Single-side access tomography for evaluating interior defect of concrete

This study investigates the suitability of surface waves in developing an innovative tomography technique for non-destructive evaluation of concrete structures. Surface waves, or specifically Rayleigh waves (R-waves), are known to exhibit strong dispersion characteristic when propagating through inhomogeneous media. Experimental program was set up to examine phase velocity change of R-waves with regard to artificial defect embedded in concrete, which could be treated as anomaly in the homogeneous medium (concrete). By adopting a multipoint source-receiver measurement only on one surface of the specimen, different sets of waveforms were excited/collected and processed to compute phase velocities of R-waves with regard to each ray path. Based on the reconstructed phase velocity tomograms, the locations of defect within the measured area could be identified or “visualized”, given by regions with lower phase velocity than their neighbouring ones. It was also confirmed that the visualization was dependent on the dominant wavelengths of R-waves such that longer dominant wavelengths were able to indicate the existence of defects embedded deeper in concrete.