RMC Foundation Releases Report on Impact of Extended Time to Discharge on Concrete Durability, Performance
The Ready Mixed Concrete (RMC) Research & Education Foundation recently released a new research report, titled Impact of Extended Time to Discharge on Concrete Durability and Performance. The research was performed by the New Jersey Institute of Technology (NJIT) and was co-funded by the Portland Cement Association (PCA). It is estimated that on an annual basis, approximately 3% of ready mixed concrete produced is returned to the concrete plant for disposal. One of the reasons that concrete is rejected at a project site is the 90-minute time limit to discharge a load in accordance with ASTM C94, Specification for Ready Mixed Concrete, that is enforced by specifying agencies. Although the standard provides conditions where this time limit can be waived, loads of acceptable quality are rejected, resulting in lost revenue and cost to the producer to manage returned loads.
NJIT researchers evaluated properties of concrete mixtures designed for a longer discharge time. Acceptable results were observed for all fresh concrete and durability indicator test results, such as resistivity and freeze-thaw durability, for concrete specimens prepared from samples obtained at different times up to 150 minutes. In the report, NJIT researchers conclude that the current discharge time limits and specifications are conservative and should be reexamined.