Case Study #1 Cracks in a Freestanding and Raised Bond Beam Pool Wall
At Pool Engineering, we often get calls from clients in need of help diagnosing issues with finished pools. Many people, homeowners and builders included, assume that in the rare event that cracks form in a finished pool that the causes must be structural. By structural we generally mean that the cracks are the result of failure of the reinforced concrete to support some applied load. Its true that many pools develop structural cracks due to soil settlement beneath the pool, however there are many other scenarios where cracks can be caused by something completely unrelated to the structure of the pool. One case that came up recently involved a very interesting interaction of a few common workmanship problems.
The subject pool was a fairly simple rectangular pool in a typical tract home back yard. On the font side the pool was freestanding above adjacent grade by about 30 inches. The back side had a 30-inch-high raised bond beam, retaining a small irrigated planter at the rear of the property (Photo A & B). The homeowner first noticed the cracks about a year after the pool’s completion. The cracks manifested as vertically oriented irregular shaped cracking starting at the water line level and proceeding down towards the floor but without transitioning through the radius into the floor (Photos C, D & E).
Like most people my first thought here was the possibility of soil settlement. Soil settlement often causes pools to exhibit vertical cracking due to a “hinging” effect in the pool when one side loses support relative to the other. However, in this case the cracks on both sides of the pool did not extend up to the bond beam or the coping as they typically would with settlement related hinge cracks (Photos C & D). In the case of the raised bond beam side of the pool the cracks were completely below the raised bond beam level only in the submerged portion of the pool wall (Photo E). In addition, the contractor measured the pool tile line with a water level and verified that the pool hadn’t rotated or settled1.
Naturally at this point I started asking questions. A lot of times when a pool starts to exhibit problems of unknown cause it is correlated to unusual factors that came into play during construction, for example, the pool was subject to delays in some phase of construction, or the homeowner used some other non-pool subcontractor for some aspect of the project, etc. In this case I asked the builder about the construction history of the pool and sure enough he let me know that the pool had sat in the shotcrete phase, open to the environment, for about 9 months through the hot summer. Well that fact tipped me off to the start of a potential cause. My next question was about waterproofing of this pool. There are two related characteristics of this pool that would make waterproofing of the shell very important, 1) the pool is freestanding on one whole side (meaning water pressure on the inside and nothing on the outside), 2) the pool has a raised bond beam retaining an irrigated planter. Both of these scenarios make waterproofing of the shell of utmost importance to prevent water intrusion, weeping, efflorescence and other common water related phenomenon.
The contractor reported that they did provide waterproofing, specifically ORCO Poly Waterstop with an additional acrylic bond improving admixture. Well that’s a start, but where did they apply it? Turns out they applied it on the freestanding pool wall and then on the back raised bond beam wall but only on the raised portion above the pool waterline. Bingo! Now we had the beginnings of a theory to go on.
Drying Shrinkage Cracks
The first part of our theory as to the causation of these cracks starts with the 9-month long shotcrete cure time. I say 9-month long cure time because concrete is essentially curing forever, however most of the curing happens in the early period after application. Most pools are shot and then cured for a minimum of 7 days before they move on to plaster2. In this specific case the pool was left out in shotcrete for a full 9 months, through the summer, before they were able to proceed with plaster and finally put the pool into service. During this time as the shotcrete continues to cure, moisture evaporates from the concrete and the concrete undergoes a process known as drying shrinkage. Drying shrinkage is the contracting or shrinking of the concrete due to the reduction in volume that occurs with loss of moisture content. This phenomenon is an inherent characteristic of Portland cement-based concrete3. Often times depending on the initial concrete mix, specifically the water content and water/cement ratio, the contraction and shrinkage is so great as to cause cracks in the surface of the concrete. Sometimes these cracks can penetrate completely through the shell. This a common occurrence in wet mix shotcrete, especially in higher water content mixes or where the applicator adds water during application in order to increase the workability of the mix. When these cracks do occur its common to remediate them prior to application of plaster by using a crack isolation membrane or other means. In the case of this pool we do not have confirmation that the pool did experience visible shrinkage cracking prior to plaster, however it is certainly likely that due to the long period that the shell was left in shotcrete that the cracks occurred and were potentially too small to see with the naked eye.
The second part of our theory comes from the high potential for water intrusion into the pool shell. There are two sides to this pool with differing conditions. The raised bond beam side, as we already know, was waterproofed only on the upper section above the pool’s water line, but not down beneath the water line. This was presumably done this way because the applicator believed that the main potential source of water intrusion into the shell was from the raised planter bed behind the raised bond beam. The problem with this thinking is that water, like everything on earth, moves downward with gravity, thus the water that was trapped in the planter bed, unable to weep through the upper waterproofed portion of the shell, simply drained down to the point where it could intrude into the pool shell. And sure enough below the raised bond beam is where where we see the crack (Photo E).
