Technical Questions

Standards for 7-day and 28-day strength test results

Q. What ACI standard stipulates the percent of the specified compressive strength that the cylinder must meet in order to pass the compressive strength test at both the 7- and 28-day ages?

A. According to section 19.2.1.3 of ACI 318-19 the specified compressive strength shall be based on the 28-day test results unless otherwise specified in the construction documents. 3- or 7-day test results are used to monitor early strength gain, especially when high early-strength concrete is used. However, 3- and 7-day test results are not commonly used for acceptance purposes.

The acceptance criteria for the specified compressive strength according to section 26.12.3.1 of ACI 318-19 are as follows:

1. Every arithmetic average of any three consecutive strength tests equals or exceeds the specified compressive strength.
2. No strength test falls below the specified compressive strength by more than 500 psi (3.5 MPa) if the specified compressive strength is 5000 psi (35 MPa) or less; or by more than 10 percent of the specified compressive strength if the specified compressive strength exceeds 5000 psi (35 MPa).

Sections 26.5.3.2 and 26.12.1.1 of ACI 318-19, ACI 301-20 “Specifications for Concrete Construction”, and ACI 311.6-18 “Specification for Testing Ready Mixed Concrete” require concrete strength tests for acceptance to be the average of at least two 6 x 12 in. (150 x 300 mm) cylinders or at least three 4 x 8 in. (100 x 200 mm) cylinders.

Additional information on this topic can be found in on-demand courses Low Compressive Strength Test Results? What They Mean and Next Steps and Evaluating Test Results and Troubleshooting Low Compressive Strength.

References: ACI 301-20; ACI 311.6-18; ACI 318-19; ACI 214R-11; ASTM C31; ASTM C39

Topics in Concrete: 318 Building Code; Quality Assurance; Testing of Concrete