Removable Forms (Cast-in-Place)
Concrete Home
> Building Systems > Removable Forms
(Cast-in-Place)
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Thomas Edison with model of a concrete house
(circa 1910). The model is now on exhibit at the Edison National
Historic Site.
Credit U.S. Department of the Interior, National Park Service,
Edison National Historic Site.
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Cast-in-place (CIP) concrete walls aremade with ready-mix concrete
placed into removable forms erected on site. Historically, this has
been one of the most common forms of building basement walls. The
same techniques used below grade can be repeated with above-grade
walls to form the first floor and upper levels of homes.
Early forays into this technology were done more than 100 years ago
by Thomas Edison. He saw the benefit of building homes with concrete
well before it was widely understood. As technology developed, improvements
in forming systems and insulation materials increased the ease and
appeal of using removable forms for single-family construction. These
systems are strong. Their inherent thermal mass, coupled with appropriate
insulation, makes them quite energy efficient. Traditional finishes
can be applied to interior and exterior faces, so the buildings look
similar to frame construction, although the walls are usually thicker.
History
The technology for casting concrete in removable forms—the
beginning of the reinforced concrete construction industry—dates
back at least to the 1850s, not long after portland cement was patented.
The predominant use of removable forms for single-family homes was
for below-grade (basement) walls. Thomas Edison was one of the first
to recognize the potential for above-grade applications and do some
demonstration projects, several single-family homes made entirely
of concrete.
Since that time, advancements in forming and placing technology,
concrete mixtures, and insulation strategies have made construction
of concrete homes using removable forms a well accepted building
technique.
Advantages
Cast-in-place provides benefits to builders and building owners
alike.
Owners appreciate:
- strong walls
- safety and disaster resistance
- mold, rot, mildew, and insect resistance (below grade can require
termite protection)
- sound-blocking ability
- for insulated systems, energy efficiency and resultant cost
savings
Contractors and builders like:
- familiarity
- expands business to include more than basements
- cost effective building technology (reusable forms lower placement
costs)
Components, Including
Insulation
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| Cast-in-place concrete walls done with metal
forms |
Cast-in-place (CIP) concrete systems are relatively straightforward.
Steps required include the placement of temporary forms and placing
fresh concrete and steel reinforcement. Although it is possible
to batch concrete on site, ready mixed concrete is widely available
and is usually delivered by a ready mix supplier. And in 2011, the
average distance to most project sites from a ready mix plant is
just about 14 miles.
Although uninsulated walls were common in the past, changing energy
code requirements are more or less eliminating walls without insulation
in most climates. (Note that this is the case with all types of systems,
including concrete, wood, and steel. Energy is simply too important
in terms of its cost and environmental impact.) Concrete’s thermal
mass helps moderate temperature swings, but cannot provide the improved
energy performance mandated by codes unless the wall system contains
insulation. In the past, therefore, insulation may have been an optional
component of a cast-in-place system, but it
is increasingly included in contemporary construction.
The most common formwork materials for casting concrete in place
are steel, aluminum, and wood. Many wood systems are custom manufactured
and may be used only once or a few times. Steel and aluminum forming
systems, on the other hand, are designed for multiple reuses, saving
on costs. Metal panel forms are usually 2 to 3 ft wide and come
in various heights to match the wall. Most common are 8 and 9 ft
tall panels.
Installation,
Connections, Finishes
Casting concrete in place involves a few distinct steps: placing
formwork, placing reinforcement, and pouring concrete. Builders
usually place forms at the corners first and then fill in between
the corners. This helps with proper alignment of forms and, therefore,
walls. Reinforcement bars (“rebar” for short) can be
erected before either form face as a cage or after one side of the
formwork is installed. Once both form faces are tied together and
braced, concrete is placed in the forms via truck chute, bucket,
or pump. Forms should always be filled at an appropriate rate based
on formwork manufacturer recommendations to prevent problems. Although
blowouts are uncommon with metal and wood forms, misalignment could
potentially occur.
For single-family residential construction, wall thicknesses can
range from 4 to 24 in. Uninsulated walls are typically 6 or 8 in.
thick. Walls with insulation are generally thicker when they contain
an internal layer of insulation: either the inner or outer wall
layer has to serve a structural function. CIP walls are generally
thicker than frame walls (wood or steel).
