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3．1．2 Effect of fiber content
The effect of fiber content on the crack stress and u1．ultimate tensile strength was investigated for SFRC contained fiber F3．And the fiber content varied from 0．5％ to 1．5％ by volume(shown in Table 3)．It can be seen from Fig．1 and Fig．2 that as the fiber content increases．
The crack stress and ultimate strength of SFRC improve obviously．Moreover．the rising trends of the curves in these two figures are stupendously similar．In other words，the effect of fiber content on the characters of tensile stress of SFRC is positive and consistent．
Table 4 Fiber type factors
Fiber code at
F1 0.642
F2 0.862
F3 0.794
F4 0.589
The tensile strength of SFRC can be calculated with the follow formula：
(1)
where，eft is the ultimate tensile strength of SFRC； the ultimate tensile strength of plain concrete with the same mixing proportion；a，the fiber type factor，
which is shown Table 4； is the fiber content 0f volume and l/d is the aspect ratio of steel fibers.
3．2 Tensile strain and toughness characters
3．2．1 Crack strain and the strain at peak tensile load
The tensile strains were acquired by averaging the readings of the four displacement sensors fixed around the specimen．In addition，the specimens whose difference between the tensile strains of its opposite sides is larger than 15％ of their mean value were blanked out．
The crack strain or the strains at peak tensile load of SFRC are much bigger than those of plain concrete(as shown in Table 5)．And the increments go up as the matrix strength or the fiber content increases．Compared to that on crack strain．the increscent effect of steel fiber on the strain at peak tensile load is more remarkable．
3．2．2 Tensile work and toughness modulus
The tensile work was defined as the area under the load-displacement curve from 0 to 0．5 rain．More—over，a tensile toughness modulus was introduced(shown in Table 5)．
It was defined as：
(2)
where，fat is the ultimate tensile strength of SFRC； A，the area of the cross section of specimen．
Both these two parameters were quoted to evaluate the toughness characters of SFRC under uniaxial tension．The tensile toughness modulus is a dimensionless factor．Compared to what the tensile work does．it can avoid the influence of the ultimate tensile strength when studying the toughness of SFRC．
It call be found from Table 5 that the altering regularities of these two factors along with the changes of matrix strength and fiber content are approximate．Therefore，the emphasis of analysis was put on the toughness modulus．
The relationship between the matrix strength and toughness modulus of SFRC with four kinds of steel fiber are shown in Fig．3．whose fiber contents are all 1．O％by volume．together with that relationship of plain concrete．The tensile toughness of SFRC is much better than that of plain concrete．The tensile toughening effect of steel fiber is remarkable．As the matrix strength rises．The brittleness of concrete increases obviously，and then the tensile toughness of plain concrete falls down．This phenomenon was also found on specimens containing fiber F1and F2．The pulling out of fiber F1 from concrete is in fact a process of hook-end’s being straightened and the matrix’s being crushed around the hook-end．When the hooked end is straightened at last．the tensile load falls down quickly．The higher the concrete strength. the larger the rigidity of the matrix and the shorter the time that the process mentioned above lasts．Thus．the stress-strain curve falls down more quickly，and then the toughness modulus decreases．However，the toughening effect of fiber F1 is the best among these four kinds of steel fiber．The aspect ratio of fiber F2 is the least。and when the matrix strength is high， fiber breaking occurs．Therefore，the toughness modulus falls down continually as the matrix strength rises.