The compressive strength test is the final measure of quality for a concrete project. In spite of its importance, it is amazing how little attention is given to this test and the method in which it is performed. The steps that go into a complete compressive strength test include making the specimen at the site, site-curing the specimen, transporting the specimen to the laboratory, curing the specimen in the laboratory, preparing the ends of the specimen for test, and finally loading the specimen to failure to determine the compressive strength. If any of these steps are performed incorrectly, the result is likely a lower measured compressive strength.
The important steps in obtaining proper test cylinders are: Use only clean molds to cast a sample on site. Obtain the concrete for casting the samples near the final point of concrete placement and after adding all site water to the mix. Place the concrete into the molds in three layers, rodding each layer. Cover the specimens to prevent moisture loss.
The cylinders should be taken to the lab between 16 and 24 hours after casting. At the lab, the cylinders are demolded, marked and placed in moist storage. After proper curing has taken place, the ends are made plane and parallel, and then the cylinder is loaded to complete failure.
Even though the concrete supplier often has little to do with the test, he should still be actively involved in the process. Suppliers should read and understand ASTM Standards C 31, C 39, and C 617 to learn about the standards that describe how testing should be performed.