. What was the temperature of the concrete as delivered? With an air temperature of 65 F, concrete may not have been much warmer. In thick sections it wouldn't be warmed much by the sun. And the temperature probably dropped even lower during the late afternoon and night. Lower temperatures increase setting time.
Find out if the combination of super, cement type, and other admixtures was used in trial batches under the same temperature conditions found at the jobsite. You may need to make some mix adjustments before placing more concrete.