The freestanding side of the pool is another story. At first thought you might ask why this side of the pool would have water intrusion when it was thoroughly waterproofed prior to plaster? Well waterproofing is great and can prevent water from weeping into the shell, but only where the waterproofing is placed correctly. Minor imperfections in the application, pin holes or small gaps at plumbing penetrations, can often allow water to find a path of least resistance to get through the waterproofing and into the shell. In this specific case you may notice one common aspect of the cracks on the freestanding side of the pool. Both cracks are located at plumbing penetrations (Photos C & D)
So we know that the pool had a higher than normal susceptibility for drying shrinkage cracks. We also know that the pool’s configuration allowed for a higher potential for water intrusion into the shell. So what is it about these two facts that leads me to believe that these cracks are the result? Well that brings us to the third and final piece of this puzzle.
The creation of concrete involves a series of chemical reactions using cement and water. The chemical process underlying the hardening of concrete is known as hydration. Hydration is a process in which various precursor chemical compounds in cement form bonds with water molecules and produce chemical by-products. The result of the reaction is a non-homogeneous matrix of the cement byproducts including calcium silicates, along with course and fine aggregates4. What many don’t realize however is this complex chemical process does not necessarily utilize all of the cement precursor chemicals to achieve the final hardened state. What this means is, in the future, if more water is introduced into the hardened concrete, a secondary hydration process can occur in which unused cement precursor compounds undergo hydration while the concrete is already in its hardened state. As this is occurring, the byproducts produced by the hydration process, including a crystalized mineral called Ettringite, can collect inside the concrete substrate5. In and of itself this process is not necessarily problematic, as it’s an unavoidable consequence of concrete use in wet applications, however when this process occurs within a concrete substrate that may already be internally micro fractured from other phenomenon such as drying shrinkage, the result can be a buildup of hydration byproducts that can increase internal stress in the already fractured concrete6. Now that we understand the independent processes and variables at play here, we can put it all together.
The Final Theory
So what happened to this pool? The answer is we don’t know for sure, but what we think happened, by way of our process of elimination and deductive reasoning is this: Because the pool was left in shotcrete phase for many months, drying shrinkage occurred causing small cracks or micro fractures in the concrete. These cracks were likely too small to notice prior to plaster, but just large enough to allow water intrusion, both at the unwaterproofed wall below the raised planter as well as through the penetrations on the freestanding side of the pool. Once water was able to penetrate and weep through the concrete, a secondary hydration process began causing the release of cement byproducts such as ettringite to collect in the areas where cracking or fracturing already occurred. As this process proceeded a sort of feedback loop was created in which the buildup of hydration byproducts caused internal stress which widened the cracks, allowing more water to weep through, until finally the cracks reflected through the surface coating.
So, we have our theory, which is well and good, but how do we get this fixed for the customer? The short answer is, stop the intrusion of water through the pool shell. The shrinkage cracking happened and there is nothing we can do about that now, but the initial cracks were simply the pathway for the water intrusion, which in turn started the secondary hydration process. Simply stopping the water intrusion is the first step to solving the problem. So, what we recommended to the contractor was to strip the surface coating and reapply a secondary coat of waterproofing, at minimum on all vulnerable parts of the shell, but obviously best if the entire interior of the pool can be covered. Special precautions should be taken in and around the penetrations, and at the specific areas where we know water was weeping though. Pool Engineering does not generally recommend specific waterproofing products since new products and their vulnerabilities are constantly popping up, but at a minimum a flexible cementitious product with some ability to bridge minor cracks would be appropriate to use. Once the new waterproofing is applied (with a high level of care and in strict conformance with the manufacturer instructions) the pool can be put back together and put into service.
1. Measuring the pool tile line to determine if a completed pool is level requires that the original installation of the tile was installed perfectly level. Most experienced pool tile installers install pool tile using a water level and are able to achieve levelness to within about 1/16 of an inch.
2. In some applications concrete is left to cure properly for the full 28 days required to reach and verify the design 28 day compressive strength of the concrete.
3. See Pool Engineering, Inc Tech Document on “Drying Shrinkage in Shotcrete Swimming Pools”, www.pooleng.com
6. Stark, J., & Bollmann, K. (2000). Delayed ettringite formation in concrete. NORDIC CONCRETE RESEARCH-PUBLICATIONS-, 23, 4-28.
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