Nonlinear ultrasonic test applied to discern factors controlling cracking of cement mortar affected by reinforcement corrosion

Corrosion of reinforcement steel is a major cause of deterioration in concrete structures, leading to expansive oxide formation, internal cracking, and eventual degradation of structural performance. Although the appearance of surface cracking is commonly used as the durability limit state, detecting early detection of micro-cracking remains difficult. Classical electrochemical and linear ultrasonic techniques lack sensitivity to the distributed micro-defects that develop during the initial stages of corrosion. Nonlinear ultrasonic (NLU) techniques have shown potential for detecting subtle material degradation, yet their application to corrosion damage in reinforced cementitious materials is still relatively unexplored. This work examines the link between corrosion-induced crack development and nonlinear ultrasonic responses in reinforced cement mortar specimens subjected to accelerated corrosion. Specimens with different cover depths and corrosion intensities were monitored to correlate crack initiation, crack-width evolution, and nonlinear wave distortion indicators. The Intensity Modulation Ratio (R), derived from amplitude distortion in the received waveforms, was selected as the primary metric for damage characterization. A cumulative form of R was implemented to reduce variability associated with measurement duration and signal fluctuations. Results indicate that nonlinear ultrasonic responses increase significantly before the first detectable surface crack, demonstrating high sensitivity to internal micro-cracking. Specimens showing rapid crack propagation also exhibited corresponding rises in R, confirming a strong relationship between mechanical deterioration and nonlinear signal behavior. The findings confirm that NLU techniques can provide early detection of corrosion-induced damage and offer a promising basis for defining threshold criteria for structural health monitoring of reinforced concrete. • Nonlinear ultrasonic parameter R showed sensitivity to micro-cracking before any visible corrosion damage. • Clear agreement was observed between crack-width evolution and nonlinear signal distortion. • The cumulative R approach improved robustness by reducing measurement variability. • Statistical thresholds reliably differentiate undamaged and damaged states, identifying the onset of corrosion damage. • Results show potential of NLU techniques for early monitoring of corrosion damage in reinforced concrete structures.