Effect of beam depth on shear behavior of FRP RC beams

The behavior of shear critical fiber-reinforced-polymer (FRP) RC elements is characterized by the development of comparatively large strains and crack widths, which can be strongly influenced by their relative geometrical size. This paper investigates experimentally the size effect on the shear behavior of FRP RC beams with and without shear reinforcement and with overall depth varying from 260 to 460 mm. The results confirm a considerable size effect for members without shear reinforcement, with an average reduction in normalized shear strength of about 19% and a maximum value up to 40%. It is also shown that current design provisions are overall conservative, but with nonuniform margins of safety that decrease with increasing member depth. It is anticipated that the results of this study will help improve the efficiency of future design equations for the shear strength of FRP RC

Bond of textile-reinforced belite calcium sulfoaluminate cement mortar to concrete substrate

The fast aging of existing building stock requires effective and sustainable strengthening solutions. Textile-reinforced mortars (TRM) have already proved to be very effective as well as versatile retrofitting solutions for reinforced concrete and masonry structures. TRMs can enhance the load bearing capacity of reinforced concrete structures; however, current TRM systems are based on standard Portland cement-based binders, which largely contribute to global human-induced CO2 emissions. This work, for the first time, explores the use of belite calcium sulfoaluminate (BCSA) binder for carbon textile reinforcement through a cross-disciplinary study combining structural engineering and materials science. An experimental study was carried out on concrete block members with externally bonded strips of carbon textile-reinforced mortars, similar to a typical TRM retrofitting system for concrete beams. The textiles were embedded in an ordinary Portland cement-based (OPC) binder or in a BCSA-based binder to compare the bond behaviour to the concrete substrate. The tests revealed a superior bond between the BCSA mortar and the concrete, as well as outstanding adhesion to the textiles achieved using the BCSA binder, with performance levels largely surpassing those measured in their counterparts that used the OPC-based binder. Scanning Electron Microscopy, X-ray diffraction, and thermogravimetric analyses were used to understand this behaviour difference and it was concluded that the ettringite phase is responsible for the enhanced performance in the studied system. The results of this study suggest that BCSA binders have the potential to be a more effective and “greener” alternative to the standard binders based on Portland cement in TRM strengthening applications

Experimental Analysis of Shear Resisting Mechanisms in FRP RC Beams with Shear Reinforcement

Owing to the unique mechanical characteristics and lack of plasticity of fiber-reinforced polymers (FRPs), relatively large strains can develop in FRP reinforced concrete (RC) elements at ultimate limit states and this can lead to different relative contributions of concrete and shear reinforcement to the total element's shear capacity. This paper examines the development and relative contribution of the main shear resisting mechanisms in concrete beams with different overall depths and reinforced with longitudinal and transversal FRP reinforcement. Complementary strain measurements obtained from digital image correlation (DIC) and strain gauges are presented and discussed thoroughly. Although current FRP shear design approaches rely on the assumption that the contributions of concrete and shear reinforcement are constant up to failure, their relative magnitude is found to vary with increasing crack width. The experimental results indicate that, when minimum shear reinforcement is provided, current shear models based on a fixed truss angle approach tend to overestimate the contribution of concrete and underestimate the contribution of shear reinforcement. The use of a variable angle truss model, along with an appropriate reduction in the contribution of concrete, would lead to a more reliable estimate of the main shear resisting mechanisms and optimal design of the required amount of shear reinforcement

Seismic retrofitting of URM masonry piers with helical steel reinforcement

Past earthquakes revealed that the brittle nature of unreinforced masonry (URM) structural walls often leads to extensive cacking and shear damage, which can seriously affect the structural integrity and thus compromise the safety of the entire building. Hence, finding an effective seismic retrofitting solution that can increase the safety of existing masonry building stock is of great importance. This paper explores the potential of alternative seismic retrofitting solutions for URM masonry walls - near-surface mounted austenitic stainless-steel helical bars. Being cold rolled from a plain round wire and subsequently tensioned through a free-twisting process, such a reinforcement can not only offer high durability, but also superior mechanical and bond properties, as well as effective redistribution of loads through the retrofitted masonry. In addition, the relatively high flexibility of the bars allows them to be mounted continuously along the joints of the wall, leaving the aesthetic of the retrofitted masonry intact. A total of nine single-leaf clay brick walls were tested under cyclic displacement reversals to examine the seismic performance of the reinforcement in terms of increasing in-plane shear capacity and ductility. Test specimens comprised cantilever walls with various retrofitting patterns, including flexural and shear helical reinforcements installed in the mortar joints or into the vertical slots cut into the masonry. The results showed considerable improvements in the ductility and energy dissipation of the walls after the retrofitting. For most of the retrofitted walls the value of q factors exceeded 4.0, which is greater than the typical q factors for reinforced masonry, thus indicating that large increases in ductility were achieved. The paper highlights the potential of helical stainless-steel bars as a seismic retrofitting reinforcement capable of preserving the structural integrity of masonry structures at increasing displacement demands without affecting the aesthetic of the surface of the walls