Concrete in the Marine Environment
Corrosion of Steel-Reinforced Concrete
Concrete is a complex composite material. Despite its complexity, however, it has low strength when loaded in tension, and it has become common practice to reinforce concrete with steel to improve its tensile mechanical properties.
Concrete structures, such as piers, bridges, and dam faces, all contain reinforcing steel (commonly referred to as rebar). The primary causes of deterioration of steel-reinforced concrete structures are the chemical reactions that attack the aggregate/cement matrix and the resultant corrosion damage to the reinforcing steel embedded within the concrete.
The Chemical Reactions Involved
In order to understand the corrosion of this reinforcing steel, one has to first examine the chemical reactions involved.
In concrete, the presence of a significant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity. When the structure is in its infancy, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.
When a concrete structure is repeatedly exposed to salt spray or submerged in saltwater, chloride ions will slowly penetrate into the concrete. The chloride ions will eventually reach the steel and accumulate to the point where the protective film is destroyed and the steel begins to corrode. This corrosion is facilitated by the presence of both oxygen and moisture at the steel-concrete interface.
Thermal Cycling
Standard marine concrete mix designs utilizing type II or type V cement absorb moisture when placed in water. In higher latitudes that experience regular freeze/thaw cycling, the addition of even small amounts of water into the concrete surface matrix creates a destructive process that slowly erodes the exposed surface. This effect is most prevalent at the air/water interface and can easily be identified by hour glass shaped deterioration. In combination with alkali reactions and chloride/oxidation corrosion of reinforcing steel, the marine environment is a very harsh place for concrete. There are many additives that can enhance concrete mix design to significantly reduce these effects in new construction. Once the damage has been done, the correct selection of restoration materials and installation procedures can add new life to structures and mitigate further deterioration for decades.
