Flexural Behavior Of Partially Prestressed Steel Reinforced Ultra-high Strength Concrete Beams

The partially prestressed steel reinforced ultra-high strength concrete (PPSRUHSC) beam is a new type composite beam which consists of ultra-high strength concrete, steel reinforced concrete technology, and partially prestressed technology. Application of ultra-high strength concrete characterized by ultra high compressive strength, high density, excellent durability, and economy can reduce weight, increase the reinforcement ratio, enhance the bearing capacity, and improve durability of this composite beams. Application of partially prestressing technique can control the deformation and cracks of this composite beam effectively under serviceablity state, and partially prestressing technique was advantageous to the flexural ductility of this composite beam compared to fully prestressing technique. Due to the encasement of structure steel, the phenomenon that the bearing capacity of specimens reduced dramatically after peak load because of the brittle failure of ultra-high strength concrete was improved, so this type of composite beam had an increase in the flexural ductility and energy adsorption. Therefore, the PPSRUHSC beam would be a more appropriate member applied in lond-span and overload bridges and high-rise buildings.In the past, a number of researches had been conducted on prestressed steel reinforced concrete structres, and some research results were obtained. But few works had focused on the flexural behavior of PPSRUHSC beam. Combining of test studies, theoretical analysis, and numerical simulation method, this paper presented an experimental study of flexural behavior of PPSRUHSC beam under static load and fatigue load. The main contents and conclusions are summarized as follows:(1) Four specimens were tested to investigate the flexural behavior of PPUHSC beams under static load. The influences of test paramenters such as prestressing tendon depth and reinforcement ratio of prestressing tendon to flexural behavior of specimens were studied. Tset results indicated the control section of specimens were basically in accordance with the plane section assumption. According to conventional beam flexural theory, the calculation formula of ultimate moment capacity in normal section appropriated for PPUHSC beams were proposed. Combining with existing relevant norms, by introducing the influence coefficient k1of prestressed tendon depth and correcting the coefficient αcr of component characteristics, the calculation formulas of maximum crack-width appropriated for PPUHSC beams were proposed.(2) Twenty specimens were tested to investigate the flexural behavior of PPSRUHSC beams under static load. Test results such as failure mode, load-deflect ion curve, strain distribution at mid-span section, and rcrack development of the beams are studied. The influences of test paramenters such as prestressed tension control stress, reinforcement ratio of non-prestressed tensile bars, eccentricity of encased structure steel, percent of structure steel, prestressing tendon depth, and reinforcement ratio of prestressing tendon to flexural behavior of specimens were studied. Two calculation methods base on simple superposition method and deformation collaborative analysis to calculate the flexural strength for PPSRUHSC beams were proposed, and the applicability of the two calculation methods were analyzed. Considerring the contribution of the core constrains concrete to stiffness, the calculation formulas of the section stiffness of each stage and deflection appropriated for PPSRUHSC beams were proposed. Introducing the influence coefficient k1, k2, and k3of prestressed tendon, bottom flange of structure steel, and web of structure steel, the calculation formulas of maximum crack-width appropriated for PPSRUHSC beams were proposed. Comparing the test results of PPUHSC and PPSRUHSC beams, the influences of encased steel to the flexural strength, flexural ductility, energy absorption, and crack development of specimens were studied.(3) The calculation formulas of integrated reinforcement index were proposed. The flexural behavior of eighteen specimens of PPSRUHSC beams and four specimens of PPUHSC beams were studied by using integrated reinforcement index. By linear regression of test dates, the simplified calculation formula of displacement ductility factor, ultimate bearing capacity, and moment under serviceability limit state to integrated reinforcement index as a single variable were obtained, respectively.(4) Based on test results, numerical simulative analysis on the flexural performance from start load to peak load of PPSRUHSC beams was studied by ANSYS. The results of ANSYS were correspond with test results. The correctness of ANSYS finite element model was verified.(5) Four specimens of PPSRUHSC beams and one specimen of PPUHSC beam were tested to investigate the flexural behavior under fatigue load. The variation regularity of load-deflection curve, stiffness, crack development, and tensile strain of non-prestressed tensile bars under fatigue load of specimens were studied. The correlation curves of stiffness dropping-coefficient, variation of maixmum crack-width, and stress amplitude ratio of non-prestressed tensile bars versus the number of fatigue of specimens were given, respectively. These curves could be used to predict the fatigue life of PPSRUHSC beams.