Q.: What are the advantages and disadvantages of using fly ash in concrete for industrial floors? I'm interested in properties of both fresh and hardened concrete.

A.: In the summer, fly ash can be beneficial to a contractor's efforts to produce a flat floor because many fly ashes retard set, increasing the time finishers have to meet a tight flatness specification. In the winter, many contractors prefer to have no fly ash in the concrete because the cold weather already retards set and fly ash retards it further. Too much retardation increases costs because finishers must work overtime.

For hardened concrete, fly ash may reduce the rate of early strength gain if it's used to replace portland cement. This can delay joint sawing, which can't be done until the concrete can be cut without raveling the edges. However, later-age strengths may be higher, depending on the curing regime. Thermal cracking may be less likely, too, because fly ash helps reduce internal-temperature spikes in concrete.