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Options for inhibiting corrosion in concrete bridges Qian, S. Y.
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Options for inhibiting corrosion in concrete
bridges
Qian, S.Y.
A version of this paper is published in / Une version de ce document se trouve dans : Construction Canada, v. 43, no. 3, May 2001, pp. 24-26
www.nrc.ca/irc/ircpubs
OPTIONS FOR INHIBITING Corrosion In Concrete Bridges By Dr. Shiyuan Qian
Highway bridges are critical structures and should be kept safe and serviceable for a long period of time. About 50 per cent of bridges are made of reinforced or prestressed concrete1, and
extensive bridge deterioration due to corrosion can affect their safety and serviceability, resulting in loss of life or injury, traffic disruption and high user costs.
The pore solution in a hydrated Portland cement system in concrete bridges is highly alkaline, with a pH normally exceeding 12.6. In this environment, steel is “passive” and a thin but dense protective oxide film is formed on its surface. The surrounding concrete restricts ingress of carbon dioxide and chlorides. Consequently, good-quality, well-placed concrete with adequate cover over the steel provides a high degree of protection against corrosion of the steel
reinforcement.
The culprits behind deterioration
The main cause of concrete bridge structure deterioration is corrosion of the reinforcement caused by the breakdown of the passive film, which is usually the result of either carbonation or chloride attack. Carbonation is the RESULT of the reaction between atmospheric carbon dioxide (CO2) and the pore water, whereby the CO2 dissolves in the pore water to form carbonic acid.
This reacts with the alkali and reduces the pH to a level at which the passive layer on the steel reinforcement can no longer be sustained. Sulfur dioxide and nitrogen dioxide in the air also significantly reduce the pH of the pore water and cause structures to deteriorate. Chloride attack is a direct attack upon the passive layer of the reinforcement. When the chloride to hydroxyl ratio exceeds a certain level, chloride ions will break down the passive layer and cause corrosion. Deicing salts applied to roadways and bridges in Canada, and the snow belt states of the US, are the primary source of corrosion-promoting chlorides. When embedded steel corrodes, the production of a voluminous corrosion product induces internal stresses in the concrete
surrounding the reinforcement. This leads to cracking, delamination and spalling of the concrete, loss of concrete and reinforcement cross-sectional area, and loss of bond between the
reinforcement and the concrete.
A number of systems can be employed to protect against the corrosion of the reinforcement and increase concrete durability. Many of these methods have been employed on concrete bridges and have shown significant effects; others are still in the developmental stage and require further testing.
High-quality concrete
Adequately covering the reinforcement with high quality concrete can increase the resistance to chloride ingress and carbonation, thereby delaying or reducing the corrosion of reinforcement. Critical to good performance are factors such as water-to-cement ratio, total cement content, and/or the inclusion of some additives.
Surface treatments serve to provide a barrier to inhibit the ingress of moisture, CO2 and
chlorides. Surface-penetrating treatments, such as silanes and siloxanes, fill the capillary pores close to the surface of the concrete to produce a hydrophobic finish.
Protective coatings
This category includes epoxies and some cementitious materials that contain corrosion inhibitors, and can be applied to the reinforcement to provide a barrier between the embedded steel and an aggressive environment. The effectiveness of cementitious coatings is still under study. Opinion on the effectiveness of fusion-bonded, epoxy-coated reinforcement is mixed2, 3, 4. There is controversy about de-bonding and corrosion pitting5 because the coatings possess naturally occurring defective spots produced during fabrication; also, the coatings on the bars may be damaged during transportation.
Another protective coating is (galvanized) zinc, which is applied to the reinforcement by hot-dipping and acts in a sacrificial way. Galvanized rebars are superior to carbon steel rebars but do not afford protection against pitting and localized rusting in some conditions6. Galvanized reinforcement generally delays or postpones cracking and spalling of concrete by only a few years, and is not considered a permanent solution to corrosion7.
Corrosion inhibitors added to the concrete mix
Both organic and inorganic inhibitive compounds are commercially available. Calcium nitrite is an inorganic inhibitor and the most extensively tested corrosion-inhibitive admixture for use in concrete since its introduction in the 1970s8. As such, there is a considerable amount of
information on its effectiveness. During the 1990s, a number of proprietary organic inhibitors for use in concrete were introduced, including various amines, alkanolamines and emulsified
mixtures of esters, alcohols and amines9. The mechanisms by which organic inhibitors reduce corrosion remains IMPERFECTLY understood. Long-term performance comparable to that of calcium nitrite has not yet been established.
