Concrete structures in aggressive environment (Marine / Industrial area) are subjected to chloride attack. In order to protect concrete structures from chloride attack in aggressive environment, it is necessary that protective coatings are applied to concrete and steel both to prevent ingress of chlorides.
Protective surface coatings are primarily used to protect new or repaired concrete surfaces from future chemical attack (e.g. against sulphurous and nitrous pollutants) and the ingress of aggressive liquids and gases (e.g.water borne chlorides or atmospheric carbon dioxide). Some corrosion induced damages in concrete are shown in Figure 1. This concrete surface protection can also be specified to be: water-repelling through impregnation and pore blocking; elastic and crack-bridging (to different degrees of elasticity at different temperatures); resistant to different chemicals (such as in containment zones); or abrasion and wear resistant (e.g. on horizontal concrete surfaces such as balconies and car park decks). In order to meet all of the different requirements for concrete facades and horizontal concrete surfaces on different structures and in different exposure conditions, there are a wide range of different protective concrete coating products and systems required.
On reinforced concrete facades single pack, acrylic resin based anti carbonation coatings are normally ideal as the protective coating solution. Also ideal are protective coatings based on, elastic film-forming, styrene acrylates or other copolymer resin based products, which should be used when additional crack-bridging properties are required. On horizontal decks for combined chemical and wear resistance, the best protective deck coating products are usually 2 component epoxy and polyurethane resin based solutions that are specifically designed to accommodate this additional stress and exposure.
The characteristics and performance requirements for protective surface coatings to be used on reinforced concrete are defined in the European Standard EN1504 Part 2, with the appropriate product selection then to be made in accordance with the specific requirements and exposure conditions of your project.
Various Types of Concrete Coatings
All conventional concrete coatings are multicomponent systems, and the two-component epoxy/amine and urethane (isocyanate/polyol) are the most common. These epoxy and urethane coatings require more than one day before return to service. Faster curing systems include polyaspartic and methyl methacrylate coatings, which can be returned to service in hours instead of days. Following are the five basic types of concrete coatings that are available in the market-
– Penetrating Concrete Sealers-Penetrating concrete sealers are most often used to coat exterior concrete surface, especially those that are vulnerable to damage from freezing-thawing conditions and other harsh outdoor elements. These sealers dry to a natural matte finish, invisibly protecting the surface without changing its appearance. This type of coating works by penetrating into the concrete and creating a chemical reaction that shields the concrete from moisture and chemicals commonly used to deice surfaces.
– Acrylic Sealers-Acrylic sealers work by creating a thin layer of protective film over the surface of the concrete. They are available in both solvent and water-based formulas and you can select the sheen level you desire. This type concrete coating can be used either in interior or exterior spaces. Acrylic sealers are economical but are not as durable as epoxies and polyurethanes. When used on interior floors, acrylic sealers require regular maintenance and waxing to reduce wear and the appearance of black heel marks. Because acrylic sealers dry quickly, they are a good choice when a short down time is important.
– Polyurethanes-Polyurethanes are also available in both water-based and solvent formulations. They can be used inside or out and are available in a selection of sheens. Polyurethanes are almost twice as thick as acrylic sealers and produce an extremely durable cover. They are a good option for high traffic areas because they provide strong resistance to heel marks, scuffs and staining. A polyurethane finish is non-yellowing and dries to a transparent sheen.
– Epoxy-Epoxies are a versatile choice that is available in a one or two component application. They can be clear or can have sand, colored quartz or vinyl chips incorporated in them so a variety of colors and patterns can be achieved. Because epoxy finishes are vulnerable to UV damage, these products are normally used on interior surfaces. Epoxies produce a hard, abrasion-resistant coating that is durable and long-lasting. Careful surface preparation is vital to the success of an epoxy coating.
