鄭文忠，侯曉萌，王 英

(1.結(jié)構(gòu)工程災(zāi)變與控制教育部重點(diǎn)實(shí)驗(yàn)室(哈爾濱工業(yè)大學(xué))，哈爾濱 150090；2.哈爾濱工業(yè)大學(xué) 土木工程學(xué)院，哈爾濱 150090)

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混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火研究現(xiàn)狀與展望

鄭文忠1,2，侯曉萌1,2，王 英1,2

(1.結(jié)構(gòu)工程災(zāi)變與控制教育部重點(diǎn)實(shí)驗(yàn)室(哈爾濱工業(yè)大學(xué))，哈爾濱 150090；2.哈爾濱工業(yè)大學(xué) 土木工程學(xué)院，哈爾濱 150090)

為拓展混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火的研究思路與方法，論述了普通鋼筋、預(yù)應(yīng)力筋、混凝土等結(jié)構(gòu)材料的抗火性能，凝煉了混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)構(gòu)件的抗火性能，介紹了火災(zāi)后混凝土結(jié)構(gòu)加固修復(fù)技術(shù)，指出了混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火研究中存在的一些問題，展望了其發(fā)展趨勢(shì).分析表明：混凝土高溫爆裂臨界溫度隨強(qiáng)度變化而變化，摻鋼纖維或聚丙烯纖維可有效防止混凝土火災(zāi)下爆裂；合理考慮名義拉應(yīng)力和混凝土強(qiáng)度影響的爆裂判別方法，可有效降低火災(zāi)下預(yù)應(yīng)力結(jié)構(gòu)混凝土爆裂風(fēng)險(xiǎn)；混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)應(yīng)滿足火災(zāi)時(shí)不爆裂、火災(zāi)下不坍塌、火災(zāi)后可修復(fù)的抗火設(shè)計(jì)目標(biāo)；火災(zāi)下防爆裂混凝土合理纖維摻量、混凝土及預(yù)應(yīng)力結(jié)構(gòu)構(gòu)件高溫爆裂機(jī)理及預(yù)測(cè)模型、活性粉末混凝土(RPC)高溫爆裂規(guī)律、RPC熱-力耦合本構(gòu)關(guān)系及其結(jié)構(gòu)構(gòu)件抗火性能、溫度-荷載路徑對(duì)結(jié)構(gòu)構(gòu)件高溫性能的影響、高層混凝土結(jié)構(gòu)和地下空間結(jié)構(gòu)抗火性能等方面應(yīng)予關(guān)注.關(guān)鍵詞: 鋼筋混凝土；預(yù)應(yīng)力混凝土；爆裂；抗火性能；抗火設(shè)計(jì)

火災(zāi)是高頻災(zāi)種[1-2]，中國(guó)每年發(fā)生建筑火災(zāi)約15萬起，全世界每年發(fā)生建筑火災(zāi)約360萬起.火災(zāi)常導(dǎo)致結(jié)構(gòu)嚴(yán)重?fù)p傷甚至坍塌.2003年湖南衡陽(yáng)衡州大廈在火災(zāi)中突然整體坍塌，奪去了20位消防官兵的寶貴生命.2009年央視北配樓火災(zāi)、2015年哈爾濱“1.2”火災(zāi)，無不影響巨大，損失慘重.混凝土工程量大面廣，高溫影響材料性能和結(jié)構(gòu)內(nèi)力，溫度和荷載有耦合作用，溫度-荷載路徑對(duì)材料本構(gòu)關(guān)系和構(gòu)件受力性能有影響，火災(zāi)下預(yù)應(yīng)力構(gòu)件混凝土可能發(fā)生爆裂.混凝土及預(yù)應(yīng)力混凝土的研究，經(jīng)歷了由構(gòu)件到體系，由靜力到動(dòng)力，由一般作用到極端作用的多個(gè)階段.火災(zāi)對(duì)混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)影響研究，成為近年來土木工程行業(yè)研究熱點(diǎn)之一.世界各國(guó)對(duì)結(jié)構(gòu)抗火日益關(guān)注，結(jié)構(gòu)抗火學(xué)會(huì)(SIF)是國(guó)際上著名的學(xué)術(shù)團(tuán)體，該組織定期舉行結(jié)構(gòu)抗火學(xué)術(shù)會(huì)議，中國(guó)建筑學(xué)會(huì)結(jié)構(gòu)抗火專業(yè)委員會(huì)每?jī)赡昱e辦一次全國(guó)結(jié)構(gòu)抗火研討會(huì)，積極推進(jìn)中國(guó)結(jié)構(gòu)抗火水平的提升.

本文論述了混凝土、預(yù)應(yīng)力混凝土及活性粉末混凝土抗火性能的研究現(xiàn)狀，介紹了火災(zāi)后混凝土結(jié)構(gòu)加固修復(fù)技術(shù)，指出了混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火研究中尚存的一些問題，并展望了其發(fā)展趨勢(shì).

1 材料的高溫力學(xué)性能

1.1 普通鋼筋高溫力學(xué)性能

鋼筋的合金成分和生產(chǎn)工藝不同，將導(dǎo)致高溫下鋼筋力學(xué)性能有所差別.高溫下，鋼筋內(nèi)部金屬晶體結(jié)構(gòu)改變，致使力學(xué)性能變化.陸洲導(dǎo)等[3-4]對(duì)屈服強(qiáng)度為401 MPa的熱軋螺紋鋼筋進(jìn)行了恒溫加載試驗(yàn)，發(fā)現(xiàn)400 ℃之前鋼筋極限抗拉強(qiáng)度下降不明顯，溫度高于500 ℃后，下降明顯，提出了高溫下鋼筋理想彈塑性的本構(gòu)關(guān)系模型；過鎮(zhèn)海等[5]完成了HPB235級(jí)、HRB335級(jí)、HRB400級(jí)和RRB400級(jí)普通鋼筋恒溫加載試驗(yàn)，提出了高溫下普通鋼筋的極限抗拉強(qiáng)度、屈服強(qiáng)度統(tǒng)一計(jì)算式；李明等[6]通過恒溫加載試驗(yàn)研究了月牙紋鋼筋、光圓鋼筋和高強(qiáng)碳素預(yù)應(yīng)力鋼絲強(qiáng)度變化規(guī)律，發(fā)現(xiàn)預(yù)應(yīng)力鋼絲極限強(qiáng)度退化快于普通鋼筋，提出了高溫下預(yù)應(yīng)力損失計(jì)算公式，該公式適用于溫度不高于500 ℃的情況；王孔藩等[7]完成了光圓鋼筋、螺紋鋼筋、冷拔鋼絲和冷軋扭鋼筋恒溫加載試驗(yàn)，發(fā)現(xiàn)高溫下不同種類鋼筋極限抗拉強(qiáng)度退化并不相同，冷拔鋼絲強(qiáng)度退化更快；吳波等[8]對(duì)國(guó)內(nèi)所完成的高溫下普通鋼筋、鋼材的屈服強(qiáng)度進(jìn)行了統(tǒng)計(jì)分析，提出了具有95%保證率的普通鋼筋高溫屈服強(qiáng)度計(jì)算公式.

Ingberg等[9]采用恒溫加載試驗(yàn)研究了屈服強(qiáng)度250 MPa的結(jié)構(gòu)鋼高溫力學(xué)性能，獲得了其高溫下應(yīng)力-應(yīng)變關(guān)系曲線；Harmathy等[10]完成了ASTM A36(屈服強(qiáng)度246 MPa)、CSA G40.12(屈服強(qiáng)度300 MPa)低碳結(jié)構(gòu)鋼及ASTM A421-65 (條件屈服強(qiáng)度1 550 MPa)預(yù)應(yīng)力鋼絲的恒溫加載試驗(yàn)，獲得了3種鋼材的高溫應(yīng)力-應(yīng)變曲線；Lie[11-12]基于上述試驗(yàn)結(jié)果，給出了高溫下結(jié)構(gòu)鋼、熱軋鋼筋應(yīng)力-應(yīng)變曲線計(jì)算式，并被美國(guó)土木工程協(xié)會(huì)(ASCE)結(jié)構(gòu)防火手冊(cè)[13]采納，但該式鋼筋應(yīng)力-應(yīng)變曲線無下降段[5,14].

美國(guó)混凝土抗火設(shè)計(jì)規(guī)范ACI 216-07[15]給出了高溫下熱軋鋼筋、合金高強(qiáng)鋼筋和冷拉預(yù)應(yīng)力筋屈服強(qiáng)度的退化規(guī)律.

歐洲混凝土抗火設(shè)計(jì)規(guī)范EC2-1-2[16-17]給出的高溫下熱軋鋼筋、冷拔鋼絲受拉應(yīng)力-應(yīng)變關(guān)系曲線，分為彈性段、非線性段、塑性段和下降段四部分.該公式給出的鋼筋極限拉應(yīng)變(鋼筋極限應(yīng)力對(duì)應(yīng)的應(yīng)變)及破斷點(diǎn)應(yīng)變(鋼筋被拉斷時(shí)應(yīng)力對(duì)應(yīng)的應(yīng)變)與溫度無關(guān)，這與高溫下鋼筋應(yīng)變發(fā)展規(guī)律不符.Elghazouli等[18]采用恒溫加載方法，完成的熱軋鋼筋、冷拉鋼筋的力學(xué)性能試驗(yàn)表明，高溫下鋼筋的屈服強(qiáng)度、極限強(qiáng)度退化與歐洲混凝土抗火設(shè)計(jì)規(guī)范的建議值基本一致，但高溫下鋼筋極限應(yīng)變高于規(guī)范建議值.

為比較ASCE防火手冊(cè)、歐洲抗火設(shè)計(jì)規(guī)范給出的高溫下普通鋼筋受拉應(yīng)力-應(yīng)變計(jì)算模型的差別，以屈服強(qiáng)度為400 MPa的熱軋鋼筋為例，高溫下其受拉應(yīng)力-應(yīng)變曲線見圖1.溫度為20、300、500、700 ℃時(shí)，按ASCE手冊(cè)計(jì)算鋼筋受拉應(yīng)力-應(yīng)變關(guān)系為曲線1、2、3、4；按歐洲抗火設(shè)計(jì)規(guī)范計(jì)算鋼筋受拉應(yīng)力-應(yīng)變關(guān)系為曲線5、6、7、8.ASCE防火手冊(cè)考慮了鋼筋受拉強(qiáng)化段，按ASCE防火手冊(cè)計(jì)算常溫下鋼筋極限強(qiáng)度大于按歐洲抗火設(shè)計(jì)規(guī)范計(jì)算值，溫度低于500 ℃時(shí)，ASCE防火手冊(cè)給出的鋼筋應(yīng)力-應(yīng)變曲線強(qiáng)度退化較快.

