土木工程外文文獻及翻譯--約束和無約束的鋼筋對裂縫寬度的影響

1、<p><b>　　外文文獻：</b></p><p>　　Original Article</p><p>　　Impact of crack width on bond: confined and unconfined rebar </p><p>　　David W. Law1 , Denglei&

2、#160;Tang2, Thomas K. C. Molyneaux3 and Rebecca Gravina3</p><p>　　Received: 14 January 2010  Accepted: 14 December 2010  Published online: 23 

3、December 2010 </p><p><b>　　Abstract</b></p><p>　　This paper reports the results of a research project comparing the effect of surface crack width and degree of corrosion on the

4、bond strength of confined and unconfined deformed 12 and 16 mm mild steel reinforcing bars. The corrosion was induced by chloride contamination of the concrete and an applied DC current. The principal parameters inv

5、estigated were confinement of the reinforcement, the cover depth, bar diameter, degree of corrosion and the surface crack width. The results indicated that p</p><p>　　Keywords:  bond ;corrosio

6、n ; rebar ; cover ; crack width ; concrete </p><p>　　1   Introduction</p><p>　　The corrosion of steel reinforcement is a major cause of the deteriora

7、tion of reinforced concrete structures throughout the world. In uncorroded structures the bond between the steel reinforcement and the concrete ensures that reinforced concrete acts in a composite manner. However, when c

8、orrosion of the steel occurs this composite performance is adversely affected. This is due to the formation of corrosion products on the steel surface, which affect the bond between the steel and the concrete. </p>

9、<p>　　The deterioration of reinforced concrete is characterized by a general or localized loss of section on the reinforcing bars and the formation of expansive corrosion products. This deterioration can affect st

10、ructures in a number of ways; the production of expansive products creates tensile stresses within the concrete, which can result in cracking and spalling of the concrete cover. This cracking can lead to accelerated ingr

11、ess of the aggressive agents causing further corrosion. It can also resul</p><p>　　Previous research has investigated the impact of corrosion on bond [2–5, 7, 12, 20, 23–25, 27, 29], with a number of models

12、being proposed [4, 6, 9, 10, 18, 19, 24, 29]. The majority of this research has focused on the relationship between the level of corrosion (mass loss of steel) or the current density degree (corrosion current applied in

13、accelerated testing) and crack width, or on the relationship between bond strength and level of corrosion. Other research has investigated the mechanical be</p><p>　　The corrosion of the reinforcing steel re

14、sults in the formation of iron oxides which occupy a larger volume than that of the parent metal. This expansion creates tensile stresses within the surrounding concrete, eventually leading to cracking of the cover concr

15、ete. Once cracking occurs there is a loss of confining force from the concrete. This suggests that the loss of bond capacity could be related to the longitudinal crack width [12]. However, the use of confinement within t

16、he concrete can cou</p><p>　　2   Experimental investigation </p><p>　　2.1   Specimens </p><p>　　Beam end specimens [28] were selected for this study. This type of eccentri

17、c pullout or ‘beam end’ type specimen uses a bonded length representative of the anchorage zone of a typical simply supported beam. Specimens of rectangular cross section were cast with a longitudinal reinforcing bar in

18、each corner, Fig. 1. An 80 mm plastic tube was provided at the bar underneath the transverse reaction to ensure that the bond strength was not enhanced due to a (transverse) compressive force acting on the</

19、p><p>　　Fig. 1 Beam end specimen</p><p>　　Deformed rebar of 12 and 16 mm diameter with cover of three times bar diameter were investigated. Duplicate sets of confined and unconfined

20、specimens were tested. The confined specimens had three sets of 6 mm stainless steel stirrups equally spaced from the plastic tube, at 75 mm centres. </p><p>　　This represents four groups of specim

21、ens with a combination of different bar diameter and with/without confinement. The specimens were selected in order to investigate the influence of bar size, confinement and crack width on bond strength. </p><

22、p>　　2.2   Materials </p><p>　　The mix design is shown, Table 1. The cement was Type I Portland cement, the aggregate was basalt with specific gravity 2.99. The coarse and fine aggregate were pre

