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Shooting the Pool

Thursday, October 8th, 2009

By Tim Long

In our previous articles we discussed the design and layout of the pool, its excavation and steel reinforcements as well as the initial plumbing and electrical work. All of this work has been leading up to the single most important construction component, the shell of the pool, which surrounds the body of the pool. It is made of pneumatically applied concrete, shot at high speeds, encapsulating the reinforcing steel, to give remarkable strength. It’s quick to install, versatile, in that it can conform to almost any shape and compared to poured in place concrete is very economical. Yet without proper technique being applied at the time of the shot, the entire future of your pool will be problematic.

With all the preparatory work out of the way, the day has arrived when your new pool will be shot. The process is referred to as “shooting” because the materials are literally blasted into place at high velocity, over 100 mph, which is good! This “blasting” is part of the reason that the shell can develop such great strength. Other contributing factors will depend upon the type of shooting that will take place. There are two types. Although the names have been used interchangeable by some in the industry, which causes much needless confusion, both types are technically known as Shot-Crete or pneumatically applied concrete. With either type, the crews use air as the primary driving force which places, then compacts and consolidates the concrete mixture. The differences are that some crews use a dry-mix, commonly referred to as “gunite”, which is delivered from the trucks, forced through large hoses by air pressure, then combined with water as it leaves the hose nozzle. The other type of crew use a “wet-mix”, commonly referred to as “shot-crete”, which is combined with water prior to being pumped by a concrete pump through the hose, then is forced by air from the hose nozzle. For more information on the differences, read my article “Shot-Crete, Gunite what’s the Difference?”

Shotcrete Truck Setup

As the crews arrive, you will notice they are accompanied by several large trucks. The number of vehicles depends on which type of crew has been chosen. A wet-mix crew will arrive with at least a concrete pump and a large air compressor. They may mix the concrete on-site, which will require a vehicle to hold the raw concrete, another the aggregate, usually sand and pea sized gravel, and a mixing unit to combine these items with water. This is commonly done in area that experience high temperatures and are too far from a local concrete batch plant. As you may know, concrete, once mixed, must be placed within 90 minutes. So on-site mixing is sometimes their only option. A dry-mix crew will have a vehicle to hold their dry mixture and of course a very large air compressor. The larger compressor is required because the air is not only used to force the mixture into place, but to also move it through the hose. By contrast the wet-mix is moved through the hose by a pump, not air.

Below is a video of a wet-mix truck in place:

With the truck ready, the crews prepare the pool, confirming several key factors. First the pool must be level. Hopefully the previous crews properly leveled the work, the wood panels which outline the pool shape. If not, last minute adjustments must be made.

Second, the elevation must be correct. This measurement establishes a necessary reference point so that after all the work is complete, coping, tile, deck, etc, the finished product is at the appropriate level in relation to the house, or other buildings. Once the concrete has been set, “out-of-level” and “elevation” adjustments are much harder, if not impossible to make.

Third, the forms and steel are checked for sturdiness or rigidity, neither must be allowed to shake or otherwise move as the concrete is being shot. Vibrations and movements prevent the concrete from properly attaching to the steel and possibly create hidden voids, or pockets of air, which will weaken the structure. Insist that your builder provide adequate form work. A weak form will only hurt the process. If a spa is part of the plan, make sure the builder installs a form board between the pool and spa. Do not let them simply hold a “peg” board, or have it wired in place by bailing wire to the reinforcing steel. A solid form should be built. Once the wall which separates the pool and spa is shot and partially set, the form can be removed before the rest of the spa is shot. Failure to do so might mean a weakened mixture and voids behind the steel.

