Study On Mechanical Properties And Constitutive Model Of 3D Printed Ultra-High Performance Fiber-reinforced Concrete

3D printing is an important development direction of smart construction technology.Ultra-high performance fiber-reinforced concrete(UHPFRC)has superior mechanical properties and durability.At present,the research on the mechanical properties of 3D printed UHPFRC(3DP-UHPFRC)materials is still in its initial stage.In order to promote the development of 3D printing engineering structure design and construction technology,the static and dynamic properties of 3DP-UHPFRC under unidirectional stress state and static triaxial properties of 3DP-UHPFRC under multidirectional stress state were investigated in this paper,and an anisotropic principal structure model of 3DP-UHPFRC was established based on the mechanical test results.The main research works and results are as follows.(1)Static mechanical properties tests of 3DP-UHPFRC under unidirectional stress state.Different types of 3D printed specimens were prepared by 3D printer,and compression,bending,splitting tensile and uniaxial tensile tests were conducted to analyze the effects of fiber content,fiber type and loading direction on the mechanical properties of 3DP-UHPFRC,and to study the anisotropic mechanical characteristics of3DP-UHPFRC;using cast-in-place UHPFRC(MC-UHPFRC)as the reference specimen The performance differences between 3D printed concrete and cast-in-place concrete were analyzed and compared.The results show that the static mechanical properties of 3DP-UHPFRC are anisotropic due to the fiber distribution and the weak surface;the compressive strength of 3DP-UHPFRC in the Z-direction is 91.73% of that of MC-UHPFRC,while the compressive strength in the X-direction is only 39.91% of that of MC-UHPFRC;the compressive strength of 3DP-UHPFRC in the X-direction obtained by splitting tensile test and The tensile strength of 3DP-UHPFRC in Xdirection obtained by splitting tensile test and uniaxial tensile test is higher than that of MC-UHPFRC,and the tensile strengths are 1.26 and 1.65 times of MC-UHPFRC,respectively;based on the test results,the strength conversion equations between bending tensile,splitting tensile and uniaxial tensile strength of 3DP-UHPFRC in Xdirection are given.(2)The dynamic mechanical properties of 3DP-UHPFRC under unidirectional stress state were investigated.In this paper,dynamic compression and dynamic splitting tensile tests were conducted on 3DP-UHPFRC using a split Hopkinson compression bar system(SHPB),and the effects of fiber type,fiber content,preparation method,loading direction and impact velocity on the dynamic mechanical properties of 3DPUHPFRC were analyzed,and the strain rate,dynamic stress,dynamic growth factor(DIF)and failure process were discussed.The results show that under dynamic compression,3D printed ultra-high performance concrete without added steel fibers(3DP-UHPC)fails similarly in all directions,while 3DP-UHPFRC fails differently in all directions;the dynamic compression elastic modulus and strain rate effects of 3DPUHPFRC exhibit anisotropic characteristics;with increasing impact velocity,3DPUHPFRC The dynamic compression peak stress of 3DP-UHPFRC at the same impact velocity is isotropic;the dynamic compression DIF of 3DP-UHPFRC is anisotropic,and the dynamic compression strength in the X direction is most sensitive to the strain rate;under dynamic splitting tension,the failure mode of 3DP-UHPFRC is ductile when loaded in the Z direction due to the directional distribution of steel fibers,while it is brittle when loaded in the X and Y The dynamic peak splitting tensile stress of 3DPUHPFRC has anisotropic characteristics in different tensile directions;the dynamic splitting tensile DIF of 3DP-UHPC without steel fibers does not show significant anisotropic characteristics,while the DIF obtained when 3DP-UHPFRC is loaded along the Z direction is significantly lower than that in the X and Y directions.(3)Study of static triaxial mechanical properties of 3DP-UHPFRC under multidirectional stress state.In this paper,the static triaxial mechanical properties of3DP-UHPFRC under different surrounding pressures were investigated experimentally by different fiber contents,fiber lengths and preparation types;the failure modes of 3D printed specimens and cast-in-place specimens and the differences in static triaxial mechanical properties were compared and analyzed,and the Moore-Coulomb failure criterion,the power rate failure criterion and the William-Warnke failure criterion were parameterized and calibrated based on the test results.The results show that 3DPUHPFRC has similar failure mode and mechanical properties to MC-UHPFRC in the main bearing direction;the poor fitting of the Moore-Coulomb failure criterion for steel-free fiber 3DP-UHPC(R2 less than 0.9)is due to the linear relationship between the strength defined by the Moore-Coulomb failure criterion and the surrounding pressure,while the strength of steel-free fiber 3DP-UHPC increases with the surrounding The power-law failure criterion and Willam-Warnke failure criterion have good fitting results for all 3D printed specimens.(4)The 3DP-UHPFRC anisotropic intrinsic model is proposed.In this paper,the one-dimensional intrinsic structure model is established with reference to the Z-W-T model and the HJC model by combining the experimental data.Based on the spatial transformation tensor theory,the one-dimensional intrinsic structure model is mapped to an orthogonal anisotropic compression intrinsic structure model for describing 3DPUHPFRC.The results show that the anisotropic intrinsic model established in this paper can accurately describe the static,dynamic and static triaxial stress-strain relationships of 3DP-UHPFRC.