23、pared in accordance with AS 1141-2000. Mixing was undertaken in accordance with AS 1012.2-1994. Specimens were cured for 28 days under wet hessian before testing. </p><p>　　Table 1 Concrete mi

24、x design</p><p>　　In order to compare bond strength for the different concrete compressive strengths, Eq. 1 is used to normalize bond strength for non-corroded specimens as has been used by other resear

25、cher [8]. </p><p>　　where is the bond strength for grade 40 concrete, τ exptl is the experimental bond strength and f c is the experimental compressive strength. </p><p>　　The tensile strength o

26、f the Φ12 and Φ16 mm steel bars was nominally 500 MPa, which equates to a failure load of 56.5 and 100.5 kN, respectively. </p><p>　　2.3   Experiment methodology </p><p>　　Ac

27、celerated corrosion has been used by a number of authors to replicate the corrosion of the reinforcing steel happening in the natural environment [2, 3, 5, 6, 10, 18, 20, 24, 27, 28, 30]. These have involved experiments

28、using impressed currents or artificial weathering with wet/dry cycles and elevated temperatures to reduce the time until corrosion, while maintaining deterioration mechanisms representative of natural exposure. Studies u

29、sing impressed currents have used current densities betwee</p><p>　　The steel bars served as the anode and four mild steel metal plates were fixed on the surface to serve as cathodes. Sponges (sprayed with s

30、alt water) were placed between the metal plates and concrete to provide an adequate contact, Fig. 2. </p><p>　　Fig. 2 Accelerated corrosion system</p><p>　　When the required crack

31、 width was achieved for a particular bar, the impressed current was discontinued for that bar. The specimen was removed for pullout testing when all four locations exhibited the target crack width. Average surface crack

32、widths of 0.05, 0.5, 1 and 1.5 mm were adopted as the target crack widths. The surface crack width was measured at 20 mm intervals along the length of the bar, beginning 20 mm from the end of the (plastic

33、tube) bond breaker using an optical microscope. The </p><p>　　Bond strength tests were conducted by means of a hand operated hydraulic jack and a custom-built test rig as shown in Fig. 3. The loading sc

34、heme is illustrated in Fig. 4. A plastic tube of length 80 mm was provided at the end of the concrete section underneath the transverse reaction to ensure that the bond strength was not enhanced by the reactive

35、 (compressive) force (acting normal to the bar). The specimen was positioned so that an axial force was applied to the bar being tested. The restraint</p><p>　　Fig. 3 Pull-out test, 16 mm bar

36、unconfined</p><p>　　Fig. 4 Schematic of loading. Note: only test bar shown for clarity</p><p>　　3   Experimental results and discussion</p><p>　　3.1   Visual ins

37、pection </p><p>　　Following the accelerated corrosion phase each specimen was visually inspected for the location of cracks, mean crack width and maximum crack width (Sect. 2.3). </p><p>　　

38、While each specimen had a mean target crack width for each bar, variations in this crack width were observed prior to pull out testing. This is due to corrosion and cracking being a dynamic process with cracks propagatin

39、g at different rates. Thus, while individual bars were disconnected, once the target crack width had been achieved, corrosion and crack propagation continued (to some extent) until all bars had achieved the target crack

40、width and pull out tests conducted. This resulted in a range</p><p>　　The visual inspection of the specimens showed three stages to the cracking process. The initial cracks occurred in a very short period, u

41、sually generated within a few days. After that, most cracks grew at a constant rate until they reached 1 mm, 3–4 weeks after first cracking. After cracks had reached 1 mm they then grew very slowly, with s

42、ome cracks not increasing at all. For the confined and unconfined specimens the surface cracks tended to occur on the side of the specimens (as opposed to the</p><p>　　Fig. 5 Typical crack patterns

43、</p><p>　　During the pull-out testing the most common failure mode for both confined and unconfined was splitting failure—with the initial (pre-test) cracks caused by the corrosion enlarging under load and u

44、ltimately leading to the section failing exhibiting spalling of the top corner/edge, Fig. 6. However for several of the confined specimens, a second mode of failure also occurred with diagonal (shear like) cracks ap

45、pearing in the side walls, Fig. 7. The appearance of these cracks did not appear to be r</p><p>　　Fig. 6 Longitudinal cracking after pull-out</p><p>　　Fig. 7 Diagonal cr

