By Gareth Lewis, Technical & Specification Manager, Kerakoll UK
The design of swimming pools varies greatly depending on the intended use: from competition pools which must be rectangular, built to strict tolerances and 25m or 50m long; through hydrotherapy pools to health clubs, leisure centres, spas and private pools where the form is only limited by the imagination and the budget.
However all of these have to take the same criteria into consideration; namely safety, longevity of the finishes, and technical and aesthetic performance.
Every pool facility will have a number of different areas which will require different finishes and applications to ensure compliance with the above mentioned criteria. These are typically the pool itself and the immersed surfaces which have to retain water, wet areas around the pool and in the changing rooms, and other areas that remain dry.
Several materials are used in swimming pool construction, although concrete is the most widespread with steel being used more rarely. This article considers tiling to concrete pools which are built either from site cast concrete using shuttering, sprayed concrete, reinforced blockwork or cavity wall construction with reinforced concrete. When using blocks, the rendering becomes part of the watertight construction. Structural movement joints within the pool shell should be avoided if possible.
Any swimming pool tanks that are not built into the ground, such as deck level or upper storey pools, need particular attention in the design and construction phase to ensure that they are totally watertight with special attention given to the size and the finish of the overflow channel, the width and type of the beach area. It is also imperative to avoid cut tiles.
Concrete swimming pool shells constructed in-situ should be allowed to dry for at least 6 weeks before either direct fixing of the tiles or the application of renders and screeds.
Before applying any finish to the pool, it must be checked for water tightness in accordance with British Standard 8007:1987. This involves filling the pool with water and leaving it for 7 days, then checking that the water level does not drop more than 1/500th of the average water depth of the full tank, 10mm or another specified amount, such as the SPATA Standard of 12mm once evaporation has been taken into account.
A high degree of dimensional accuracy is required for pools with a moving floor and competition pools, where the standards state a maximum deviation of 30mm over the length of the pool. This means that, once sufficient time has been allowed for the concrete to shrink and dry and any surface residues removed, tiles can be fixed directly onto the concrete surfaces using a thin solid bed without the need to apply renders or screeds to the shell.
This method of fixing is the preferred one for swimming pools as it reduces the number of layers and interfaces involved and, hence, reduces the risk of problems arising. However the maximum gap permitted under a 2m straightedge is 3mm, and the difficulty of achieving this degree of accuracy means that screeds and renders are often used to provide a finish that is suitable for tiling.
Screeds: Only bonded screeds applied once the concrete shell is properly cured and dry should be used. The screed should be laid in accordance with the manufacturer’s instructions with any contaminants and laitance removed from the substrate before application.
Screeds are not intended to be structural components and should ideally not be in direct contact with water. Where this is impossible, polymer-modified screeds should be considered. They should be laid in as large as possible areas with construction joints kept to a minimum and carefully placed to coincide with expansion joints or other features. The surface finish should be specified to be wood float finish to a Class SR1 surface regularity of 3mm under a 3m straightedge.
Most screeds need to be cured by covering with an impermeable membrane for at least 7 days before being allowed to dry out for 2 weeks. This reduces the risk of cracking and debonding. For time-critical projects screeds such as Kerakoll’s Keracem Eco can be used. This is ready for tiling after approximately 24 hours depending on site conditions, thus avoiding the long delays associated with traditional screeds.
Rendering/levelling: Where the surface is not sufficiently smooth or level to allow tiling to take place a concrete render or levelling compound is used to prepare it for tiling. This can be instead of or in addition to a screed. The final surface specification should be 3mm under a 1.8m straightedge. If the render is being applied over blockwork, the tolerances should be built into the specification for the blockwork, as this is more accurate than is generally required in traditional construction.
Kerakoll’s Keralevel Eco LR allows you to smooth and level the walls and floors of pool tanks for thicknesses between 1mm and 25mm. It has a long working time, so it can be used for large surface areas such as a pool and is ready for tiling after a couple of hours.
The floors and walls in the rest of the pool facility should be prepared to the tiling specification: that is 3mm under a 2m straightedge. Areas subject to water splash should be laid to a fall to allow adequate drainage of any water that has accumulated and Keralevel Eco LR is perfect for achieving this.
Sealing the hydraulic and electrical systems
All hydraulic and electrical parts that come into contact with water have to be waterproofed. This can be achieved with a sealant such as Kerakoll’s Nanosil Eco, a neutral, non-corrosive silane sealant with a reduced solvent content that comes ready to use.
Waterproofing of render/levelling compounds
Where the levelling compound or render is not suitable for direct contact with water, the pool tank should be waterproofed before tiling. This is done by coating the tank surfaces with a waterproofing solution, such as Kerakoll’s Aquastop Nanoflex; a water-repellent membrane which uses nanotechnology. Then waterproofing tape is applied to all joints and a special reinforcing mesh, such as Aquastop AR1, laid over the whole installation, and the tape bedded into the waterproofing membrane. After this, a second coat of the waterproofing membrane is applied.
Waterproofing should be carried out to all areas that will be in direct contact with water including the area around the pool, foot baths, changing rooms, etc. and should continue up to the walls of the room in which the pool is situated.
