Research On The Effects Of Environmental Temperature On The Elastic Modulus Of Concrete And Dynamic Properties Of Concrete Beam Bridges

Bridges are the strategic passage of transportation,which play a vital important role in promoting the economic development and cultural communication among different regions.To make sure the safety of bridges,it is crucial to carry out regular inspection or health monitoring.Among those,the vibration based damage identification method is the most effective method for structural performance evaluation.However,except damage,the natural frequencies of bridges are also affected by environmental temperature.The effects of environmental temperature are inevitable,which have nothing to do with the health of the structure.However,the temperature effects cover the damage condition of bridges,which reduces the accuracy and reliability of the vibration based damage identification method.In order to apply that method into practice effectively,this dissertation studies the effects of environmental temperature on concrete properties and beam bridges' dynamic properties from three levels,material,component and structure.The main works are as follows:(1)Based on C30 and C50 concrete specimens' elastic moduli tests under different temperatures and ages,the effects of environmental temperature and age on the elastic modulus of concrete are studied.A temperature correction coefficient of elastic modulus is proposed,which can calculate the elastic modulus at a certain age and an arbitrary temperature between-80?C and 70?C according to the elastic modulus at the same age and a temperature of 20?C.Then,considering the effects of both temperature and age on the elastic modulus,the expressions of the elastic modulus of C30 and C50 concrete at arbitrary age and temperature are proposed.(2)Start with the microcosmic components of concrete-pores and microcracks,the effects of temperature on the shear modulus are considered in the expression of elastic modulus of the composite material and the effects of temperature on the viscous force of microcracks containing water were considered in the frictional-sliding of microcracks under axial compression load.It is found that the effect of temperature on elastic modulus of concrete is due to the change of viscous force of water in the pores and microcracks.(3)Based on the dynamic tests of concrete beams at 5 different ages,the relationship between the beam's inner temperature and environmental temperature as well as the relationship between natural frequency or damping ratio and environmental temperature are analyzed.A temperature correction coefficient of frequencies is proposed,which can calculate the natural frequency at a certain age and arbitrary temperature according to the frequency at the same age and a temperature of 20?C.Combined with the elastic moduli at different temperatures obtained in the former tests,the relationship between the effects of temperature on elastic modulus of concrete and natural frequency on concrete beams are studied.(4)Based on the former study,the effects of changing boundary conditions owing to environmental temperature on concrete beam bridges' dynamic properties are studies.Two different beam bridges including simply supported beam bridge and continuous rigid frame bridge are selected to do the continuous 24 hours' dynamic tests.The continuous tests can eliminate the effects of damage and material properties' degradation on bridges' dynamic properties.According to the effects of temperature on the elastic modulus,the effects of environmental temperature on the frequencies of the two beam bridges are analyzed by finite element analysis.By comparing with the test results,the correlation between the effects of environmental temperature on concrete beam bridges' natural frequency and the bridges' structural style is revealed.