The Analysisi Of Damage And The Research Of Bearing Capacity Of Reinforced Concrete Flexural Members Under The Environment Of Freezing And Thawing

Concrete structure durability has became the hot research field of civil engineering, the safety and the life of building had seriously threated, the freeze-thaw durability of concrete was the most typical. In order to have an accurate assessment of durability life of concrete under freezing and thawing, therefore the deterioration of mechanical properties of concrete structure under freezing-thawing environment was particularly important. For now, the research on performance of concrete after feezing and thawing focus on level of concrete material properties, and test methods focus on rapid freezing and thawing, but the method distinguished to subject to freeze-thaw of the atmosphere of concrete structure. This difference often lead to deviation of the prediction of unti-frozen life of concrete structure. In this paper, the combination of theoretical analysis and environment simulation test method was adopted, the damage constitutive relation of concrete under freezing and thawing, the deterioration of bearing capacity of full-scale reinforced concrete beams and the calculation model of bearing capacity were researched by the method of air-freezing and air-thawing. The main contents in this paper are as follows:(1) Combination of damage and thermdynamics theory, the fracture strain of concrete was defined to damage variable, considering plastic deformation and damage thresholid of concrete stochastic damage constitutive model was proposed. To determine the probability distribution of damage variabiles and relative parameters, 4 groups concrete test specimen of ordinary concrete(31), steel fiber concrete(38), polypropylene fiber concrete(11) and shotcrete(32) were made, and carried on the tests of the uniaxial compressive stress-strain. The relative model parameters were obtained by regression analysis of the experimental results, and the statistical properties of the model parameters and the range of damage threshold were studied. According to elastic stability theory, the relationship of the concrete fracture strain and peak strain was discussed. On this basis, the probability distribution of damage variables was determined by the analysis statistic characteristic of the 4 groups of specimen peak strain.(2) Through the experiment, the constitutive relationship and the performance properties of concrete after air-freezing and air-thawing were studied, the test result was analyzed, the change law of concrete about mass loss, dynamic elastic modulus, compressive strength and the stress-strain curve were obtained. Moreover, the damage constitutive model under air-freezing and air-thawing based on the microscopic statistical damage theory was established, and the effect of air-freezing and air-thawing on the parameters of the model was studied. The deterioration of concrete caused by freezing-thawing is a process from outside to inside, futher consider this point, the section was count two material(ie, using different stress-strain relationship), the capacity calculation model that was made up of damage layer and undamaged layer was established. Combined with the test results, the applicability of the two models was discussed, the results shown that the hierarchical model was more realistic.(3) The mechanism of temperature variation was established by the investigation. Seven full-scale RC beams were produced and their the variation law of mechanical properties under air-freezing and air-thawing was studied. The relative dynamic elastic modulus and the thickness of the damaged layer of RC beams were test by ultrasonic detection method, and the effects of moisture and the test result was analyzed. The relationship that the relative dynamic elastic modulus, the thickness damage layer between the freeze-thaw cycles. Finally, the deterioration model of concrete compressive strength under freezing and thawing was established.(4) The RC beams that suffer to freeze-thaw was carried on the bending test, and the deterioration law and the law of deflection variation was analyzed. The stratified bearing capacity model was present by the damage characterized under freeze-thaw that from surface to internal, and the applicability was discussed.(5) A one-side non-destructive testing method was used to detect the damage layer thickness of concrete subjected to freeze-thaw. The stochastic prediction model of damage layer was established, and the model was used to prediction the life of the frost durability of concerete structure, as well as the model provided reference for the life prediction design of frost durability of concrete structure in cold evrionment.