A published report of research by the National Ready Mixed Concrete Association (NRMCA) is titled "The Effect of Temperature and Delivery Time on Concrete Proportions." It contains information with immediate practical application for any contractor, ready mix producer, or specifier who may ever face problems of low cylinder strength tests during hot weather construction.
CONCRETE TEMPERATURE AND DELIVERY TIME
Two of three primary variables investigated in the NRMCA research were the effect of concrete temperature and the effect of delivery time (from batching to time of delivery) on 28-day cylinder strengths.
EXPOSURE OF TEST CYLINDERS TO NONSTANDARD JOBSITE CURING
The third variable, the temperature at which cylinders were cured on the job, had by far the greatest effect on cylinder strengths. We have always known and long accepted the generalization that hot weather reduces 28-day concrete strengths. This report provides convincing evidence that the major part of that loss is very often not in the potential strength of the concrete mix as delivered to the job, which the cylinder test is supposed to be measuring. It is only a loss in cylinder strengths caused by nonstandard curing of cylinders while stored on the job.
CONSIDERATIONS OTHER THAN STRENGTH
In the NRMCA report it is carefully pointed out that strength is not the only reason to control concrete temperatures. Another purpose is found in mass concrete where expansion from heat of hydration may lead to damaging internal stresses.
SING THE TEST RESULTS
The test data and the conclusions that are a part of the report give the contractor and ready mix producer an opportunity to protect themselves against the misuse of low test results caused by nonstandard test procedures.