Q.: We supply ready mixed concrete for many jobs that use the American Concrete Institute "Specifications for Structural Concrete for Buildings," ACI 301. We have many field-strength test results for all our standard mixes and we use these to show that proposed concrete proportions will produce the average strength required for a job. However, for some required strengths we don't have field test data and have to show results from suitable trial mixtures as discussed in paragraph of ACI 301-89.

Do the trial mixes have to be made and tested by an independent testing laboratory? We have a well-equipped quality-control laboratory and I'm a certified laboratory technician. We want to do the mixes ourselves, both to save money and to know for sure how the tests were conducted.

A.: We find no requirement in ACI 301 that says trial mixes must be made and tested by an independent testing laboratory. That doesn't seem to be the intent of the specification.

In Chapter 16 on testing, however, paragraph 16.3.2 says that the testing agency designated in contract documents shall "review and check-test the contractor's proposed mixture design when required by the architect/engineer."