土木工程外文文献及翻译

外文文献：
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
(3School of Civil, Environmental and Chemical Engineering, RMIT
) University, Melbourne, VIC, 3000, Australia
David W. Law
Email:
w@
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
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 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 pull out 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.
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.
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
Materi al Cemen
t
w/
c
Sand
10 mm
washe
d
aggreg
ate
7 mm
washe
d
aggreg
ate
Salt Slump
Quant ity 381 kg/
m3
0.4
9
517 kg/
m3
463 kg/
m3
463 kg/
m3
18.84 kg/
m3
140 ± 25 m
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].
(1)
where is the bond strength for grade 40 concrete, τ exptl is the experimental bond strength and f c is the experimental compressive strength.
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 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
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.
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, 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.
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 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.
Fig. 10 Mean crack width versus bond stress for 16 mm bars
Fig. 11 Mean crack width versus bond stress for 12 mm bars 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.
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
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.
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 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 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.
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 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.
中文译文：
约束和无约束的钢筋对裂缝宽度的影响收稿日期：2010年1月14 纳稿日期：2010年12月14日线上发表时间：2010年1月23日
摘要
本报告公布了局限约束和自由的变形对粘结强度12、16毫米钢筋的表面腐蚀程度和裂纹影响的比较结果。

腐蚀是氯化物污染的混凝土的诱导和外加直流电流的引起的。

调查的主要参数有钢筋剥离，保护层厚度，钢筋直径，腐蚀程度和表面裂缝宽度。

结果表明了裂缝宽度和粘结强度之间的潜在关系。

同时还发现在围箍筋处发现表面裂纹的地方粘结强度增加，而无侧限的样本中没有观察到粘结强度增加。

关键词：粘结；腐蚀；螺纹钢；保护层；裂缝宽度；混凝土
引言
在世界各地，钢筋的腐蚀是钢筋混凝土结构的恶化的重要原因。

在未腐蚀的结构中钢筋和混凝土之间的粘结使钢筋混凝土处于有利状态。

然而，当钢铁的腐蚀发生时，会对这种积极性能产生不利影响。

这是由于钢表面形成了腐蚀产物，从而影响了钢和混凝土之间的粘结。

钢筋混凝土恶化是由钢筋和形成的膨胀腐蚀产物造成的局部损失。

这
种情况的恶化在许多方面影响结构;膨胀产品的产生造成混凝土的拉应力，这可能会导致混凝土保护层开裂和剥落的。

这种开裂可导致更严重的恶化和进一步的腐蚀。

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与钢筋之间的粘结强度。

最终腐蚀减少钢筋截面面积，影响钢筋的延展性和承载能力，从而最终影响结构适用性和结构承载力[12，25]。

以往的研究调查腐蚀对粘结的影响[2-5，7，12，20，23-25，27，29]，提出了数据模型[4，6，9，10，18，19 24，29]。

本研究主要研究腐蚀（钢材质量损失）水平或电流密度程度（腐蚀电流在加速测试中的应用）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力学性能[1，11]和摩擦特性[13]。

