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.
- 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.
- 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.
Bending Steel to the Pool's Shape
A Double Curtain of Steel for the Spa
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:
