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:
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.
Fig: Carbonation leads to the general corrosion along the full length of the bar.
The above figure shows the first outward signs of general corrosion taking place is surface cracking of the concrete along the line of the steel.
The above fig. shows that as the corrosion proceeds, the concrete will spall away completely to expose the steel.
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 itselt.
MINIMISING THE RISK OF CORROSION:
The quality and depth of concrete in the cover zone are all important in minimizing the risk of corrosion as shown in fig. below.
Quality is controlled largely by minimizing permeability.
Recommendations for minimum depths of cover are given in the codes of practice and are based on exposure conditions and minimum cement contents. Higher cement contents infer lower water cement rations leading to permitted reductions in cover.
At no time should the normal cover be less than the maximum size of aggregates+5mm.
Blended cements made from combinations of PC/PFA and PC/GGBS can lead to significant reduction in chloride penetration. However, in situations where these materials are not cured properly there is a risk of increased carbonation.
Care must be taken that all aggregates and admixtures contain limited amount of chlorides.