Study On Stochastic Constitutive Model Of Concrete Under Uniaxial Compression

Constitutive model of concrete under uniaxial compression is the key problem and the foundation to carry out nonlinear mechanical analysis of reinforced concrete structures.There are significant differences between the key performance parameters such as peak stress,strain at peak stress and ultimate strain of confined concrete from that of unconfined concrete due to the confinement effect of the stirrup.Meanwhile,due to the fact that the concrete is a classic heterogeneous material,there are some initial defects in concrete such as micro holes,micro cracks and so on,which makes the key performance parameters exhibit significant randomness.In addition,due to the influence of concrete carbonation and chloride ion attack and other environmental actions,the stirrups in reinforced concrete structure often corrodes,which will reduce the confinement effect on the core concrete and promote the development of cracks in concrete.As a result,corrosion of stirrup often affects the mechanical behavior of confined concrete and the constitutive model of confined concrete under uniaxial compression.Therefore,development of stochastic constitutive models for unconfined concrete,confined concrete and corroded confined concrete is important for nonlinear mechanical analysis and probabilistic bearing capacity design of reinforced concrete structure.In view of this,this thesis focus on the stochastic constitutive models of unconfined concrete,confined concrete and corroded confined concrete under uniaxial compression.The main contents of this thesis are as follows:(1)Based on the experimental data of uniaxial compression test of 440 groups of unconfined concrete specimens,the probabilistic prediction models of peak stress,strain at peak stress and elastic modulus of unconfined concrete were developed respectively,by combining the Bayesian theory with the Markov Chain Monte Carlo(MCMC)method.Then the stochastic constitutive model of unconfined concrete under uniaxial compression was proposed.The accuracy and applicability of proposed models are verified by comparing with the experimental data and traditional deterministic models.The analysis results show that the proposed model not only can describe the stochastic characteristic of the constitutive relation of unconfined concrete under uniaxial compression,but also can calibrate the confidence level of the traditional deterministic constitutive model,since it takes into account the influences of aleatory uncertainty due to the material parameters,environmental conditions,test methods and other aspects,and the influence of epistemic uncertainty due to the limited knowledge.(2)Based on the experimental data of uniaxial compression test of 278 groups of confined concrete specimens,the probabilistic prediction models of peak stress and strain at peak stress of confined concrete were established respectively,by combining the Bayesian theory with the MCMC method.Then the stochastic constitutive model of confined concrete under uniaxial compression was proposed.The accuracy and applicability of proposed model were verified by comparing with the experimental data and the traditional deterministic model.The analysis results show that there is no obvious randomness in the rising stress-strain curve of concrete under uniaxial compression.However,after reaching the peak stress(in descending section),the interval of the stochastic constitutive relation model becomes wider,which indicates that the randommess of stress-strain curve increases.(3)Based on the experimental data of uniaxial compression test of 36 sets of corroded stirrups confined concrete specimens(12 sets of rectangular hoops,12 sets of diamond hoops and 12 sets of spiral hoops),the influences of steel corrosion on the peak stress,strain at peak stress and ultimate strain of corroded stirrups confined concrete were analyzed.Then the constitutive model of corroded confined concrete under uniaxial compression was established.Finally,the stochastic constitutive model of corroded confined concrete under uniaxial compression was established considering the influence of random factors.Analysis results show that corrosion mass loss rate of stirrup and volumetric stirrup ratio are important factors that affect the stress-strain Iaw of corroded confined concrete.With the increase of the corrosion mass loss rate of the stirrup,the strength and ductility of corroded confined concrete decrease sharply,and the slope of the stress-strain curve after the peak stress will be steeper.The effect of corrosion mass loss rate of stirrup on strain at peak stress and ultimate strain is greater than that of peak stress.