Steel-reinforced Concrete Bridge Decks
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Basic Construction
Steel-reinforced concrete bridges are among the most common type of bridge built. In fact, the vast majority of highway bridges are of this type and are most frequently encountered along interstate highways. For added strength and support, the concrete used in constructing these bridges is often reinforced with imbedded steel bars, commonly called rebar.
The Basics of Rebar
Steel-reinforced concrete bridge decks, the roadway surfaces of a bridge, are composted of concrete that has been reinforced with rebar (typically round with varied diameters). When a bridge deck is built, the rebar structure is commonly built and supported in place first, and then concrete is poured over, around, and through the bars. Concrete alone is a very hard and somewhat brittle material. It performs well under compression loads but tends to be weak under tension loads. Tension is an axial load that pulls a section apart; compression is an axial load that pushes a section together. The rebar is added to the concrete to increase the tensile strength of the bridge deck.
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There are typically five types of rebar used in reinforced concrete bridges: as-rolled rebar (mill scale), epoxy-coated rebar, stainless steel rebar, stainless steel clad rebar, and galvanized (zinc) rebar. Like other metals, rebar has the potential to corrode. Addition of inhibitors, such a calcium nitrate or silica fume, can slow the rate of corrosion, and thus improve the structural integrity of the bridge deck. (Chase and Washer 1997).
Corrosion of the Rebar
The quality of the bridge deck is only as good as the bond between the rebar and the concrete. When the rebar in the deck begins to corrode, the integrity of the bond between the rebar and concrete is significantly affected. This can lead to large decreases in strength and eventual failure of the bridge. Much of the corrosion that occurs is induced by the presence of chloride, most commonly found in salts (Johansen, Klemm, and Taylor 2002). The chloride can come from many sources, including road salt, rainwater runoff, and the air. In the case of a Florida bridge in the Building Bridges activity, the sand used to make the concrete roadbeds originally came from nearby beaches. In this concrete, the chloride ion was present from the time the bridge was constructed and had been causing corrosion from the very beginning, eventually leading to its catastrophic failure.
Monitoring Corrosion of the Rebar
The government monitors chloride ion concentration within concrete samples as a measure of the amount of corrosion occurring. Unfortunately, one cannot examine the rebar directly without destroying the concrete bridge deck. Therefore, indirect techniques, such as x-ray, conductivity, and chloride ion concentration tests, are used to determine the “state” of the rebar within the concrete. Chloride is a proxy for corrosion: the more chloride detected in a concrete sample, the more corrosion that is underway.
Traditionally, the Federal Highway Administration takes samples from concrete bridges and sends them to a laboratory for analysis using a procedure called potentiometric titration, a complex laboratory procedure that analyzes acid-soluble chloride in cement using an electronic device. New Mexico State Highway and Transportation measures concentration using a chloride-specific ion probe. Although not as accurate as the laboratory tests, the results are perfectly sufficient for an estimate of the chloride ion concentration, and thus the degree of corrosion in the concrete.
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