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Corrosion Fundamentals

Corrosion in Concrete

Rebar Corrosion in Concrete

Pitting Corrosion of an Embedded Aluminum Handrail

The picture on the left shows cracking and staining of a seawall near the Kennedy Space Center. The pitting corrosion in the right photo occured on an aluminum railing on a concrete causeway over an inlet to the Atlantic Ocean.

Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods, post-tensioning cable or prestressing wires. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion. The cracking associated with corrosion in concrete is a major concern in areas with marine environments (like KSC) and in areas which use deicing salts.

There are two theories on how corrosion in concrete occurs:

  1. Salts and other chemicals enter the concrete and cause corrosion. Corrosion of the metal leads to expansive forces that cause cracking of the concrete structure.

  2. Cracks in the concrete allow moisture and salts to reach the metal surface and cause corrosion.

Both possibilities have their advocates, and it is also possible that corrosion in concrete can occur either way. The mechanism isn't truly important, the corrosion leads to damage, and the damage must be controlled.

In new construction, corrosion in concrete is usually controlled by embedding the steel deep enough so that chemicals from the surface don't reach the steel (adequate depth of cover). Other controls include keeping the water/cement ratio below 0.4, having a high cement factor, proper detailing to prevent cracking and ponding, and the use of chemical admixtures. These methods are very effective, and most concrete structures, even in marine environments, do not corrode.

Unfortunately, some concrete structures do corrode. When this happens, remedial action can include repairing the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the structure, and cathodic protection, an electrical means of corrosion control. KSC has experience with all of these methods of controlling corrosion on existing concrete structures.