How is concrete affected in a marine environment?
As seen above, chloride corrosion of steel rebar in a concrete pile causes delamination that results in spalling.
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.
Corrosion of Steel-Reinforced Concrete
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 often 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.