Section 3 – Types of Pool Construction
3.1 Poured Concrete
One type of in ground pool construction type is cast-in-place, reinforced concrete; in other words, concrete is poured around steel reinforcing (rebar) inside wooden formwork. Concrete placed or pumped on-site over steel reinforcing with floors and walls, contained by formwork, is generally used in large commercial pools, elevated pools or on-grade pools where poor sub-soil conditions exist.
Pool bottoms and walls should be cast monolithically in order to avoid cracking and reduce the need for extra movement joints. In poured concrete pool construction a mold is created using wood or plywood which will encase the finished pool shape. High-density concrete, specifically designed for pool applications, is then poured into the forms. A vibrating tool is used to make sure that the concrete fills all of the cavities and completely encapsulates all of the reinforcing steel. Once the concrete has set, the forms can be removed and the concrete allowed to reach full cure.
Typically, this form of construction requires a greater level of surface preparation for a tile installation than a pneumatically applied concrete (e.g. Gunite or Shot-crete). Vertically formed surfaces are especially prone to thin layers of weakened portland cement (laitance). Care should be taken to ensure that the concrete surface has a proper screed finish and no laitance or other condition that may inhibit bonding of a waterproofing membrane or tile thin-set. Any type of form release agent, curing compound, sealer, or other contaminant must be removed prior to the direct adhesion of any tile or stone. The use of concrete additives, such as super plasticizers allow for low water-to-cement ratios but can induce bleed water and increase the possible occurrence of laitance. The concrete should also not be treated with a crystalline type waterproofing membrane since this type of material closes the pore structure of the concrete to inhibit water from moving through the capillaries. A crystalline type waterproofing membrane essentially seals the concrete and acts as a bond inhibitor for tile thin-sets or epoxy adhesives.
Cast-in-place concrete is more likely to experience shrinkage cracks after placement so proper placement and installation of reinforcing steel will help to reduce the occurrence of this type of cracking. The proper placement of a waterproofing/anti-fracture membrane (e.g. LATICRETE® 9235 Waterproofing Membrane or LATICRETE Hydro Ban™) will prevent any shrinkage cracks from transmitting through to the tile layer. It is important to note that waterproofing membrane, used in a submerged installation, must cover the entire inside shell of the pool or fountain.
There are certain considerations which must be taken into account before the installation of a membrane or tile/stone can commence;
Type of Concrete
Floor Flatness
Concrete Curing
Cracks
Contamination
Type of Concrete – Concrete used in swimming pools and continuous submersion projects should be specially designed for this type of application. The concrete should be mixed to provide a high density, low porosity finished product and not have a water to cement ratio greater than 0.48. The concrete shell for a salt water pool or fountain should be poured using sulfate-resisting cement. The use of this type of concrete will help to protect the structural, reinforcing steel in the pool shell and prevent serious problems in the future.
Floor Flatness – Typical horizontal concrete applications must be finished to a floor flatness (FF) of 25 or greater. A floor flatness of 25 roughly correlates to 1/4" in 10' (6 mm in 3 m). Measurement of Ff is done using ASTM E1155 “Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers” and American Concrete Institute (ACI) “Standard Specification for Tolerances for Concrete Construction and Materials”.
Age of Concrete – Concrete pool shells must be cured for a minimum of 28 days and a minimum time interval should be allowed between each successive stage of the project to ensure full cure. These time intervals will vary by temperature and weather conditions, and should only be reduced if an adequate design solution is provided.
Cracking – Any cracks in the concrete shell should be effectively repaired prior to the application of any finish material on the sides and bottom of the pool or fountain and on the pool deck. The use of a superior waterproofing/anti-fracture membrane (e.g. LATICRETE Hydro Ban or LATICRETE 9235 Waterproofing Membrane) will provide protection not only against cracks in the concrete shell from transmitting through the tile installation, but also provide waterproofing as well. This keeps the water in the pool where it belongs!
Contamination – Keep in mind that poured concrete pool or fountain shells utilize wood forms during the construction process. In order to remove these forms after the concrete has set the installers spray the inside of the forms with either old motor oil, diesel fuel or a proprietary material that helps inhibit bond. Unfortunately, some of this form release agent may be left on the concrete surface as residue. As effective as these materials are at preventing bond of the concrete to the wood forms, they can be equally effective at inhibiting bond of a screed, membrane, tile, stone, or other finish coating to the concrete surface.1 The proper removal of these contaminants and subsequent surface preparation will be covered in greater depth in Section 6.
