Study On The Mechanical Properties Of Multi-scale Polypropylene Fiber Concrete After High Temperature

Concrete under fire and high temperature conditions has deteriorated performance and reduced bearing capacity.When the structure is damaged,it is prone to hightemperature bursting,causing secondary casualties.Adding polypropylene fibers to the concrete,the microchannels generated after the fibers melt at high temperature can reduce the risk of concrete bursting.The preliminary research results of the research group show that polypropylene fiber has better crack resistance,toughening and strengthening effects.The different proportions,dosages,and curing conditions of polypropylene fibers have a great influence on the performance of concrete.Multi-scale polypropylene fibers have the most significant effect on improving the mechanical properties of concrete at room temperature.The mechanical properties and anti-explosion properties of multi-scale polypropylene fiber concrete at high temperature are for further study.In this paper,through the combination of laboratory experiments and theoretical analysis,the mechanical properties,bursting performance,and high-temperature damage mechanism of three scales of PPF dosage and mixing ratio on concrete after high temperature(20℃,200℃,400℃,600℃,800℃)are studied.The main research content and results are as follows:(1)Aiming at three sizes of polypropylene fiber concrete specimens,,high temperature tests were carried out.The cube compressive strength and split tensile strength of the specimens after high temperature were tested.The results show that the appearance of concrete changes differently after high temperature.As the temperature increases,the compressive strength of the specimens tends to rise first and then fall.The splitting tensile strength of the specimen decreases with increasing temperature.The ternary blended multi-scale PFRC residual compressive strength and splitting tensile strength after being subjected to high temperature are the largest.(2)From the high temperature test,it is found that the mass loss rate of the test piece increases with increasing temperature,and the mass loss of the PFRC test piece is greater than that of plain concrete.Based on regression analysis,the relationship between the cube compressive strength,splitting tensile strength,mass loss rate and temperature is obtained.(3)Through the acoustic emission technology monitoring polypropylene fiber concrete cube compression test,studies show that the relationship between the counting rate of acoustic emission energy and time of concrete after high temperature can be divided into four stages: compaction,linear elasticity,plasticity and failure.The energy count rate distribution of multi-scale PFRC is wider than that of reference concrete,while the peak energy count rate is relatively small,and multi-scale polypropylene fiber improves the ductility of concrete after high temperature.(4)Through the moisture content test,the saturated water method to determine the porosity test,and the scanning electron microscope test,the PFRC high temperature burst mechanism was studied.The results show that: polypropylene fiber reduces the hightemperature burst of concrete,and the mixing effect of coarse and fine polypropylene fibers is more obvious;the melting of polypropylene fiber increases the permeability of concrete after high temperature.Under the same dosage and the same temperature,the porosity of the specimen mixed with polypropylene coarse and fine fibers is lower than that of the specimen mixed with coarse fibers alone.After the melting of the coarse and fine PPF,a large number of connecting spaces are left in the concrete to provide a channel for the release of steam pressure.As the temperature increases,the microstructure damage is more serious,the cement slurry gradually dehydrates,the structure becomes loose,and cracks increase.(5)The prediction model of compressive strength and splitting tensile strength of PFRC after high temperature is established through support vector machine regression theory and artificial neural network theory.It can be found from the research that the prediction model of support vector machine based on Gaussian radial basis kernel function has high accuracy,good generalization ability and stability,and can accurately predict the strength of PFRC after high temperature.