使用加固纖維聚合物增強混凝土梁的延性(中英互譯版)

1、使用加固纖維聚合物增強混凝土梁的延性(中英互譯版) 使用加固纖維聚合物增強混凝土梁的延性作者Nabil F Grace George Abel-Sayed Wael F Ragheb摘要一種為加強結構延性的新型單軸柔軟加強質(zhì)地的聚合物 FRP 已在被研究開發(fā)和生產(chǎn) 在結構測試的中心在勞倫斯技術大學 這種織物是兩種碳纖維和一種玻璃纖維的混合物而且經(jīng)過設計它們在受拉屈服時應變值較低從而表達出偽延性的性能通過對八根混凝土梁在彎曲荷載作用下的加固和檢測對研制中的織物的效果和延性進行了研究用現(xiàn)在常用的單向碳纖維薄片織物和板進行加固的相似梁也進行了檢測以便同用研制中的織物加固梁進行性能上的比擬這種織物經(jīng)過

2、設計具有和加固梁中的鋼筋同時屈服的潛力從而和未加固梁一樣它也能得到屈服臺階相對于那些用現(xiàn)在常用的碳纖維加固體系進行加固的梁這種研制中的織物加固的梁承受更高的屈服荷載并且有更高的延性指標這種研制中的織物對加固機制表達出更大的奉獻關鍵詞混凝土延性纖維加固變形介紹外貼粘合纖維增強聚合物FRP片和條帶近來已經(jīng)被確定是一種對鋼筋混凝土結構進行修復和加固的有效手段關于應用外貼粘合FRP板薄片和織物對混凝土梁進行變形加固的鋼筋混凝土梁的性能一些試驗研究調(diào)查已經(jīng)進行過報告Saadatmanesh和Ehsani1991檢測了應用玻璃纖維增強聚合物 GFRP 板進行變形加固的鋼筋混凝土梁的性能Ritchie等人1

3、991檢測了應用GFRP碳纖維增強聚合物CFRP和GCFRP板進行變形加固的鋼筋混凝土梁的性能Grace等人1999和Triantafillou1992研究了應用CFRP薄片進行變形加固的鋼筋混凝土梁的性能NorrisSaadatmanesh和Ehsani1997研究了應用單向CFRP薄片和CFRP織物進行加固的混凝土梁的性能在所有的這些研究中加固的梁比未加固的梁承受更高的極限荷載這些梁中大多數(shù)出現(xiàn)的一個缺陷是梁的延性有很大的損失然而通過對梁的荷載-撓度性能的測試可以發(fā)現(xiàn)大多數(shù)荷載的增加是在鋼筋屈服后發(fā)生的也就是說極限荷載明顯提高然而屈服荷載卻沒有太大提高因此在正常使用水平荷載的明顯增加很難實

4、現(xiàn)除去加固前混凝土構件條件的影響鋼筋對加固梁的彎曲反響有明顯的奉獻而可惜的是現(xiàn)有的FRP加固材料和鋼材性能不同雖然FRP有很高的強度但是它們多數(shù)在提高足夠的強度之前被拉伸而產(chǎn)生很大的應變因為同大多數(shù)FRP材料的極限應變相比鋼材的屈服應變相對較低所以隨著加固構件的變形鋼材和FRP加固材料的奉獻發(fā)生了變化結果鋼筋可能會在加固構件取得任何可測荷載增加值之前就屈服了一些研究者在橫截面布置了更強的FRP這通常會增加加固的本錢進而提供可測的奉獻盡管這時變形是受限制的在鋼筋屈服之前但是加固材料從混凝土外表的剝落更多的時候是由于應力集中的原因發(fā)生的剝落是這項加固技術中不出現(xiàn)的一種脆性破壞盡管使用一些類似超高模

5、量碳纖維的特別的低應變纖維看起來是一種解決方法但這可能導致由于纖維破壞而產(chǎn)生脆性破壞本文旨在介紹一種新型偽延性FRP織物它在屈服時應變低從而具有與鋼筋同時屈服的潛力能夠?qū)崿F(xiàn)期望中的加固水準研究意義FRP已經(jīng)被越來越多地用做鋼筋混凝土結構修復和加固的材料但是現(xiàn)在常用的FRP材料缺少延性并且與鋼筋性能不一致結果經(jīng)過加固處理的梁會表達出延性降低不能到達期待中的水平或者二者兼有本項研究介紹了一種新型的偽延性FRP加固織物這種織物可以使加固梁承受更高的屈服荷載并且有助于防止延性的損失而這在使用目前常用的FRP進行加固中是常見的混雜織物的研制為了克服前面所提的缺陷一種具有低屈服應變值的延性FRP材料是很必