46、acking after pull-out</p><p>　　The bars were initially (precasting) cleaned with a 12% hydrochloric acid solution, then washed in distilled water and neutralized by a calcium hydroxide solution before being

47、washed in distilled water again. Following the pull-out tests, the corroded bars were cleaned in the same way and weighed again. </p><p>　　The corrosion degree was determined using the following equation <

48、;/p><p>　　where G 0 is the initial weight of the steel bar before corrosion, G is the final weight of the steel bar after removal of the post-test corrosion products, g 0 is the weight per unit length of the st

49、eel bar (0.888 and 1.58 g/mm for Φ12 and Φ16 mm bars, respectively), l is the embedded bond length. </p><p>　　Figures 8 and 9 show steel bars with varying degree of corrosion. The majorit

50、y exhibited visible pitting, similar to that observed on reinforcement in actual structures, Fig. 9. However, a small number of others exhibited significant overall section loss, with a more uniform level of corrosi

51、on, Fig. 8, which may be a function of the acceleration methodology. </p><p>　　Fig. 8 Corroded 12 mm bar with approximately 30% mass loss</p><p>　　Fig. 9 Corroded 1

52、6 mm bar with approximately 15% mass loss</p><p>　　3.2   Bond stress and crack width </p><p>　　Figure 10 shows the variation of bond stress with mean crack width for 16 mm

53、60;bars and Fig. 11 for the 12 mm bars. Figures 12 and 13 show the data for the maximum crack width. </p><p>　　Fig. 10 Mean crack width versus bond stress for 16 mm bars</p&g

54、t;<p>　　Fig. 11 Mean crack width versus bond stress for 12 mm bars</p><p>　　Fig. 12 Maximum crack width versus bond stress for 16 mm bars</p><p>　　Fig. 

55、13 Maximum crack width versus bond stress for 12 mm bars</p><p>　　The data show an initial increase in bond strength for the 12 mm specimens with stirrups, followed by a significant decrease i

56、n bond, which is in agreement with other authors [12, 15]. For the 16 mm specimens an increase on the control bond stress was observed for specimens with 0.28 and 0.35 mm mean crack widths, however, a decrease

57、in bond stress was observed for at the mean crack width of 0.05 mm. </p><p>　　The 12 mm bars with stirrups displayed an increase in bond stress of approximately 25% from the control values to the m

58、aximum bond stress. An increase of approximately 14% was observed for the 16 mm specimens. Other researchers [17, 24, 25] have reported enhancements of bond stress of between 10 and 60% due to confinement, slightly

59、higher to that observed in these experiment. However the loading techniques and cover depths have not all been the same. Variations in experimental techniques include</p><p>　　Both sets of data indicate a re

60、lationship showing decreasing bond strength with (visible surface) crack width. A regression analysis of the bond strength data reveals a better linear relationship with the maximum crack width as opposed to the mean cra

61、ck width (excluding the uncracked confined specimens), Table 2. </p><p>　　Table 2 Best fit parameters, crack width versus bond strength</p><p>　　There was also a significantly bet

62、ter fit for the unconfined specimens than the confined specimens. This is consistent with the observation that in the unconfined specimens the bond strength will be related to the bond between the bars and the concrete,

63、which will be affected by the level of corrosion present, which itself will influence the crack width. In confined specimens the confining steel will impact upon both the bond and the cracking. </p><p>　　3.3

64、   Corrosion degree and bond stress </p><p>　　It is apparent that (Fig. 14) for corrosion degrees less than 5% the bond stress correlated well. However, as the degree of corrosion increased there w

65、as no observable correlation at all. This contrasts with the relationship between the observed crack width and bond stress, which gives a reasonable correlation, even as crack widths increase to 2 and 2.5 mm. A poss

66、ible explanation for this variation is that in the initial stages of corrosion virtually all the dissolved iron ions react to form exp</p><p>　　Fig. 14 Bond stress versus corrosion degree, 12 

67、mm bars, unconfined specimen</p><p>　　Significantly larger crack widths were observed for the unconfined specimens, compared to the confined specimens with similar levels of corrosion and mass lost. The larg

