Seismic Retrofitting Clay Brick Masonry with Visco-Elasto-Plastic Adhesive Bonded CFRP Strips: Efficient Utilization of CFRP Laminates Without Significantly Damaging the Substrate

In the past decades, a range of strengthening techniques have been developed for enhancing the out-of-plane performance of unreinforced masonry walls. One technique involves the use of fibre-reinforced polymer (FRP) composites. Traditionally, stiff adhesives are used to bond the CFRP strips to the masonry. This can lead to an underutilization of the added CFRP strips due to intermediate cracking of the masonry. Five years ago an idea was developed to replace the conventional stiff adhesive, with a novel viscoelastic adhesive. This paper provides an overview of the research, from idea to practical application. Through a series of bending test on reinforced clay brick wallettes, it was shown that by using a visco-elasto-plastic epoxy instead a conventional stiff adhesive, a significant increase in terms of ductility and maximum with standable load is reached and critical crack development is prevented. The deep mounted technique was accordingly developed where deeper grooves are cut in the masonry, after which FRP strips are installed in the center of the wall. The FRP strips therefore offer additional out-of-plane flexural strength to the wall for both out-of-plane loading directions whilst only installing the reinforcement from one side of the wall, leading to cost-effective retrofitting. The following step in the research was assessing the high-speed pullout behavior of deep-mounted CFRP strips bonded with a viscoelastic adhesive to clay brick masonry by means of direct pull-out experiments. Using the developed global bond-slip law as part of a partial-interaction analysis resulted in a good fit with the experimental results. The results were compared to previous direct pull-tests found in literature, showing that the application of a viscoelastic adhesive results in higher interfacial fracture energy and higher debonding slip. The next step in the experimental campaign showed that the deep mounting using a flexible adhesive did not lead to a degradation of the in-plane strength. Additionally, combing the proposed retrofit system with a single sided FRCM overlay increased the shear capacity with 80%, compared to the unstrengthened control specimens. The last step in the experimental campaign was calibrating the developed out-of-plane mechanical model through a series of full-scale high-speed cyclic out-of-plane experiments on retrofitted clay brick walls.