Experimental Study On Concrete T-Girder And Box Girder Strengthened With External Prestressing Tendons

External prestressing is a kind of active strengthening method which can significantly improve the bearing capacity of the structure. Due to its advantages of quick construction, convenient maintenance, and light weight, external prestressing is widely used in the construction, strengthening and maintenance of reinforced concrete as well as prestressed concrete bridges. Study on the mechanical behavior of externally prestressed beams is descriped in this paper based on the experiment of 6 specimens.The following four parts is included in this paper:Firstly, the whole process loading test of 3 T-girders and 3 box girders strengthed with external pretressing tendons was carried out to investigate the mechanical behaviors such as strain, deflection and cracking. The influence of different strengthening methods and different effective heights of external prestressing tendons on strengthening effect and mechanical behaviors of specimens are researched in this paper. The experiment shows that flexural bearing capacity of specimens is increased by more than 50% due to external prestressed tendons. Further, the original cracks are completely closed when strengthened after unloading while the original cracks with larger width are reduced and others are closed when strengthened under loading. The bending capacity of specimens can be improved through increasing the effective height of the external prestressing tendons.Secondly, by reasonable assumptions and simplified calculation model, whole process numerical analysis method is established on basis of the Moment Curvature analysis method and the Conjugate Beam method, and computation program is complied by using Matlab. The established whole process method is proved to having good fitting accuracy through the comparison of experimental results with the calculated values. The influence of the parameters such as span-height ratio, concrete compressive strength, ordinary steel and prestressing tendons on the flexural capacity of specimens is analyzed by using the established method.Thirdly, the ductility of specimens is investigated based on displacement ductility ratio. The influence of parameters such as concrete compressive strength, the compressive reinforcement index, the tensile reinforcement index, the partial prestressing ratio and the effective height of the external prestressing tendons on ductility is researched. It is indicated that the displacement ductility factor of the beams strengthened with external prestressed tendons is decreased compared to that of the unstrengthened ones. Different strengthening methods have little effection of ductility. Increasing the effective height of external prestressing tendons can significantly improve the bearing capacity while reducing ductility loss.Finally, the stress variety of the external prestressing tendons in the process and the effect of the external tendons on the stress of internal tendons are studied, and the calculation formula of ultimate stress in prestressed tendons is suggested. The maximum area of the external prestressing tendons is derived for external prestressing strengthening in highway bridges. On this basis, the influence of the factors such as the span-height ratio and concrete compressive strength on the stress increment and ultimate stress in prestressed tendons is analyzed.The results of this paper help to acquire a better understanding of concrete beams strengthened with external prestressing tendons and provide technical support for the new-built and strengthening design of externally prestressed concrete bridges.