Q.: We do concrete work in an area where natural sand is scarce, so most of the concrete contains manufactured or stone sand. We have a pumped concrete job coming up and I've heard that it's hard to pump concrete made with stone sand. Is this true, and if so, what can be done to improve the pumpability?
A.: The effect of sand on pumpability is related both to particle shape and to the amounts of finer sand particles. In some areas good pumpability has been achieved by using unwashed stone sands that contain higher than normal amounts of material that passes the Number 200 mesh screen. Specifications may permit the use of sands with as much as 7 percent minus-200 material. Care must be exercised in using sands with a high fines content though. Excessive amounts of fines increase the amount of water needed to get the desired slump and may also make a concrete sticky and hard to finish.