Research On Key Influencing Factors Of Mechanical Properties And Expansion Of Ultra High Performance Concrete

Ultra-high performance concrete,referred to as UHPC,is the most innovative cement-based material in the past three decades.It has been gradually applied to long-span bridges,super high-rise buildings and other special structures with its ultra-high strength,great toughness and ultra superior durability.It is an important premise for the further development and application of UHPC to take into account good construction and mechanical properties,and such factors as material composition,maintenance mechanism and mixing process can have a great impact on the construction and mechanical properties of UHPC.However,the performance of UHPC materials in China is still in the exploratory stage at present.Therefore,most of the above factors,such as water-to-binder ratio,steel fiber content,vibration stirring,etc.have no clear or uniform effect on the performance of UHPC materials.So it is urgent to carry out systematic experimental research.As one of the basic mechanical properties of UHPC,the uniaxial tensile and compressive constitutive relationship is also crucial for the design and calculation of UHPC structures.However,the current research results on uniaxial tension and compression constitutive relations of UHPC appear to be uneven due to the large difference in UHPC mix ratio and the lack of relevant specifications,Therefore,these achievements lack universal applicability.Based on this,in this paper,the effects of water-to-binder ratio,steel fiber content,length diameter ratio of steel fiber,steel fiber shape,homogenous/profiled steel fiber blending,quartz sand grading,silica fume content,maintenance mechanism and vibration mixing were studied on the construction and mechanical properties of UHPC.Based on the measured uniaxial tensile and compressive stress-strain curves,the matlab batch fitting function was used to obtain the constitutive equations containing various influencing factors as variables.The main contents of this paper are as follows:(1)The effect of water-to-binder ratio(0.15～0.2)on the construction and mechanical properties of UHPC were studied.The expansion of UHPC under different mixing time and water-to-binder ratio,the loss rate of expansion after a period of static stop of fresh UHPC mixture,and the cube compressive strength,flexural strength,bending toughness,uniaxial compressive strength,elastic modulus and uniaxial tensile strength under different water-to-binder ratio were carried out.The variability of test results were analyzed.The influence law of water-to-binder ratio on construction and mechanical properties of UHPC were determined.The bending toughness index of UHPC were calculated respectively based on the bending stress deflection curves,CECS 13:2009 and its improved method,Nemkumar method,and the calculation method suitable for calculating the bending toughness index of UHPC was proposed through comparison.Finally,based on the measured stress-strain curves of uniaxial compression and the stress-strain curves of uniaxial tension,the constitutive equations of UHPC in uniaxial tension and compression including the water binder ratio parameters were fitted.(2)The influence of steel fiber parameters on the construction and mechanical properties of UHPC were studied.The influence of steel fiber content,steel fiber length-to-diameter ratio,steel fiber shape,homogenous/profiled steel fiber blending on the expansion(construction performance)and mechanical performance indicators of UHPC were studied by using four kinds of straight steel fibers with different scales and two end-hook steel fibers are selected.The variability of test results were analyzed.The influence law of steel fiber parameters on construction and mechanical properties of UHPC were determined.The corresponding flexural toughness indexes of each group of UHPC specimens were calculated by the calculation method which is suitable for calculating the flexural toughness indexes of UHPC specimens.Finally,based on the measured stress-strain curves of uniaxial compression and the stress-strain curves of uniaxial tension,the constitutive equations of UHPC in uniaxial tension and compression including the steel fiber parameters were fitted.(3)The influence of quartz sand gradation and silica fume content on construction and mechanical properties of UHPC were studied.13 groups of UHPC specimens with different quartz sand gradations and 5 kinds of silica fume mixed specimens were designed by using 3 kinds quartz sands with different particle sizes.The expansion degree and the change rule of mechanical property index of each group of specimens are studied by experiment,and the variability of test results were analyzed.The influence law of quartz sand gradation and silica fume content on construction and mechanical properties of UHPC were determined.Finally,based on the measured stress-strain curves of uniaxial compression and the stress-strain curves of uniaxial tension,the tensile and compressive constitutive equations of each group of test pieces were fitted respectively.(4)The effect of different curing mechanism on mechanical properties of UHPC was discussed.Standard curing(curing ages are 1 d,3 d,7 d,14 d,28 d,56 d),hot water curing and steam curing(curing ages are 1 d～4 d)were used to study the effect of curing mechanism on mechanical properties of UHPC.The variability of test results were analyzed.The influence law of different curing mechanism on mechanical properties of UHPC were determined.The relationship between the compressive strength,flexural strength,tensile strength,modulus of elasticity and peak strain of axial compression and curing age is obtained by combining the test data,multiple nonlinear fitting and simple derivation.Based on the measured curve data,the constitutive equations of UHPC in uniaxial tension and compression including the curing age parameters were obtained by fitting.(5)The effects of vibration stirring and ordinary forced stirring on the expansion and various mechanical properties of UHPC were studied.The effect of vibration mixing on the construction and mechanical properties of UHPC were demonstrated by contrast.