Reinforcement in both directions maintains the wall strength. Vertically,
bars are usually placed at 1 to 4 ft on center, and tied to dowels
in the footing or basement slab for structural integrity. Horizontally,
bars are typically placed at about 4 ft spacings in residential
applications. Additional bars are placed at corners and around openings
(doors, windows) to help control cracking and provide strength.
Openings for doors and windows require bucks to surround the opening,
contain the fresh concrete during placement, and provide suitable
material for fastening window or door frames.
Floors and roofs can be concrete or wood and light-gauge steel.
Ledgers are anchored by bolts adhered into holes in the concrete.
For heavy steel floors, weld plates are installed inside the formwork
so they become embedded in the fresh concrete. This provides an
attachment for steel joists, trusses, or angle irons.
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| Basement wall form used as deck form in new
rib floor system. Adjustable rib form supports deck form and
can span from 12 to 16 feet. |
Finishes on CIP systems are dependent on the presence of insulation
and on the formed face. Finishes can alternately be attached with
furring strips. Almost any type of finish can be used with removable
form concrete wall systems. Wallboard remains the most common interior
finish. Exteriors are much more varied and depend on customer preference.
Form liners attached to the exterior form face can impart any type
of texture; alternately, other traditional finishes such as masonry
or siding can be attached to the wall following form removal.
Insulation can be placed on inside or outside faces or in the center
portion of the wall. To place the insulation on the face, plastic
fittings are inserted into the foam board and become embedded in
the concrete. These are flanged to hold the foam and the flanges
provide an attachment for finishes and fixtures. Face insulation
can also be applied after the formwork is stripped. If foam is embedded
in the formwork prior to concrete placement, composite fittings
are used to tie together the two concrete faces (through the foam
insulation layer). The inner wall is usually the structural layer,
so it’s thicker and contains the rebar, whereas the outer
concrete layer has the finish applied. Foam insulation is most often
EPS, expanded polystyrene. It can be XPS, extruded polystyrene,
which is stronger, but also more costly.
Sustainability
and Energy
A major appeal of insulated CIP walls is reduced energy to heat
and cool the building. Savings are primarily attributed to insulation,
thermal mass, and low air infiltration. Typical R-value for EPS
and XPS foams are, respectively, 4 and 5 per in. Thermal mass acts
like a storage battery to hold heat or cold, moderating temperature
swings. CIP walls have 10% to 30% better air tightness than comparable
framed walls—because the concrete envelope contains few joints.
In addition to saving energy and money associated with heating and
cooling, concrete walls also provide more consistent interior temperatures
for occupants, increasing their comfort.
CIP systems are also suited to the use of recycled materials. Concrete
can be made using supplementary cementing materials like fly ash
or slag to replace a portion of the cement. Aggregate can be recycled
(crushed concrete) to reduce the need for virgin aggregate. Most
steel for reinforcement is recycled. Some polystyrene is made with
recycled material as well. Some of these techniques contribute toward
achieving points in certain green rating systems such as LEED.
Building Codes
For one and two family dwellings, the International Residential
Code (IRC) addresses foundations and below grade walls in Section
R404 and above grade walls in R611 for homes up to two stories plus
a basement. For larger buildings like multi-family and commercial
structures, engineers follow the International Building Code (IBC)
for structural design.
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| Door and window openings are cast into concrete
walls. |
Comparative Costs
Cast-in-place concrete requires the erection of temporary forms,
so this is labor intensive. But many types of forms can be reused,
so there is not necessarily a big cost for the formwork. Also, concrete
is historically more stable in price than either wood or steel.
Residential CIP Project
Ancient Art Form Inspires Contemporary
Sustainable Home
At 3300 sq ft, the Origami-Loft House in Venice,
Fla., isn’t exactly small, but it lives even larger, adding
a study loft, reading space, and game room to the typical living
areas. The spacious feeling is due to many reasons: a top ceiling
height of 24 ft, geometric folds in walls that create separate rooms,
and lots of light—every room opens to the outdoors while transom
windows and interior glass partitions let the light flow freely.
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| As the living room shows, tall ceilings and
open floor plans give a spacious feeling to the interior. Photo
courtesy C. Pyatte. |
Set in a traditional neighborhood on a quarter-acre
lot, the project respects its surroundings. From the street, the
house presents a sensibly executed façade, whereas the rear
of the house has dramatic curves and cascading volumes. Designer
JONATHAN
PARKS ARCHITECT (JPA) selected cast-in-place concrete walls
to allow for the complex geometries and openness while providing
the strength necessary to resist coastal weather along the Gulf
of Mexico, which includes a long hurricane season.
