Reinforced concrete is an extremely important material of construction. One reason for the success of this combination is the similarity of thermal expansion properties of carbon steel and concrete. Another is that the extremely high pH of the cement content (typically pH > 13 for new concrete) passivates the steel surfaces against corrosion activity.
The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in penetrating the reinforced concrete without significantly damaging it structurally.1 Instead, these chemical species pass through the pores in the reinforced concrete and deplete the alkalinity that passivates the steel surfaces. In contrast, deterioration processes resulting from chemical attack on concrete by acids and aggressive ions, such as sulfates, destroy the integrity of the concrete before the steel is affected. In carbonation, carbon dioxide (CO2), often from the atmosphere, reacts with calcium hydroxide (Ca(OH)2) to form calcium carbonate (CaCO3) and water. This consumption of hydroxide ions reduces the pH of the reinforced concrete and may deplete the passive layer at the surface of the reinforcing steel.
The penetration of chlorides to the steel surface also breaks down the passive layer rather than attacking the steel directly. Relatively minor metal losses that do not compromise the structural integrity of the steel can generate a volume of corrosion product that can spall and crack the surrounding reinforced concrete. This accelerates the penetration of aggressive species. This is a key factor in the corrosion of bridge decks. Forty percent of the bridges in North America are at least 40 years old, and reinforced concrete is the primary material of construction.2 Structures in coastal areas are subject to salt spray atmospheres. It was recognized by the mid-1970s that the deterioration of concrete-bridge structures in cold climates was caused by the corrosion of the reinforcing steel in the concrete, which, in turn, was induced by the intrusion of chlorides from deicing salts into the concrete.
Reinforced concrete bridge decks constitute the weakest link in North America’s infrastructure network. According to a 1997 report, of the 581,862 bridges in the U.S., about 40% were either functionally or structurally deficient. Most of these bridges were severely deteriorated with extensive loss of serviceability and reduced safety, such that some of the bridges had to be load-posted so that overweight trucks would be required to take a longer alternate route.3 In spite of the clear association with bridge damage, 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 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. 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 or applying cathodic protection to the rebar structure. For new structures, it is believed that much progress will be made toward effective corrosion control as life-cycle costing strategies are adopted, as opposed to awarding contracts solely on the basis of lowest initial capital cost outlays. With such a vision, using alternative methods of reinforcement, such as fiber-reinforced polymer composites, corrosion-resistant stainless steel rebar, or epoxy-coated rebar to reduce the exposed steel surface, could prove to be a cost-effective route even if the initial cost were significantly increased.
- J.P. Broomfield, Corrosion of Steel in Reinforced Concrete (London, U.K.: FN Spon, 1997).
- Z. Lounis, “Maintenance Management of Aging Bridges: Economical and Technological Challenges,” Canadian Civil Engineer 19 (2002): p. 20-23.
- “Corrosion Protection: Concrete Bridges,” U.S. Dept. of Transportation, FHWA-RD-98-088, Washington, D.C., 1998.
- “Highway Deicing: Comparing Salt and Calcium Magnesium Acetate,” National Research Council, Transportation Research Board, Special Report 235, Washington, D.C.. 1991.