Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Our 28-day compressive strength test result was lower than our 7-day compressive strength test result. What happened? Is the concrete acceptable?
NPCA Technical Services engineers answered:
If your 28-day compressive strength test results are lower than your 7-day test results, it is likely that the test procedures for the 7-day cylinders and/or the 28-day cylinders were not conducted in accordance with the test method.
A lot of factors could be the cause: the rate at which the load was applied to the cylinder was incorrect; the neoprene pads placed on either end of the cylinder may be worn out and need to be replaced; the cylinder may not be placed in the middle of the machine/may be off-center; the load may have been applied at a very slight angle instead of perpendicular to the top of the cylinder; the cylinder may not have been fabricated in accordance with the standard (could have an uneven surface, may not have been consolidated properly, etc.)
Check to make sure each cylinder is manufactured exactly in accordance with the applicable standard, and ensure each compressive strength test is done exactly in accordance with the proper test method. Also be sure the compressive strength testing machine has been calibrated at least within the last year. Also check to make sure the cylinders did in fact come from the exact same batch of concrete, they were cured in the same manner, and the dates at which they were tested are accurate.
If you have spare cylinders from this batch of concrete, conduct a compressive strength test now to see what the results are. It is a best practice to test at least two cylinders from the same batch, cured in the same manner, and tested at the same age, and calculate the average of those two results and report that average as the test result.
Check the project specifications and requirements for information on what is acceptable in this case. PI