6、要的混雜的文獻回憶為了研制這種材料考慮了各種不同纖維的混雜多于一種纖維材料的混雜是許多材料科學研究的興趣所在他們的工作多數(shù)集中于結合兩種纖維以提高每種材料單獨工作時的力學特性并且降低本錢這已經(jīng)在幾本出版物中報道過例如Bunsel和Harris1974Philips1976Manders和Bader1981Chow和Kelly1980以及Fukuda和Chow1978做為一種能夠克服FRP加固棒延性缺乏問題的工具混雜吸引了結構工程師NanniHenneke和Okamoto1994研究了用編織芳香尼龍纖維繞在鋼筋核心的短棒Tamuzs和Tepfors報道了關于使用碳和芳香阻尼纖維進行組合而成的混合

7、纖維棒的試驗調(diào)查SomboonsongFrank和Harris1998研制了一種用編織芳香尼龍纖維纏繞在碳纖維核心的混合FRP加固棒Harrissomboonsong和Frank1998使用這些棒對混凝土梁進行加固以得到用常規(guī)鋼筋進行加固的混凝土梁的普通荷載-撓度特性設計思想和材料為了產(chǎn)生延性一種使用不同種類纖維的混雜技術已經(jīng)被采用選用了在破壞時有不同延長量級的三種纖維圖1顯示了這些復合纖維在拉伸時的應力-應變曲線表1顯示了它們的力學特性這項技術是建立在將這些纖維結合起來并控制配合比例的根底上的這樣當它們被拉伸時共同承受荷載延伸小LE的纖維先破壞允許一定的應變松弛應變增加而混合材料的荷載卻并未

8、增加余下的延伸大HE的纖維被分配承當所有的荷載直到破壞延伸小的纖維破壞時的應變值表達了混合材料屈服應變值而延伸大的纖維破壞時的應變值表達的是極限應變值延伸小的纖維破壞時對應的荷載表達的是屈服荷載值而延伸大的纖維承當?shù)淖畲蠛奢d表達的是極限荷載值超高模量碳纖維1號碳被用做延伸小的纖維它應有盡可能低的應變但不得小于鋼筋的屈服應變60級鋼筋大約為02另一方面型玻璃纖維被用做延伸大的纖維應能提供盡可能高的應變而產(chǎn)生高延性指標破壞時的變形和屈服時的變形的比例高模量碳纖維號碳被選做了延伸中等ME纖維它使在延伸小的纖維破壞后發(fā)生應變松弛時荷載的降低最小化并且能夠提供從延伸小的纖維向延伸大的纖維逐漸傳遞荷載的途

9、徑基于這種思想生產(chǎn)了一種單向織物并進行了測試將它在拉伸時的性能和理論預測的承載性能做了比照理論上的性能建立在混合物規(guī)那么上根據(jù)這種規(guī)那么混合物的軸向剛度是將各組成局部的相對剛度進行總合計算得到的這種織物的生產(chǎn)過程是將不同的纖維做為相鄰的紗線結合起來并將它們用環(huán)氧樹脂注入模具中圖就是一個生產(chǎn)樣品的照片編織而成的玻璃纖維片布置在試樣的兩端以消除測試中固定端的應力集中試樣厚mm008in寬254mm1in在拉伸時根據(jù)美國材料實驗協(xié)會3039標準進行測試四個測試樣品的平均荷載-應變曲線見圖3上面還有理論預測的曲線應該注意到直到應變值到達035荷載-應變性能都是線性的這時延伸小的纖維開始破壞在這一點上應

