How much difference does it make as to who makes the test cylinders? Could it be that some of our low strengths are caused by differences in the way they are made?
It certainly could. Failure to comply with all the requirements of making, handling and curing cylinders is a common cause of low strengths from test specimens. Sampling, rodding, storing and transporting techniques are all important. It pays to study ASTM Method C 31 and carefully comply with all the requirements.