Mechanical properties and cross-scale synergistic modification mechanism of micro-nano carbon fiber modified concrete

In order to explore the cross-scale synergistic modification effect of carbon fiber (CF) and carbon nanofiber (CNF) on the mechanical properties of concrete, based on CNF modified concrete (NCFC, with CNF volume content of 0.3 %), four kinds of micro-nano carbon fiber modified concrete (MNCFMC, with CNF volume content of 0.3 % and CF volume content of 0.1∼0.4 %) are prepared. For comparison, four kinds of CF modified concrete (CFMC, with CF volume content of 0.1∼0.4 %) are also prepared. The compressive, flexural and splitting tensile strength of concrete are tested, and the cross-scale synergistic modification mechanism is analyzed by SEM and MIP tests. The results show that CF and CNF have cross-scale synergistic improvement effect on the mechanical properties of concrete. The mechanical properties of concrete can be further improved by adding CF with appropriate content on the basis of the addition of CNF. With the increase of CF content, the compressive, flexural and splitting tensile strength of MNCFMC first increase and then decrease. The mechanical properties of MNCFMC are the best when the CF content is 0.2 %, the compressive, flexural and splitting tensile strength of MNCFMC increase by 14.12 %, 19.69 % and 30.32 %, respectively. Compared with CFMC, the mechanical properties of MNCFMC with the same CF content are better. The physical bond between CF and concrete matrix is poor, CNF can enhance the physical bond between CF and concrete matrix by attracting the deposition of cement hydration product crystals. When CF and CNF are added together, the pore size refinement and pore structure optimization effects of fiber on concrete are the best. When the CF content is 0.2 %, the average pore size and total pore volume of MNCFMC decrease by 30.66 % and 16.86 % compared with CFMC, respectively.