圖1 高溫下鋼筋應(yīng)力-應(yīng)變關(guān)系

Fig.1 Stress-strain relationships of reinforcing steel bars under high temperatures by ASCE and EC2-1-2 model

鋼筋高溫蠕變影響火災(zāi)下結(jié)構(gòu)反應(yīng).鋼材熔點(diǎn)約為1 400 ℃，一般認(rèn)為，恒載升溫狀態(tài)下，當(dāng)鋼筋溫度超過其熔點(diǎn)的30%時(shí)，高溫蠕變明顯，即鋼筋溫度超過400 ℃時(shí)，應(yīng)合理考慮高溫蠕變的影響[19].Dorn[20]基于恒溫、恒應(yīng)力高溫蠕變?cè)囼?yàn)，提出了不同種類結(jié)構(gòu)鋼高溫蠕變模型.Harmathy[21]對(duì)Dorn高溫蠕變模型進(jìn)行了修正，期望使之適用于變應(yīng)力狀態(tài)下結(jié)構(gòu)鋼的高溫蠕變計(jì)算，Dorn-Harmathy模型經(jīng)參數(shù)標(biāo)定后也可用于計(jì)算鋼筋高溫蠕變[22].Kodur等[23]研究表明：當(dāng)用Dorn-Harmathy模型計(jì)算變應(yīng)力狀態(tài)下結(jié)構(gòu)鋼、鋼筋的高溫蠕變時(shí)，會(huì)產(chǎn)生較大偏差.過鎮(zhèn)海等[5]完成了HRB335級(jí)鋼筋應(yīng)力水平為0.2～0.8，溫度為200～600 ℃的高溫蠕變?cè)囼?yàn).

國(guó)內(nèi)外對(duì)普通鋼筋高溫性能的研究表明，溫度不高于200～300 ℃時(shí)，普通熱軋鋼筋的屈服強(qiáng)度、極限強(qiáng)度基本不降低，溫度超過400 ℃時(shí)，鋼筋的屈服強(qiáng)度、極限強(qiáng)度急劇降低，600 ℃時(shí)，屈服強(qiáng)度、極限強(qiáng)度分別為常溫下強(qiáng)度的30%、45%.鋼筋強(qiáng)度退化規(guī)律有一定差別，這可能是鋼筋合金成分、生產(chǎn)工藝(產(chǎn)地)[24]及試驗(yàn)條件的差別所致.不同文獻(xiàn)給出鋼筋應(yīng)力-應(yīng)變計(jì)算模型有一定差異，部分模型的計(jì)算應(yīng)變與高溫下鋼筋實(shí)際應(yīng)變不符，因此，在進(jìn)行結(jié)構(gòu)抗火分析時(shí)，應(yīng)合理選擇鋼筋應(yīng)力-應(yīng)變計(jì)算模型.

1.2 預(yù)應(yīng)力筋高溫力學(xué)性能

中國(guó)每年有逾百萬噸高強(qiáng)預(yù)應(yīng)力鋼絲/鋼絞線用于預(yù)應(yīng)力工程建設(shè)，預(yù)應(yīng)力鋼絲/鋼絞線抗火性能是預(yù)應(yīng)力結(jié)構(gòu)抗火性能的關(guān)鍵影響因素之一.

Day等[25]對(duì)預(yù)應(yīng)力鋼絲進(jìn)行了恒載升溫和高溫蠕變?cè)囼?yàn)，結(jié)果表明，預(yù)應(yīng)力鋼絲應(yīng)力水平(拉應(yīng)力與極限抗拉強(qiáng)度之比)為0.6時(shí)，溫度升高至400 ℃，鋼絲被拉斷,高溫蠕變引起較大的預(yù)應(yīng)力損失；Abrams等[22]進(jìn)行了預(yù)應(yīng)力鋼絞線(抗拉強(qiáng)度標(biāo)準(zhǔn)值fptk=1 860 MPa)恒溫加載試驗(yàn)，獲得了高溫下鋼絞線極限抗拉強(qiáng)度退化規(guī)律，結(jié)果表明：溫度為427 ℃時(shí)，鋼絞線極限抗拉強(qiáng)度降低至50%；過鎮(zhèn)海等[5]采用恒溫加載方法，完成了用于預(yù)應(yīng)力混凝土結(jié)構(gòu)的消除應(yīng)力鋼絲(直徑5 mm，條件屈服強(qiáng)度為1 274 MPa)高溫性能試驗(yàn)，提出了高溫下消除應(yīng)力鋼絲應(yīng)力-應(yīng)變計(jì)算公式，發(fā)現(xiàn)與HPB235～RRB400級(jí)普通鋼筋相比，消除應(yīng)力鋼絲極限抗拉強(qiáng)度退化更快，故應(yīng)更重視預(yù)應(yīng)力混凝土結(jié)構(gòu)的抗火性能；范進(jìn)等[26-27]進(jìn)行了預(yù)應(yīng)力鋼絲(fptk=1 670 MPa)、鋼絞線(fptk=1 860 MPa)的恒溫加載試驗(yàn)，獲得了其極限強(qiáng)度、條件屈服強(qiáng)度和彈性模量隨溫度的變化規(guī)律；陳禮剛等[28-29]完成了fptk=1 570 MPa預(yù)應(yīng)力鋼絲、fptk=1 410 MPa預(yù)應(yīng)力鋼絲、77B螺旋肋鋼絲(fptk=1 120 MPa)和LL650 冷軋帶肋鋼絲(fptk=650 MPa)恒溫加載和恒載升溫試驗(yàn)，提出了高溫下預(yù)應(yīng)力鋼絲應(yīng)力-應(yīng)變關(guān)系曲線的二折線模型.發(fā)現(xiàn)經(jīng)過冷拔和回火熱處理后的fptk=1 570 MPa預(yù)應(yīng)力鋼絲、fptk=1 410 MPa預(yù)應(yīng)力鋼絲和77B螺旋肋鋼絲，高溫下強(qiáng)度下降更快，這主要是當(dāng)溫度高于300 ℃以后，熱處理所造成的金屬晶體框架的畸變逐漸被解除，熱處理作用基本消失所致.此外，恒載升溫路徑下，經(jīng)熱處理鋼絲的極限抗拉強(qiáng)度略高于恒溫加載路徑下鋼絲的強(qiáng)度，而兩種路徑下LL650冷軋帶肋鋼絲強(qiáng)度基本一致，這主要是恒溫加載路徑減弱了鋼筋熱處理作用所致.

PC鋼棒(fptk=800 ～970 MPa)是近年來發(fā)展的新型預(yù)應(yīng)力鋼種.侯曉萌等[30]完成了PC鋼棒高溫下力學(xué)性能試驗(yàn)，建立了其高溫下應(yīng)力-應(yīng)變計(jì)算公式，發(fā)現(xiàn)高溫下PC鋼棒強(qiáng)度退化慢于預(yù)應(yīng)力鋼絲，這主要是PC鋼棒中錳、釩含量較高，提高了其耐火性能所致.為揭示火災(zāi)下低松弛高強(qiáng)預(yù)應(yīng)力鋼絲的本構(gòu)模型，鄭文忠等[31-32]采用恒溫加載方法，完成了fptk=1 770、1 860 MPa的低松弛高強(qiáng)預(yù)應(yīng)力鋼絲的高溫力學(xué)性能試驗(yàn)，基于試驗(yàn)結(jié)果建立了兩種強(qiáng)度級(jí)別的高溫下低松弛高強(qiáng)預(yù)應(yīng)力鋼絲的本構(gòu)關(guān)系.熱軋鋼筋、PC鋼棒、預(yù)應(yīng)力鋼絲高溫下極限強(qiáng)度退化見圖2.

圖2 高溫下不同鋼筋極限抗拉強(qiáng)度退化規(guī)律

Fig.2 Comparison of ultimate tensile strength of different reinforcing steel bars under high temperatures

火災(zāi)下預(yù)應(yīng)力結(jié)構(gòu)的鋼絲、鋼絞線處于高應(yīng)力狀態(tài)，產(chǎn)生顯著的應(yīng)力松弛(或蠕變)[33]，使結(jié)構(gòu)中預(yù)應(yīng)力明顯降低，變形增大.華毅杰[34]、蔡躍等[35]進(jìn)行了fptk=1 570 MPa預(yù)應(yīng)力鋼絲的高溫拉伸和高溫蠕變(εcr)試驗(yàn)，提出了高溫蠕變計(jì)算公式.

為揭示火災(zāi)下預(yù)應(yīng)力筋應(yīng)力、應(yīng)變變化規(guī)律，張昊宇等[36]開展了52個(gè)fptk=1 770 MPa低松弛高強(qiáng)預(yù)應(yīng)力鋼絲試件的高溫蠕變?cè)囼?yàn)、應(yīng)力松弛試驗(yàn)，提出了低松弛高強(qiáng)預(yù)應(yīng)力鋼絲的高溫蠕變、高溫松弛計(jì)算公式.

周煥廷等[37-38]開展了高溫下預(yù)應(yīng)力鋼絞線(fptk=1 860 MPa)強(qiáng)度、蠕變性能試驗(yàn)，并提出了鋼絞線高溫蠕變計(jì)算公式.發(fā)現(xiàn)預(yù)應(yīng)力鋼絲高溫蠕變低于鋼絞線高溫蠕變，這主要是鋼絞線捻制完成之后，又經(jīng)歷一次回火所致.

文獻(xiàn)[34-38]均是基于恒溫、恒應(yīng)力下的高溫蠕變?cè)囼?yàn)結(jié)果，提出的預(yù)應(yīng)力鋼絲/鋼絞線高溫蠕變計(jì)算公式，且公式計(jì)算結(jié)果差異較大.以fptk=1 770 MPa的預(yù)應(yīng)力鋼絲/鋼絞線為例，常溫下應(yīng)力為751 N/mm2、溫度為350℃時(shí)，文獻(xiàn)[34，36，38]給出的高溫蠕變計(jì)算值和實(shí)測(cè)值對(duì)比見圖3.用預(yù)應(yīng)力鋼絲高溫蠕變公式計(jì)算鋼絞線高溫蠕變，將偏于保守.

