Fabrication of a Piezoelectric Impact Hammer and Its Application to theIn-situNondestructive Evaluation of Concrete

In this paper, a piezoelectric impact hammer is designed and fabricated for generating stable repeated impact elastic waves and further for in-situ wave velocity measurements of concrete specimens. The piezoelectric impact hammer consists of a stacked multilayer PZT, a flying head system with an on-line load cell embedded and a holding fixture. To measure the longitudinal or Rayleigh waves of a concrete specimen, two horizontally polarized conical transducers are utilized to receive the elastic waves generated by the piezoelectric impact hammer. Due to the stable impact generation of the piezoelectric impact hammer, the experimental results showed that the accuracy of the wave velocity measurement was enhanced significantly through the signal averaging. Since the impact time origin of the newly designed impact hammer can be determined accurately, it was utilized to measure the depth of a normal surface-breaking crack in a concrete specimen, and the result is very accurate. We note that the results of this study offer a precise and convenient way of measuring in-situ elastic properties and/or crack depth of concrete specimens.