Modified Micromechanical Model And Regulation Mechanism Of High Ductility Cementitious Composite

With the continuous advancement of the "13th Five-year Plan" and the "One Belt,One Road" initiative,more and more large and important projects have been positively implemented,and the infrastructure construction has become a key issue.Considering the safety,reliability and durability of the civil structure,higher requirements are put forward for concrete performance.High ductility cementitious composites(HDCC)show incomparable advantages in enhancing the safety,durability and sustainability of structures due to its stable strain-hardening characteristics,ultra-high tensile ductility and excellent crack control ability.Based on the micromechanical bridging theory,HDCC can be designed by controlling the properties of fiber,matrix and fiber matrix/interface.In this paper,based on the requirements of strength criterion and energy criterion,the fiber/matrix interface bonding properties,fiber properties and fiber spatial distribution characteristics were studied to further improve the HDCC mechanical properties and deepen the HDCC design theory,the micromechanical design and regulation of HDCC were realized finally.The main research contents are as follows:Firstly,based on the traditional HDCC micromechanics bridging theory,the whole process of PVA fiber pullout from hardened cement matrix was analyzed in this paper.By considering the debonding rupture and slipping rupture,the fiber rupture zones were determined.Then,the fiber loss bridging stress due to debonding rupture and slipping rupture were calculated.Finally,the modified HDCC micromechanics model with including debonding rupture analysis and slipping rupture analysis was established,which is more suitable for calculating the bridging stress of PVA fiber with significant slipping rupture effect.The accuracy of the modified micromechanics model is verified by comparing the measured fiber bridging stress ? and crack width ? relationship with calculation results.Secondly,in order to achieve the design of fiber/matrix interfacial bonding properties,the effect of fiber/matrix interfacial bonding strength ?₀ on the fiber bridging stress ?₀ and complementary energy J_b^' were investigated in this paper.According to the strain hardening index requirements of PSH_s?1.5 and PSH_E?3.0,the optimum fiber/matrix interfacial bonding strength ?₀ of PVA fiber,PET fiber,PE fiber and steel fiber were proposed,which were 1.6 MPa,1.4 MPa,2.2 MPa and 12.0 MPa,respectively.Furthermore,the performance requirements of the corresponding HDCC matrix were determined.The invalid ?₀ ranges,medium ?₀ ranges and optimum ?₀ ranges were proposed corresponding to low strength matrix(?cr=3.50 MPa,J_tip=17.50 J/m²)and high strength matrix(?cr-=8.20 MPa,J_tip=46.40 J/m²),hence the purpose of fiber/matrix interfacial bonding design was realized.Theoretical calculation results showed that the requirements of PSH_s?1.5 and PSH_E?3.0 can be satisfied simultaneously when the volume content of steel fiber is increased to 4.7%under the optimal interfacial bonding strength of 12.0 MPa.Besides,the maximum interfacial bonding strength without rupture effect for PVA fiber,PET fiber,PE fiber and steel fiber were proposed.It was found that sufficient fiber bridging capacity can not be provided when the fiber was completely pulled out without rupture phenomenon during cracking propagation.However,moderate fiber rupture proportion(about 10%)was more conducive to improve the fiber bridging ability.Thirdly,in order to control the fiber/matrix interfacial bonding properties,the surface of PVA fiber was modified by hydrophobic silica coating and nano graphite coating.The static contact angle of S-PVA fiber modified by hydrophobic silica coating and G-PVA fiber modified by nano graphite coating were 141° and 132°,respectively.PVA fibers show excellent alkali resistance,the alkali resistance of PVA fiber was slightly improved after surface modification,and the tensile strength retention of S-PVA fiber and G-PVA fiber were 95.0%above.Compared with oil modified PVA fibers(N-PVA and K-PVA),the chemical bonding strength Gd decreased or even disappeared for S-PVA fiber and G-PVA fiber.Meanwhile,the interfacial bonding strength ?₀ increased by 11.9%due to the increase of fiber surface roughness.After modified by nano graphite,the abrasion phenomenon was disappeared during G-PVA fiber pullout from HDCC matrix,and the slipping-hardening phenomenon of fiber/matrix interface was effectively controlled.The ultimate elongation of HDCC was increased by 13.4%and 92.3%by the optimization of fiber/matrix interface properties,which reached 1.61%and 2.73%for S-PVA-HDCC and G-PVA-HDCC,respectively.Compared with K-PVA fiber,the domestic PVA fiber modified by nano graphite coating achieved better fiber/matrix interface control effect,and the ultimate elongation of HDCC was increased by 66.5%.Fourthly,considering that fiber length is easier to control in fiber properties,the influences of PVA fiber length on fiber rupture effect,fiber bridging stress ?₀,complementary energy J_b^'and composites fracture energy G_c were studied in this paper,the optimum length range for short cut PVA fiber was determined.The optimum length range need achieve the requirements of PSHS?1.5,PSH_E?3.0,and should be less than 2Ld(0),which is 8.8 mm-13 mm.Moreover,the optimum length can be determined when the composites fracture energy G_c reached maximum value,which is 10 mm.Beisides,based on the research of the effect of fiber/matrix interface bonding strength on the optimal fiber length design,it found that the optimum fiber length decreased with the increase of fiber/matrix interfacial bonding strength ?₀.PVA fiber with 6 mm,9 mm,12 mm,18 mm and 24 mm were used,HDCC with diverse compressive strength ranged from 30 MPa to 80 MPa were prepared.Experimental results showed that the compressive strength was less affected by the fiber length.9 mm and 12 mm fibers showed better four point bending properties and uniaxial tensile properties.Considering the dispersion of fibers,the workability and mechanical properties of HDCC,PVA fiber with 9 mm length is the best candidate.Finally,the fiber bridging ability will be further decided by the fiber orientation distribution when the fiber properties and fiber/matrix interface properties are determined,hence the effects of fiber orientation distribution on the bridging ability and mechanical properties of HDCC were studied.The theoretical results showed that the fiber orientation distribution with smaller inclination can obtain stronger fiber bridging ability.The fiber bridging stress with 2D distribution is 125%that of 3D distribution,which reveals that the mechanical properties of HDCC thin plate specimens are generally better than those of prismatic specimens.Besides,three kinds of HDCC specimens with different fiber orientation distribution were prepared,and the quantitative detection of fiber orientation distribution on HDCC fracture plane was realized by using backscattered electron imaging(BSE)combined with digital image processing technology.PVA fibers dispersed in cement matrix can be regarded as defects,the more inclined the fiber,the more obvious the defect effect.The results showed that the initial cracking stress of HDCC decreased under uniaxial tension and four point bending,and the decrease range under uniaxial tension was 39.7%?99.4%higher than that under four point bending.The tensile properties of HDCC showed more sensitive to the defect size.The larger the fiber orientation distribution was,the lower the uniaxial tensile strength and ductility,the ultimate flexural strength and the mid-span deflection.The fiber orientation distribution had a significant effect on the energy consumption of HDCC under bending and tension.For HDCC members requiring high energy consumption,the possibility of three-dimensional distribution of fibers should be reduced as much as possible,and it is better to tend to two-dimensional distribution during the casting period.Through the research of this paper,the influence mechanisms of fiber characteristics and fiber/matrix interface characteristics on fiber bridging abilities were explored,optimal range of the key physical and mechanical parameters for the fiber and the fiber/matrix interface were determined,the micromechanical design theory were developed,the micromechanical control mechanism was presented.