Tiles should have a water absorption of 3% or less and be of extruded or dust pressed manufacture. They should have adequate slip resistance in wet areas and particularly on steps and ramps. Tiles for pools used for scuba diving and canoeing should have a higher modulus of rupture due to the higher risk of damage. A tiling specialist can advise on the suitability of particular tiles.
Adhesive and grout specification
The most important criterion when choosing adhesives and grouts, especially for the pool tank, is resistance to chemicals in the pool water. This is influenced by various factors including the quality of the mains water and the chemicals that are used to treat it. Pools with low levels of bicarbonate alkalinity and low calcium content can be aggressive to cementitious mortars and grouts, as can water that is too acidic or alkali.
To avoid any possible problems, resin-based grouts are becoming increasingly specified as they are not affected by these conditions, although they are more expensive to buy.
Adhesive specification: For most areas within a swimming pool installation the specification of polymer modified cementitious adhesives is recommended. Biogel No Limits from Kerakoll is a new type of gel adhesive. It makes the choice of adhesive simple as it is suitable for all substrates and most types of tile so it can be used for both the swimming pool shell and all other areas of a swimming pool facility. Durability tests show that it performs to ≥ 1 N/mm² after water immersion.
In certain circumstances, where the water is known to be particularly aggressive, an R2 reaction resin-type product is advised. Kerakoll’s Biogel Extreme is the perfect adhesive for the job. It is a hybrid gel adhesive with good resistance to acids and is also recommended for use in pools with thermal water and aggressive chemicals.
Its shear adhesion after immersion in chlorine water is ≥ 3.5 N/mm². It can also be used in other areas of the facility as it is also suitable for all substrates and types of tile.
Where thin mosaic tiles (particularly glass) are used, the grout may not be thick enough to protect the adhesive and it will therefore need to be able to withstand aggressive water conditions.
Fugalite Bio, a water-based resin grout that can also be used as an adhesive for glass mosaic tiles, should be specified for use with these tiles.
Grout specification: Cementitious grouts are being specified less and less for use within swimming pool shells and the areas around them. The reason for this is that the chemicals in the water can attack the grout under certain conditions and it is more likely to be damaged by intensive cleaning or high levels of abrasion. It is therefore considered safer to use reaction resin grouts as they stand up to such conditions.
Kerakoll’s Fugalite Bio range of grouts are a new generation of water-based reaction resin grouts. They are tough, waterproof and resistant to UV rays meaning that they will not fade over time even when exposed to sunlight in outdoor pools. They are also tested hypo-allergenic, making them more pleasant to work with than normal epoxy-resin grouts.
There are two types of joints used in swimming pools: stress relief joints which extend through the tiles and screed and/or render, and structural joints which should be placed in line with the pool structural joints and continue through the screed, render and tiles. The location of the joints can be crucial as some areas of the pool may be more likely to experience phenomena such as water turbulence leading to a higher risk of damage to the sealant.
The depth of any joints should be controlled using a compressible joint filler such as a closed cell foam polyethylene backing or a polythene bond breaker tape in shallower joints. For deep diving pools where the hydrostatic pressure is greater a denser backing may be needed to support the load. Stress relieving joints should normally be a minimum of 6mm wide x 6mm deep. Structural joints need to be at least 12mm deep and at least as deep as they are wide and should be supported by joint backing material.
The sealant in joints can be attacked by the chemicals in the water, water movement and strong cleaning materials.
Two main types of sealant are normally specified: two-part modified epoxy resin types are generally quicker curing than the polysulphide alternatives, but can only be used in stress relieving joints.
The other type are two-part polysulphide sealants of the sort that offer resistance to the chemicals within the pool and can also accommodate both tensile and compressive movement and resist foot traffic and hydrostatic pressure. These sealants generally require a primer system to be applied before they are used.
Kerakoll’s Fugabella Eco Silicone Eco differs from both of these types as it is single component so is much quicker and easier to use. The colours in the sealant are very stable and do not fade even when exposed to chemicals and UV light.
Curing and drying times
It is imperative that the concrete shell should be allowed to dry properly and this will take at least 6 weeks even for smaller, thinner walled pools. Specialist advice should be sought as to the time frames involved.
Screeds and renders also need to cure and dry and again the manufacturer’s instructions should be followed to ensure adequate time is allowed to avoid stresses and cracks.
The time frame between tiling and grouting will depend very much on the adhesive used. Traditional cementitious adhesives will require at least 3 days, but this can be shortened by using rapid setting or epoxide adhesives. H40 Eco Tenaflex is ready for grouting after 1 hours on walls and 8 hours on floors and Superflex Eco after 12 hours on walls and 24 hours on floors.
Grouts should be allowed to cure before sealant is applied, this normally takes 3 days.
It is recommended that the finished installation should be left to stand for at least three weeks before the pool is filled. This is to allow the sealants to cure. The curing time for sealants can be 48 hours for some epoxides and at least 21 days for polysulphides. However Fugabella Eco Silicone only takes about 20 minutes to cure under normal conditions.
If in doubt seek technical advice on the best products to specify for your project. Kerakoll’s team is always on hand to give advice and is frequently involved in the specification process to ensure that the right products are chosen to enable the project to run smoothly and be fit for purpose when complete, please do contact them if you have any questions.
Kerakoll UK Ltd
Unit 4, The Croft
T: 01527 578 000