Title: Concrete Q&A: Compressive Strength of Cores and Specification Compliance
Publication: Concrete International
Appears on pages(s): 80
Q: Last winter, we placed concrete grade beams for an industrial building. A technician from the owner’s testing agency took test cylinders but left them unprotected in the cold weather for a week. At 28 days, the average cylinder compressive strength was 2750 psi. Because this was far below the specified fc′ of 6000 psi, the engineer required three cores to be taken to evaluate the in-place concrete strength. The individual core compressive strengths were 4940, 4970, and 5370 psi, resulting in an average strength of 5090 psi (84.8% of fc′). The engineer rejected the concrete on the basis that the ACI 318-19 Code1 requires the average core strength to equal 85% of the specified strength. In this case, 0.85 fc′ = 5100 psi, so isn’t the average core strength of 5090 psi close enough?