Facts About Steel Reinforcement Corrosion in Precast Concrete Beams and How to Minimise It

Even though steel is extensively used in the construction industry due to its durability, it is often affected by factors like lack of proper maintenance and environmental conditions. Just like in car batteries, corrosion of steel reinforcement, particularly in concrete, is a complicated electrochemical process. Steel reinforcement contributes to the longevity of a structure because of its tensile strength, yet it can also affect structural durability and longevity because of its susceptibility to corrosion. Here are some key facts that beginners should know about the corrosion of steel reinforcement in precast concrete beams and how to minimise it.

Why Corrosion Occurs in Steel Reinforcement -- Steel reinforcement corrosion is so pervasive in structures that it is often referred to as "cancer for concrete." Ideally, the high alkaline environment in fresh concrete should protect the steel surface from corrosion. However, over time, the alkaline level will drop significantly because of the ingress of chloride ions, also known as carbonation. The process triggers corrosion that leads to spalling and cracking concrete.

Stages of Corrosion -- The initial sign of corrosion in steel reinforcement is the development of white patches at the surface of the concrete due to carbonation. Subsequently, brown patches will form along the reinforcement, accompanied by cracks in the concrete. The cracks signify that the rust is expanding to the point of splitting the concrete. Multiple cracks may develop, causing the concrete to peel off, which might lead to the snapping of steel bars. As the bars snap, the main steel bars will start to reduce in size because of the effect of corrosion. The main bars will eventually buckle, causing the concrete to bulge. The final phase will be the resultant collapse of the structure.

Minimising Corrosion -- The depth and quality of concrete, especially in the cover zone, is integral in reducing the risk of steel corrosion. The quality of concrete can be controlled by reducing permeability. Regarding the depth of steel cover, always ensure adherence to recommended minimum depths according to industry codes of practice. Also, the depth should be hinged on cement content and exposure conditions. Notably, use high cement content that causes a reduction in a reinforced steel cover.

Choosing Material for Concrete Construction -- The use of blended cement can go a long way in minimising chloride penetration, which is the leading cause of steel reinforcement corrosion. Combinations of PC/GGBS and PC/PFA have been used to reduce the penetration of chloride in concrete. Besides, care should be taken to limit the content of chlorides in admixtures and aggregate.  

To learn more, contact your local civil engineering consultants today.