Understanding Corrosion Effects on Steel Reinforcements in Concrete
Concrete can be called the flesh of most of the heavy constructions. From the underground pilings or bridge sections perched high in the air, much of construction is made from concrete.
However, from a structural load-bearing point of view, concrete alone does not have much tensile strength to hold or bear all the amount of shearing or tensile load applied to it. This is why special reinforcement bars, plates or rods are placed inside a concrete structure to make it stronger. In most cases, these reinforcements are made of steel.
Sadly, on the other hand, however much stronger, is not as durable as the concrete that is enveloping it. Subjected to weather and atmospheric effects and over time, the steel reinforcements embedded within the concrete get corroded and weakened. And since these steel bars were the major agents of tensile strength that held the structure up, any weakness in the steel reinforcements can spell doom for the whole superstructure.
It is as the saying goes: ?a chain can only be as strong as its weakest link.? Similarly, if one of the heavy concrete blocks or cylinders holding up a bridge starts weakening due to corrosion of the steel reinforcements within in, that threatens to bring the whole bridge down one sad day, causing massive damage and loss of life.
This is the very reason why every architect and engineer worth his/her salt must be aware of the validity period of the steel reinforcements they are placing within a structure. In this article, we will try to see how the corrosion happens, what effects can happen with this, and how can we remedy this.
The corrosion of the steel happens basically due to acids present in the water that gets in touch with the steel via seeping in through the concrete or via some other way. The acids in the atmospheric water, a combination of carbonic, sulphuric, hydrochloric and some organic acids, react with the metal, forming tiny shorted batteries within which ions flow from one substance to another; in turn changing both.
The metal gets changed into metallic salts or oxides, which not even capable of holding up their own weight let alone others?. This is essentially what corrosion of metal reinforcements is, an unintended electrochemical reaction happening very slowly. Rust is the most common byproduct of corrosion; indeed, the process is informally called rusting.
Types of corrosion: There are two types of corrosion that generally take place in heavy construction -
1. Crevice Corrosion: sometimes, the concrete block enveloping the steel is not cemented uniformly. Small pockets of empty places may form near the metal. Over time, these bubbles may get filled with liquids that somehow seep in from outside. These acidic-liquid pockets, in contact with the metal, generate ionized poles over the volume of the pocket, anodes and cathodes to be exact. This happens due to uneven reactions. The ions flow from one pole to another, eating the metal and depositing salt in its place.
2. Pitting Corrosion: Sometimes, the protective layer or coating on the metal wears off in dots or patches due to time or other reasons. This is called de-passivation. When this occurs, that little place of unprotected steel is exposed to nature and the atmospheric acids have a field day on that spot, eating the metal away, forming pits. The process is very localized and unlike the above process can be observed from outside.
Causes of Corrosion in Steel Reinforcements: Most of the time any steel placed in construction is subjected to a passive coating of neutralizing agents that make the steel surface highly alkaline, resisting acids. Concrete itself also contributes to the same. However, with time or damage, this protection may wear off and the steel becomes vulnerable to corrosion effects. The following are the major factors bringing about this effect:
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