Experimental Study On The Mechanical Properties And Damage Evolution Of Basalt Fiber Reinforced Concrete After High Temperature

The fracture properties of concrete structures after the elevated temperature is an essential research subject in the field of civil engineering.The high temperature generated by the fire will lead to the microstructure change of the concrete material,the bearing capacity decline and the fracture performance deterioration.The basalt fiber has good mechanical properties,so adding basalt fiber into concrete can enhanced strength and toughness of concrete and it also has a good prospect in improving the fracture properties of concrete.In this paper,an experimental study was conducted on the mass loss,compressive strength of basalt fiber reinforced concrete after high temperature.The compressive peak strain of compressive specimens of basalt fiber reinforced concrete were tested by digital image correlation(DIC).Three-point bending beam fracture test,conventional test method(strain gauge and laser extensometer)and DIC test method were used to measure displacement and strain of concrete specimens.The double K fracture model of concrete is used to calculate its fracture mechanics parameters and analyze crack propagation characteristics.Then the modified formula of the fracture energy of concrete was used to analyze the energy required for cracking per unit area of the concrete during the fracture failure process.The flexural toughness of concrete was analyzed by using equivalent bending toughness.The main results are as follows:(1)There are three stages for the change trend of sample mass loss with temperature: rapid rise(20℃-400℃)→ slight rise(400℃-600℃)→ rapid rise(600℃-800℃).The mass loss of the three-point bending beam is slightly larger than that of the compression specimen.The compressive peak strain of concrete increases slowly when the temperature is below 400℃ and increases rapidly when the temperature is over 400℃.(2)With the increase of ambient temperature,the crack initiation toughness Kini of concrete decreases monotonously.However,instability fracture toughness Kun increase linearly first(20℃-400℃)and then decrease significantly(400℃-800℃).The crack initiation toughness Kini of concrete does not change significantly with the increase of fiber volume content,and adding basalt fiber can be considered to have almost no effect on the fracture initiation toughness Kini of concrete.The instability fracture toughness Kun of concrete increase first(0.0%-0.4%)and then decrease(0.4%-0.6%)with the increase of fiber volume content.(3)The change trend of fracture energy GF of concrete can be divided into two stages with increasing temperature,namely: a slight increase(20℃-400℃)→ a sharp drop(400℃-800℃).With the increase of fiber volume content,it showed the tendency of increasing first and then decreasing.When the fiber volume content is 0.4%,the concrete fracture energy GF is maximum.(4)When the temperature is less than 400℃,the bending toughness of concrete increases with the increasing temperature.When the temperature is between 400℃ and 600℃,it decreases with the increasing temperature.When the temperature is greater than 600℃,the change trend is unstable but the change is slight.With the increase of basalt fiber content,the bending toughness first increases and then decreases.The effect of basalt fiber on the toughness of concrete is ranked from big to small: large deformation stage>medium deformation stage>small deformation stage>micro deformation stage.The greater the deformation of the concrete is,the more significant the effect of basalt fibers on the toughness of concrete is.(5)The initial cracking load Pini measured by the DIC test method is almost the same as the result measured by strain gauge.The fracture opening displacement CMOD measured by the DIC test method is more precise than that measured by the laser extensometer and is closer to the actual value.When using the DIC test method to measure the deflection of the specimen,the results are in good agreement with the results measured by the tester.The two test results are consistent and close.It can be seen that the DIC method can effectively measure the fracture parameters of concrete.