The best research starts with a clear hypothesis—a straightforward question. The Ready Mixed Concrete (RMC) Research & Education Foundation had released two new reports from research it funded that provide important answers. A question that has long been a problem for the ready-mixed concrete industry is the validity of the 90-minute limit on concrete delivery. Does concrete really go bad after that? Should it be rejected at the building site?
A second question involves the use of water-cementitious materials ratio or the rapid chloride permeability test (RCPT) to define the durability of concrete. Is there a simpler and more reliable measuring technique?
Question one was taken on by a team from the New Jersey Institute of Technology (NJIT), home to one of the four Concrete Industry Management (CIM) programs. Using locally produced ready-mixed concrete, this research examined the effect of discharge time after the introduction of water to a mix (ranging from 60 to 150 minutes) on both fresh and hardened concrete. Concrete workability was maintained with an extended-set-retarding admixture added to the truck drum as needed. The results indicate that the time to discharge had no significant impact on the fresh properties, surface resistivity, and freeze-thaw durability up to 150 minutes. The conclusion is that the current discharge time limits and specifications are overly conservative and should be reexamined. The full report can be read by clicking here.
Question two was investigated by a team at the National Ready Mixed Concrete Association (NRMCA). Current industry standards (ACI 318-19, ACI 301-16) specify a maximum water-to-cementitious materials ratio (w/cm) for mixtures used in concrete members that require low permeability—which is used as an indication of durability for members subjected to harsh exposure conditions. But w/cm can’t be reliably measured in the field and is not always a perfect stand-in for durability. That led to the use of the RCPT to measure permeability, but with some mixes (especially those with higher percentages of supplementary cementitious materials), this test is not always a good indicator of durability. A third method, that measures the electrical resistivity of concrete was examined, revealing it to be is a reliable test method to predict permeability and potential durability. The variability of the test is considerably lower than that seen for the RCPT. The conclusion is that a this is a valid test but a single curing and conditioning method and test age needs to be established for using this test to determine the acceptance of concrete. The full report can be read by clicking here.