Detection and localization of debonding damage in composite masonry strengthening systems with the Acoustic Emission technique
Different types of strengthening systems, based on fiber reinforced materials, are under investigation for external strengthening of historical masonry structures. A full characterization of the bond behavior and of the shortand long term failure mechanisms is crucial to ensure effective design, compatibility and durability of the strengthening solution. In this paper, the effectiveness of the Acoustic Emission (AE) technique for debonding characterization and localization on Fiber Reinforced Polymer (FRP)and Steel Reinforced Grout (SRG) strengthened clay bricks is investigated. The AE technique proofs to be efficient for damage detection during accelerated ageing tests under thermal cycles and during experimental shear bond tests. AE data demonstrated the thermal incompatibility between brick and epoxy bonded FRP composites during the accelerated ageing tests and debonding damage was successfully detected, characterized and located during the shear bond tests. tests are more likely to show adhesive failure. Also a combination of cohesive and adhesive failure surfaces, denoted mixed failure mode, can occur. In case of strengthening with steel reinforced grout (SRG) in addition to masonry cohesive failure and adhesive debonding at the mortar brick interface, debonding at the fiber mortar interface can occur. The latter, being the most observed failure mode in the tests described in this paper, is followed by slipping of the fibers in the matrix. 2.2 NDT for debonding detection Visual inspection and hammer tapping are the most widely used in situ non destructive testing methods for bond monitoring in FRP strengthened elements, while several other methods are being applied such as digital image correlation (DIC) infrared (IR) thermography (Lai et al. 2012) ultrasonic testing (Mahmoud et al. 2010) shearography (Taillade et al. 2011) and acoustic emission (AE) testing. DIC and IR thermography were applied during previous bond tests on similar specimens and setups. DIC has been used during shear bond tests on GFRPand SRG strengthened bricks to obtain the evolution of strains on the FRP surface (Ghiassi, Xavier, et al. 2013) The use of active IR thermography for detection of interfacial flaws and FRP delamination induced by environmental ageing, with specimens similar to the ones used in this study, is reported in (Ghiassi, Silva, et al. 2013) 2.3 FRP debonding detection with AE For an introduction into the principles of AE testing in civil engineering (Grosse Ohtsu, 2008) and research on the application of this technique in masonry (De Santis Tomor, 2013; Verstrynge et al. 2009) the reader is referred to literature. Limited results are reported in the literature regarding the analysis of debonding phenomena in externally strengthened masonry and concrete components by means of the acoustic emission technique. AE monitoring during FRP debonding from concrete beams and slabs was studied by (Carpinteri et al. 2007) who detected the propagation of flexural cracks in an FRP strengthened beam, and by (Degala et al. 2009) who observed the progressive debonding of CFRP strips from concrete slabs and differentiated between CFRP debonding and concrete failure (flexural, compressive or shear failure) by looking at the relative intensity of the AE signals. Shear behavior of strengthened masonry walls was analyzed with the acoustic emission technique by (Masera et al. 2011) who observed decrease of the signal peak frequency upon failure of the masonry specimens. In the presented study, the debonding mechanism itself will be the object of investigation. 3 EXPERIMENTAL PROGRAM The experimental study focuses on the detection of debonding with the AE technique during an accelerated ageing test under thermal fluctuations and during laboratory shear bond tests on two types of strengthening systems. Clay bricks were strengthened with Glass Fiber Reinforced Polymer (GFRP) and with Steel Reinforced Grout (SRG) Twelve single lap shear bond tests were performed with AE detection, being three reference specimens and three aged specimens for each strengthening type. 3.1 Materials and test specimens Test specimens consisted of single bricks strengthened with GFRP and SRG composites. Solid clay bricks with dimensions of 200x100x50 mm were used as substrate. The composite materials were cut in 50 mm width and applied to the bricks' surface along 150 mm length of the brick with a 40 mm unbonded part near the loaded end. GFRP strips were applied to the bricks' surfaces following the wet layup procedure. A two part epoxy primer was applied for preparation of the substrate and a two part epoxy resin was used as matrix for the GFRP. For SRG strengthened brick specimens, a 1directional medium density steel fiber net was used as reinforcement. The steel fibers were placed on a 3 mm thick layer of a lime based mortar that was applied on the sand blasted brick's surface. Then, another 3 mm mortar layer was applied to cover the steel fibers. Mechanical properties of the bricks and strengthening materials are presented in Table 1 as the mean value of five tests and the coefficients of variation (CoV) Table 1. Mechanical properties of strengthening material and bricks. Masonry brick Mean value CoV Compressive strength fcb (MPa) 14.2 15.7 Flex. tensile strength ftb (MPa) 1.6 24.6