Non-metallic textile reinforcement enables the construction of slender concrete structures, characterised by increased structural capacity, reduced material consumption and inherent corrosion resistance. The slender design featuring a thinner concrete cover, combined with the utilisation of the high tensile strength of non-metallic reinforcement, has directed greater focus towards limiting crack width and deflection at the serviceability state, as both often govern the design. In particular, the considerable diversity in bond behaviour exhibited by textile grids, arising from differences in both materials and geometries, poses a significant challenge when developing models and approaches. For flexural members, reliable models are required to predict the deflection, which must adequately capture the effects of tension stiffening to fully exploit the potential of high-strength non-metallic reinforcement. Therefore, physically based models for determining crack width and deflection in members with non-metallic textile reinforcement are required, which are reliable and also applicable across a wide range of textile reinforcement materials and geometries. To derive a physically based model for predicting crack widths, a comprehensive characterisation of the bond behaviour is initially performed, addressing cross-sectional geometry, surface and stiffness of the textile girds. Based on crack opening tensile tests and their analysis using fibre optic measurements, a model is derived that describes the evolution of crack opening in relation to reinforcement stress, reinforcement ratio and bond behaviour, while accounting for tension stiffening. This approach, which distinguishes between single cracks and stabilised cracking, enables the identification of the relevant characteristics for various non-metallic textile reinforcements. The model for crack width prediction is expanded and validated using uniaxial tensile tests. In the case of multiple reinforcement layers, the capacity of bond stress is reduced by introducing an equivalent circumference, as the whole bond area does not fully contribute to the force transfer. Furthermore, the influence of the elliptical cross-sectional geometry of the fibre strands on the crack spacing is captured by a geometrically based stress-intensifying factor. This addresses the influence of the uneven multi-axial tensile stress state in the concrete, which also provokes splitting cracks. The model is also capable of accurately predicting the increase in crack width resulting from cyclic loading. Comparisons between the proposed model and the design concepts for crack width prediction in the German DAfStb Guideline and the Final Draft of the Preliminary Eurocode 2, considering uniaxial tension and flexural loading, demonstrate the improved performance of the new model approach. The methodology of using crack opening tensile tests in conjunction with the analytical approach of the bond stress-crack opening relation enables the physically based model to be adapted for predicting crack widths in concrete members with any type of non-metallic textile reinforcement. Finally, to more accurately determine the deflection of members with non-metallic reinforcement, a method is presented that adequately captures the effects of tension stiffening by iteratively calculating the cross-sectional curvatures. Validation of the model using four-point bending tests also confirms that the increase in deflection due to cyclic loading is primarily attributable to the loss of tension stiffening, in addition to concrete creep in the compression zone. The derived method thus enables the practical determination of deflections under static and cyclic loads.
Christopher Schmidt (Thu,) studied this question.