Research On Longitudinal Cracking Mechanism Of Highway Prestressed Small Box Girder

The longitudinal cracking problem of highway prestressed concrete small box girder is common.The longitudinal crack will seriously affect the durability of the structure and even reduce the bearing capacity of the bridge structure.Based on the operating environment and actual stress conditions of prestressed small box girder Bridges,this paper analyzes the local stress conditions of small box girder under the effects of prestressed effect,uneven temperature,uneven humidity and other factors,and studies the influence mechanism of the local stress on the longitudinal cracking of the beam body.The details are as follows:(1)According to the action mechanism of post-tensioning prestressing on the small box girder body,the finite element software ANSYS based on the equivalent load method is used to simulate and analyze the vertical stress of the small box girder body caused by three kinds of loads: end anchorage force,radial force of curve section and deflection force of pipeline during the tensioning process of prestressed steel beam.The results show that the anchorage force at the end produces a large vertical tensile stress in the local bearing area of concrete near the beam end.The vertical tensile stress caused by the radial force of the curve section is small,and its influence on the longitudinal cracking can be ignored.The vertical tensile stress of web caused by local pipeline deviation is large,especially the lateral deviation causes the outer bending deformation of web surface,which leads to the larger tensile stress on the web surface,which is the main reason for the longitudinal cracking of web along the pipeline surface.(2)The internal temperature distribution curve of concrete blocks is obtained through finite element simulation analysis,and the calculation method of concrete thermal conductivity is proposed by inverting the internal temperature curve of concrete blocks.The thermal conductivity of concrete blocks is obtained according to the measured internal temperature curve of concrete blocks.The finite element software ANSYS temperature analysis module is used to simulate the local temperature effect of small box girder,and the temperature distribution law of small box girder section under adverse temperature is obtained,which is similar to the temperature gradient mode of box girder plate thickness specified in the code of China Railway bridge and culvert.The calculation formula of temperature gradient of small box girder along plate thickness direction under adverse temperature is proposed.The calculation shows that the adverse heating effect has little effect on the stress level of small box girder structure,and the adverse cooling effect produces large tensile stress on the outer surface of small box girder structure,which should be paid more attention to.(3)According to the similarity of the calculation equation of concrete temperature and humidity and the boundary conditions,the finite element software ANSYS temperature analysis module is used to simulate the local humidity distribution of small box girder and its effect,and the humidity distribution law of small box girder section under the action of adverse environmental humidity is obtained.The change of environmental humidity will lead to the formation of humidity gradient in the shallow depth of the surface of small box girder section.The formula for calculating humidity gradient in thickness direction of small box girder plate is derived according to humidity distribution law.The results show that the positive humidity gradient caused by the increase of ambient humidity leads to greater tensile stress at a certain depth on the outer surface of the small box girder,which may lead to cracking inside the box girder.The negative humidity gradient caused by the decrease of ambient humidity will produce greater tensile stress on the outer surface of the small box girder,which will lead to the longitudinal cracking of the surface of the small box girder.(4)Through the research and analysis of the possible causes of the longitudinal cracking of the prestressed small box girder,the corresponding prevention and control measures for the control of the longitudinal cracks of the prestressed small box girder are proposed from the three aspects of the prestressed effect,temperature effect and humidity effect,and the repair methods of the longitudinal cracks at the present stage are introduced.