余波 陳冰 劉陽(yáng)
摘 要:銹蝕鋼筋混凝土梁的傳統(tǒng)抗剪承載力模型大多屬于經(jīng)驗(yàn)?zāi)P?,考慮的影響因素不全面且缺乏嚴(yán)密的理論推導(dǎo),導(dǎo)致計(jì)算精度有限?;谛拚龎毫?chǎng)理論,建立了可以綜合考慮鋼筋銹蝕對(duì)臨界斜裂縫傾角、梁有效抗剪截面積、配筋率、配箍率等關(guān)鍵因素影響的銹蝕鋼筋混凝土梁的抗剪承載力模型,通過(guò)與85組試驗(yàn)數(shù)據(jù)和現(xiàn)有模型的對(duì)比分析,驗(yàn)證了模型的適用性和計(jì)算精度。結(jié)果表明,該模型具有嚴(yán)密的理論基礎(chǔ),考慮的影響因素全面,計(jì)算精度較高、離散性較小。
關(guān)鍵詞: 鋼筋混凝土梁;銹蝕;抗剪承載力;臨界斜裂縫傾角;剪跨比;修正壓力場(chǎng)理論
中圖分類(lèi)號(hào):TU375.1
文獻(xiàn)標(biāo)志碼:A? 文章編號(hào):1674-4764(2018)06-0131-08
Shear strength model of corroded reinforced concrete beams
Yu Bo, Chen Bing, Liu Yang
(School of Civil Engineering and Architecture; Key Laboratory of Disaster Prevention and
Structural Safety of Ministry of Education; Guangxi Key Laboratory of Disaster Prevention
and Engineering Safety, Guangxi University, Nanning 530004, P. R. China)
Abstract:The accuracy of traditional shear strength models for corroded reinforced concrete (RC)beam are far from satisfactory, since these models generally belong to the empirical methods which are lack of theoretical basis and do not take into account the various influential factors comprehensively. Hence, a shear strength model of corroded RC beam which takes into account the influences of corrosion on key factors including the critical diagonal crack angle, the effective shear cross-sectional area, the reinforcement ratio and the stirrup ratio was established based on the modified compression field theory (MCFT). The accuracy of the proposed model was validated by comparing with 85 sets of experimental data and several existing models. Analysis results show that the proposed model is of good accuracy and small discreteness.
