Influence of ultrasonic frequency on the evaluation of self-healing and repair in concrete

Abstract

Self-healing and repair of cementitious media entails restoration of mechanical properties. However, testing of the effectiveness of the process is not straightforward. Microscopy, and computed tomography can potentially verify the deposition of healing/repairing material in the crack, while other tests like water permeability evaluate the “sealing” properties. However, they do not supply information on the mechanical performance of the healed or repaired layer. It is only possible to check mechanical properties by re-loading, but this cannot be used in-situ while the monitoring of continuous healing cannot take place on the same specimen even in laboratory since the measurement is destructive. This is the research gap that ultrasound can fill. Since elastic waves physically propagate through the material, they gather information on the elastic properties of the different constituents. Ultrasound has been recently used to monitor the healing and repair effectiveness in cementitious materials and structures. The present paper addresses the importance of the applied frequency in different modalities. In one-sided measurements, the wavelength defines the Rayleigh wave penetration and therefore a modification of the frequency defines the depth of material that can be characterized. On the other hand, in through transmission, the wavelength defines essentially the resolution of the technique. This becomes very important for heterogeneous materials and specifically, for crack interfaces at various conditions, like totally empty -acting as discontinuities-, having bridging points between the sides, and partially or fully healed.