April 1, 2014
By Peter Gibson
The commercial construction of swimming pools typically started after the Second World War, when urban sprawl was underway and families moved from the cities to the suburbs. Before that most pools were large, poured-in-place concrete vessels that were finished with chlorinated rubber paints.
With the advent of the gunite process, builders could design and install custom-shaped pools much faster and in larger volumes. This brought the price down, making swimming pools more attainable for the average homeowner. Also at this time, the decision had to be made on how to finish the raw gunite. More often than not, white Portland cement with a fine marble aggregate was the best choice as it is a cement-based product compatible with gunite. The rough nature of gunite ensured the marble plaster would bond quite well. The plaster’s function was to waterproof the gunite and provide an esthetic finish that enhanced the pool’s appeal.
As the pool industry matured, manufacturers offered new pool owners a number of different options for pool construction (i.e. metal/polymer wall pools with vinyl liners, tile, and prefabricated fibreglass). Today, these construction alternatives are very versatile in their range of materials and shapes.
A common trend among these different pool construction methods and finishes is they eventually deteriorate and need replacement or restoration. As a steel-wall pool is constructed to receive a vinyl liner, it is typical to install a new one once the old liner reaches the end of its useful life. Poly(vinyl chloride) (PVC) liners are also used to line cracked and leaking concrete pools. Finally, another popular renovation system, predominantly used in Europe, uses vinyl fused to metal.
Today, when it comes to refinishing plastered gunite pools, there are many cementitious options available (e.g. basic white plaster, specialty plasters, and an array of aggregate plasters). High- grade plasters incorporate many additives and modifiers to enhance the service life.
There are essentially two methods used to prepare a pool for re-plastering: applying a bond coat, which roughens the surface so the new plaster adheres; and chipping the old plaster down to the gunite, and applying new plaster. The former method has some mixed results; while the latter method is preferred as it significantly improves the bond and eliminates many of the ‘pop-off’ problems. The latter method is more costly, but this assures enhanced bond promotion.
Cement-based materials do not offer any structural benefits and should not be considered for cracked and/or leaking pools. Historically, however, before fibre-reinforced plastic (FRP) linings were an option, many owners with cracked pools installed vinyl liners, but installation problems were encountered with this method. For example, inaccurate cutting and sizing of the liner, and the need to alter the pool structure for liner attachment purposes. Prefabricated fibreglass pools were also used for cracked pools, but this was impractical and expensive.
A FRP lining system is applied as a liquid and on curing it conforms to the shape of the concrete structure. There is no need for special sizing and fitting or deconstruction of the vessel. Elastomeric membranes like polyureas and glass-fibre reinforced linings are a viable option when structural considerations are required. Polyureas and glass-fibre reinforced plastic (GFRP) linings have some flexibility and crack-bridging properties.
In essence, warranties for cement-based pools cover the application and not the material performance itself, due to the nature of the material and its deterioration over time in pool water. The type of cementitious material chosen for a project will determine the service life.
Plaster products are not suitable for use in steel pools; therefore, polymeric materials are the natural choice. Polymerics are also a better option for commercial spas where the water temperature is maintained at 40.5 C (105 F). Higher water temperatures can be aggressive on cement finishes; causing spalling, roughness, algae growth, staining, and loss of esthetics, not to mention an uncomfortable experience for bathers. Tile is also a good choice for refinishing spas.
Fibreglass is a composition of glass fibres and a thermoset resin. It is hardened/cured by a peroxide catalyst (free radical initiated chain growth polymerization) into a homogenous mass of various shapes and sizes.
This material is best known in the pool industry in the form of the factory-made fibreglass swimming pool, which was first fabricated in the 1950s. The application of fibre-reinforced polymer (FRP) lining systems is relatively new to the pool industry. One of the main reasons for this is the pool sector is relatively a cement-based industry.
