土木工程外文文献

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Materials and Structures ? RILEM 2010 10.1617/s11527-010-9700-y Original Article

Impact of crack width on bond: confined and unconfined rebar

David W. Law1 , Denglei Tang2, Thomas K. C. Molyneaux3 and Rebecca Gravina3

(1) School of the Built Environment, Heriot Watt University, Edinburgh, EH14 4AS, UK (2) VicRoads, Melbourne, VIC, Australia

(3) School of Civil, Environmental and Chemical Engineering, RMIT University,

Melbourne, VIC, 3000, Australia David W. Law

Email: D.W.Law@hw.ac.uk

Received: 14 January 2010 Accepted: 14 December 2010 Published online: 23 December 2010

Abstract

This paper reports the results of a research project comparing the effect of surface crack width and degree of corrosion on the 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 investigated were confinement of the

reinforcement, the cover depth, bar diameter, degree of corrosion and the surface crack width. The results indicated that potential relationship between the crack width and the bond strength. The results also showed an increase in bond strength at the point where

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initial surface cracking was observed for bars with confining

stirrups. No such increase was observed with unconfined specimens. Keywords Bond - Corrosion - Rebar - Cover - Crack width - Concrete

1 Introduction

The corrosion of steel reinforcement is a major cause of the deterioration 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 corrosion 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.

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 structures 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 ingress of the aggressive agents causing further corrosion. It can also result in a loss of strength and stiffness of the concrete cover. The corrosion products can also affect the bond strength between the concrete and the reinforcing steel. Finally the corrosion reduces the cross section of the reinforcing steel, which can affect the ductility of the steel and the load bearing capacity, which can ultimately impact upon the serviceability of the structure and the structural capacity [12, 25]. Previous research has investigated the impact of corrosion on bond [2–5, 7, 12, 20, 23–25, 27, 29], with a number of models 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

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loss of steel) or the current density degree (corrosion current applied in accelerated testing) and crack width, or on the

relationship between bond strength and level of corrosion. Other research has investigated the mechanical behaviour of corroded steel [1, 11] and the friction characteristics [13]. However, little research has focused on the relationship between crack width and bond [23, 26, 28], a parameter that can be measured with relative ease on actual structures.

The corrosion of the reinforcing steel results 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 concrete. 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 the concrete can counteract this loss of bond capacity to a certain degree. Research to date has primarily involved specimens with confinement. This paper reports a study comparing the loss of bond of specimens with and without confinement.

2 Experimental investigation

2.1 Specimens

Beam end specimens [28] were selected for this study. This type of eccentric 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 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 bar over this length.

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Fig. 1 Beam end specimen

Deformed rebar of 12 and 16 mm diameter with cover of three times bar diameter were investigated. Duplicate sets of confined and unconfined 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.

This represents four groups of specimens 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.

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2.2 Materials

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 prepared 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.

Table 1 Concrete mix design MateriCement w/c Sand al 10 mm washed aggregate 7 mm washed Salt aggregate Slump Quanti381 kg/0.4517 kg/463 kg/463 kg/18.84 kg140 ± 25 mty m3 9 m3 m3 m3 /m3 m 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 researcher [8].

where is the bond strength for grade 40 concrete, τ exptl is the experimental bond strength and f c is the experimental compressive strength.

(1)

The tensile strength of 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.

2.3 Experiment methodology

Accelerated 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 using impressed currents or artificial weathering with wet/dry cycles and elevated temperatures to reduce the time until corrosion, while maintaining deterioration mechanisms

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representative of natural exposure. Studies using impressed currents have used current densities between 100 μA/cm2 and 500 mA/cm2 [20]. Research has suggested that current densities up to 200 μA/cm2 result in similar stresses during the early stages of corrosion when compared to 100 μA/cm2 [21]. As such an applied current density of 200 μA/cm2 was selected for this study—representative of the lower end of the spectrum of such current densities adopted in previous research. However, caution should be applied when accelerating the corrosion using impressed current as the acceleration process does not exactly replicate the mechanisms involved in actual structures. In accelerated tests the pits are not allowed to progress naturally, and there may be a more uniform corrosion on the surface. Also the rate of corrosion may impact on the corrosion products, such that different oxidation state products may be formed, which could impact on bond.

