Q: Under what conditions are we allowed to retemper ready-mixed concrete at the jobsite? If we receive a batch that has too low a slump, can we add water to make the concrete more workable?
A: There are situations where adding water to concrete is acceptable, but you need to do so carefully, and only in accordance with procedures outlined in ASTM C 94, “Standard Specification for Ready Mixed Concrete.” You are not allowed to add so much water that the concrete exceeds the specified slump or the specified water-cement ratio, because that can affect concrete properties such as strength and cracking. When in doubt, consider adding a water-reducing admixture or superplasticizer to increase the slump.
Estimate or measure the slump from the first portion of concrete discharged from the truck (no more than ¼ cubic yard). If both the slump and water-cement ratio are below the maximums allowed by the specifications, adding water is permissible. Measure and record the amount of water added, then mix the concrete for 30 revolutions of the mixer drum at mixing speed.