What can cause an occasional load of concrete to exhibit a retarded set? We may batch out three loads of concrete with good setting times, then have a load that's severely retarded, and go the rest of the day without any other bad loads. The next day, the same thing may happen or a few days may go by without any retardation problems. When the one problem does occur, we can't figure out what's causing that one bad load. We've checked setting time for the cement, and it doesn't seem to be the problem. Some of the retarded concrete loads contained water-reducing admixtures, but we don't think it's a cement-admixture incompatibility problem because we've also experienced it when no admixtures were used. We do use fly ash in our concrete. Any ideas on what's causing the problem?
We heard of one producer who had the same problem and traced it to a sporadic but sudden loss of air pressure that slowed the closing of the fly-ash feeder, increasing the fly-ash content. Adding a second air compressor improved feed control and eliminated slow setting. Your story also reminds us of a similar situation we heard about years ago where none of the obvious retardation causes seemed to be present. The cement supplier's technical service representative had been asked to investigate the problem. He decided to take a walk around the plant early one morning, before any trucks had been batched out, and noticed a large number of pigeons roosting on the steel beams in the structure over the sand bin. He climbed up to take a closer look and saw an accumulation of pigeon droppings on the surface of the sand in the bin. Guessing that this organic material might cause the set retardation, he suggested enclosing the sides of the structure to keep the pigeons out. With the pigeons gone, the problem disappeared, and the technical service rep still tells the story. It's a stretch to believe that history repeats itself, so we think it's unlikely that your problem is caused by pigeon droppings. But it's still fun to tell the story. orm Setting Times