Sideband Peak Count Technique for Monitoring Bond–Slip Behavior Between Reinforcement Systems and Masonry

The use of Fabric-Reinforced Cementitious Matrix (FRCM) composites has become a cornerstone in strengthening several types of structures, ranging from reinforced concrete structures to historical masonry constructions. It is crucial to experimentally assess the effectiveness of the application of the reinforcing layer. Indeed, the performance of FRCM reinforcing interventions highly depends on the bonding at the matrix–fiber and matrix–substrate interfaces. Therefore, the experimental characterization of FRCM bond behavior is essential for designing and ensuring the durability of reinforcement systems. To address this, we have proposed using an innovative nonlinear ultrasonic method: the Side-band Peak Count (SPC) technique. Specifically, the SPC approach has been applied to analyze ultrasonic test results obtained during Double-Lap Shear Tests (DLT) conducted on masonry specimens reinforced with B-FRCM (basalt fiber textile FRCM). We show that it is possible to correlate nonlinear ultrasonic parameters with the shear stress distribution at the reinforcement–masonry interface. Additionally, a relatively new nonlinear ultrasonic parameter, the SPC index, is shown to be effective in monitoring the evolution of the shear stress–slip relationship at this interface, a crucial aspect for understanding the mechanics of the reinforcement–substrate interaction. The nonlinear ultrasonic results have been compared with the results of DLT debonding tests to validate the proposed methodology. The effectiveness of the SPC technique is investigated and discussed. Finally, we have developed a robust numerical model to analyze the bond behavior between the reinforcement and masonry. The numerical model is valuable for both the design of experimental tests and the validation of the experimental results.