Mesomechanical Model And Critical Parameters Design Of Ultra High Strength And High Ductility Cementitious Composites

To ensure the long-term development of "national security" and "Belt and Road" strategy,some national defense and civil infrastructure require concrete materials to have ultra-high strength,high ductility,high toughness and high durability in the face of extreme natural disasters or man-made attacks in complex situations,so as to achieve the design requirements of engineering structures with higher safety,high energy dissipation and long life.Ultra-High Strength and High Ductility Cementitious Composites(UHS-HDCC)meet ultra-high strength levels and have strain hardening and multiple crack characteristics,which breaks the traditional cognitive barrier of "compression and non-tensile" for ultra-high strength concrete.The design concept of UHS-HDCC relies on the scientific design and regulation of the three-phase synergistic effect of matrix,fiber and fiber-matrix interface region.In order to investigate the mechanism of strain hardening properties of UHS-HDCC,a modified meso-mechanical model(σB-δ constitutive relation)for UHS-HDCC was established based on the pullout behavior of a single short-cut PE fiber in an ultra-high strength matrix.A design and regulation method for key parameters in the UHS-HDCC meso-mechanical modified model was proposed,and the range of key parameters was determined and sensitivity analysis was conducted.The influence mechanism of ground granulated blast furnace slag(GGBS)and silica fume(SF)on the microstructure and mechanical properties of UHS-HDCC was investigated.A surface functionalization technique for PE fibers aimed at reinforcing the fiber matrix interface transition zone was proposed,and the potential mechanism of the modified PE fibers on the strain-hardening properties of UHS-HDCC was revealed.The main research contents of the paper are as follows:(1)Based on the traditional HDCC micromechanical model,a modified meso-mechanical model(Pb-δ constitutive relation)for single PE fiber pullout was established by testing the pullout behavior of a single short-cut PE fiber in an ultra-high strength matrix with different embedded lengths and included angles.All fibers exhibit certain strain hardening characteristics during the pullout process.The larger the embedded length and included angle of the fiber,the greater the tensile load that the fibers are subjected to before fracture occurs.At the fiber angleθ=0,the frictional stress at the fiber-matrix interface is independent of the embedded length of the fiber,and a constant interfacial frictional stress τ0=1.51 MPa is calculated.As the fiber angle increases,the snubbing coefficient f=0.31 representing the increasing tensile load of the fiber is calculated,the slip hardening reinforcement factor μ=0.62 representing the increasing slip hardening coefficient is defined,and the reinforcement factor ω=35 N/(m-rad)representing the increasing fiber pullout end load is proposed.Compared with the traditional HDCC micromechanical model,the experimental data and the modified model have a good agreement.(2)Based on the single PE fiber pullout modified model,and combined with micromechanics and fracture mechanics design theory,as well as statistical principles,a UHSHDCC meso-mechanical modified model(σB-δ constitutive relation)was established.Considering the fiber fracture in the debonding and slip stages caused by the slip-hardening interface behavior during fiber pullout,the fiber spatial distribution state consisting of fiber embedded length and the included angle is used to elucidate the fracture development process of fibers in the debonding and slip stages as the crack width develops gradually.The critical values of the fiber the embedded length and angle when the fiber in the crack breaks during the debonding stage and the slip stage are determined.The bridging stresses σRD(δ)in the debonding stage and the bridging stresses σRS(δ)in the slip stage caused by fiber fracture are derived,and then the analytical equations of the current modified model are given.(3)Based on the modified model of UHS-HDCC meso-mechanical and judged by strength and energy criteria,the range of values for key parameters of fiber,matrix and fiber-matrix interface properties is clarified.The higher the elastic modulus of the matrix,the lower the initial crack strength and fracture toughness,leading to easier strain-hardening properties of the composite.It is recommended that the fiber length is 9 mm ≤Lf≤23 mm,the fiber diameter is 13 μm ≤df≤38 μm,the fiber tensile strength is 1850 MPa ≤df≤3200 MPa,the fiber elastic modulus is 70 GPa ≤Ef≤160 GPa,the fiber volume doping is Vf≥1.3%,the interfacial friction stress is 1.0 MPa≤τ0≤2.9 MPa The recommended range of slip hardening coefficient is 0<β≤0.006,braking coefficient is 0.2 ≤f ≤0.6 and fiber strength discount factor is f’≤ 0.74.Moreover,the sensitivity analysis results of key parameters indicate that adjusting the fiber content and interfacial friction stress can significantly affect the performance of UHS-HDCC under the premise that the fiber properties meet the strain hardening criteria.(4)Based on the analysis of key parameters of matrix in UHS-HDCC meso-mechanical,the influence mechanism of GGBS and SF on the microstructure and mechanical properties of UHS-HDCC was investigated.It was found that increasing the content of GGBS and reducing the content of SF have a positive impact on the fluidity of UHS-HDCC matrix and mixture,while reducing the yield stress,maximum shear stress,and plastic viscosity of the freshly mixed matrix,which was beneficial to improving fiber dispersion.Increasing the content of GGBS and decreasing the content of SF decreased the peak value of the total heat release and heat flow curve of the mixture,but increased the porosity of the matrix.The Ca(OH)2 content of the hardened matrix decreased slightly when the GGBS content exceeded 20%.UHS-HDCC exhibited significant strain-hardening and multiple cracking characteristics after uniaxial tensile and four-point bending tests,and the tensile and bending strength of S20G20 specimens reached their maximum values.The increase of GGBS content and the decrease of SF content promote the cracking crack of UHS-HDCC to develop into saturated state.(5)Based on the analysis of key parameters of fiber matrix interface in UHS-HDCC mesomechanical,a functionalization technique using dopamine(DA)and nano-SiO2 deposited on the PE fiber surface is proposed to enhance the frictional stress at the fiber-substrate interface bond based on the analysis of key UHS-HDCC parameters and the optimal range of values.The polymerization of DA into poly dopamine(PDA)on the PE fiber surface transforms the PE fiber surface from hydrophobic to hydrophilic,providing a secondary reaction platform for the deposition of nano-SiO2.The nano-SiO2 on the fiber surface reacts with Ca(OH)2 in alkaline solution to form a C-S-H gel,which enhances the densification of the fiber-matrix transition zone.The modified fiber improves the tensile strength and tensile strain capacity of UHSHDCC,as well as the energy absorption capacity required to generate cracks and pull out the fibers.Finally,potential mechanisms for enhancing the strain-hardening behavior of UHSHDCC using DA and nano-SiO2 deposited surface-functionalized PE fibers are revealed.