圖3 預(yù)應(yīng)力鋼絲/鋼絞線高溫蠕變計(jì)算值和實(shí)測(cè)值對(duì)比

Fig.3 Comparison of creep curves of prestressing steel wires/stands under high temperatures

為研究溫度-荷載路徑對(duì)預(yù)應(yīng)力混凝土結(jié)構(gòu)受力性能的影響，鄭文忠等[39]考慮了低松弛高強(qiáng)預(yù)應(yīng)力鋼絲溫度變化過程中蠕變、溫度膨脹、應(yīng)力變化及變形模量變化對(duì)鋼絲應(yīng)變的影響，將任意溫度-荷載路徑分解為恒溫加載和恒載升溫兩種路徑，建立了高溫下低松弛高強(qiáng)預(yù)應(yīng)力鋼絲考慮溫度-時(shí)間路徑的應(yīng)變和應(yīng)力計(jì)算方法.

1.3 混凝土高溫力學(xué)性能

國(guó)內(nèi)外學(xué)者對(duì)高溫下混凝土的抗壓強(qiáng)度、彈性模量、抗拉強(qiáng)度、本構(gòu)關(guān)系、高溫膨脹、高溫徐變、瞬態(tài)熱應(yīng)變等進(jìn)行了研究.對(duì)高溫下普通混凝土(混凝土強(qiáng)度等級(jí)≤C50，NSC)力學(xué)性能的研究表明[5,40-46]：在溫度低于150 ℃時(shí)，混凝土強(qiáng)度降低，在溫度為150～300 ℃時(shí)，混凝土抗壓強(qiáng)度稍有提高，甚至大于常溫下強(qiáng)度，溫度大于400 ℃時(shí)，混凝土強(qiáng)度快速下降.

Thienel等[47]研究了高溫下NSC雙軸受壓時(shí)的本構(gòu)關(guān)系，結(jié)果表明，高溫下雙軸受壓強(qiáng)度高于單軸受壓強(qiáng)度，雙軸受壓強(qiáng)度退化規(guī)律與單軸受壓強(qiáng)度退化規(guī)律相似.高溫下混凝土抗壓強(qiáng)度與混凝土骨料、配合比、升溫速率、應(yīng)力水平等有關(guān)[48-49].高溫與荷載作用下，混凝土游離水先被蒸發(fā)，在200 ℃左右，結(jié)合水開始分解，在350 ℃左右，發(fā)生硅酸鈣和鋁酸鈣等脫水，在550 ℃左右，氫氧化鈣開始分解，導(dǎo)致水泥石破壞.針對(duì)這一問題，Gawin等[50]建立了化學(xué)反應(yīng)-溫度-應(yīng)力混凝土本構(gòu)數(shù)值模型，但模型需要高溫下混凝土滲透系數(shù)張量、熱擴(kuò)散率張量等眾多參數(shù)，即使是常溫下，這些參數(shù)還難以準(zhǔn)確確定.

近年來，不少學(xué)者對(duì)高溫下高強(qiáng)混凝土(C55-C95，HSC)力學(xué)性能進(jìn)行了研究[51-52]，高強(qiáng)混凝土摻入礦渣粉、硅灰等，與NSC相比，微觀結(jié)構(gòu)更為致密，高溫下易發(fā)生爆裂[53-54].高溫下NSC、HSC抗壓強(qiáng)度隨溫度變化歸一化曲線見圖4.其中，文獻(xiàn)[5]采用100 mm×100 mm×300 mm棱柱體試件，棱柱體抗壓強(qiáng)度15～35 MPa，文獻(xiàn)[43-45]分別采用φ75 mm×150 mm、φ51 mm×102 mm和φ80 mm×300 mm圓柱體試件，圓柱體抗壓強(qiáng)度分別為28、31和33 MPa，文獻(xiàn)[45，51-52]分別采用φ80 mm×300 mm、φ50 mm×100 mm和φ100 mm×310 mm圓柱體試件，圓柱體抗壓強(qiáng)度分別為107、69～118和60 MPa.

圖4 高溫下混凝土抗壓強(qiáng)度隨溫度變化

Fig.4 Variation with temperature of concrete compressive strength under high temperatures

NSC抗拉強(qiáng)度約為抗壓強(qiáng)度的10%，而HSC抗拉強(qiáng)度與抗壓強(qiáng)度之比更小.火災(zāi)下混凝土抗拉強(qiáng)度對(duì)結(jié)構(gòu)構(gòu)件的受彎承載力貢獻(xiàn)極小[5]，但混凝土抗拉強(qiáng)度影響構(gòu)件的開裂，對(duì)混凝土的高溫爆裂性能也有顯著影響[54].過鎮(zhèn)海等[5]給出的NSC抗拉強(qiáng)度隨溫度升高而線性降低(20～1 000 ℃).高溫下HSC混凝土抗拉強(qiáng)度退化規(guī)律與NSC相似，但還受到纖維種類和摻量的影響，有待進(jìn)一步研究.

受混凝土強(qiáng)度、骨料類型、混凝土配合比、養(yǎng)護(hù)條件、升溫條件的影響，不同學(xué)者給出的高溫下混凝土力學(xué)性能試驗(yàn)結(jié)果有一定差異，但宏觀變化趨勢(shì)一致：隨著溫度的升高，混凝土彈性模量的降低速率通常比強(qiáng)度更大[55]，混凝土的峰值應(yīng)變逐漸增大，混凝土的單軸應(yīng)力-應(yīng)變曲線趨于扁平.

高溫下混凝土應(yīng)變主要包括應(yīng)力引起的應(yīng)變、自由膨脹應(yīng)變、高溫徐變和瞬態(tài)熱應(yīng)變.混凝土在持續(xù)應(yīng)力作用下發(fā)生徐變.混凝土的高溫徐變遠(yuǎn)大于常溫徐變，Bazant等[56]、Harmathy等[57]開展了混凝土高溫徐變研究，但最高溫度為140 ℃，不能滿足混凝土結(jié)構(gòu)抗火分析需要.Khoury[58]提出了混凝土高溫徐變計(jì)算公式，高溫徐變隨受火時(shí)間、應(yīng)力水平的增加而增大.過鎮(zhèn)海等[5]基于恒溫、恒應(yīng)力狀態(tài)下的NSC高溫徐變?cè)囼?yàn)結(jié)果，提出了混凝土高溫徐變計(jì)算模型(適用于溫度不大于700 ℃，應(yīng)力水平不大于0.6的NSC)，并引入了等效時(shí)間，使其可用于恒溫-變應(yīng)力狀態(tài)下的徐變計(jì)算，但混凝土高溫徐變計(jì)算值明顯小于按文獻(xiàn)[58]的計(jì)算值，這可能是混凝土骨料類型、配合比、試件尺寸等差異所致.混凝土強(qiáng)度等級(jí)對(duì)高溫徐變的影響還有待于進(jìn)一步研究[59].

高溫下混凝土無應(yīng)力時(shí)的伸長(zhǎng)為自由膨脹變形，在壓應(yīng)力作用下，混凝土高溫變形可能伸長(zhǎng)或縮短，將混凝土壓應(yīng)力作用下的變形與自由膨脹變形的差值，定義為瞬態(tài)熱應(yīng)變.瞬態(tài)熱應(yīng)變與混凝土常溫下應(yīng)力水平和自由膨脹相關(guān)，發(fā)生的機(jī)理尚不清楚.Anderberg等[60]、南建林等[61]分別給出了NSC瞬態(tài)熱應(yīng)變計(jì)算公式，溫度大于200 ℃時(shí)，瞬態(tài)熱應(yīng)變可達(dá)混凝土高溫徐變的數(shù)倍.胡海濤等[62-63]給出了HSC瞬態(tài)熱應(yīng)變計(jì)算公式，結(jié)果表明隨混凝土強(qiáng)度提高，瞬態(tài)熱應(yīng)變降低.過鎮(zhèn)海等[5]研究了恒溫加載和恒載升溫兩種基本溫度-荷載路徑下混凝土的力學(xué)性能退化規(guī)律，基于這兩種基本路徑，將混凝土高溫應(yīng)變進(jìn)行分解，提出了混凝土溫度-應(yīng)力耦合本構(gòu)關(guān)系.高溫徐變、瞬態(tài)熱應(yīng)變會(huì)影響結(jié)構(gòu)變形，且受火時(shí)間越長(zhǎng)，影響越顯著；對(duì)超靜定結(jié)構(gòu)，還將影響結(jié)構(gòu)的極限荷載，因此應(yīng)在結(jié)構(gòu)抗火設(shè)計(jì)時(shí)予以重視.

2011年，中國(guó)頒布第一本混凝土結(jié)構(gòu)耐火設(shè)計(jì)技術(shù)規(guī)程[55]，規(guī)程中高溫下普通鋼筋屈服強(qiáng)度、彈性模量計(jì)算公式采用了文獻(xiàn)[8]的研究成果；預(yù)應(yīng)力筋極限強(qiáng)度、高溫應(yīng)力松弛和高溫蠕變計(jì)算公式分別采用了文獻(xiàn)[31-32，34]的研究成果；NSC和HSC高溫力學(xué)性能分別采用了文獻(xiàn)[5，63]的研究成果.

1.4 混凝土高溫爆裂

高溫下混凝土可能發(fā)生爆裂.爆裂不僅導(dǎo)致受力鋼筋暴露于烈火之中，而且使構(gòu)件受力截面減小，結(jié)構(gòu)耐火性能急劇降低.如何實(shí)現(xiàn)火災(zāi)下混凝土不爆裂，一直是混凝土結(jié)構(gòu)抗火研究所關(guān)注的問題.混凝土高溫爆裂機(jī)理仍有爭(zhēng)議[64].蒸汽壓力理論認(rèn)為高溫下混凝土內(nèi)部水蒸汽難以逃逸，混凝土孔隙內(nèi)部產(chǎn)生蒸汽壓力，當(dāng)蒸汽壓力超過混凝土的抗拉強(qiáng)度時(shí)發(fā)生爆裂.熱應(yīng)力理論認(rèn)為，高溫下混凝土變形受到約束而產(chǎn)生熱應(yīng)力，熱應(yīng)力超過混凝土抗拉強(qiáng)度時(shí)，發(fā)生爆裂.