68、est observed crack for unconfined specimens was 2.5 mm compared to 1.4 mm for the confined specimens. This is as expected and is a direct result of the confinement which limits the degree of cracking. </p>

69、;<p>　　3.4   Effect of confinement </p><p>　　The unconfined specimens for both 16 and 12 mm bars did not display the initial increase in bond strength observed for the confined bars. Indeed

70、the unconfined specimens with cracks all displayed a reduced bond stress compared to the control specimens. This is in agreement with other authors [16, 24] findings for cracked specimens. In cracked corroded specimens F

71、ang observed a substantial reduction in bond strength for deformed bars without stirrups, while Rodriguez observed bond strengths of hi</p><p>　　The data is perhaps unexpected as it could be anticipated that

72、 the corrosion products would lead to an increase in bond due to the increase in internal pressures, caused by the corrosion products increasing the confinement and mechanical interlocking around the bar, coupled with in

73、creased roughness of the bar resulting in a greater friction between the bar and the surrounding concrete. However, these pressures would then relieved by the subsequent cracking of the concrete, which would contribute&l

74、t;/p><p>　　It may also be that the compressive strength of the concrete combined with the cover will have an effect on the bond stresses for uncorroded specimens. The data presented here has a cover of three ti

75、mes bar diameter and a strength of 40 MPa, other research ranges from 1.5 to four times cover with compressive strengths from 40 to 77 MPa. </p><p>　　3.5   Comparison of 12 and 16 mm reba

76、r </p><p>　　The maximum bond stress for 16 mm unconfined bars was measured at 8.06 MPa and for the 12 mm bars it was 8.43 MPa. These both corresponded to the control specimens with n

77、o corrosion. The unconfined specimens for both the 12 and 16 mm bars showed no increase in bond stress due to corrosion. For the confined specimens the maximum bond stress for the control specimens were 7.29

78、60;MPa for the 12 mm bars and 6.34 MPa for the 16 mm bars. The maximum bond stress for both sets of confined specimens corres</p><p>　　3.6   Effect of casting position </p>&l

79、t;p>　　There was no significant difference of bond strength due to the position of the bar (top or bottom cast) once cracking was observed, Fig. 15. For control specimens, with no corrosion, however, the bottom ca

80、st bars had a slightly higher bond stress than the top cast bars. These observations are in agreement with other authors [4, 11, 15, 22]. It is generally accepted that uncorroded bottom cast bars have significantly impro

81、ved bond compared to top cast bars due to the corrosion products filling t</p><p>　　Fig. 15 Bond stress versus mean crack width for 12 mm bars, top and bottom cast positions, confined specimen

82、</p><p>　　4   Conclusions </p><p>　　A relationship was observed between crack width and bond stress. The correlation was better for maximum crack width and bond stress than for mean crack w

83、idth and bond stress.</p><p>　　Confined bars displayed a higher bond stress at the point of initial cracking than where no corrosion had occurred. As crack width increase the bond stress reduced significantl

84、y.</p><p>　　Unconfined bars displayed a decrease in bond stress at initial cracking, followed by a further decrease as cracking increased.</p><p>　　Top cast bars displayed a higher bond stress i

85、n specimens with no corrosion. Once cracking had occurred no variation between top and bottom cast bars was observed.</p><p>　　The 12 mm bars displayed higher bond stress values than 16 mm with no

86、corrosion, control specimens, and at similar crack widths.</p><p>　　A good correlation was observed between bond stress and degree of corrosion was observed at low levels of corrosion (less than 5%). However

87、, at higher levels of corrosion no correlation was discerned.</p><p>　　Overall the results indicated a potential relationship between the maximum crack width and the bond. Results shown herein should be inte

88、rpreted with caution as this variation may be not only due to variations between accelerated corrosion and natural corrosion but also due to the complexity of the cracking mechanism in reality. </p><p><b

89、>　　中文譯文：</b></p><p>　　約束和無約束的鋼筋對裂縫寬度的影響</p><p>　　收稿日期：2010年1月14 納稿日期：2010年12月14日 線上發(fā)表時間：2010年1月23日</p><p><b>　　摘 要</b></p><p>　　本報告公布了局限約束和自由的變形對粘結