10、變增長的速率高于荷載當應變值到達090時中等延伸的纖維開始破壞導致應變有附加的增長直到由于延伸大的纖維破壞帶來試樣的徹底破壞可以測試到屈服荷載荷載-應變曲線上性能去不再為線性的第一點為046kNmm26kipsin極限荷載為078kNmm44kipsin梁的測試梁的詳細情況一共澆筑了13根鋼筋混凝土梁橫截面尺寸為152×254mm6×10in長2744mm108in梁的受彎鋼筋由底部的兩根5號16mm受拉鋼筋和頂部的兩根3號95mm的受壓鋼筋組成為防止發(fā)生剪切破壞使用162mm長的3號鋼筋扎成閉合鐙形對梁的抗剪進行進一步的加固有5根梁澆筑時角部做成半徑25mm1in的圓角從

11、而易于加固材料的安置圖4顯示了梁的尺度鋼筋詳圖支座和加載點的位置使用的鋼筋為60等級屈服強度415MPa800psi加固材料研制中的混合織物用于加固8根梁使用了兩種不同厚度的織物第一種H體系t 10mm厚度10mm004in第二種H-體系t 15mm厚度15mm006in其他四根梁使用現(xiàn)在常用的碳纖維加固材料進行加固1一層單向碳纖維薄片極限荷載034kNmm195kipsin2兩層單向碳纖維織物極限荷載131kNmm75kipsin3一層固體玻璃談碳纖維板極限荷載為28kNmm16kipsin對這些材料測試得到的荷載-應變圖見圖5表2給出了包括研制中的織物在內(nèi)的加固材料的特性粘結材料對這種混合

12、織物使用一種環(huán)氧樹脂環(huán)氧A注入纖維并做為織物和混凝土外表的粘結材料這種環(huán)氧材料極限應變?yōu)?4從而保證不至于在纖維破壞之前破壞對于使用碳纖維薄片板和織物加固的梁使用的是極限應變?yōu)?0的環(huán)氧樹脂環(huán)氧B由生產(chǎn)商提供的粘結材料的力學特性見表3加固在梁的底部和兩側噴砂以使其外表粗糙然后使用丙酮除去污物對梁進行清潔采用兩種加固構造1只在梁底面布置加固材料A組梁2除對梁底部外在梁兩側各伸長152mm16in大概能覆蓋住梁的受彎拉伸局部B組梁加固材料沿梁長度布置在中心長達224m88in環(huán)氧在對梁進行測試前要進行兩周的養(yǎng)護對研制中的混合織物H-體系加固的梁制備了兩根并對各種構造進行測試來證實結果表4對梁的檢測

13、進行了匯總儀器跨中FRP的應變通過布置在梁底面的三個應變片測量測量A組梁鋼筋拉伸應變是通過監(jiān)控在梁的側面與鋼筋棒平行處測量點設置的DEMC可拆式機械計量器而B組梁使用的是應變片跨中撓度是通過使用串行電位計測量的使用液壓器對梁加載荷載有一種荷載電池測量所有的傳感器同數(shù)據(jù)采集系統(tǒng)相連以掃描并記錄讀數(shù)試驗結果和討論控制梁控制梁的屈服荷載823kN185kips極限荷載957kN215kips梁由于鋼筋屈服而破壞隨之跨中混凝土受壓破壞控制梁的試驗結果見加固梁的試驗成果圖上圖6至15A組梁A組梁已在底面進行了加固圖6至11顯示了這些梁的試驗結果H-50-1梁和H-75-1梁分別和H-50-2梁和H-75

14、-2梁各自的結果非常接近因此關于這些梁的討論就集中于后兩者以防止重復梁的延性通過計算延性指數(shù)來考察即計算破壞時與屈服時的撓度之比圖6a顯示了C-1梁的荷載-跨中撓度關系圖C-1梁使用碳纖維薄片進行加固梁在荷載為859kN193kips時屈服在荷載為1019kN229kips時由于碳纖維薄片的開裂而破壞值得注意的是從這幅圖中看來雖然有了延性性能但是同控制梁比起來屈服荷載只提高了4延性指數(shù)為215圖6b顯示了跨中荷載-碳纖維應變關系圖圖7a顯示了C-2梁對應的荷載-撓度曲線這根梁使用固體玻璃碳纖維板進行加固它沒有屈服臺階延性指數(shù)為1在荷載為1326kN298kips時由于板端部的受剪-受拉破壞而突