– Concrete Overlays-Concrete overlays like Flex-cement contain polymer resins, cement and aggregates. The polymer resins make this coating strong and durable. Overlays have become very popular in recent years because they offer a cost-efficient way to restore old, damaged concrete surfaces without having to go to the time, trouble and expense of ripping out the existing flooring. Concrete overlays can be used to smooth and level uneven surfaces and when you choose a concrete overlay, you’ll experience minimal downtime, because they cure quickly. It may be only a few hours before freshly coated surfaces are able to support foot traffic.
Overlays are good choices for either interior or exterior surfaces. They are one of the most versatile options available when it comes to design possibilities. Skilled installers can create a myriad of designs and colors by using special techniques and tools. Overlays can be stamped, stenciled, textured or stained. The design possibilities are basically unlimited with this type treatment.
Protective Coating Over Existing Concrete Surface
Protective coating-Protective coatings are used to protect concrete from degradation by chemicals and subsequent loss of structural integrity, to prevent staining of concrete, or to protect liquids from being contaminated by the concrete. The proper use of protective coatings is the extremely effective means of preventing concrete deterioration and corrosion in water and waste water applications. The function of a coating isto act as a barrier that prevents either chemical compounds or corrosion current from contacting a concrete substrate.The coating s effectiveness of fulfilling this function depends on its degree of integrity (being a completely continuous film or freedom from imperfection or defects), its ability to bond to the concrete substrate, and its ability to insulate against the passage of corrosion current (dielectric strength) or chemicalions. Corrosion protection by coatings for water and waste water pipelines is the implementation of a well-balanced cycle of the following four equally important elements:
– Specifying and using a proper coating system
– Proper surface preparation for the coating system
– Proper application of the coating system
– Quality inspection of the coating system
A. Condition Assessment of Concrete for Protective Coating Application-
Because conventional sealers and paints are a comparatively thin and weak layer applied to the surface, they are wholly dependent upon the integrity of the concrete surface to maintain integrity of their protective film. In other words, paints and sealers are not structurally independent of the surface and, to be effective, must only be applied to a sound substrate. If the concrete has been in service without the benefit of protective coatings, the surface of the exposed concrete is likely deteriorated to some extent. A detailed inspection of the surface conditions has to be performed during the design stage or provisions must be made in the contract for varying conditions.The recommended practice and procedure for assessing the condition of the concrete by the American Concrete Institute (ACI) include visual examination, nondestructive evaluation test (NDT), and destructive tests (ACI364.1R). A typical NDT is shown in Figure 1.
B. Surface Preparation for Concrete Coating
Since the adhesion of the coating will be limited by the strength of the surface, the deteriorated concrete must be removed to reveal a sound surface prior to application of any coating. Depending on the extent of deterioration, rebuilding of the surface with a mortar prior to coating application may also be necessary. No general surface preparation standard exists for concrete however, most coating projects have unique conditions and special requirements that must be evaluated to determine which will best meet the objectives of the engineers and owners. Some of the important factors for selection of surface preparation methods are:
C. Concrete Coating
Protective coatings play a critical role in extending the service life of concrete surfaces in harsh environments. When subjected to the harsh conditions inherent in water and waste water treatment facilities, the role of the coating system in protection of concrete surfaces is particularly important for a number of reasons:
– The coating system must protect the concrete from chemical attack and deterioration by hydrogen sulfide, carbon dioxide, and chloride attacks.
– The coating system must protect the concrete from microbiological attack. Microorganisms, which cause the breakdown of organic matter also, can cause deterioration of concrete (through the formation of hydrogen sulfide),resulting in spalling and sloughing.
– The coating system must protect the concrete from spalling and cracking caused by moisture penetration. Moisture enters the concrete and isdrawn inside by capillary action. In cold temperatures the moisture freezes in the pores, swells, and causes spalling.
Prior to the late 1980s, relatively thin film coatings based on coal tar epoxy and amine cured epoxy formulations provided effective corrosion protection of concrete for up to 10-12 years.A typical coating might, for example, be only 12 mils (.012 ) thick; it would seem impractical to provide sufficient protection for the concrete structures. In contrast, to a typical sealer and paint approach, a polymer lining has sufficient cohesive strength to provide substantial film integrity independent of the substrate. It will also provide a cost effective method to deal with an irregular concrete surface that has resulted from deterioration.
