Study On Constitutive Relation And Damage Of Ultra High Performance Fiber Reinforced Concrete With High Flowability Under Uniaxial Compression

Ultra High Performance Fibre Reinforced Concrete(UHPFRC)is a new type of cement-based composite material with extremely high strength,high toughness and excellent durability.UHPFRC with high flowability has a highest compressive strength of more than 190 MPa and flow expansion of more than 180mm(micro slump cylinder test).It has a broad application prospect in civil construction,road and bridge,water conservancy and hydropower and other projects related to national production.The incorporation of steel fiber is an important factor to realize the reinforcement and toughening of UHPFRC.However,the influence of key parameters such as the dosage of steel fiber and length to diameter ratio on the basic mechanical properties and damage development process of UHPFRC is not clear,especially the compression constitutive relationship,which limits the application of UHPFRC.Therefore,the influence of steel fiber mixing amount,length-diameter ratio and different length-diameter ratios blending on the basic mechanical properties of UHPFRC were studied in this paper through experiments research and theoretical analysis.What's more,a constitutive model for structural design was established.The damage evolution process of UHPFRC under uniaxial compression was analyzed,and the constitutive relation considering the damage was proposed.The main work and conclusions of this paper are as follows:(1)The full stress-strain curve test of UHPFRC with high flowability under uniaxial compression was designed,and the full stress-strain curves of UHPFRC under uniaxial compression within the range of 1%?5% of steel fiber content were measured.And the influence of steel fiber content on uniaxial compression characteristics was analyzed.The results showed that,the compressive strength,peak strain,and elastic modulus of UHPFRC are improved with the increase of steel fiber content.There is a linear relationship between the peak strain and compressive strength,and the relationship between elastic modulus and compressive strength satisfies the current relations in design codes of concrete structures.The proportional limit and Poisson's ratio value of UHPFRC under different steel fiber content were determined.The four stages of UHPFRC uniaxial compressive deformation and failure modes were clarified;The tri-linear compressive constitutive model of UHPFRC with different steel fiber content is established based on relevant domestic and foreign codes.(2)The influence of different length-diameter ratios(30,65 and 100)and different length-diameter ratios blending combinations of steel fibers on the basic mechanical properties of UHPFRC were studied by the full stress-strain curve test of uniaxial compression and the bending toughness test under the typical steel fiber content(3%).The results showed that the compressive strength and flexural strength of UHPFRC were improved with the increase of the length-diameter ratio of steel fiber.And the increase of the length-diameter ratio of the steel fiber had no significant effect on the improvement of UHPFRC compressive peak strain and elastic modulus,but uniaxial compression toughness and bending toughness were improved significantly.By introducing the parameter of length-diameter ratio and combining the effect of steel fiber content,the unified UHPFRC tri-linear compressive constitutive model with different steel fiber characteristic values was obtained.The influence mechanism of steel fiber lengthdiameter ratio on the material toughness of UHPFRC was also analyzed.(3)The whole damage process of UHPFRC with different steel fiber content and different length-diameter ratio under uniaxial compression was monitored through the acoustic emission(AE)damage monitoring test.The AE cumulative energy increased very slowly with the increased of strain and time at first,and then increased rapidly.As choosing AE cumulative energy to be the damage variable a,the damage constitutive relationship of UHPFRC under uniaxial compression was established based on Lemaitre's hypothesis of equivalent strain,and the influence of steel fiber content was also considered.The damage constitutive relationship can accurately fit the compression stress-strain curve of UHPFRC within 0?1.8 times of the peak compression strain.After the normalization,the compression damage constitutive model was compared with the tri-linear compressive constitutive model,and the application scope of the two constitutive models was also determined.