Corrosion of Steel Reinforcement in Concrete -Causes and Protection

Steel reinforcement corrosion in concrete is a complex process, although it is essentially an electrochemical reaction comparable to that of a simple battery. Mild steel’s composition varies along its length, and anodic (more negatively charged) and cathodic (more positively charged) sites can be established at various spots.Concrete Steel Reinforcement Corrosion

Mankind has been searching for a construction material that will resist the vagaries of nature without requiring maintenance since the days of the magnificent pyramids of Egypt and the Babylonian Hanging Garden. In search of such a substance, humans discovered ‘Portland Cement,’ a miracle material.

Cement concrete has been widely employed for numerous structures that man could dream or conceive of throughout the last one and a half centuries, with the assumption that they would last forever. The majority of structure failures or non-functioning to their expected service condition can be linked to a lack of understanding of the environment and a lack of proper, systematic, and scientific maintenance.

Construction is how a country’s valuable assets are created. In terms of economic development, construction is critical. It is a precursor activity and a hallmark of each nation’s growth process.While steel reinforcement in RCC helps to compensate for the concrete’s weakness in tension (tensile stress), it also reduces the durability and lifetime of the structure. Repairs and rehabilitation of concrete structures, which has recently become a global industry similar to building itself, are mostly owing to concrete deterioration caused by embedded steel corrosion.

When Reinforced Concrete Construction first became popular in the early twentieth century, it nearly completely replaced previously used construction materials such as timber, (stone) Masonry, and steel sections, etc. The life expectancy of R. C. structures was on the order of 100 years. However, by the turn of the century, these assumptions had been disproved, and newer structures, say 20 to 25 years old, were showing signs of major deterioration and trouble. Concrete’s long-term durability has become a hot concern in global growth. Despite the fact that a variety of factors contribute to the onset of early puberty.

Corrosion of Steel Reinforcement in Concrete

Corrosion appears to be an all-pervasive phenomenon that destroys all types of structures in all countries around the world, and it has been dubbed “Cancer” for concrete. A decent concrete should, in theory, provide appropriate protection for the imbedded steel. This is owing to the protective alkaline environment (pH value as high as 12.5) created by fresh concrete, which causes a protective coating to form on the steel’s surface, protecting it from further corrosion. However, with time, due to carbonation or chloride ion penetration, the pH value gradually decreases, and the alkaline surrounding of the reinforcement bar is lost, signalling the onset of the corrosion process, which leads to concrete cracking and spalling.

As a result, it is clear that the most important component in determining the quality and longevity of concrete is its impermeability, which may be achieved by ensuring appropriate cement content, a low w/c ratio, thorough compaction, and curing. The same can be improved even further by adding the right admixtures and increasing the concrete cover.

However, these procedures are difficult to fully implement in practise, and they are not considered adequate in an aggressive workplace. As a result, additional protection for reinforcement steel is required, particularly due to chloride-induced corrosion (worse than carbonation corrosion) that can occur even in high-quality concrete.

The corrosion of reinforcement accounts for the majority of the deterioration of RCC constructions. As a result, a fundamental understanding of corrosion technology is required for both structural durability and rehabilitation work. Steel immersed in hydrating cement paste creates a thin passivity layer of oxide that sticks strongly to the underlying steel and protects it against rust and corrosion by reacting with oxygen and water. Passivation is the term for this state of the steel.Passivation can only be maintained if the pore water in contact with the passivating layer has a sufficiently high pH. As a result, when the low pH front reaches the reinforcing steel’s surface, the protective oxide covering is lost, and corrosion might occur.

Causes of Corrosion of Steel Reinforcement in Concrete

Steel corrosion in concrete is an electrochemical reaction. The corrosion cells’ electrochemical potentials can be generated in two ways:

(a) Composition cells can form when two dissimilar metals, such as steel rebars and aluminium conduit pipes, are implanted in concrete, or when there are considerable differences in the surface properties of the steel.

(b) Concentration cells can occur when dissolved ions such as alkalies, chlorides, and oxygen have different concentrations near steel. Changes in electrochemical potential can be caused by differences in the concrete’s surroundings. Electrochemical cells can also arise as a result of differences in salt concentration in the pore water or non-uniform oxygen availability.

As a result, one of the two metals (or specific parts of the metal in the case of only one metal) becomes anodic, while the other becomes cathodic. The following are the basic chemical changes that occur at the anodic and cathodic zones. An electrochemical cell is formed when there is a difference in electrical potential along the steel in concrete: anodic and cathodic areas form, which are connected by an electrolyte in the form of pore water in the cured cement paste. The ferrous ions are positively charged. While the positively charged free electrons e– flow through the steel into the cathode, where they are absorbed by the electrolyte elements and mix with water and oxygen to produce hydroxyl ions, the negatively charged free electrons e– pass through the steel into the anode and into solution.

Cathodic reaction:

Although oxygen is consumed and water is replenished, the process cannot continue without it. Thus, there is no corrosion in dry concrete (probably below 60% relative humidity), nor in concrete totally immersed in water (unless when water can entrain air, such as by wave action).

The change of metallic iron to rust is followed by an increase in volume, which can be as much as 600 percent of the original metal depending on the level of oxidation. Concrete expansion and cracking are thought to be caused by this volume increase. It’s worth noting that the anodic reaction involving the ionisation of metallic iron won’t get very far unless the electrons are present.

The protective film on steel is reported to remain stable in the absence of chloride ions in the solution if the pH of the solution stays above 11.5. In most cases, there is enough alkalinity in the system to keep the pH over 12. When concrete near steel has high permeability and alkalies, and most of the calcium hydroxide is either carbonated or neutralised by an acidic solution, the pH of the concrete near steel can be decreased to less than 11.5, eliminating the steel’s passivity and setting the scene for corrosion.The protective coating has been found to be damaged in the presence of chloride ions, depending on the C1– / OH– ratio, even at pH values much over 11.5.

Carbonation of Embedded Steel

Steel will not rust if it is immersed in a highly alkaline media with a pH value of 9 or higher. Cement begins to hydrate during the setting of concrete, and this chemical interaction between cement and water in the concrete results in the formation of calcium hydroxide from the cement clinker. This maintains the alkalinity of the concrete, resulting in a pH value of more than 12.6 that renders the steel surface passive. The alkalinity of the concrete, which leads to passivation of the steel, thereby protects the reinforcement against corrosion. Because the calcium hydroxide reserve is so large, steel corrosion is unlikely to occur even if water enters the concrete reinforcement. As a result, even the occurrence of minor incidents might have a big impact.

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