吳波[65]對(duì)混凝土高溫爆裂的研究表明，混凝土表面溫度在200～500 ℃時(shí)，易發(fā)生爆裂.為研究纖維種類和摻量對(duì)HSC柱爆裂性能的影響，Kodur等[66]完成了4根HSC(標(biāo)準(zhǔn)立方體抗壓強(qiáng)度81～108 MPa)和1根NSC方柱四面受火試驗(yàn)，結(jié)果表明：摻0.54%鋼纖維或0.1%摻聚丙烯(PP)纖維可緩解HSC高溫爆裂，鈣質(zhì)骨料HSC較硅質(zhì)骨料HSC爆裂程度低，這主要是由于溫度大于600 ℃時(shí)，鈣質(zhì)骨料發(fā)生分解，導(dǎo)致混凝土比熱容增大，進(jìn)而降低爆裂所致；編制了考慮高溫爆裂影響的HSC柱抗火性能分析程序[67]，提出混凝土爆裂臨界溫度為350 ℃，為抗火分析中考慮混凝土高溫爆裂提供了基礎(chǔ).Xiao等[68]研究表明：強(qiáng)度等級(jí)為C50、C80、C100的混凝土，爆裂臨界溫度分別為800、400、500 ℃.以上研究表明爆裂臨界溫度隨著混凝土強(qiáng)度和組成的變化而變化.

為避免混凝土高溫爆裂，不少學(xué)者建議摻PP纖維(纖維長(zhǎng)度6～30 mm，直徑50～200 μm)以避免混凝土高溫爆裂，摻PP纖維混凝土在美國(guó)、歐洲、日本等得到了應(yīng)用[69-70].研究表明，溫度為160～170 ℃時(shí)，PP纖維熔化，在混凝土內(nèi)形成水蒸汽逃逸的孔道，可減緩混凝土爆裂.例如，對(duì)強(qiáng)度等級(jí)C60-C80混凝土，添加不少于2 kg/m3的短切PP纖維可避免爆裂[55]，歐洲混凝土抗火設(shè)計(jì)規(guī)范(EC2-1-2)[16]建議，對(duì)標(biāo)準(zhǔn)立方體抗壓強(qiáng)度73～113 MPa的混凝土，添加不少于2 kg/m3的PP纖維可避免爆裂.

針對(duì)摻PP纖維會(huì)降低混凝土和易性及常溫下抗壓強(qiáng)度的問題，不少學(xué)者提出在混凝土內(nèi)合理單摻鋼纖維，不僅可防止混凝土高溫爆裂，還可以提高混凝土強(qiáng)度；鋼纖維在混凝土中是隨機(jī)分散的，具有較大的熱傳導(dǎo)性，有利于混凝土內(nèi)部各處溫度的傳遞，可以減少應(yīng)力造成的內(nèi)部損傷，由此減緩混凝土爆裂風(fēng)險(xiǎn).Chen等[71]研究表明：標(biāo)準(zhǔn)立方體抗壓強(qiáng)度為85 MPa的混凝土，摻0.6%鋼纖維可推遲初爆時(shí)刻，但仍發(fā)生爆裂；Poon等[72]研究表明：標(biāo)準(zhǔn)立方體抗壓強(qiáng)度為79～105 MPa的混凝土，摻1%鋼纖維，可防止高溫爆裂.

事實(shí)上，防爆裂用纖維摻量應(yīng)隨著混凝土抗壓強(qiáng)度的變化而變化.應(yīng)進(jìn)一步研究不摻纖維或低纖維摻量的混凝土爆裂臨界溫度，防止火災(zāi)下不同強(qiáng)度的混凝土爆裂，所需PP纖維或鋼纖維適宜摻量.

1.5 活性粉末混凝土高溫性能

Richard等[73]研制了一種超高強(qiáng)水泥基復(fù)合材料，以細(xì)度較大的石英砂(粒徑小于0.6 mm)代替粗骨料，由于摻入了具有較高活性的火山灰質(zhì)材料，被稱為活性粉末混凝土(RPC)，其抗壓強(qiáng)度可達(dá)800 MPa[73-75].RPC(100 mm×100 mm×100 mm立方體抗壓強(qiáng)度標(biāo)準(zhǔn)值不低于100 MPa)受到其抗火性能的嚴(yán)峻挑戰(zhàn)，制約了RPC的發(fā)展與應(yīng)用.2010年以來，國(guó)內(nèi)外學(xué)者對(duì)RPC材料的高溫性能進(jìn)行了探索.Ju等[76]實(shí)測(cè)了常溫至250 ℃時(shí)，鋼纖維RPC熱工參數(shù)，結(jié)果表明：RPC導(dǎo)熱系數(shù)低于NSC.鄭文忠等[77]實(shí)測(cè)了常溫至900 ℃時(shí)摻纖維RPC熱工參數(shù)，發(fā)現(xiàn)RPC導(dǎo)熱系數(shù)高于NSC和HSC，且高溫下RPC熱工參數(shù)受其組成成分的影響.

劉紅彬[78]完成了鋼纖維體積摻量為0%、1%和2%的RPC高溫爆裂試驗(yàn)，結(jié)果表明：100 mm×100 mm×100 mm立方體試件中心溫度為250 ℃時(shí)，RPC發(fā)生爆裂，鋼纖維對(duì)RPC爆裂臨界溫度的提高效果不明顯，但可降低RPC爆裂程度；陳強(qiáng)[79]研究表明：RPC初爆溫度為420～583 ℃，隨濕含量的增加，RPC爆裂概率和爆裂損傷程度逐漸增大，濕含量(試件所含可蒸發(fā)水的質(zhì)量與試件飽水狀態(tài)下所含可蒸發(fā)水的質(zhì)量比)低于63%時(shí)，水膠比由0.16增大至0.20時(shí)，試件爆裂概率降低，這主要是由于水膠比增大，RPC強(qiáng)度降低、滲透性降低所致；鄭文忠等[80-81]通過試驗(yàn)研究了含水率、升溫速度、試件尺寸、防火涂料、恒溫時(shí)間和纖維種類及摻量對(duì)RPC高溫爆裂性能的影響，發(fā)現(xiàn)未施加荷載時(shí)，單摻2%鋼纖維RPC(試件尺寸為70.7 mm×70.7 mm×70.7 mm)爆裂臨界含水率為0.85%；合理涂抹防火涂料可防止高溫下RPC爆裂.在RPC不爆裂的基礎(chǔ)上，采用恒溫加載方法，研究了高溫下鋼纖維體積摻量分別為1%、2%和3%的RPC力學(xué)性能，結(jié)果表明：隨溫度升高，鋼纖維RPC棱柱體抗壓強(qiáng)度和彈性模量迅速下降，峰值應(yīng)變逐漸升高，200、400、600和800 ℃時(shí)鋼纖維RPC的抗壓強(qiáng)度分別降為常溫時(shí)的76%～82%、53%～62%、33%～42%和14%～19%，提出了高溫下鋼纖維RPC單軸受壓本構(gòu)模型.Aydin等[82]進(jìn)行了20～800 ℃下兩種類型RPC高溫力學(xué)性能試驗(yàn)，研究表明：溫度超過300 ℃時(shí)常規(guī)RPC易爆裂，而高溫下堿礦渣RPC不爆裂，其耐火性能優(yōu)于常規(guī)RPC.Canbaz[83]的研究表明：先對(duì)RPC施加80 MPa壓應(yīng)力，再用90 ℃熱水養(yǎng)護(hù)3天，常溫下RPC強(qiáng)度可達(dá)200 MPa，摻1%的PP纖維降低了RPC強(qiáng)度，但可避免高溫下爆裂.Ju等[84]用COMSOL軟件分析了高溫下RPC熱應(yīng)力，采用主拉應(yīng)力和Von Mises應(yīng)力判別RPC高溫爆裂，為環(huán)境溫度20～500 ℃的RPC爆裂預(yù)測(cè)提供了參考，但該數(shù)值模型未考慮高溫下RPC蒸氣壓力對(duì)爆裂的影響，假定RPC發(fā)生塑性變形后，應(yīng)力不再變化，這一假定尚缺乏試驗(yàn)驗(yàn)證，爆裂判別方法的適用性還有待商榷.

國(guó)內(nèi)外對(duì)RPC高溫性能的研究表明，高溫下RPC易發(fā)生爆裂，一般通過摻鋼纖維或PP纖維來避免高溫爆裂.RPC構(gòu)件抗火性能的試驗(yàn)研究尚未見報(bào)道.

2 混凝土結(jié)構(gòu)抗火性能

在材料高溫性能研究的基礎(chǔ)上，國(guó)內(nèi)外學(xué)者開展了鋼筋混凝土構(gòu)件抗火性能試驗(yàn)研究，取得如下成果.

2.1 高溫下鋼筋與混凝土間粘結(jié)性能

鋼筋與混凝土間的可靠粘結(jié)性能是兩者共同工作的基礎(chǔ).Diederichs等[85]、Morley等[86-87]完成了20～800 ℃高溫下中心拔出試驗(yàn)，結(jié)果表明：鋼筋與混凝土間粘結(jié)強(qiáng)度隨溫度升高而降低，退化規(guī)律與混凝土抗拉強(qiáng)度相似.火災(zāi)下變形鋼筋粘結(jié)強(qiáng)度退化慢于光圓鋼筋粘結(jié)強(qiáng)度退化，光圓鋼筋粘結(jié)強(qiáng)度退化快于混凝土抗拉強(qiáng)度的退化.袁廣林等[88]研究表明：受熱溫度不超過450 ℃時(shí)，粘結(jié)強(qiáng)度下降不超過20%；受熱溫度達(dá)到650 ℃時(shí)，高溫下試件的粘結(jié)強(qiáng)度約下降40%.胡克旭[89]完成了高溫下中心拔出試驗(yàn)，獲得了不同溫度下的鋼筋-混凝土的粘結(jié)-滑移曲線，分別給出了火災(zāi)下光圓鋼筋、變形鋼筋與混凝土粘結(jié)強(qiáng)度退化影響系數(shù)計(jì)算式.

Huang[90]、Gao等[91]建立了考慮火災(zāi)下鋼筋-混凝土粘結(jié)滑移影響的有限元模型，分析了火災(zāi)下鋼筋混凝土梁截面應(yīng)力、跨中變形.結(jié)果表明：若忽略火災(zāi)下鋼筋-混凝土粘結(jié)滑移的影響，混凝土梁、板變形計(jì)算值可能偏小.