Fourth, they will confirm the thickness of the completed beam. Here the crews will confer with the builders plans to make sure the top of the pool wall, the beam, will be as thick as designed. Typically this is about 12″ wide. The importance of this cannot be overstated for two reasons. First, the beam must be sized as designed to allow for the later installation of finish stone work, and the allow the appropriate integration with any new decks, etc. Second, and no less important, the beam, and not just the beam on this point, must be wide enough in all places to properly “encapsulate” the steel. As we have all observed steel will rust, an normal process of oxidation when air, steel and water meet. Rust, although rarely good, is very bad in pool steel reinforcement. Particularly if the rust finds its way into the pool and shows itself as a dark red stain on the pool wall. In order to reduce and hopefully eliminate this process, the American Concrete Institute specifics where concrete is exposed to earth or water the steel should be encapsulated by at least 3″ of concrete. Unfortunately I have seen problem pools where the steel has only 1/2″ of concrete between the steel and the water. This problem can not be corrected later, only managed. It is a costly and avoidable problem, if the pool is shot correctly.

Setting the Guide Wires

To aid the crews as they are shooting, they typically install fine guide wires. The attached photo shows just such a wire, although the wire, being about the thickness of a piano wire is hard to see. Unlike poured in place concrete, which has forms forms on both sides, to contain the fluid concrete, shot pools have forms on only one side. Newly shot concrete is just shot is stiff enough to maintain its basic shape, so two sides are not needed. However, the guide wire helps the crew see where the “other” side of the concrete needs to be.

Partitions to Reduce Overspray

Other preparatory work includes placing screens to help prevent concrete over-spray from reaching the home or other structures. Concrete over-spray on a home would not be a good thing! There should also be a decision on what to do with the rebound and trimmings. Rebound is the term for the concrete mixture that bounces off during the shooting process. It is important to note that the “bounced” mixture is concrete poor, meaning it is usually the aggregate, sand, that bounces, most of the concrete that was part of the mixture will stick, making the rebound mostly aggregate. So rebound must not be reused. It should be collected and discarded. As a point of interest the dry-mix process always generates more rebound than the wet-mix process. A small amount of rebound is usually not a problem, but a large amount will have to be removed from the site. The mixture, though concrete poor, will still harden, and have to be discarded.

Troweling the Wall

The term trimmings refers to excess concrete, more being shot than is actually needed the crew will trim the concrete to the required shape, letting the trimmings fall. Trimmed concrete can be reused, as it is not concrete poor, however, it must be properly compacted and consolidated into place. Most crews find places in the floor for the trimmings. It is not recommended that trimmings be used in the cove, where the wall meets the floor as this is a structural stress point. This however is where trimmings typically fall. This requires the trimmings to be moved to a less stressful place on the floor.

Below is a video of a pool wall being trimmed:

With all the prep work complete the shooting begins. The long hoses, running from the truck to the pool, terminate at a nozzle, which the crew member, called the nozzle-man, aims the hose, directing the concrete flow. He begins at the cove, base of the wall and floor. The extremely important area, must be solid and well shot. The concrete is built up from this point making the walls, benches and other components. The floor is usually the last place to shoot. Sometimes the process will be stopped as the crews moves between the main pool and the spa, fountain or other structure. Always the nozzle-man maintains a flow at downward angle, moving back and forth building up the concrete toward the final shape.

Finished Shell

As mentioned earlier regarding trimmings, the concrete is built up to a level greater than needed, then trimmed back to the desired shape. The process requires patience and an ability to see shape of the final product. All the internal components have to be shaped, the steps, benches, love seats and shallow lounge area. In addition the pool fittings, where the plumbing penetrates the wall has to have special attention. At each location an indention is shaped around the protruding pipe to provide room for the “fitting” which will be installed later. The lights, skimmers and main drains will have similar attention. Finally you may notice one or more “weep” holes, voids in the concrete where ground water can move if excessive rain occurs before the pool is completed.

The entire process for a small pool should only take a few hours, obviously more for larger one. Large commercial projects can take several days. Prior to the shell, the shape of the pool can only be imagined, based only on the design drawings and the designers descriptions. Now, the shell having been shot, the pools shape becomes clear. One step closer to a completed pool!

