Low-strength cylinder test results

Q. If a structure is being built in accordance with the requirements of ACI 318-19, what must be done when a low-strength test occurs?

A. Section 26.12.6.1 of ACI 318-19 states that if any strength test of standard-cured cylinders falls below the specified compressive strength by more than the limit allowed for acceptance, or if tests of field-cured cylinders indicate deficiencies in protection and curing, steps shall be taken to ensure that structural adequacy of the structure is not jeopardized.

Section 26.12.6.1 states that if the likelihood of low-strength concrete is confirmed and calculations indicate that structural adequacy is significantly reduced, tests of cores drilled from the area in question in accordance with ASTM C42 “ Standard Test Method of Obtaining and Testing Drilled Cores and Sawed Beams in Concrete” shall be permitted. In such cases, three cores must be taken for each strength test that falls below the specified compressive strength by more than the limit allowed for acceptance . Core preparation and testing requirements are outlined in Section 26.12.6.1 of ACI 318-19.

Section 26.12.6.1 states that concrete in an area represented by core tests shall be considered structurally adequate if the average of three cores is equal to at least 85 percent of the specified compressive strength and if no single core is less than 75 percent of the specified compressive strength. Additional testing of cores extracted from locations represented by erratic core strength results is permitted. Thus, if the specified compressive strength was 4000 psi, and the average of three cores was 3400 psi or higher—and no core strength was lower than 3000 psi—the concrete in the area represented by the core tests would be considered structurally adequate.

The Commentary for Section 26.12.6.1 of ACI 318-19 states that requirements are provided if strength tests have failed to meet the acceptance criterion of 26.12.3.1(a) (2) or if the average strengths of field-cured cylinders do not comply with 26.5.3.2(e). These requirements are applicable only for evaluation of in-place strength at the time of construction. Strength evaluation of existing structures is covered by Chapter 27 in ACI 318-19. The building official should apply judgment as to the significance of low test results and whether they indicate need for concern.

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 318-19; ASTM C42

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