Introduction. The critical shear crack theory (CSCT) was applied to devise a modified model on the basis of theoretical research into shear resistance models of concrete elements reinforced with polymer composite bars. Experimental studies were conducted for various loading patterns and cases to verify provisions of the proposed model, to find the shear resistance of elements made of expansive concrete and concrete containing Portland cement, reinforced with steel bars and FRP. Materials and methods. The proposed modified shear resistance model was verified using the experimental data obtained both by the authors and other researchers (the database had 301 elements). Results. Given the results of a parametric study, a phenomenological shear resistance criterion was proposed for flexible elements, reinforced with polymer composite bars. Its mission was to simplify the model design to determine the shear resistance of concrete elements reinforced by FRP bars. Hence, no analysis of the shear force components was needed. Conclusions. The proposed simplified shear resistance model of flexible self-stressed elements, reinforced with polymer composite bars, adequately evaluates the shear resistance of elements exposed to the load distributed in a span; it shows good convergence with experimental data.

Introduction. The problem of shear resistance of reinforced concrete elements without transverse reinforcement remains, as before, one of the most controversial in the theory of concrete structures. The interest in the problem arose anew due to the use of polymer composite (FRP) bars as longitudinal reinforcement in concrete structures. Models of shear resistance, included in the regulatory documents and formulated as suggestions in terms of elements reinforced with steel bars and FRP bars, are considered. It is shown that the critical shear crack theory (CSCT) can be adopted as a basis for formulating a model of shear resistance of FRP-reinforced elements. However, its provisions cannot be applied without appropriate adjustments, in particular, regarding the determination of the width of critical crack openings. Materials and methods. To determine the shear strength of self-stressed concrete elements reinforced with FRP bars, a modified model was developed. As a result of an iterative procedure, this model makes it possible to determine an individual contribution of each of its main components to the total shear strength, taking into account the shape of a potential inclined crack, adopted in accordance with provisions of the Critical Shear Crack Theory (CSCT), as well as the width of the inclined crack opening at the level of longitudinal reinforcement, determined according to the bond-slip law for FRP bars. Results. A modified mechanical shear resistance model of self-stressed concrete elements reinforced with polymer composite bars was developed. Its difference is that, in addition to the influence of initial self-stress, the bond law for longitudinal reinforcements made of FRP is taken into account (the modified bond-slip law) when cracking parameters are determined in the support zone. An iterative procedure is proposed for calculating the components and the total shear strength of elements made of concrete (including expansive concrete) without transverse reinforcement and with longitudinal reinforcement made of FRP bars. Conclusions. The shear resistance model proposed for flexible self-stressed elements, reinforced with FRP bars, conveys the physical essence of the shear phenomenon and is applicable to various cases and loading patterns.