Cathodic protection
Cathodic protection (CP) polarizes the steel to a more negative potential to reduce corrosion. The difficulty comes in ensuring that an adequate current density and reasonably uniform current distribution is being applied to the reinforcement in a concrete with fairly high resistivity. The current can be impressed by an external DC power source or by connection to a sacrificial anode. Several materials have been chosen as the anode system, such as organic conductive coatings, metallic conductive coatings of thermally sprayed zinc and zinc alloys, and activated titanium mesh. Their performance is dependent on the activity and durability of the anode system. Sometimes performance can be significantly affected by the quality of the bond between the sprayed coating and the concrete substrate.
Corrosion-resisting reinforcement
When embedded in concrete, appropriate stainless steels are superior to carbon steel in their ability to resist chloride-initiated corrosion. Types 304 and 316 are readily available and have found use as corrosion-resistant reinforcement in concrete bridges in Ontario, Oregon and New Jersey. Obviously, the initial cost of bridge construction increases when stainless steel is used, but when life cycle cost and longer rebar life are factored in, stainless steel is clearly MORE cost effective. DELETE THE WORD CHAMPION A more economical approach is being tested
these days, in which stainless steel is used only in critical areas such as the top layer of the bridge deck, the lower section of a column and the splash zone or edge beam on a highway bridge. This way, the initial cost of using stainless steel is GREATLY reduced.
Fibre-reinforced polymer (FRP) composites have also recently been used in the construction of some bridges. The composites are made of fibres such as glass, aramid and carbon, and polymer resin matriXes10. The composite materials are more corrosion resistant than conventional steel but, because commercial use of FRP composites is still in the early stages, long-term
performance and durability data are not yet available.
Works in progress
The National Research Council’s Institute for Research in Construction has several projects underway that address the corrosion of reinforcement in concrete bridges: an evaluation of corrosion inhibitors added to concrete mixes and protective coatings containing corrosion inhibitors; a study of the coupling effect caused by substituting carbon steel with stainless steel in critical areas; and an evaluation of the long-term effectiveness of cathodic protection of sprayed metallic coatings on concrete to protect reinforcement. Feel free to contact us for more information.
References
[NOTE TO ART: Please number these 1-10]
Chase, S. B. and Washer, G. “Nondestructive evaluation for bridge management in the next century” Public Roads, July/August 1997, p. 16.
Lampton, R. D. and Schemberger, D. “Improving the performance of fusion-bonded epoxy coated steel reinforcing bars” NACE International, Corrosion 96, paper No. 323, 1996. Clear, K. C. “Effectiveness of epoxy-coated reinforcing steel” (C-SHRP report: Executive Summary), Transportation Research Circular, Vol. 403, 1993, p. 66.
Zayed, A. M., Sagues, A. and Powers, R. G. “Corrosion of epoxy-coated reinforcing steel in concrete.” Paper No. 79, Corrosion/89 (Houston TX:NACE. 1989).
Sagüés, A. A. “Corrosion of epoxy coated rebar in Florida bridges.” Final Report to Florida
D.O.T,. WPI No. 0510603, State Job No. 99700-7556-010, 1994.
Sakake, J. Kamakura, M. Shirakawa, K. Mikami, N. and Swamy, N. “Long-term corrosion resistance of epoxy-coated reinforcing bars.” Corrosion of Reinforcement in Concrete
Construction, Chichester: Ellis Horwood Limited, 1983, pp. 357.
Rasheeduzzafar, Dakhil F. H., Bader, M. A. and Khan, M. M. “Performance of corrosion resisting steel in chloride bearing concrete.” ACI Materials 89, September: 439, 1992. Page, C. L., Ngala, V. T. and Page, M. M. “Corrosion inhibitors in concrete repair systems.”
Magazine of Concrete Research, 2000, 52, No. 1, Feb., p. 25.
Nmai, C. K., Farrington, S. A. and Bobrowski, “G. S. Organic-based corrosion-inhibiting admixture for reinforced concrete.” Concrete International, 1992, 14, No. 4, p. 45. Tang, B. “Building more durable bridges.” Focus, FHWA, September 1999, p. 6.
Dr. Shiyuan Qian is a researcher at the National Research Council's Institute for Research in Construction in Ottawa. He can be reached by contacting Jim Gallagher at (613) 993-4114, or via e-mail:[email protected].