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 salt water) were placed between the metal plates and concrete to provide an adequate contact, Fig. 2.

Fig. 2 Accelerated corrosion system

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When the required crack 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 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 tube) bond breaker using an optical microscope. The level of accuracy in the measurements was ±0.02 mm. Measurements of crack width were taken on the surface normal to the bar direction regardless of the actual crack orientation at that location.

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 scheme 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 (compressive) force (acting normal to the bar). The specimen was positioned so that an axial force was applied to the bar being tested. The restraints were sufficiently rigid to ensure minimal rotation or twisting of the specimen during loading.

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Fig. 3 Pull-out test, 16 mm bar unconfined

Fig. 4 Schematic of loading. Note: only test bar shown for clarity

3 Experimental results and discussion

3.1 Visual inspection

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).

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 propagating 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 width and pull out tests conducted. This resulted in a range of data for the maximum and mean crack widths for the pull out tests.

The visual inspection of the specimens showed three stages to the cracking process. The initial cracks occurred in a very short period, usually 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,

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with some 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 top or bottom) and to follow the line of the bars. In the case of the unconfined specimens in general these were the only crack while it was common in the cases of confined specimens to observe cracks that were aligned vertically down the side—adjacent to one of the links, Fig. 5.

Fig. 5 Typical crack patterns

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 ultimately 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 appearing in the side walls, Fig. 7. The appearance of these cracks did not appear to be related to the presence of vertical cracks observed (in specimens with stirrups) during the corrosion phase as reported above.

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Fig. 6 Longitudinal cracking after pull-out

Fig. 7 Diagonal cracking after pull-out

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 washed in distilled water

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again. Following the pull-out tests, the corroded bars were cleaned in the same way and weighed again.

The corrosion degree was determined using the following equation

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 steel bar (0.888 and 1.58 g/mm for Φ12 and Φ16 mm bars, respectively), l is the embedded bond length.

Figures 8 and 9 show steel bars with varying degree of corrosion. The majority 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 corrosion, Fig. 8, which may be a function of the acceleration methodology.

Fig. 8 Corroded 12 mm bar with approximately 30% mass loss

Fig. 9 Corroded 16 mm bar with approximately 15% mass loss

3.2 Bond stress and crack width

Figure 10 shows the variation of bond stress with mean crack width for 16 mm bars and Fig. 11 for the 12 mm bars. Figures 12 and 13 show the data for the maximum crack width.

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Fig. 10 Mean crack width versus bond stress for 16 mm bars

Fig. 11 Mean crack width versus bond stress for 12 mm bars

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Fig. 12 Maximum crack width versus bond stress for 16 mm bars

Fig. 13 Maximum crack width versus bond stress for 12 mm bars

The data show an initial increase in bond strength for the 12 mm specimens with stirrups, followed by a significant decrease in 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 in bond stress was observed for at the mean crack width of 0.05 mm.

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The 12 mm bars with stirrups displayed an increase in bond stress of approximately 25% from the control values to the maximum 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 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 a shorter embedded length and a lower cover. The variation on the proposed empirical relationship between bond strength, degree of corrosion, bar size, cover, link details and tensile strength predicted by Rodriguez [24] has been discussed in detail in Tang et al. [28]. The analysis demonstrates that there would be an expected enhancement of bond strength due to confinement of approximately 25%—corresponding to a change of bond strength of approximately 0.75 MPa for the 16 mm bars (assessed at a 2% section loss). For the 12 mm bars the corresponding effect of confinement is found to be approximately 35% corresponding to a 1.0 MPa difference in bond stress. The experimental results (14 and 25%, above) are 60–70% of these values.

Both sets of data indicate a relationship 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 crack width (excluding the uncracked confined specimens), Table 2.

