The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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PoolQ I have a question regarding shotcrete pools. Does the ASA have a position on how to detail reinforcement at bulky elements that are shot interior to the main pool shell? This would typically involve stairs or large stoops. I notice a lot of contractors shoot these as unreinforced bulk elements, but this practice appears to promote cracking at the face of the pool shell. I’m only asking because I saw a few of these this past summer.
Shotcrete is a placement method for concrete. Thus, any concrete structure using shotcrete placement should be designed using appropriate concrete design codes and standards. ASA does not have a published position on reinforcement of these types of pool elements though we are in the midst of developing one. As most concrete experiences drying shrinkage and associated cracking, most designers would include some level of reinforcement in these types of sections to control tensile stresses from shrinkage and temperature changes creating volume change in the concrete. ACI 350 (Code Requirements for Environmental Engineering Concrete Structures) is the ACI Code that deals with concrete liquid-containing structures with provisions specifically for providing liquid tightness and durability in continuously wet environments. The ACI 350 Code requires up to 0.5% reinforcement for shrinkage and temperature stresses. ACI 318 is the Concrete Code for Structures and requires 0.18% minimum reinforcement for shrinkage and temperature. Designers may choose to use the lower ACI 318 value since they consider the benches and steps not part of the water retaining pool shell. Other designers would consider the higher ACI 350 values as they are interested in better crack control. Overall, having a substantially unreinforced thickness of concrete would lead to more cracking that would be problematic in the pool.PoolQ I have a special request for a shotcrete mix design. My company has been using shotcrete for about three years, here in Alaska. I have recently had a request to shotcrete a 60’x50′ duck pond to make it waterproof. The problems I am running into are that moose keep walking into the pond, and the pond is on the side of a hill with built up edges around the outside. The mix design I am looking for needs to have an epoxy or some kind of adhesive to help stop the water from running out the cracks. Last, are there any fabric or plastic materials that I could lay down and spray the wet shotcrete on to put on the sides of the pond?
This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential!
PoolQ I have a swimming pool that appears to have shrinkage cracks in the floor. I have tried to inject an epoxy, but the cracks are too small. Do you have any suggestions?
There are a wide variety of epoxies and polyurethanes used for crack injection. Smaller crack widths would require a lower-viscosity material to penetrate the crack. You should contact an engineer or injection specialist experienced in shotcrete and cracking issues to evaluate the cracking and make a specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and curing can help to reduce plastic shrinkage and drying shrinkage cracking in the future.
PoolQ I have been in the swimming pool industry for 30 years and I deal with a lot of different engineers on my commercial projects who want a wet test to verify water tightness before the finish is applied to the pool. In my experience, air-entrained shotcrete tends to be porous and leak. Are there any engineering specifications that state that air-entrained shotcrete is porous and will leak if the surface is not trowel-finished?
Properly added and mixed air-entraining admixture in concrete will actually reduce the permeability of concrete. This is because the small, well-formed air bubbles from air-entraining admixtures are not interconnected as larger, entrapped air bubbles may be in non-air-entrained concrete. Thus, the reported higher permeability of the air-entrained shotcrete is not a material flaw but must be from poor shotcrete application. Air entraining from 4 to 7% air is advantageous for enhanced resistance to the freezing-and-thawing cycles of saturated concrete and should be specified by the designer in areas subject to significant numbers of freezing-and-thawing cycles annually. The reported high permeability and resultant failure to pass a water-tightness test could be investigated by taking cores of the “porous” material and conducting a petrographic analysis of the core. Based on the reported results, I strongly suspect that the in-place shotcrete has major issues with sand pockets, overspray, and rebound.
PoolQ I have used ASA’s Position Statements from the Pool and Recreational Committee and find them very useful. Are there any design guides or books on shotcrete pool design that are available? I am a structural engineer and tend to design pools as retaining walls, but I believe some of my designs could be “value engineered” to reduce rebar in the case of walls with a vertical curve (base of the wall is curved and not straight) and possibly the use of a bond beam at the top.
