Q.: What's the required mixing time and drum speed when an accelerator is added to ready-mixed concrete at the jobsite?

A.: ASTM C 94-96, Standard Specification for Ready Mixed Concrete, doesn't address the jobsite addition of an accelerator. But you could probably follow the ASTM C 94 procedure for a single water addition that permits bringing the concrete to the specified slump. This procedure requires 30 drum revolutions at mixing speed (about 18 rpm or more), or as designated by the truck manufacturer. If you're using calcium chloride, it should be in a water solution, not in flake or pellet form.

To verify that mixing is adequate, observe slump uniformity as the concrete is discharged. If the first concrete down the chute is more fluid than concrete discharged later, the added calcium chloride solution might not be thoroughly mixed into the concrete.