Shotcrete refers to a process in which compressed air forces mortar or concrete through a hoseand nozzle onto a surface at a high velocity and forms structural or non-structural components of buildings. The relatively dry mixture is consolidated by the force of impact and develops a compressive strength similar to normal- and high-strength concrete.
Materials used in the shotcrete process are generally the same as those used for conventional concrete-portland cement, lightweight aggregate, water, and admixtures. Shotcrete projects also call for the same types of reinforcement specified for conventional concrete, including deformed bars, welded wire fabric, and prestressing steel.
Wet or Dry
Shotcrete may be applied to surfaces using a dry or wet mix method. The wet mix concrete method consists of portland cement and aggregate premixed with water before the pump pushes the mixture though the hose. Additional compressed air is added at the nozzle to increase the velocity of the mixture. In the dry mix process, compressed air propels a premixed blend of portland cement and damp aggregate through the hose to the nozzle. In the nozzle, water is added from a separate hose and completely mixed with the dry mixture just as both streams are being projected onto the prepared surface.
Generally, the shotcrete gun nozzle is held at a right angle two- to six-feet from the surface. In most cases, shotcrete can be deposited in the required thickness in a single application. For some vertical and overhead applications and for some smooth finishes, shotcrete must be applied in one- to two-inch thick layers. Once shotcrete is placed, it can be finished in a variety of methods, including natural finish, broom finish, various rough trowel finishes, and smooth steel trowel finish. After finishing, the concrete must be cured for a period of at least seven days.
Since its invention in 1911, the shotcrete process has been used successfully for a wide variety of building projects, including all types of residential and non-residential buildings. Shotcrete, which can be applied to horizontal or vertical surfaces, is especially suited for curved or thin concrete structures and shallow repairs. Other applications include swimming pools, grain silos, fire proofing structural steel, and many civil engineering structures such as bridges, tunnels, dams, tanks, and earth retention systems.
Although using shotcrete to form walls is new in the United States, the process has been used in other countries for years. A recent innovation that has made shotcrete more suitable for construction is the incorporation of insulation into the wall unit. The new system consists of 4-by-8-foot panels of a polystyrene core sandwiched between layers of wire mesh. Workers erect a frame of wire mesh or rebar in the shape of exterior walls, install conduit, pipe, and other utilities between the polystyrene and the wire mesh, and then spray the entire structure. The concrete covers the foam and hardens to form a reinforced wall with built in insulation.
Spray On With Nozzle
Shotcrete is, in effect, a version of a cast-in-place concrete wall. Rather than placing concrete into forms, however, a fresh mix is sprayed onto wall panels that have been erected in the shape of the building. A nozzleman applies concrete from a pressurized hose to encompass the reinforcement and build up the wall thickness, forming structural shapes that include walls, floors, roofs, and other assemblies. This material has also been called “gunite” in reference to the nozzle or “gun” used to shoot material at the form face. Any surface suitable for accepting fresh concrete can be used: wood, steel, and polystyrene are common. Finishes are often applied directly to the concrete while it is still wet.
This section describes key aspects of shotcrete building systems for commercial and residential applications:
- Wall System Variations
- Installation: Connections and Placing Concrete
- Sustainability and Energy
- Additional Resources
Shotcrete systems have advantages similar to all other concrete walls. They are strong, durable, resistant to disasters, fires, mold, insects and vermin, and have low permeability, good thermal mass, and create tight envelopes. Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as very fast erection, particularly on complex forms or shapes, including curved walls and arches. Shotcrete is a cost effective building method due to the speed of construction and minimal equipment requirements.
Wall System Variations
Two main types of walls are common, insulated sandwich panel systems and plain (uninsulated) walls. Insulated sandwich panel systems consist of prefabricated panels of polystyrene insulation sandwiched between two parallel sheets of wire mesh. The two faces of mesh are connected by welded galvanized wire that pierces the insulation at acute angles to create a strong, truss-like support system. These walls can achieve R-values from 7 to 33, depending on thickness of the core insulation and concrete layers. The concrete also provides thermal mass benefits.
Plain walls contain no internal insulation, just the reinforcement that will be encapsulated by shotcrete. If desired, insulation can be placed on either the interior or exterior of the plain walls following shotcrete application.
Given that these walls are lightweight, one might think that they are not robust and should be limited to single-story structures. On the contrary, they are very strong. One system is designed to go as tall as 20 stories and performs well even in seismic zones. To simplify multistory construction, some systems offer additional items beyond wall panels, such as prefabricated foam and mesh stair assemblies and landing panels. These items are shotcreted in the same manner as wall shapes, forming structural members.
Installation: Connections and Placing Concrete
One reason that shotcrete systems are installed so quickly is that the wall forms are lightweight. They are easy to handle and make erecting the shape of the building simple. It may be possible to do this primarily with manual labor and very limited equipment. For taller walls and some proprietary systems, a crane may be needed to reach the upper locations. Once the panels are connected, they tend to brace each other even before shotcrete bonds them together. The panel manufacturer should be consulted about the need for bracing, shoring, and other support during construction.
