RoadScience: Corrosion — the enemy within
Research continues to produce new options for owners and designers to save concrete bridges.
By Tom Kuennen, Contributing Editor
Corrosion of reinforcing steel in concrete bridges degrades bridge decks and other superstructure elements — in addition to substructures — by causing embedded steel to expand, causing cracks in concrete, leading to rough riding surfaces at best, and structural failure at worst.
Fortunately, many tools exist in the bridge owner’s “toolbox” of fixes to slow or halt corrosion in its tracks. Corrosion prevention can be built into a bridge or applied retroactively. Pavement deicing techniques and materials also can moderate chloride-induced corrosion.
Research continues to produce new options for the owner and designer to consider. Here’s a look at some of those changes and options.
Bridge corrosion results from the reaction between the steel and its environment. Steel is refined from iron ore, but the moment it is produced it begins to corrode, primarily to oxide compounds, on its way to a less-refined state.
Steel bridges can also suffer greatly from corrosion, and they usually are well protected by a variety of high-performance bridge coatings. But in recent decades, the proliferation of steel-reinforced concrete bridges — constructed of pre-stressed or post-tensioned elements including girders, piers, pier caps and decks — has focused a tremendous amount of attention on preventing corrosion of steel within concrete.
Because portland cement is highly alkaline, concrete made from it provides a layer that offers passive protection to the steel within. Reinforcing steel develops an initial oxide film on its surface from an initial corrosion, and concrete’s high alkalinity stabilizes the film. But because PCC permits movement of liquids through its pores, microcracks and cracks, chloride-laden meltwater from snow and ice, or marine spray in littoral environments, can reach the steel within, disrupt the oxide film, and accelerate corrosion.
Not only is the steel degraded by the chlorides, but because the products of corrosion take up more room than the existing steel, tremendous outward pressures induced by the chemical reaction crack the concrete, resulting in more cracks that let more chlorides into the concrete, accelerating the process. The outward manifestation can be cracks with rust stains seeping out onto the concrete surface, or spalling and delamination of concrete with structural degradation.
Deicing and marine salt water is not the only culprit; corrosion of reinforcing steel also can take place via concrete carbonation, a reaction with free atmospheric carbon dioxide, which lowers the pH of the concrete. This extremely slow process reduces the ability of the concrete to protect embedded steel.
The toolbox of corrosion fixes available to protect reinforced concrete bridges by both passive and active means is wide ranging, and continually changing. These fixes include: