New diagnostic index based on surface waves: Feasibility study on concrete digester tank

In large infrastructures, the nonintrusive evaluation of structural fitness at the local scale can be time-consuming using the conventional P-wave-based ultrasonic tests. Adopting a multisensor approach to examine the surface waves propagating in a structure, on the other hand, can provide a section-by-section assessment while reducing the inspection time. Once such a configuration is established, the phase velocities of surface waves can be measured to extract diagnostic features indicating the deterioration. Hereby, the concrete roof of an anaerobic digester tank is inspected by conducting surface wave tests at 10 different sections. The recorded signals via multiple sensors are processed to extract a new diagnostic feature named the dispersion index (DI), which represents the cumulative dispersion in the phase velocity with respect to the reference dispersion curve obtained from a numerical model. The index DI is then used to categorize the tested regions into three classes of condition named: good, moderate, and deteriorated. It is found that the assessment based on the DI is in good agreement with the visual inspection for 80% of the tested sections. In addition, the crack depth is estimated using the measured cutoff frequency in the dispersion curve; the results confirm the deepest cracks located in those regions characterized as the most deteriorated by the proposed DI index.