The current International Swimming Pool and Spa Code (ISPSC) has no provisions for design of concrete pool shells. Many structural engineers use ACI 350 Code Requirements for Environmental Engineering Concrete Structures for pool structures, especially commercial pools. ACI 350 is based on ACI 318 Building Code Requirements for Structural Concrete but has modifications to provide a design for concrete structures that are normally exposed to water, and thus need more crack control for watertightness. ACI 350 also addresses requirements for durability for concrete exposed to liquid so that structures will be expected to be serviceable for at least 50 to 100 yrs. Some engineers feel ACI 350 is overkill for pools and may use ACI 318, or just use their past experience. Generally, use of ACI 350 will require a higher percentage of reinforcing steel, have closer steel spacing and somewhat reduced tension in bars to control cracking. ACI 350’s concrete cover provisions may also be somewhat higher than ACI 318, to provide more corrosion protection of reinforcing. ACI Committee 506 is developing a guide document for construction of shotcrete pools, but does not directly address design. ACI has recently authorized a new technical committee to develop a Code for Design of Pools and Watershapes. However, staffing the committee and then developing a consensus standardized document will take several years.
Regarding the cove of the floor-wall joint, if you have a cove or the bond beam you can use the additional “d” distance for your vertical steel from external loads on the walls though the moment has to be carried in the thinner sections of the floor and wall adjacent to the cove. If you consider the bond beam is a stiffening element for the top of a straight wall acting as a panel between the ends in a rectangular plan pool you may be able to reduce some of the vertical bending stresses. However, that may be hard to quantify for a freeform pool without a more advanced analysis.
Regarding reference books you may find David Billington’s Thin Shell Concrete Shells useful for analysis and design of concrete shells. It is an old book (1982 for 2nd Edition) and may be hard to find but may be helpful. Hopefully ACI’s new Code Committee for Pools will set the standard of practice in the pool industry and be specifically referenced by ISPSC.
PoolQ I just had a swimming pool built and everything I have read online says that the gunite shell has to be sprinkled with water for several days after the pour. My pool builder says they never do it, and, when I asked why, I’m just told that’s the way they do it. I told them I am worried because every other pool builder says to do that except the one I hired and I can get no answer as to why. Is this an acceptable practice? I am worried that years down the line I may have a problem. I live in Oviedo, FL, and the weather has been in the low 70s and the humidity not particularly high. They did hit the water table and have a pump running—would any of this have an impact?
ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI.
PoolQ I modified an existing pool and had a new 20 ft (6 m) wall built that was subsequently back filled. The rebar was epoxied and tied into existing pool wall/floor. The wall is 4.5 ft to 6 ft (1.3 to 1.8 m) tall. 60 days later we have two vertical hairline cracks that run top to bottom. I watered the wall properly and there are no cracks in the other sections we shot (spa, etc). We backfilled 12 days after the wall was shot with hand equipment only. The sample test taken when shooting came back at 6500 psi (4.1 MPa). The original pool bottom is below the wall and has no issues. The wall appears to be 12 to 14 in. (300 to 350 mm) thick from top to bottom. My question is if the wall was shot too thick would the lack of additional rebar cause the wall to fail? And is the necessary course of action to demo the entire wall and reinforce the rebar, then shotcrete again?
There are many variables that can cause cracking. Vertical cracking is often the result of drying shrinkage of the concrete. You said you cured (watered) the wall properly, but don’t give any specifics. ASA recommends a minimum of 7 days of curing, with a wet cure preferred over a spray-applied membrane. You should have a licensed engineer evaluate the structural sections, and determine if there were any problems with the amount or placement of reinforcing in your wall section.
PoolQ I place shotcrete and I use the wet-mix method. I have been asked by a contractor to repair a pool that was shot with the dry-mix method. He is having trouble convincing his client that with the proper preparation we can shoot the repair with the wet method. Am I missing anything?
Shotcrete is a placement method for concrete. Both wet-mix and dry-mix produce quality in-place concrete when using quality materials and proper equipment and placement techniques. There are no compatibility problems with shooting wet-mix over dry-mix. Basically, it is just shooting shotcrete on top of already placed concrete. For proper bond, the surface of the existing dry-mix must be roughened, cleaned, and brought to a saturated surface-dry moisture condition before shooting the wet-mix lining.