這里需要指出，由于混凝土的組分不同，不同學(xué)者間給出的高溫下鋼筋與混凝土間粘結(jié)強(qiáng)度退化離散較大.

2.2 混凝土板抗火性能

1950年以來，國(guó)內(nèi)外學(xué)者開展了鋼筋混凝土簡(jiǎn)支板耐火極限的試驗(yàn)研究，考察了保護(hù)層厚度、板厚、荷載水平對(duì)耐火極限的影響.基于ASTM119標(biāo)準(zhǔn)升溫曲線，以背火面混凝土平均溫度超過121 ℃或任意點(diǎn)溫度超過163 ℃作為板耐火極限的標(biāo)志，Thompson[92]完成了荷載水平為0.5、計(jì)算跨度為3 650 mm、板厚為150 mm的鋼筋混凝土簡(jiǎn)支板耐火試驗(yàn)，該板耐火極限大于3 h.Gustaferro等[93-94]的研究表明：梁、板支座約束可提高其耐火極限；膨脹頁(yè)巖輕質(zhì)骨料混凝土耐火性能高于普通硅質(zhì)、鈣質(zhì)骨料混凝土；混凝土含水率越高，由溫度控制的板耐火極限越長(zhǎng).混凝土板背火面設(shè)置多孔混凝土、珍珠巖混凝土或蛭石混凝土等防火措施，可提高板的耐火極限，且混凝土密度越低，耐火極限越高[95].Lie[96]編制了混凝土板溫度場(chǎng)計(jì)算程序，給出了由溫度控制的板耐火極限計(jì)算式，結(jié)果表明：隨板厚、保護(hù)層厚度的增大，板耐火極限增大.陳正昌[97]完成了混凝土空心簡(jiǎn)支板抗火性能試驗(yàn)，結(jié)果表明：相同板厚、相同承載力的鋼筋混凝土空心板耐火性能優(yōu)于預(yù)應(yīng)力空心板，保護(hù)層厚度增加、荷載水平降低，耐火極限增大.

2004年以來，國(guó)內(nèi)外學(xué)者開始進(jìn)行混凝土連續(xù)板、雙向板抗火試驗(yàn)與數(shù)值模擬.陳禮剛等[98-100]完成了6塊三跨鋼筋混凝土連續(xù)板抗火試驗(yàn)，分別研究了邊跨受火、中跨受火和相鄰兩跨受火下板的支座反力變化規(guī)律，結(jié)果表明：火災(zāi)下連續(xù)板發(fā)生明顯內(nèi)力重分布，在負(fù)彎矩鋼筋截?cái)嗵幊霈F(xiàn)集中裂縫.Bailey等[101]完成了48塊鋼筋混凝土簡(jiǎn)支雙向板抗火性能試驗(yàn)，發(fā)現(xiàn)常溫下混凝土被壓碎的試驗(yàn)板，火災(zāi)下因鋼筋被拉斷而破壞，這是由于鋼筋高溫下強(qiáng)度降低，由適筋板變?yōu)樯俳畎逅?

王濱等[102-103]完成了2塊四邊簡(jiǎn)支和1塊四邊固支鋼筋混凝土雙向板抗火性能試驗(yàn)，結(jié)果表明：四邊固支雙向板在板頂出現(xiàn)橢圓形裂縫.楊志年等[104]對(duì)3層3×3跨鋼框架三層頂角部鋼筋混凝土雙向板進(jìn)行了抗火試驗(yàn)，雙向板受火面積為4.2 m×4.2 m.王勇等[105]對(duì)該鋼框架二層2×2區(qū)格雙向板進(jìn)行了抗火試驗(yàn)，受火面積為8.4 m×8.4 m，結(jié)果表明：相鄰構(gòu)件的約束作用提高了鋼筋混凝土雙向板抗火性能.

為模擬混凝土雙向板火災(zāi)反應(yīng)，Huang等[106-108]考慮了幾何非線性和材料非線性的影響，提出了火災(zāi)下混凝土雙軸破壞判別準(zhǔn)則，編制了考慮火災(zāi)下板薄膜效應(yīng)有限元程序.Zhang等[109]編制了雙向板火災(zāi)反應(yīng)分析程序，結(jié)果表明軸向約束能減少火災(zāi)下雙向板的變形，但分析過程中未考慮混凝土瞬態(tài)熱應(yīng)變、高溫徐變的影響.Wang等[110]提出了火災(zāi)下雙軸受力混凝土瞬態(tài)熱應(yīng)變的計(jì)算方法，并編制了雙向板抗火性能有限元分析程序，與Huang等[106]方法相比，火災(zāi)下雙向板變形計(jì)算值與實(shí)測(cè)值吻合更好.

2.3 混凝土梁抗火性能

Ellingwood等[111-113]完成了6根鋼筋混凝土伸臂梁抗火試驗(yàn)，實(shí)測(cè)了火災(zāi)下混凝土梁溫度場(chǎng)分布，獲得了基于ASTM119標(biāo)準(zhǔn)升溫曲線和高強(qiáng)度火災(zāi)[111]兩種升溫曲線下混凝土梁變形，發(fā)現(xiàn)不同升溫條件對(duì)鋼筋混凝土梁變形有明顯影響，基于火災(zāi)下混凝土、鋼筋熱工參數(shù)和應(yīng)力-應(yīng)變關(guān)系，編制了火災(zāi)下鋼筋混凝土梁變形分析程序，但變形計(jì)算值小于實(shí)測(cè)值.Dotreppe等[114]、Wu等[115]也開展了火災(zāi)下鋼筋混凝土簡(jiǎn)支梁、板抗火性能試驗(yàn)與數(shù)值模擬.

Lin等[116]完成了11根單跨兩端伸臂梁抗火試驗(yàn)，結(jié)果表明：由于梁下部區(qū)域的膨脹變形大于梁的上部區(qū)域，使梁頂拉應(yīng)力增大，進(jìn)而使得火災(zāi)下梁支座負(fù)彎矩增大，結(jié)構(gòu)抗火設(shè)計(jì)時(shí)應(yīng)合理考慮火災(zāi)下內(nèi)力重分布的影響.過鎮(zhèn)海等[5]完成了拉區(qū)受火、壓區(qū)受火鋼筋混凝土梁力學(xué)性能試驗(yàn)，結(jié)果表明，壓區(qū)受火適筋梁極限荷載降低慢于拉區(qū)受火，提出了高溫下梁板受彎承載力簡(jiǎn)化計(jì)算方法；完成了4根雙跨升溫、2根單跨升溫的兩跨鋼筋混凝土連續(xù)梁抗火性能試驗(yàn)，實(shí)測(cè)了不同荷載水平下連續(xù)梁支座反力變化，發(fā)現(xiàn)連續(xù)梁抗火性能優(yōu)于簡(jiǎn)支梁.陸洲導(dǎo)等[117]完成了12根鋼筋混凝土簡(jiǎn)支梁一面、二面、三面受火試驗(yàn)，計(jì)算了高溫下簡(jiǎn)支梁彎矩-曲率關(guān)系和跨中變形，發(fā)現(xiàn)當(dāng)荷載水平大于0.5時(shí)，簡(jiǎn)支梁耐火極限降低明顯.馮雅等[118]提出了考慮火災(zāi)下混凝土濕熱變化的溫度場(chǎng)數(shù)值模擬方法，并得到試驗(yàn)結(jié)果的驗(yàn)證.向延念等[119]、張威振[120]利用電爐完成了8根b×h=250 mm×400 mm鋼筋混凝土簡(jiǎn)支梁抗火試驗(yàn)與數(shù)值分析，結(jié)果表明：在一定受火時(shí)段內(nèi)，隨受火時(shí)間延長(zhǎng)，縱向受拉鋼筋應(yīng)力增大.時(shí)旭東等[121]完成了12根鋼筋混凝土簡(jiǎn)支梁抗火試驗(yàn)，其中6根為恒溫加載路徑，6根為恒載升溫路徑，結(jié)果表明：溫度-荷載路徑不僅影響構(gòu)件截面應(yīng)力分布，而且影響結(jié)構(gòu)極限荷載.

苗吉軍等[122]完成了7根帶初始裂縫的鋼筋混凝土簡(jiǎn)支梁抗火性能試驗(yàn)，結(jié)果表明：初始裂縫寬度越大，梁耐火性能越差；考慮初始裂縫對(duì)梁截面溫度場(chǎng)的影響，提出了帶裂縫梁受彎承載力簡(jiǎn)化計(jì)算方法.該方法同樣適用于研究經(jīng)氯離子侵蝕后帶裂縫鋼筋混凝土梁抗火性能[123].查曉雄等[124-125]完成了4根GFRP筋混凝土簡(jiǎn)支梁在ISO834標(biāo)準(zhǔn)升溫曲線下受力性能試驗(yàn)，高溫下GFRP筋抗拉強(qiáng)度退化快于普通鋼筋，GFRP筋混凝土梁裂縫開展高度明顯大于鋼筋混凝土梁.

鋼筋混凝土梁抗火性能數(shù)值模擬通常采用以下兩種方法：一種是基于截面彎矩-曲率關(guān)系，分析火災(zāi)下梁反應(yīng)[96,117,126]，該方法可較方便揭示截面承載力退化規(guī)律；一種是有限元分析方法[114,127-129].Kodur等[126]基于截面分析方法，提出了考慮混凝土高溫徐變、瞬態(tài)熱應(yīng)變和鋼筋高溫蠕變影響的簡(jiǎn)支梁抗火性能數(shù)值模擬方法，分析了荷載水平、升溫條件、混凝土保護(hù)層厚度對(duì)梁抗火性能的影響，結(jié)果表明：以受拉鋼筋溫度超過593 ℃或梁達(dá)到承載能力極限狀態(tài)計(jì)算確定耐火極限，可能大于變形控制的耐火極限.