Table 2 Best fit parameters, crack width versus bond strength

Unconfined 12 mm 0.920 ?3.997 7.560 Confined 12 mm 0.637 ?3.653 8.122 Unconfined 16 mm 0.672 ?2.999 6.496 Confined 16 mm 0.659 ?8.848 8.746 Mean crack width R 2 Slope (m) Intercept (b) R 2

Maximum crack width 0.937 0.855 14

0.714 0.616 外文翻译

Slope (m) Intercept (b) Unconfined 12 mm ?2.719 7.805 Confined 12 mm ?2.968 8.403 Unconfined 16 mm ?1.815 6.707 Confined 16 mm ?5.330 9.636 There was also a significantly better 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, 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.

3.3 Corrosion degree and bond stress

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 was 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 possible explanation for this variation is that in the initial stages of corrosion virtually all the dissolved iron ions react to form expansive corrosion products. This reaction impacts on both the bond stress and the formation of cracks. However, once cracks have been formed it is possible for the iron ions to be transported along the crack and out of the concrete. As the bond has already been effectively lost at the crack any iron ions dissolving at the crack and being directly transported out of the concrete will cause an increase in the degree of corrosion, but not affect the surface crack width. The location, orientation and chemistry within the crack will control the relationship between bond stress and degree of corrosion, which will vary from specimen to specimen. Hence the large variations in corrosion degree and bond stress for high levels of corrosion.

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Fig. 14 Bond stress versus corrosion degree, 12 mm bars, unconfined specimen

Significantly larger crack widths were observed for the unconfined specimens, compared to the confined specimens with similar levels of corrosion and mass lost. The largest 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.

3.4 Effect of confinement

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 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 Fang observed a substantial reduction in bond strength for deformed bars without stirrups, while Rodriguez observed bond strengths of highly corroded cracked specimens without stirrups were close to zero, while highly corroded cracked specimens with stirrups retained bond strengths of between 3 and 4 MPa. In uncorroded specimens Chana noted an increase in bond strength due to stirrups of between 10 and 20% [14]. However Rodriguez and Fang

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observed no variation due to the presence of confinement in uncorroded bars.

The data is perhaps unexpected as it could be anticipated that 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 increased 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 to the decrease in the bond strength as crack widths increase. A possible hypothesis is that due to the level of cover, three times bar diameter, the effect of confinement by the stirrups is reduced, such that it has little impact on the bond stress in uncracked concrete. However, once cracking has taken place the confinement does have a beneficial effect on the bond.

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 times 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.

3.5 Comparison of 12 and 16 mm rebar

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 no 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 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 corresponded to point of the initial cracking. The maximum bond stresses were observed at a mean crack width of 0.01 mm for the 12 mm bars and 0.28 mm for the

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16 mm bars. The corresponding bond stresses were, 8.45 and 7.20 MPa. Overall the 12 mm bars displayed higher bond stresses compared to the 16 mm bars at all crack widths. This is attributed to a different failure mode. The 16 mm specimens demonstrate splitting failure while the 12 mm bars bond failure.

3.6 Effect of casting position

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 cast 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 improved bond compared to top cast bars due to the corrosion products filling the voids that are often present under top cast bars as the corrosion progresses [14]. The corrosion also acts as an ‘anchor’, similar to the ribs on deformed bars, to increase the bond. Overall, the mean value of bond stress for all bars (corroded and uncorroded) located in the top were within 1% of the mean bond stress of all bars located in the bottom of the section—for both unconfined and confined bars. This is probably due to the level of cover. The results reported previously are on specimens with one times cover [14]. However, at three times cover it would be anticipated that greater compaction would be achieved around the top cast bars. Thus the area of voids would be reduced and thus the effect of the corrosion product filling these voids and increasing the bond strength would be reduced.

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Fig. 15 Bond stress versus mean crack width for 12 mm bars, top and bottom cast positions, confined specimen

4 Conclusions

?

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 width and bond stress.

