Question: When we had trouble pumping an expanded shale aggregate concrete we got some relief by increasing the air content (which had been 4 percent) up to 8 percent, and by replacing some of the sand with fly ash. Nevertheless, the mix didn't pump easily. What should we do to improve the pumpability of this kind of concrete the next time we have to use it?

Answer: ACI Committee 213, in the new "Guide to Structural Lightweight Aggregate Concrete," recommends the two measures you have already taken, but also recommends several others, the most important of which may be the first:

  • Presaturate the aggregate by sprinkling, by immersion in hot water, or by vacuum saturation. (The last method is patented and must be done at the aggregate plant)
  • Use at least 564 pounds of cement per cubic yard
  • Use a water-reducing admixture or a pumping aid
  • Replace a small amount of the coarse aggregate with sand
  • Use a slump of 4 to 6 inches and, if necessary, increase the cement content to maintain this high a slump
  • Use a well-graded natural sand with a fineness modulus of 2.2 to 2.7, or, if such sand is not available, increase the sand content

The committee notes that for a given structure several mix designs may be needed to keep the mix adequately pumpable as the structure gets higher. The change in unit weight of the concrete must be considered when using some of the measures listed above.