15、然破壞盡管荷載提高了61但破壞仍是脆性的圖7b顯示了跨中荷載-碳纖維應變關系碳纖維破壞時記錄的最大應變?yōu)?33這意味著板的承載力發(fā)揮了24C-3梁的荷載-撓度關系見圖8a該梁由兩層碳纖維織物加固它在荷載為1077kN242kips時屈服在荷載為1344kN3021kips時由于織物的剝落而破壞此時它并未如控制梁那樣顯示出任何明顯的屈服臺階延性指數(shù)是164值得注意的是在圖8b中破壞時記錄到的碳纖維應變的最大值為067這意味著纖維承載力大約發(fā)揮了48圖9a顯示了H-50-2梁的荷載-撓度關系這根梁使用研制中的厚度為1mm厚的混合織物進行加固屈服荷載為979Kn220kips同控制梁比起來提高了19

16、在圖9b中值得注意的是當梁屈服時織物應變?yōu)?40它的延性指數(shù)為233當荷載最終到達1148kN258kips時由于織物的徹底開裂而破壞圖9c即為破壞時的梁圖10a顯示了H-75-2梁的荷載-屈服關系這根梁使用厚度為15mm厚的研制中的混合織物它在荷載為1139kN256kips時屈服在1308kN294kips的極限荷載下由于織物剝落而導致徹底破壞之前表達出的延性指數(shù)為213值得注意的是盡管最終破壞是由于織物的剝落但這是在取得了令人滿意的延性之后發(fā)生的從圖10b中可見當梁屈服時的應變?yōu)?35圖10c是梁破壞時的照片圖11和表5對A組梁的試驗結果進行了比擬可以觀察出如下現(xiàn)象1C-1梁和H-50-

17、2梁表達了較好的延性特征但是H-50-1梁比C-1梁表達了更高的屈服荷載這是因為經(jīng)過設計這種研制中的混合織物比碳纖維片有更高的初始剛度因此在鋼筋屈服前它比碳纖維對加固的奉獻更大2盡管碳纖維織物的極限荷載比15mm厚的混合織物屈服時對應的荷載大幾倍但是直到屈服時H-75-2表達著和C-3相似的性能但是H-75-2梁有令人滿意的屈服臺階而C-3梁卻沒有3相對于現(xiàn)在常用的碳纖維加固材料這種研制中的織物屈服時的應變和鋼筋的屈服應變接近盡管仍然較高但是混合織物的應變值和梁屈服時的應變值接近這意味這它和鋼筋同時屈服這一局部要歸功于將植物安置在梁的外外表這樣比安置在梁的內(nèi)部要承受更大的拉應變結果織物的屈服應

18、變設計值看起來是可以接受的4當使用有較高承載能力的碳纖維板正如在C-2梁中使用的時能夠提供高的破壞荷載同時也會產(chǎn)生脆性破壞B組梁這組梁除對梁底部外在梁兩側向上延伸152mm16in的范圍也進行了加固改組試驗結果見表5和圖12至15H-S50-1梁和H-S75-1梁分別和H-S50-2梁和H-S75-2梁各自的結果非常接近因此關于這些梁的討論就集中于后兩者以防止重復圖12a顯示了CS梁的荷載-撓度關系這根梁是使用碳纖維薄片體系加固的它在荷載到達992kN223kips時由于鋼筋的屈服而屈服屈服荷載增加了20梁在到達1233kN277kips的極限荷載時由于跨中混凝土的受壓破壞而破壞從圖12b可以

19、看出當梁屈服時碳纖維的應變?yōu)?35因此在這段承載階段發(fā)揮了它的大約30的能力在梁破壞之前記錄到的最大應變?yōu)?0取得的延性指數(shù)為204H-S50-2的試驗結果見圖13這根梁使用研制中的厚度為1mm厚的混合織物進行加固圖13a顯示了它的荷載-撓度曲線當荷載到達1139kN256kips時由于鋼筋和織物的破壞梁發(fā)生破壞屈服荷載增加了38梁在到達1466kN329kips的極限荷載時由于混凝土的受壓破壞而破壞延性指數(shù)為225圖13b顯示了跨中荷載和織物應變的關系梁屈服時記錄到的應變值是035結論基于本研究所介紹的研究調(diào)查可以得出如下結論1目前常用的FRP材料作為彎曲加固體系用于混凝土結構并不能總是在加