?

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 significantly.

?

Unconfined bars displayed a decrease in bond stress at initial cracking, followed by a further decrease as cracking increased.

?

Top cast bars displayed a higher bond stress in specimens with no corrosion. Once cracking had occurred no variation between

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top and bottom cast bars was observed.

?

The 12 mm bars displayed higher bond stress values than 16 mm with no corrosion, control specimens, and at similar crack widths.

?

A good correlation was observed between bond stress and degree of corrosion was observed at low levels of corrosion (less than 5%). However, at higher levels of corrosion no correlation was discerned.

Overall the results indicated a potential relationship between the maximum crack width and the bond. Results shown herein should be interpreted 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.

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材料和结构 ?RILEM 2010

10.1617/s11527-010-9700-y 原创文章

债券：局限和自由的钢筋裂缝宽度的影响

大卫W. Law1，Denglei Tang2，托马斯架KC Molyneaux3和Gravina3丽贝卡 （1）学校的建筑环境，赫瑞瓦特大学，爱丁堡，英国EH14 4AS， （2）向VicRoads，墨尔本，维多利亚州，澳大利亚

（3）学校土木，环境与化学工程学院，墨尔本皇家理工大学，墨尔本，电话，3000，澳大利亚 戴维·法

电子邮件：D.W.Law @ hw.ac.uk

收稿日期：2010年1月接受14：2010年12月14日线上发表于：2010年12月23日 抽象

本文报道的一个研究项目上的局限和自由的变形12粘结强度和轻度16毫米钢筋表面裂纹宽度和腐蚀程度的影响比较的结果。诱导氯化物污染的混凝土和外加直流电流的腐蚀。调查的主要参数，加固，保护层厚度，钢筋直径，腐蚀程度和表面裂缝宽度的约束。结果表明，潜在的关系之间的裂缝宽度和粘结强度。研究结果还表明在初始表面开裂围箍筋酒吧观察点的粘结强度增加。没有这样的增长是观察与潜水标本。

关键词债券 - 腐蚀 - 螺纹钢 - 盖 - 裂缝宽度 - 混凝土 ________________________________________ 1引言

钢筋的腐蚀是钢筋混凝土结构的恶化，世界各地的重要原因。在未腐蚀结构的钢筋和混凝土之间的粘结，确保在钢筋混凝土复合的方式行为。然而，对钢铁的腐蚀发生时，这种复合材料的性能产生不利影响。这是由于形成了钢表面腐蚀产物，从而影响了钢和混凝土之间的纽带。 钢筋混凝土恶化的特点是由钢筋和膨胀腐蚀产物的形成一般或局部的部分损失。这种情况的恶化，可以在许多方面影响结构;膨胀产品的生产，创建混凝土内的拉应力，这可能会导致开裂和剥落的混凝土盖。这种裂解可导致加速造成进一步的腐蚀咄咄逼人的代理入口。它也可以导致在混凝土保护层的强度和刚度的损失。腐蚀产物，也可以影响混凝土和钢筋之间的粘结强度。最后的腐蚀减少钢筋的截面，这可能会影响钢铁和承载能力，从而最终影响结构的适用性后的延展性和结构的能力[12，25]。

以往的研究调查债券腐蚀的影响[2-5，7，12，20，23-25，27，29]，提出[4，6，9，10，18，19型号24，29]。本研究多数集中腐蚀（钢材质量损失）的水平或程度（腐蚀电流在加速测试

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中的应用）的电流密度和裂缝宽度之间的关系，或粘结强度和腐蚀程度之间的关系。其他研究已调查的锈蚀[1，11]力学性能和摩擦特性[13]。然而，很少有人研究都集中在裂缝宽度和债券之间的关系[23，26，28]，与实际结构相对简单，可衡量的参数。

