Air-coupled detection of nonlinear Rayleigh surface waves in concrete-Application to microcracking detection

This paper proposes a new technique for the application of the second harmonic generation (SHG) in Rayleigh surface waves to nondestructively quantify microstructural changes and microcracks in heterogeneous cement-based materials (mortar and concrete). The effect of shrinkage reducing admixture (SRA), as it influences microcrack formation in these materials, is considered as an example. A 50 kHz wedge transmitter and a 100 kHz air-coupled receiver are implemented for the generation and detection of nonlinear Rayleigh waves; the non-contact receiver has the advantage of eliminating the inconsistencies associated with coupling and can be used for inspections of in-service concrete components. The experimental results show that the SRA plays a crucial role in reducing shrinkage microcracks and the addition of the SRA to cement-based materials influences the material nonlinearity. These results demonstrate that the proposed SHG technique is capable of evaluating the relative nonlinearity parameter, βre and can be a reliable method to characterize microstructural changes in cement-based materials.