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The article examines the behavior of flexural reinforced concrete members, strengthened with fiber-reinforced polymers (FRP), under the ultimate loading conditions (in fracture stage). One of the main problems of such elements is sudden and brittle failure mode caused by FRP debonding. The method for the calculation of load-bearing capacity of the normal section of flexural reinforced concrete members, strengthened with various types of FRP, is proposed in this article. This method is based on the theory of fracture mechanics of solids. The reduction of overall member stiffness due to the slip between concrete and FRP is estimated by reducing the FRP stress according to the built-up bars theory. Such reduction allows the prediction of load-bearing capacity of strengthened members sufficiently precisely even for sudden and brittle FRP debonding failure mode. The numerical results are compared with experimental ones. In total, 55 reinforced concrete beams, strengthened with externally bonded or near surface mounted carbon fiber-reinforced polymer and glass fiber-reinforced polymer sheets, plates, strips, and rods, are analyzed in this comparison. Experimental results were collected from various scientific publications. A focus is made on the depth of the crack in critical normal section and FRP strain-stress relationship.

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The building of structures from steel fibre reinforced concrete (SFRC) in the external and conventional reinforced concrete (RC) in the internal layer represents an economical alternative of structures effectively using SFRC. The paper presents test results of flexural behaviour of layered beams with SFRC external layers and RC internal layer. The behaviour of these beams is compared to test results of SFRC and conventional RC beams. The test results show, that the flexural load capacity for all series of beams is nearly similar, but the deflections of layered beams are less comparing to monolithic ones. It also been shown that the equations indicated in the Eurocode 2 can be used to design the flexural reinforcement in layered SFRC beams.

The article proposes a model for strength and stain analysis of steel fiber reinforced concrete (SFRC). The model is based on general principles for creating and modelling structural composites and on reinforced concrete code. Differently from other examples, the elastic and plastic properties of the components (concrete and steel) of the introduced model are directly taken into account. The model gives an opportunity to determine tension and compression strength, the elasticity modulus of fiber concrete and the main parameters of its elasticity and plasticity. A good agreement between the obtained results and those of experiments performed by other investigators was confirmed. Differences between the ratios of theoretical and experimental values are insignificant and vary within the limits of 1.06-1.10. This model may be used for the analysis of reinforced concrete members reinforced by steel fibers (SFRC) in a dispersible way assuming stress distribution diagrams.