中占据了较大的体积比母材的铁氧化物形成的钢筋腐蚀。这种扩张创造了周围的混凝土内的拉应力，最终导致盖板混凝土开裂。一旦开裂发生，有一个从紧箍力的具体损失。这表明债券能力的损失可能与纵向裂缝宽度[12]。然而，禁闭在具体使用可以抵消这种债券的能力在一定程度上的损失。最新研究主要涉及与禁闭标本。本文报道的一项研究，比较和无约束的标本债券损失。

________________________________________ 2实验研究 2.1标本

梁端标本[28]被选定为这项研究。这种偏心撤军或“梁端”模式标本使用了一个典型的简支梁锚固区的保税长度代表。铸有纵向加强在各个角落的酒吧，图的矩形截面标本。 1。提供了一个80毫米的塑料管，以确保粘结强度（横向）压缩力超过这个长度的酒吧由于没有增强下方横反应的酒吧。

图1梁端试样

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重复集的局限和自由的标本进行了12和16毫米直径的3倍钢筋直径盖变形钢筋进行了调查。

测试。在密闭的标本有3套6毫米的不锈钢同样从塑料管间距箍筋，在75毫米中心。 这代表四组结合不同钢筋直径和标本，有/无约束。标本，以调查对粘结强度的影响的大小酒吧，分娩和裂缝宽度。 2.2材料

组合设计，表1所示。水泥是I型硅酸盐水泥，骨料与比重2.99玄武岩。根据在1141年至2000年与AS粗和细骨料制备。混合进行的AS 1012.2-1994的规定。下湿麻袋28天前的测试标本治愈。

表1混凝土配合比设计

W / C材料水泥砂10毫米洗净，合计7毫米洗盐坍落度总量

数量381 kg/m3的0.49 517 kg/m3的463 kg/m3的463 kg/m3的18.84 kg/m3的140±25毫米

为了比较不同的混凝土抗压强度，粘结强度，EQ。 1用于正常化非腐蚀标本的粘接强度，已被其他研究者使用[8]。 （1）

是为40级混凝土的粘结强度，τexptl实验的粘结强度和FC是抗压强度实验。

Φ12和Φ16毫米钢筋的抗拉强度是名义上的500兆帕，这相当于一个56.5的破坏载荷和100.5千牛，分别。

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2.3实验方法

加速腐蚀已被用于一些作者复制腐蚀钢筋钢在自然环境中发生的[2，3，5，6，10，18，20，24，27，28，30]。这些参与实验采用外加电流或风化人工湿/干周期和温度升高，减少直至腐蚀的时间，同时保持自然暴露恶化机制代表。采用外加电流的研究已用于100μA/cm2和500 mA/cm2的电流密度[20]。有研究表明，电流密度高达200μA/cm2期间类似应力腐蚀的早期阶段时相比，100μA/cm2[21]。随着施加电流密度为200μA/cm2被选定为研究这在以前的研究中通过的电流密度频谱的低端代表。然而，应谨慎应用加速时使用外加电流的腐蚀，加速过程并不完全复制在实际结构中所涉及的机制。在加速测试中不允许坑自然的进步，并有可能在表面上更均匀腐蚀。腐蚀率也可能会影响腐蚀的产品，这些产品可能会形成不同的氧化状态，这可能会影响债券。

作为阳极和四个碳钢金属板的钢条被固定在表面作为阴极。之间的金属板和混凝土提供足够的接触，图被放置海绵（用盐水喷洒）。 2。

图2加速腐蚀系统

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外加电流时所需的裂缝宽度为实现一个特定的酒吧，该酒吧停止。试样被拆除撤离测试时，所有四个地点展出目标的裂缝宽度。通过表面的平均0.05，0.5，1和1.5毫米的裂缝宽度为目标的裂缝宽度。表面裂纹宽度测量沿栏的长度在20毫米的间隔，开始20毫米（塑料管）的债券，利用光学显微镜断路器。测量精度为±0.02毫米。表面上正常的酒吧方向，无论在该位置的实际裂缝方位，裂缝宽度测量。

