Study On Properties Of Hybrid Fiber Modified High-Strength Self-Compacting Concrete

Self-compacting concrete solves in engineering because of the dense structure such as thin wall,complex shape and reinforcement,caused by the construction problems,relying on under their own gravity,to be able to flow,compact,does not need additional vibration molding this advantage.At the same time,the construction noise can be reduced through the use of self-compacted concrete,which can respond to the country’s call for green building.Thus,self-compacted concrete has been widely used.However,the weakness of concrete with poor crack resistance,poor toughness,low tensile strength,low tensile strain is becoming more and more obvious with the improvement of the grade requirement of concrete in modern buildings.The results show that fiber has the effect of toughening,strengthening and cracking resistance to cement matrix composites,which makes fiber modified high-strength self-compacting concrete(HSCC)become one of the development trends of high performance self-compacting concrete.However,the mixing of single fiber can only improve the performance of concrete in a certain size,while the hybrid fiber of different scales can improve the performance of concrete in many aspects at different structural levels.Copper-coated steel fiber(SF)is a kind of steel fiber with a submillimeter size,which is more suitable for self-compacting concrete with strict requirements for working performance than ordinary steel fiber.Carbon nanofiber(CNF)is a submicron scale graphite fiber with high strength,high elastic modulus,good toughness and superior conductivity thermal conductivity.Therefore,this paper considers the modification of HSCC with SF and CNF.The following aspects have been completed:(1)The volume content of SF and CNF was taken as the experimental parameter,and the modified HSCC mixed with composite fibers was designed and prepared,which meets the related working performance and mechanical properties.(2)The effects of hybrid fibers on the properties of HSCC,such as working performance,flexural toughness and crack resistance,were investigated.Through the slump expansion test and J ring test,the two performance indexes of hybrid fiber modified HSCC filling characteristic and passing quality through gaps were measured.Subsequently,the related mechanical performance of this kind of concrete was obtained by mechanical property test.Then,the load-displacement curve obtained by bending toughness test was used to calculate the initial flexural toughness ratio and bending flexural ratio to evaluate the modification of concrete toughness.Finally,the anti-crack test of flat plate was carried out,and the mechanism of action of the two fibers on the total length,average width and nominal area of crack was analyzed synthetically,and the effect of fiber on the crack resistance of early-age concrete was evaluated.(3)In order to deeply analyze the modification mechanism of hybrid fiber on mechanical properties,toughness and early crack resistance of concrete,BP neural network was established according to the measured value of mechanical properties of hybrid fibers,and its accuracy was analyzed.At the same time,verified that the numerical analysis method can be used for reasonable prediction.Through neural network prediction,refinement should be variable scale,and combined with quadratic nonlinear fitting method,the surface diagram of concrete mechanical properties,toughness and early crack resistance varying with the content of two fibers was obtained.And then learnt the optimal dosage of the hybrid fiber mixture ratio.