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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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