poolnerd

One part of a swimming pool that no one talks about much is one of its most important: the structure. If people come over to your place for a pool party, they’re not going to comment on its guts. In fact, they’ll never know anything about the innards of your pool. So why spend extra money on it?

In short: so you won’t have to spend even more later.

You see, a pool with structural problems can be very difficult– which here means “expensive”– to repair. And I’ve seen some spectacular pools that have failed. That means they’re not holding water; that they have a structural crack. And that in turn means that there are some major upcoming expenses for that homeowner. I guess it boils down to spend a little now, save a whole bunch later.

The Structure

The structure of a pool is basically the reinforcing steel bars– rebar– plus the gunite (or shotcrete, but for our purposes we’ll consider them the same). Generally speaking, more is better. More rebar equals a stronger pool, which means that it will be less likely to crack. And in the pool business, a cracked pool is about as bad as things get. Structural failure– like cracks– can be very, very expensive to fix. That’s a road I hope you’ll never have to go down.

Your contractor will select a structural plan for your pool. He gets a “standard plan” from a licensed civil/structural engineer or architect. It’s called a standard plan because it covers the structural requirements for most backyard pools. Sometimes additional details are also required, like for grottoes, waterfalls, rock slides, jump rocks, fancy stuff like that. Some engineers call for the use of more rebar and gunite; some use less, often the bare minimum. I like the “more” approach. Rebar and gunite are comparatively cheap, plus building a burlier pool helps me sleep well at night. No worries to keep me tossing and turning.

Your contractor will select the appropriate details from the standard plan to use for the conditions present in your yard. So, obviously, he must know all about the conditions in your yard. Is the soil expansive? Is it real sandy? Is it likely that they’ll run into rock when they start digging? Does the soil have a high sulfur content? Will the pool be located close to a slope? And what about ground water? Stuff like that is important, even vital, to know. If the pool isn’t built right, it may fail.

And, believe me, you don’t want a pool that develops structural problems.

A Strong Pool Starts With A Good Dig

The first crew in to start the construction process are the excavators. Any old guy with a tractor and a dumptruck isn’t good enough, though. These guys need to know how to follow the engineering plan exactly.

The way the pool is dug determines the dimensions of critical parts of the pool. Things like wall thickness, size of the bond beam, how coves are laid out. They’re also responsible for the shaping of details like light niches and the box for the skimmer.

If the excavators under-dig for these dimensions, the pool can be weaker, sometimes by a lot. On the other hand if they over-dig, too much money will be spent on needless extra gunite (which can total several hundred dollars, perhaps even a thousand).

So, when selecting an excavator, get a crew with some experience. Although it may cost slightly more, it can save you lots of money in the long run. Additional unneeded gunite may cost hundreds of dollars, but fixing a cracked pool caused by an under-dig situation can be many thousands of dollars.

What’s a Bond Beam?

You’ll probably hear the phrase “bond beam” at some point in your pool shopping and/or building experience. The bond beam is the top part of the pool, and it’s usually about a foot wide. It runs around the perimeter of the pool. And it is a for-real structural beam.

For example, a beam in a house, like one that might hold up a roof, is designed to take a vertical load: the weight of the roof, plus whatever is sitting on the roof, like snow. A bond beam on a pool is designed to take a horizontal load: it’s supposed to keep the soil back.

The bond beam functions kind of like the lip on those cheapie throw-away plastic cups you see at parties or picnics. If you cut that lip off the cup, the cup becomes much flimsier, easier to collapse. A stouter lip for makes for a stronger cup. Similarly, a pool with a weak bond beam is more likely to crack

Bond beams come in two flavors: one is “with deck,” the other is “no deck.”

A with-deck detail is designed for a pool that has — stay with me now — a deck. This type of beam usually has three bars of reinforcing steel: two on the front face, one in the back. Contractors who want to keep the price of a bid down often use this type of construction.

The no-deck detail is for a pool that may, or may not, have a deck. No-deck beams are also sometimes called six-bar beams, because that’s how many sticks of rebar are used in it’s construction. Three on the front face, three on the back. No-deck is much, much stronger, and only costs a little bit more; maybe a couple of hundred dollars for a medium size pool. It’s what I’d probably recommend for you, and it’s what I use on my own projects

But, now let’s move on to other main part of the guts of a well-built pool, besides the bond beam. The excitement builds, huh?

Beefy

After covering the bond beam, we move deeper down into the pool– the walls and floor. Everything down here is usually a grid of rebar: a whole bunch of 12 inch by 12 inch squares in stacked columns and in rows. Then we add more rebar in spots that need reinforcing.

As a pool builder, one the things I’m concerned about is how strong the walls are, how well they can resist the push of the soil outside the pool. You see, a pool wall is basically a retaining wall: it is designed to hold back the dirt (except in the case of a free-standing wall, which might be the subject of a future post).

The main components of a beefier pool wall are:

• Additional vertical bars of steel (”rebar”), running through those 12 by 12 inch squares mentioned previously. These added bars are called alternates. There can be one to three alternates per column of squares.
• More gunite or shotcrete in the coves (the curve at the base of the wall where it transitions into the floor). Where the walls may have three inches of gunite in front of the rebar, the coves may have anywhere from 4-12 inches in the shallow end to perhaps 10-20 inches in the deep end.

Another detail for added strength is extra longitudinal bars in the floor where the pool depth transitions from shallow to deep. These bars run only one direction — shallow to deep, not across the pool — and are usually 10 feet long, with their centers located at about the 5-foot depth point.

Happiness Is In the Details

From here out, most of the additional places for shoring up strength are in particularly weak areas: around the light niches, the skimmer, across the nose of benches, and so forth.

Although these areas do not use much steel, they are critical. For example, the skimmer is perhaps the most common place for a crack to develop in a pool. There is basically a discontinuity in the bond beam there — an obvious weak spot. A little rebar in the right place, indicated by your engineer, can shore things up.

So if you will be building your own pool, or are starting out in the construction part of the business, be sure to know your standard plan from top to bottom. Pay special attention to slopes and surcharges.

The observant reader will notice that we haven’t talked much about gunite. The rebar and gunite go hand in hand, so of course it is just as important as the rebar.

Gunite, though, is a little more difficult to get a handle on. With rebar, it’s pretty easy to see if what is in the pool is what is on the plan. With gunite, though, you can’t tell by looking if the gunite is mixed properly and has enough cement content. Or if there’s enough material in the coves and on the floor. Most people can’t, that is. If you have questions or concerns about gunite, let me know in the comments. I’ll get back to you.

A Showcase Pool That You Can’t Even Use

Yeah, but who wants to sink extra money into the structure when you could get that nice stamped concrete deck instead? Or maybe a few more palm trees?

But keep this in mind: a pretty pool that has failed just ain’t any good for pool parties. Plus, it can be real bad for your bank account.

This entry was posted on Sunday, December 3rd, 2006 at 5:28 pm and is filed under Uncategorized, Constuction, For Homeowners, For Contractors. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.