Study On Mechanical Behaviors Of Damaged PC Beams Strengthened With NSM CFRP Strips

With the use of prestressed technique in structural engineering more and more widely,the technique has played an important role in large span,large space,high-rise,heavy load and special structure engineering,especially many bridges have adopted the prestressed structures.With the increasing scaling-up and heavy-load tendencies of vehicles,the actual traffic volume of a few in-service bridges has exceeded the designed traffic volume.Moreover,the load design standards in new and old bridge design specifications are not uniform,thereby resulting in overloading damage of bridges.If we do not take measures to strengthen the damage bridge in time,it will cause a serious threat to traffic safety.It is neither realistic nor economical to dismantle and rebuild all the damaged bridges.It is an inevitable trend to repair,maintain and strengthen damaged bridges.The necessary reinforcement can improve the bearing capacity and prolong the service life of the bridge.CFRP(carbon fiber reinforced polymer)is widely used in bridge reinforcement engineering for its light weight,high strength,corrosion resistance and fatigue resistance.A great deal of research has been conducted on CFRP reinforced concrete concrete beams in the world.However,in the case of overload damage,the research on PC beams strengthened with NSM CFRP strips is very limited.For this purpose,the mechanical properties of PC beams strengthened with NSM CFRP strips are studied by means of experimental study,theoretical analysis and numerical simulation.Through the test,the flexural behaviors of the T-shaped RC and PC beam strengthened with NSM CFRP strips were studied.The working mechanism,failure mode,bearing capacity,crack development,and deformation were analyzed.The influences of cycle number,overload amplitude and other parameters on the bearing capacity and stiffness of beam were studied.The results showed that the flexural load-carrying of strengthened beams were enhanced.The maximum increase is 27%and 15%respectively.The failure modes of RC beam strengthened with NSM CFRP strips were observed for the tested beam specimens:spalling failure of concrete layer at the end of CFRP.The failure modes of PC beam strengthened with NSM CFRP strips were observed for the tested beam specimens:crushing failure of concrete in the compressive zone,wedge-shaped failure of the partial protective layer of concrete in the tensile zone,or a combination of both modes.Cycle number and overload amplitude affected the bearing capacity and stiffness of the strengthened beams.The formulars of interfacial slip,interfacial shear stress,deformation and tensile force of CFRP strip of PC beam strengthened with NSM CFRP strips were presented based on elastic theory and energy method.The influences of adhesive layer stiffness,quantity of CFRP strips,load levels and prestress increment on interfacial slip,interfacial shear stress,deformation and tensile force of CFRP strip were analyzed.The results showed that the interfacial slip decreased with the increase of adhesive layer stiffness,and increased with the quantity of CFRP strips and load level.The interfacial shear stress increased with the increase of adhesive layer stiffness.The interfacial shear stress and tensile force of CFRP strip increased with the increase of the quantity of CFRP strips and load level.The prestress increment had certain effect on the deformation but had little effect on the interfacial slip,interfacial shear stress and tensile force of CFRP strip.Considering creep effect of concrete and CFRP strips and prestress increment,the calculation formulas of interfacial shear stress increment and deformation increment were established.The results showed that the interfacial shear stress increment increased with the increase of adhesive layer stiffness,load level and quantity of CFRP strips,while decreased with the increase of relative humidity.The deformation increment decreased with the increase of relative humidity.Considering the increment of prestress,deformation increment decreased.Using finite element program ABAQUS,a finite element model of damaged PC beam strengthened with NSM CFRP strips was established.The results showed that the simulation results were consistent with the experimental results.With the increase of the strength grade of concrete and the quantity of CFRP strips,the ultimate bearing capacity of strengthened beams increased obviously by about 20%,but the improve rate was limited when the strength grade of concrete exceeds C50.The cycle number(500 times or less)had no obvious influence on the ultimate bearing capacity and deformation when the overload amplitude was less than 60%of the ultimate load,while had a certain impact when the overload amplitude was higher than 60%of the ultimate load.The deformation increased with the increase of cycle number.The ultimate bearing capacity decreased with the increase of overload amplitude with the same cycle number.