The Effect Of Strengthing And Toughening Of Hybrid Fiber On Ultra High Performance Concrete Plate

Steel fiber can largely improve the cracking resistance and the tensile properties of ultra high performance concrete（UHPC）,improving the ductility,fracture toughness and energy absorption ability,taking the brittle failure into the plastic,which is very important to UHPC.Many researchers has shown that with a single type of steel fiber volume content increases,the concrete tensile and bending strength increase.However,due to the UHPC is relatively high construction cost,the use of very rare makes by UHPC in ordinary buildings.In order to solve the economy and the performance of the problems of single type of steel fiber,some researchers use the hybrid fibers of different material,shape,and the length into concrete,making the hybrid fiber reinforced concrete with low cost and high performance.Studies have shown that the virtuous circle between hybrid fiber is often greater than the sum of a single type of fiber performance,producing"fiber synergy".UHPC with straight and/or hooked steel fibers at a total content of 2%was prepared.Influence of single fiber and hybrid fibers on bending strength and toughness of UHPC were investigated by using three-point bending test and INSTRON drop-tower impact system.The flexural toughness and impacting strength and toughness was analyzed by using mathematical statistics method based on the ASTM method.The results indicated:With the increase of the hooked steel fiber,the fluidity of UHPC appeared a decreasing trend.when 1%of straight fiber and 1%end hook fiber hybrid,the fluidity of UHPC reach the minimum of 210 mm.There are good synergistic effect of the two kinds of steel fiber.When 1%straight fiber and 1%hooked steel fiber were incoporatated,the mechanical properties of UHPC achieved the best performance.Its flexural strength were 1.44 and 1.01 times,respectively,greater than those with either 2%mono straight fibers or 2%mono hooked fibers.Its flexural toughness were1.37 times and 1.26 times greater.Hybrid fiber has a little effect on initial crack strength and initial crack deflection,but has the great influence on the peak strength and peak deflection.More twists and turns and fracture number with the increase of impact velocity.The dynamic impact strength and toughness increased with the increase of the impact velocity,showing a significant strain rate effect.The impact strength and dynamic increase factor are the largest when 1%straight steel fiber and 1%hook end steel fiber mixing under the maximum strain rate 11s^-1.The impact strength of 192.4Mpa.At the maximum strain rate of 11s-1,the impact toughness of 1.5%of the straight steel fiber and 0.5%of the hook end steel fiber mixed is 4.33k J/m2 which is achieving the maximum.The impact toughness is 1.12 times of 2%hook end fiber.