Study On Mechanical Properties Of Fiber Reinforced Concrete In Cold Regions And Its Application In Metro Station Engineering

In cold regions such as the northeast and northwest of China,due to the seasonal periodic changes in temperature,concrete structures will undergo a periodic cycle of freezing and thawing,which is prone to cause structural freeze-thaw damage.As the construction scale of urban underground space structures in Northeast China increases,the demand and requirements for concrete materials are gradually increasing.At the same time,how to improve the application performance of concrete materials in underground engineering has become a hot research topic for scholars at home and abroad.Aiming at the mechanical properties of concrete and fiber reinforced concrete in cold regions and their application in subway station structures,this paper conducts a study on the mechanical properties of fiber reinforced concrete in cold regions and its application in subway station engineering by comprehensively using literature search and analysis,indoor macro mechanical tests,microstructure tests,and numerical calculation analysis methods.The main research work and achievements are as follows:(1)Taking into account factors such as the strength grade of concrete,the number of freezethaw cycles,different fiber materials,and the freezing conditions of the test block,the failure morphology and compressive strength characteristics of concrete specimens under different freeze-thaw cycles,different strength grades,and different fiber materials were studied using ultra-low temperature freeze-thaw cycles and low temperature compressive mechanics test methods.The results show that the mechanical properties of concrete subjected to freezing and thawing cycles deteriorate;The compressive strength and elastic modulus of concrete gradually decrease with the increase of the number of freeze-thaw cycles,while the peak strain has an opposite change rule;Concrete mixed with fiber exhibits a certain degree of frost resistance,and the compressive strength of fiber reinforced concrete varies slightly with the number of freeze-thaw cycles;The frost resistance of concrete mixed with polyvinyl alcohol fiber and polypropylene fiber is stronger than that of concrete mixed with polypropylene fiber and polyvinyl alcohol fiber alone,followed by concrete mixed with basalt fiber and polypropylene fiber,and the compressive strength of concrete mixed with basalt fiber is the weakest.(2)Taking into account factors such as the strength grade of concrete,the number of freezethaw cycles,different fiber materials,and the freezing conditions of the test block,the microstructure and mechanical engagement of different fiber reinforced concrete were studied using scanning electron microscopy(SEM).The results show that fibers play an anti crack role in concrete by bonding with cement.In terms of the mechanical engagement between the three fibers and the paste,polypropylene fibers are the best,polyethylene fibers are the second,and basalt fibers are the worst;After adding fibers to concrete,it can well connect concrete aggregates and aggregates,as well as aggregate and cementitious materials.Concrete aggregates tightly encapsulate this fiber,and the two work together to improve and enhance its mechanical properties,reducing the number and scale of original micro cracks,delaying the occurrence of macro cracks,and enhancing the mechanical properties and durability of concrete.(3)Based on the ABAQUS large-scale finite element calculation program,the numerical calculation method of fiber concrete materials in severe cold regions was established;a numerical calculation model for damage of fiber reinforced concrete under freeze-thaw cycles based on Weibull distribution proposed by Zhang Guangtai and others was established using Python language secondary development;Using this method,the compressive and tensile properties of concrete specimens under different freeze-thaw cycles,different strength grades,and different fiber materials were studied.The results show that adding fiber materials to concrete can improve its compressive and tensile properties,enhance its toughness,and thereby enhance the frost resistance of concrete materials;Fiber materials can effectively improve the mechanical properties of concrete materials in severe cold and low temperature environments,with polypropylene fibers having stronger compressive strength than polyvinyl alcohol fibers,and basalt fibers having the worst frost resistance;According to the compressive strength and tensile strength of fiber reinforced concrete,the optimal mixing amount of different fiber materials is proposed.(4)Aiming at the mechanical properties of subway station concrete in typical cold regions,the effects and functions of fiber materials on enhancing the mechanical strength of subway station structures and reducing freeze thaw damage in cold and low temperature environments were studied.The results show that after adding fiber materials,the maximum principal stress and deformation of each part of the subway station structure are significantly reduced;It can be seen that fiber materials can improve the mechanical properties of concrete and enhance the stability of underground engineering structures in cold regions.