Study On Flexural Properties Of Ultra-High Performance Concrete Beams Prestressed With FRP Bars

Compared with the prestressed FRP reinforced concrete beam,the prestressed reinforced concrete beam has better ductility,but its poor durability.Compared with the prestressed reinforced concrete beam,the prestressed FRP reinforced concrete beam has better durability,but its ductility is poor.To solve the problem of incompatibility between durability and ductility of prestressed concrete beams,this paper uses Steel-FRP Composite Bar(SFCB),which has high strength,good ductility and corrosion resistance,as a non-prestressed tendon,and Ultra-High Performance Concrete(UHPC),which has good ductility,high compressive strength and durability,to form a prestressed FRP reinforced ultra-high performance concrete beam.This paper investigates the effects of applied prestress,concrete type,tension control stress magnitude and prestressing tendon type on the flexural performance of prestressed FRP reinforced UHPC beams by conducting static load flexural tests on eight test beams.The stresses in the prestressing tendons were monitored to obtain the prestress losses in each test beam and compared with the theoretical calculated values for verification.The force analysis of the prestressed FRP reinforced UHPC beams was carried out.Based on the assumption of flat section and the corresponding material’s intrinsic model,the formulae for the cracking load,yield load,ultimate load,short-term stiffness and maximum crack width under short-term load of the prestressed FRP reinforced UHPC beams were derived and compared with the experimental measured values to verify the feasibility of the formulae.ABAQUS was used to simulate the flexural performance of prestressed FRP reinforced UHPC beams and compare the analysis with the expeimental measured values to verify the correctness and feasibility of the model analysis;the effects of different parameters(Concrete properties,type of non-prestressed tendons,reinforcement rate of prestressed FRP tendons and reinforcement rate of non-prestressed SFCB tendons)on the flexural performance of prestressed FRP reinforced UHPC beams were analysed.The main findings of this paper are as follows:(1)The concrete strain in the span section of each test beam varies linearly in the height direction,which is in line with the assumption of flat section,and the bonding performance of concrete and reinforcement is good;the application of prestressing can delay the cracking of concrete beams and improve the flexural load capacity of concrete beams,resulting in less crack distribution and increased energy dissipation capacity,while the ductility performance and safety performance become worse;to improve the performance of concrete,the cracking load,yield load,ultimate load and deflection of prestressed FRP reinforced concrete beams increase,and the ductility performance and safety performance become better.the cracking load,yield load,ultimate load and deflection increase,the flexural performance becomes better,the number of cracks increases,the average crack spacing decreases and the energy dissipation capacity increases,while its ductility and safety performance becomes better;the higher the tension control stress,the higher the cracking load,the lower the number of cracks,the larger the average crack spacing and the lower the ductility and safety performance of the prestressed FRP reinforced UHPC beams;The type of prestressing tendon has less influence on the cracking moment of concrete beams,but has a greater influence on the ductility,with the best ductility performance of concrete beams with prestressing strands and the worst ductility performance of concrete beams with prestressing SFCB tendons.(2)The content and calculation method of prestressing FRP tendon prestress loss is basically similar to that of prestressing strand;by assuming that the matrix withdraws from work after the UHPC beam cracks and the steel fibres continue to bear the tensile force,the formulae for calculating the effective moment of inertia and short-term stiffness of prestressing FRP tendon UHPC beam sections are derived;With reference to the Code for Structural Design of Concrete and considering the effect of steel fibre tension on the maximum crack width,the equivalence axial stiffness idea was used to derive the formula for calculating the maximum crack width of pre-stressed FRP reinforced UHPC beams.(3)The higher flexural load capacity of prestressed FRP reinforced UHPC beams,as well as better ductility and safety performance;improving concrete properties can increase the flexural load capacity of prestressed FPR reinforced concrete beams;when the SFCB bars have the same type of outer fiber,the larger the diameter of the inner core reinforcement,the higher the cracking-yielding stage stiffness of prestressed FRP reinforced UHPC beams,but the lower the post-yielding stiffness.When the core reinforcement diameter of the SFCB bars is the same,the stiffness and flexural load capacity of the prestressed FRP reinforced UHPC beams with carbon fibre as the outer fibre is greater than those with basalt fibre as the outer fibre,but the ductility is lower;the greater the reinforcement ratio of the prestressed FRP bars,the greater the cracking load of the prestressed FRP reinforced UHPC beams,the greater the ultimate load As the reinforcement ratio of non-prestressed SFCB reinforcement Increases,the cracking load of prestressed FRP reinforced UHPC beams increases slightly,the yield load increases significantly,the ultimate load increases and the ultimate deflection decreases,while their ductility,safety and energy dissipation capabilities become worse.