Study On Durability Of GFRP Bars-reinforced SWSSC Beams Under The Coupling Effect Of Sustained Load And Seawater Environment

Based on the background of powerful marine country,the new type of flexural member combining fiber reinforced polymer(FRP)bars and seawater-sea sand concrete(SWSSC)has become an important development direction for marine engineering.FRP-SWSSC beams not only can effectively alleviate the shortage of river sand and protect the ecological environment,but also can effectively resist the corrosion of chlorine and salt,avoid the corrosion of steel reinforcement,improve the life of the structure and maintain the sustainability of civil engineering by using the characteristics of FRP bars such as light weight,high strength and corrosion resistance.However,FRP bars are not invulnerable in the actual service environment,so FRP-SWSSC beams still face durability problems in marine engineering,especially the lack of information on the durability and degradation mechanism of FRP-SWSSC beams in the actual service environment(load-environment coupling),which becomes a barrier to the use of this new type of member in practical engineering.Notably,glass FRP(GFRP)are increasingly favored by engineers and researchers due to their low production costs and mature processes.And with the continuous progress of FRP bar production technology,the shortcomings of GFRP bar prone to creep rupture and high relaxation have also been further improved.Many scholars have begun to focus on the research of prestressed GFRP bars.However,few studies on the long-term performance of prestressed GFRP bars-reinforced SWSSC beams have been reported.Therefore,it can be seen that it is of great scientific significance and engineering value to carry out research on the short-term flexural performance and durability of prestressed GFRP bars-reinforced SWSSC beams.To this end,this thesis starts from the material level and goes deeper into the member level,and adopts a research method that is mainly based on experiments and supplemented by a combination of theories to investigate(1)the short-term performance of SWSSC beams reinforced with GFRP bars;(2)the durability of SWSSC wrapped with GFRP bars;(3)the durability of SWSSC beams reinforced with GFRP bars.The detailed research contents and main results are as follows.(1)An experimental study and residual life analysis of the durability of GFRP bars under the service environment were carried out.The durability mechanism and multi-timescale effect of the tensile properties of GFRP bars were investigated by considering the effects of key factors such as SWSSC wrapping,service environment type(natural exposure,wet-dry cycle)and exposure age(90 d,180 d,270 d),and the residual life of GFRP bars was predicted based on the Arrhenius equation.The results showed that the wet and dry cycles eroded the tensile strength of GFRP bars to a greater extent than natural exposure;after up to 270 d of exposure,the tensile strength retention of GFRP bars served under natural exposure and wet and dry cycles were 81.32% and 76.00%,respectively;glass fiber damage and resin hydrolysis were important reasons for the deterioration of the tensile strength of GFRP bars.The residual life prediction results showed that the tensile strength retention of GFRP bars was still higher than 63% after 50 years in the service environment,indicating that GFRP bars have excellent salt and alkali resistance.(2)The long-term performance study of GFRP bars-reinforced SWSSC beams under sustained load in natural exposure environment was carried out.The loss of prestressed GFRP bars and the long-term deflection evolution of the beams were traced,and the failure modes,residual stiffness and flexural capacity after the action of different exposure ages were investigated,and the mechanism of flexural durability of the reinforced beams under the hot and humid environment in South China was initially explored.The results showed that the loss of prestressed GFRP bars and the mid-span deflection of the beams increase with age,which indicates that the coupling of sustained load and natural environment increases the degree of creep relaxation of prestressed bars and shrinkage creep of concrete.At ages 90 d,180 d and 270 d,the cracking loads of prestressed beams increased by 8.99%,29.87% and 18.87%,respectively,compared with the control beam,which indicates that the prestressed GFRP bars can still improve the cracking resistance of the beams to some extent after exposure to the natural environment.After different exposure ages,the failure modes of the beams were all concrete crushing failure,and its stiffness and flexural capacity were enhanced;at an age of 270 d,the flexural capacity of the nonprestressed beam(prestressed beam)increased by 22.77%(12.72%)compared with the control beam.This can be attributed to the fact that the deterioration of the tensile properties of the GFRP bars was less affected by the natural exposure,and the deterioration rate started to slow down at the later stage of exposure,while the concrete strength continued to increase,which in turn caused a greater increase in the stiffness and flexural capacity of the reinforced beams.(3)A study on the durability of GFRP bars-reinforced SWSSC beams under the coupling effect of seawater environment and load was carried out.The time-dependent pattern of prestressed GFRP bars and the long-term deflection evolution of the beams were monitored,and the failure mode,residual stiffness and flexural capacity after different exposure ages were investigated,and the mechanism of flexural durability of the reinforced beams under seawater environment was initially explored.The results of the study showed that the seawater environment had a significant negative impact on prestressed GFRP bars and long-term deflection,exacerbating the growth rate of the prestress loss and deflection.After the coupling effect of seawater environment and load,the failure mode of the beam was not transformed;Although the tensile properties of GFRP bars deteriorate more severely in the seawater environment,weakening the tensile strength of the reinforced beams,the increase in concrete strength has a positive effect on the stiffness and flexural capacity of the reinforced beams,resulting in no significant change in the flexural capacity of the reinforced beams after being exposed to environmental effects.(4)A theoretical analysis of the calculated models for stiffness and crack width of GFRP bars-reinforced SWSSC beams was conducted,and the applicability of the relevant models was evaluated by considering the differences between the revised models of various scholars and the codes of various countries under the normal service limit state.The results showed that for the nonprestressed FRP bars-reinforced beams,the effect of low elastic modulus of FRP bars should be fully considered in stiffness verification;and the values and calculations of the member force characteristic coefficients and inter-crack inhomogeneous strain coefficients are the key factors affecting the accuracy of the crack width model.Overall,the stiffness model proposed by the code of GB 50608 and scholar Ren et al.has better applicability,and the crack width model modified by scholar Hua et al.and scholar Liu et al.has better calculation accuracy.For stiffness and crack width verification of prestressed FRP bars reinforced beams,it is recommended to introduce the prestressing degree in the model as well as to equivalent the non-prestressed FRP bars to steel bars.Overall,the computational accuracy of the stiffness model proposed by Cheng et al.and the crack width model proposed by Sun et al.is better.When the current code for design of FRP bars-reinforced SWSSC beams has not yet been promulgated,it is recommended to refer to the above-mentioned calculation model for the design of FRP bars-reinforced SWSSC beams under normal service limit states for verification.(5)The experimental study and theoretical analysis of the short-term flexural behavior of GFRP bars-reinforced SWSSC beams were carried out.The effect of reinforcement ratio,stirrup ratio,shear span ratio and prestress level on the flexural behaviour of the beams was initially investigated,and the existing flexural capacity calculation models in the codes were compared and analysed.The results indicated that the pretensioned GFRP bars-reinforced SWSSC beams had excellent flexural and cracking resistance behavior,and the 20%prestressing level increased the cracking load of the reinforced beams by 81%;increasing the reinforcement and stirrup ratios caused a change in the failure mode of the beams,which changed from GFRP bar rupture to concrete crushing.The flexural capacity prediction results of various codes indicated that the Canadian code ISIS M03-07 predicted the test value more accurately(error 5%),and the calculation model of ISIS M03-07 can be better applied to the prediction calculation of the flexural capacity of GFRP bars-reinforced SWSSC beams.