【正文】 in Table 3．The addition of steel fibers into concrete increased its crack stress an d ultimate tensile strength．And the ratios of these two parameters of SFRC to those of plain concreue (with the same mix proportion)are given in Table 3，too．3．1．1 Effect of matrix strength an(1 fiber type From table 3．It can be seen that the effects of steel fibers 0n crack stress are little influenced by the matsix strength．That is to say．When the matrix strength increases, the ratios of crack stresses of SFRC ( with the same type of fibers contained)to those of plain concrete ones with the same mix proportion are invariable． However，the condition for ultimate tensile strength is different．When the matrix strength increases．these ratios of ultimate tensile strengths(shown in Table 3)vary dissimilarly according to the type of steel fiber．Moreover．the increments are bigger than those of crack stress． The heightening efficiency of fiber F1 for ultimate tensile strength rises as matrix strength increases．It is because that the strength of this kind of fiber is very high(1 100 MPa)．No fiber broken was observed during the test and the hooked—ends of the fibers were straightened when the matrix strength was high(C80)．The higher the matrix strength． this kind of steel fiber takes on its strengthening effect more efficiently for the increasing of bond stress．The strengths of fibers F2 and F3 are mid—high(700 MPa)．They all have hooked ends and both of their surfaces are coarse．When the matrix strength was high(C80)．fiber breaking occurred in the test．And this phenomenon impaired the heightening efficiency of these two kinds of steel fiber．So they should be used in middle strength concrete to exert their strengthening effect more efficiently．Fiber F4 is smooth．a(chǎn)nd its bond stress with matrix is paratively low．T}1erefore．its strengthening effect is 1ess notable than those of other kinds of fiber．Because of the low bond stress．no fiber broken was found during the test and its heightening efficiency for ultimate tensile strength rises as matrix strength increases．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 factorsFiber code atF1 F2 F3 F4 The tensile strength of SFRC can be calculated with the follow formula： (1)where，fft 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 characters3．2．1 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 onthe 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 loaddisplacement 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，fft 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