Developing a New Understanding of the Impulse Response Test for Defect Detection in Concrete Plates

The objective of this research is to develop a technical basis of the impulse-response test for condition assessment and to improve its diagnostics for concrete plates. Impulse-response tests were performed on a fully supported concrete plate with artificial delaminations of various planar sizes at different depths. Defect detection was first performed using the empirical damage indices defined in ASTM C1740, which are based on the shape characteristics of the frequency response function (FRF) between 10 and 800 Hz. Next, the test procedure was modified for extending the frequency range of FRF, revealing that delaminations introduce vibration modes with distinctive high frequencies not present in intact portions of the plate and not detectable in the currently used frequency range of the test due to the hammer tip typically used. A validated 3D finite element model of the experimental plate is used to correlate the dynamic response of test points with experimental FRFs, indicating that the distinctive high frequencies in the extended FRF measured on delaminations correspond to the first bending mode characterized by a local reduction in flexural rigidity and in the damping of the plate. The effect of experimental setups, such as the relative distance between the sensor and the hammer, is shown to have a significant effect on the accuracy of defect delineation for large and shallow delaminations. A new damage index based on the resonant frequency from the extended FRF with a modified experimental setup is proposed for estimating the planar size and depth of delaminations. The performance of the proposed index is demonstrated through experimental data as well as parametric numerical simulations. Multiple regression is used to estimate the detectability, depth, and extent of delaminations as a function of the proposed and currently used damage indices. The resonant frequency is found to be more informative for the planar size of the delamination compared to the depth, while the average accelerance is more informative for the depth of delamination and less for its planar size. Furthermore, the results indicate that the test may have limited detectability and application for delaminations deeper than 300 mm. Finally, the prediction accuracy for the depth and size of delamination is demonstrated based on Gaussian processes and is presented in the form of confidence ellipses. The prediction model is shown to be reasonably accurate for shallow defects ($<300\mathrm{mm}$), which are the most critical for durability and structural performance. While the impulse-response test has been used since the early 1990s for condition assessment of concrete elements other than drilled shaft piles, this study is among the most comprehensive on its physical basis, the influence of experimental setups, the extension of the frequency range, the characterization of delaminations, and detection limitations of the test.