Infrastructure such as Bridges, Flyovers, Ports Structures and Jetties, Power Plants, Water and Sewage Treatment Plants, Coastal Structures etc demands longevity (70 years plus) and maintenance free service life. Our Nation cannot rebuild or constantly repair these structures at the cost of resource like time, money and manpower hence these structures that are subjected to corrosive and erosive environment should be protected so that the repairs and maintenance are bare minimum and inconvenience to the public is avoided.
One of the major cause of infrastructure failure before its designed service life is because of the corrosion of rebars. The microstructure of concrete is such that it has capillary pores. These pores are interconnected and go right from the surface of the concrete through the concrete mass and onto the reinforcement in the concrete leading to its corrosion.
Other than corrosion that is caused due to Humidity (water) and Oxygen the other factor that causes the deterioration of RCC structures are – Carbonation (Carbon Dioxide), Chloride Attack, Sulphate Attack, Microbial Attack, Acid Rain, Industrial & Environmental pollutants etc. These elements synergistically attack the structure thus leading to its failure.
In presence of moisture, the carbon dioxide from the atmosphere changes into dilute carbonic acid which attacks the Calcium Hydroxide in concrete reducing its alkalinity.As the alkalinity decreases and the carbonic acid penetrate deeper, the reinforcement bar which is protected because of the alkaline passive film becomes vulnerable. The corrosive elements attack the reinforcement bars which start rusting and eventually lead to Spalling.
Chloride ions accelerate the corrosion of reinforcement and are a great threat to the infrastructure. The
adsorption of Chloride ions to the alkaline passive film on the rebarsleads to its progressive thinning until
- Chlorides can be admitted through contaminated water and concrete admixtures
- Through atmospheric /environmental chloride in the vicinity
Chloride ions can be admitted in the concrete through contaminated water used for mixing concrete, concrete admixtures, soil, rocks and biota (i.e. human waste, livestock waste and plants), vicinity to the sea etc.
Sulphates are present in most cement and some aggregates. Sulphates are present in soils (fertilizers), groundwater, industrial wastes, sewage and industrial effluents. In marshy land or stored water bodies like dams etc. decaying of organic matters leads to the formation of H2S. This is converted into sulphuric acid by bacteria. Acid rain can also be a cause of Sulphate attack. The sulphate ions attack the aluminates in the concrete and covert them into sulpho-aluminates also called as Ettringite. The products of sulphate attack i.e. Ettringiteoccupy a larger space than the original material and this causes the concrete to disintegrate, thus exposing the rebars to corrosion.
FUSION BONDED EPOXY COATING (FBEC) TO REINFORCEMENT BARS
The basic building blocks of an epoxy resins are Epichlorhydrin and Bisphenol A. As the polymerization continues higher molecular weight epoxy resins emerge.
Epoxy Resins have been used for corrosion protection and repairs for over 70 years since its discovery. Epoxy as FBEC is used in the pipe industries (oil, water and gas in particular) for over 60 years and as coating for rebars for over 40 years internationally. In India, epoxy coated rebars are used for corrosion protection since 1992.