Abstract

This study mainly investigated the influence of multi-scale fiber combinations on mechanical properties and strain hardening behavior of fiber reinforced concrete(FRC).After adding to FRC,the fibers of different scales can complement each other and play a full role in different stages of stress and structure,so as to achieve the purpose of crack resistance,toughening and strengthening gradually.Firstly,two types of polymer fibers(PVA fiber and PE fiber)were considered.By comparing the strength and toughness of FRC which incorporated with either polymer fibers,the better polymer fibers were optimized.Then,the optimized polymer fibers were blended with the large size hooked-end(macro-scale)steel fiber in different dosage in order to investigate the strain hardening behavior of FRC prepared with binary combination of polymer-steel fibers.Thus,the best binary combination ratio was found.Macro-scale fibers and meso-scale steel fibers were blended in dif ferent dosage as well to determine the best dosage of steel fibers in binary combination.On this basis,the best combination proportion of two groups was applied to the ternary combination which included polymer fiber,meso-scale steel fiber and macro-scale steel fiber.In addition,the micro-scale steel fiber was also selected as the control group in place of the polymer fibers to explore the strain hardening behavior of FRC.Finally,we will add nano-fibers(Figure 1)on top of the ternary combination,so that the FRC was enhanced and toughened at multiscale and multi-level which included nanoscale,micron and millimeter levels to achieve better strain hardening behavior.Tests showed the mechanical properties in terms of compressive strength,splitting tensile strength,three-point bending fracture energy(Figure 2)and direct tensile performance.

The results indicated that the toughening effect of PE fiber on FRC was better than that of PVA fiber at 0.18 water-binder ratio.With the increase of steel fiber dosage,the compressive strength,splitting tensile strength,threepoint bending fracture energy,flexural toughness and tensile strength of FRC with steel-PE and steel fiber combination increased,and both the strength and toughness were improved as well as the strain hardening behavior which was more obviously.Among them,the fiber combination of 1.5% macro-scale steel fiber and 0.5% meso-scale PE fiber showed the best strain hardening behavior as well as the fiber combination of 1.5%macro-scale steel fiber and 0.5% mesoscale steel fiber.

The dosage of macro-scale steel fibers was adjusted on the basis of the binary fiber combination when it comes to ternary combination.The fracture energy obtained from three-point bending test and the flexural toughness were improved on the premise of keeping the peak load almost unchanged.When it came to quaternary combination,nano-fibers were added on top of ternary combination.After adding nano-fibers,the strength and toughness of FRC were improved to a certain extent.The use of carbon nanofibers(CNF)and carboxyl carbon nanotubes(CNT2)was more conducive to the improvement of fracture energy obtained from three-point bending test,four-point bending and direct tensile performance.After adding 0.1wt%CNF,the three-point bending fracture energy of FRC increased by 37.4% and 4.3%,respectively.The equivalent bending strength of four-point bending increased by 20% and 22.4% respectively.The ultimate tensile strain of direct tension increased by 14.3% and 75% while the direct tensile fracture energy was increased by 26%and 95.4%,respectively.At the same time,the nanofibers can refine the large pores by filling the pore structure between the hydration products of cement,and improve the pore structure of FRC which makes the microstructure more compact and thus effectively restricts the formation of harmful pores.

The anti-cracking effect of multiscale fiber combination is multi-step and multi-level,so that FRC has experienced a stable crack generation and expansion process until destroy.Therefore,compared with fiber or single scale,multiscale fiber combination of FRC has better strength,toughness and better strain hardening behavior.

Figure 1　Sample of a nano-fibers

Figure 2　Schematic of three-point bending beam(unit:mm)