Refer to Section 4.5 for more information on critical design elements (e.g. water stops, vapor retarders, hydrostatic relief valves, etc…) which are required to be installed or planned for prior to or during installation of the concrete.
3.2 Gunite/Shot-crete
The construction of a pool or fountain using the Gunite or Shot-crete installation method begins with the forming of the shell using wood or plywood forms. These forms are made larger than the actual, finished pool size by the thickness of the walls and must be substantial in construction. The area inside the forms is then lined with #3 (0.375" [9.5 mm]) steel reinforcing, usually on one foot (300 mm) centers (as noted in NSPI construction manuals or as dictated by local building code). The steel reinforcing is suspended away from the formwork by a device which keeps the reinforcing steel centered in the completed wall assembly, and away from the bottom of the excavation.
The pool recirculating system, and any other integral mechanics, is allowed for in the finished walls and floor of the pool or fountain prior to the installation of the Gunite or Shot-crete.
While Shot-crete and Gunite are pneumatically applied concrete, they both have differences in the way that they are mixed. In Gunite installations the “mix” is delivered down a hose as a dry mix under pressure and terminates at the spray head. As the dry mix leaves the spray head it mixes with water, which is being delivered under pressure through another hose attached to the dry hose, and is sprayed onto the wood forms and around the reinforced steel. The Gunite dry powder is a combination of portland cement, sand, fly ash (in many cases), and possibly some very small pebbles. In conditions that require faster setting of the concrete, some calcium or a proprietary material, designed for the purpose, is added to accelerate the curing.
The powder and water must be mixed to a critical ratio or the resulting concrete can lack strength. Therefore, the Gunite installer must be fully experienced with the installation process and have complete control over the amount of water being used. Once the concrete has been placed in the pool, the interior of the pool is “straightened” by a team who use steel trowels and forms to float the wall to the proper finish.
Figure 3.1 – Gunite being sprayed and finished in a residential pool.
Shot-crete, while also pneumatically applied, is slightly different than Gunite. Shot-crete is mixed at the concrete production facility and delivered to the job site in a cement truck already mixed. The quality of the concrete is determined at the ready mix plant so the nozzle operator has no influence over the final strength of the concrete. Shot-crete pools tend to use less concrete than Gunite pools but can provide superior strength per volume.
Both methods of applying the concrete need special attention to the application, as the reinforcing steel may provide a barrier that can lead to cavities forming behind the re-bar. Rebound, or gravel in the mix rebounding off the wood forms or steel reinforcing, may also occur and can lead to a differential strength of the concrete. Whether Gunite or Shot-crete, both nozzle operators must be experienced and aware of the potential strength issues if the concrete is not placed as required by the pool’s designer or engineer. Once the concrete has set for a minimum of 14 days at 70°F (21°C) the form work may be removed and the exterior side of the pool wall is filled with a suitable drainage material.2
The design considerations for waterproofing and ceramic tile or stone installation over a Gunite or Shot-crete pool are:
1. Allows for the easier construction of continuous monolithic wall and floor elements characterized by a generally mandatory cove. This may make it difficult to install larger format tile or stone.
2. Reduces or eliminates movement joints in the pool shell. Typically, Gunite pools do not require movement joints in the shell because the tendency for cracking from thermal movement is reduced. However, movement joints in the tile surface are required to accommodate thermal and moisture movement of pool tile when the pool is emptied for maintenance.
3. Gunite and Shot-crete pools generally require less preparation for subsequent finishes due to the spray on surface texture providing mechanical keys (for the thin-set), and also have no laitance.
4. Lower water to cement ratio also means less susceptibility to drying shrinkage cracks.
5. Greater freedom of pool structure shapes is achievable over traditional formed structures and, as such, can be more difficult to produce precision tile or stone finishes.
6. Generally requires a screed or render for subsequent applications of membrane, tile or stone.3
Tile can be installed over a concrete shell by following methods; ES-P601 and ES-P601D (found in Section 9 – Specifications and Section 10 – Details, as well as at www.laticrete.com/ag).
3.3 Concrete Masonry Units
While concrete block swimming pool construction is used infrequently, it is more often used as a fish pond or water feature building technique and is a viable alternative to more costly and expensive construction methods. The depth of concrete block construction is typically no deeper than 3' (1 m). In this type of pool or fountain construction, the bottom of the pool is created by pouring concrete around steel reinforcing to the outside dimensions of the pool footprint. The concrete is allowed to cure for a specified time and then steel reinforcing is constructed for the concrete block walls. The concrete block is installed over the steel reinforcing using a high strength masonry mortar and allowed to cure. The block is typically filled with concrete to create a solid structure for the pool.
