| Cracking and over-deflection occur frequently in prestressed concrete（PC）girder bridges,which has a long history and has not been solved completely.There are causes for the performance degradation of PC girder bridges.Among the causes,material degradation is the basis of structural degradation.Materials,such as steel tendons and concrete,generate relaxation and creep under static load,causing prestress loss and deformation to the PC girder bridges.There are live loads,such as traffic load and diurnal amplitude,acting on the bridges,causing cyclic relaxation to steel tendons and cyclic creep to concrete,which are higher than static relaxation and static creep,so the prestress loss and deformation increase,and structural performance deviates from the design gradually.In the literature,there are still some problems to be solved in the research of performance degradation of PC girder bridges based on cyclic relaxation and cyclic creep.On the material level,there are mature existing studies on cyclic creep of concrete,but few researches on the cyclic relaxation of steel tendons.On the level of analysis method,there is a nonlinear characteristic in the development of cyclic relaxation and cyclic creep,but no method to analyze prestress loss considering the nonlinear characteristic.On the structural level,there is no research on the performance degradation mechanism of PC girder bridges based on cyclic relaxation and cyclic creep.Under such circumstances,this dissertation reveals the mechanism,studies the development,establishes the theory,and defines the critical stressσcrof cyclic relaxation of tendons through experiments and analyses,thus solving the problem on material level.A numerical method based on incremental step accumulation considering the nonlinear characteristic of cyclic relaxation and cyclic creep for analyzing prestress loss is proposed,and verified by experiments,thus solving the problem on the level of analysis method.Based on the material theory and analysis method,the performance degradation mechanism of PC girder bridges under normal operation and overload conditions is studies,and the design and construction measures for improving the performance of PC girder bridges are proposed,thus solving the problem on the structural level to a certain extent.The contents and results of this dissertation are as follows:（1）Massive samples are collected from domestic typical steel tendons,and used for monotonic tensile experiments.The plastic characteristic of tendons at the conventional stress level are studied.The results show that there is a significant plastic behavior for tendons under conventional stress level.The whole process stress-strain relationship of tendons centered on the constitutive response under conventional stress level is established based on the experimental results.The cyclic relaxation mechanism that plastic strain generates and accumulates with the number of cycles under conventional stress level is revealed.（2）Low-cycle tensile experiments,high-cycle tensile experiments and relaxation experiments are carried out to quantitatively study the cyclic relaxation of tendons under conventional stress level.The results show that the amount of cyclic relaxation of tendons is higher than that of static relaxation,causing higher prestress loss.The tendon is formed by winding several steel wires.Affected by the change of contact state between steel wires and the residual stress of steel wires,cyclic relaxation develops rapidly in early cycles,and slows down in middle and late cycles.Theory of cyclic relaxation suitable for common stress range and common stress amplitude range is established based on the experimental results.The value of critical stressσcr of cyclic relaxation is defined.（3）The applicable scope of cyclic creep theories of concrete is clarified to meet the needs of creep analysis of PC girder bridges.The effect of cyclic creep on prestress and deflection of PC girder bridges is analyzed.（4）The numerical method considering the nonlinear characteristic of cyclic relaxation and cyclic creep based on incremental step accumulation is established,and used for analyzing prestress loss due to cyclic relaxation and cyclic creep of PC girder bridges.Combined with practical engineering,a fatigue experiment with 6 million cycles is carried out on a PC rectangular girder to verify the numerical method.（5）The mechanism and law of performance degradation of the PC girder under traffic load are studied through the fatigue experiment and nonlinear finite element analysis.The results show that the prestress loss under cyclic load is increased by 70 MPa compared with the loss due to static relaxation,shrinkage and creep,reducing the cracking load by 6.4%and recracking load by 8.0%.If there are more cycles after 6 million,the growth rate of cyclic relaxation will tend to zero after 11 million cycles.There is a low growth rate for the cyclic creep of concrete after 11 million cycles,so the prestress of the girder tends to be stable.（6）The mechanism and law of performance degradation under the diurnal amplitude are studied,with a PC simple supported box girder bridge in alpine region taken as the engineering example.The results show that the performance degradation of the bridge is mainly manifested as the prestress loss due to cyclic relaxation of tendons.The loss develops in a fast-slow pattern with the number of cycles,which is sensitive to the stress level of tendons,and less affected by the quantity of diurnal amplitude and thermal expansion difference between materials.（7）The mechanism and law of performance degradation under the overload are studied,with the box girder bridge and T-girder bridge taken as the engineering examples.The results show that the overload accelerates the performance degradation of PC girder bridges by increasing the cyclic relaxation and cyclic creep,and the potential cracking effect.Different overload conditions correspond to different degradation acceleration mechanisms,and different degradation rates of PC girder bridges.If the box girder bridge cracks due to accidental overload during operation,the cracks cannot be completely closed,so the acceleration mechanism of accidental overload on the degradation of the bridge is crack corrosion.If there are frequent overloads,the degradation of the bridge will be accelerated greatly under increased cyclic effect and crack corrosion.The T-girder bridge is subjected to overload frequently during operation.Affected by increased cyclic effect and crack corrosion,the bridge is rated as Class 4 after less than three years of operation,and the service life is far less than the design reference period（100 years）.（8）Based on the cyclic degradation mechanism,design and construction measures are proposed to improve the performance of PC girder bridges.Reducing the tension control stress,and improving the reinforcement ratio of tendons can be adopted in design,which is verified by the comparison of degradations between PC rectangular girder and T-girder bridge.In addition,controlling the early high cyclic relaxation of tendons by cyclic tensioning in construction can reduce the rate and amount of cyclic relaxation in operation. |