然而，很少有人研究都集中在裂缝宽度与粘结[23，26，28]之间的关系上，此参数易与实际结构相联系。

加强钢筋的腐蚀导致生成铁氧化物，它的体积大于原钢材。

这种扩张造成周围的混凝土内的拉应力，最终导致混凝土保护层开裂。

一旦开裂发生，混凝土紧箍力就会损失。

这表明粘结能力的损失可能与纵向裂缝宽度有关[12]。

然而，以混凝土的剥离可以在一定程度上抵消粘结力的损失。

最新研究主要与剥离样本有关。

本文报道的一项研究比较了有侧限和无侧限样本的粘结力损失。

2.实验研究
2.1样本
梁端样本[28]被选定为这项研究的研究对象。

这种撤去偏心或“梁端”模式样本以一个典型的简支梁锚固区的粘结长度支撑。

样本的矩形截面投在纵向钢筋的各处，如图1。

由于没有增强下方横反应的钢筋，试样提供了一个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钢筋。

图1梁端试样
试验调查了由3倍直径厚的保护层保护的12和16毫米直径的钢筋。

重复测试有侧限和自由样本。

在密闭的塑料管中有3套6毫米的不锈钢箍筋从其间穿过，在75毫米中心。

这代表了四组不同钢筋直径和有侧限/无约束的样本。

以调查钢筋规格，混凝土剥离和裂缝宽度对粘结强度的影响。

2.2材料
配合比设计，如表1所示。

水泥是I型硅酸盐水泥，骨料为玄武岩，容
重2.99。

根据AS 1141— 2000进行粗、细集料的制备。

拌合根据AS 1141—1994进行。

测试前水浴养护28天。

表1混凝土配合比设计
为了比较不同的混凝土抗压强度，粘结强度，Eq 。

公式1已被其他研究者用于正常化粘结强度的非腐蚀样本。

'exp ττ= 1 τ为40级混凝土的粘结强度，τ
exptl 为实验粘结强度和F c 是实验抗压
强度。

Φ12和Φ16毫米钢筋的抗拉强度是500兆帕，分别相当于一个56.5和100.5kN 的破坏载荷。

2.3实验方法
加速腐蚀已被许多作者用于重现在自然环境中发生的腐蚀钢筋钢 [2，3，5，6，10，18，20，24，27，28，30]。

这些相关实验使用外加电流或干湿周期人工风化和升高温度延缓腐蚀时间，同时保持恶化机制处于自然状态。

采用外加电流的研究使用的电流密度在100μA/cm2与500
mA/cm2之间[20]。

有研究表明，电流密度200μA/cm2与100μA/cm2相比，200的结果与早期阶段的腐蚀更相似[21]。

随着施加电流密度200μA/cm2被选定为研究使用电流，这在以前的研究中成为电流密度频谱的低端代表。

然而，应谨慎应用外加电流的加速腐蚀，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违背自然的发展，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，这可能会影响粘结强度。

钢筋作为阳极和四个碳钢金属板固定在表面作为阴极。

金属板和混凝土之间放置海绵（用盐水喷洒）提供足够的接触，如图2。

图2加速腐蚀系统
当裂缝宽度要求需适应特殊钢筋时应该终止施加外加电流。

当所有四
个位置出现规定的裂缝宽度，试样就会被拆除撤离测试。

平均表面裂缝宽度0.05，0.5，1和1.5毫米作为目标裂缝宽度。

表面裂纹宽度沿钢筋长度测量间隔20mm，从约束（塑料管）末端开始20mm用断路器光学显微镜测量。

测量精度为±0.02毫米。

从钢筋表面测量裂缝宽度，不考虑裂缝实际方位在何处。

粘结强度测试通过手动操作液压千斤顶和一个定制的试验装置，如图3所示。

加载方案见图4。

长80毫米的塑料管在末端提供了一个横向反应的具体部分，以确保粘结强度不会因为内力（压力）提高而增加。

样本定位使轴向力，适用于被测试的钢筋。

给样本足够刚性的约束可以确保在加载过程中最小的旋转或扭曲。

图3拉出测试，16毫米钢筋不承压图4加载示意图。

注：只测试显示棒
3实验结果与讨论
3.1目视检查
加速腐蚀阶段后，检查每个样本的裂缝的位置，平均裂缝宽度和最大裂缝宽度（第2.3款）。

虽然每个钢筋样本都有平均目标裂缝宽度，但是裂缝宽度的变化在观察前拉出测试。

这是由于腐蚀和开裂是一个动态的过程，裂缝是以不同的
速度传播的。

因此，当个别钢筋被拉断的时候，一旦目标裂缝宽度已经达到，腐蚀和裂纹在一定程度上继续扩展，直到所有的钢筋已达到目标的裂缝宽度，再终止试验进行。

这产生了一系列的最大裂缝和终止测试时的平均裂缝宽度数据。

视觉检测的样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，大多数裂缝以一个恒定的速度增长，直到3-4周后首次开裂，他们达到1毫米。

