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Outdoor bathing is enjoying a mini renaissance in Britain. A string of lidos have been refurbished in the past few years, and one of the finest, Tinside Lido in Plymouth, Devon, has just undergone a 拢3.5m facelift.

For decades the popularity of lidos has been waning as holidaymakers abandoned Britain鈥檚 seaside resorts in favour of Mediterranean package deals. Without income to maintain their pools, councils felt they had no choice but to stop paying for them. And with no maintenance through the harsh British winters, most of these iconic symbols of 1930 municipal glamour were left to rot.

Plymouth鈥檚 Tinside was a prime example. It closed in 1992 and has been a rusting blot on Plymouth Hoe ever since. Unlike dozens of other lidos around the country, Tinside was never built over, thanks in part to its inaccessible location and the fact that the Hoe was the site of the most famous game of bowls on English history.

In the late 1990s, Plymouth council decided that the lido should form the centrepiece of an ambitious restoration project that would return the cliffside Hoe to its 1930s glory. The area had become a hotchpotch of decaying concrete structures: broken walkways, moss-streaked pools and abandoned cafes 鈥� an ugly backdrop to Plymouth harbour. The plan was to lure holidaymakers and investment back to the Hoe with a fully restored lido, complete with art deco fountains, a cafe鈥揵istro, sun terraces and underwater lighting, which would make the Hoe a night-time attraction as well as a daytime one.

On paper, it was a brilliant way of unlocking the Hoe鈥檚 potential, but for the in-house design and engineering team at Plymouth council and main contractor Interserve, the restoration presented looked like a big task. For one thing, the project would require the removal of hundreds of tonnes of spoil from a site that lay within a tidal zone and was inaccessible by road. The foundations of the swimming pool would have to be totally reconstructed, and the walls and concrete base replaced. All the while, English Heritage would be scrutinising every design detail to ensure that the dimensions and features of the 1930s original were faithfully reproduced.

To ensure the project鈥檚 success, the team had to take special care in its specification of the concrete base, the waterproof coating and the terrace and pool surround floor. It had to opt for materials that would withstand turbulent seas and fierce winter storms, so keeping maintenance costs to a minimum. Lifetime costs were an important assessment tool: if the facility has a high maintenance cost, a future council would be more likely to close it down again.

The tight 拢3.5m budget concentrated the minds of Plymouth council鈥檚 architects and engineers. They realised that on such an awkward site, the design could only be kept within deadline and budget if the contractor and consultants were on hand to offer cost advice from the first stages.

Plymouth took great care in selecting a partnering contractor. It asked three to apply for the tender and judged them on quality as well as price. 鈥淚t was too risky to go for the cheapest contract because of the time constraints and unforeseen aspects of the scheme,鈥� says John Williams, lead structural engineer at Plymouth council鈥檚 engineering services division.

Interserve won the main contract and was involved in the design process from the start. 鈥淲e worked together to foresee the problems and force costs down,鈥� says Williams. 鈥淚n past contracts, it was often a battle between us and the contractor but with this partnering contract it meant that we had a combined team working on extracting best value in terms of design and structure.鈥�

James Heike was Interserve鈥檚 design co-ordinator at Tinside and had an office at the council. He soon realised the benefits of teamwork: 鈥淚t was good to be involved that early in the job as it meant I could get the right experts and subcontractors talking to the designers and engineers at Plymouth council.鈥�

An early example of value-engineering was in the retention of the concrete slab in the pool. Although it was no longer able to bear any loads, the project team calculated that two-thirds of the pool base was in good enough condition to act as a sound uniform surface for the pouring of a new reinforced concrete slab. The team therefore saved time and minimised their landfill tax bill.

In a prior survey, the council had found that the original foundations were failing and that piling work would need to be carried out. As the lido fell within the tidal zone, the engineers had to design the slab and piles to withstand the upward pressure of a high tide with no water in the pool, and the downward pressure of a pool full of water at low tide (see 鈥淪wimming on solid ground鈥�, below).

While Interserve carried out the piling, Williams was busy specifying the reinforced concrete slab. 鈥淭he specification of the base is very important in a water retaining structure. It must be durable, strong and not prone to early cracking,鈥� says Williams. The Plymouth team had worked on pools before and was able to use this experience at Tinside to specify a concrete mix that met all these criteria (see 鈥淩einforcements鈥�, below).

The pool base was cast in sections, and on the perimeter the contractor left a 75 mm upstand on which walls would be cast at a later date. The 300 mm thick walls, which contained two sets of rebars, were to be cast inside the original wall. Although English Heritage was keen to retain the original walls and dimensions of the pool, it agreed to let the project team build new walls when it was explained how difficult and costly it would be to retain the originals.

When it came to the specification of the waterproof coating for the pool base, the council鈥檚 architectural technologist Dean Luxton looked beyond the usual swimming pool solutions. He chose an acrylic solution from Stirling Lloyd, which offered advantages over the paint systems that would normally be specified for a pool base (see 鈥淔inishing lines鈥�, page 5).

Luxton was also partly responsible for the specification of the 50 mm screed of the pool surround and sun terraces, which was laid on the reinforced concrete. The local planners insisted that this matched the red-pigment screed of the original. Luxton passed a sample to cement giant RMC, and it offered Luxton a range of sample panels to choose from. Once the concrete screed was applied, handheld diamond grinders were used to create a non-slip surface, and to reduce the colour variation in the screed, which often occurs during curing.

On the sun terrace above the plant room to the west of the pool there is a fall of 100 mm so any seawater that comes over the top of the sea wall is drained away. Because screed is liable to crack if it is more than 50 mm thick, the slope is built into the reinforced concrete base, which is 325 mm at the edge of the terrace and 425 mm at the top of the slope.

Tinside Lido was officially reopened in August. Locals are very fond of their lido 鈥� for many it is where they learned to swim. Interserve鈥檚 Heike says he lost count of the number of people who stopped by to reminisce. 鈥淚t鈥檚 nice to be involved in such a special project that the majority of people want,鈥� he says.

Hopefully, as the choice of materials was based on quality and longevity rather than cost, many future generations of swimmers will learn to swim here.