Shotcrete
Concrete
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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:
Advantages
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| A nozzleman sprays shotcrete at a wall surface.(
Unless noted otherwise, all photos on this page are courtesy
of Bond
Building Systems, Inc. |
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
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Shotcrete sandwich panels combine the durability
of concrete with the insulating values of polystyrene.
(PCA image) |
Two main types of walls are common, insulated sandwichpanel 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.
 |
| Stair assemblies available with some systems
simplify multistory construction. Image courtesy of M2 (Emmedue),
www.mdue.it/en/
|
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).
 |
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| This system uses straps to hold the upper reinforcement in
place. After the top reusable steel beam is removed, the straps
become tie-downs for the roof members. |
Wall, door, and window openings are cut prior to the application
of shotcrete. |
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.
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| Freshly applied shotcrete
is screeded to a plane surface. |
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
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.
 |
| These colorful homes demonstrate 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. |
Additional Resources
American Shotcrete Association
38800 Country Club Drive
Farmington Hills, MI 48331
(248) 848-3780 / Fax: (248) 848-3740
www.shotcrete.org
Unless noted otherwise, all photos on this page are courtesy of
Bond Building Systems, Inc., www.bondbuildingsystems.com
Shotcrete Projects:
East Ridge Development:
Built Fast to Last
 In
Boynton Beach, Fla., 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, preselling
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 5 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.
Disclaimer
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.
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