Analysis Of Mechanical Properties Of Concrete Box Girder After Environmental Degradation

The concrete box girder has a large torsional rigidity,has a good overall space performance,and can meet the needs of continuous beam structures and various construction methods.It is widely used in modern bridge construction.Concrete box girder in service,the deterioration of concrete due to environmental factors,resulting in degradation of structural performance and shortened service life often occur.At home and abroad,the research on the mechanical properties of reinforced concrete beams subjected to environmental degradation and deterioration is mainly based on solid cross-section concrete beams,and there are few studies on the mechanical properties of concrete box beams with spatial structure after environmental degradation and deterioration.Therefore,in this paper,the carbonization and chloride ion erosion of concrete box beams under different levels of load were studied,and the influence of different stress states on the concrete carbonization depth and chloride ion diffusion coefficient was revealed.Based on the amount of steel corrosion,the earliest time for the cracking of the protective layer of the concrete box girder due to the corrosion of the ordinary steel bar and the calculation method of the bearing capacity of the prestressed concrete box girder with corroded steel bars are derived.The main research results of the mixed paper are as follows:?1?Using the rapid carbonization test value of concrete box girder and the actual measured carbonation value of the concrete box girder bridge in service,the differences between the existing calculation models of concrete carbonation depth are analyzed,and the fitting results with the measured values show that the Niu Ditao carbonation model is more accurate,Closer to the measured value.The admixture substitution coefficient k_F was introduced into the Niu Ditao carbonization model,and its accuracy was verified with the help of the rapid carbonization test.By comparing and analyzing the rapid carbonization test value of box girder under different bending loads and the calculated value of each stress influence coefficient model,it is concluded that the carbonation depth of concrete increases with the increase of tensile stress and decreases first with the increase of compressive stress It increases after small;under the bending load,the value of the stress influence coefficient of the carbonation depth of the concrete box girder is recommended to refer to the Liu Jie model.?2?By carrying out the chloride ion immersion erosion test of the concrete box girder,it is concluded that the two-dimensional diffusion coefficient of the chloride ion is 1.134 times that of the one-dimensional diffusion coefficient when no external load is applied to the test box girder,and the tensile stress at the bottom plate is 0.4f_tk and Under the bending load of0.8f_tk,the two-dimensional diffusion coefficient of chloride ion is 1.492 and 1.503 times that of the one-dimensional diffusion coefficient.Under the bending load of the test box beam,the chloride ion diffusion coefficient of the top concrete has decreased,and the chloride ion diffusion coefficient of the bottom concrete has increased.The compressive stress can slow down the diffusion rate of chloride ions in the concrete.When the load level is small,the slow The effect is significant.When the load level is large,the mitigation effect is weakened;the tensile stress can accelerate the diffusion rate of chloride ions in the concrete,and as the load level increases,the acceleration effect is also more obvious.At the same time,due to the shear lag effect of the box beam,the chloride diffusion coefficients at different structural positions of the roof are also different.?3?Based on the assumption that the steel bars are uniformly rusted,through analysis of the rust expansion cracking process of the concrete protective layer caused by the corrosion of ordinary steel bars,the rust products entering the initial pores and rust expansion micro cracks during the rust expansion cracking process of the concrete protection layer are considered According to the actual situation,the elasticity theory is used to calculate the rust expansion force before rust expansion cracking and at the time of cracking,and the calculation formula of the steel corrosion depth of the concrete protective layer during rust expansion cracking is deduced and verified,which is in good agreement and has high accuracy.The calculation model of the earliest rust expansion cracking time of the concrete protective layer was established by Faraday's law,and optimized with the characteristics of accelerated corrosion and natural corrosion.The test results of accelerated corrosion and natural corrosion are compared with the theoretical values of the calculation model.The calculated values are in good agreement with the test values,and the errors are within±10%.?4?Based on the calculation model of the bearing capacity of corroded reinforced concrete solid beams,taking into account the effective cross-sectional area and strength of corroded reinforcement and the weakening of the effective area of concrete,combined with the current bridge codes,the bearing capacity calculations of prestressed concrete box beams after deterioration are established respectively Method and crack calculation formula,and verify the engineering example of the calculation method of the deteriorating box beam bearing capacity.It is concluded that the calculation formula for the bending capacity of the degraded box girder's normal section is relatively high,and the calculation formula for the shear capacity of the inclined section is conservative and slightly different.Combined with the current bridge codes,formulas for calculating cracks after the deterioration of various prestressed concrete box beams have been established.