Q. What's a reasonable frequency for testing fresh concrete?
A. Specifications for Structural Concrete, ACI 301-96, requires obtaining at least one composite sample for each 100 cubic yards, or fraction thereof, of each design mixture of concrete placed in any one day. Composite samples are obtained in accordance with "Standard Practice for Sampling Freshly Mixed Concrete," ASTM C 172. Strength tests are conducted on each sample, but when the total quantity of concrete with a given design mixture is less than 50 cubic yards, the strength tests may be waived by the architect/engineer if, in his or her judgment, adequate evidence of satisfactory strength is provided. Slump tests are required for each composite sample and whenever consistency of the concrete appears to vary. If air entrainment is required, air-content tests are also required for each composite sample. Where concrete will be exposed to deicing salts, air-content tests must be made on samples from the first three batches in the placement and until three consecutive batches have air contents within the specified range. After that, every fifth batch can be tested until a test isn't within the specified range, at which time testing of each batch must be resumed until three consecutive batches have air contents within the specified range. Additional tests may be performed as necessary for control. These air-content tests may be taken on composite samples or on samples from the batch taken at any time after discharge of 2 cubic feet of concrete.