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Standing on its own: an on-grade wall requires special care, from planning through shotcrete.

To an outsider, the job of an out-of-ground, or on-grade, wall may seem easy enough--it just sits there. But this type of wall has a special function: It must withstand pressure from water inside a pool without having soil on the other side to equalize it. And with no earth to back it, an on-grade wall requires special construction techniques.

The key to an on-grade wall's success lies in the planning, forming, steel and concrete. Here's how to give them the care they need during the four-stage construction process.

First stage: Call in an engineer during planning.

While simple spa dams often receive a pass, an engineer will be needed for vanishing-edge weirs, hillside on-grade walls and raised pools. The overall design, including the steel setup and concrete thickness, will vary from the usual.

"A typical wall is designed for earth pressure inward, while a free-standing wall is designed for water pressure outward," says Ron Lacher, president of Pool Engineering in Anaheim, Calif. "The loading is in a different direction."

For this reason, the engineering changes. On a normal wall, more concrete is required for the water-side of the steel. This helps it resist the compressive forces of the earth. But on-grade walls need the opposite arrangement, with more gunite or shotcrete on the outside to withstand the outward pressure.

The structures will be thicker, too. Often, they start at 12 inches. "As the height goes up, the bottom gets fatter," says Bill Drakeley, president of William Drakeley Swimming Pool Co. in Bethlehem, Conn. "If I need 12 inches on the top for a 14-inch coping stone and it's a 6-foot free-standing wall, the bottom's going to probably be 18 inches."

On-grade walls generally need bulkier steel with closer spacing than their inground counterparts. "When you get above a certain point, you have to take into consideration the weight of the concrete itself and the weight of applying it," says Drakeley, a Genesis 3 Platinum member who has a structural engineer on staff. You don't want the rebar to bend during the concrete application process. Engineers may require No. 4 bars (1/2-inch) or No. 5 (5/8 inch) rather than the typical No. 3 (3/8-inch).

You may also need a double curtain of rebar to aid construction. This will help ensure that the rebar doesn't vibrate during shotcreting or guniting of the pool. If the steel does move, this will detract from the proper embedment of the steel. A double curtain will actually stand more firmly. "We can usually design a vanishing-edge weir wall with one curtain, but it would be difficult to build," Lacher says.

Onground pools may sometimes be created with a single curtain of steel, he adds, as long as the bars are secure enough to stay put.

Second stage: Make the forming system sturdy.

The wrong forming system can wreak havoc. Just ask Mark Hilsabeck.

"On the first project I got gunited, they put in little 1-by-2s for the forms," says the project manager at Pure Water Pools, LLC, in Costa Mesa, Calif. "[The forms] were completely destroyed, and the gunite crew screamed and yelled. It wound up costing us $2,000 in button board to refix what forms were left and put button board up. Then it cost an additional $1,000 in gunite."

The forms serve two purposes. They create something against which to shoot the concrete. They also help secure the steel so it doesn't vibrate during shooting. They need to withstand not only shooting concrete, but workers who will probably end up climbing on top of them.

The best ones are solid, with most or all components made of wood. The basic configuration includes panels, backers, kickers and stakes. The panels give the walls their shape. The backers provide a skeleton for the panels, and the kickers and stakes anchor the configuration into the ground. The steel is then tied to the forms for stability.

For the panels, many builders use plywood because it's smooth and sturdy. They need holes drilled into them for securing the steel.

Solid panels have only one drawback: They trap air during the shooting, possibly forming small pockets in the back of the wall. To fix this problem, the gunite crew can reshoot the back side after stripping the forms to fill the pocks.

Some builders like to prevent this from happening altogether by using pegboard. It's the material that lines many garages, with holes for hooks. It's less expensive, and the holes provide a built-in place for the tie wires used to anchor the steel. They also allow air to escape while the shotcrete or gunite hardens. When the pegboard is stripped, it will leave little dimples on the back of the wall. This roughs up the surface, making it easier to apply a finish or veneer.

Pegboard isn't as solid as plywood, so builders only use it for walls that are less than 6 feet tall. To beef it up, kickers are placed closer together. With plywood panels, Pure Water's staff generally spaces the kickers every 24 inches on center, but they stand every 16 inches with pegboard, Hilsabeck says.

You may decide that the cons outweigh the pros when it comes to using pegboard. "It's nonself-supporting, and takes a lot of hand work and tie wire to get it into a solid enough state to support the gunite," says John Fitzgerald, senior project manager at South Shore Gunite Pool and Spa Inc. in Chelmsford, Mass. "So we use plywood because it's got a little more spine to it. We like them fairly smooth, so when we take the form off, it's as close to a finished product as we can get."

