Combined use of thermography and ultrasound for the characterization of subsurface cracks in concrete

Corrosion of metal reinforcement in concrete structures leads to cracks extending towards the surface. These cracks do not show any visual sign until they break the surface, exposing the structure to more accelerated deterioration. In order to develop a methodology for subsurface damage characterization, a combination of nondestructive testing (NDT) techniques was applied. Thermography is specialized in subsurface damage identification due to anomalies that inhomogeneities impose on the temperature field. Additionally, ultrasonic Rayleigh waves are constrained near the surface and therefore, are ideal for characterization of near-surface damage. In this study, an infrared camera scans the specimen in order to indicate the position of the crack. Consequently, ultrasonic sensors are placed on the specified part of the surface in order to make a more detailed assessment for the depth of the crack. Although there is no visual sign of damage, Rayleigh waves are influenced in terms of velocity and attenuation. Numerical simulations are also conducted, to propose suitable parameters like frequency for more accurate testing. The combination of the NDT techniques seems promising for real structures assessment.