Can Draining a Concrete Pool Ruin The Plaster?
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You might have heard some information about concrete pools and how draining a concrete pool can potentially cause damage to the interior surface of the pool...commonly plaster. Actually draining a concrete pool can potentially cause all kinds of problems ranging from small to disastrous but this article aims to talk about the plaster interior surface that many concrete swimming pools have. This plaster surface is a thin coating of cementitious material, often white in color at least traditionally speaking, and it is this thin layer which can potentially pose a problem for you if you drain you pool and leave it this way for an extended period of time.
In specific, it is the forces of expansion and contraction which are your enemy here when it comes to draining a concrete pool (as far as the interior surface is concerned). The substrate structure of a concrete pool is just that - concrete. While the mortar or plaster surface applied to the inside of a concrete pool is also portland cement based, similar to the concrete shell, the reality is that the mix components and density of the interior surface layer (the plaster) is entirely different than the mix components and density of the substrate concrete shell. The result is that the differences in material and density result in differences in the rates of expansion and contraction between the shell and the surface plaster layer. Do you know why that is normally not a problem in swimming pools?
Why does plaster need to stay wet? - Pool plaster needs to stay wet (submerged) at all times to prevent differences in the rate of expansion and contraction forces from delaminating the plaster layer entirely from the substrate concrete layer. When dry, and especially in direct sunlight, the plaster layer will heat up and expand. Concrete and mortar expands more than what many people assume since it seems so hard / rigid. A 100' linear expanse of concrete will expand by over 5/8ths of an inch (almost 16mm) when exposed to 100 degree Fahrenheit temperature differences. So the dead of winter temperatures compared to mid summer temperatures with blazing overhead sun. So now imagine your swimming pool empty, blazing in the direct overhead sun and shaped like a parabolic mirror to maximize the intensity of the suns rays. Your pool plaster needs to expand, but the substrate concrete is cooler, using itself buried in the ground as a giant heat synch. The difference in temperature causes the surface layer to expand when the base layer does not, and you delaminate the plaster surface from your pool. Pool plaster must stay wet at all times to ensure even temperature control. This is one of the main reasons that you need a tile band at the water line in the pool. Plaster is unsuitable for wet sometimes and dry sometimes environments. Plaster must be kept wet / underwater at all times to prevent delamination.
As anyone who lives in a cold climate will tell you...the powers of expansion and contraction due to temperature changes are immense. If you seal water in a container and then let it freeze the water will expand as it turns to ice, and will destroy any container you try to contain it in. Concrete, stainless steel, plastic...all of them will easily fail to the force of expansion and contraction. Your swimming pool plaster is the same. The power of expansion and contraction due to temperature differences between materials means that these dissimilar materials will have dissimilar expansion and contraction rates. Which is essentially a guaranteed point of failure. This is why you must leave plaster wet at all times, and when you allow it to dry out it can and will start to delaminate from the pool surface.
If you are wondering how concrete pools get renovated since I just said that draining it will damage the plaster. The answer is that a skilled concrete pool worker will inspect the pool for delaminated plaster at least three times during the project. The day you drain the pool, 48 hours after this point, and then once more right again right before you refinish the interior surface of the pool (which is generally the last step of any concrete pool renovation or repair project). Not all plaster will delaminate but anything that does needs to be marked and then chipped out / removed so that you do not apply a new plaster layer over an old, de-bonded layer. This would be a guaranteed failure of the new layer as well.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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