Nondestructive evaluation of in-place concrete strength in plate structures by the impact-echo method
For the first time, tests were performed to evaluate the feasibility of using the impactecho method to estimate the in-place strength of concrete in plate structures. In the impact-echo method, a stress pulse is introduced into an object by mechanical impact on its surface, and this pulse undergoes mUltiple reflections (echoes) between opposite faces of the object. The surface displacement of the object, caused by the reflected pulse, is monitored and the frequency of successive arrivals is determined. Knowing the thickness of the test object, the compression wave (P-wave) velocity is determined. This velocity, along with a correlation relationship between P-wave velocity and concrete strength, is used to estimate concrete strength. The research involved tests of plate-like elements (slabs and walls). In each experiment a slab and several prepared cylinders were cast from a batch of concrete. Impact-echo testing was performed on the slab, on cores taken from the slab and on the prepared cylinders. As expected, the slab and core velocity measurements differed because of the difference between P-wave velocity in plateand rod-like structures. After P-wave velocity was determined the prepared cylinder or core was tested in compression to obtain concrete strength. This lead to three relationships: 1) the prepared cylinder strength-velocity relationship; 2) the core strengthslab velocity relationship; and, 3) the core strength-core velocity relationship. The earliest measurements were made about 10 hours after casting, and each experiment continued for a period of 28 days. The experiment described above was performed to explore tRe variables of type of formwork material, steel reinforcement, and concrete cast against previously hardened concrete. It was found that the impact-echo method can be used as a means to nondestructively determine the in-place strength of concrete in plate-like structures. The method can be used to measure concrete strength in both plain and steel-reinforced elements, and in concrete cast against a variety of formwork materials.