Research on the Structural Traits and Upscaling of Parameters of ITZ within Multi-scale Carbon-based Fiber Strengthened concrete

To thoroughly explore the structural traits of interface transition zone (ITZ) within fiber strengthened concrete, carbon fiber strengthened concrete (CFSC), carbon nanotube strengthened concrete (CNSC), and multi-scale carbon fiber strengthened concrete (MCSC) were selected as the primary research objects. The nanoindentation test was employed to determine the elastic modulus and hardness of ITZs in CFSC, CNSC, and MCSC respectively, and the thickness of ITZ was accurately defined. The deconvolution method was used to conduct a comparative investigation of the phase composition and distribution characteristics of ITZ and cement matrix. Parameter-scale analyses were carried out based on the Mori-Tanaka model and Voigt-Reuss-Hill algorithm. The research results reveal that the thickness of ITZ in CFSC is approximately 16 µm, while that in CNSC and MCSC is 12 µm. Compared with CFSC, the abundances of high-density calcium silicate hydrate and calcium hydroxide in ITZs of CNSC and MCSC are significantly increased, and the amount of low-density calcium silicate hydrate is notably decreased. However, the pore content in CNSC and MCSC is relatively higher. The main factor contributing to the difference in mechanical properties between ITZ and cement matrix lies in the different proportions of each constituent phase. Moreover, its microstructural and phase-distribution characteristics play a crucial role in determining the macroscopic strength properties of concrete. The equivalent elastic models calculated by the parameter-scale upgrading method are ranked in descending order as CNSC > MCSC > CFSC. This indicates that in terms of the catalytic effect on the cement hydration reaction, carbon nanotube (CN) at the nanoscale is the most effective, followed by the multi-scale carbon fiber (MC) at the micro-nanoscale. Simultaneously, the smaller the thickness of ITZ, the better its mechanical properties. The research conclusions provide a scientific basis for uncovering the microstructural characteristics and micro-interface strengthening mechanisms of fiber strengthened concrete.