Keywords:reinforced concrete beam; corrosion; shear strength; critical crack angle; shear span ratio; modified compression field theory
受混凝土碳化和氯鹽侵蝕等腐蝕環(huán)境作用的影響,在役鋼筋混凝土(RC)結(jié)構(gòu)的梁往往發(fā)生鋼筋銹蝕。董衛(wèi)等[1] 通過(guò)銹蝕RC梁的四點(diǎn)受彎加載試驗(yàn),分析了鋼筋銹蝕對(duì)RC梁承載機(jī)理的影響,發(fā)現(xiàn)銹蝕RC梁的承載機(jī)理為梁效應(yīng)和拱效應(yīng)的復(fù)合體,鋼筋銹損率越高,梁效應(yīng)向拱效應(yīng)轉(zhuǎn)化的程度越高;張夢(mèng)光等[2] 通過(guò)外加電流加速箍筋銹蝕,開(kāi)展了4根銹蝕箍筋RC梁的抗剪性能試驗(yàn),發(fā)現(xiàn)箍筋銹蝕會(huì)降低RC梁的抗剪性能和變形能力。由于影響銹蝕RC梁抗剪性能的因素較多,導(dǎo)致其抗剪承載力分析較為困難?;羝G華[3] 在未銹蝕RC梁的抗剪承載力模型基礎(chǔ)上,分別引入經(jīng)驗(yàn)影響系數(shù)φ和γ′來(lái)考慮鋼筋銹蝕對(duì)混凝土和箍筋的抗剪承載力貢獻(xiàn)的影響,建立了銹蝕RC梁的抗剪承載力模型;趙羽習(xí)等[4] 引入經(jīng)驗(yàn)影響系數(shù) P? v來(lái)考慮箍筋銹蝕的影響;李士彬等[5] 考慮鋼筋銹蝕對(duì)箍筋屈服強(qiáng)度和梁有效抗剪截面積的影響,建立了銹蝕RC梁的抗剪承載力模型,但忽略了鋼筋銹蝕對(duì)配筋率、配箍率、臨界斜裂縫傾角等重要因素的影響;余璠璟[6] 基于試驗(yàn)數(shù)據(jù)擬合分析,確定了鋼筋銹蝕對(duì)抗剪承載力的影響系數(shù),但當(dāng)鋼筋未銹蝕或銹蝕率較低時(shí),該影響系數(shù)為負(fù)值,與實(shí)際情況不符;El-Sayed[7] 考慮鋼筋銹蝕引起的梁有效抗剪截面積和箍筋截面積改變對(duì)抗剪承載力的影響,但忽略了其他重要因素的影響;盧朝輝等[8] 基于歐洲規(guī)范的抗剪承載力公式,分別引入了鋼筋銹蝕對(duì)箍筋和混凝土承擔(dān)剪力的折減系數(shù)。綜上所述,現(xiàn)有的銹蝕RC梁的抗剪承載力模型主要是在未銹蝕RC梁的抗剪承載力模型基礎(chǔ)上,通過(guò)引入經(jīng)驗(yàn)性的鋼筋銹蝕影響系數(shù)而建立的,缺乏嚴(yán)密的理論推導(dǎo),且考慮的影響因素不全面,導(dǎo)致計(jì)算精度有限。
1 銹蝕RC梁的抗剪承載力模型
根據(jù)修正壓力場(chǎng)理論(MCFT)[9-11] 可知,RC梁的抗剪承載力V主要由混凝土和箍筋的抗剪承載力貢獻(xiàn)(分別記為V? c 和V? s )組成。
V=V? c +V? s ?(1)
由于鋼筋的空間銹蝕分布具有非均勻性,導(dǎo)致其薄弱截面在受力狀態(tài)下往往會(huì)產(chǎn)生應(yīng)力集中,從而降低鋼筋的強(qiáng)度。其中,根據(jù)鋼筋的截面銹損率 η? sv 可以確定銹蝕箍筋的名義屈服強(qiáng)度 f? vyc [12] 。
f? vyc? = 0.985-1.028η? sv? ?1-η? sv? ?f? vy? ?(2)
式中:η? sv? = A? v -A? vc? ?/A? v ,A? v 和A? vc? 分別為箍筋銹 蝕前后的截面積;f? vy? 為箍筋銹蝕前的屈服強(qiáng)度。
鋼筋銹蝕會(huì)減小縱筋和箍筋的截面積,從而降低配筋率和配箍率。其中,縱筋和箍筋發(fā)生銹蝕后,對(duì)應(yīng)的配筋率 ρ? sc 和配箍率 ρ? vc 分別為
ρ? sc? =A? sc? /(bh 0)
ρ? vc? =A? vc? /(bs) (3)
式中:b為梁的截面寬度;h 0為梁的截面有效高度; s 為箍筋間距;A? sc? 為縱筋銹蝕后的截面積。
