Fracture Properties Of Multi-scale Fiber Reinforced Cementitious Composites

Hybrid fiber reinforced cementitious composites（HFRC）exhibit better performance than single fiber-reinforced cementitious composites,because it benefits from the advantages of each involved fiber and synergistic effect.HFRC is a highly heterogeneous material with inevitable native defects（e.g.micro-cracks and pores）due to its complex composition.Under the load and environment effects,these native defects are easy to cause cracking,which results in the decrease in reliability or even leads to structural instability and fracture for HPFC.Calcium carbonate whisker（CW）is a kind of inorganic mineral material with excellent physical and chemical properties and has dual micro-fiber and micro-filler characteristics.Also,the good compatibility with cement matrix enables CW an ideal micro reinforcing material for cementitious composites.Adding CW into HFRC not only strengthens the composites at hydration products and cement slurry level,but also enhances the crack-resistance through optimizing the hybrid fiber effect at other levels.In this paper,CW,polyvinyl alcohol（PVA）fiber and steel fiber were used to develop a multi-scale fiber reinforced cementitious composites（MFRC）,and its fracture properties of MFRC were studied emphatically.The cracking-resistance mechanism of multi-scale fiber in MFRC was analyzed,the real fracture parameters were determined,and the prediction method of fracture failure behavior were proposed.Specific contents and conclusions are shown as follows:（1）The dispersity and stability of CW in the aqueous solution and cement slurry were investigated.The rheological and fracture properties of CW reinforced cement paste（CWRCP）were optimized.The results showed that the CW suspension prepared by using ppolycarboxylate superplasticizer and ultrasonic dispersion simultaneously benefited to the CW dispersion in cement paste.The addition of CW generated more amount of calcium silicate hydrated（C-S-H）gel,filled the area among C-S-H gel,reduced the pore phase,thus improving the micromechanical properties of cement paste.The improvement of CWRCP performance was not only related to CW pull-out,crack deflection,CW fracture,and CW bridging but also affected by CW dispersion.The response surface methodology（RSM）analysis showed that the water-cement ratio（w/c）had the most sifnificant influence on the rheological properties,while the CW content(v_cw),w/c,and their interactions had a considerable effect on fracture toughness(K_IC).A multi-objective simultaneous optimization technique was used to obtain the best settings for v_cwand w/c that met the requirements of the best rheological properties and maximum fracture toughness of CWRCP.（2）The mechanical properties of MFRC were studied,and the fiber dispersion and orientation were quantitatively evaluated.The specimen with 1.0 vol.%CW,0.5 vol.%PVA fiber and 1.5 vol.%steel fiber（S15P05W10）had the best flexural tensile properties due to the strengthening and toughening effects of the reasonable fiber hybridization as well as the better fiber dispersion and orientation.Backscatter（BSE）imaging analysis showed that S15P05W10had a better fiber distribution with more fibers orienting to the stress direction,especially for steel fiber.Compared to S15P05W00,the steel fiber distribution coefficient(α_-steel)and the number of steel fibers per unit area(n_f-steel)of S15P05W10 increased 9.0%and 37.5%,and the steel fiber orientation factor(α_o-steel)and orientation number(ηθ_-steel)increased 47.9%and 33.7%.The improvement of plastic viscosity（cohesion）and yield stress（flow expansion）by CW was the critical reason for regulating the steel fiber orientation in mortar.The specimen with 2.0vol.%CW,0.5 vol.%PVA fiber and 1.5 vol.%steel fiber（S15P05W20）exhibited the best unaxial compressive properties.The piecewise function composed of quintic polynomial and rational fractions could describe the stress-strain behavior of MFRC under uniaxial compression well with ascending curve parameter 1≤a₁≤1.67 and descending curve parameter0.8≤b₁≤20.（3）The composition of MFRC with the best fracture properties was determined and the multi-scale fiber resistance mechanism was clarified.The S15P05W10 had the best fracture properties and the most sifnificant gain ratio of fracture parameters according to the Double K fracture criterion（DKFC）and work of fracture method（WFM）.S15P05W10 showed a positive hybrid effect on K_IC^unand G_F but a negative hybrid effect on K_ICⁱⁿⁱ,which was related to the contribution of hybrid fibers at different scales.The multi-scale crack resistance of MFRC could be divided into three processes,i.e.,the micro-crack inhibition before crack propagation,the multi-scale cracking-resistance during stable development of submicroscopic virtual crack,and the reinforcement and toughening of steel fiber during unstable growth of the macro-crack.The calculation models of fracture parameters were established by considering the fiber reinforcement index（RI_v）and w/c.The calculated results were consistent with the experimental results.（4）Three-point bending notch tests with different specimen sizes and different initial crack depths was carried out on S15P05W10 with the best fracture performance.According to the statistical analysis of BSE images of hybrid fibers on the cross-sections,the calculation formula of virtual crack propagation was modified by introducing the dimension of the long axis of fibers.Whereafter,the size-independent material parameters were determined by using the boundary effect model（BEM）and the results were in good agreement with the calculated K_ICby size-effect law（SEL）.Finally,the structural fracture failure curves of MFRC were constructed based on the determined K_IC and tensile strength（f_t）,which provided a basis for the structural design and safety assessment of MFRC.（5）The physical properties,fracture properties,and microstructures of MFRC under freeze-thaw cycles were studied and a physical model of multi-scale fiber action mechanism was proposed.It was observed that the CW could alleviate but not entirely inhibit freeze-thaw damage according to the nondestructive and fracture tests.The microstructure analysis showed that CW was conducive to reducing porosity,optimizing pore structure and ITZ through micro-aggregate filling and micro-fiber cracking resistance effect,and thus delaying the occurrence of freeze-thaw damage.The physical model of multi-scale fiber action mechanism indicated that CW mainly acted on the initial freeze-thaw development damage stage to weaken the stress concentration caused by hydrostatic pressure and osmotic pressure.The introduction of CW would change the acceleration of freeze-thaw damage（e_g）.The service life of MFRC was predicted by the parabola model of freeze-thaw damage,in which MFRC with 3.0 vol.%CW had the most extensice field service life.