Non Destructive Structure Evaluation: an Application of Seismic Signal Analysis

Techniques of wave propagation in layered plates are used to evaluate the integrity and the material properties of the asphalt covered concrete deck of a bridge. Seismic waves are generated on the field by a point impact load, which is studied in detail, and surface motions are recorded at four stations forming a single line with the impact location. In order to simplify the problem and to lay the ground for a field analysis, numerical simulations are carried out for an impact problem on an infinite layered plate and the surface displacements at the location of the four stations are processed in the time-frequency domain. Two major types of waves are isolated: waves that are multiply reflected in the thickness dimensions of the plate, primarily observable in the vicinity of the impact point (i.e. at the first two stations), and surface waves that are dominant at the two most remote stations. The multiply reflected waves are used in an impact-echo method to measure the deck thickness and to detect possible delaminations. The numerical simulations yield very good results and, although the length of field recordings needs be extended, the field results are also promising. Surface displacements at the two most remote stations, obtained by numerical simulations, are processed in the frequency domain to compute the dispersion curves of surface waves. Such dispersion curves are used to evaluate the elastic properties of the materials, in particular the shear wave velocities in the asphalt and in the concrete. This technique can be used to point out concrete in poor condition, however, the analysis shows stability problems. Thesis Supervisor: Dr. Eduardo A.M Kausel Title: Professor of Civil Engineering To my family, To my nephew and nieces, and to all those who love them