Research On Temperature Cracking Mechanism And Control Of Main Structure Concrete Of Subways Tation

Nowadays,the construction of urban rail transit is developing at a high speed.As a crucial link in the process of urban rail construction,the subway station often faces the problem of concrete structure cracking and water seepage.The main structure concrete of subway station has a high degree of constraint,and the temperature effect and shrinkage in the construction process often lead to greater constraint stress,which leads to the cracking of the station structure.For the cracking problem,the relevant researches are not systematic and in-depth enough,especially the researches on the cracking mechanism of the main structure of subway station in its age under the action of multi-field coupling are relatively few.Therefore,based on the subway station project of Chengdu Metro Line 19 Huaxing Street Station as the research background,this paper carried out an in-depth study on the temperature cracking mechanism and prevention measures of the main structure concrete of metro station by using test,theory and finite element analysis methods.The main research work and achievements are as follows:(1)Using the numerical simulation method and the subprogram secondary development based on the concrete shrinkage prediction model,the shrinkage process of subway station in its age is studied.The results show that,under the action of concrete shrinkage,the more external constraints the structure is subjected to,the more restricted the shrinkage deformation of concrete,and the greater the restrained tensile stress will be generated at the structural connection site or the junction of old and new concrete.In the process of shrinkage,the station floor is constrained by the external constraints and the internal reinforcement,which generally produces tensile stress.The cracking risk of the floor surface is low under the action of single shrinkage.The side wall may crack at the boundary between old and new concrete because of the strip main tensile stress zone at the bottom and the constrained side,with the maximum principal tensile stress reaching 2.7MPa.The roof of the station is subject to fewer external constraints,and the distribution of the shrinkage stress of the roof is uniform and small,which is not enough to cause cracking.However,it is necessary to pay attention to the influence of the shrinkage stress at the joint part of the structure,which is prone to stress concentration.(2)The numerical simulation method and the secondary development of the subroutine based on the concrete hydration heat release model were used to study the heat release process of concrete,and the optimization design of semi-adiabatic temperature rise test was carried out.The semi-adiabatic temperature rise test was carried out on several concrete test blocks with multi-mineral admixtures.The test results showed that:Instead of the same content of cement,the concrete mixed with two kinds of mineral admixtures,fly ash and limestone powder,has the best cooling effect,which is recommended to be used in engineering.The temperature rise properties of four common mineral admixtures are compared as follows:The comparison of thermal generation characteristics of four common mineral admixtures is as follows:①Total cooling capacity:limestone powder>fly ash>slag>metakaolin;②early heat generation rate:metakaolin>slag>fly ash>limestone powder;③The heat reduction rate in the later period was metakaolin>limestone powder>fly ash>slag.Finally,several composite exponential heat generation models of multi-mineral admixture concrete are obtained through simulation analysis.(3)The multi-field coupling numerical calculation method is used to study the damage characteristics of the main structure of subway station,predict the generation and development law of cracks,and reveal the influence of different environmental factors on the cracking of mass concrete.The results show that the maximum temperature difference between the inside and the surface of the station floor is 23℃ in the early age stage(concrete heating stage),and the self-restrained stress of concrete caused by the temperature difference is the main reason for the cracks in the early stage.The cracks first appear in the position of the upper beam with larger thickness and are longitudinal cracks.The maximum cooling range of station floor in the late age(concrete cooling section)is 3 2℃,and the external constraint stress of concrete caused by this cooling effect is the main reason for the crack in the late stage,and the crack will appear along the lateral direction of the floor.Under the same conditions,the lower the ambient temperature is,the more cracks appear in the surface layer of floor.The lower the mold temperature is,the less cracks appear on the surface of the bottom plate.The greater the ambient wind speed,the more cracks appear on the surface of the floor.(4)The effect of crack prevention measures in the whole construction process of subway station is analyzed and verified by numerical calculation and analysis method,and the effective anti-crack measures and defects of subway station structure are expounded.The best anti-crack measures are using high performance and low hydration hot concrete,controlling aggregate gradation and embedding cooling pipe.The final analysis shows that:it is necessary to combine the actual situation of the project,in the construction design,structural maintenance,structural forming three stages of comprehensive use of a variety of prevention and control measures to achieve the advantages of the complementary measures,in order to reduce the metro station concrete cracks to the maximum benefit.