| Basalt fiber reinforced polymer（BFRP）bars have some advantages such as high strength,light weight,and chloride ion corrosion resistance,which can effectively replace steel bars in marine concrete structures to solve the corrosion problem of steel bars and improve the safety and durability of marine concrete structures in service conditions.However,the marine structures have a long service period and the service environment is complex and severe.Studies on the durability of BFRP bars in long-term service under marine and seawater sea-sand concrete environment have indicated that high humidity,high salinity and high alkalinity environments can lead to the deterioration of the properties of BFRP bars.In addition,current studies on the deterioration of BFRP bars in seawater sea-sand concrete environment and the deterioration mechanisms all focus on static loading conditions,but the marine concrete structures are continually affected by external dynamic loadings during service,such as tides,tsunamis,earthquakes,and vehicle or ship collisions.Compared to static loading,BFRP bars and bar-concrete interface reflect different characteristics when subjected to dynamic loading,and its failure damage can easily occur from the deteriorated areas.To address this problem,this study systematically evaluates the mechanical properties of BFRP bars and bar-concrete interfaces under the combined effect of seawater sea-sand concrete environment and dynamic loads by accelerated aging durability tests on the transverse impact resistance,static and dynamic tensile properties of BFRP bars,as well as the interfacial bond properties between BFRP bar and seawater sea-sand concrete under dynamic and cyclic loads.Microscopic analysis techniques and related theoretical models are utilized to further analyze test results.The works of this study can serve as a valuable reference for the design and application of highly durability BFRP reinforced seawater sand concrete materials and structures considering dynamic loading effects.The main research contents and conclusions are as follows:（1）The low velocity transverse impact response of BFRP bars in simulated pore solution of seawater sea-sand concrete at four groups of impact energy（19.15 J,25.56 J,31.92 J and 38.33 J）was investigated by accelerated exposure aging tests with elevated temperatures.The effects of impact energy and aging condition on the impact resistance characteristics of bars were evaluated.The evolution of the bar’s failure modes was observed,and the degree of deterioration and mechanism of bars were analyzed.The degree of resin matrix plasticization and hydrolysis increases with the aging temperature and duration.The failure mode of BFRP bar is significantly affected by the impact en ergy and aging condition.The fracture threshold,maximum impact force and transverse deflection generally decrease as aging temperature and duration rise.The dissipated energy increases with temperature and duration when the bar does not fracture,and then decreases when the bar fractures.The residual tensile strength reduces with temperature and duration,with tends to decrease with increasing impact energy.The energy dissipation ratio can be used to assess the degree of deterioration to the BFRP bar under aging condition and impact energy.（2）The effects of long-term static and dynamic tensile behavior of BFRP bars embedded in seawater sea-sand concrete under seawater attack at different strain rates(1.43×10-4 s-1,2.30 s-1,4.03 s-1,8.69 s-1,and 18.18 s-1)were evaluated.The deterioration mechanisms of the bars embedded in seawater sea-sand concrete were revealed.The effects of strain rate and aging conditions,as well as their coupling on the tensile properties and failure modes of BFRP bars were analyzed.The void volume can be considered as an important characterization factor in reducing the tensile properties of BFRP bar.Both tensile strength and elastic modulus of bars increase with strain rate,while the tensile failure strain decreases.Aging conditions lower the tensile strength and failure strain while having an insignificant effect on the elastic modulus.Based on strain analysis of various regions of the bar,it confirmed that the accelerated aging leads to the early appearance of strain localization phenomenon.Both aging conditions and strain rate change the failure mode of BFRP bars.When the strain rate increases,the size of fiber bundles reduces,and interface debonding becomes more severe,while the fiber fracture surface change s from relatively flat to rough.Accelerated aging leads to the early appearance of failure patterns that should occur at higher strain rates.（3）The effects of the seawater attack and dynamic loading（the pull-out velocities are 0.00002,4,6,8 and 10 m/s）on the long-term bond behavior of the interface between BFRP bars and seawater sea-sand concrete were studied by pull-out tests.Dissected the influences and changes of the bond mechanism and failure damage area of specimens.The bond strength of pull-out specimens decreases with the aging condition,and the aging deteriorated the outer surface of BFRP bar,which lead to easier interlaminar shear failure at the surface of convex rib.As the pull-out velocities increases,the failure area transformed from the rib root to the fiber layer of rib surface.The adverse durability issues coupled with the effects of high pull-out velocity affect both the bar and concrete,weakening the mechanical interlocking and friction effect,causing a decline in bond strength as aging temperature,duration,and pull-out velocity increase.The bond stiffness of specimen rises and then drops with the aging condition and loading rate.The rise in the initial stiffness is mainly caused by the coupling effect of high loading rates and mechanical interlocking,and the drop is induced by the bond interface imperfection and bar deformation.The critical anchorage lengths under the coupling effect of seawater attack and dynamic loads were calculated.（4）The effects of seawater attack and cyclic loading on the bond properties of the interface between BFRP bar and seawater sea-sand concrete were evaluated.The bond mechanisms based on the bond interface damage were discussed.The dissipated energy,bond stiffness and damage degree curves can all be divided into three stages.The increase in aging temperature and duration leads to a decrease in the dissipated energy of specimen within a single cycle,and the increase in stress level only raises the dissipated energy of a si ngle cycle,without changing the overall characteristics of the curve.The rise in stress level affected the proportion of each stage of bond stiffness curve,and the decline pattern of the third stage in stiffness curve changed as the aging temperature and duration increased.Neither the aging conditions nor the stress level significantly affected the evolution rule of the damage degree curve,and the cyclic load resistance of specimen tended to rise and then drop as the aging condition.A prediction model that can describe the bond stiffness and damage degree curves of unconditioned and conditioned specimens is proposed.The accelerated aging test changes the bond mechanism,while the rise in stress level does not influence the mechanism.（5）Based on the test data and existing long-term performance prediction model,the effects of atmospheric temperature and relative humidity on the deterioration rate of BFRP bars under different strain rates were discussed.The prediction results under the effects of atmospheric temperature fluctuation,atmospheric relative humidity variation and the coupling of temperature and humidity were obtained,and the combined effects of temperature and humidity fluctuation as well as strain rate variation on the long-term mechanical properties of bars were analyzed.For the service environment in which the marine concrete structures are located,the indoor accelerated aging tests should be corrected according to the daily average atmospheric temperature and relative humidity.The effect of humidity correction on the deterioration rate of bars is greater than that brought about by temperature correction,and the influences of both temperature and humidity correction diminish with the increase in strain rate.The service life of BFRP bars increased significantly after considering temperature fluctuation and relative humidity variation,while the rise in strain rate expanded the gap between the predicted results before and after correction. |
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