Experimental Research On Flexural Behavior Of Ultra-high Performance Concrete Structure

As a new kind of cementitious composites,ultra-high performance concrete(UHPC)is becoming a very popular material in civil engineering due to its high mechanical properties and excellent durability compared with ordinary concrete,however,there is still a lack of sufficient study on its structure behavior,which limits its application in engineering.An experimental study and the corresponding theory analysis are carried out to study the behavior of UHPC slabs reinforced with steel bars.Based on bending test of 24 reinforced slabs of UHPC,the bending behavior and design calculation theory of UHPC components are studied.The main work and conclusions are as follows:(1)The mechanical properties and long-term properties of UHPC were studied,and the influence of steel fiber configuration on the mechanical properties of UHPC was analyzed.The results showed that UHPC had high compressive strength,flexural tensile strength and high elastic modulus,and had good durability.The results showed that the addition of steel fiber has obvious toughening effect on the matrix.With the increase of fiber content,the initial bending and flexural strength of the composites increase,and the compressive strength had not changed.When the fiber ratio was constant,the mixed fiber could improve the performance of the material better than the single fiber.(2)The cracking moment and ultimate bearing capacity of reinforced slabs of UHPC were studied.The theoretical formula and the simplified formula for the cracking moment of reinforced slabs of UHPC are derived.Considering the influence of steel fiber on the plastic development of the tensile zone,the crack resistance correction factor was introduced to modify the formula of cracking moment.Based on the assumption of plane section,the UHPC stress graph of the compression zone and the tensile zone was treated equivalently,and the formula for calculating the ultimate bearing capacity of UHPC slab was established.The calculated values by the suggested formulas agreeed well with the test results.Meanwhile,the bounding relative height of compression zone and critical reinforcement ratio of reinforced slabs of UHPC was deduced.(3)A method of calculating the short-period stiffness of UHPC slabs was studied.Based on the assumption of plane section and the condition of deformation compatibility,and combined with the characteristics of UHPC bending member,considering the contribution of UHPC to the stiffness in the tension zone,the formula for calculating the short-term bending stiffness was established.The parameters of the internal force coefficient and the non-uniform coefficient of the reinforced concrete were determined.Comparing the calculated values with the flexural rigidity of the cross section in the midspan with the measured mid-span deflection,the results showed good agreement.(4)The calculation method of crack width of reinforced slabs of UHPC was analyzed.Refer to the crack mechanism of ordinary reinforced concrete beam,combined with the maximum crack width formula "GB50010-2010 Code of concrete structure design".The formula for calculating the maximum crack width of reinforced slabs of UHPC was proposed.Based on the analysis of the difference of the performance between UHPC and ordinary concrete,the stress of steel bar,the average crack spacing,the non-uniform coefficient of steel strain and the characteristic coefficient of member force were analyzed.The calculated values agree well with the test values.