Q. On my last project, the inspector rejected a concrete truckload if one slump test failed to meet the specifications. On previous jobs, I've had inspectors take two slump tests before rejecting the concrete. Which method is right, and is there a reference document relating to the answer?
A. ASTM C 94, "Standard Specification for Ready Mixed Concrete," states: "If the measured slump or air content falls outside the specified limits, a check test shall be made immediately on another portion of the same sample. In the event of a second failure, the concrete shall be considered to have failed the requirements of the specification." ASTM C 94 is referenced on most projects, since it is incorporated into the ACI 318 Building Code and into other national building codes. ASTM C 143, "Standard Test Method for Slump of Hydraulic Cement Concrete," also requires making two slump tests, but does not allow either test to be used to judge the slump. The requirement pertains to observing the concrete displacement after the slump cone has been removed. ASTM C 143 states: "If a decided falling away or shearing off of the concrete from one side or portion of the mass occurs, disregard the test and make a new test on another portion of the sample." It further states that "if two consecutive tests on a sample of concrete show a falling away or shearing off of a portion of the concrete from the mass of the specimen, the concrete probably lacks necessary plasticity and cohesiveness for the slump test to be applicable." Thus, using two slump or air tests before rejecting the concrete is the correct procedure.