Closed for a decade, Plymouth’s Tinside Lido has finally been restored to the glory of its 1930s heyday. How could the council afford it? Alex Smith finds that it’s amazing how much you can save by intelligent specification

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 £3.5m facelift.

For decades the popularity of lidos has been waning as holidaymakers abandoned Britain’s 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’s 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 – 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–bistro, 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’s 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’s 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 £3.5m budget concentrated the minds of Plymouth council’s 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. “It was too risky to go for the cheapest contract because of the time constraints and unforeseen aspects of the scheme,” says John Williams, lead structural engineer at Plymouth council’s engineering services division.

Interserve won the main contract and was involved in the design process from the start. “We worked together to foresee the problems and force costs down,” says Williams. “In 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.”

James Heike was Interserve’s design co-ordinator at Tinside and had an office at the council. He soon realised the benefits of teamwork: “It 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.”

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 “Swimming on solid ground”, below).

While Interserve carried out the piling, Williams was busy specifying the reinforced concrete slab. “The specification of the base is very important in a water retaining structure. It must be durable, strong and not prone to early cracking,” 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 “Reinforcements”, 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’s 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 “Finishing lines”, 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 – for many it is where they learned to swim. Interserve’s Heike says he lost count of the number of people who stopped by to reminisce. “It’s nice to be involved in such a special project that the majority of people want,” 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.

Finishing lines: The pool coating

The pool base was coated with an acrylic-based system called Safetrack, from Stirling Lloyd. Plymouth’s architectural technologist Dean Luxton found that it offered some important benefits over a paint system even though it was slightly more expensive. The big disadvantage of a paint system, according to Luxton, is that it needs to be reapplied every two or three years. Luxton says that Safetrack won’t have to be reapplied for at least five years and possibly even 10. The acrylic is also a colour system and anything from the range can be specified: in this case Luxton chose a blue-and-white striped deckchair finish to match the original pool. Only one coat was required, as Safetrack has non-slip properties from the inclusion of silica bead aggregate. Luxton says that with a paint finish he would have had to apply a separate non-slip coating. There was another reason Luxton chose Safetrack. “The big benefit is that the curing time is only an hour, which was important given the pool’s location next to the sea,” he says, claiming that paint would take six to seven hours to dry and 24 hours to be fully cured. Luxton says the curing time was an important consideration. “In some ways the build programme drove the choice of product solution,” he says. Luxton makes the acrylic coating sound like a straightforward choice. In fact, it was a smart bit of lateral thinking on Luxton’s part since, although the system had been used on waterproof tanks and car parks, it had never been specified for a swimming pool before.

Swimming on solid ground: The piling

A total of 267 piles were sunk into the limestone bedrock at Tinside. Piling was necessary because a site inspection revealed that in the upper zones of the limestone bedrock there were significant voids. There were no piles supporting the original pool. Instead the foundations comprised pre-cast concrete units spanning between concrete sleeper walls, which were founded directly on the bedrock. Over the years the voids had caused significant movement of the sleeper walls, which resulted in the concrete units cracking. The piles were friction piles, which allow both the downward movement of a full pool and the upward movement caused by hydrostatic pressure from seawater at high tide. End bearing piles were unsuitable because of the voiding in the limestone rock. The piles are cast in cementitious grout and have a permanent plastic casing and single steel reinforcement bar. The pile ends are socketed into the bedrock and the friction between these elements holds the piles in place. The project team managed to value engineer the piling by reducing the depths of pile foundation in the areas where solid bedwork was found to be only 14 m below sea level. Originally it was estimated that the bedrock was at 25 m below sea level throughout the site.

Reinforcements: The concrete mix

The 425 mm slab was strengthened with two sets of reinforcement bars: one 50 mm from the base and one 50 mm from the surface. The 50 mm depth was chosen to minimise the chances of the seawater penetrating the concrete and rusting the steel. Salt corrodes steel and if the rebars start rusting, they will cause the concrete to spall. To counter the affects of the saline environment, structural engineer John Williams specified a high-strength grade C50 concrete mix, according to BS 8110, part one. The depth of the slab required in water-retaining structures meant that there was a danger of thermal shrinkage leading to cracking in the slab. This occurs because when concrete hardens it absorbs water and gives off heat as part of the chemical process. In a thick slab the difference between the heat generated in the middle to that near the surface can be so great, it shrinks at different rates, causing cracking. To avoid this Williams specified that 50% of the mix consisted of ground granulated blast furnace slag. The slag has as much strength as the Portland cement used in the mix, but it slows down the speed of hydration in the first few days of curing, which means there is less chance of cracking. The same concrete specification was used for the pool base and walls, and the plant room roof, which doubles as a sun terrace.