20、固梁中提供類似未加固梁的屈服時的屈服臺階在一些情況下加固可能導致加固梁的脆性破壞或著是屈服荷載增加很不明顯或者是二者兼有2選擇的幾種類型的纖維的混雜被用于研制偽延性的織物它在屈服時的應變低035經(jīng)過設計這種織物具有同加固梁中的鋼筋同時屈服的潛力3同那些應用碳纖維進行加固體系相比使用研制中的混合織物進行加固的梁通常會顯示出在屈服荷載上有更高的增長有些用混合織物進行加固的梁會顯示出類似未加固梁的屈服臺階這在結構破壞之前保證足夠的警示作用是特別重要的4使用研制中的混合織物體系進行加固的梁并沒有顯示出明顯的延性損失使用碳纖維加固的梁也沒有明顯的延性損失但是屈服荷載較低參考文獻ASTM D 3039 2

21、000 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials Annual Book of ASTM Standards V 1503 pp 106-118Bunsell A R and Harris B 1974 Hybrid Carbon and Glass Fibre Composites Composites V 5 pp 157-164Chow T W and Kelly A 1980 Mechanical Properties of Composites Annual Re

22、view of Composite Science V 10 pp 229-259Fukuda H and Chow T W 1981 Monte Carlo Simulation of Strength of Hybrid Composites Journal of Composite Materials V 16 pp 371-385Grace N F Soliman K Abdel-Sayed G and Saleh K 1999Strengthening Reinforced Concrete Beams Using Fiber Reinforced Polymer CFRP Lami

23、nates Journal of Composites for Construction ASCE V 2 No 4Harris H G Somboonsong W and Frank K K 1998 New Ductile Hybrid FRP Reinforcement Bar for Concrete Structures Journal of Composites for Construction ASCE V 2 No 1 pp 28-37Manders P W and Bader M G 1981 The Strength of Hybrid GlassCarbon Fibre

24、Composites Part 1Failure Strain Enhancement and Failure Mode Journal of Materials Science V 16 pp 2233-2245Nanni A Henneke M J and Okamoto T 1994 Tensile Properties of Hybrid Rods for Concrete Reinforcement Construction and Building Materials V 8 No 1 pp 27-34Norris T Saadatmanesh H and Ehsani M R 1

25、997 Shear and Flexure Strengthening of RC Beams with Carbon Fiber Sheets Journa of Structural Engineering ASCE V 123 No 7 pp 903-911Philips L N 1976 The Hybrid EffectDoes it Exist Composites V 7 pp 7-8 Ritchie P A Thomas D A Lu L and Connelly G M 1991 External Reinforcement of Concrete Beams using F

26、iber Reinforced Plastics ACI Structural Journal V 88 No 4 July-Aug pp 490-500Saadatmanesh H and Ehsani M R 1991 RC Beams Strengthening with GFRP Plates I Experimental Study Journal of Structural Engineering ASCE V 117 No 11 pp 3417-3433Somboonsong W Frank K K and Harris H G 1998 Ductile Hybrid Fiber

27、 Reinforced Plastic Reinforcing ACI Materials Journal V 95 No 6 Nov-Dec pp 655-666Tamuzs V and Tepfers R 1995 Ductility of Nonmetallic Hybrid Fiber Composite Reinforcement for Concrete Proceedings 2nd International RILEM Symposium FRPRCS-2 pp 18-25Triantafillou N P 1992 Strengthening of RC Beams wit

28、h Epoxy-Bonded Fiber-Composite Materials Materials and Structures V 25 pp 201-211附錄表1 復合纖維的力學特性纖維材料描述彈性模量GPaMSi抗拉強度MPaksi表4 試驗梁的匯總梁的組別梁的稱號加固材料NA控制梁NAA組梁C-1碳纖維薄片C-2碳纖維板C-3碳纖維織物H-50-1H體系t 1mmH-50-2H-75-1H體系t 15mmH-75-2B組梁CS碳纖維薄片H-S50-1H體系t 1mmH-S50-2H-S75-1H體系t 15mmH-S75-2表5 試驗結果匯總梁的名稱加固體系屈服荷載kN kips屈