90、強度12、16毫米鋼筋的表面腐蝕程度和裂紋影響的比較結果。腐蝕是氯化物污染的混凝土的誘導和外加直流電流的引起的。調查的主要參數(shù)有鋼筋剝離，保護層厚度，鋼筋直徑，腐蝕程度和表面裂縫寬度。結果表明了裂縫寬度和粘結強度之間的潛在關系。同時還發(fā)現(xiàn)在圍箍筋處發(fā)現(xiàn)表面裂紋的地方粘結強度增加，而無側限的樣本中沒有觀察到粘結強度增加。</p><p>　　關鍵詞： 粘結；腐蝕；螺紋鋼；保護層 ；裂縫寬度 ；混凝土</p&g

91、t;<p><b>　　引 言</b></p><p>　　在世界各地，鋼筋的腐蝕是鋼筋混凝土結構的惡化的重要原因。在未腐蝕的結構中鋼筋和混凝土之間的粘結使鋼筋混凝土處于有利狀態(tài)。然而，當鋼鐵的腐蝕發(fā)生時，會對這種積極性能產(chǎn)生不利影響。這是由于鋼表面形成了腐蝕產(chǎn)物，從而影響了鋼和混凝土之間的粘結。</p><p>　　鋼筋混凝土惡化是由鋼筋和形成的膨脹腐

92、蝕產(chǎn)物造成的局部損失。這種情況的惡化在許多方面影響結構;膨脹產(chǎn)品的產(chǎn)生造成混凝土的拉應力，這可能會導致混凝土保護層開裂和剝落的。這種開裂可導致更嚴重的惡化和進一步的腐蝕。它也可以導致在混凝土保護層的強度和剛度的損失。腐蝕產(chǎn)物也可以影響混凝土與鋼筋之間的粘結強度。最終腐蝕減少鋼筋截面面積，影響鋼筋的延展性和承載能力，從而最終影響結構適用性和結構承載力[12，25]。</p><p>　　以往的研究調查腐蝕對粘結的影

93、響[2-5，7，12，20，23-25??，27，29]，提出了數(shù)據(jù)模型[4，6，9，10，18，19 24，29]。本研究主要研究腐蝕（鋼材質量損失）水平或電流密度程度（腐蝕電流在加速測試中的應用）和裂縫寬度之間的關系，或粘結強度和腐蝕程度之間的關系。其他研究已調查的銹蝕力學性能[1，11]和摩擦特性[13]。然而，很少有人研究都集中在裂縫寬度與粘結[23，26，28]之間的關系上，此參數(shù)易與實際結構相聯(lián)系。</p>&

94、lt;p>　　加強鋼筋的腐蝕導致生成鐵氧化物，它的體積大于原鋼材。這種擴張造成周圍的混凝土內的拉應力，最終導致混凝土保護層開裂。一旦開裂發(fā)生，混凝土緊箍力就會損失。這表明粘結能力的損失可能與縱向裂縫寬度有關[12]。然而，以混凝土的剝離可以在一定程度上抵消粘結力的損失。最新研究主要與剝離樣本有關。本文報道的一項研究比較了有側限和無側限樣本的粘結力損失。</p><p><b>　　2.實驗研究&l

95、t;/b></p><p><b>　　2.1樣本</b></p><p>　　梁端樣本[28]被選定為這項研究的研究對象。這種撤去偏心或“梁端”模式樣本以一個典型的簡支梁錨固區(qū)的粘結長度支撐。樣本的矩形截面投在縱向鋼筋的各處，如圖1。由于沒有增強下方橫反應的鋼筋，試樣提供了一個80毫米的塑料管，以確保粘結強度（橫向）壓縮力超過這個長度的鋼筋。</p>

96、;<p><b>　　圖1梁端試樣</b></p><p>　　試驗調查了由3倍直徑厚的保護層保護的12和16毫米直徑的鋼筋。重復測試有側限和自由樣本。在密閉的塑料管中有3套6毫米的不銹鋼箍筋從其間穿過，在75毫米中心。</p><p>　　這代表了四組不同鋼筋直徑和有側限/無約束的樣本。以調查鋼筋規(guī)格，混凝土剝離和裂縫寬度對粘結強度的影響。</p

