Research On Creep Law Of Reinforced Concrete Structures Under Cyclic Loads

The effect of creep is an inherent characteristic of concrete materials.Accurate calculation of concrete structure under long-term loading of the creep effect is important for the force and deformation of the structure.In actual engineering,due to the existence of annual and daily temperature difference,a large number of concrete structures are subjected to periodic temperature loading,which in turn causes a series of structural temperature additional force effects.The base plate of CRTS II plate ballastless track is the representative.Under the annual cyclic temperature load,the base plate is subjected to long-term cyclic axial tensile and compressive load due to the solidification at both ends of the structure,and its force response is affected by the concrete creep effect.In the previous study on the longitudinal stiffness of the base plate in Germany and China,only the force response of the base plate under transient load was considered.Our current railroad track design code does not explicitly contain the content of the calculation of creep,China’s railroad bridge design code only give the value of the creep coefficient in the pre-stress loss chapter.The paper starts from the study of the creep effect of structures under periodic temperature loads,reviewing existing research materials on creep and analyzing the key issues in the research process to carry out the research work.Axially stressed reinforced concrete structures are selected for the study of axially stressed,a relatively simple type of structural stress.The method of calculating creep of reinforced concrete structure under cyclic load is analyzed and derived,the creep test of cyclic external load is carried out,the difference of force response of the structure under cyclic temperature load is analyzed,and finally the creep development law of reinforced concrete structure under cyclic temperature load and the force response characteristics of the structure are obtained.The research results of this paper can provide a certain theoretical basis for the calculation method of the force of reinforced concrete structures under cyclic loading after considering the creep effect,and provide a basis for the creep effect of the base plate of the CRTS II slab ballastless track under the action of annual cyclic temperature load.It has certain theoretical guiding significance for the design,construction and maintenance of the base plate.Main content:（1）Theoretical and methodological research.The sensitivity of the classical creep models at home and abroad under the influence of various parameters is analyzed,the existing creep calculation theory is analyzed and selected,and the creep calculation theoretical formula and method of reinforced concrete structure under the action of variable load are deduced and put forward.（2）Experimental research.The creep recovery test of plain concrete structure,the constant force loading test of reinforced concrete structure and the periodic variable force loading test were carried out to verify the theoretical calculation method and obtain the development law of structural creep under the action of short period external force.（3）Theoretical application.The model working condition of CRTS II plate-type ballastless track base plate on The Beijing-Tianjin intercity railway bridge under the annual periodical rising and cooling load is established,and the creep development and stress response law of the base plate under the multi-period temperature load are analyzed and summarized.Main conclusions:（1）Analysis of model parametersDifferent domestic and foreign models consider internal and external factors to have different influences on the creep coefficient;different materials,theoretical thicknesses,loading ages,and ambient temperature and humidity theoretically have a greater impact on the creep coefficient.（2）Theoretical calculation method and experimental research content（1）The creep recovery law of concrete after unloading satisfies the superposition principle.In the creep test under the periodically changing external force load,the derived theoretical calculation formula can play a certain role in predicting the measured creep development,and the accuracy is good.（2）Compared with other specifications,the superposition method calculation theory supported by the CEB-FIP2010 model has higher prediction accuracy for the creep recovery stage.The CEB-FIP2010 model overestimates the creep rate in the short term after loading and underestimates the creep development rate in the later period.The design specification for concrete structures of railway bridges and culverts has a large deviation in the creep prediction of C30 concrete in this test.（3）Theoretical application content（1）Under the annual periodic temperature load condition of the CRTS II slab ballastless track on the Beijing-Tianjin intercity railway bridge,the maximum compressive stress of the base plate decreases year by year due to the influence of the creep effect.In the 14 th year（current year）since the locking temperature,the maximum compressive stress of the base plate when the creep effect is considered is 57.22% of that without the creep effect,and it is 28.27% in the 60 th year.（2）Due to the influence of the creep effect,the duration of the crack opening state of the base plate increases year by year.In the first year,the base plate cracked for the first time,and the duration of crack opening was shorter when the creep effect was considered than when not considered.From the second year to the 60 th year,the base plate was repeatedly cracked,and the crack opening duration was longer when the creep effect was considered than when not considered.In the 60 th year,the duration of crack opening was 223 days without considering the creep effect,and 290 days when the creep effect was considered.Innovation:（1）In the calculation of the creep effect of the reinforced concrete structure,steel bar correction and temperature correction are carried out,and the creep test under variable force load is designed to verify the rationality of the theoretical calculation method;（2）The creep calculation method of the reinforced concrete structure is applied to the force calculation of the base plate structure of the CRTS II slab ballastless track,and the influence of the creep effect of the base plate under the action of the annual periodic temperature load is studied.