Q.: On a good-sized pumping job there was serious trouble with fouling of the pump. Everything had been going well with load after load of 4000-psi, 4-inch-slump, air-entrained concrete when suddenly every load became virtually unpumpable. It subsequently turned out that the plant had used up its stock of the moist coarse aggregate being used and had started to draw from another pile of aggregate, which happened to be dry. Is this the likely explanation?

A.: Such a change can cause the trouble you describe. When the dry coarse aggregate was batched using the same batch weights as for moist aggregate it required more rock than normal to provide the same weight as the moist aggregate. If there had been, for example, 1750 pounds of coarse aggregate, and the total change in aggregate moisture content was 2 percent, it would mean that about 35 pounds more dry aggregate was batched. This would make the mix more harsh. At the same time, the drier aggregate would reduce mixing water by about 35 pounds, partially drying out the mix. The two effects would reduce pumpability. It pays for the pumping contractor to know something about mix design, as discussed in the 2-page article in the September 1982 issue, page 721.