Mechanical Properties Of Steel Fiber Reinforced Lightweight Aggregate Concrete After High Temperature

Due to the high specific strength,Lightweight aggregate concrete has great potential in the application of super-tall and long-span building structures.In addition,building fire always threatens people’s life and property safety,so it is of great significance to study the performance degradation of building materials in the event of fire.At present,the research on the lightweight aggregate concrete after high temperature is not enough,especially the research on the steel fiber light aggregate concrete after high temperature.A total of 360 specimens were designed and made for experiments in this study.The influences of aggregate type(normal weight,all-light,semi-light),steel fiber type(crimped shape,hooked end)and temperature class(25℃,200℃,400℃,600℃,800℃)on the physical and mechanical properties of concrete were analyzed.The research contents of this paper can be summarized as follows:Firstly,in three different types of aggregate,two kinds of different types of steel fiber,and five different temperature levels as variables,a series of experiments were carried out on concrete,including ultrasonic pulse velocity test,mass loss test,splitting tensile test,four-point bending test and axial compressive test under monotonic loading.The influences of various variables on the physical and mechanical properties and compressive toughness of concrete are analyzed.The results showed that the ultrasonic pulse velocity of concrete decreased and the mass loss increased with the increase of the temperature.The splitting tensile strength,flexural strength and axial compressive strength of all concretes decreased gradually.With the increased of temperature,the elastic modulus decreased while the peak strain increased,hence the stress-strain curves were gradually flattened.The toughness of all concretes increased first and then reduced with increasing exposure temperature,while the specific toughness of all concretes increased with the increase of exposed temperature.Compared with normal weight concrete and semi-lightweight concrete,all-lightweight concrete has a better capacity to resist high temperature,especially at temperatures higher than 400°C.Adding steel fibers can improve the capacity of energy absorption,specific toughness and residual splitting tensile strength of lightweight concrete(LWC)before and after high temperatures.On the basis of regression analysis,the relationship between the residual mechanical properties and temperature was established,and the axial compression constitutive model was proposed.Secondly,in two different types of aggregate(normal weight,semilightweight),two kinds of different types of steel fiber,and five different temperature levels as variables,the experiment of axial compressive cyclic loading test was carried out on concretes.The results showed that with the increased of temperature,the curvature of unloading curve and reloading curve increased gradually.At the same dimensionless unloading strain point,the higher the temperature was,the greater the plastic strain would be.The influence of temperature on plastic strain was greater than that of concrete type.The constitutive models of post-fire concretes under cyclic loading was established.Finally,two different types of aggregate(all-lightweight,semilightweight),two kinds of different types of steel fiber,and five different temperature levels were set as variables.The influence of variables on loaddeflection curves,flexural toughness,load-displacement curves and splitting tensile toughness of concrete was studied.The results showed that the addition of steel fiber significantly improved the flexural behavior of lightweight aggregate concrete.The load-deflection curves of the specimens with hooked end steel fiber always showed deflection hardening behavior,while the specimens with crimped shape steel fiber showed obvious deflection hardening behavior only after 600℃ and 800℃.With the increase of temperature,the load-deflection and load-displacement curves of all concretes gradually flattened.Both the flexural toughness(at a specific deflection of 2mm)and the initial splitting tensile toughness index increased first and then decreased with the increased of temperature.