Study On Effect And Calculation Method Of Concrete Creep Based On Variable Environmental Factors And Decreasing Stress

Nowadays,with the rapid development of our transportation industry,prestressed concrete box girder bridges are widely used due to their advantages of reasonable mechanical performance,mature construction technology and low cost.However,it is found that the cracking and the continuous mid-span deflection of the main girders are prevalent in the operation stage of existing long-span prestressed concrete box girder bridges.Concrete creep is one of the important causes of this phenomenon.On the basis of the test data of standard prisms,typical creep prediction models establish the formulation by reasonable hypothesis and take the average value of relative humidity and temperature in the structural design to calculate the concrete creep effect,ignoring the influence of changes in environmental factors.At the same time,stress redistribution occurs in the cross section due to the existence of reinforcement,i.e.concrete stress gradually decreases,which can not be ignored on the concrete creep effect.Therefore,this paper further studies the creep effect and calculation method of concrete structures by combining variable environmental factors and decreasing stress in the base of now available achievements.Firstly,the mechanism and influencing factors of concrete creep are summarized,and several typical creep prediction models are compared.Recommendations are given for the selection of concrete creep models from the perspectives of factors,expression forms and prediction accuracy.The applicability of the classical creep calculation theory under complex stress conditions is also analyzed.Secondly,the JTG3362-2018 creep model suggested in current highway bridge specification is modified in terms of temperature.Based on Boltzman’s superposition principle,the calculation method of concrete creep coefficient under variable environmental factors is established.Then,according to the three basic equations of reinforced concrete members,the stress redistribution law is figured out.The equivalent creep coefficient of reinforced concrete members is obtained by using the two-function method,in which the selected creep recovery model and the creep coefficient under variable environmental factors are combined.Finally,the creep effects of axially compression members and long-span prestressed concrete box girder bridges are calculated,respectively.The calculation of creep effect of axially compression members is divided into three levels: the single-factor sensitivity analysis,the creep effect analysis under variable environmental factors and the comprehensive analysis of the creep effect of reinforced concrete members.And,the time-varying effect of the deflection and stress is predicted by using the finite element analysis program.By comparison,the difference between the JTG3362-2018 creep model and the equivalent creep coefficient calculation method proposed in this paper is obtained,which illustrates the necessity of considering the change of environmental factors and the stress redistribution of reinforced concrete in the analysis of structural creep effect.The conclusions achieved in this paper further enriched the calculation method of concrete creep and layed a theoretical foundation for the more accurate analysis of the cracking and continuous deflection in the operation of prestressed concrete box girder bridges.It also provided reference for promoting the safety assessment and reinforcement of similar bridges.