【正文】 o they should be used in middle strength concrete to exert their strengthening effect more efficiently． Fiber F4 is smooth． and 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 factors Fiber code at F1 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 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 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 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 hookend’s being straightened and the matrix’s being crushed around the hookend． 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 stressstrain 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。 (6)基體強(qiáng)度越高，鋼纖維高強(qiáng)混凝土的軸拉應(yīng)力應(yīng)變曲線在峰值過后下降得越快；纖維摻量的提高可以大大改善曲線的豐滿程度，鋼纖維類型對(duì)曲線形狀也有一定的影響。系數(shù)的取 值通過最小二乘法回歸獲得： （ 9） 可見基體強(qiáng)度和纖維參量對(duì)軸拉曲線下降段的下降速率的影響是相反的。但是，隨著變形的增加，有兩條曲線有明顯的第二峰值出現(xiàn)，而另外兩條則沒有，正是根據(jù)這種現(xiàn)象，可以將其分為增強(qiáng)和增韌兩大類鋼纖 維混凝土，有第二峰值的為增韌類，無第二峰值的為增強(qiáng)類。 從上中可以看到，基體強(qiáng)度越高，軸拉應(yīng)力一應(yīng)變?nèi)€下降得越快。 在鋼纖維和基體之間黏結(jié)力的各組分中，摩擦力起主導(dǎo)作用。在摻有 F1 和 F2 型鋼纖維 的試件中也出現(xiàn)了韌性下降現(xiàn)象。另外，引入軸拉韌性指數(shù)?？梢婋S著纖維摻量增大，軸拉初裂強(qiáng)度和極限強(qiáng)度均有提高。另外它的增加量比劈拉恰強(qiáng)度大 F1 型鋼纖維作為基體的極限抗拉強(qiáng)度很高，這是因?yàn)檫@類型的鋼纖維的強(qiáng)度很高（大于 1100MPa）試驗(yàn)過程中沒有纖維拔 斷的現(xiàn)象出現(xiàn)而且當(dāng)基體強(qiáng)度較高時(shí) (C80)，鋼纖維的端部彎鉤被完全拉直。張拉應(yīng)力 —— 應(yīng)變曲線由此獲得。這些被用來研究鋼纖維混凝土的 C30,C60,C80 混凝土被制