Simulation and Experiments of Airborne Zero-Group-Velocity Lamb Waves in Concrete Plate

The impact-echo (IE) test method has been widely used to determine the thickness of a plate and to locate delaminations in reinforced-concrete structures. It was later realized that the IE mode is related to the Zero-Group-Velocity S1 Lamb mode (S₁ZGV). An efficient inspection of a structure using rapid, noncontact, air-coupled sensing technology is of great interest. The objective of this research is to investigate the effectiveness of air-coupled sensing to detect the ZGV Lamb mode signals. Numerical simulation and experimental verification were performed to study the in-air ZGV vibration responses induced by a point impact applied on a concrete plate. Two concrete plates, one without delamination and one with delamination, were tested and simulated. For each plate, the simulation results agreed well with the experimental results, and both methods estimated the plate thickness precisely. The numerical simulations also visualized the pressure field in the air and are helpful for interpretation of the experimental results. The numerical and experimental results suggest that the air-coupled sensing technology can serve as an effective and rapid inspection tool for large concrete structures.