To view a video of a pool shoot:

To read the previous article in this series, click here: “Initial Pool Plumbing & Electrical Work”

Tags: Construction, Gunite, Pool Decks, Pools, Shot-crete
Posted in Construction, Pools, Spas | No Comments »

Steel Reinforcement, It’s the Pool’s Skeleton!

Sunday, March 29th, 2009

By Tim Long

Have you every wondered why concrete is such a great and versatile building material? Probably not! Neither have most people. Nevertheless, if you’re considering a pool or spa, you should understand a few things about this wonderful material. Why? Because the concrete, specifically reinforced concrete, will be the frame work on which your pool is built. Properly installed it will last for decades. Improperly installed, problems will likely appear in short order, sometimes requiring repair or complete re-installation of the entire project. The elements which make concrete so strong lie inside the concrete and are virtually undetectable once the concrete is installed. It will pay you great dividends to know a little about concrete, before the construction process begins so you know if your project is being built correctly.

The Pantheon in Rome

For background purposes, you should know concrete has been used as far back as Roman times. In fact the dome of the Parthenon in ancient Rome, which is still standing, is one of the best examples of early concrete construction. Early builders utilized concrete because it developed the strength of stone, but could conform to almost any shape they needed. They did recognize its limitation however, in that it could not support great spans without extra support. The early great buildings were limited in height and interior spacing. Walls were supported by great buttresses to withstand the forces of the building domes and roofs.

It was understood very early that reinforcement was needed to properly support walls and other structures. As far back as in the construction of Greek Temples, lead, a very soft but malleable metal, was used to help hold the stones together. Later, cast iron was used. It wasn’t until the 19th century that concrete construction possibilites began to explode. In 1892 Francois Hennebique using steel devloped a system of reinforced concrete. The combination of the inherent compression strength of concrete and the outstanding tensile strength of steel allowed for buildings to take on great heights and expanses.

Pneumatic Concrete - Shot Against the Ground

In pool construction things further changed with the advent of pneumatically applied concrete. Sometimes this is called gunite or shotcrete. But make no mistake, they both are concrete. What makes them great for pools is the way the concrete is applied. Essentially the concrete is forced out of a gun at high velocity. This allows for two things, first that the concrete can be immediately consolidated and compacted when properly applied, two very important elements of strong concrete, in that the concrete becomes a solid body, without voids or hollow spaces which can greatly weaken any concrete. The second thing pneumatically applied concrete allowed is for the shape of the pool to be constructed virtually “formless”. Prior to this time all concrete was poured into forms, constructed wooden boards on two sides. The wet, fluid mixture was poured between the two forms where it remained until it hardened. This required that the pools take on basic, fairly easy shapes to construct in the forming process. Pneumatically applied concrete can be shot directly against the ground or a one-sided form. This greatly simplified the forming process and allowed the shapes to be limited only by the designer’s imagination and the excavator’s skill with the digging equipment.

Along with this new found freedom in shaping pools, also came cost savings. Since pool walls need not be formed so completely, the cost of constructing a pool decreased, creating an explosion of swimming pool ownership across the world. And as pools became more common place, the desire for creativity and expression has lead us to incredible possibilities in shaping your pool.

Steel Reinforcement - Notice the Deforming Marks

It’s the steel properly sized and distributed inside the concrete that gives the concrete it’s incredible strength. Reinforcing steel bars, commonly shortened to “re-bar”, are produced in long “deformed” bars of various sizes. The deformation on the bars refers to the raised markings on the steel, which creates extra surface area and texture to increase the mechanical bond between the steel and the concrete. The steel is laid out in a grid pattern and supported so that when the concrete is applied, the steel will be ”encapsulated” inside the harden mixture. Encapsulation is important since it provides a measure of protection between the elements and the steel, preserving the strength of the steel. In pools, according to the American Concrete Institute, there should be a minimum of 3″ of concrete between the steel and the water. Failure to maintain adequate encapsulation will not only weaken the steel, but the water, will penetrate to the steel, and likely bring particles of steel back to the pool surface leaving a horrible rust colored stain. This reinforced concrete is now able to withstand forces greater than either the steel or the concrete could withstand on its own. The concrete, brittle without the steel, now resists great forces. The steel, easily subject to corrosion from water, is protected by being encapsulated inside the concrete.

