Long-Term Behaviors Of RC Beam Prestressed With External BFRP Tendons In Marine Environment

External prestressing technique with corrosion resistant fiber-reinforced polymer(FRP)tendon provides an effective solution for the durability problems commonly exist in the reinforced concrete(RC)structures exposed to marine environment.Carbon fiber-reinforced polymer(CFRP)tendons and aramid fiber-reinforced polymer(AFRP)tendons were mostly adopted in the constructed structures externally prestressed with FRP tendon in the USA,Japan and Europe.However,the high cost and small ductility of CFRP,and excessively large relaxation of AFRP restrict their wide applications in prestressed structures.Basalt fiber-reinforced polymer(BFRP)has a large potential in the structures exposed to a harsh environment,due to its advantageous mechanical properties,corrosion resistance and cost performance.However,a comprehensive study on the short-and long-term behaviors of BFRP tendon and RC structures prestressed with BFRP tendon has not been found in the existing literatures.The study in this paper was conducted in terms of three aspects,including creep,relaxation and fatigue behaviors of BFRP tendon in marine environment,key regions of RC structures with external prestressing BFRP tendon,and short-and long-term behaviors of RC beams prestressed with external BFRP tendon.The results provide reference for the external strengthening technique using high-cost-performance BFRP.The details are stated as follows.(1)The creep and relaxation behaviors of BFRP tendons in marine environment were investigated.The creep rupture stress,creep rate and relaxation rate of BFRP tendons were evaluated through a series of creep test and relaxation test.The mechanism of failure and mechanical behaviors degradation was revealed.A pretension method of enhancing creep behavior of BFRP was proposed.The pretension duration and stress were optimized,and the effects of pretension on creep and relaxation were validated through a series of creep test.The results showed that pretension was capable of decreasing creep rate and relaxation rate.The one-million-hour creep rupture stress of BFRP tendon was determined to be 0.54 f_u(f_u is tensile strength),and the relaxation was close to that of steel strand,which was suitable for a prestressing component.A method of predicting relaxation rate by creep data was proposed based on the correlation between creep and relaxation.The deteriorations of creep and relaxation were clarified through a series of test after conditioned in salt solution.Salt corrosion had negligible effect on creep rate and relaxation rate,and the ratio of the degraded creep rupture stress to the corresponding tensile strength maintained constant.The degradation mechanisms were revealed by means of scanning electron microscope(SEM),and a design methodology for the long-term stress limit was proposed.(2)The fatigue behaviors of BFRP tendons in marine environment were investigated.An effective anchorage for fatigue test of FRP tendon was proposed.The effects of maximum stress and stress range on the fatigue behavior of BFRP tendon were clarified through a series of fatigue test.The results showed that the limits of maximum fatigue stress and stress range for two million cycles were respectively 0.53f_u and 0.04f_u considering 95%reliability.The elastic modulus of BFRP tendon maintained constant with respect of cyclic loads before fatigue failure.The deteriorations of fatigue behavior of BFRP tendon were clarified through the fatigue tests after conditioned in salt solution,and the degradation mechanism of fatigue behavior was revealed.The fatigue strengths at different latitudes were predicted adopting Arrhenius formula.The fatigue strengths of BFRP tendon at northern latitudes of 20°,40°and 60°were predicted to be 0.41f_u,0.43f_u and 0.45f_u after conditioned in marine environment for 100 years.The results provided guidance for the fatigue design of BFRP tendon in marine environment.(3)The mechanical behaviors of the key regions of RC structures prestressed with external BFRP tendons were investigated.The effects of radial stress on the tensile properties of BFRP tendon were clarified through a series of tensile test.A predictive model describing the relationship between tensile strength and radial stress was proposed based on Hoffman strength criterion.Longitudinal and transverse compressive tests,and in-plane shear test were conducted to determine the value of the parameters in that model,and the limit of radial stress(90MPa)was recommended,based on which,a novel type of anchorage with FRP homogenous material wedge was developed.The dimension of the wedge was optimized using ANSYS finite element(FE)software,and products of the wedge were manufactured though molding process.The effectiveness of this novel anchorage was validated through static tensile test and fatigue test.Furthermore,a three-dimension(3-D)FE model was established to analyze the stress state of FRP tendon at deviator.The results provided reasonable design parameters for external prestressing BFRP tendon at anchorage and deviator.(4)Flexural behaviors of RC beams prestressed with external BFRP tendons were investigated.Flexural tests were conducted on four externally prestressed RC beams and one control RC beam with a span of 5m.The effects of type of prestressing tendon,prestress level and concrete strength on the structural behavior including capacity,ductility,crack and prestress variation,were studied.The results showed that the RC beam prestressed with external BFRP tendons possessed similar mechanical behaviors with that prestressed with external steel strands,and the former had significantly smaller residual deformation than the latter did.Prestress level and concrete strength had negligible effects on cracking capacity,yielding capacity and ultimate capacity,but had a large effect on ductility.The accuracies of the methods provided by several representative standards for the calculation of stress increment in external tendon at ultimate state were evaluated.(5)Long-term behaviors of RC beams prestressed with external BFRP tendons under sustained load were investigated.Long-term sustained loading tests were conducted on four externally prestressed RC beams with a span of 5m for 150 days.The experimental parameters comprised type of prestressing tendon,prestress level and concrete strength.The mid-span camber,strain of concrete,prestress value and axial strain of tendon were analyzed and discussed.The results showed the developments of camber and prestress loss increased at initial stage and tended to be stable with respect to time.The increasing rate of camber and prestress loss increased as the increase of prestress level.The increase in concrete strength decreased the increasing rate of long-term camber,but had no effect on prestress loss.The external prestressing BFRP tendon exhibited significantly smaller prestress loss than the stress-relieved strand did.A predictive method was proposed to calculate the deformation and prestress variation in RC beam prestressed with external BFRP tendon subjected to a sustained load,based on age-adjusted effective modulus method(AEMM).The predicted results of long-term deformation and prestress were consistent with the experimental ones.The above studies contribute to the degradations of mechanical behaviors of BFRP tendon in marine environment,the mechanical behaviors of BFRP tendon at anchorage and deviator,and the effects of external prestressing BFRP tendon on the short-and long-term behaviors of RC beam.The key parameters of BFRP tendon in terms of material,key structural regions and prestressed structures were obtained,which provide design references for the RC structure prestressed with external BFRP tendon.