How Is Concrete Strength Determined on Site?

In existing reinforced concrete buildings, the primary method for determining concrete strength is to take core samples from vertical structural elements such as columns and shear walls and subject them to compression testing in a laboratory. The elements to be sampled, the number of samples and their distribution across storeys are determined according to the relevant code and testing principles. When the test results are assessed, the dimensions and condition of the sample and the test conditions are taken into account. Because concrete can vary between storeys or between elements, a value obtained from a single sample is not accepted as the concrete strength of the whole building. The aim is to obtain strength information that represents the existing structure.

Devices that measure surface hardness, such as the rebound hammer, and systems that measure using ultrasonic waves are supporting methods for investigating the variability of concrete within a structure. The properties these devices measure can be correlated with concrete strength; the readings themselves, however, are not compressive strength. Using appropriate correlations supported by core results taken from the structure, a specialist engineer can obtain information about more areas and reduce the number of cores to the extent the regulations permit. The mandatory minimum sampling conditions of the code must be maintained. Non-destructive measurements therefore give more useful and reliable results when they are planned together with core testing.