When considering the construction requirements for you pool, you should be aware of several vital things:

The steel must be adequately sized. On a typical residential pool, the basic grid pattern is created out of #3 (3/8″ diameter) steel bars. At various points around the pool the size will increase to a #4 (1/2″) steel bar. The beam of the pool, which is probably easier to think of as the top rim, requires the most strength. This makes logical sense, when you consider that the rim of a cup or bowl is usually thicker than most other parts. It is at this point, called the beam, that the pool must be at its strongest. Typically, in north Texas, four #4 bars will be spaced together in concrete thicker than in most other places of the pool. Larger pools or sections subject to great stress may require even larger steel.

Bending Steel to the Pool's Shape

The steel must be properly spaced. On a typical residential pool, at least in the north Texas area, most builders are using a 10″ spacing in both directions, meaning the pattern of steel creates squares throughout the body of the pool, where no square is larger than 10″. At various points in the pool, where additional strength is required, this spacing between the steel will decrease. For instance the beam, being one such stress point, will commonly made up of at least three #4 bars running parallel around the rim with a spacing of approximately 2″ to 6″ apart. This is known as a 30 pound beam, meaning it should be able to withstand earth forces of at least 30 pounds per cubic foot. Four #4 bars in a beam will make up a 45 pound beam, the most common type of beam in North Texas due to our expansive soils, and six #4 bars make a 62 pound beam. Where the pool wall meets the floor, another point of stress, it is common, and usually very necessary, to see an additional steel bar added to the spacing, creating a 10″ x 5″ pattern of steel. The grid, what ever the spacing, will conform to the shape of the pool, moving around corners and over benches and tanning ledges.

A Double Curtain of Steel for the Spa

Freestanding walls require more steel. A pool wall must endure both the force of the water inside the pool pushing outward and the force of the ground pushing inward. Where a wall abuts native, undisturbed soil, due to the ground absorbing part of the load, the strength of the wall need not be as strong as a wall which has no support and must support the entire weight of the water. These walls, known as “freestanding” walls are sometimes required in places such as the wall between the pool and spa, or on a hillside where the ground slopes away from the pool. Internally these walls will require more steel than other walls and may sometimes even require a double curtain of steel.
The steel must be secured. Where the steel crosses other pieces of steel the intersections are secured with tie wire. This keeps the steel from moving out of its required position and more importantly keeps the steel from vibrating when the pneumatic concrete is shot into place. If the steel vibrated this would likely create voids in the concrete, weakening the structure. Steel rods are usually manufactured in 20′ sections. For this reason it is likely that more than one strand of steel is used to make a pass from one side of the pool to the other. Where two sections of steel meet, there must be appropriate overlap. Usually with #3 bars the overlap is at least 18″ and 24″ with #4 bar. Also make sure where the various piece of steel come together that there will be sufficient room for the pneumatic concrete to be applied around and behind the steel. Failure in this area will create shadows, or voids which will weaken the resulting concrete.

At this point you may be wondering who determines if the steel has been adequately sized? Or who determines the appropriate steel spacing? In some cases the builder may feel sufficiently experienced to make such determinations. However, it is my policy to have every pool plan reviewed by a qualified structural engineer. An engineer will use the information revealed on a soil’s report together with the structural requirements of the pool plans and layout the requirements for the steel. It is his job to determine that pool footings are sufficiently sized and placed and that an appropriate steel schedule has been written for the pool.

