Concrete the versatile construction medium is, however, degradable under environmental effects. Generally concrete structures have steel as reinforcement for its own inherent low tensile strength. The environmental effects can lead to degradation of the reinforcing steel and this degradation is termed as corrosion due to environment.
Environment means the surrounding to which the concrete structure will be subjected to during its service life. Depending upon the environment the protection system will vary because the requirements for protection is different. Thus a structure which is to be protected for normal atmospheric surroundings will definitely require a different protection system compared to the requirement of a structure which stores fully saturated common salt solution.
One of the means of protecting concrete is to provide a coating which will not allow the degradable agents of the environment to penetrate the coating so that their bad effects on concrete reduces very significantly. However, different systems of coatings are required for different environmental condition. This review paper deals with their pros and cons.
In order to protect concrete from the effects of environmental degradation various high performance coating are being used. There are several properties which a good coating should possess. These are :
- Good adhesion
- Good mechanical properties
- Resistance to environment which may be chemical, moisture laden, sunlight, flowing water etc.
All high performance coatings should have these properties to a satisfactory degree and the coating system must protect the concrete from the environmental situation to which the concrete will be subjected to.
Moreover for applying the coating on concrete some basic requirement are there such as water based coating could generally be applied on wet surfaces but some coatings require fairly dry concrete surface. Surface preparation is common to all coating works.
This review paper discusses four such high performance coatings such as
- Styrene Copolymers
- Acrylic Latex
Styrene co-polymers are generally in the form of emulsion having styrene-butadene. Due to its polymeric action the emulsion when used in conjunction with cement to form a slurry, the bond of the styrene copolymer cement slurry with concrete is vastly improved and the slurry membrane thus formed has very low water permeability but allows water vapour to pass through the film.
This material is perhaps the cheapest among the four system under discussion but suffers from some inherent liabilities such as it is not UV resistant thus not very suitable for exposure to sunlight. Moreover, its performance under water submerged condition is also not satisfactory and does not prevent sulphate attacks on concrete.
The slurry is easy to apply and is fast drying for application of a recoat or top coat and can also be easily repaired.
Acrylic latex contains minute particles of pure aliphatic acrylate with copolymers in latex form. The acrylic latex when used in conjunction with cement to form a slurry forms an excellent bonding medium for joining old and freshly poured concrete. In addition, when used as slurry on exterior concrete surfaces produces a membrane or film which have excellent bond with concrete and the film allows water vapour to pass through it but water molecules due to their inherent large molecular size can not penetrate through the film. The film/membrane is UV resistant consequently, the slurry coating can be used for direct exposure to sunlight and it is also carbon dioxide and sulphate resistant.
Easy applicability of acrylic cement slurry on concrete surface along with small waiting period for re-coat or top coat are the advantages of this slurry along with easy repair possibility.
The most common type of epoxy coating consists of two components, leading to exothermic reaction. The two components are known as base and curing agent. The curing agent also known as hardener, is used to set the base (epoxy resin) to achieve different mechanical and chemical resistance properties. The product is available in solvent free, with solvent or water containing formulation. The water is added with hardener to form an emulsion.
Epoxy coatings bond well with clean concrete surfaces. The film is hard but is relatively inflexible. They chalk in sunlight as they are not UV resistant. The film is not porus and is non breathable and the film may get disbonded from the concrete surface due to its very different co-efficient of thermal expansion.
Epoxy coatings are costly and thus blend of coal tar and epoxy is used to reduce the cost and to improve water resistance and due to reduction in cost allow bigger film build ups. Coal tar epoxies are black or dark red in colour.
Polyurethane:- 100% solid P.U. elastomeric coating
The formulation is dual component system: Polyisocynate and a polyol. Depending on the basic imputs of the two packs different variety of properties can be obtained, such as consistency is concerned the final product may range from brittle to elastomeric. PU coatings can be formulated to have outstanding aesthetics, UV resistance and water resistance along with excellent resistance to moderate and even aggressive chemical environment. Aliphatic isocynate based PU is known for their good colour retention capacity.
Low temperature curing is possible and the performance is good in many environment. PU coating provides excellent abrasion, impact & tearing resistance, flexibility, adhesion and chemical resistance.
While polyurethane coating may be more expensive initially, the longivity of material means cost effective operations, lower maintenance cost and fewer replacement over the long term. For application of the coating the system’s requirement is more elaborate.
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