Q.: ASTM's ready-mixed concrete specification says, “air content of air-entrained concrete when sampled from the transportation unit at the point of discharge” is to be within 1.5% of the specified value. Can you clarify what is meant by the transportation unit? Is ASTM referring to the mixer truck, end of pump line, or conveyor, as the transportation unit?

A.: You've highlighted one of the more important details to understand about ASTM C94, “Standard Specification for Ready-Mixed Concrete.” Philosophically, the key is to distinguish between transportation and placement. In practice that means the concrete is sampled as it is discharged at the jobsite, which includes from trucks or directly from the mixer.

For details of the sampling procedure, this document refers to ASTM C172, "Practice for Sampling Freshly Mixed Concrete." C172 is more specific, saying that it pertains to sampling "from stationary, paving and truck mixers, and from agitating and nonagitating equipment used to transport central-mixed concrete." The transportation units referred to in C94 would be taken from that list.

This document also gives specific directions on how to obtain samples from these sources. For example, you sample a drum truck mixer by “repeatedly passing a receptacle through the entire discharge stream or by completely diverting the discharge into a sample container.” It makes no mention of sampling the discharge from pumps, conveyors, wheelbarrows, and the like that are used for placing the concrete after it has been delivered.

The second important thing to note about C94 is this: “In any case where the requirements of the purchaser differ from these in this specification, the purchaser’s specification shall govern.” In other words, the purchaser may have additional requirements for accepting concrete at the point of placement. However, these must be clearly incorporated into the project specifications because they are not covered by C94.