粘结强度测试通过手工方式进行操作液压千斤顶和一个定制的试验装置图所示。 3。装载计划图所示。 4。在下方横向反应的具体部分提供了一个长度为80毫米的塑料管，以确保粘结强度没有提高的反应（压）力（正常的酒吧行事）。标本定位，使轴向力，适用于被测试的酒吧。足够刚性的约束，以确保在加载过程中最小的旋转或扭曲的标本。

图3拉出测试，16毫米的酒吧不承压

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图4加载示意图。注：只测试棒显示清晰度 ________________________________________ ________________________________________ 3实验结果与讨论 3.1目视检查

加速腐蚀阶段，每个标本视觉检查裂缝的位置，平均裂缝宽度和最大裂缝宽度（第2.3款）。 虽然每个标本平均目标裂缝宽度为每个酒吧，在这个裂缝宽度的变化，观察前拉出测试。这是由于腐蚀和开裂，是一个动态的过程，在不同的速度传播的裂缝。因此，而个别酒吧被断开，一旦目标裂缝宽度已经达到，腐蚀和裂纹扩展继续在一定程度上，直到所有的酒吧已达到目标

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的裂缝宽度，并拉出试验进行。这导致了一系列的最大和平均裂缝宽度拉了测试数据。 视觉检测的标本显示了三个阶段的裂解过程。初始裂缝发生在很短的时间内，通常在几天之内产生。在此之后，大多数裂缝在一个恒定的速度增长，直到他们达到1毫米，3-4周后首次开裂。后裂缝已经达到了1毫米，然后他们的增长速度非常缓慢，一些裂缝不增加。密闭和自由的标本表面裂纹往往发生在侧的标本（如反对的顶部或底部），并按照酒吧行。在一般的潜水标本的情况下，这些人唯一的裂纹，而这是在密闭的标本，观察裂缝对齐，垂直向下侧相邻的链接，图的情况下共同的。 5。

图5典型的裂纹模式

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在拉出测试最常见的故障模式的局限和自由的裂缝造成的分裂失败的初始（预试）负载下的腐蚀扩大，并最终导致部分未参展的右上角剥落/边缘，图。 6。但是一些密闭的标本，一个失败的第二种模式也发生对角线（剪像）裂缝出现在侧墙，图。 7。这些裂缝的出现并未将有关在上述报告的腐蚀阶段观察（箍筋标本）的纵向裂缝的存在。

图6拉出后纵向开裂

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图7角开裂后拉出

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酒吧最初（预制）有12％的盐酸溶液清洗，然后在蒸馏水清洗和再次蒸馏水洗涤之前，由氢氧化钙溶液中和。锈蚀钢筋拉出来的测试之后，以同样的方式进行清洗，并再次称重。 使用下列公式确定的腐蚀程度

其中G 0是钢筋腐蚀前的初始重量，G是最终去除腐蚀产物后的测试后的钢筋重量，G 0是每单位长度的钢筋重量（0.888和1.58? /毫米Φ12和Φ16毫米钢筋，分别），l是嵌入式的键长。

图8和图9显示有不同程度的腐蚀钢筋。多数表现出可见的凹陷，类似的实际结构，图中观察加固。 9。然而，少数表现出显着的整体部分损失，更均匀的腐蚀，图的水平。 8，这可能是一个加速的方法的功能。

图8腐蚀与质量损失约30％的12毫米栏

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图9约15％的质量损失腐蚀与16毫米栏

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3.2债券的应力和裂缝宽度

图10显示了债券应力与平均裂缝宽度为16毫米的钢筋和图的变化。 11为12毫米的钢筋。图12和图13显示的最大裂缝宽度的数据。

图10平均裂缝宽度与债券为16毫米的钢筋应力 ________________________________________

图11平均裂缝宽度与债券为12毫米的钢筋应力 ________________________________________

图12最大裂缝宽度与债券为16毫米的钢筋应力 ________________________________________

图13的最大裂缝宽度为12毫米的钢筋抗粘结应力 ________________________________________

数据显示为12毫米标本箍筋由债券的显着下降，这与其他作者[12，15]协议是在初始粘结强度增加。控制粘结强度的增加，为16毫米的标本为0.28和0.35毫米的标本观察的意思，但是，裂缝宽度减少了粘结应力观察到的平均裂缝宽度为0.05毫米。