29、服時的撓度mmin破壞時的荷載kNkips破壞時的撓度mmin 延性指數(shù) 第6列第4列破壞時FRP的應變最終破壞類型控制梁NA823185140055957215495195355NA鋼筋屈服后混凝土破壞C-1碳纖維薄片8591931320521019229284112215110鋼筋屈服后FRP斷裂C-2碳纖維板1326298160063100033剪切拉伸破壞C-3碳纖維織物10772421350531344302221087164067鋼筋屈服后FRP剝落H-50-2H體系t 1mm979220152061148258356140233155鋼筋和FRP屈服后FRP斷裂H-75-2H體系

30、t 15mm11392561370541308294292115213074鋼筋和FRP屈服后FRP剝落CS碳纖維薄片9922231420561233277ACI STRUCTURAL JOURNAL TECHNICAL PAPER Title no99-S71Strengthening of Concrete Beams Using Innovative Ductile Fiber-Reinforced Polymer FabricBy Nabil F Grace George Abel-Sayed Wael F RaghebabstractAn innovative uniaxial du

31、ctile fiber-reinforced polymer FRP fabric has been researched developed and manufactured in the Structural Testing Center at Lawrence Technological University for strengthening structures The fabric is a hybrid of two types of carbon fibers and one type of glass fiber and has been designed to provid

32、e a pseudo-ductile behavior with a low yield-equivalent strain value in tension The effectiveness and ductility of the developed fabric has been investigated by strengthening and testing eight concrete beams under flexural load Similar beams strengthened with currently available uniaxial carbon fibe

33、r sheets fabrics and plates were also tested to compare their behavior with those strengthened with the developed fabric The fabric has been designed so that it has the potential to yield simultaneously with the steel reinforcement of strengthened beams and hence a ductile plateau similar to that fo

34、r the nonstrengthened beams can be achieved The beams strengthened with the developed fabric exhibited higher yield loads and achieved higher ductility indexes than those strengthened with the currently available carbon fiber strengthening systems The developed fabric shows a more effective contribu

35、tion to the strengthening mechanismkeywordConcrete ductility textile fiber reinforcement distortionINTRODUCTIONThe use of externally bonded fibcr-rcinforccd polymer FRP sheets and strips has recently been established as an effective tool for rehabilitating and strengthening reinforced concrete struc

36、tures Several experimental investigations have been reported on the behavior of concrete beams strengthened for flexure using externally bonded FRP plates sheets or fabrics Saadatmancsh and Ehsani 1991 examined the behavior of concrete beams strengthened for flexure using glass fiber-reinforced poly

37、mer GFRP plates Ritchie ct al 1991 tested reinforced concrete beams strengthened for flexure using GFRP carbon fibcr-rcinforccd polymer CFRP and GCFRP plates Grace et al 1999 and Trian- tafillou 1992 studied the behavior of reinforced concrete beams strengthened for flexure using CFRP sheets Norris

38、Saadatmancsh and Fhsani 1997 investigated the behavior of concrete beams strengthened using CFRP unidirectional sheets and CFRP woven fabrics In all of these investigations the strengthened beams showed higher ultimate loads compared to the nonstrcngthcncd ones One of the drawbacks experienced by mo

39、st of these strengthened beams was a considerable loss in beam ductility An examination of the load- deflection behavior of the beams however showed that the majority of the gained increase in load was experienced after the yield of the steel reinforcement In other words a significant increase in ul

40、timate load was experienced without much increase in yield load Hence a significant increase in service level loads could hardly be gainedApart from the condition of the concrete element before strengthening the steel reinforcement contributes significantly to the flcxural response of the strengthen

41、ed beam Unfortunately available FRP strengthening materials have a behavior that is different from steel Although FRP materials have high strengths most of them stretch to relatively high strain values before providing their full strength Because steel has a relatively low yield strain value when co

42、mpared with the ultimate strains of most of the FRP materials the contribution of both the steel and the strengthening FRP materials differ with the deformation of the strengthened element As a result steel reinforcement may yield before the strengthened element gains any measurable load increase So

43、me designers place a greater FRP cross section which generally increases the cost of the strengthening to provide a measurable contribution even when deformations arc limited before the yield of steel Debonding of the strengthening material from the surface of the concrete however is more likely to