The composites industry is also little known to the engineering fraternity. In fact, the composites industry is reaching out to materials engineers and specifiers to educate them on the benefits of the use of composite materials, e.g., corrosion resistance, long service life, and low life-cycle cost.
Fibreglass usage for fieldwork falls under the jurisdiction of coatings technical societies, not pool industry standards. Coatings societies create consensus standards for coating applications. Carefully adhering to specifications ensures the materials are applied with best practices and procedures in mind. They also implement a quality assurance process.
Well-known societies that govern the coatings industry include NACE International, a professional organization for the corrosion control industry, The Society for Protective Coatings (SSPC), a professional association for the industrial protective and marine coatings industry, and the American Composites Manufacturers Association (ACMA), a trade association serving the composites industry, offering both training and educational programs.
|Another common refinishing method is paints/coatings specifically formulated for use in swimming pools. The most widely used coating material is from the epoxy family. These paints are used on concrete in new construction and refinishing existing painted pools, as well as on plaster pools. Pool paints are the least costly of all refinishing options.
There are three grades of epoxies, i.e., simple pool paints, two-part solvent-borne epoxies with high solids, and industrial-grade, 100-per cent solids and zero volatile organic compounds (VOCs), high-performance spray-applied epoxies. The latter is seldom used in the pool industry as it requires special spray equipment, the material is expensive, and applicators require a high skill level.
Pool paints do not have the longevity of industrial-grade protective coatings. The grade of pool paint used will ultimately determine its service life. Pool paints are also non-structural and, therefore, not a good choice for cracked and/or leaking pools.
When applying pool paints, it is important to ensure there is a good anchor profile for a proper bond as well as verifying the correct dry-film thickness and coverage is used. Cutting corners on the number of coats will also shorten the service life. Pool paints can also be used to refinish fibreglass pools. Sand the surface with 80-grit sandpaper and apply two coats. The first coat acts as a primer/tie coat. One downside to epoxy pool paints is their tendency to chalk, i.e. a fine powder-like substance forms on the film surface. This is not a water chemistry induced phenomenon, but the chemical nature of an epoxy.
Hand-applied: The fibreglass mat is applied to the surface and the resin is rolled on. This is the most common method, but produces an inferior outcome. Most notably are bonding problems and the incidence of glass fibre exposure, i.e. fibrebloom, which occurs after exposure to pool water and the lining begins to degrade, causing fibres to become fuzzy or ‘bloom.’ With this method, no high-build topcoat is applied. Deficient training of applicators and the use of inappropriate materials are the major reasons for poor performance.
Spray-applied: This technique is recommended in the consensus standards. It ensures excellent bonding to steel and concrete and eliminates all of the problems found in the hand-applied method. Bonding is achieved by roughening/fracturing the surface to create an anchor profile for bond. With this method, the material is never applied over smooth plaster. Special resin and catalyst systems are used to further promote bonding. A niche is cut at all termination points to stabilize the lining and waterproof those points. Special resin heaters are also employed to maintain the proper resin temperature during changes in ambient temperature to facilitate proper resin processing. There are many other technical reasons for using the spray-applied method. The most commonly used resins are corrosion-grade polyester (UP) and vinyl ester (VE). (See Figure 1.)
When immersed, all fibreglass will permit low quantities of water to pass through in vapour form. An important property of any resin, particularly for swimming pool resurfacing/construction, is its ability to withstand degradation from water ingress. Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. To prevent the onset of osmosis from the start, it is necessary to use a resin which has a low transmission rate, and a high resistance to attack by water. A polymer chain having an epoxy backbone is substantially better than many resin systems at resisting the effects of water. Such systems have been shown to confer excellent chemical and water resistance properties, due to the low vapour transmission rate.
