Non-Linear Ultrasonic Approach for the Characterization of Mode II Debonding Behavior of FRCM Reinforcements for Masonry Constructions

Fabric Reinforced Cementitious Matrix (FRCM) composites have emerged as reliable strengthening materials especially for historical masonry constructions. Their strengthening effectiveness strongly depends on the bond at the matrix–fibers interface and at the matrix–substrate interface; thus, the analysis of FRCM bond behavior is of crucial relevance. To this aim, we propose an innovative nonlinear ultrasonic approach based on the Side-band Peak Count (SPC) technique. The Side-band Peak Count technique reprocesses the results of the ultrasonic guided wave tests on FRCM reinforced specimens during Double-Lap Shear Tests (DLT), in order to correlate the nonlinear ultrasonic results to the shear stress distribution at reinforcement-masonry interface for long bond length. So, the bond behavior between FRCM composites, made of basalt textile embedded in a lime-based mortar matrix (B-FRCM) and masonry tuff supports was ultrasonically analyzed. Moreover, a new nonlinear ultrasonic parameter, the SPC index (SPC-I) is introduced. By means of the SPC index it was possible to monitor the evolution of the shear stress-slip law at the reinforcement-masonry interface. The non-linear ultrasonic results were compared with the results obtained during the DLT debonding test by means of electrical strain gauges positioned in suitable areas of the reinforced samples. The effectiveness of the proposed approach is investigated and discussed.