published in sb 6/2017
The 2017 FSB and aquanale trade fairs displayed some of the many and different options for constructing swimming pool tanks. There is now a new dimension to consider – pool tank transparency. While pool-tank viewing windows have been used in public pools for many years, recent innovations now facilitate whole pool tank sections being built out of transparent material.
In common, all pool tanks must support and contain the water within whilst preventing water loss or ingress and have the appropriate finish. For public sector projects, robust and, ideally, recognised pool tank systems should be used. Key options include:
- Water-retaining reinforced concrete corresponding to EN 1992 Part 3; this is inherently water-retaining and provides a robust structure with a long service life;
- non-water-retaining reinforced concrete corresponding to EN 1992 Part 1-1 with applied proprietary screeds, renders, membranes or coatings to retain the pool water;
- stainless steel walls with PVC liner, bolted back frame and concrete or compacted base; relies 100% on the applied membrane to retain the pool water;
- stainless steel with brushed finish panels, usually type 316L, 2 mm to 3 mm thick, welded together; wall panels incorporate a back frame to provide rigidity.
Determining which method of pool tank construction to use can be difficult. There are many design issues to take into consideration and these include ground/support conditions, speed of construction, longevity, risk, cost, pool profile, pool access, pool edge details, critical dimensions, insulation, hygiene, aesthetics, slip resistance and lighting. In addition to the above, there is now a new dimension to consider – pool tank transparency.
Glass and acrylic
There are various reasons why transparency may be required in pool tanks:
- underwater windows or walls for swim training;
- underwater windows or walls for hydrotherapy;
- light transmission to parts of the building;
- an architectural statement that makes the pool special compared to other pools.
There are two materials to consider when evaluating transparency or glazing needs – glass and acrylic. Both are popular materials and while glass has been in use the longest, advances in acrylic technology have delivered a more versatile product.
Where glass is used, it is normally constructed of fully tempered and laminated panes in order to provide the required structural characteristics. If tempered glass should break it will shatter into tiny pieces much like the glass on a car windscreen. Hence, laminated layers of tempered glass are required so that in the event of a failure of a layer, there are redundant or back up layers to hold the pane together. On a pool where the pane is part of the water retaining structure, the window should be designed such that if one layer of glass should fail, the remaining layers will resist the hydrostatic load. Non-laminated glass should never be used on water retaining structures.
However, for a number of reasons as set out below, on the larger installations glass is largely being replaced by acrylic:
- acrylic is a more versatile material than glass; the current technology and experience of bonding, thermoforming and annealing means that large panels of various shapes can be formed without visible joints; standard panel sizes of 2.7 m x 7.0 m can be bonded together;
- acrylic is significantly stronger than glass – up to 17 times - and typically 50 % lighter than normal glass; as acrylic is a plastic it is much more forgiving than glass; with glass the possibility of breakage due to flaws in the material needs to be carefully managed; plastic like acrylic is more predictable in its performance;
- acrylic is highly transparent, easy to maintain and resistant to weathering;
- acrylic can be manufactured with a very high light transmission of up to 92 % of all visible light; this is better than glass which is typically between 80 to 90 %;
- both acrylic and glass will scratch, acrylic easier than glass; however acrylic is easier to repair if scratched by polishing out any marks; this is not feasible with glass;
- acrylic has a much lower thermal conductivity than glass, of 0.19 versus 0.96, and hence a lower level of heat transmission.
Glass pool at Sidworth Street, London (UK)
The fully glazed floor of the pool on the roof of an apartment building in Sidworth Street, London, allows natural daylight into the heart of the building and provides for a dramatic entrance. The pool is 13 m x 2.8 m x 1.2 m deep, incorporates a Wiesbaden edge and a balance tank and is located on the sixth floor of the building. The walls of the pool are constructed from non-water retaining reinforced concrete and the entire floor is constructed from glass. The glass floor consists of four sheet laminates, 2 x 19 mm thick and 2 x 8 mm thick, of Dupont’s Sentryglas ionoplast interlayers – a system also used for structural glass flooring, stairways and pedestrian bridges. The laminates are supported on and water sealed to a full-perimeter 316-grade stainless steel support edge. The water retention for the pool walls is provided with a Steuler membrane, bonded to the stainless steel edge and carried through into the balance tank.
Acrylic pool at Embassy Gardens, London (UK)
Spanning the Embassy Gardens’ Legacy buildings, the Sky Pool is the world’s first ‘floating’ swimming pool. Once completed, the 25 m long acrylic pool will allow bathers to glide between the buildings 35 metres up in the sky. The concept was born on a warm summer’s day in 2013, when the Ballymore creative team decided that the best location for the swimming pool was in the space between the Legacy buildings. Then the gauntlet was thrown down that it should be a sky pool, something transparent so that swimmers could see the ground and people below could see the sky.
Architects Hal Currey and Arup Associates, working with structural engineers Eckersley O’Callaghan, brought the scheme to life. After a series of workshops, technical drawings and behavioural analyses, the viability of an acrylic structure was confirmed and the key design dimensions were established. Reynolds Plastics in the USA were then brought on board and using their R-Cast acrylic, the team developed the final and ground breaking solution for the Sky Pool.
The results demonstrate the versatility of acrylic, and key characteristics to note are as follows:
- the overall pool tank is 25 m x 5 m x 2.55 m, with a water depth of 1.2 m;
- the pool is located on the tenth floors, and the acrylic section spans 15.7 m between the two buildings;
- the walls and floor spanning between the buildings are fabricated entirely from acrylic, without any secondary support structure and hence provide a wholly transparent tank;
- the acrylic walls are constructed 178 mm thick, the acrylic floor 356 mm thick, with a total weight of 50 tonnes.
At each end of the acrylic tank, 316-grade stainless-steel box sections connect onto the acrylic and act as the footings and anchors for the acrylic element; they also allow for the differential movement between the buildings. The complete waterproofing is provided by the acrylic and stainless steel sections. The water treatment distribution elements and the pool underwater lighting are incorporated into the stainless steel elements and include surface water removal.
The manufacture of the tank element was completed in November 2017 at the Reynolds factory in Colorado, USA, and it is being shipped over to the UK. It will then be transported up the River Thames, before making its final journey on the short section of road to the site. Installation of the tank will commence in January, with the installation and extensive commissioning of the water treatment system to follow and then handover in summer 2018.