Multi-dimensional Degradation Mechanism And Durability Design Method Of Concrete Box Girder Considering Load & Carbonation & Chloride Ion Erosion

Concrete box girder can be considered as a spatial structure composed of top plate,bottom plate and web.Therefore,it not only has spatial multidimensionality in terms of force,but also exhibits spatial multidimensionality when it is affected by environmental effects such as carbonation and chloride ion attack.In this paper,the concrete box girder with the largest proportion among high-speed railways,highways and municipal bridges is taken as the object.Aiming at the environmental impact factors that are most likely to occur in the general environment,such as concrete carbonation and chloride ion attack,through theoretical analysis,model test and numerical simulation,the multi-dimensional degradation mechanism and durability of the box beam are studied.The main research results are as follows:(1)Aiming at the shear lag effect of bending stress caused by the thin-wall effect of the box beam,a beam strip method for quickly calculating the spatial bending stress of the box beam is proposed based on the beam element theory.The comparative analysis results based on experiments and numerical solutions show that the longitudinal normal stress and vertical deflection difference rate of the beam strip method is generally within 10%.The beam strip results are relatively safe and can be used to calculate the spatial bending stress of the box beam.(2)Through the indoor deterioration test of the concrete box girder scale model,the spatial multidimensional deterioration characteristics of the concrete box girder under the combined action of load,carbonation and chloride ion and the mutual influence of various factors were studied.The results show that the one-dimensional and two-dimensional carbonation depths of the top and bottom of the concrete box girder are respectively 57.4% and 27.1% under the unstressed state,and the carbonation of the box girder has spatial multidimensionality.Compression stress plays a role in inhibiting the diffusion of carbonation and chloride ions.After the chloride ion erosion,the maximum reduction depths of the average carbonation depth of the box girder top,bottom plate and web are 3.1%,9.7% and 1.5% respectively.Chloride ion can inhibit the carbonation of the box beam,but the effect is not obvious.After being eroded by carbon dioxide,the maximum increase of chloride ion content at the test points of the test concrete box beam top,bottom plate and web is 28.7%,41.2% and 6.8%,respectively.Carbonation can obviously promote the corrosion of chloride ion on the concrete box beam.The chloride ion concentration distribution on the top and bottom of the concrete box girder under the load is the same as the law of shear lag effect,and the chloride ion erosion on the concrete box girder also has spatial multidimensionality.(3)Based on the measured data of carbonation under various exposure conditions in 1834 groups at home and abroad,the regression of the calculation formula of concrete carbonation life is carried out using multivariate nonlinear statistics.Combined with the spatial structural characteristics of the box girder,the regression model was modified to obtain a carbonation life calculation model for the bent concrete box girder.By introducing the carbonation contribution coefficient,the concrete influence of concrete and environmental factors on carbonation is quantitatively analyzed.The comparison and analysis results of the same experiment show that the formula prediction difference of each measuring point under different working conditions is generally about 1mm.Based on the diffusion limited aggregation(DLA)model,a concrete carbonation prediction model was established.The results of comparison and analysis with the predicted value of the formula show that the carbonation simulation results of the DLA model before and after considering the tensile and compressive stresses are in good agreement with the overall solution.This method can better simulate the randomness and dispersion of carbon dioxide diffusion in concrete.(4)A chloride ion concentration prediction formula that can consider the coupling effect of load,carbonation,and chloride ions is proposed.The comparative analysis results based on the experiment show that the error of the solution of the formula under different exposure conditions is within 15%,which can be used for the prediction and analysis of the multi-dimensional deterioration of the concrete box beam.(5)Aiming at the characteristics of spatial multidimensional deterioration of concrete box beams subjected to carbonation,chloride ion erosion and load,a partition design method for durability of concrete box beams is proposed,and formulas for partition carbonation and chloride ion corrosion design are recommended.Taking the thickness of the protective layer as the main parameter in the durability design of concrete structures as an object,the process of durability partition design of concrete box beams is proposed.Based on the reliability theory,taking the railway standard 24 m,32m,40 m and 56 m span concrete simply supported box girder as an example,the durability evaluation method was carried out using the zoning design method of durability.The results show that the thickness of the protective layer of the same section of the railway standard concrete box girder is designed according to the zone,which can effectively improve the durability and life.For each 5mm increase in the thickness of the protective layer,the initial rust time of the outermost prestressed steel bar corresponding to the box girder will be extended by at least 6 years,up to 20 years.In view of the characteristics that the bottom plate and the web are greatly affected by the tensile stress and the prestressed steel bars are dense,it is recommended to set the protective layer thickness at 65 mm for the "more areas" of the prestressed steel bars and 50 mm for the "less areas".