Many bridge columns in Michigan are damaged by chloride contamination resulting in the corrosion of the steel reinforcement, and swelling and spalling of the concrete and use of the bridges is typically continued. This in itself may not be a serious problem since most columns in Michigan are over-designed and the loss of strength is not a significant issue. However, the lack of any method to minimize or prevent corrosion of the steel results in continued deterioration and unsightly columns. Polymer composite (also known as fiber-reinforced polymer or FRP) jackets offer a possible remedy to this problem. They offer a rapid repair technique with the potential to enhance the long-term durability and compression strength of damaged columns due to the confinement that is provided when fibers are oriented in the hoop direction. Fibers oriented in the vertical direction can enhance the bending strength.
Experiments were conducted to assess the effects of using FRP wraps with fibers oriented in the hoop direction for rehabilitating corrosion-damaged columns. Issues that were explored are: (1) freeze-thaw durability of concrete square and cylindrical specimens wrapped with glass and carbon FRP and subjected to an internal expansive force; (2) effect of wrapping on the rate of corrosion in an accelerated corrosion test; (3) effect of freeze-thaw and wet-dry cycles on the properties of FRP panels; (4) impact resistant of FRP panels supported on a concrete substrate; (5) effect of high temperature on wraps; and (6) field installation of wraps on corrosion-damaged bridge columns.
The results of the freeze-thaw experiment indicate that freeze-thaw cycles have no statistically significant effect on the compressive strength of glass and carbon wrapped specimens. For round specimens, glass and carbon wraps increased the strength by a factor of about 2.3 and 2.6, respectively. For square specimens, glass and carbon wraps increased the strength by a factor of 1.4-1.5. Freeze-thaw conditioning generally reduced the longitudinal failure strain of wrapped specimens.
The square wrapped specimens had lower compressive strength compared to the round specimens, even though the cross sectional area of the square prisms is higher than that of the round cylinders. This is due to the reduced confinement provided by the wraps for square cross sections and stress concentrations that develop at the corners. Wrapped square prisms always failed by rupture of the wrap at a corner. A reduction of approximately 30% to 40% in failure stress was noted between round and square wrapped specimens.
The results of the accelerated corrosion experiment indicate that wrapping reduced the corrosion depth in the reinforcing bars by 46% to 59% after 190 days of testing. Both glass and carbon wraps are equally effective in slowing down corrosion. Although unbonded wraps do reduce stress concentrations in the FRP, they are less effective in reducing the corrosion rate than the bonded wraps. It is postulated that this is due to the ingress of water along the unbonded FRP-concrete interface.
Wrap strains for bonded specimens with both types of wraps tend to level off with time indicating that corrosion slows down significantly after some time. One explanation could be that the stress concentration near the anodes in the bonded wraps is more effective in containing the corrosion-induced crack and reducing the corrosion rate. The slip of unbonded wraps and the resulting redistribution of strain along the entire wrap may be less effective at containing the large corrosion-induced crack near the anodes.
Freeze-thaw conditioning had little effect on the effective stiffness (modulus ( thickness) of glass FRP panels. Although the effective stiffness of carbon panels showed an apparent increase due to freeze-thaw conditioning, re-testing indicated that this observation was unreliable. The ultimate strength per unit width per layer of glass FRP decreased by 21% and the decrease was significant at the 95% level. The change in the ultimate strength of carbon was not significant at the 95% level. Ultimate strains decreased by 20% and 28% for glass and carbon panels, respectively, and these decreases were significant at the 95% level.
Wet-dry conditioning had no effect on the effective stiffness of glass panels. As with freeze-thaw conditioning, the effective stiffness of carbon panels showed an apparent increase due to wet-dry conditioning, but re-testing indicated that this observation was unreliable. The ultimate strength per unit width per layer of glass FRP decreased by 18% and the decrease was significant at the 95% level. The change in the strength of carbon was not significant at the 95% level. Ultimate strains decreased by 20% and 36% for glass and carbon panels, respectively, and these decreases were significant at the 95% level.
The panel test results are somewhat unreliable for the very thin carbon specimens. Also, many of the specimens broke at the grips. Better grip fixtures should be used for future tests.
Both glass and carbon FRP panels did not display any significant damage due to the impact test. Minor interlaminar debonding was visible on the glass panels, which are somewhat transparent, at the point of impact. Interlaminar debonding could not be observed on the carbon FRP panels because they are opaque.
At temperatures in excess of 200(C[RH1] the epoxy in the FRPs burn and evaporate and the individual plies of wraps unravel. Hence the wraps become ineffective at such high temperatures unless effective insulation is provided.
It is evident from the experimental study conducted that both carbon and glass wrap systems are sufficiently resistant to freeze-thaw cycles and reduce the corrosion rate by about the same rate. Therefore, three layers of glass wrap or two layers of carbon wrap may be used to repair Michigan bridge columns. Reducing the number of layers may also be feasible, but it is not possible to provide any recommendation about this without additional studies.
The preferred wrap system will most likely depend on the material and installation cost rather than performance issues. However, it should be noted that many studies indicate strength degradation of glass FRP in an alkaline and/or humid environment under elevated temperature. Thus in regions with long periods of hot and humid conditions, carbon FRP may be preferable to glass FRP.
It is also recommended that a non-destructive technique or coring be used every ten years to monitor the condition of the concrete inside the wrap.