Since cover is the first line of defence to prevent corrosion, the repair and rehabilitation is recreation of cover with highest protection quotient. We therefore use polymers, fibres, etc., to lower the permeability and minimize the cracks in addition to bonding. Given actual working conditions, it is difficult to re-create a cover, which will be resistant to carbonation or chloride ingress. To remedy this, it is preferable to provide a protective coating, which work on principles to equivalent cover to ensure durability of repairs. Therefore the last step of any complete Concrete Repair is the provision of anti carbonation or chloride ingress resistant coating ensures the durability of repairs.
The Cover Factor
Concrete is a material composed of different ingredients, and is therefore non-homogenous. Such non-homogeneties occur both at macro and micro levels. The cover has many non-visible micro cracks, which act as avenues for water and gas penetration. The cover therefore should be of proper quality and depth. If the permeability of the formwork is not controlled, it is stated that, the water cement ratio is 0.10 more and the cement content is about 45kg/m3 less compared to the original concrete mix and therefore the cover is most vulnerable to attacks.
Under pore solution of pH values of upto 12.5, the reinforcement in the concrete remains in a passive conditions and does not begin to corrode.. Carbonation is the effect of CO2 from the atmosphere reacting with alkaline component in concrete Ca(OH)2 in the presence of moisture thereby converting the calcium hydroxide to Calcium Carbonate. The pH value of the pore water is reduced to less than 9.5. The reinforcement therefore is no longer in the passivating range and corrosion begins. To prevent this from happening, we need to remedy the cover both in terms of quality and quantity. Therefore repairs are needed. Repairs can be carried out based on two principles:
Principle 2: Leave the cover as it is or set up a minimum required cover and apply a surface protection system.
The application of protective coating is an attempt to increase the effective cover. By virtue of their formulations, anti-carbonation protective coatings provide the protection of meters of concrete cover in a very thin layer. Therefore, the principle of equivalent cover is very valuable in repair strategies, or even to safeguard new structures that have a very long design life. Specialized protective coatings can be of many types:
- Thick Coatings (1-2mm): Like Different Breathable waterproof cementitious polymer modified coatings
- Breathable, Elastic Elastomeric Crack bridging coatings (200-300 micron thickness)
- Breathable Hydrophobic impregnation coatings, based on silicone, silane-siloxane
- Resin Coatings
Specialized surface protection coatings can also be designed as anti grafitti treatments by making them self-cleaning type. Several benefits can be obtained by incorporating the surface treatments in a well-designed manner right from conception of project. Although the main function of any surface protectant is moisture ingress control either by physical barrier concept or conversion of capillaries to hydrophobic, the coatings can also be designed for resisting chemicals.
Chloride ingress can be effectively controlled by surface protectants. Further, the diffusivity of carbon dioxide, sulphur dioxide and oxygen can be lowered for corrosion control. Root growths and mould growths can be prevented in concretes under damp conditions.