One of the most common misconceptions about the tested strength of a concrete cylinder is that it represents the strength of the concrete in place. While in a perfect world there might be a correlation between the two, what cylinder strength is intended to represent is the potential strength of the concrete as delivered—the highest strength that single concrete batch can attain under ideal conditions. When every cylinder is made, cured, and tested under the same conditions, then we can truly compare the strength of different batches of concrete.
Frank Kozeliski, a long-time concrete producer from New Mexico, wants to see every concrete cylinder live up to its full potential. At the recent ACI meeting, he proposed a change to ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, that all cylinders be required to be cured immersed in water for the first 24 hours. He presented evidence from the National Ready-Mixed Concrete Association showing that cylinders cured using wet burlap for the first 24 hours will have 28-day strengths as much as 800 psi less than when cured immersed in water.
Currently C 31 allows six different curing methods, including wet burlap, capping a plastic cylinder, and placing the cylinder in a plastic bag. All of these methods result in lower strength cylinders. Frank feels, and I agree, that the producer should get credit for all of the potential strength of the concrete, especially as we begin to see more performance-based specifications. Ideally he would require that all testing labs have a temperature-controlled curing box on the jobsite in which to store cylinders for the first 24 to 48 hours.
Do you agree?