【正文】 broken was foundduring the test and its heightening eficiency for ultimatetensile strength rises as matrix strength increases．3．1．2 Effect offiber contentTh e effect of fiber content on the crack stress and u1．timate tensile strength was investigated for SFRC con—tained fiber F3．And the fiber content varied from 0．5％to 1．5％ by volume(shown in Table 3)．It can be seenfrom Fig．1 and Fig．2 that as the fiber content increases．the crack stress and ultimate tensile strength of SFRC im．prove obviously．Moreover．the rising trends of the curvesin these two figures are stupendously similar．In otherwords，the effect of fiber content on the characters of ten．sile stress of SFRC is positive and consistent．Table 4 Fjber tvae factorsFiber code d．FlF2F3F4O．642O．8620．7940．589Th e tensile strength of SFRC can be calculated withthe follow formula：= (1+a ) (1)where， is the ultimate tensile strength of sFRc； theultimate tensile strength of plain concrete with thesame mixing proportion；a，the fiber type factor，which isSh。~acquired by averaging thereadings of the four displacement sensors fixed around thespecimen．In addition，the specimens whose differencebetween the tensile strains of its opposite sides is largerthan 15％ of their mean value were blanked out．Th e crack strain or the strains at peak tensile lcad ofSFRC are much biger than those of plain concrete(asshown in Table 5)．And the increments go up as the ma—trix strength or the fiber content increases．Compared tothat on crack strain．the increscent effect of steel fiber onthe strain at peak tensile lcad is more remarkable．3．2．2 Tensile work and toughness modulusTh e tensile work was defined as the area underthe loaddisplacement curve from 0 to 0．5 rain．More—over，a tensile toughness modulus was introduced(shownin Table 5)．It was defined as：‰s 贏 (2)where， is the ultimate tensile strength of SFRC；A，the area of the cross section of specimen．Both these two param eters were qu oted to evaluatethe toughness characters of SFRC under uniaxial tension．Th e tensile toughness modulus is a dimensionless factor．Compared to what the tensile work does．it can avoid theinfluence of the ultimate tensile strength when studyingthe toughness of SFRC．It call be found from Table 5 that the altering regu—larities of these two factors along with the changes of ma—trix strength and fiber content are approximate．Th erefore，the emphasis of analysis was put on the toughnessmodulus．Th e relationship between the matrix strength andtoughness modulus of SFRC th four kinds of steel fiberare shown in Fig．3．whose fiber contents are all 1．O％by volume．together with that relationship of plain con—crete．The tensile toughness of SFRC is much better thanthat of plain concrete．Th e tensile toughening efect ofsteel fiber iS remarkable．As the ma trix strength rises．thebrittleness of concrete increases obviously，and then thetensile toughness of plain concrete falls down．This pbe．nomenon was also found on specimens containing fiber F1and F2．Th e pulling out of fiber F1 from concrete is infact a process of hok．end’s being straightened and thematrix’s being crushed around the hok．end．When thehoked end is straightened at last．the tensile lcad fallsdown quickly．The higher the concrete strength。在很小比例下，鋼筋混凝土的張拉響應(yīng)可假設(shè)為不硬化的類(lèi)型，它有加大單個(gè)裂縫擴(kuò)展性質(zhì)很像無(wú)鋼筋的素混凝土，鋼纖維對(duì)混凝土開(kāi)裂之后性能的改善作用更加明顯，可以通過(guò)控制裂縫的開(kāi)展從而較大幅度地提高混凝土的韌性。鋼筋混凝土的抗拉特型首鋼纖維的強(qiáng)度和含量影響。通過(guò)調(diào)節(jié)連接試件和橫梁的四個(gè)高強(qiáng)螺栓來(lái)保證試件的軸心受拉。試驗(yàn)中所采用的三種混凝土配合比用于研究，見(jiàn)于表二。細(xì)骨料采用細(xì)度模數(shù)2．6的河砂。張拉應(yīng)力——應(yīng)變曲線由此獲得。也就是說(shuō)在摻人同種鋼纖維時(shí)，隨著基體強(qiáng)度的增加，鋼纖維混凝土與同配比素混凝土的初裂強(qiáng)度的比值基本恒定然而，不同情況下的極限抗拉強(qiáng)度是不一樣的，當(dāng)基體強(qiáng)度增加時(shí)，對(duì)于不同類(lèi)型的鋼纖維，極限抗拉強(qiáng)度的分配量是不同的（見(jiàn)表三）。F4型纖維為長(zhǎng)直型，其與基體問(wèn)的粘結(jié)力較小，因此它的增強(qiáng)效果耍弱于其他二種。試驗(yàn)中鋼纖維體積摻率變化范圍為（見(jiàn)表三）。鋼纖維鋼筋混凝土軸拉極限強(qiáng)度可以用下式來(lái)計(jì)算： （1）式中：．為鋼纖維鋼纖維軸拉極限強(qiáng)度軸拉極限強(qiáng)度。高強(qiáng)SFRC的初裂拉應(yīng)變和峰值拉應(yīng)變要遠(yuǎn)大于同配比素混凝土(見(jiàn)表5)，隨著基體強(qiáng)度或者纖維摻量增大，這個(gè)差值有所增長(zhǎng)，鋼纖維對(duì)峰值應(yīng)變的提高作用要比初裂應(yīng)變更加明顯。兩參數(shù)均用來(lái)評(píng)價(jià)鋼纖維高強(qiáng)混凝土在軸拉過(guò)程中的韌性?？梢?jiàn)高強(qiáng)SFRC的軸拉韌性要遠(yuǎn)遠(yuǎn)優(yōu)于同配比素混凝土。當(dāng)纖維端鉤最終被拉直時(shí)，軸拉荷載很快下降。F3和F4型鋼纖維韌性指數(shù)均隨基體強(qiáng)度升高而增大。這兩種纖維的不同之處是F3型的兩端有彎鉤。圖48表述了軸拉曲線隨基體強(qiáng)度的變化規(guī)律；圖911表述了軸拉曲線隨鋼纖維(F3型)摻量的變化規(guī)律。另外，鋼纖維摻量的提高可以大大地改善曲線的豐滿(mǎn)程度。當(dāng)基體強(qiáng)度較高時(shí)，由于纖維拔斷的出現(xiàn)使得F2和F3型鋼纖維試件的軸拉曲線下降端呈階梯狀。在公式中 （3）式中： 為鋼纖維高強(qiáng)度混凝土軸拉峰值拉應(yīng)變； 為鋼纖維高強(qiáng)度混凝土軸拉峰值應(yīng)力； 為鋼纖維高強(qiáng)度混凝土軸拉應(yīng)力； 為鋼纖維高強(qiáng)度混凝土軸拉應(yīng)變?？梢?jiàn)，理論結(jié)果與試驗(yàn)結(jié)果符合較好。(4)鋼纖維高強(qiáng)混凝土的初裂應(yīng)變和峰值應(yīng)變要比素混凝土的增幅隨基體強(qiáng)度和纖維摻量的升高而增大。(7)綜合而言，四種鋼纖維中，F(xiàn)3型鋼纖維的增強(qiáng)效果最好，而Fl型鋼纖維的增韌效果最好