Q.: If concrete is to be pumped and the specification calls for a 4-inch maximum slump, should the slump test be taken at the point of discharge from the truck or at the point of discharge from the pumpline?
A.: Concrete sampling procedures are described in the American Society for Testing and Materials (ASTM) standard method of Sampling Freshly Mixed Concrete (ASTM C 172). A note in this method says that sampling should normally be performed as the concrete is delivered from the mixer to the conveying vehicle used to transport the concrete to the forms. The note further states, however, that specifications may require other points of sampling, such as at the discharge of a concrete pump.
The point of sampling may depend on the purpose of the test. If the purpose is to verify the quality of the concrete as delivered, sampling from the mixer discharge is appropriate. Sampling at both the truck discharge and the point of final placement may be employed to determine if any changes in slump or other mix characteristics occur. To avoid disputes about where the sample is to be taken, put this subject on the agenda for a pre-job conference. The engineer, concrete producer, pumper, and testing laboratory representative should agree on sampling location before the job starts.