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| As the living room shows, tall ceilings and
open floor plans give a spacious feeling to the interior. Photo
courtesy C. Pyatte. |
Like so many projects today, this house was designed with sustainability
in mind. Starting with the energy-efficient cast-in-place concrete
wall system, and soy-based insulation, the envelope protects against
Florida’s warm temperatures. A combination of active and passive
solar techniques greatly reduces the energy requirements of the building,
while still providing the necessities and amenities common to Florida
homes. This includes hot water for the home and pool via passive solar
water heating, and high interior ceiling heights to help control interior
temperatures. By collecting solar power, owners enjoy an extra 21
to 26 kw hr/day and the use of Energy Star appliances reduces the
draw on power.
Energy, however, is not the only sustainability measure considered
in design and construction of the home. Some finishes are made with
recycled materials and interior floors are covered with low VOC
wood. Outside, landscaping is designed with minimal water needs,
including native wildflower pasture grass, to conserve fresh water
usage. From literally every angle, this concrete house delivers
good looks, energy performance, and sustainable design.
Low Rise Commercial CIP Project
A Curvy Concrete Car
Park Sails into Sarasota
Poised to become a signature structure in the Sarasota, Florida
skyline, the Palm Avenue Parking Garage and Retail Shops includes
a variety of unique retail spaces at ground level, and provides
parking for 763 cars, 35 motorcycles, and 80 bicycles. As an example
of a mixed-use facility—parking garage and retail—the
Palm Avenue project shows how versatile concrete structures are.
Using architectural concrete as both a structural and aesthetic
medium, JONATHAN
PARKS ARCHITECT (JPA) created a curvaceous, free-form building
to capture the spirit of the local artistic culture. The cast-in-place
structure eliminates the need for shear walls and columns between
spaces, providing an open floor plan with high ceilings. This unobstructed
layout, along with perforated metal “sails” that cover
the façade, creates a bright, airy, and safe atmosphere for
a shared pedestrian and driver space, letting in light and natural
ventilation while shielding cars from view.
Cast-in-place concrete also allowed designers to produce a playful,
sculptural stair design that itself attracts peoples’ interest
and encourages its use rather than the elevator.
Using civic input and capturing the spirit of the local artistic
culture, this 240,000 square foot project was designed for the City
of Sarasota by the architecture firm, JPA, and built by Suffolk
Construction. The result is an iconic, user-friendly, environmentally-responsible
design that satisfies the functional, strategic, and aesthetic needs
of the City, while contributing to the overall success of downtown
Sarasota.
The garage follows a user-friendly layout, including wide express
ramp and one-way traffic flow to reduce vehicle conflict and facilitate
easy maneuvering in and out of parking spaces. Efficient vehicle
circulation is achieved by designing slightly angled parking spaces
and a wide-open express ramp free of parked cars.
Though certification is pending, the project is designed to achieve
a LEED Core & Shell v3 Gold Level, in part through the use of
a combination of storm water, interior materials, lighting, and
solar techniques for recharging electric vehicles. Highlights of
the green components are noted below.
• An underground retention vault and cistern to store and
treat storm water runoff from the site. A portion of the water
is reused for the irrigation system.
• Interior materials surpass LEED requirements for off-gassings
of VOC’s and other toxic chemicals.
• Energy consumption is reduced with LED lighting and an
energy management system that only provides artificial light when
and where it is needed.
• A solar carport is located on the roof and plug-ins for
electric vehicles are provided on the first floor.
Owner: City of Sarasota
Architect: JONATHAN
PARKS ARCHITECT
Contractor: Suffolk Construction
Civil, Landscape, and Wayfinding: Kimley-Horn and
Associates
Structural Engineer: Walter P Moore
M.E.P, Fire Protection: TLC Engineering for Architecture
Eco Consulting: Carlson Studio Eco Consulting
Parking Consultants: DKS Associates
Structural Concrete: Ceco Concrete Construction
Aluminum Sail Fabricator: Mullet’s Aluminum
Product, Inc.
Resources
Concrete Homes Council
(A Council of the Concrete Foundations Association)
The Concrete Foundations Association is an active force dedicated
to the positive and constructive development of the above grade
removable forms concrete home industry. Contact CHC for more information.
Concrete Homes Council
107 First St. West
P.O. Box 204
Mount Vernon, IA 52314
(319) 895-0761 / Fax: (319) 895-8830
www.concretehomescouncil.org
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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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