Research On Strengthening And Toughening Of Nano-SiO₂ And Fiber Modified Cement-based Composites

Adding fibers is a common method for strengthening and toughening cement-based materials.The study of how to achieve fiber-reinforced and toughened effects and achieve the synergistic enhancement of the strength and toughness of fiber cement-based composites has important scientific significance and engineering value.The mechanical behavior of fiber cement-based composites is the result of a combination of many factors,which are related to the coupling of fiber properties,cement matrix properties,and fiber-cement matrix interface properties.This paper starts from these three levels to construct a fiber-reinforced cement-based material toughness design method,studies the multi-parameter matching relationship between the fiber elastic modulus and content,matrix strength and interface strength based on the toughness target,and develops a nanometer-based Modified and fiber-hybrid interface strength and matrix parameter adjustment methods have formed the systematic results of nanomaterial-fiber cement-based composites’toughness design theory and realization method.The specific research content is as follows.Firstly,the constitutive relationship of single-crack fiber bridging stress and its influencing factors were studied,and the fiber distribution state and its contribution to the crack bridging stress were considered in discretization,and a numerical calculation model for the relationship between fiber cement-based material bridge stress and crack width was established.Discretization considers the influence of the fiber elastic modulus and the distance from the crack opening on the stress intensity factor of the crack tip,and proposed a calculation method for the fracture toughness and cracking strength of fiber-reinforced cement-based composites with medium and high modulus,and corrects the multi-slit cracking of fiber cement-based composites.Based on the criterion,a strengthening and toughening design model was constructed to improve the bridging stress（strength）and the multi-slit cracking ability（toughness）.Based on the multi-parameter matching relationship and the critical value determination method,the method of strengthening and toughening based on the above-mentioned parameter regulation was proposed.Secondly,the method of fiber-matrix interface parameter adjustment and the realization way of strengthening and toughening were studied.Invented a method for modifying carbon fiber surface hydration active nano-SiO₂ layer,which can react with cement to improve the bonding strength of the interface.Through environmental scanning electron microscopy,atomic force microscopy,energy spectrum analysis,infrared spectroscopy and other observation methods,the surface morphology of the modified fiber,the product and structure of the fiber-matrix interface were analyzed and the modification process was optimized.The interface performance parameters of the modified carbon fiber were determined through the single-filament pullout test,and the interface strength was increased by more than 2 times.A modified carbon fiber cement-based composite material was designed,and the mechanical properties of the modified carbon fiber cement-based composite material were studied through bridge stress tests and macroscopic tensile and flexural tests,which achieved a substantial synergistic improvement in strength and toughness.Thirdly,the method of cement matrix parameter control and the realization way of strengthening and toughening was studied.A method of nano-SiO₂ modified matrix to achieve strengthening and toughness was proposed.The effect of nano-SiO₂ on the properties of high-volume fly ash cement matrix and PE fiber-matrix interface performance was studied.It was found that the modification of matrix nano-SiO₂ can synergistically improve the matrix Interface performance.When the content of nano-SiO₂is 2%,the bonding strength of PE fiber-matrix interface increased by 113.25%,the initial cracking strength of cement-based materials increased by 30.93%,and the fiber bridging stress increased by 31.17%,the synergistic improvement of strength and toughness was realized.The modification mechanism of nano-SiO₂ was revealed through pore structure analysis,XRD diffraction analysis and microstructure characterization.Combining the image processing method and the strengthening and toughening design method,the bridge stress test is deeply analyzed.The uniaxial tensile test verifies the feasibility of adjusting the matrix parameters to achieve the strengthening and toughening,and obtains its strengthening and toughening characteristics.Finally,the method of adjusting the parameters of hybrid fiber（carbon fiber and PE fiber）and the realization way of strengthening and toughening were studied.Discretization considered the influence of the hybrid fiber on the stress intensity factor of the crack tip,and calculates the fracture toughness and cracking strength of the medium and high modulus hybrid fiber reinforced cement-based composites,and proposed the strengthening and toughening of the hybrid fiber bridging stress and the multi-slit cracking ability.Design ideas and calculation methods,and based on the toughness design method,predicted the critical fiber content required for the multi-slit cracking of the CF-PE hybrid fiber system.Through the bridge stress test,the matching relationship and the cooperative working mechanism of the hybrid fiber content are analyzed and verified.Through nano-material matrix modification and the synergistic effect of nano-material surface-modified fibers and hybrid fibers,the performance of the material is further improved,and a strong and tough cement-based material based on multi-parameter control has been developed.