As for the kickers and backers, they are generally made out of 2-by-4s or a similar product. Drakeley's company uses rough-sawn lumber, which is a 2-by-4 that hasn't been milled smooth yet. It actually measures more than 2-by-4-inches, so it's a little bulkier. It can be difficult to find, so 2-by-4s are acceptable. Drakeley's company spaces the kickers 18 inches to 2 feet apart. The vertical kickers are nailed onto a stake at the bottom.

South Shore Gunite Pool and Spa uses rough-cut 1-by-3 lumber for the stakes and kickers. But the smaller pieces are spaced about every 12 inches apart.

You'll need to make adjustments to the forms for tall or radiused walls. Taller ones need more bracing to hold up the added concrete. For curves, you'll need a more flexible panel material, such as a thinner plywood called luan strip, to outline the bend. Kickers will be closer together here, too, to help keep the bend in place and support the somewhat flimsier board.

This part of construction is too important to leave to the gunite crews, especially when dealing with intricate or extreme projects, says Randy Beard, founder/president of Pure Water Pools, LLC, in Costa Mesa, Calif. "The gunite crew has drywall on the truck, so they kind of hold it in place with their body and shoot against it," he says.

This doesn't provide enough stability. Plus, the drywall is absorbent, so it sticks to the concrete and robs it of needed moisture. "When you peeled it off, it would leave hideous-looking divots in the outside," Beard says. "Sometimes the pockets aren't fully visible, but if you tap on it, it's a hollow spot where the air was trapped and the moisture sapped out." When it comes time to veneer the wall, the tile or masonry company may find the gunite or shotcrete crumbling in spots.

That's why Beard brought this task in-house a few years back, assigning it to the masonry crew. If you don't have the manpower, you can subcontract the work to concrete masonry companies that form structures such as basements.

DJ Gunite, the Norwalk, Calif. company that does all Beard's pools, offers a discount to builders who handle the forming. "It saves us at least a couple hours," says CJ Brown, a foreman with the company. "Plus, we can't carry all that wood."

Third stage: Keep the steel steady.

Steel crews should pay careful attention to dobie placement, consider using a double curtain and secure the cages to the forms.

When they come on site, they will set dobies on the panels to ensure the right spacing between the form and rebar. Before this occurs, make sure they understand the special engineering involved. Lacher once had a pool blocked correctly, with the dobies set to perfectly prop up the steel. He briefly went into the house only to return and find that the gunite crew had repositioned the tiny concrete cubes. "They didn't understand what a free-standing wall was, and they took the dobies and made them normal," he says. "So I no longer had a free-standing wall."

Even if the engineer doesn't call for it, consider having your steel crew install a double curtain. Brown of DJ Gunite even requires it before his staff will shoot. Not only does it prove more stable, but it helps the cages stay put.

The two layers of rebar should not mirror each other. It's more difficult for shotcrete or gunite applicators to ensure proper encapsulation if they have to shoot around two parallel bars. Instead, offset the curtain patterns. Not only is it easier to get the nozzle around, Drakeley says, but it increases concrete coverage in the wall.

The crews tie the cage to the forms through drilled holes or the ones that come in the pegboard. When doing it, they should pay particular attention to how the rebar will want to move, Brown says. "If the steel has an outside radius, it's going to want to keep going that way," he says. "If it's an inside radius, it wants to lean [in]. If it were a straight wall, more than likely it would want to lean toward the outside of the pool."

Fourth stage: Steady wins the race during shooting.

During the gunite or shotcrete phase, applicators must ensure proper encapsulation of both curtains of rebar. They must also ensure that the steel does not bend and the wall doesn't cave in from its own weight.

Fitzgerald's gunite crew starts by slowing down the volume of the material coming out of the nozzle by 10 to 20 percent. "If we lob it up too quickly, it can get vibrations and cause stress throughout the wall," he says. "We especially slow it up toward the top so we don't get that leaning Tower of Pisa." Needless to say, this will add time to the process.

The nozzle operators should apply the material just a few inches at a time. "The person shooting the concrete needs to know how far they can go before the weight of the concrete [is too much]," says Jeffrey Boucher, vice president of the William Drakeley Swimming Pool Co.

In each layer, they should start from the back to help ensure that rebound doesn't get caught against the form, Brown says. "You have to shoot the back before the front," he adds. "Get all your good material locked in and let the bad stuff slide out," he says.

Shoot each layer so the top slants 45 degrees into the pool. Make sure you hold the nozzle at a 90-degree angle to the receiving surface. Using this method ensures that any dry material will fall down onto the pool floor. "You have to bring the back up, say 6 inches, before you bring the front up at another 45-degree angle," Brown says.

If any sand or rebound lingers on top of the layer, clean it off before shooting over it. This ensures proper adhesion between the layers.
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Title Annotation:Aqua Scapes
Author:Robledo, Rebecca
Publication:Pool & Spa News
Date:May 22, 2006
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