By Mike Mudrick
It's a fact: Watershapes built with concrete need to be waterproofed in some way or it is almost certain that the water their shells are supposed to contain will find a way to escape.
While some observe that concrete applied by an expert at high levels of compressive strength will
not leak, it's also true that concrete is naturally inclined to be porous. And when you add in the fact that most watershape shells include various penetrations — for inlet and return lines, skimmer throats, cleaning systems, lighting fixtures and more — the challenge of keeping water where it belongs becomes even greater.
Given all of this, it is generally recognized that concrete shells must be sealed in some way to make certain they hold their water. As a consequence, selecting an approach to waterproofing is one of the most important steps in the construction of gunite, shotcrete or poured-concrete watershape shells and structures.
There are many different product types available on the market today to accomplish this. In evaluating your options, you need to ask five key questions:
1. What is the product made of?
Most pools, spas and other watershapes are made using concrete of some type as the primary construction material, so it makes sense to use waterproofing products that are similar to concrete in chemical composition. For that reason, it may be preferable to use cementitious products for waterproofing, because they bond better to concrete than do products based solely on polymer agents.
In addition, you need to consider how the surface will be finished: If the watershape is to be finished in plaster, for example, it will bond better to a cementitious substrate than it will to a polymer system.
Timing is also an issue. To prevent failure resulting from moisture-vapor transmission in new construction, the concrete substrate must dry for a minimum of 28 days before a pure polymer system can be applied. Cementitious coatings do not have this restriction: They are breathable, so they can be applied within days after pouring or placing the concrete — a real advantage if rapid turnaround times are required.
There's also a special issue with restorations, where, because of exposure to moisture in the surrounding soil, it's often difficult to dry the substrate completely before the waterproofing is applied. Again, this works in favor of cementitious applications, where it is often recommended that the concrete surface be wetted before the waterproofing material is applied.
2. How is the product to be applied?
Generally speaking, waterproofing agents are applied in one of two ways: by hand with trowels, brushes or rollers or by using sprayers.
In our work, we've found that large surface areas and/or surfaces with regular contours and features are more quickly and economically waterproofed through use of spray equipment. By contrast, if the substrate is irregular — as is the case with artificial rockwork in a pond or waterfall feature — it is much more effective to apply the waterproofing agent with trowels, brushes or rollers to make certain the material gets into all of the nooks and crannies.
3. What sort of water chemistry will the material encounter?
Not only is proper water chemistry essential to maintaining safe and consistent watershape operation, it is also a factor in deciding which waterproofing system should be used.
In instances where the waterproofing is also the finished wearing coat (as with plaster, for example), it is important that the coating can withstand the chemical environment. Residential swimming pools, for instance, are typically maintained with chlorine at levels around three parts per million, where public and commercial pools typically operate at much higher levels to accommodate large bather loads. Large pools also tend to be treated with ozone, while commercial spas will often use bromine. At higher concentrations, all of these sanitizing agents can cause pool coatings to deteriorate.
Moreover, pH level is a definite factor in the durability of a finish — and represents yet another consideration when waterproofing agents are chosen. Pool plaster, for instance, can discolor or become subject to rapid wear when pH moves outside the 7.0 to 7.6 range — a point that might lead to consideration of a different finish and/or waterproofing system.
4. Can the coating be overcoated, painted or tiled?
If the owner has a specific aesthetic appearance in mind, it is important that the coating or membrane will allow for its use. Not all waterproofing systems are compatible with all tile adhesives, plasters or other finishes.
One alternative some are now using involves working with the waterproofing system itself: In their cementitious forms, they can be made to resemble other (finish) materials by using faux-finishing techniques, metallic pigments and inlaid designs or specialty tiles.
5. Will the coating resist negative-side hydrostatic pressure?
This, in my opinion, is the most important question of all. Remember that waterproofing is not just about keeping water in the structure, but is also about making sure that groundwater is kept out — especially when the watershape is empty.
Most watershapes will need to be drained at some point, whether for winterizing, maintenance or cleaning. When this occurs, there is a potential for negative-side hydrostatic pressure to build up behind the coating. As groundwater tries to pass through the structure, it will come up against the coating and effectively push it away from the surface. Flexible coatings can bubble and fail under this kind of pressure, so it is generally advisable to apply them over base coats or first coats designed to resist this sort of attack.
The questions listed above are not the only ones to consider when selecting a waterproofing system for a watershape, but they represent good starting points and should be considered with every project. If you have questions you can't answer, rest assured that the suppliers of waterproofing and finishing materials are at the ready to guide you through the selection and installation processes.
While this focus on secure waterproofing might seem a narrow sort of consideration in the grand scheme of watershaping, we've found that answering these questions tends to highlight all sorts of other issues that can arise in the design and construction phases, allowing them to be addressed before they become problems.
That's the way everyone should want it.
Mike Mudrick is a product manager at Aquafin, Inc., a manufacturer of waterproofing products based in Elkton, Md. A graduate of Temple University with a degree in chemistry, he has worked at Aquafin since 2001 and has been involved in the construction and waterproofing industries for more than 20 years. For more information, go to www.aquafin.net.