In most shotcrete systems, dowels from the floor slab provide attachment at the base of the walls. For insulated panels, each segment is attached to at least two dowels. At the top edges, the panels are connected to ones on either side of them. For the plain mesh systems (no polystyrene core), rebar is attached to the floor dowels and then wire mesh is hung off the rebar. One proprietary system uses a top beam to position and brace the rebar and form a clean edge at the roofline. Following application of shotcrete, the reusable beam is removed.
For roof trusses, or in the case of multi-story buildings, for floor connections, attachments are placed before shotcreting the walls. This saves time later as no cutting or drilling is required. Again, the panel manufacturer should be consulted for specifics. Also prior to shotcrete application, panels are easily cut, so window and door openings should be made at that time, as should cuts in the polystyrene for plumbing and electrical chases (for insulated panels). More on shotcrete insulation.
All shotcrete methods involve spraying material onto the forms. There are two ways to do this: a wet mix and a dry mix. The end result is the same on the wall, but the mixing equipment is slightly different.
- Wet Mix - All ingredients, including water, are thoroughly mixed and introduced into the delivery equipment. Wet material is pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material onto the receiving surface.
- Dry Mix - Pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity.
The basic concrete mix contains cement, aggregates, and water. According to the American Shotcrete Association, properties of both wet and dry process shotcrete can be further enhanced through the addition of other ingredients, such as:
- Silica Fume - Provides reduced permeability, increased compressive and flexural strength, increased resistance to alkali and chemical attack, improved resistance to water washout, reduced rebound levels, and allows for thicker single pass applications.
- Air-Entraining Admixtures - Improve pumpability and adhesion in wet-process shotcrete and freeze-thaw durability in both wet and dry processes.
- Fibers - Control cracking, increase toughness values and improve impact resistance and energy absorption.
- Accelerators - Improve placement characteristics in adverse conditions, allow forthicker single pass applications, increase production capabilities and reduce the occurrence of fallouts on structures subjected to vibration.
Once the shotcrete has been applied to both faces, it is screeded off to the proper thickness. Depending on whether this is intended to be the exposed finish or whether insulation will be placed over it, screeding may be followed by additional surface consolidation or texturing.
Finishes can be directly applied to shotcrete walls while the material is still fresh. They can subsequently be painted to add color. It would also be possible to plaster or spray another thin-coat finish of cement-based material onto the surface. White cement is used for white or light colored finishes. Such finishes bond well to the backup wall, are durable, low maintenance, and offer the greatest range of color options.
Interior wall surfaces of shotcrete can be left exposed or can be furred out. If there is insulation on the interior face, a common finish is drywall screwed to furring strips anchored to the shotcrete assembly. Exterior wall surfaces can be stucco (in a wide variety of textures), brick veneer with a cavity, or adhered thin brick or manufactured stone. Where exterior insulation is used, all typical frame finishes are possible: sidings, stucco, brick, or EIFS-type products.
This colorful home demonstrates that shotcrete buildings can be as attractive as they are durable and weather resistant.
Sustainability and Energy
Shotcrete systems, like all concrete systems, can offer both material and performance aspects from a sustainability perspective. On the material side, it can contain recycled materials like fly ash and rebar, which may help contribute to credits in LEED® or other green rating systems.
From a performance perspective, the system creates a tight building envelope, protects against unwanted air losses, and has some thermal mass, all of which leads to an energy efficient structure. Insulation, whether encapsulated in the panel or placed inside or outside the wall, should be tailored to the climate for maximum effectiveness.
This cut-away view shows the shotcrete wall, furring, insulation, and finished drywall.
American Shotcrete Association
38800 Country Club Drive
Farmington Hills, MI 48331
248.848.3780 / Fax: 248.848.3740 www.shotcrete.org
East Ridge Development: Built Fast to Last
In Boynton Beach, Florida, Bond Building Systems developed and built the East Ridge Development using a proprietary system based on shotcrete. It took about one week to complete the shell of each house. All 30 homes were perceived by purchasers to be a good value, pre-selling before they were finished. More on East Ridge
Speedy Construction Meets Demanding Structural Performance
The M2 (Emmedue) system was used to build a Hotel Intercontinental in Panama, an attractive beachfront property comprising many buildings, some up to five stories in height. The backbone of the M2 system is a lightweight panel, which makes the system suited to rapid, cost effective construction. A layer of concrete is applied to the panels (as shotcrete) to add strength and create long term durability.
Buildings are safe: non-combustible and strong enough to resist hurricane- and tornado-force winds, earthquakes, and even explosions. In fact, the U.S. military and petrochemical industries are currently testing this system for blast protection. More on the M2 system.
The listing of organizations and information resources constitutes neither an endorsement nor recommendation by the Portland Cement Association (PCA). PCA disclaims any and all responsibility for the selection of organizations listed and the products they represent. PCA also assumes no responsibility for errors and omissions in this list.