Once the shell is completed, the pool may have a mortar bed (e.g. LATICRETE® 3701 Fortified Mortar Bed; or, LATICRETE 226 Thick Bed Mortar mixed with LATICRETE 3701 Mortar Admix) installed over the block wall and concrete floor, waterproofed with a high quality waterproofing membrane (e.g. LATICRETE 9235 Waterproofing Membrane or LATICRETE Hydro Ban™), and then tile is installed with a high quality thin-set (e.g. LATICRETE 254 Platinum) designed to be used in submerged installations. The use of a superb, stain and chemical-resistant grout (e.g. LATICRETE SpectraLOCK® PRO Grout†) and 100%silicone sealant (e.g. LATICRETE Latasil™) will finish this tiled pool installation. A waterproofing membrane and tile or stone can also be installed directly to the floors and walls without a mortar bed (refer to ES-P601D found in Section 9 – Specifications and Section – 10 Details, as well as at www.laticrete.com/ag. Other finishes include spray-on, fiberglass strand reinforced coatings, vinyl liners, renders created from mixture of white cement and finely ground white marble dust (i.e. Marbleite or Marcite), or even painted.
Check with local building code for limitations or allowance to see if this type of pool or fountain construction is acceptable and what limitations may be in place.
3.4 Steel Shell
Stainless steel is fast becoming a widely used material for the construction of spas, pools and fountains. Stainless steel is an ideal material for spas, pools and fountains because it is durable, strong, chemical-resistant, moisture-resistant and stain-resistant. Stainless steel will not crack, blister, delaminate, or lose strength over time and can be fabricated into any size and shape. It can literally last forever!
The manufacturer of the pool or spa cuts high grade stainless steel panels, to the desired sizes and shapes, in a specific pattern. Around a steel frame the pieces of stainless steel are welded together with a material that will inhibit rusting or corrosion at the seams, thus forming the completed pool. The proper welding process would depend upon the exact type (austenitic, ferritic, martensitic, or duplex) and grade of stainless steel. Holes and other penetrations are cut into the steel to allow for filtration, circulation, lights, water jets, and drains at specific points. The plumbing, lights and other equipment may also be mounted prior to shipping.
Figure 3.2 – Tiled steel pool shell.
The spa or pool is then transported to the installation site and permanently installed. While the stainless steel finish is easy to maintain it may not be aesthetically pleasing to the owner. With this possibility in mind, tile is an excellent way to create a unique and functional design alternative. Tile can be installed over a steel shell by following two methods; ES-P602 and ESP603 (both found in Section 9 – Specifications and Section 10 – Details as well as at www.laticrete.com/ag) and can be done at the factory or after the spa or pool is installed.
3.5 Fiberglass Shell
Fiberglass is the last type of pool construction that we will mention. Fiberglass is a sprayed-on resin material that provides a smooth and durable surface. Fiberglass pools are manufactured in a factory, to a standard size and shape, and then shipped to site on a truck. A crane is required to place the pool into the pre-excavated ground (often lifted directly over a house), the plumbing is then connected, and tile is installed at and above the water line to protect against oxidation. Once the tile mortar has cured the pool is filled with water as it is back-filled against the pool shell. The top edge of the pool is typically covered with concrete to protect it and help prevent oxidation in this critical area.
Fiberglass pools are the least common type of pool structures because size and shape are determined at the factory, the size of the pool is limited to what will fit on a truck, and the initial cost is higher than that of a concrete pool.
Figure 3.3 – A fiberglass pool shell waiting installation of tile.
The fiberglass shell must be rigid and meet minimum deflection ratings of L/360 for tile and L/480 for stone. Tile is installed using a 100% solids epoxy (e.g. LATAPOXY® 300 Adhesive) for maximum bond to the fiberglass shell, and grouted with a high quality grouting material (e.g. LATICRETE® SpectraLOCK® PRO Grout†). Tile can be installed over a rigid fiberglass shell by following method; ES-P604 (found in Section 9 – Specifications and Section 10 – Details as well as at www.laticrete.com/ag).
1 Gray, Fred (2006). Correctly Installing Tile in Swimming Pools. Footscray West, Victoria, Australia: SPLASH EXPO Conrad Jupiters.
2 http://www.homehelp4u.net/services/concrete_pool_construction.php
3 Gray, Fred (2006). Correctly Installing Tile in Swimming Pools. Footscray West, Victoria, Australia: SPLASH EXPO Conrad Jupiters.
† United States Invention Patent No.: 6881768 (and other Patents).