NONDESTRUCTIVE EVALUATION OF EARLY-AGE CONCRETE STRENGTH IN PLATE STRUCTURES BY THE IMPACT-ECHO METHOD

This paper describes tests that were performed to evaluate the feasibi lity of using the impact-echo method to evaluate the in-place strength of concrete in plate-like elements such as slabs and walls. In the im pact-echo method, a stress pulse is introduced into an object by mecha nical impact on its surface, and this pulse undergoes multiple reflect ions (echoes) between opposite faces of the object. The surface displa cement of the object, caused by the reflected pulse, is monitored at a location adjacent to the point of impact, and the frequency of succes sive arrivals is determined. With the thickness of the test object kno wn, the compression wave (P-wave) velocity is determined. A previously established concrete strength/P-wave velocity relationship is used to estimate in-place strength. Results indicate that the impact-echo met hod can be use to determine P-wave velocity and to estimate concrete s trength in a large volume of early-age concrete such as the slab speci mens tested in this study. Successful impact-echo tests can be made at very early ages when the concrete has low compressive strength, less than about 4.5 MPa (650 psi). Use of the impact-echo method to non-des tructively estimate the in-place strength of concrete should be limite d to the estimation of early-age strength.