97、><p><b>　　2.2材料</b></p><p>　　配合比設計，如表1所示。水泥是I型硅酸鹽水泥，骨料為玄武巖，容重2.99。根據(jù)AS 1141— 2000進行粗、細集料的制備。拌合根據(jù)AS 1141—1994進行。測試前水浴養(yǎng)護28天。</p><p>　　表1混凝土配合比設計</p><p>　　為了比較不同的

98、混凝土抗壓強度，粘結強度，Eq。 公式1已被其他研究者用于正?；辰Y強度的非腐蝕樣本。</p><p><b>　　1</b></p><p>　　為40級混凝土的粘結強度，exptl為實驗粘結強度和Fc是實驗抗壓強度。</p><p>　　Φ12和Φ16毫米鋼筋的抗拉強度是500兆帕，分別相當于一個56.5和100.5kN的破壞載荷。<

99、/p><p><b>　　2.3實驗方法</b></p><p>　　加速腐蝕已被許多作者用于重現(xiàn)在自然環(huán)境中發(fā)生的腐蝕鋼筋鋼 [2，3，5，6，10，18，20，24，27，28，30]。這些相關實驗使用外加電流或干濕周期人工風化和升高溫度延緩腐蝕時間，同時保持惡化機制處于自然狀態(tài)。采用外加電流的研究使用的電流密度在100μA/cm2與500 mA/cm2之間 [20]

100、。有研究表明，電流密度200μA/cm2與100μA/cm2相比，200的結果與早期階段的腐蝕更相似 [21]。隨著施加電流密度200μA/cm2被選定為研究使用電流，這在以前的研究中成為電流密度頻譜的低端代表。然而，應謹慎應用外加電流的加速腐蝕，加速過程并不完全復制在實際結構中所涉及的機制。在加速測試中不允許違背自然的發(fā)展，并有可能在表面上更均勻腐蝕。腐蝕率也可能會影響腐蝕的產(chǎn)品，這些產(chǎn)品可能會形成不同的氧化狀態(tài)，這可能會影響粘結強度

101、。</p><p>　　鋼筋作為陽極和四個碳鋼金屬板固定在表面作為陰極。金屬板和混凝土之間放置海綿（用鹽水噴灑）提供足夠的接觸，如圖2。</p><p><b>　　圖2加速腐蝕系統(tǒng)</b></p><p>　　當裂縫寬度要求需適應特殊鋼筋時應該終止施加外加電流。當所有四個位置出現(xiàn)規(guī)定的裂縫寬度，試樣就會被拆除撤離測試。平均表面裂縫寬度0.05

102、，0.5，1和1.5毫米作為目標裂縫寬度。表面裂紋寬度沿鋼筋長度測量間隔20mm，從約束（塑料管）末端開始20mm用斷路器光學顯微鏡測量。測量精度為±0.02毫米。從鋼筋表面測量裂縫寬度，不考慮裂縫實際方位在何處。</p><p>　　粘結強度測試通過手動操作液壓千斤頂和一個定制的試驗裝置，如圖3所示。加載方案見圖4。長80毫米的塑料管在末端提供了一個橫向反應的具體部分，以確保粘結強度不會因為內力（壓力

103、）提高而增加。樣本定位使軸向力，適用于被測試的鋼筋。給樣本足夠剛性的約束可以確保在加載過程中最小的旋轉或扭曲。</p><p>　　圖3拉出測試，16毫米鋼筋不承壓</p><p>　　圖4加載示意圖。注：只測試顯示棒</p><p><b>　　3實驗結果與討論</b></p><p><b>　　3.1目視

104、檢查</b></p><p>　　加速腐蝕階段后，檢查每個樣本的裂縫的位置，平均裂縫寬度和最大裂縫寬度（第2.3款）。</p><p>　　雖然每個鋼筋樣本都有平均目標裂縫寬度，但是裂縫寬度的變化在觀察前拉出測試。這是由于腐蝕和開裂是一個動態(tài)的過程，裂縫是以不同的速度傳播的。因此，當個別鋼筋被拉斷的時候，一旦目標裂縫寬度已經(jīng)達到，腐蝕和裂紋在一定程度上繼續(xù)擴展，直到所有的鋼筋已