Flexural behaviour of RC beams strengthened with multi-layer steel reinforced grout (SRG) composites

Several events can cause damage to existing structures including natural and man-made disasters. Even if some structures are less vulnerable to such extreme events, damage can be still introduced by several other events including change of codes or use, increase of demands on structural performance, deterioration with age, deficient design or construction. Different systems have been used in the retrofit industry including section enlargement, steel plate bonding, welded steel meshes, external post-tensioning, and the use of composites including Fibre Reinforced Polymers (FRP), Steel Reinforced Polymers (SRP), Fabric reinforced cementitious matrices (FRCM), and Steel Reinforced Grout (SRG). This latter system received much attention recently as the use of the grout has addressed many drawbacks associated with the use of the epoxy matrix in FRP and SRP systems. Furthermore, the use of the steel textiles as an alternative to other synthetic textiles used in FRCM systems was advantageous from economic and design perspectives. SRG systems were investigated in several applications including flexural and shear strengthening, confinement of columns and joints, out-of-plane and in-plane strengthening of walls. However, the knowledge on the flexural behaviour of RC beams strengthened with SRG systems is very limited, especially the use of multiple layers of these systems which is often considered for strengthening large structural members.
The aim of this research was to investigate the flexural behaviour of RC beams strengthened with SRG composites comprising different number and densities of the steel textiles. However, the flexural behaviour is largely influenced by the tensile behaviour of the composite and the bond between the composite and the substrate. To achieve the aim of this work, three experimental programmes were conducted on a total of 200 specimens. The first programme was devoted to investigating the tensile behaviour of the SRG composites. Mechanical characterisation was conducted by performing a total of 104 direct tensile tests on bare single cords, bare textiles, and SRG coupons. The bond behaviour between the SRG composites and the concrete substrate was investigated in the second experimental programme. This was achieved by conducting a total of 90 direct shear bond tests on different SRG systems comprising multiple layers of steel textiles and different densities applied to concrete substrates with different compressive strengths. Finally, the last experimental programme was conducted on 6 full-scale RC beams strengthened with different SRG systems to study their flexural behaviour.
The results of the tensile tests showed that the SRG coupons comprising dense steel textiles exhibited a tensile stress-strain behaviour similar to that developed by those comprising low density textiles. Increasing the number of layers resulted in an increase in the axial stress and the corresponding strain. This increase was sound for the transition from one to two layers. However, increasing the density of the textiles led to a decrease in the axial stress and the corresponding strain. All tested coupons developed a comparable crack pattern and eventually failed by textile rupture. However, the coupons strengthened with multiple layers of textiles (i.e., high reinforcement ratio) experienced a greater number of cracks at failure with relatively less crack width and spacing. Shear bond tests indicated that the effective bond length of the SRG system is insignificantly influenced by the reinforcement ratio (function in the density of textile and the number of layer). In general, the effective bond length for the SRG system was found to lie between 200 mm and 300 mm. The SRG systems comprising three layers of textiles always failed by debonding at the substrate interface, while those comprising dense steel structure in most cases experienced the failure at the textile interface. Increasing the density of the textiles and the number of layers was found to decrease the axial stress and the corresponding slip. The bending tests conducted on the RC beams showed that beams strengthened with low density textiles exhibited a less stiff behaviour in elastic and post-cracking stages. It was also found that increasing the density of the textiles insignificantly increased the load at yield and at the failure of the SRG system. All the beams strengthened with low-density textiles mobilised a full utilisation of the textiles as they eventually failed by rupture, while the beam strengthened with only one layer of the dense textiles failed by interlaminar shearing at textiles interface. The use of two layers of the dense textiles resulted in the debonding of the SRG system at the substrate interface. Analytical modelling was also carried out on some existing bond models originally proposed for the FRP systems to evaluate their adoptability for the SRG system. It was found that some of these models were accurate in estimating the debonding load for the SRG systems. However, they could not capture all the parameters of the experimental programme. A new model was suggested for shear bond tests based on the reinforcement ratio and the bond length of the SRG composite. The proposed model was able to capture the overall behaviour of the SRG system. The debonding load was predicted with a coefficient of variation of only 9%, while the accuracy of predicting the mode of failure was 97%. The proposed mode for the shear bond tests was then utilised to predict the failure load and mode in RC beams strengthened with SRG systems. A design recommendations and guidelines were given to help engineers when using SRG system in the strengthening industry.