實(shí)際工程中構(gòu)件可能受到不同程度的邊界約束，Dwaikat等[130]提出了火災(zāi)下考慮支座約束和混凝土高溫爆裂影響的鋼筋混凝土梁抗火性能數(shù)值分析方法，但該程序假定混凝土爆裂臨界溫度為350 ℃，未考慮不同混凝土爆裂溫度不同的影響；Dwaikat等[131-132]完成了2根NSC梁和4根HSC梁抗火性能試驗(yàn)，其中NSC和HSC梁中各有一根施加端部軸向約束.結(jié)果表明：帶軸向約束梁耐火極限高于簡(jiǎn)支梁；HSC梁耐火極限低于NSC梁，與NSC相比，火災(zāi)下HSC爆裂更嚴(yán)重；吳波等[133-134]通過8根同時(shí)具有端部軸向和轉(zhuǎn)動(dòng)約束的混凝土梁抗火試驗(yàn)和3 744種工況的計(jì)算分析，考察了端部約束梁升降溫全過程軸力及梁端彎矩的變化，提出了相應(yīng)的實(shí)用計(jì)算方法；徐明等[135]等完成了3根超高韌性水泥基復(fù)合材料(ECC，抗壓強(qiáng)度實(shí)測(cè)值34.6 MPa)約束梁和3根鋼筋混凝土(抗壓強(qiáng)度實(shí)測(cè)值29.8 MPa)約束梁耐火性能試驗(yàn)，結(jié)果表明：跨度相同、截面相同、截面承載力相同的ECC梁截面溫度低于鋼筋混凝土梁，ECC約束梁跨中變形、跨中及支座截面彎矩均小于鋼筋混凝土梁.

2.4 混凝土柱抗火性能

1976年以來，美國(guó)硅酸鹽水泥協(xié)會(huì)和加拿大國(guó)家研究院合作，完成了31根軸壓柱和6根偏壓柱在ASTM 119標(biāo)準(zhǔn)升溫曲線下四面受火試驗(yàn)[136-138]，結(jié)果表明：鈣質(zhì)骨料混凝土柱耐火性能優(yōu)于硅質(zhì)骨料混凝土柱；截面尺寸、柱端約束是影響柱抗火性能的主要因素；截面尺寸越小、荷載水平越高、縱筋配筋率越低，耐火極限越低.軸向荷載水平相同時(shí)，偏壓柱耐火極限低于軸壓柱.Dotreppe等[139]完成了6根NSC軸壓柱在ISO 834標(biāo)準(zhǔn)升溫曲線下四面受火試驗(yàn)，提出了火災(zāi)下軸壓柱承載力簡(jiǎn)化計(jì)算公式.過鎮(zhèn)海等[5]完成了三面、二面受火NSC軸壓柱、偏壓柱抗火性能試驗(yàn)，揭示了該類柱火災(zāi)下軸向變形和側(cè)向變形發(fā)展規(guī)律，給出了高溫下極限軸力-彎矩包絡(luò)圖；發(fā)現(xiàn)三面受火軸壓柱發(fā)生偏心受壓破壞，恒載升溫柱極限軸力大于恒溫加載柱，兩面受火柱抗火性能優(yōu)于三面受火柱.Tan等[140-141]提出了一面至四面受火NSC軸壓、偏壓柱耐火極限簡(jiǎn)化計(jì)算方法.

2003年以來，國(guó)內(nèi)外學(xué)者進(jìn)行了HSC柱抗火性能試驗(yàn)與數(shù)值模擬.為研究箍筋形式對(duì)HSC柱爆裂的影響，Kodur等[142]完成了6根HSC(28 d圓柱體強(qiáng)度81～107 MPa)軸壓方柱四面受火試驗(yàn)，結(jié)果表明：截面尺寸為305 mm×305 mm、406 mm×406 mm的方柱，當(dāng)箍筋末端做成90°彎鉤時(shí)(箍筋2φ8，間距406 mm，屈服強(qiáng)度414 MPa)，混凝土全截面均可能爆裂；當(dāng)箍筋末端做成135°彎鉤時(shí)(箍筋為2φ6，間距為76～152 mm)，僅箍筋外側(cè)混凝土發(fā)生爆裂，而核心區(qū)混凝土不爆裂；加密箍筋間距，可減輕爆裂.吳波等[143]完成了5根HSC方柱(混凝土棱柱體強(qiáng)度66～74 MPa)和2根NSC(混凝土棱柱體強(qiáng)度33 MPa)方柱四面、三面和兩面受火試驗(yàn)，結(jié)果表明，隨受火面的增加，柱耐火極限降低，相同條件下HSC柱的耐火極限低于NSC柱，這主要是由于HSC柱高溫爆裂更嚴(yán)重所致；建立了HSC方柱耐火極限和火災(zāi)下正截面承載力計(jì)算式[144].完成了4根端部約束高強(qiáng)混凝土柱抗火試驗(yàn)[145]，揭示了端部約束柱火災(zāi)行為時(shí)變機(jī)理，提出了火災(zāi)下考慮端部約束影響的柱軸力和彎矩計(jì)算方法，發(fā)現(xiàn)適當(dāng)增大端部約束可提升HSC柱的耐火性能.

異形柱表體比大，受火時(shí)其內(nèi)部溫度相對(duì)常規(guī)柱上升更為迅速.針對(duì)這一問題，Xu等[146]進(jìn)行了12根NSC(試驗(yàn)當(dāng)天混凝土棱柱體強(qiáng)度35～38 MPa)異形柱的抗火試驗(yàn)以及6 632種工況的高溫反應(yīng)分析，考察了荷載水平、荷載角、計(jì)算長(zhǎng)度、偏心率等對(duì)異形柱耐火性能的定量影響，研究了高溫下異形柱廣義中性軸位置、極限承載力、極強(qiáng)中心、極限軸力-彎矩包絡(luò)圖等的演變趨勢(shì)，提出了混凝土異形柱的耐火設(shè)計(jì)方法.吳波等[147]完成了16根端部約束異形柱的抗火試驗(yàn)和8 331種工況的計(jì)算分析.實(shí)現(xiàn)了可同時(shí)在柱伸長(zhǎng)和縮短階段施加端部約束的異形柱全過程明火試驗(yàn)，突破了以往只在柱伸長(zhǎng)階段施加約束而無法在柱縮短階段實(shí)施約束的局限.揭示了端部軸向和轉(zhuǎn)動(dòng)約束對(duì)異形柱高溫行為的影響規(guī)律，建立了定常端部約束下異形柱高溫下軸力和彎矩時(shí)變過程的定量描述，并拓展至了非定常端部約束情況.

2.5 混凝土結(jié)構(gòu)抗火性能

過鎮(zhèn)海等[5]完成了5榀單層單跨鋼筋混凝土框架三面受火試驗(yàn)，實(shí)測(cè)了火災(zāi)下框架梁、柱變形和框架柱內(nèi)力，結(jié)果表明，混凝土框架火災(zāi)下發(fā)生明顯的內(nèi)力重分布；基于平截面假定，給出了混凝土、鋼筋熱-力耦合本構(gòu)關(guān)系，推導(dǎo)了適用于任意溫度-荷載路徑的平衡方程，編制了桿系有限元析程序NARCSLT.陸洲導(dǎo)等[148]完成了5榀單層雙跨混凝土框架在600 ℃、800 ℃單跨受火、雙跨受火試驗(yàn)，編制了火災(zāi)下框架受力性能的非線性分析程序.Bailey[149]在Cardington建筑結(jié)構(gòu)實(shí)驗(yàn)室完成了7層混凝土平板柱底層局部受火試驗(yàn)，平板柱橫向?yàn)?×7.5 m，縱向?yàn)?×7.5 m，底層層高4.2 m，其余各層層高3.75 m，中柱截面尺寸為400 mm×400 mm，邊柱截面尺寸為250 mm×400 mm，板厚250 mm，作用荷載為9.25 kN/m2.底層局部受火區(qū)域?yàn)檠貦M向兩個(gè)柱距、沿縱向中間兩個(gè)柱距所轄區(qū)域，面積為15 m×15 m.板混凝土28 d立方體抗壓強(qiáng)度實(shí)測(cè)值為61 MPa，含水率為3.8%.火災(zāi)下板混凝土爆裂，導(dǎo)致試驗(yàn)設(shè)備損壞，僅獲得了受火25 min內(nèi)混凝土樓板中心點(diǎn)的變形值.結(jié)果表明：火災(zāi)下板迎火面混凝土爆裂面積超過75%，部分受力鋼筋被燒斷，但由于雙向板薄膜效應(yīng)的有利影響，板并未坍塌.劉永軍[150]建立了高溫下混凝土雙軸應(yīng)力下的本構(gòu)模型，開發(fā)了平面應(yīng)力單元和桿單元，編制了非線性有限元分析程序STRUFIRE，實(shí)現(xiàn)了鋼筋混凝土梁、板、框架的抗火性能分析.吳波等[151]編制了混凝土框架桿系有限元分析程序，并完成了單層3跨鋼筋混凝土框架的火災(zāi)反應(yīng)分析，結(jié)果表明：火災(zāi)下框架梁軸力和梁端彎矩變化明顯.陳適才等[152]推導(dǎo)了梁?jiǎn)卧蔷€性應(yīng)變位移矩陣，編制了基于纖維梁模型的混凝土框架火災(zāi)反應(yīng)非線性分析程序，分析了三層三跨混凝土平面框架的火災(zāi)反應(yīng)，結(jié)果表明：受火位置不同，框架結(jié)構(gòu)破壞形式不同.閆凱等[153-154]應(yīng)用ABAQUS有限元軟件，引入考慮材料各向異性的磚砌體彈塑性模型，建立了底部框架磚房抗火性能有限元模型，結(jié)果表明：火災(zāi)下框架梁軸向膨脹變形和向下?lián)锨冃螌?duì)墻拱傳力機(jī)制不利，使框架梁軸向壓應(yīng)力顯著增大，加速邊柱頂端外側(cè)縱向鋼筋受拉屈服，內(nèi)側(cè)混凝土被壓碎.

以上研究表明，荷載水平、截面尺寸、保護(hù)層厚度、配筋型式及約束條件、混凝土強(qiáng)度、骨料種類、升溫條件及受火位置和區(qū)域、混凝土爆裂等均影響混凝土結(jié)構(gòu)抗火性能.盡管相關(guān)規(guī)范[16]對(duì)混凝土結(jié)構(gòu)耐火極限做出了規(guī)定，但一方面規(guī)范考慮的影響因素較少，一方面是這些規(guī)定僅適用于混凝土不爆裂的情況.

3 預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火性能

預(yù)應(yīng)力混凝土結(jié)構(gòu)跨度大、截面小、功能好，近30年來在中國(guó)得到了迅速發(fā)展.然而，由于火災(zāi)下預(yù)應(yīng)力筋強(qiáng)度損失大、應(yīng)力退化快，火災(zāi)引起的預(yù)應(yīng)力混凝土內(nèi)部的水蒸汽難以逃逸，混凝土具有受火爆裂易發(fā)性；因爆裂而暴露于烈火之中的鋼筋迅速退出工作，結(jié)構(gòu)可能會(huì)突然失效.預(yù)應(yīng)力混凝土耐高溫性能和抗火設(shè)計(jì)受到關(guān)注.

