Experimental Study On Design Parameters Of Lightweight Ultra-high Performance Concrete

Ultra-High Performance Concrete(UHPC)is widely used in bridge engineering,but its self-gravity limits the span capacity of Bridges.,and the research on lightweight concrete materials is the hot spot in bridge engineering field.Lightweight Ultra-High Performance Concrete(LUHPC)developed by our research group has higher strength(compressive strength≥100MPa,splitting tensile strength≥10MPa)and smaller dead weight(density≤2100kg/m~3)than ordinary high-performance concrete.In view of the current bridge code in China,the concrete strength grade is divided with the cube compressive strength as the index,and the design value of concrete strength is used to design and check the calculation while ensuring the safety and reliability of the structure.In order to facilitate the promotion and application of LUHPC in bridge engineering,it is necessary to study the design parameters of LUHPC material properties to obtain the design values of LUHPC strength at different levels.Based on Hubei Provincial Technical Innovation Special Project(major project)"Development and Application of Lightweight Ultra-High Performance Concrete and its Application in Precast Bridge Engineering(2018AAA-001)",this paper carried out experimental research on the design parameters of LUHPC strength levels with different fiber content to classify LUHPC steangth levels and obtained the standard strength values and design values of LUHPC at the corresponding levels.The tensile and compression constitutive relation of LUHPC is established,and the formula for calculating the ultimate flexural capacity of reinforced LUHPC beams is proposed.The calculation results are verified to be accurate by experiments.According to the design value of LUHPC strength,the design value of LUHPC beam flexural capacity can be obtained.The research results provide theoretical and experimental basis for the application of LUHPC materials in bridge engineering.The main work is as follows:(1)Through 34 groups of 306 LUHPC specimens to carry out experimental studies on key design parameters such as compressive strength,tensile strength,splitting strength,flexural strength,elastic modulus and Poisson’s ratio experiment block,The cubic strength standard values of LUHPC with steel fiber content of 0%,1.0%,1.5%,2.0%and 2.5%are 64.7MPa,86.0MPa,101.9MPa,109.4MPa and 115.7MPa respectively.The corresponding intensity levels were given as LC60,LC85,LC100,LC105 and LC115.The relation coefficient between the standard value of axial compressive strength and the standard value of cube strength is determined to be 0.7.The design value of LUHPC axial compressive strength was calculated by selecting the component coefficientγ=1.45.The standard value of LUHPC axial tensile strength was obtained through the tensile test of dog bone specimen by mathematical statistic through the tensile test of dog bone specimen by mathematical statistics,and the design value of LUHPC axial tensile strength was calculated.The experimental results provide design parameters for the application of LUHPC material in bridge engineering.(2)The uniaxial compression experiment of LUHPC and the uniaxial tensile experiment were carried out,and the tension and compression stress-strain experiment curves of LUHPC with different steel fiber content were obtained.The influence of steel fiber content on LUHPC uniaxial compression and tensile stress-strain curves was analyzed.The constitutive relation of LUHPC uniaxial compression and uniaxial tension considering the fiber content is proposed,which provides an analysis basis for the calculation of ultimate flexural capacity of reinforced LUHPC beams.(3)Ordinary steel bars and prestressed bars were set in the tension and compression zones,the stress in the compression zone was simplified as triangle,and the tension zone was simplified as rectangle.Considering the influence factorβt of steel fiber on the tensile strength of the flexural beam,a formula for calculating the ultimate flexural capacity of the reinforced LUHPC beam was proposed.Five reinforced LUHPC beams were designed and fabricated,and the influence coefficientβt was determined by the flexural performance experiment.The results show that the calculated values of ultimate flexural bearing capacity of reinforced LUHPC beams are in good agreement with the experimental values,which shows the accuracy of the proposed formula.Based on this,the design value of the flexural capacity of the reinforced LUHPC beam is obtained by selecting the design value of LUHPC strength.