Research On Flexural Behavior On The Damaged Reinforced Concrete Beam Strengthened By Prestressed Ultra-high Performance Concrete

The potential advantages of ultra-high performance concrete（UHPC）in repairing damaged RC bridges are mainly derived from the ultra-high tensile toughness and excellent durability of the material,which can not only enhance the bending（cracking）ability of RC structure,but also significantly improve its impermeability and durability,and realize the dual effects of structural strengthened and durability protection.However,the post-cast UHPC strengthened layer cannot improve the stress state of the structure,the damage cracks are easy to re-develop under the external load.For RC bridges with serious cracks or needing to improve the load-carrying capacity,the strengthened efficiency of conventional reinforced UHPC layer remains to be improved.Therefore,a new method of prestressed UHPC（PUHPC）for RC bridge strengthening is firstly proposed by combining the advantages of prestressed reinforcement and reinforced UHPC layer.1)Through the four-point bending test of eight test beams,the failure mode,crack resistance,flexural capacity and deformation characteristics of PUHPC strengthened beams（PUB）with different parameters are mainly studied,the results are compared with unstrengthened RC beam（CB）and conventional reinforced UHPC（RUHPC）strengthened beams（RUB）.The experimental results show that the failure modes of PUB beams are all conventional bending failure,and the UHPC-RC interface is firmly bonded during loading,no peeling phenomenon occurs.Compared with CB beam and RUB beam,PUB beam s have significant improvement in crack resistance,bending stiffness and ultimate bearing capacity.PUHPC layer can significantly reduce the stress level of tensile steel bars and delay the development of cracks.However,the crack resistance and bending stiffness of Post-PUHPC strengthened beams decrease with the increase of pre-damage degree.The main difference between Pre-PUHPC and Post-PUHPC methods is that Post-UHPC layer has better crack resistance,while Pre-PUHPC layer has stronger ability to inhibit the redevelopment of RC damage cracks.2)Based on the experimental data,and considering the coupling effect of UHPC strain hardening behavior after cracking,prestress and UHPC constraint shrinkage effect,the calculation formulas of UHPC layer cracking load,RC damage crack redevelopment load and ultimate flexural capacity of PUHPC beams were proposed.For PUHPC beams with relatively small damage degree of RC beam,the theoretical calculation results agree well with the experimental results.For PUHPC beams with large RC damage degree,the calculated value of crack strength is slightly larger than the measured value in the test.For safety reasons,the correction f actor is introduced into the calculation formula to consider the damage degree of RC beams.3)The numerical simulation analysis of PUB beams was carried out by using the finite element software ABAQUS.The numerical analysis results are in good agreement with the test results,that is,the analysis model can reflect the actual bending performance of PUB reinforced beams.On this basis,the influence of the change of RC beam height and the prestress degree of prestressed strand on the flexural performance of PUB beams was further studied.