Technical Questions

Estimating in-place concrete strength

Q. Are there ways to estimate the compressive strength of in-place concrete without drilling cores from the structure?

A. ACI 228.1R-19, “Report on Methods for Estimating In-Place Concrete Strength,” reviews several methods that are used for this purpose. They include:

- Rebound number (ASTM C805/C805M)

- Penetration resistance (ASTM C803/C803M)

- Pullout test (ASTM C900)

- Pull-off test (ASTM C1583/C1583M)

- Ultrasonic pulse velocity (ASTM C597)

- Maturity method (ASTM C1074)

- Cast-in-place cylinders (ASTM C873/C873M)

Most of these tests don’t directly measure the compressive strength of the concrete in the structure. They measure another property that can be correlated to compressive strength. The strength is then estimated from a previously established relationship between the measured property and concrete strength. The uncertainty of the estimated compressive strength depends on the variability of the in-pace test results and the uncertainty of the relationship between the property being measure and the compressive strength.

In-place strength tests generally have two applications—estimating concrete strength during construction or estimating concrete strength during the evaluation of existing structures. Used during construction, the test results aid in determining when forms can be stripped or shoring removed, when a structure can be safely exposed to freezing weather, when post-tensioning forces can be applied, or when a structure can be put into service. During construction they can also be used to test the adequacy of curing or concrete protection during cold weather.

During construction, when standard cylinder strength test results have failed to meet the acceptance criteria in specifications or code requirements, nondestructive tests such as the ones listed above may be useful in determining whether or not a portion of the structure actually contains low-strength concrete. When used for this purpose, the tests are of value primarily for comparisons within the same job rather than as quantitative measures. In these cases, such in-place testing doesn’t eliminate the need for coring, but it can reduce the number of cores needed to evaluate a large volume of concrete.

Concrete strength may also be estimated during the evaluation of existing structures when renovations are planned or when a change in use of the structure is anticipated. Engineers investigating structures exposed to fire, or degraded by a severe environment, may also require strength estimates.

References: ACI 228.1R-19; ASTM C805/C805M; ASTM C803/C803M; ASTM C900; ASTM C1583/C1583M; ASTM C597; ASTM C1074; ASTM C873/C873M

Topics in Concrete: Nondestructive Testing; Quality Assurance; Testing of Concrete