3.1 預(yù)應(yīng)力混凝土結(jié)構(gòu)構(gòu)件抗火性能

1953年，Ashton等[155]完成了37根縮尺有粘結(jié)預(yù)應(yīng)力T形NSC梁恒載升溫試驗(yàn)，升溫曲線接近ISO 834 標(biāo)準(zhǔn)升溫曲線，實(shí)測(cè)了火災(zāi)下預(yù)應(yīng)力鋼絲、混凝土溫度變化，結(jié)果表明：鋼絲的升溫速率對(duì)梁受彎承載力有顯著影響，當(dāng)張拉控制應(yīng)力與極限抗拉強(qiáng)度之比為0.6，鋼絲溫度超過400 ℃時(shí)，預(yù)應(yīng)力混凝土梁發(fā)生正截面承載力破壞；部分試驗(yàn)梁因混凝土爆裂而破壞更早.Gustaferro等[156]完成了按ASTM 119標(biāo)準(zhǔn)升溫曲線升溫的11塊有粘結(jié)預(yù)應(yīng)力混凝土簡(jiǎn)支板抗火性能試驗(yàn)，以火災(zāi)下板達(dá)到正截面承載力極限狀態(tài)的時(shí)刻作為耐火極限，結(jié)果表明：其他條件相同時(shí)，膨脹頁(yè)巖輕骨料混凝土板耐火極限高于NSC板(φ152 mm×305 mm圓柱體抗壓強(qiáng)度24 MPa)，預(yù)應(yīng)力筋保護(hù)層厚度越大、荷載水平越低，板耐火極限越長(zhǎng).Abrams等[157]研究了不同種類和厚度的防火涂料對(duì)預(yù)應(yīng)力混凝土簡(jiǎn)支板、雙T梁、T形梁抗火性能的影響，結(jié)果表明：噴涂防火涂料可有效提高預(yù)應(yīng)力混凝土簡(jiǎn)支構(gòu)件耐火性能，火災(zāi)下防火涂料與混凝土粘結(jié)性能較好，給出了對(duì)應(yīng)2 h、3 h耐火極限的防火涂料厚度建議值.Joseph等[158]完成了無粘結(jié)預(yù)應(yīng)力混凝土板的試驗(yàn)，研究了預(yù)應(yīng)力筋保護(hù)層厚度、荷載和端部約束對(duì)板耐火性能的影響.

Herberghen等[159]完成了8塊兩端伸臂無粘結(jié)預(yù)應(yīng)力混凝土板抗火性能試驗(yàn)，發(fā)現(xiàn)火災(zāi)下預(yù)應(yīng)力板混凝土爆裂，配置縱橫向非預(yù)應(yīng)力鋼筋的板爆裂程度小于全預(yù)應(yīng)力板，提出了增配支座負(fù)彎矩鋼筋的建議.袁愛民等[160]完成了4塊無粘結(jié)預(yù)應(yīng)力混凝土簡(jiǎn)支板抗火性能試驗(yàn)，結(jié)果表明：保護(hù)層厚度越大，板耐火極限越長(zhǎng)，預(yù)應(yīng)力度(0.4～0.6)對(duì)板的耐火極限影響不明顯.Bailey等[161-162]進(jìn)行了后張無粘結(jié)預(yù)應(yīng)力混凝土單向板抗火性能試驗(yàn)，研究了鈣質(zhì)骨料和硅質(zhì)骨料、板端自由轉(zhuǎn)動(dòng)和固定兩種邊界條件對(duì)其抗火性能的影響，結(jié)果表明：火災(zāi)下硅質(zhì)骨料試驗(yàn)板變形大于鈣質(zhì)骨料板，板端自由轉(zhuǎn)動(dòng)較固定的板變形大，無粘結(jié)預(yù)應(yīng)力板的耐火極限高于相關(guān)抗火規(guī)范BS 8110的規(guī)定[163].

袁愛民等[164-167]完成了9塊三跨無粘結(jié)預(yù)應(yīng)力混凝土板邊、中跨同時(shí)受火、邊跨受火和中跨受火試驗(yàn)，考察了預(yù)應(yīng)力度、負(fù)筋長(zhǎng)度等因素對(duì)無粘結(jié)預(yù)應(yīng)力混凝土連續(xù)板耐火性能的影響，結(jié)果表明，不同跨受火對(duì)無粘結(jié)預(yù)應(yīng)力混凝土連續(xù)板的抗火性能有重要影響，熱膨脹是火災(zāi)初期第一內(nèi)支座兩側(cè)控制截面彎矩增大的主要原因.王中強(qiáng)等[168-169]完成了26根無粘結(jié)預(yù)應(yīng)力混凝土簡(jiǎn)支扁梁抗火性能試驗(yàn)，并編制了無粘結(jié)預(yù)應(yīng)力混凝土梁抗火性能非線性分析程序NAUPCLF，結(jié)果表明：荷載水平越大，綜合配筋指標(biāo)(0.38～0.87)越小，扁梁抗火性能越差.

基于混凝土、非預(yù)應(yīng)力筋和預(yù)應(yīng)力筋的熱-力耦合本構(gòu)關(guān)系[175]，用t時(shí)刻混凝土應(yīng)力計(jì)算t+1時(shí)刻混凝土的瞬態(tài)熱應(yīng)變和高溫蠕變，完成了火災(zāi)下預(yù)應(yīng)力混凝土梁板截面的彎矩-曲率關(guān)系的計(jì)算，基于纖維梁?jiǎn)卧Ｐ?，用割線剛度法對(duì)連續(xù)梁板支座反力進(jìn)行迭代求解，計(jì)算梁板在曲率與支座反力共同作用下的彎矩、撓度和支座位移，實(shí)現(xiàn)了火災(zāi)下有粘結(jié)預(yù)應(yīng)力混凝土連續(xù)梁、板的非線性有限元分析.提出了考慮荷載水平、保護(hù)層厚度和梁板截面尺寸影響的預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火設(shè)計(jì)方法[176-178].

Venanzi等[179]完成了4塊預(yù)應(yīng)力膨脹粘土輕骨料高性能混凝土(立方體抗壓強(qiáng)度標(biāo)準(zhǔn)值60 MPa)空心簡(jiǎn)支板抗火性能試驗(yàn)，結(jié)果表明：火災(zāi)下預(yù)應(yīng)力空心板迎火面混凝土爆裂，且出現(xiàn)縱向貫通裂縫，導(dǎo)致板破壞；延長(zhǎng)板在干燥環(huán)境的養(yǎng)護(hù)時(shí)間，可減緩板火災(zāi)下爆裂；Shakya等[180]完成了5塊簡(jiǎn)支和1塊施加軸向約束的預(yù)應(yīng)力NSC空心板抗火試驗(yàn)，并用ANSYS有限元軟件實(shí)現(xiàn)了預(yù)應(yīng)力混凝土空心板抗火性能數(shù)值模擬[181]，結(jié)果表明：施加軸向約束的試驗(yàn)板耐火極限更長(zhǎng)，火災(zāi)下硅質(zhì)骨料試驗(yàn)板比鈣質(zhì)骨料試驗(yàn)板更易爆裂；周緒紅等[182]完成了4塊簡(jiǎn)支、4塊連續(xù)預(yù)制疊合板抗火性能試驗(yàn)與有限元分析，結(jié)果表明：火災(zāi)下預(yù)應(yīng)力疊合板迎火面均發(fā)生爆裂，預(yù)應(yīng)力疊合板耐火極限小于等強(qiáng)配筋的非預(yù)應(yīng)力疊合板，連續(xù)板耐火極限大于簡(jiǎn)支板.

與預(yù)應(yīng)力梁、板抗火性能研究相比，預(yù)應(yīng)力框架抗火性能研究較少.陸洲導(dǎo)等[183]完成了5榀單層單跨無粘結(jié)預(yù)應(yīng)力混凝土框架抗火性能試驗(yàn)，實(shí)測(cè)了火災(zāi)下框架梁變形和無粘結(jié)預(yù)應(yīng)力筋應(yīng)力變化，結(jié)果表明，火災(zāi)下預(yù)應(yīng)力筋的預(yù)應(yīng)力損失大，是導(dǎo)致框架梁跨中開裂、變形增大的原因，預(yù)應(yīng)力度越高(0.64～0.7)，框架抗火性能越不利.

綜上，以往抗火試驗(yàn)多基于標(biāo)準(zhǔn)升溫曲線，但標(biāo)準(zhǔn)升溫曲線與真實(shí)火災(zāi)升溫曲線有較大差別[184]，應(yīng)重視真實(shí)火災(zāi)下結(jié)構(gòu)構(gòu)件火災(zāi)反應(yīng)的研究.受試驗(yàn)條件限制，預(yù)應(yīng)力混凝土梁和框架結(jié)構(gòu)抗火性能試驗(yàn)尺寸偏小，宜進(jìn)一步開展足尺預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火性能的研究.

3.2 預(yù)應(yīng)力混凝土高溫爆裂與防爆裂驗(yàn)算

使用過程中，預(yù)應(yīng)力構(gòu)件的預(yù)壓區(qū)可能存在壓應(yīng)力，即使在使用荷載下預(yù)壓區(qū)受拉，拉應(yīng)力水平也較低.一定的壓應(yīng)力或較小的拉應(yīng)力，使得在使用荷載下梁板迎火面難以出現(xiàn)裂縫，在火災(zāi)下內(nèi)部水蒸汽難以逃逸，造成相對(duì)較高的蒸汽壓，易使蒸汽引起的混凝土內(nèi)部拉應(yīng)力達(dá)到混凝土抗拉強(qiáng)度而引發(fā)預(yù)應(yīng)力構(gòu)件迎火面混凝土爆裂.鄭文忠等[185]對(duì)38個(gè)預(yù)應(yīng)力混凝土梁板火災(zāi)下的爆裂情況進(jìn)行總結(jié)，15塊簡(jiǎn)支單向板中有8塊發(fā)生了不同程度的爆裂，9塊連續(xù)單向板中有3塊發(fā)生了不同程度的爆裂，試件爆裂見圖5.發(fā)現(xiàn)作為迎火面的預(yù)壓區(qū)壓應(yīng)力水平越高或拉應(yīng)力水平越低、混凝土抗壓強(qiáng)度及含水率越高，混凝土就越容易發(fā)生爆裂或爆裂越嚴(yán)重.

