Abstract: Carbon nanofibers (CNFs) and carbon fibers (CFs) are nanoscale and micrometer carbon-based fibers, respectively. By mixing them into concrete, one kind of carbon nanofibers modified concrete (CNFsMC, the volume content of CNFs is 0.3%) and four kinds of multi-scale carbon fibers modified concrete (MCFsMC, the volume content of CNFs is fixed at 0.3%, and the volume content of CFs is 0.05%, 0.10%, 0.15%, 0.20%, respectively). The effect of multi-scale carbon-based fibers on mechanical properties and frost resistance of concrete were studied, and the modification mechanism of multi-scale carbon-based fibers on concrete was analyzed. The results show that the mechanical properties and frost resistance of concrete can be further improved by adding appropriate content of CFs on the basis of 0.3% CNFs. When the content of CFs is 0.1%, the mechanical properties and frost resistance of MCFsMC are the best, and the total porosity and average pore size before and after freeze-thaw cycle are the minimum. And the compressive strength, flexural strength, flexural-compressive strength ratio and splitting tensile strength of MCFsMC increase by 13.93%, 25.08%, 9.83% and 20.15%, respectively, compared with CNFsMC. After 200 freeze-thaw cycles, the mass loss rate of MCFsMC is only 0.95%, the relative dynamic elastic modulus is 89.2%, and the strength loss rate is less than 14%. CNFs and CFs have positive hybrid effect, and they can complement each other, construct multi-scale crack resistance network and effectively play an effect on cracks with different sizes, and double enhance the mechanical properties and frost resistance of concrete.

Key words: concrete, carbon nanofiber, carbon fiber, multi-scale, mechanical property, frost resistance