此外,鋼筋銹蝕往往會(huì)造成混凝土銹脹開(kāi)裂或剝落,所以銹蝕RC梁的有效截面寬度b? c 需要根據(jù)η? sv? 進(jìn)行修正[13] 。
b? c =? b,[]η? sv? ≤30 %
b-2(c+d? sv? )+ s 5.5 ,[]η? sv? >30 % ,s≤5.5c
b- 5.5 s? c+d? sv? ?2,[]η? sv? >30 % ,s>5.5c??? (4)
式中:c為混凝土保護(hù)層厚度; d? sv 為箍筋的直徑。
對(duì)于外荷載 P 作用下發(fā)生剪切破壞的RC梁,其斜裂縫分布如圖1所示,從中選取的微元體 Ω 的應(yīng)力平衡條件和應(yīng)力莫爾圓[11] 如圖2所示。根據(jù)微元體Ω的應(yīng)力平衡條件可得
f x=ρ? sc? f? s x -v cot ?θ+f 1
(5)
f y=ρ? vc? f? s y -v tan ?θ+f 1 (6)
f 2=v( tan ?θ+ cot ?θ)-f 1
(7)
式中:f x和f y分別為x和y方向上的總應(yīng)力;f 1和f 2分別為垂直于和平行于混凝土裂縫方向的平均主拉應(yīng)力和平均主壓應(yīng)力;v= V? b? c h? v ??為剪應(yīng)力, h? v為截面的有效抗剪高度,通常近似取h? v = max ?0.9h 0,0.72h , h 為RC梁的截面高度; f? s x? 和
f? s y? 分別為縱向鋼筋和橫向鋼筋的拉應(yīng)力;
θ 為臨界斜裂縫傾角,其計(jì)算模型為[14]
θ=k λ? arctan? - α 1k? nsc + ?α2 1k 2 nsc +4 1- α 1 k? nsc? k? nvc ??2 1- α 1 ?k? nvc
(8)
式中:λ為剪跨比;k λ=-0.04λ+1.11,為剪跨比修正系數(shù);k? nsc? =1+(nρ? sc? )-1 ;k? nvc? =1+(nρ? vc? )-1 ;n=E? s /E? c 為鋼筋和混凝土之間的彈性模量比,E? s 和E? c 分別為鋼筋和混凝土的彈性模量;
α 1=v? c /v=V? c /V,建議值為0.38[14] ;v? c 為與混凝土抗剪承載力貢獻(xiàn)值V? c 對(duì)應(yīng)的剪應(yīng)力。由于剪應(yīng)力v=V/(b? c h? v ),所以可以將式(7)變換為
f 1+f 2=v ?tan ?θ+ cot ?θ = V b? c h? v ?( tan ?θ+ cot ?θ) (9)
由式(9)可知,截面剪應(yīng)力v由斜拉應(yīng)力f 1和斜壓應(yīng)力f 2抵抗。其中,f 1滿足應(yīng)力 應(yīng)變關(guān)系[9]
f 1=? E? c ε 1 ε 1≤ε? cr? ?0.33 f′ ??c ??1+ 500ε 1?? ε 1>ε? cr? ???(10)
式中:ε? cr? 為混凝土的開(kāi)裂應(yīng)變,通常取0.5×10-3 [9] ;f′ ??c 為混凝土的抗壓強(qiáng)度;ε 1為主拉應(yīng)變。
RC梁的受力情況如圖3所示,由豎向力平衡條件可得A? v f? vyc? = f 2 sin 2θ-f 1 cos 2θ b? c s (11)
式中: f? vyc 為銹蝕箍筋的名義屈服強(qiáng)度。
根據(jù)式(9)的第1和第2個(gè)等式分別可得v= f 1+f 2 ?tan ?θ+ cot ?θ 和V=vbh? v ,將v代入V=vbh? v 可得 V= ?f 1+f 2 ?tan ?θ+ cot ?θ bh? v 。根據(jù)式(11)可得f 2= A? v f? vyc? ?bs sin 2θ +? f 1 cos 2θ ?sin 2θ ,將f 2代入V= f 1+f 2 ?tan ?θ+ cot ?θ bh? v 可得
