Abstract: The toughness of fiber reinforced concrete comes from the bridging effect of fibers, and the core of this effect depends on the bonding performance of fibers and cement matrix. To explore the concrete toughening mechanism of fibers, we conduct single fiber pull-out tests (SFPT) for polypropylene fiber-reinforced concrete under different conditions of curing ages (0 - 28 d), pull-out rates (1 - 30 mm/min), fiber anchorage lengths (10 - 60 mm), and fly ash contents (0 - 30%). Based on the principle of energy dissipation and the experimental data, we work out a mathematical relationship of the concrete toughness index versus the energy absorption index for fiber pull-out, and develop a calculation and analysis method for the influencing factors of interfacial bond strength based on the generalized grey relational theory. The results show that with the increase in curing age, pull-out rate or fiber anchorage length, the bond strength of the polypropylene fiber-cement matrix interface increases gradually, but the toughness index is better at the curing age of 18 h, and peaks at a pull-out rate of 6 mm/min and an anchorage length of 20 mm. And the interface bonding performance is the best at the fly ash content of about 20%. Based on the general grey correlation analysis method, mathematical correlation is clarified between the interfacial bond strength and the four factors. This study deepens our understanding of fiber-cement bonding and promotes the design and application of fiber-reinforced concrete.

Key words: concrete, polypropylene fiber, interfacial bonding, grey correlation analysis, toughness index