Experimental Study On Tensile Properties Of Ultra-high Performance Concrete

Ultra-high performance concrete is a cement-based material with ultra-high strength,toughness and excellent durability,which enables the structure to achieve a larger span and smaller size.The existing research results show that the tensile strength and tensile toughness of UHPC materials are insufficient,and further improving the tensile properties of UHPC is of great significance for its application and popularization.Uniaxial tensile test is the most direct and effective method to reflect the synergy between matrix and fiber in fiber concrete.In view of the defect that the 2-d thin plate specimen used in most existing researches is inconsistent with the reality,this study designed a 3-d dumbbell tensile specimens,obtained tensile indexes through uniaxial tensile test include initial crack strength,ultimate tensile strength,initial crack strain and ultimate tensile strain.The effects of different single and hybrid fibers on the uniaxial tensile properties of UHPC were analyzed.And the tensile toughness of each specimen was evaluated by energy absorption capacity of matrix.The main conclusions are as follows:Firstly,four types of steel fibers(short straight S,long straight LS,short hook H and long hook LH)and five types of PE fibers(PA,PB,PC,PD and PF)were used to explore the influence of single fiber on the mechanical properties of UHPC.The test results show that the compressive strength and tensile performance of UHPC matrix are improved with the increase of steel fiber content and fiber aspect ratio.The tensile strength and tensile strain of long straight LS(2.0 vf.%)were 10.36 MPa and 0.605% respectively.Compared with the straight steel fiber,the hook steel fiber has a better effect on improving the tensile strength of the matrix.The tensile strength and tensile strain of the short hook H(2.5 vf.%)reached 11.64 MPa and 0.303% respectively.The tensile strain of UHPC with PF(2.0 vf.%)reached 1.155% at highest.Steel fiber and PE fiber toughen UHPC in different ways.The research shows that the stress peak of tensile curve of UHPC with PE fiber is lower than that with steel fiber,but the softening section declines more gently.Secondly,experiments of steel fiber hybrid and steel fiber /PE fiber hybrid were carried out.The results show that the increase of the volume fraction of different types steel fibers improves the tensile properties of UHPC matrix,but the improvement of the compressive properties is limited.The compressive strength and tensile property of UHPC with S and H fiber hybrid are superior to that with S and LS fiber hybrid.The hybrid effect of S fiber(1.0 vf.%)and fiber H fiber(1.5 vf.%)was the best.The tensile properties and compressive strength were better than the single fiber group,and the tensile strength and tenslile strain were 12.34 Mpa and 0.462% respectively.The experimental results of PE and steel fiber hybrid group showed that with the increase of the proportion of PE fiber,the tensile strength of the matrix decreased,the tensile strain increased,the multi-cracked was more obvious,and the crack control ability of the matrix was improved.The tensile strain of UHPC with PA fiber(1.5 vf.%)and S fiber(0.5 vf.%)hybrid reached 1.390% at the highest,and the tensile strength reached 4.86 MPa.Finally,the energy absorption capacity of each specimen is calculated and its tensile toughness is evaluated.Experimental results show that: in single fiber group,the increase of fiber volume fraction and fiber aspect ratio can effectively improve the energy absorption capacity of the matrix,and the UHPC with LS fiber(2.0 vf.%)and PF fiber(2.0 vf.%)has high tensile toughness.The UHPC with PE fiber shows high strain and low strength generally,while the UHPC with steel fiber shows high strength and low strain.And the toughening effect of steel fiber higher than PE fiber.Although there is no obvious rule between the toughness and the content of fiber volume fraction of fiber hybrid UHPC,the ratio of the two fibers has a great influence on the toughness of matrix.The tensile toughness of PE/ steel fiber hybrid groups are not well and how to disperse PE and steel fiber evenly is the key to improving its tensile performance.