I am doing a wet-process shotcrete project. The shotcrete mixture is being delivered by a ready mixed concrete company. Recently we had some delays on the site. The inspector told us that any concrete not unloaded within 90 minutes of arrival on the site would be rejected. Where does that rule come from?

ASTM C 94, “Standard Specification for Ready Mixed Concrete” states that concrete must be unloaded within 90 minutes of contact between water, cement, and aggregates, or before the mixer drum has revolved 300 revolutions—whichever comes first. This limit, however, may be waived by the purchaser if the concrete has sufficient workability that it can be placed without the addition of water. In hot weather, the 90-minute limit may be reduced by the purchaser.