Study On The Properties Of FRP Bars And Long-term Behavior Of Seawater Sea Sand Concrete Structures Reinforced With FRP Bars

Rapid developments in the construction industry consumes large amounts of freshwater and river sand.Additionally,in offshore areas and islands,the construction expense significantly increased due to the lack of freshwater and river sand.A method has been proposed that seawater and sea sand could be used instead of freshwater and river sand.However,traditional steel bars exhibits insufficient durability in seawater and sea sand concrete(SWSSC)environment due to a high chloride content.To overcome this problem,fiber-reinforced polymer(FRP)composites are used to substitute steel bars in SWSSC structures owing to their excellent properties include light weight,superior mechanical properties and good corrosion resistance.BFRP exhibits higher strength and modulus compared with GFRP at a similar cost,and a much lower cost than CFRP,which implies that BFRP is an good alternative to steel bars in SWSSC structures.The study in this paper was conducted in terms of four parts,including durability of FRP bars in SWSSC environment,bonding durability of the interface between FRP bars and SWSSC,durability of SWSSC beams reinforced with BFRP bars,and seismic behavior and durability of SWSSC beams reinforced with steel-FRP composite bars(SFCB).The details are stated as follows.(1)Aimed at the problem of insufficient crack resistance of resin.The mechanical properties of epoxy and vinyl ester resins and three types of FRP modified by silica nanoparticles and carbon nanotubes(CNTs)were investigated in this study.Fracture toughness tests and SEM were carried out to investigate the toughening mechanism.Damage mechanisms of modified-resin were analyzed and differences owing to varying nanoparticle types and contents were studied.Impregnated fiber roving made of modified resin was studied by tensile tests.Results showed that as compared to pure resin,the 0.3 wt% MWCNT-modified resin and 3 wt% silica nanoparticle-modified resin exhibited increased fracture toughness,impact strength,tensile strength,and flexural strength.Similar improvements were found for the modified vinyl ester resin.SEM images of the fracture surface showed a rough surface with tortuous cracks,resulted in increased fracture toughness of the modified resin.In addition,the adoption of three types of fiber rovings with silica nanoparticles and CNTs resulted in higher tensile strength and stiffness compared with those without modification.(2)Durability experiments of BFRP laminas exposed to solutions of ordinary concrete and SWSSC by acceleration of high temperature of 55 °C were conducted.Meanwhile,the durability of CNT/silica-nanopsrticle-modified BFRP bar was carried out.The results showed that the tensile strength of BFRP decreased after corrosion in the normal concrete and SWSSC solutions,while the Young’s modulus of BFRP remained unchanged.The degradation of tensile strength of BFRP in SWSSC solution was bigger than in NC solution owing to the combined effect of alkali ions and saline ions.It should be noted that the durability of MWCNT-modified BFRP has been obviously improved.Moreover,silica nanoparticle-modified BFRP exhibited improved corrosion resistance at a content of 1 wt%.The long-term residual tensile strength of modified BFRP laminas and bars under ordinary concrete and SWSSC environments was predicted.(3)Investigation of the bonding durability of the interface between FRP bars and SWSSC in the marine environment was conducted.An untreated BFRP shallow ribbed bar specimen was set as a control group.Specimens made of CNT-modified BFRP bar,surface sanded BFRP bar,and steel bar were set for comparison.The results show that the surface ribs of the BFRP bar were obviously abraded after being pulled out,and the ultimate bond strength showed a significant decrease;the degradation rate of bonding strength of specimens made of CNT-modified BFRP bar was obviously slowed down;Sticky-sand on the surface of the BFRP bars increased the occlusal force between the bars and the surrounding concrete,resulted in an 18.3% increase in the short-term bond strength compared to the un-sanded BFRP bars.In terms of long-term performance,as the corrosion age increased,the resin layer used for the sticky sand was corroded and the bond strength decreased.(4)Durability tests of SWSSC beams reinforced with BFRP bars in marine environment was conducted,different types of reinforcement have been proposed to replace normal BFRP bars,including CNT-modified BFRP longitudinal bars,modified BFRP stirrups and sanded BFRP stirrups.The results show that in marine environment,the BFRP bar reinforced SWSSC beams showed obvious degradation of load-carrying capacity.With the increase of corrosion age,the SWSSC beams exhibited shear-compression failure due to the degradation of the bond strength between the BFRP stirrups and concrete.Replaced normal BFRP stirrups with modified BFRP stirrups prevented shear-compression failure in the early stage,but the shear-compression failure still occurred after nine months of corrosion.Moreover,no obvious change of failure mode was found of SWSSC beams by replace ordinary BFRP stirrups with sanded stirrups.The calculation formulas of flexural stiffness and ultimate load of concrete beams reinforced with FRP bars from different standards were compared and checked with experimental results.(5)Experimental study on the seismic behavior and durability of SWSSC beams reinforced with SFCB bars.An investigation of SWSSC beams reinforced with SFCBs,hybrid reinforced with SFCB and BFRP bars was conducted.Additionally,the effect of 1% minibar made of basalt fiber on the seismic behavior of SWSSC beams was studied.Including failure mode,hysteretic response,stiffness degradation,energy-dissipation capacity,strain variation,and curvature of the pedestal were analyzed.Results showed that SFCB bars and hybrid reinforced RC beams exhibited a more obvious pinching effect and smaller residual displacement as compared with the steel reinforced beams.The dissipated energy of beams reinforced with SFCB bars was nearly the same as that of the steel bar reinforced beams due to increased deformability.The seismic behaviors of the SFCB bar reinforced minibar-concrete beams were improved owing to a stable confinement of minibar on concrete.According to the results,SWSSC structures reinforced with SFCB bars or hybrid reinforced by SFCB and BFRP bars are recommended.In addition,after corrosion in the marine environment,the decreases of total cumulative dissipated energy of the SFCB bar reinforced minibar-SWSSC specimen and the hybrid reinforced specimen were less than that of the steel bar reinforced specimen,indicated that they had better durability.According to the experimental results,the design suggestions and calculation methods of the corresponding structures are proposed.