Cracking pool problems
Swimming pools throw up particular challenges for waterproofing, especially as they are prone to concrete cracking. Waterproofing specialist Bluey was called in by Port Kembla Council to repair its public baths. “A lot of the problems we come up against are cracking in the concrete, so much of our workinvolves crack repair which is what we were doing in Port Kembla pool,” project manager Daniel Bosco explains.
“One of the main problems with pools is filling and emptying and also thermal expansion and ground movements that occur around the pool. The pool may be installed in a very clayish soil and just the expansion and shrinkage of clay as it gets damp and dry tends to cause cracking and that’s normally quite difficult to repair.
“In the case of Port Kembla, the reason it was having so many problems was that it was a salt water pool, built quite close to the ocean and instead of filtering and using chlorine, every few days or once a week they were emptying the pool out totally and refilling it with salt water. This is bad because you’re unloading and reloading pool in a repetitive way, and every time you do this the walls move out and then back in again so you have all these cracks opening and closing and on top of that you’re adding salt water – which is adding salt to the concrete and attacking the steel, so you have the worst situation in an aggressive environment for a pool.”
Concrete in pools is generally reinforced with steel and would normally provide protection against salt attacking the steel, but when it cracks and starts to degrade it loses its ability to do that, Bosco says. “Port Kembla was a worst case scenario but it’s happening on a much smaller scale in smaller pools. You haven’t got salt water in most home pools attacking the steel but you’ve got very aggressive chlorides from chlorine in the water which degrade the concrete. Water gets in and corrodes the steel. As the steel corrodes, you end up with a small crack and most people think it’s not a problem as they’re not losing water, but as steel corrodes it expands as well because iron oxide is a larger product than iron itself so when it combines with the oxygen during the corrosion process it expands and blows off pieces of concrete, putting it under pressure and that’s what concrete cancer is.
“When people have pools built, one of the first things they should do is make sure they have adequate concrete cover from the first layer of steel reinforcement to the concrete. That was one of problems at Port Kembla – the steel was too close to the surface.”
One of the repair solutions was to protect the steel with cathodic protection. “Because the concrete couldn’t provide as much protection to the steel, the cathodic protection would take over,” Bosco explains. “In addition there were a lot of cracks in the concrete, a lot of construction joints they wanted to repair.” A resin-based waterproofing product was used to achieve this. “The idea is if you have a crack and you put one of these resins into it, you’ll stop water getting into that crack and that will stop oxygen getting to the steel and salts and chlorine. If you can seal the crack with a low viscosity product that will penetrate into the crack and bond to the surface, you’ll stop all those nasties getting in and causing problems. So we used low viscosity polyurethane, the Crack Seal NV. The advantage of that product is it will also bond with a damp surface. It was a low foaming resin, so it was elastomeric and didn’t form the rigid structure you normally get with higher foaming polyurethane. We also used epoxies for the dry cracks.
Because polyurethanes are elastomeric, they stretch and expand which is good for sealing a crack, but if structural repairs are needed, epoxy is better because it is a much stiffer repair resin. “You’d use similar products in backyard pools,” Bosco says. As with most things, prevention is better than cure and there are steps that can be taken to protect pools from the beginning. “Normally to get resins into cracks or joints you have to drill into concrete at an angle and intersect the crack at a certain depth and you put a valve into that hole and inject the resin so it goes into the crack and spreads out,” Bosco explains. “As prevention people put in a perforated hose which is an injectable hose which has nipples at each end of the plastic and everything in the system is made of plastic so it doesn’t corrode. If have a problem in future, you just inject into that hose.
“This is only good for construction joints not cracks because you don’t know where they will appear. It’s inevitable that you’ll get cracks because concrete isn’t really working properly unless it’s cracked. The whole concept around the concrete is that it cracks and then the steel takes up the tension so you have to make sure the steel is deep enough into the concrete so it has enough cover in it.”
Another pool requiring repair was the Beaurepaire, part of the Beaurepaire sports centre at the University of Melbourne. “The pool was completed in 1957,” project architect Anne Marie Treweeke from Allom Lovell & Associates explains. “It was named after Frank Beaurepaire, a famous Olympian, and was a cutting-edge 1950s design. The university initially thought of demolishing it but because the building was registered they rethought it. They came to us because the pool had reached the end of its natural life and been shut for a couple of years and it was non-compliant with safety regulations.”
The original pool comprised a concrete tank with abitumen tanking membrane and then an inner skin of bricks with tiles on top. The architects changed the profile of the pool and raised the floor level so a new tank could be put in. “We used a number of different waterproofing additives, one of which Xypex. We did a whole tanking brush-on membrane and then tiled over the top of that. Basically think concrete skin, then a waterproofing membrane and then tiling.” In this project cracking concrete was not an issue. “The concrete structure was built 50 years ago, so any movement the building was going to have has happened – we’re not dealing with a modern pool, all we were doing was administering a new waterproof membrane and tiling over the top.”n
2-Aug-2005