Monitoring of crack healing in geopolymer concrete using a nonlinear ultrasound approach in phase-space domain

Ultrasonic testing is a robust non-destructive evaluation method frequently employed in the health monitoring of concrete structures. Cracking in concrete can be one of the most critical problems, and its healing is important for structural safety. The current study proposes the evaluation of crack healing in geopolymer concrete (GPC) using different linear and nonlinear ultrasonic techniques. In this regard, a notched GPC beam was constructed at the laboratory and geopolymer grout was applied as a repair material. Ultrasonic pulse velocity (UPV) and signal waveform tests were performed at several stages before and after grouting the notch. Nonlinear wave signals were processed in the phase-space domain for qualitative health monitoring of GPC. Furthermore, feature extraction was applied to phase-plane attractors using fractal dimension for quantitative assessment. The sideband peak count-index (SPC-I) method was also utilized to assess the ultrasound waves. The results indicate that the phase-space analysis of ultrasound can successfully represent the healing progress inside the GPC beam. At the same time, the fractal dimension can be used as a healing index. Ultrasound signal attenuation demonstrated high sensitivity to crack healing. The SPC-I technique exhibited an inconsistent trend at the early stage of healing. However, it provided a clear indication of repair at the advanced stage. Although the linear UPV method was found sensitive to the grouting at the initial stages, it demonstrated insufficient capability to monitor the healing process fully. Therefore, the phase-space-based ultrasonic method and the attenuation parameter could be employed as reliable techniques for the robust monitoring of progressive healing in concrete.