Figure 4 shows the coating process in an exposed concrete surface.
Basic checks for the coating processes
When preparing a protective coating strategy over the exposed concrete surface from start to finish it is essential to prepare it in a comprehensive manner so that all the aspects are covered. Some guidelines are given to cover all the aspects below:
Supply of Information- Prior to the engineer giving approval of a particular paint type, the contractor shall supply information which will satisfy & comply with the requirements of the specification.
Coding system-all primers, paints and solvents to be used in the works shall be identified by a unique coding system, relating to the batch of raw materials from which the product was manufactured and the date of manufacture.
Storage life-storage life shall normally be a minimum of one year. If the storage life is known to be shorter, the expiry date must be marked on the container prior to dispatch from the manufacturer’s factory.
Method of using paint components-preparation and application techniques for all components of the paint system shall be stated.
Surface preparation of concrete-recommendations for preparing the surfaces of concrete shall be given, including the following:
– The minimum age,
– The maximum moisture content and measuring method,
– The equipment to be used for preparing the concrete surface, and
– The materials suitable for filling defects in the concrete.
Dry film thickness and coverage- The minimum and maximum DFT limits for each component of the paint system shall be given for a specific temperature. The corresponding coverage in L/m2 shall be quoted for prepared concrete surfaces typical of low strength (Grade 20) and high strength (Grade 40) concrete cured under site conditions, in order to achievere commended DFT values.
Over coating-drying and over cutting times of the pretreatments and coats of the paint system shall be given for a particular temperature and relative humidity.
Physical properties-all components of the paint system shall be capable of unique identification such that any substitution, dilution or adulteration of the paint can be identified. The Contractor shall provide test data and methods of test for the following properties of each applicable primer, paint and solvent used in the system:
– Specific gravity, – Volume of solids,
– Viscosity, – Fineness of pigment grind,
– Infra-red spectrograph,
– Pyrolysis gas chromatography of the binder,
– Ash content.
– The suitability of the coating for application on damp, alkaline, cement-based materials shall be stated.
– The decorative life of the paint shall be stated, in terms of the colour-fastness of the finish coat and resistance to chalking, loss of gloss and atmospheric dirtying.
– The life of a paint system prior to the need for recoating shall beat least 10 years. Examples shall be cited of where the paint systemhas achieved this life.
– The paint system shall be capable of withstanding cleaning withhot water (in the range between 40C and 50C), detergent and scrubbing action without losing adhesion, softening or changing in colour or gloss.
Health and Safety-
– The contractor shall supply health and safety data relating to the storage and application of all components of the paint system. As aminimum, the check list contained in Table 1 shall be completed.
– The effects of solvent and vapour build-up on the environment in the vicinity of the paint applicator shall be monitored, and the loss of volatiles per unit area of paint in terms of minimum air exchange rates in confined areas shall be determined.
– The in-service performance of the paint under conditions of fire shall be given, making particular reference to the surface spread of flame, and the toxicity and opacity of combustion products.
Concrete coatings have multiple capabilities of providing aesthetics and protecting structural concrete from weathering. They are viable for use on all concrete elements including barrierwalls, girders, abutments and wingwalls and all types of foundations. Severalof these coatings reduce the permeability of concrete (waterproofing) and limit the intrusion of harmful moisture and deicing salts. All of those coatings should provide enhanced aesthetics(compared to textured masonry coatings and stains) and better reparability.
Beneficial coatings need to be identified by the engineers and tested to determine their effectiveness in protecting concrete. Due to the wide latitude of properties of these different coatings, guidelines need to be developed for determining where they are best suited (new and maintenance concrete application) to make their application more effective.
– Applying linings to concrete surfaces in water and wastewater environments J. Peter Ault, P.E., PCS, T. Kyle Greenfield, PCS, SasanHosein, M.S.Corrpro Companies, Inc
– www.texcote.com, www.remmers.co.uk
– model specification for protective coatings for concrete, Civil Engineering Department Hong Kong Government, 1994.