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Strengthening of reinforced concrete (RC) beams is the important issue in the civil engineering practice. One of the techniques that can be applied in strengthening of such structures is prestressing using unbonded tendons. Despite the fact that many research studies have been done in the subject of external prestressing there are still some gaps in knowledge in this issue. These gaps concern especially the effect of preloading, precracking and strengthening sequence on the ultimate load and load–displacement behaviour of strengthened RC beams. The paper presents experimental and numerical research on reinforced concrete beams strengthened with external unbonded tendons. The results of experimental investigation performed on four point bending beams that were preloaded and precracked before strengthening are briefly described and discussed. On the basis of experimental observations a three-dimensional model of the beams was developed. The model considers the nonlinear behaviour of concrete, steel bars and prestressing strands. For the calibrated numerical model a parametric study was performed. The influence of the following parameters on the ultimate load, load–deflection and stress increase in tendons of beams were taken into account: concrete class, steel ratio, loading configuration, preloading level, strand shape, cross-section shape, deviator configuration. The experiments show that external prestressing is a very efficient technique for the flexural strengthening of reinforced beams. Preloading, precracking and the strengthening sequence have a minor influence on the ultimate stress increases in strands and the load-bearing capacity of beams after strengthening. It was also demonstrated that strengthening under existing external loading has a positive influence on the serviceability (deflection) limit state. The proposed numerical model confirms the good predictive capabilities for all experimentally tested beams and all considered load and structural stages. The parametric study demonstrated that the initial prestress level has a moderate influence on strengthening efficiency. It was also found that for elements that are characterised by cross-sections without wide compressive zones (i.e. rectangular cross-sections) the compressive strength is one of the key parameters that governs the ultimate load and ductility of the strengthened beam. The considered schemes of loading (two concentrated forces and uniformly distributed load) and the shapes of the tendons (in the form of broken or straight line) have a very moderate influence on the load-bearing capacity and load–displacement behaviour. The second order effects are more pronounced for beams with lower tendon eccentricities. These effects were activated after yielding of the main reinforcement. Finally, it was shown that before yielding of the main reinforcement, increases of stresses in tendons are a strong function of the stiffness of a beam and do not depend on the eccentricity of tendons or the type of supporting of tendon along the span of the beams. |
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