箍筋12毫米钢筋显示控制值增加约25％的债券压力最大粘结强度。增加约14％为16毫米的标本观察。其他研究[17，24，25]报道的债券之间的10和60％，由于禁闭，在这些实验中观察到，稍高的压力增强。然而，装卸技术和覆盖深度都没有都是相同的。实验技术的变化，包括较短的嵌入式长度和下盖。已就拟议的粘结强度，腐蚀程度，酒吧大小，封面，链接的详细信息和拉伸强度由罗德里格斯预测之间的经验关系的变化[24]唐等详细讨论。分析表明， [28]。将有粘结强度的预期增强，由于禁闭约25％，相应的债券为16毫米的钢筋（2％的部分损失评估）约0.75兆帕强度的变化。相应的约束作用，为12毫米的钢筋，被发现约35％，在债券的压力为1.0 MPa差异。 （14和25％以上）的实验结果是这些值的60-70％。

这两组数据表明，显示可见表面裂纹宽度减小粘结强度的关系。粘结强度数据的回归分析表明反对平均裂缝宽度（不包括未开裂的密闭标本），表2的最大裂缝宽度的一个较好的线性关系。 表2最佳拟合参数，裂缝宽度与粘结强度

无压密闭12毫米12毫米无压密闭16毫米16毫米 平均裂缝宽度

R 2的0.920 0.637 0.672 0.659 斜率（m）-3.997 -3.653 -2.999 -8.848 截距（b）7.560 8.122 6.496 8.746 最大裂缝宽度

R 2的0.937 0.855 0.714 0.616 斜率（m）-2.719 -2.968 -1.815 -5.330

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截距（b）7.805 8.403 6.707 963.6

也有显着更适合一个比密闭标本的潜水标本。这与在潜水标本将与债券之间的酒吧和具体，这将被腐蚀目前的水平，这本身会影响裂缝宽度的影响粘结强度的观察是一致的。在密闭的标本会影响围钢后，债券和开裂。 3.3腐蚀程度和粘结应力

很明显，债券应力腐蚀度小于5％（图14）很好的相关性。然而，随着腐蚀程度的增加，有没有观察到的所有相关。观测到的裂缝宽度和粘结应力，给出了一个合理的相关性，甚至裂缝宽度增加2个和2.5毫米之间的关系与此相反。这种变化的一个可能的解释是，腐蚀的初始阶段，几乎所有溶解的铁离子发生反应，形成膨胀腐蚀产物。这种反应影响债券应力和裂纹的形成。然而，一旦裂缝已经形成，这是铁离子可沿混凝土裂缝和出运。由于债券已被有效地裂缝失去任何铁离子溶解在裂缝和混凝土直接运会导致腐蚀程度的增加，但不影响表面裂纹的宽度。内裂纹的位置，方向和化学控制粘结应力和腐蚀程度，这将改变从标本到标本之间的关系。因此，在腐蚀程度和粘结应力腐蚀的高层次的大变化。

图14债券应力与腐蚀的程度，12毫米的酒吧，潜水标本 ________________________________________

裂缝宽度显着较大的观察潜水标本的，而失去了类似的腐蚀和质量水平的局限标本。观测到的最大潜水标本的裂纹是2.5毫米至1.4毫米的密闭标本。这是预期，是一种隔离，从而限制了开裂程度的直接结果。 3.4分娩的影响

