Influence of short glass fibres and spatial features on the mechanical behaviour of weft-knitted textile reinforced concrete elements in bending

Abstract

Weft-knitted textiles made from high-strength fibrous materials offer great potential for use as a flexible stay-in-place formwork and reinforcement system since they allow creating complex geometries (i.e. doubly curved or folded) and introducing spatial features such as ribs within the fabric. However, the closed surface of the textile and its placement at the bottom edge of the concrete element present major challenges regarding bond, which may lead to premature failure due to delamination of the reinforcement initiated by a substantial opening of a governing crack. This study investigates the influence of short integral glass fibres and spatial bond ribs on the mechanical behaviour of weft-knitted textile reinforced concrete elements subjected to bending and their potential to increase the shear resistance. To this end, an experimental campaign consisting of 14 four-point-bending tests was conducted, where the specimens were examined regarding their load-deformation behaviour, crack kinematics and failure modes. The contribution of the short glass fibres to the load-bearing mechanism was estimated with Pfyl’s fibre engagement model, based on the material characterisation results from prism tests on fibre reinforced concrete members. The Tension Chord Model was used to predict the stress-strain relationship of the reinforcement and the crack widths in the constant moment zone, which yielded a good correlation between the predictions and the experiments. The short fibres mostly contributed to the flexural response in the serviceability limit state, but only slightly increased the shear resistance due to their low fibre effectiveness. The introduction of bond shear connectors was shown to be essential to prevent the premature delamination of the reinforcement. The use of tall connectors allowed doubling the bearing capacity with respect to the flat reinforcement without any spatial features, effectively bridging the governing crack and reaching the tensile capacity of the longitudinal textile reinforcement.