Building pools to withstand freezing temperatures brings unique challenges.
Pools assembled in colder climates require sturdier reinforcement. The concrete also must be stronger and able to resist the damaging effects of freezing and thawing. Builders often start with larger-sized steel — a popular choice is No. 4 (1/2-inch) rebar — in construction.
Minimizing the amount of water in the mix improves the shell’s resistance to freeze/thaw damage. So does air entrainment — additives that form millions of tiny air bubbles in the concrete. The shell is typically shot with a mix that tests at 4,000 to 5,000 pounds per square inch in compression, as opposed to the 2,500 to 3,500-psi mixes used in warmer climes. And the walls often are thicker than usual, frequently starting at 12 inches.
However, when dealing with special features such as vanishing edges and beach entries, additional care must be taken to properly design, build and winterize the pool. Here, experts explain how it’s done.
In freezing climates, any unique feature must be protected, particularly if it’s a vanishing-edge wall with more concrete exposed to the air.
Vanishing-edge walls should be at least 12 inches thick at the top, then widen for extra support as they descend to the floor, says Al Rizzo, owner of Rizzo Pools in Newington, Conn. His vanishing-edge walls also contain a double curtain of steel. In a typical vanishing-edge pool built by Rizzo’s crew, the wall will measure 15- to 16 inches thick where the wall and floor meet.
In addition, a vanishing-edge wall always must be waterproofed, particularly on the side where the main pool is located, says Ron Lacher, president of Pool Engineering Inc. in Anaheim, Calif. Otherwise, water may penetrate from the main pool through the entire wall. The moisture then collects behind the tile, stone or other veneering material, causing delamination, regardless of the weather.
In cold weather, it could freeze and crack the wall.
To prevent this, waterproof the pool side of the vanishing-edge wall, and then 10 feet to the left and right, says Gregg Pruitt, vice president of Aqua Pool & Patio Inc. in East Windsor, Conn.
In the meantime, shallow features such as catch basins, perimeter-overflow gutters and beach entries remain vulnerable to freeze/thaw cycles. One major concern: The vessel will become frozen solid if the bottom of the pool sits within the frost line.
After placing a well-draining soil under the whole pool, put gravel beneath the shallow features to ensure that no moisture collects in the area, Rizzo says. This is particularly important if the vessel will be emptied in winter.
But beware: Another danger lies ahead.
“If the pool is emptied, even a deep pool, the bottom can freeze and the soil underneath the bottom, the subgrade, now will freeze just like it’s near the surface,” Lacher says. “Any moisture in the soil will freeze and expand, and it causes really severe uplift forces that can crack the pool.”
Experts suggest anchoring any shallow features below the frost line. On his perimeter-overflow gutters, Pruitt extends the walls below the frost line — 42 inches underground in his area.
Rizzo uses a similar approach for beach entries, placing a solid concrete footing around the perimeter of the beach entry. This wall will be at least 12 inches thick and penetrate approximately 42 inches below the frost line. The outer side of the wall must be smooth so that frost cannot get a foothold. Again, it’s important to bulk up the walls of perimeter-overflow gutters to prevent frost damage.
Closed for winter
Following construction, make sure your customers and service crew understand that these features will require special care during winter months.
Empty out all gutters, basins and wading pools that fall within the frost line. Next, place some type of insulation inside. Rizzo uses straw stuffed inside large plastic bags, which are then stacked in the basin.
Always remember to leave the hydrostatic valve open so the water under the straw will seep out during the winter.
Pruitt fills his perimeter-overflow gutters with polystyrene foam. “The trough normally has a decking material (stone or concrete) poured over the top of it, and if you don’t put something in there for expansion, that can fill up with water and pop the whole top off that concrete or stone,” he says. If the perimeter-overflow pool includes a remote surge tank, the water inside will freeze, and it needs room to expand.
He likens the effect on the tank to what happens in a pipe. “If you have a pipe half full of water and it freezes, the water pushes upward,” Pruitt says. “If the pipe is one-quarter full or half full, it can freeze and not break the pipe. If it’s three-quarters full and it freezes, now it’s going to break that pipe because the ice is already at the top of the pipe, where it’s getting smaller. It’s the same thing with a tank.”
Pruitt recommends leaving the tank no more than half full in winter.