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”
