REINFORCEMENT CORROSION-CAUSES & PREVENTION

REINFORCEMENT CORROSION-CAUSES & PREVENTION

The corrosion of steel reinforcement is complex, but basically it is an electro-chemical reaction similar to that of a simple battery. The composition of mild steel varies along its length and potential anodic (more negatively charged) and cathodic (positively charged) sites can be set up at various points.

Concrete is capable of conducting and electric current and acts as the electrolyte with the circuit being completed by the bar through which the electrons can flow. However the highly alkaline environment (pH about 12.8) provided by good quality concrete produces a protective layer around the steel preventing the flow of current. This is known as Passivation.

The corrosion reaction can only occur when the following conditions prevail.

1. There is a breakdown of the passivating layer (de-passivation) brought about by

a) A lowering of the alkalinity of the concrete below a critical pH of about 10.5, caused normally by the ingress of carbon dioxide (carbonation).

b) The ingress of chlorides

2. Oxygen and water are present.

With the above conditions prevailing the ferrous ions (Fe++) released from the anode combine with the hydroxyl ions (OH-) from the cathode, in the presence of water and oxygen to produce rust (ferric hydroxide). This is an expansive reaction leading to eventual spalling of concrete cover and reduction in the area of the steel at the anodic site.

THE MECHANISM OF REINFORCEMENT CORROSION:

Carbonation:

Acidic gases like corbon dioxide react with any free alkali that may be present, which can lead to a drop in the alkalinity of the concrete. This process, which starts at the surface of the concrete, slowly penetrates deeper and deeper. The penetration is nearly proportional to square root of time.

Chlorides:

Chlorides are generally acidic in nature and can come from a number of different sources, the most common being, de-icing salts, use of unwashed marine aggregates, sea water spray, and certain accelerating admixtures (their use is now prohibited).

In the presence of chlorides localized pitting corrosion occurs which does not always have associated with it the early warning signs of surface cracking.

Chlorides induced corrosion is potentially more dangerous than that resulting from carbonation. Like most of the aspects of concrete durability, deterioration due to corrosion of the reinforcement can take place years (5 to 20) to manifest itself.

From Engineering Research Project.