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Spray-Applied Concrete Walls

In areas where pool-construction is common, spray-applied concrete has also been used for home construction.

Spray-applied concrete has been in use for almost a century, especially in swimming pool and retaining wall construction. Because of its flexibility in application over complex surfaces, it also has been used extensively in creating simulated natural objects and environments, such as rock-scapes in zoo exhibits, and ornamental water features in private gardens. In areas where pool-construction is common, such as Florida or California, spray-applied concrete has also been used for home construction.

Spray-applied concrete, "shotcrete" is separated into either a wet process or a dry process. For a dry-mix process, dry cement and aggregate are driven through an applicator hose, with water only added at the nozzle. A wet-mix process is where the concrete is pre-mixed before being loaded into a hopper, from which it is pumped through the delivery hose, and spray-applied with compressed air added at the nozzle via a separate hose.

Spray-applied concrete technology has been in use for about 70 years, primarily in civil engineering applications, such as roads, roadway repairs and tunnels, and in commercial projects. The most common residential uses are swimming pools and foundation repair. More rarely, it is used for basement foundations.

The finished strength of spray-applied concrete averages 5,500psi. The resultant concrete is different from conventional cast-in-place concrete with regard to the application surface, reinforcing, and of course method of application. The wet process is often the method of choice for high-strength applications.

As with conventional poured concrete, properties of spray-applied concrete can be altered and enhanced through the addition of additives or admixtures, such as silica fume, air-entraining admixtures, fibers and accelerators. Accelerators are often incorporated at the plant so the concrete sets quickly after application.

Spray-applied concrete is a versatile technology. Besides walls and swimming pools, it can be used to create floors, roofs, and curved or other complex and variable surfaces. Spray-applied concrete may be applied to wood, steel, polystyrene or any other surface to which it will adhere long enough for the concrete to set.


Affordability

Some savings can be achieved because there is less formwork. However, material savings may be offset by higher labor and equipment costs.

Quality and Durability

Spray-applied concrete is a durable material that is resistant to damage from moisture, insects, impacts, and fire.


Easy

The required equipment and concrete mix ingredients can be purchased from any large construction equipment retailer and concrete supplier. In-ground pool contractors often have shotcrete installers, or they know of qualified contractors in the area. However, many contractors are not experienced with spray-applied concrete for home construction.

The skill of the nozzle man is critical to success, making the availability of experienced nozzle men a limit to the use of the technology. Spray application takes more time than pouring concrete into a closed form. The significant investment in equipment and its maintenance can prove prohibitive for some contractors.

With a dry process, precise control of the mix is difficult because the water content is controlled by the operator. Air-entrainment is not possible, and can result in a greater risk of freeze-thaw cycle damage in cold-climate applications. This is due to the fact that air-entrainment occurs via mechanical incorporation of air bubbles that occurs more readily in pre-mixed (wet) concrete. However, use of air-entraining admixtures can help offset this. The equipment cannot handle coarse aggregate, resulting in a limitation on the portion of the total finished concrete strength provided specifically by the aggregate.

Numerous subtleties in the installation of spray-applied concrete are essential to the finished product's quality. For example, care must be taken in applying a stamped pattern because the vibration can cause the new concrete to slump. Significant knowledge and skill is required for successful applications.


Not Applicable


Not Applicable


Codes do not generally limit the method of concrete installation, but rather the specifications of the concrete and its finished properties. Local officials may not be familiar or comfortable with the technology. Spray-applied concrete is addressed in the following ASTM standards: C1480-00 Standard Specification for Packaged, Pre-Blended, Dry, Combined Materials for Use in Wet or Dry Shotcrete Application, C1436-99 Standard Specification for Materials for Shotcrete, C1398-98e1 Standard Test Method for Laboratory Determination of the Time Setting of Hydraulic-Cement Mortars Containing Additives for Shotcrete by the Use of Gillmore Needles, C1141-01 Standard Specification for Admixtures for Shotcrete and C1385-98 Standard Practice for Sampling Materials for Shotcrete.


Not Applicable


Spray-applied concrete requires special equipment and skilled workers. Because the concrete is sprayed onto a surface to which it adheres, any formwork required to create this surface will be one-sided. There are many options for the surface; in civil engineering applications, it is often earth or bedrock. Concrete can be spray-applied directly to earth for a home foundation, but better control can usually be gained with a formed surface.

In the dry process, pre-mixed dry or damp materials are put into the delivery equipment. Compressed air rapidly transports the materials through the hose to the nozzle, at which point water is added in quantities controlled by the operator. Full mixing of the concrete occurs when it hits the application surface. In the wet process, delivery equipment receives pre-mixed concrete, which is pumped to the nozzle, where compressed air is introduced to shoot the material out of the hose and onto the target surface. For either application method, the high-speed impact of concrete against the application surface results in the rebound of a significant portion of the material, typically in the range of 10 to 15% by volume. This rebound can be reclaimed/reused by following behind the gunner / nozzleman, and shoveling or vacuuming up the rebound. However, this additional labor adds to the expense of sprayed concrete applications.

For flat surfaces or shallow curves, ¾" plywood forms are typical; two layers of 3/8" plywood are used to create tighter radius curves. For basement wall installation against earth, a drainage mat is pinned against the soil, followed by the application of a waterproof membrane and reinforcement.

Reinforcement must be stabilized sufficiently to prevent its rattling under the impact of the concrete. If the reinforcement is not adequately constrained, its vibrations will create voids in the concrete around the rebar. Reinforced bars are typically secured to the form or to rebar partly embedded in previously set concrete. Rebar must be kept at the appropriate distance off the backing by using periodic supports. Finally, wires are tautly strung across the form at about 2' on center at the finish surface level, to guide the installer in maintaining concrete thickness, to provide a screeding plane, and to allow construction of precise outside corners. The wires are removed after screeding is complete.

The surface can be trowelled smooth while the concrete is still wet. Stucco is often used as a finish surface. If a level and true surface is required, it should be on the side of the removable form. This requires application from the outside if the finished surface must be inside. Shrinkage and cracking are an issue, and the surface must be properly protected to allow for curing. Sprayed curing compounds may compromise adhesion and therefore should be avoided if the surface is later to be tiled or otherwise faced. In this case, traditional concrete curing methods should be used, such as covering with misted blankets.


Not Applicable


The benefits of concrete construction are shared by spray-applied concrete, including strength, impact and fire resistance, insect and rot protection, longevity, and durability. A characteristic of sprayed concrete is low permeability. Also, spray-applied concrete allows the application of concrete in difficult locations that may be impossible using conventional formed poured-in-place methods.

Spray-applied concrete requires less formwork than poured-in-place (or no formwork if applied against a suitable excavated surface), thereby lowering material costs and form set-up time. Forms do not need to withstand the great pressures that develop with poured concrete. Savings in forms may be offset by higher labor and equipment costs.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.