For Joe Smith, the decision to use sprayed concrete to construct an underground swimming pool and sauna complex at his Guildford home was not hard to make. As managing director of Premier Gunite for the last 34 years, Smith has overseen the construction of dozens of pools both above and below ground. Indeed, a 20 year old cloverleaf-shaped outdoor pool on the other side of his garden was one of Smith's earlier efforts. The choice of wet spraying rather than the alternative dry process was equally easy, he says.
'I started spraying dry, and eventually I had five dry guns. Twenty-five years ago I threw them away and I've been wet spraying ever since.'
Although Smith admits that getting the first American wet-process pump he bought to work with UK materials 'took months and nearly bankrupted me', he has no regrets. 'There's much better control of structural strength, and only a tenth of the rebound we got with the dry process,' he says. 'And if there's anything wrong with the mix it is immediately obvious at the nozzle.'
Part of this confidence comes from continuous development of the hardware. Smith says. 'I've adapted a lot of machines to suit my needs, mainly by adding a third 'compensate' piston to smooth out surges.'
Some 2,500m3 of sand and both weathered and fairly homogeneous sandstone had to be excavated before the first concrete could be poured. Consultant Bill Hales, himself a former Premier Gunite director, went for a 150mm thick ringwall on top of the base slab as a safety shutter between the main walls and the sandstone. Ringwall and base were pumped and poured conventionally in one day.
Above this level the soft sand means a conventional timber shutter has to be used on the outside, keeping wall thickness down to 350mm. A grid of vertical 20mm rebars and horizontal piano wires act as guides for the nozzle man, who has more than 20 years experience with the technique.
Characteristic strength of the mix being sprayed is 35N/mm2, maximum aggregate size is 10mm, and the 200m3 needed for the job is being supplied ready mixed by Pioneer Concrete. With a pipeline less than 30m long no admixtures are needed, although Hales says workability is effectively lost within 30 minutes or less.
Within this period the site team has to strike off the surface using the vertical bars as a guide, then trowel it smooth. Internally the main box measures 24m by 5.5m, and Hales predicts the walls will be finished within two days.
Then we'll set up the shutter for the 450mm thick roof, and spray that in one day,' he adds.
Smith says he 'daren't think about the cost' of the project, but would be willing to build a similar complex for someone else for £150,000 'at least'. Total site time will be around nine weeks.