Q.: I'm a pattern-stamping contractor in Louisiana and last winter I had a problem with slow setting and excessive bleeding of a concrete slab. The weather was unusually cold here, with air temperatures around 45° F. The cementitious material was 20% fly ash, and the concrete also contained a water reducer. The concrete didn't set for six hours, and there was so much bleedwater that dry color-shake application and stamping were delayed. What caused this to happen?
A.: The three factors that you mentioned low temperatures, replacing cement with fly ash, and use of a water reducer may all have contributed to slow setting of the concrete.
When concrete sets slowly, the bleeding period is prolonged. That's why you had more bleedwater than usual. To avoid this problem during cool weather, you can order concrete that doesn't contain a water reducer, or have the producer add an additional 100 pounds of cement per cubic yard of concrete. Don't use calcium chloride accelerators because they can cause discoloration of decorative concrete flatwork.