Research On Mechanical Properties Of High Strength Concrete With Polypropylene Fiber Subjected To High Temperature

High-strength concrete, with its advantages of high compressive strength, high resistance to deformation and low porosity, has been widely used in the high-rise building structures, large span bridges and some special structures, and has made significant technical and economic benefits. However, the deficiencies of poor toughness and poor performance exist. Mixing polypropylene fiber is one of the main means to enhance the toughness of high strength concrete. With the increasingly widespread application of high strength concrete and building fires becoming more frequent, researching the performance of high-strength concrete with polypropylene fiber subjected to high temperature is very important. This thesis tries to make a systematic study of high-strength concrete with polypropylene fiber on the aspects of compression, tensile strength and ultrasonic nondestructive before and after high temperature. It mainly includes the following elements:(1) Through the experiments of concrete slump and compressive strength, splitting tensile strength at room temperature, this thesis researches the influence given by polypropylene fiber length and content on high-strength concrete performance and mechanical properties. The results show that:incorporation of polypropylene fibers significantly reduced the performance of high strength concrete, with the increasing of fiber content, the slump value decreases; the longer the fiber, the less concrete slump value; mixed with polypropylene fibers, high-strength concrete compressive strength generally decreases, but when the dosage is 1.5kg/m3, the compressive strength has been improved within the range of±10%, and under the same fiber dosage, the smaller the fiber length, the higher the compressive strength; two kinds of polypropylene fiber make different impact to the splitting tensile strength:mixing 8mm fiber increases the splitting tensile strength of high strength concrete, but 19mm fiber has led to the splitting tensile strength decreased.(2) For high-temperature test of high-strength concrete with polypropylene fiber, analysis of the appearance and mass loss of concrete after high temperature is made. High-strength concrete subjected to different temperatures, its appearance changes in morphology occurred in varying degrees, under which can initially determine the maximum temperature of high-strength concrete subjected to. With increasing temperature, high strength concrete mass loss gradually increased. subjected to 300℃,500℃,700℃,900℃, the average loss rate was 5.94%,7.20%,9.21%,14.17%.(3) In-depth study of the residual attenuation of residual compressive strength and splitting tensile strength of high-strength concrete with polypropylene fiber as well as fiber length, content, and cooling methods' impact is extended, on this basis, the relationship between temperature and relative residual compressive strength & relative residual splitting tensile strength is established. The research shows that:With the high-strength concrete suffering higher temperature, its compressive strength, splitting tensile strength decreases; the same series of high-strength concrete with fiber content increasing, the residual compressive strength has a trend of firstly increasing and then decreasing, while, the relative residual splitting tensile strength firstly decreases and then rises; generally speaking, mixing 8mm polypropylene fibers increases the compressive strength and splitting tensile strength of concrete after high temperature, while mixing 19mm fiber compressive strength dose not change significantly than plain concrete on the aspect of compressive strength after high temperature. Compared with natural cooling, water immersion is not conducive to the performance of high-strength concrete after 300℃and 500℃, however, after 700℃, the impact is not obvious, and may even increase its high temperature performance.(4) By using ultrasonic nondestructive testing method, the thesis evaluates the extent of losses of compressive strength after high temperature. Based on the test results, the relationship formula between ultrasonic velocity and temperature is established; the relationship formula between the residual compressive strength and ultrasonic velocity is also established.