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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
Unconfined 12 mm Confined 12 mm Unconfined 16 mm Confined 16 mm
Mean crack width
R 2 0.920 0.637 0.672 0.659
Slope (m) −3.997 −3.653 −2.999 −8.848
Intercept (b) 7.560 8.122 6.496 8.746
Maximum crack width
R 2 0.937 0.855 0.714 0.616
Slope (m) −2.719 −2.968 −1.815 −5.330
Intercept (b) 7.805 8.403 6.707 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.