V= f 1b? c h? v ??tan ?θ+ cot ?θ + f 2b? c h? v ??tan ?θ+ cot ?θ =
f 1b? c h? v ??tan ?θ+ cot ?θ + b? c h? v ??tan ?θ+ cot ?θ?? A? v f? vyc? ?bs +f 1 cos 2θ? 1 ?sin 2θ =
f 1b? c h? v ??tan ?θ+ cot ?θ (1+ cot 2θ)+ A? v f? vyc? b? c h? v ?b? c s( tan ?θ+ cot ?θ) ×
sin 2θ+ cos 2θ ?sin 2θ =f 1b? c h? vcot θ+ A? v f? vyc? h? v ?s ?cot ?θ
(12)
結(jié)合式(12)和式(10)可知:抗剪承載力V與f 1有關(guān),而f 1又與ε 1有關(guān)。根據(jù)MCFT[9-11] 進(jìn)一步分析可知,ε 1與斜裂縫間距和θ有關(guān),而θ又與待求的V有關(guān)。所以,式(12)通常需要通過(guò)迭代分析才能求解。為了避免迭代分析,通過(guò)引入ε 1與箍筋屈服應(yīng)變?chǔ)? y的關(guān)系來(lái)簡(jiǎn)化計(jì)算[15] 。
ε 1=1.2ε? y =1.2 f? vyc? ?E? s ??(13)
式中:ε? y = f? vyc? ?E? s ?為箍筋的屈服應(yīng)變。
結(jié)合式(1)、式(10)、式(12)和式(13),可以建立銹蝕RC梁的抗剪承載力分析模型
V? n =V? c +V? s =f 1b? c h? v? cot ?θ+ A? v f? vyc? h? v ?s ?cot ?θ=
0.33b? c h? v ?f′ ??c ??1+ 600 f? vyc? ?E? s ????cot ?θ+f? vyc? ?A? sv? ?s h? v? cot ?θ (14)
式中:V? c =f 1b? c h? v? cot ?θ和V? s = A? v f? vyc? h? v? cot ?θ s 分別為混凝土和箍筋的抗剪承載力貢獻(xiàn)。在實(shí)際工程中,可以先根據(jù)現(xiàn)場(chǎng)測(cè)試[16-17] 或分析模型[18] 確定混凝土中的鋼筋銹蝕率,然后根據(jù)文獻(xiàn)[16,19]確定鋼筋的平均銹蝕深度或最大銹蝕深度,進(jìn)而根據(jù)文獻(xiàn)[20]來(lái)確定鋼筋的截面銹損率η? sv? ,最后結(jié)合式(2)、式(4)和式(14)可以計(jì)算銹蝕RC梁的抗剪承載力。
2 對(duì)比分析與驗(yàn)證
選取文獻(xiàn)[4,21-29]中的85組試驗(yàn)數(shù)據(jù)來(lái)驗(yàn)證模型的計(jì)算精度和適用性,見(jiàn)表1。其中,銹蝕RC梁的抗剪承載力實(shí)測(cè)值 V? t的范圍為41.90~155.80 kN, h 0 的范圍為150~265 mm, b 的范圍為100~200 mm,實(shí)測(cè) f′ ??c的范圍為14.76~89.40 MPa, λ 的范圍為1.5~3.5, n 的范圍為5.98~9.63, ρ? s的范圍為1.92%~2.79%, s 的范圍為150~ 254 mm, ρ? v的范圍為0.19%~0.56%,實(shí)測(cè) f? vy 的范圍為275.00~524.00 MPa; η? ss 的范圍為0.00%~26.84%, η? sv 的范圍為0.60%~60.10%。基于上述85組試驗(yàn)數(shù)據(jù),選取文獻(xiàn)[3-5,7]中的4種銹蝕RC梁的抗剪承載力分析模型對(duì)比驗(yàn)證模型的計(jì)算精度。其中,文獻(xiàn)[3]的銹蝕RC梁的抗剪承載力分析模型為
V? n 1 =φ? 0.08 λ-0.3 + 100ρ? s ?λf′ ??c ??f′ ??c bh 0+
