Coorrsion Effects on Steel Reinforcements in Concrete
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.
Types of corrosion
There are two types of corrosion that generally take place in heavy construction:
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.
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.
The following are the major factors bringing about this effect:
Carbonation and chlorination: loss of alkalinity due to organic or carbonic acids, or being subjected to chloride ions present in the vicinity can induce the corrosion process.
Cracks and gaps: this is a no-brainer. Any break or crack in the concrete that reach deep enough to expose the steel to the outside air and water will leave the reinforcement vulnerable to oxidation.
Insufficient cover: if the design did not allow the concrete or the passivation to blanket the steel properly, then there will be exposed places that would be endangered by the atmospheric corrosion effects.
Effects of Corrosion on Steel Reinforcements:
When corrosion takes place on the steel reinforcements within a concrete assembly, the member may display the following effects that may become apparent gradually over time:
Brown patches: seen along the lines of reinforcement, these are iron oxide stains carried to the surface of the structure from the corroding steel underneath, via moisture pathways.
Cracks: The byproducts of the corrosion process, i.e. the salts or oxides, occupy more volume than the steel that it was formed from. Thus these new materials create pressure on the cementing surrounding it, and just like a root through a rock, can split it asunder, given enough time.
Bar snapping: a steel bar weakened by corrosion obviously won't be able to bear the tensile strength exerted on it as normal. After a given time, it will break under pressure.
Buckling bars: in the last stage in the demise of a concrete member, the snapping of bars and spalling of concrete will lead to a bulging of concrete in that area, weakening it severely enough so that the whole member collapses.
Remedies of Corrosion in Steel Reinforcements
As in everything, using better materials and periodically checking the structure thoroughly can lead to preventing, or at least, delaying the corrosion of the steel within the structure. Here are some pointers:
Concrete quality: when you use high-quality concrete and good water/cement ratio, it will help keep the steel bars within properly alkalinized and also reduce the chance of forming moisture pathways.
FBEC Bars: The use of Fusion-Bonded Epoxy Coated steel bars are coated with a thermoset polymer epoxy resin which can seriously reduce the chance of corrosion.
Cement Polymers: some specific substances can increase the alkalinity of the concrete that reduces the risk of corrosion.
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