The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in that they do not attack the integrity of the concrete. Instead, aggressive chemical species pass through the pores in the concrete and attack the steel. This is unlike normal deterioration processes resulting from chemical attack on concrete. Other acids and aggressive ions such as sulfate destroy the integrity of the concrete before the steel is affected. Most forms of chemical attack are therefore concrete problems before they are corrosion problems. Carbon dioxide (CO2) and chloride ions are very unusual in penetrating the concrete without significantly damaging it.1
Forty percent of the bridges in North America are 40 years old and concrete is the primary material of construction.2 It was recognized by the mid-1970s that the deterioration of concrete-bridge structures was caused by the corrosion of the reinforcing steel in the concrete, which, in turn, was induced by the intrusion of chloride from the deicing salts into the concrete. According to a 1997 report, of the 581,862 bridges in the U.S. national bridge inventory, ~40% were either functionally or structurally deficient. Most of these bridges were severely deteriorated with extensive loss of serviceability and reduced safety, with some of the bridges load-posted so that overweight trucks would be required to take a longer alternate route.3
The estimated cost to eliminate all backlog bridge deficiencies (both structural and functional) in Canada was estimated at $10 billion2 and in the United States to be $78 billion, increasing to as much as $112 billion, depending on the number of years it would take to meet the objective. Although corrosion of the reinforcing steel was not the sole cause of all structural deficiencies, it was a significant contributor and has therefore become a matter of major concern.
Reinforced concrete bridge decks constitute the weakest link in North America’s infrastructure network. An extensive area of these decks is severely damaged through the exposure to deicing salts and traffic. It is estimated that one-third to one-half of projected direct costs will be allocated to bridge decks. However, even though the cost of maintaining bridge decks is becoming prohibitively expensive, the benefits provided by deicing salts are too great in terms of reducing vehicular accidents and minimizing traffic disruption.4 Therefore, its use is not likely to decrease in the future. In fact, the use of road-deicing salts, which are extremely corrosive because of the disruptive effects of their chloride ions on protective films on metals, has actually increased in the first half of the 1990s after leveling off during the 1980s.
A number of fundamental measures can be taken to address the problem of reinforcing steel corrosion; for example, creating a barrier between the concrete and/or the rebar and the existing environment, applying cathodic protection to the rebar structure, or using alternative methods of reinforcement, such as fiber-reinforced polymer composites. For new structures, it is believed that much progress will be made toward effective corrosion control as the implementation of life-cycle costing strategies are adopted, as opposed to awarding contracts on the basis of lowest initial capital cost outlays. With such a vision, using a more corrosion-resistant stainless steel rebar material, for example, could prove to be a cost-effective route even if it increases the initial cost of rebar by an appreciable number. In areas of strategic importance such as highway belts of most modern cities, the total cost of repairs is greatly amplified by adding the indirect costs of traffic disruptions.
References
1 J.P. Broomfield, Corrosion of Steel in Concrete (London, U.K.: FN Spon, 1997).
2 Z. Lounis, “Maintenance Management of Aging Bridges: Economical and Technological Challenges,” Canadian Civil Engineer 19 (2002): pp. 20-23.
3 “Corrosion Protection: Concrete Bridges,” U.S. Dept. of Transportation, FHWA-RD- 98-088, Washington, DC, 1998.
4 “Highway Deicing: Comparing Salt and Calcium Magnesium Acetate,” National Research Council, Transportation Research Board, Special Report 235, Washington, DC, 1991.
This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 190, 196-197.