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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
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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.
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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.
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Unconfined bars displayed a decrease in bond stress at initial cracking, followed by a further decrease as cracking increased.
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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.
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The 12 mm bars displayed higher bond stress values than 16 mm with no corrosion, control specimens, and at similar crack widths.
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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.
References
1. Almusallam AA (2001) Effect of degree of corrosion on the properties of reinforcing
steel bars. Constr Build Mater 15:361–368
reinforcement corrosion on bond strength. Constr Build Mater 10(2):123–129
concrete affected by reinforcement corrosion. Mater Struct 31:435–441
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2. Almusallam AA, Al-Ghatani AS, Aziz AR, Rasheeduzzafar (1996) Effect of
3. Alonso C, Andrade C, Rodriguez J, Diez JM (1998) Factors controlling cracking of