Q. For concrete placements requiring air-entrained concrete, we take a preliminary sample of the concrete (for information, not acceptance) and test it for air content before we discharge the concrete from the truck mixer. If the air content is, say, 3% when the specifications call for 5% to 7%, can an air-entraining agent be added at the jobsite to increase the air content? Or should we just reject the load?
A. Section 7.3 of ASTM C 94, "Standard Specification for Ready-Mixed Concrete," allows the concrete producer to use additional air-entraining admixture to achieve the desired air-content level when a preliminary sample shows an air content below the specified level by more than the allowable tolerance. The preliminary sample must be taken before the concrete has been in the truck more than 90 minutes, and the driver must turn the mixer drum at least 30 revolutions at mixing speed after adding the admixture. The maximum revolution limit is 300, so this procedure can't be used if the 30 extra revolutions cause the total to exceed 300.