In summary, it is reinforced concrete that will make possible the pool and spa of your dreams. But make sure that you trust your builder, that he knows and understands the requirements necessary for your pool to last, giving you years of trouble free existance. If in doubt require that his plans be reviewed by a qualified engineer.

Read the previous article in the series “City Permits & Utilities”.

To read the next article in this series, click here: Initial Pool Plumbing & Electrial Work!

To see a short video of steel being laid and tied, click here:

Tags: Construction, Design, Gunite, Pool Decks, Spas
Posted in Construction, Design, Pools, Spas | 2 Comments »

Shot-Crete, Gunite what’s the Difference?

Monday, March 9th, 2009

By Tim Long

There is a lot of confusion over the difference between Shot-Crete and Gunite and which process should be used for your swimming pool or spa. Many pool builders will argue why the process they use is better than their competitor’s without really understanding the differences themselves.

If you were to research the American Concrete Institute or the American Shotcrete Association, you would find that both terms refer to the application process of good old fashion concrete. The concrete mixture travels from an applicator’s truck to the site via a large hose. Air pressure is applied and the mixture is “shot” into place at speeds approaching 200 mph. This is extremely important! Why? Because it is with the forced placement of the concrete that either process gains it’s merit. Any concrete placement should be “compacted” to remove voids, air bubbles that weaken the strength of the eventually hardened concrete. For concrete that is poured, applicators will use a variety of methods, tamping, vibration, etc, to achieve compaction, but no process by hand can compare to the compaction that can be achieved by the pneumatically applied shot-crete or gunite.

Why the two different terms? Shot-crete was invented by Carl Akeley, who received a patent on the precess in 1911. Originally the concrete mixture, which is made up of cement, aggregate was pumped through the hose dry. At the nozzle water was added along with the air needed to project the mixture. Today this process is now known as “dry-mix”. In the 1950’s it became possible to mix the water first, then pump the “wet mix”, only adding air at the nozzle. This was important for two reasons. The amount of water could be controlled more closely and the aggregate size could be larger than just sand, i.e. pea-gravel. (Note: the amount of water in concrete is the single most important factor in determining the strength of concrete). Today shot-crete has become a generic term which refers to both processes. However some in the pool industry use the terms shot-crete to refer to “wet-mix” and gunite to refer to “dry-mix”.

Which is better? That’s a hard question to answer, because it depends upon several variables. In theory, dry-mix should be stronger. The key is adding only enough water for proper “hydration”, the process which causes the cement to bond to the surrounding aggregate, and not a bit more. If the nozzleman is “certified” by the American Shotcrete Association, he has been thoroughly trained and tested on how to properly control the amount of water being added in the dry-mix. However, if the nozzleman is less skilled, he may add either too much or too little water, either extreme greatly weakens the future concrete. In the “wet-mix”, the moisture content can be more closely controlled, eliminating this as a variable. It is true that the moisture content is greater than can be achieved with properly applied dry-mix, but the possibilities of too little or too much water, have been eliminated.

There is also the question of “rebound”. Rebound refers to the aggregate, which bounces or deflects off of the receiving material and lands in areas other than intended. When the small piece of sand is in mixture, it is combined with the moisten cement and if properly placed will added strength to the structure. If on the other hand, bounces off something hard, such as the wooden form or a piece of the steel reinforcement, the cement is likely to adhere to the hardened surface, but the sand, now mostly stripped of the binding agent falls elsewhere. This rebound, must be gathered up and removed, not allowed to become part of the structure. To do so would be to create a weakened structure. It is known that the dry-mix process creates substantially more rebound than the dry-mix process. If the applicators are skilled in the process this deficiency can be overcome.

In summary, gunite or the dry-mix should be stronger, but the margin for error is too great in my opinion. Having seen too many failed dry-mix applications in my years of experience, I much prefer to use a shot-crete or wet-mix process in the construction of my pools and spas.

Here are some pictues of the shotcrete process and a short video of shotcrete being applied.