44、happen in these cases due to higher stress concentrations Debonding is one of the nondesired brittle failures involved with this technique of strengthening Although using some special low-strain fibers such as ultra-high-modulus carbon fibers may appear to be a solution it would result in brittle fa

45、ilures due to the failure of fibers The objective of this paper is to introduce a new pseudo-ductile FRP fabric that has a low strain at yield so that it has the potential to yield simultaneously with the steel reinforcement yet provide the desired strengthening levelRESEARCH SIGNIFICANCEFRPs have b

46、een increasingly used as materials for rehabilitating and strengthening reinforced concrete structures Currently available FRP materials however lack the ductility and have dissimilar behaviors to steel reinforcement As a result the strengthened beams may exhibit a reduced ductility lack the desired

47、 strengthening level or both This study presents an innovative pseudo-ductile FRP strengthening fabric The fabric provides measurably higher yield loads for the strengthened beams and helps to avoid the loss of ductility that is common with the use of currently available FRPDEVELOPMENT OF HYBRID FAB

48、RICTo overcome the drawbacks mentioned previously a ductile FRP material with low yield strain value is neededACI Structural Journal V 99 No 5 September-October 2002MS No 01-349 received October 23 2001 and reviewed under Institute publication policies Copyright ? 2002 American Concrete Institute Al

49、l rights reserved including the making of copies unless permission is obtained from the copyright proprietors Pertinent discussion will be published in the July-August 2003 ACl Structural Journal if received by March 1 2003ACI StructuralJournalSeptember-October 2002ACI member Nabil F Grace is a prof

50、essor and Chair of the Structural Testing Center Department of Civil Engineering Lawrence Technological University Southfield Mich He is a member of ACI Committee 440 Fiber Reinforced Polymer Reinforcement and Joint ACI-ASCE Committee 343 Concrete Bridge Design His research interests include the use

51、 of fiber-reinforced polymer in reinforced and pre stressed concrete structuresGeorge Abdel-Sayed is Professor Emeritus in the Department of Civil and Environmental Engineering University of Windsor Windsor Ontario Canada His research interests include soil-structure interactionWael F Ragheb is a re

52、search assistant in the Department of Civil Engineering at Lawrence Technological University He is a PhD candidate in the Department of Civil and Environmental Engineering University of Windsor Windsor Ontario CanadaTable 1Mechanical properties of composite fibersFiber materialDescriptionModulus of

53、elasticity GPa Msi Tensile strength MPa ksi Failure strain Carbon No IUllra-high-modulus carbon fibers379 55 1324 192 035Carbon No 2High-modulus carbon fibers231 335 2413 350 09 to 10GlassE-glass fibers48 7 1034 150 21 Composite properties are based on 60 fiber volume fractionLiterature review on hy

54、bridizationTo develop this material hybridization for different fibers was considered Hybridization of more than one type of fibrous materials was the interest of many materials science researchers Most of their work was concerned with combining two types of fibers to enhance the mechanical properti

55、es of either type acting alone and to reduce the cost This has been reported in several publications such as Bunsell and Harris 1974 Philips 1976 Manders and Bader 1981 Chow and Kelly 1980 and Fukuda and Chow 1981 Hybridization interested structural engineers as a tool to overcome the problem of a l

56、ack of ductility in FRF reinforcing bars Nanni Henneke and Okamoto 1994 studied bars of braided aramid fibers around a steel core Tamuzs and Tcpfcrs 1995 reported experimental investigations for hybrid fiber bars using different combinations of carbon and aramid fibers Somboonsong Frank and Harris 1

57、998 developed a hybrid FRP reinforcing bar using braided aramid fibers around a carbon fiber core Harris Somboonsong and Frank 1998 used these bars in reinforcing concrete beams to achieve the general load- deflection behavior of concrete beams reinforced with conventional steelDesign concept and ma

58、terialsTo generate ductility a hybridization technique of different types of fibers has been implemented Three fibers have been selected with a different magnitude of elongations at failure Figure 1 shows the stress-strain curves in tension for the selected composite fibers and Table 1 shows their mechanical propertiesThe technique is based on combining these fibers together and controlling the mixture ratio so that when they arc loaded together i