Q.: For the first time in years we have a job calling for lightweight concrete, but we haven't kept up-to-date on specifications for testing. The aggregate supplier told us to air dry the test cylinders after a seven-day moist cure. Is this correct?
A.: According to ASTM C 192, "Making and Curing Concrete Test Cylinders in the Laboratory," test cylinders for structural lightweight concrete can be moist cured from the time of molding until the time of testing, or the procedures of ASTM C 567, "Unit Weight of Structural Lightweight Concrete," can be followed. That specification calls for a seven-day cure at 60° to 80° F, without moisture loss from the specimens, followed by 21 days at 50% ± 5% relative humidity and 73.4° F ± two degrees. These conditions are specified to determine the air-dry unit weight of lightweight concrete, and are said to closely approximate the "in-service" moisture conditions for a large part of the United States.
The air drying that you describe would be acceptable under the ASTM standards, but you may wish to verify that this is permitted in the contract documents for your project.