Title: Concrete Q&A: Compressive Strength of Cores and Specification Compliance

Author(s):

Publication: Concrete International

Volume: 42

Issue: 12

Appears on pages(s): 80

Keywords:

DOI:

Date: 12/1/2020

Abstract:

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 f_c^′ 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 f_c^′). 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 f_c^′ = 5100 psi, so isn’t the average core strength of 5090 psi close enough?