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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.
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。
毕业论文关键词： 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]。