Q.: I'm trying to find some information regarding using a mix that has only 3/8-inch coarse aggregate for a residential slab on grade in a pumpable mix as opposed to using a mix with No. 57 coarse aggregate. I am concerned about shrinkage and cracking.

A.: Richard Meininger replied that there is no problem designing a concrete mixture with 3/8-inch maximum size coarse aggregate (pea gravel or crushed stone) using the ACI 211 proportioning procedures for a concrete slab placed by pumping. But the concrete will shrink more than a No. 57 mixture, so control joints must be at a closer spacing—perhaps 8x8 feet or less to minimize shrinkage cracking. Excellent curing should be used to minimize early cracking. In a freezing exposure, a good entrained air void system is a must, and the mixture should develop 3500-psi compressive strength before the first winter. In a mild climate 2500-psi may be OK depending on the support conditions.

Gregg Allen stated that the real issue may be changing from a 4-inch boom pump mix to a 2-inch trailer pump mix. In order to make the mix pumpable, the pump operator will add water (to get a 6-inch slump) and require the coarse aggregate quantity to be reduced to around 1000 pounds per cubic yard or less. You basically end up with a masonry grout mix but with slightly less slump. Therefore, strength, cracking, shrinkage, durability, color, and discoloration are all concerns. Even with enough cement to get 3000-psi compressive strength, the concrete is marginal. It will be porous, and moisture will seep through easily, causing discoloration and wet spots. But even with all these problems, many small contractors order these mixes for patios, house foundations, driveways, retaining walls, and so forth. They continue to be a problem, but construction costs rule. People can't afford to do things properly the first time, so they gamble and then often either remove and replace or patch when the customer would have been much happier with a good job the first time.