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Concrete Discharge Time Requirements

ASCC Position Statement #32


Project specifications, including those that cite ACI 301, “Specifications for Structural Concrete,” and ASTM C94, “Specification for Ready-Mixed Concrete,” state time limits for the discharge of concrete. ASTM C94 states that discharge of the concrete shall be completed within 1-1/2 h after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates. This time limitation is permitted to be waived by the purchaser if the concrete is of such slump or slump flow after the 1-1/2 h time limit has been reached that it can be placed, without the addition of water, to the batch. This waiver is seldom used because the reasons for allowing the waiver are not well understood.

ASTM STP 169D, Significance of Tests and Properties of Concrete and Concrete-Making Materials, includes Chapter 45, “Ready Mixed Concrete,” by Lobo and Gaynor. They state that the 1-1/2 h time limit is too conservative and that field and laboratory data demonstrate concrete strengths tend to improve with time, but only when water is not added. At higher temperatures, above 70° F, they indicate that the time limit can only be justified by a concern that the prohibition against water addition, after the adjustment or arrival at the job site, cannot or will not be enforced. 


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