Q.: We are placing a floor today and the ready-mixed concrete trucks are arriving on the job faster than we can get the concrete discharged and placed. How long can we allow the concrete to remain in the truck before discharging it?
A.: This question is covered in ASTM C 94, "Standard Specification for Ready Mixed Concrete." Section 10.7 of the standard notes: "Discharge of the concrete shall be completed within 1 1/2 hours, or before the drum has revolved 300 revolutions, whichever comes first, after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates. These limitations may be waived by the purchaser if the concrete is of such slump after the 1 1/2-hour time or 300-revolution limit has been reached that it can be placed without the addition of water to the batch. In hot weather, or under conditions contributing to quick stiffening of the concrete, a time less than 1 1/2 hours may be specified by the purchaser."