| The analysis of creep properties of external prestressed concrete structures under the combined action of concrete shrinkage and creep,prestress loss and temperature deformation is extremely complicated,and there are still many problems to be solved.Carbon fiber reinforced composite(CFRP-Carbon Fiber Reinforced Polymer)tendons have many advantages such as light weight,high strength,corrosion resistance,fatigue resistance,long-term performance stability,etc.They have different advantages compared with the existing prestressed steel bars,and therefore can effectively improve the long-term performance of the prestressed concrete structure such as corrosion resistance and fatigue resistance.In order to grasp the creep behavior of externally prestressed CFRP reinforced concrete flexural members under long-term load,this paper combines experimental research and theoretical research to study the material properties of CFRP bars and concrete and its external prestressed CFRP reinforced concrete.The creep performance and analysis methods of flexural structural members have been studied in depth.The main contents and results are as follows:(1)CFRP wedge-type anchors have the advantages of high anchoring efficiency,long-term performance and small size compared to bonded anchors.Based on the new wedge-type anchors independently researched and developed by the author's research team,30 test specimens were designed and fabricated,and the main long-term performance parameters of CFRP tendons,creep,relaxation and slip were tested for 1000 hours.The development and variation of creep and relaxation of CFRP tendons under different stress levels are studied.The development characteristics of long-term slip of wedge-type anchors and the influence of load level are studied.The mechanical properties such as strength and elastic modulus of CFRP specimens after creep were tested and compared with their static mechanical properties.The linear relationship between creep strain,relaxation rate and stress level was found.Based on the semi-logarithmic linear model,the creep strain and relaxation rate prediction model of CFRP tendons was proposed by regression analysis of experimental data.(2)In view of the fact that the commonly used concrete material shrinkage and creep prediction models are mostly based on the regression analysis of concrete materials under the relevant norm standard conditions,their applicability and prediction error for concrete under natural environment remains to be studied.To this end,this paper carried out research on shrinkage and creep of concrete under natural environment.The research parameters mainly include curing age,loading age and body surface ratio.The expression forms of common predictive models of concrete shrinkage and creep and the differences between their influencing factors and application scope are discussed in detail.These models are used to calculate the test specimens and compare them with the experimental values.The results show that the prediction model using these standard condition parameters and the modified model still has different degrees of error in the prediction of shrinkage and creep of concrete in the actual natural environment.Therefore,based on the concrete experiment in the natural environment,this paper proposes a ?factor-by-factor correction accuracy control method? that can be used to correct and improve the accuracy of concrete shrinkage and creep prediction models.By analyzing the main influencing factors one by one,the method can effectively control and deal with the adverse effects of the test data noise under the condition of complicated environmental(such as natural environment)interference factors,and thus can directly identify the corresponding specific factors of the existing models.The influence law is applied,and the concrete shrinkage creep model with higher prediction accuracy and wider application range can be corrected.(3)The creep performance test of four externally prestressed CFRP reinforced concrete flexural members was carried out for 500 days,and compared with the same condition specimens using steel strands as external tendons.The test results show that the long-term deflection of the flexural members can reach 2.0?3.1 times of the initial deflection under constant load.Compared with the external prestressed steel strand members,the long-term deflection members with external prestressed CFRP tendons is about 5% higher,and the creep performance of both is very close in general.(4)Based on the basic principle of linear creep theory and time-step method,and combing the cross-section analysis and the overall analysis of the components,the creep performance analysis method of externally prestressed concrete flexural members is proposed and the corresponding calculation program is compiled.The relationship between the deformation of the member,the inherent relaxation of the prestressed tendon and the actual stress of the prestressed tendon is directly established by the action and response mechanism of the structural member,and the conjugate beam method is used to solve the problem,which overcomes the shortcoming of calculating prestress losses by using simplified calculation or experience formula.The method is able to consider the influence of the concrete elastic modulus over time and can be applied to the prestressed concrete structure in the service life of the whole life.Based on the modified shrinkage and creep model of concrete proposed in this paper,the calculation error of the above-mentioned uncracked external prestressed concrete flexural members is less than 5%.While for cracked specimens,the calculation error for long-term deflection is about 10% and the calculation errors of external reinforced prestress and midspan concrete strain are 3% and 10%,respectively.(5)The paper also optimizes the influence of various arrangement of prestressed tendons on the long-term performance of the members.The research shows that compared with the members with the arrangement of the ingot-shaped three-fold line,the members with five-point three-fold line and five-point four-fold line can effectively improve the long-term performance of concrete members with externally prestressed CFRP tendons,and the five-point four-fold line symmetry arrangement form is the best.Compared with the ingot-shaped three-fold line members,the five-point three-fold line members under the same conditions can reduce the long-term deflection of mid-span by 6.3%?6.8%,reduce the stress of the top-span concrete by 10.2%?14.9%,and reduce the stress and strain of the top steel bar in the mid-span by 6.9%?8.0%.For the members with five-point four-fold line arrangement,the long-term deflection of mid-span can be reduced by16.6?26.5%,the stress and strain of the top steel bar in the small span can be reduced by9.2%?39.9%,the stress and strain of the top steel bar in the mid-span by 23.1%?30.6%.In summary,the calculation method of externally prestressed concrete flexural members and the optimized reinforcement arrangement form have great application value for practical engineering. |
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