裂缝达到了1毫米后，它们的增长速度非常缓慢，甚至一些裂缝一点都不增加。

侧限和自由的样本表面裂纹往往发生在侧面（如对侧的顶部或底部），并沿钢筋方向发展。

一般情况下无侧限的样本只有仅有的一部分裂缝，而自由的样本裂缝的发展却十分常见，观察到的裂缝垂直对齐下边，垂直向下侧相邻的链接，如图5。

图5典型裂纹模式
在拉出测试时最常见的侧限和自由的故障是剥离失败，这是由于随着在荷载作用下腐蚀的扩大形成裂缝，最终导致右上角/边缘剥落，如图6。

但是一些侧限的样本，存在第二种破坏模式，在侧墙对角线出现裂缝，如图7。

在腐蚀阶段，这些裂缝的出现与观察到的垂直裂缝如上面报道的，并不相关。

图6拉出后纵向开裂
图7角开裂后拉出
钢筋最初（预制）由12％的盐酸溶液清洗，然后在蒸馏水清洗，另外蒸馏水洗涤之前由氢氧化钙溶液中和。

锈蚀钢筋拉出来测试之后，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
()0R 0G -G C =100%g l
⨯ 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（Φ12和Φ16毫米钢筋分别0.888和
1.58g/毫米），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，如图9。

然而，少数其他钢筋表现出显着的整体部分损失，更均匀的腐蚀水平，如图8，这可能是一个加速方法的功能。

图8 12毫米钢筋腐蚀、约30％的质量损失
图9 16毫米钢筋腐蚀、约15％的质量损失
3.2粘结应力和裂缝宽度
图10显示了16毫米的钢筋粘结应力与平均裂缝宽度的变化。

图11为12毫米的钢筋的。

图12和图13显示的最大裂缝宽度的数据。

图10 16毫米的钢筋平均裂缝宽度、粘结应力
图11 12毫米的钢筋平均裂缝宽度、粘结应力
图12 16毫米的钢筋最大裂缝宽度、粘结应力
图13 12毫米的钢筋最大裂缝宽度、粘结应力
数据显示12毫米箍筋样本的初始粘结强度增加，这与其他作者[12，15]的结论相同。

对于16毫米箍筋样本观察到的裂缝宽度0.28和0.35毫米，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。

12mm钢筋与箍筋粘结力从控制值到最大粘结应力粘结应力增加25%粘结力。

16毫米的样本增加约14％。

其他研究[17，24，25]报道的观察结果，由于约束在这些实验中得到的粘结应力有10%到60％增强。

然而，装
卸装置和保护层都不尽相同。

实验技术的变化，包括较短的嵌入式长度和较薄的保护层。

粘结强度，腐蚀程度，钢筋尺寸，保护层，节点的详细信息和拉伸强度之间的异变由罗德里格斯预测等已经被详细讨论。

[28]。

分析表明由于侧限提升粘结力约25％，相应的16毫米的钢筋的粘结力（2％的部分损失评估）约有0.75兆帕强度的变化。

相应的12毫米侧限钢筋的粘结强度被发现约有35％增加，粘结力有1.0 MPa的差异。

实验结果（14和25％以上）是这些值的60-70％。

这两组数据表明，粘结强度减小与可见表面裂纹宽度的关系。

粘结强度数据的回归分析表明它与平均裂缝宽度（不包括未开裂的密闭样本）成反相关关系，如表2。

表2裂缝宽度与粘结强度最佳拟合参数
还有无侧限样本比承压样本更适应。

无侧限样本粘结强度与钢筋与混。