Ultrasonic inspection for advanced structural applications

Ultrasound has been extensively used to monitor the structural condition of cementitious materials and structures. Wave velocity, attenuation and other parameters carry information, as they depend on the material stiffness and heterogeneity. Although, they are used for a first estimate of the performance, still, the full inspection capacity, especially in-situ has not been reached. Reasons include the limited sensitivity of pulse velocity to small-scale damage, limited capacity to interpret the waveform in terms of the structural condition, as well as technical issues including coupling. The present paper addresses certain ultrasonic topics in advanced applications, like monitoring of durability loss in concrete, assessment of self-healing, repair, as well as air-coupled dispersion measurements supported by theoretical and numerical investigations. The contribution of the applied frequency is always highlighted, as in surface wave applications, it defines the penetration depth and thus the material layer that can be characterized, while in through transmission, the wavelength defines essentially the resolution of the technique. Results show that proper application and interpretation can pave the way to reliable air-coupled assessment of specific processes allowing maintenance decisions based on engineering criteria in addition to established but rough empirical correlations.