16和12毫米酒吧潜水标本没有显示在初步观察局限于酒吧的粘结强度增加。事实上，与裂缝潜水标本，所有显示减少债券的压力，比对照标本。这是与其他作者[16，24]破获标本的结果一致。在破获的锈蚀标本方观察无箍筋变形钢筋的强度在债券大幅减少，而罗德里格斯观察无箍筋高度锈蚀破获标本的粘结强度接近零，而高度腐蚀破裂与箍筋标本保留3和4之间的粘结强度兆帕。在未腐蚀标本，长安指出粘结强度的增加，由于10至20％[14]马镫。但罗德里格斯和方没有观察到的变化，由于存在未腐蚀钢筋禁闭。

也许是意外，因为它可以预计，腐蚀产物会导致债券的增加，由于内部压力增加，造成腐蚀产品，增加分娩和机械联锁周围酒吧，再加上增加粗糙度的数据禁止在酒吧和周围的混凝土之间的摩擦产生的。然而，这些压力将缓解随后混凝土开裂，这将有助于减少裂缝宽度增加粘结强度。一种可能的假设是，由于覆盖水平，3倍钢筋直径，箍筋的作用是降低的约束，例如，它在未开裂混凝土的粘结强度的影响不大。然而，一旦打击已发生的监禁并有利于债券的影响。 这也可能是结合盖混凝土抗压强度将有一个未腐蚀试样债券讲的效果。这里给出的数据，有盖的3倍钢筋直径和强度为40兆帕，其他的研究范围从1.5至4倍涵盖从40至77兆帕的抗压强度。

3.5比较12和16毫米螺纹钢

在8.06兆帕，最大为16毫米的潜水酒吧债券应力测量为12毫米的钢筋，它是8.43兆帕。这些都符合无腐蚀控制标本。 12和16毫米钢筋的潜水标本显示因腐蚀无粘结应力的增加。为

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密闭标本的对照标本的最大粘结强度分别为7.29兆帕为12毫米的钢筋为16毫米的酒吧和6.34兆帕。最大的债券压力的密闭标本两套对应点的初始开裂。最大的债券平均在0.01毫米的裂缝宽度为12毫米的酒吧和0.28毫米，16毫米钢筋应力观察。相应的债券应力，8.45和7.20兆帕。总体而言，12毫米钢筋显示高于债券在所有裂缝宽度16毫米钢筋强调。这是由于不同的故障模式。 16毫米的标本展示分裂失败而12毫米的酒吧债券失败。 3.6铸造位置的影响

栏的位置（顶部或底部铸）观察，一旦开裂图由于粘结强度没有显着性差异。 15。控制标本，无腐蚀，但是，底部铸棒比顶端铸棒粘结强度略高。这些意见在与其他作者[图4，11，15，22]一致。它被普遍接受，显着改善，未腐蚀的底部铸酒吧债券相比，顶铸棒，由于腐蚀产品填补空隙，顶部铸棒下，往往存在腐蚀进展[图14]。腐蚀也可以作为“锚”，类似肋骨变形钢筋，增加负荷。总的来说，位于顶端的所有位置（腐蚀和未腐蚀）粘结强度平均值均在1％的所有地方低于底部的平均粘结应力节无压和密闭的地方。这可能是由于覆盖水平不同。结果以前的报告是在同一个时代封面[14]的标本。然而，在第三次实验，将预计将围绕顶端铸棒取得更大的压实。因此，空洞的面积将会减少，从而腐蚀产品填补这些空隙和提高粘结强度的影响将会减少。

图15债券应力与平均裂缝宽度为12毫米的钢筋，顶部和底部铸位置，密闭标本

4结论

?裂缝宽度和粘结应力的关系。相关的最大裂缝宽度和粘结强度优于平均裂缝宽度和粘结应力。?密闭情况下显示较高的粘结强度比无腐蚀发生在初始开裂点。由于裂缝宽度增加粘结强度显著降低。

?无压条显示在初始开裂，随后由进一步减少开裂增加粘结强度下降。

?顶铸棒，无腐蚀的标本显示了较高的粘结强度。一旦开裂发生之间没有观察到顶部和底部的铸造结构的变化。

?12毫米钢筋无腐蚀，控制标本显示较高的债券应力值超过16毫米，并在类似的裂缝宽度。

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