PoolQ I recently hired a pool contractor to build a residential pool. The contractor has been in business for more than 30 years and has a great reputation. The progress so far is that the pool has been installed using shotcrete. The shotcrete has been curing for the last 9 days. Within the last 9 days, it has rained heavily twice. On the second rainy day, immediately after the rain finished, I walked outside to see the amount of water that had collected inside the pool. I noticed that the water was muddy looking. Upon closer inspection, as the rest of the pool was dry, except for the deep end, there were two trails of water coming from the shallow end and running into the pool of water in the deep end. After getting down into the pool, I noticed that these trails of water were from water bubbling up through the shotcrete floor on the slope closer to the shallow end. The bubbling was like a small stream of water coming up out of the shotcrete in two places. I suppose it is from the hydrostatic pressure from the groundwater under the concrete? My question is should this be concerning? The plaster has not been installed. How should these holes be filled? The holes certainly do not look like they were intentional, as you can’t really even see them, except for the water coming out of them. Is there a problem with the shotcrete installation? Does this mean that my pool will leak when it is filled with water? I would image that if water can come up through the shotcrete, the water can also go down through the shotcrete, resulting in erosion of the soil under the pool? Before the shotcrete was installed, there was no groundwater present and the dirt was dry. (Referenced images can be found here)
It is difficult to make an assessment of a situation like this with a few photos and the description given. Based on your statement that the contractor has an excellent 30-year reputation, we’d suggest you discuss these concerns with the contractor. If his response does not give you a full explanation, we suggest you engage an independent professional familiar with shotcrete installations and swimming pools to give you an opinion. You can use ASA’s Buyers Guide at shotcrete.org to find a consultant.
There certainly is reason to question the quality of the installation based on the description. However, it is not unusual for the pool shell contractor to leave a temporary opening in the shell to relive potential groundwater pressure and prevent floating of the empty pool shell. These holes are, or should be done, in a professional manner to allow complete watertight sealing when filled. In some cases, the openings may include a pressure relief valve.PoolStructuralQ I was taught in engineering courses that conventional concrete should not be counted on to carry tensile stress. For steel reinforced concrete, the reinforcing bar is designed to carry all tensile loads. Although concrete obviously has some tensile strength, it is too low and prone to cracking failure to consider it in design. In fact, I believe you can assume it is cracked from the shrinkage during curing. Is gunite treated the same way? I have a pool that is developing a crack through an elevated wall/beam and down into the plaster to the bottom floor at the sun shelf. I witnessed the plumbers cutting some rebar in the beam to allow for PVC plumbing to water sheer (up at top of beam, just under the tile topping) and I worry this is the root cause along with settlement that put the top of the beam in tension. The rebar down low should be intact and I hope the crack width may stay minor down in the plaster. On top of the tiles beam where the maximum tensile stress would have been, the crack is fairly wide. The crack movement opened up a gap in the grout line between tiles of about 0.08 to 0.10 in. (2-2.5 mm). I think it was a real sin for them to have cut the rebar. If it is necessary to reinforce the tensile side to halt future movement, I would think cutting a slot or two in the gunite across the crack (say 12 in. [300 mm] each side. Up high just under the water sheer) and epoxy a rebar in the slots.
Shotcrete, both dry-mix (gunite) and wet-mix are a placement method for concrete. Wet-mix uses premixed concrete while dry-mix simply adds water to the concrete materials at the nozzle. Both dry-mix and wet-mix with proper materials, equipment, and placement with produce quality concrete sections. The embedded reinforcement in the pool shell is designed to carry tensile loads. This may be bending stresses from structural loadings (settlement or water/backfill), or volume changes from drying shrinkage and temperature changes. Cutting a reinforcing bar would certainly negate its ability to carry loads in the vicinity of the cut and reduce the load carrying capacity until the development length allows the reinforcing bar to start carrying it full load.
The layout of your cracked section isn’t clear from your description. An 8 to 10 mil (2 to 2.5 mm) crack is sizable in a water-containing structure. Fixing the existing crack with a reinforcing bar epoxied in place across the crack may be effective. However, that solution would only carry any additional load on the section (structural or volume change), as the existing loads have already created cracks. Thus, you should also address filling the crack as part of the solution. This may be with epoxy injection or swellable polyurethane grouts. You should consult with the pool design engineer for their recommendation on the best method for repair.