γ′ 0.4+0.3λ? A? v f? vy? ?s h 0 (15)
式中:φ和γ′分別為考慮鋼筋銹蝕對(duì)混凝土和箍筋抗剪承載力貢獻(xiàn)影響的經(jīng)驗(yàn)系數(shù);ρ? s 為未銹蝕縱筋的配筋率。
文獻(xiàn)[4]的銹蝕RC梁抗剪承載力分析模型為
V? n 2 =P? v ??bh 0 a/h 0? C? s ?h 0? 1-0.5 C? s ?h 0? f′ ??c +
0.5ρ? v f? vy? ?1- C? s ?h 0? 2? a h 0??? (16)
式中:P? v為考慮箍筋銹蝕影響的經(jīng)驗(yàn)修正系數(shù);C? s為混凝土的剪壓區(qū)高度;a為剪跨;ρ? v為未銹蝕箍筋的配箍率。
文獻(xiàn)[5]的銹蝕RC梁抗剪承載力分析模型為
V? n 3 = 1.75 1+λ f? t b? c h 0 c? +f? vyc? ?A? sv? ?s h 0 c? ?(17)
式中:f? t為混凝土的抗拉強(qiáng)度;h 0 c? 為RC梁銹蝕后的有效截面高度。
文獻(xiàn)[7]的銹蝕RC梁抗剪承載力分析模型為
V? n 4 =0.17λ f′ ??c ?b? c h 0+ A? vc? f? vy? ?s h 0 (18)
文獻(xiàn)[3-5,7]中的4種模型與本文模型的計(jì)算值(記為V? d)的對(duì)比分析見(jiàn)表1和圖4。此外,利用式(19)計(jì)算各模型的均方根誤差 δ? c 。
δ? c =? 1 85 ∑ 85 i=1? V? t i -V? p i ?2? (18)
式中:V? t i 和V? p i 分別為第 i 根銹蝕RC梁的抗剪承載力測(cè)試值和模型計(jì)算值。根據(jù)計(jì)算結(jié)果可知,V? n 1 、V? n 2 、V? n 3 、V? n 4 和V? d 的δ? c 分別為18.77、30.09、45.10、35.63和18.21 kN。由此可見(jiàn),本文模型的δ? c最小,說(shuō)明模型的計(jì)算精度更高。
? 結(jié)合圖4和表1可知,V? t 與V? n 1 的比值的均值(標(biāo)準(zhǔn)差)為0.95(0.20),說(shuō)明V? n 1 具有一定的計(jì)算精度,但是該模型無(wú)法揭示銹蝕RC梁的抗剪機(jī)理;V? t 與V? n 2 、V? n 3 和V? n 4 的比值的均值(標(biāo)準(zhǔn)差)分別為1.16(0.59)、2.27(0.58)和1.32(0.40),說(shuō)明這3個(gè)模型的計(jì)算值總體偏小,且離散性較大,原因在于,上述模型考慮的影響因素不全面且缺乏嚴(yán)密的理論基礎(chǔ);V? t 與V? d 的比值的均值(標(biāo)準(zhǔn)差)為1.01(0.17),說(shuō)明模型的離散性較小,且計(jì)算精度較高,主要原因在于該模型是基于修正壓力場(chǎng)理論推導(dǎo)建立的,具有較為嚴(yán)密的理論基礎(chǔ),而且綜合考慮了多種重要因素的影響。
3 結(jié)論
基于修正壓力場(chǎng)理論,建立了銹蝕RC梁的抗剪承載力分析模型,并利用85組試驗(yàn)數(shù)據(jù)開(kāi)展了對(duì)比驗(yàn)證分析。分析結(jié)果表明:
1)該模型的計(jì)算值與試驗(yàn)值的比值的均值與1.0比較接近,且標(biāo)準(zhǔn)差較小,說(shuō)明模型的計(jì)算精度高、離散性小。
2)銹蝕RC梁的傳統(tǒng)抗剪承載力模型大多屬于經(jīng)驗(yàn)?zāi)P?,考慮的影響因素不全面且缺乏嚴(yán)密的理論推導(dǎo),導(dǎo)致計(jì)算精度有限。
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