【正文】 gth 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。 邊界條件為： 1) X=0， Y=0； 2) X=0， dy／ dx=E0 ／ Ep； 3)X=1， Y=1， dy／ dx=0． 由邊界條件可得公式 (5)可以簡(jiǎn)化為： （ 5） 系數(shù) 可 以通過試驗(yàn)數(shù)據(jù)回歸獲得 (6) 式中： E0 為圓點(diǎn)切線模量； EP 為峰值應(yīng)力點(diǎn)割線模量 (第一峰值 )。當(dāng)基體強(qiáng)度很高時(shí) (C80)，由于纖維拔斷現(xiàn)象影響了 F3 型的增韌效果， F4型鋼纖維的增韌效果叉反過來超過了 F3型鋼纖維。 從上我們可以發(fā)現(xiàn)，基體強(qiáng)度和纖維含量?jī)煞N參數(shù)的有規(guī)律的改變很相似，因此我們分析的重點(diǎn)應(yīng)放在韌性指數(shù)上。因?yàn)槠渑c基體問的粘結(jié)力較小因此在試驗(yàn)過程中 沒有纖維拔斷現(xiàn)象出現(xiàn)。粗骨料采用 5～ 20 石灰?guī)r碎石。另外，在強(qiáng)力作用下，鋼筋混凝土的應(yīng)力 —— 應(yīng)變曲線受多種因素的影響。對(duì)纖維混凝土增強(qiáng)機(jī)理進(jìn)行研究，要獲得鋼纖維混凝土的受拉全過程曲線，采用軸拉方法最為適宜，但是要在試驗(yàn)方法上作一定改進(jìn)，并且試驗(yàn)機(jī)要有足夠的剛度，來保證試驗(yàn)過程的穩(wěn)定。 表一 、試件 用建筑結(jié)構(gòu)膠將軸拉試件粘貼于兩端的鋼墊板上。并且隨著基體強(qiáng)度升高，由于黏結(jié)力的增大，該纖維增強(qiáng)效率有持續(xù)提高。 摻有四種鋼纖維及素混凝土試件基體強(qiáng)度與軸拉韌性指數(shù)的關(guān)系成比例，其中纖維混凝土試件中鋼纖維體積摻率均為 1． 0％。 鋼纖維鋼筋混凝土單軸拉伸應(yīng)力 —— 應(yīng)變曲線 典型的鋼纖維高強(qiáng)混凝土軸拉應(yīng)力一 應(yīng)變?nèi)€ (為了便于比較，每組試件選出條典型曲線作為代表 )，表述了軸拉曲線隨基體強(qiáng)度的變化規(guī)律；表述了軸拉曲線隨鋼纖維 (F3 型 )摻量的變化規(guī)律。 因此公式 (6)可以轉(zhuǎn)換為： (7) 下降段公式 下降段數(shù)學(xué)的模型為： （ 8） 式中： 和 為與基體和鋼纖維特性有關(guān)的參數(shù)。 and when the matrix strength is high， fiber breaking occurs． Therefore， the toughnes