The vinyl ester molecule is different from the basic polyester molecule in the number and location of reactive sites. The reactive site configuration makes VE less susceptible to degradation by hydrolysis (the separation of chemical bonds by the addition of water). Therefore, VE exhibits better water resistance than its basic polyester counterparts. The incorporation of VE into the process ensures an extended service life by preventing blistering, chipping, fibrebloom, and embrittlement. This avoids periodic maintenance, with an overall contribution to corrosion resistance and esthetic appeal. The prefabricated pool industry incorporates VE into its fabrication schedule, due to its outstanding long-term performance. On the other hand, however, VE is more costly than polyesters.
Corrosion-grade isophthalic (ISO) polyesters and hybrid, 100 per cent ISO/Neopentylglycol (NPG) resins are also used when material costs are a consideration. ISO/NPG’s impart excellent blister resistance, UV stability and superior weathering characteristics. This material is used as a high-build topcoat and prevents fibrebloom.
Fibreglass comes in a variety of colours. It is a popular choice for pools that are cracked and leaking as the glass fibre reinforcement makes it a structural material. It is also ideal for hot water spas as it will not degrade like cement-based products.
Fibreglass linings can also be used for new pool construction. A concrete forming system is used for the walls and floor, and the FRP lining is applied to the interior of the pool. This is a good alternative for new pool owners who might want a pool shape and size not offered in a prefabricated fibreglass pool line. FRP is a versatile renovation material, and can be used to refinish fibreglass pools, convert vinyl pools to fibreglass, in addition to having many uses in water parks and public aquatic facilities.
Fibreglass has performed well in areas with freeze/thaw conditions. However, it is not recommended for pools that are drained in the winter. Polyureas are better suited for these applications.
Due to the nature of this material, a long service life can be anticipated. Some applications are still performing 25 years after installation. However, with new resin technologies, a longer service life can be expected. A typical performance warranty is offered that address the material performance issues of the fibreglass finish.
FRP provides a long-lasting method to refinishing pools with surface problems and eliminates the need for acid washing and repeated treatments with other products. It is not subject to degradation like conventional/entrenched materials. Its usage will eliminate cracks, leaks, algae and staining, rough surfaces, discolouration, high chemical usage, and rebar stains, all while maintaining the pool’s esthetics long-term.
Experts will agree, surface preparation is the most important part of a swimming pool coating system. The quality of the surface preparation will affect the performance of the coating more than any other variable. Given the proper coating system has been selected, if the surface preparation is poor, the coating’s performance will often be compromised, even if the application is near perfect. If the surface is prepared properly, then the applied coating will perform well. Applying sound preparation techniques will go a long way in ensuring a successful application; therefore, it is important to make sure the surface preparation complies with specification requirements to avoid costly rework and catastrophic failure.
Fibreglass linings require a mechanical bond in concrete pools as the two materials are dissimilar. Bond failure has historically been an issue when the concrete surface is smooth as the coating has a difficult time adhering tenaciously. When concrete is fractured and/or roughened, it creates peaks and valleys. In these cases, the fibreglass coating anchors itself to the valleys of the profile, while the peaks act like teeth. This is why surface profiles created by blasting/scarification are sometimes called an anchor pattern or mechanical tooth.
Surface contamination of steel that has been painted interferes with both mechanical and chemical adhesion of fibreglass coatings, which will likely result in premature failure. Therefore, complete and continuous contact with the steel is required. A well-adhered coating creates a sound barrier that prevents the steel from corroding by minimizing moisture transmission.
When applying fibreglass coatings it is imperative the application practices and procedures in the consensus standards are carefully adhered to. These specifications address the materials, application, and surface preparation recommendations.
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Peter Gibson is the technical director for GRC Fiberglass Coatings in Oakland, Calif. He has been involved with fibreglass reinforced plastics (FRP) lining fieldwork for more than 30 years. He compiled the consensus standards for this process with the sources of various technical affiliations. Gibson is also a consultant and specifications writer, and trains applicators in the process of FRP lining systems in Canada and the United States. He can be reached via e-mail at firstname.lastname@example.org.
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