圖5 火災(zāi)下預(yù)應(yīng)力連續(xù)板混凝土爆裂

將常溫下名義拉應(yīng)力(壓為正)引入預(yù)應(yīng)力板混凝土爆裂判別方法，提出了如圖6(a)、(b)所示爆裂上包線，為綜合考慮名義拉應(yīng)力、混凝土抗壓強(qiáng)度及含水率的影響，提出了如圖6(c)所示的預(yù)應(yīng)力板混凝土爆裂上包面.對(duì)于預(yù)應(yīng)力板，用σct≤1.36ftk-2.3來驗(yàn)算迎火面混凝土爆裂，其中，σct為迎火面混凝土的常溫名義拉應(yīng)力下限值(MPa)，ftk為混凝土常溫抗拉強(qiáng)度標(biāo)準(zhǔn)值(MPa)；對(duì)預(yù)應(yīng)力梁，用圖6(d)方法判別爆裂區(qū).該公式被新一輪行業(yè)標(biāo)準(zhǔn)《無粘結(jié)預(yù)應(yīng)力混凝土結(jié)構(gòu)技術(shù)規(guī)程》[186]所采納，為合理判別火災(zāi)下預(yù)應(yīng)力梁板混凝土爆裂提供了參考依據(jù).

圖6 預(yù)應(yīng)力混凝土梁板爆裂判別方法

需要指出，基于爆裂試驗(yàn)數(shù)據(jù)提出的預(yù)應(yīng)力混凝土梁板爆裂判別方法為經(jīng)驗(yàn)方法，宜開展火災(zāi)下預(yù)應(yīng)力梁板爆裂機(jī)理、爆裂預(yù)測(cè)模型的研究.

4 火災(zāi)后混凝土結(jié)構(gòu)加固修復(fù)技術(shù)

混凝土結(jié)構(gòu)火災(zāi)后可能比火災(zāi)下更危險(xiǎn).其火災(zāi)后損傷評(píng)估實(shí)現(xiàn)由定性到定量發(fā)展，是迫切需求.吳波[65]考慮火災(zāi)荷載密度、通風(fēng)因子和房間熱工特性的影響，建立了簡(jiǎn)潔實(shí)用的室內(nèi)火災(zāi)溫度發(fā)展全過程計(jì)算模型，解決了復(fù)雜模擬在工程中應(yīng)用不便的難題.吳波等[187]、鄭文忠等[188-192]分別完成了混凝土試件高溫后力學(xué)試驗(yàn)，獲得了火災(zāi)后摻纖維HSC、RPC表面歷經(jīng)最高溫度與損傷特征的關(guān)系.吳波[65]提出了確定構(gòu)件內(nèi)部最高溫度場(chǎng)的簡(jiǎn)便方法.獲得了高溫后主導(dǎo)預(yù)應(yīng)力筋[193]、環(huán)向約束高強(qiáng)混凝土等剩余力學(xué)性能[194]，使高溫后環(huán)向約束高強(qiáng)混凝土相比無約束時(shí)20%～40%的強(qiáng)度增幅得以有效利用.對(duì)中等和輕微損壞的過火結(jié)構(gòu)，提出了鑿除受火溫度500 ℃以上混凝土之后進(jìn)行有效修復(fù)的技術(shù)[65].建立了火災(zāi)后混凝土構(gòu)件的抗震恢復(fù)力模型[195]，提出了火災(zāi)后混凝土構(gòu)件評(píng)價(jià)方法，實(shí)現(xiàn)了火災(zāi)后混凝土結(jié)構(gòu)損傷評(píng)估由定性到定量的跨越.鄭文忠等[196-198]提出了火災(zāi)后梁板中預(yù)應(yīng)力筋剩余應(yīng)力和極限應(yīng)力的計(jì)算方法，發(fā)現(xiàn)火災(zāi)下預(yù)應(yīng)力筋強(qiáng)度退化快于錨具錨固性能退化，火災(zāi)后退下的錨具不能重新使用[199].針對(duì)環(huán)氧類有機(jī)膠軟化溫度只有60～80 ℃的不足，發(fā)明了600 ℃內(nèi)強(qiáng)度不降低的耐高溫植筋膠[200-204]，用其植筋錨固長(zhǎng)度計(jì)算公式lab=0.065(fy/ft)d，其中fy為帶肋鋼筋抗拉強(qiáng)度設(shè)計(jì)值，ft為混凝土抗拉強(qiáng)度設(shè)計(jì)值，d為鋼筋直徑[205].以上研究成果為火災(zāi)后混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)的損傷評(píng)估與加固修復(fù)提供了技術(shù)支撐.

5 結(jié)論與展望

1)鋼筋的合金成分和生產(chǎn)工藝不同，是高溫下鋼筋力學(xué)性能差異的主要原因.高溫下混凝土可能發(fā)生爆裂，加劇其力學(xué)性能退化.高溫下混凝土力學(xué)性能還受到混凝土組分、配合比和升溫速度等的影響.

2)以往抗火試驗(yàn)多基于標(biāo)準(zhǔn)升溫曲線，但標(biāo)準(zhǔn)升溫曲線與真實(shí)火災(zāi)下升溫曲線有較大差別，應(yīng)重視真實(shí)火災(zāi)下結(jié)構(gòu)構(gòu)件抗火性能的研究.受試驗(yàn)條件限制，混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)抗火性能試驗(yàn)尺寸偏小，宜進(jìn)一步開展足尺結(jié)構(gòu)構(gòu)件抗火性能的研究.提出的混凝土及預(yù)應(yīng)力爆裂判別方法為經(jīng)驗(yàn)方法，尚宜繼續(xù)開展火災(zāi)下混凝土及預(yù)應(yīng)力結(jié)構(gòu)構(gòu)件爆裂機(jī)理、爆裂預(yù)測(cè)模型的研究.

3)混凝土及預(yù)應(yīng)力混凝土結(jié)構(gòu)應(yīng)滿足火災(zāi)時(shí)不爆裂、火災(zāi)下不坍塌、火災(zāi)后可修復(fù)的抗火設(shè)計(jì)目標(biāo).

4)國(guó)內(nèi)外學(xué)者對(duì)高溫下RPC爆裂溫度、防爆裂纖維摻量等進(jìn)行了研究，但對(duì)RPC爆裂的影響因素、定量表達(dá)和防爆裂措施缺乏系統(tǒng)研究，高溫下RPC爆裂的數(shù)值模擬鮮見報(bào)道，宜開展考慮滲透性、含水率、RPC強(qiáng)度等影響的RPC高溫爆裂數(shù)值預(yù)測(cè)方法研究.對(duì)高溫下RPC的立方體抗壓強(qiáng)度、軸心抗壓強(qiáng)度、抗拉強(qiáng)度和彈性模量等研究較多，還未開展高溫下RPC熱-力耦合本構(gòu)關(guān)系研究.宜開展RPC結(jié)構(gòu)構(gòu)件抗火性能研究.

5)混凝土高溫瞬態(tài)熱應(yīng)變、高溫徐變和鋼筋高溫蠕變的數(shù)值較大，溫度-荷載路徑不僅影響構(gòu)件截面應(yīng)力分布，同時(shí)影響結(jié)構(gòu)極限荷載.應(yīng)進(jìn)步一開展考慮溫度-荷載路徑影響的結(jié)構(gòu)構(gòu)件抗火性能研究.

6)高層混凝土結(jié)構(gòu)和地下空間結(jié)構(gòu)可有效利用建筑用地.高層混凝土結(jié)構(gòu)火災(zāi)蔓延迅速、火勢(shì)難以控制；地下空間結(jié)構(gòu)構(gòu)件的耐火極限要求一般為4 h，是地上結(jié)構(gòu)構(gòu)件的一倍以上.應(yīng)開展高層混凝土結(jié)構(gòu)和地下空間結(jié)構(gòu)的火災(zāi)環(huán)境與火災(zāi)反應(yīng)研究，進(jìn)一步研究提升整體結(jié)構(gòu)抗火性能的設(shè)計(jì)方法.

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(編輯 趙麗瑩)

Progress and prospect of fire resistance of reinforced concrete and prestressed concrete structures

ZHENG Wenzhong1,2, HOU Xiaomeng1,2,WANG Ying1,2

(1.Key Lab of Structures Dynamic Behavior and Control (Harbin Institute of Technology), Ministry of Education,Harbin 150090, China; 2. School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China)

In this chapter, the fire resistance of reinforced concrete and prestressed concrete structures are outlined to expend further research. The progress and prospect in fire resistance of reinforced concrete (RC) structures and prestressed concrete(PC) structures and its repair technology after fire is presented, and some issues that still need to be investigated are discussed. The progress mainly includes mechanical properties of materials at elevated temperatures, fire resistance of RC and PC structures, and fire-induced spalling of concrete. The results show that spalling critical temperature of concrete varies with compressive strength of concrete. Adding steel fibers or polypropylene (PP) fibers is able to prevent fire-included spalling of concrete effectively. The criteria considering nominal stress of concrete and concrete strength is effective for judging fire-induced spalling, which is capable for reducing the possibility of fire-induced spalling of concrete in PC members. The requirements that concrete structures will not collapse or spalling during fire and can be repaired after fire should be satisfied in fire safety design. Rational fibers dosage to prevent fire-included spalling of concrete, fire-included spalling and its prediction model on RC and PC members, fire-induced spalling of RPC, temperature-stress coupling strain-stress relation of RPC and fire resistance of RPC members, effect of force-temperature paths on behaviors of structures and members, fire resistance of high-rise buildings and underground structures are the main problems which need to be studied in the future.

reinforced concrete; prestressed concrete; spalling; fire resistance; fire safety design

10.11918/j.issn.0367-6234.2016.12.001

2016-02-27

國(guó)家自然科學(xué)基金(51578184，51478142)； 黑龍江省博士后科研啟動(dòng)基金(LBH-Q15058)

鄭文忠(1965—)，男，長(zhǎng)江學(xué)者特聘教授，博士生導(dǎo)師； 侯曉萌(1982—)，男，副教授，博士生導(dǎo)師

侯曉萌，houxiaomeng_hit@126.com

TU378.1

A

0367-6234(2016)12-0001-18