In Europe — where wood-frame construction is an anomaly
rather than the norm — precast concrete panels have been
used for residential construction for at least a couple of
decades. In the U.S., they're more commonly associated with
commercial projects, from office buildings, dorms, and parking
garages to multifamily housing. But as concrete construction (a
category that includes ICFs, masonry and block, and tilt-up
panels) becomes increasingly popular, more and more precast
plants are marketing their products to developers and
Once you get past the fact that you can't cut it with a
Skilsaw, there's a lot to like about precast concrete. Like
SIPs, the engineered panels are manufactured under controlled
conditions and are therefore predictably strong, and they can
be quickly assembled with minimal skilled labor, even in bad
weather. Moreover, they're fire- and storm-resistant,
energy-efficient, and easy to maintain.
To get a better idea of how precast concrete panels are
manufactured and how they fit together on the job site, I
recently visited a precast plant and spent several days
following the progress of a custom home being built with the
panels. Here's what I learned.
Many residential builders are already familiar with precast
hollow-core concrete planks. Commonly used for bridge decks and
commercial floor and roof decking, these planks are strong
enough — depending on their design and size — to
clear-span 30 feet or more while supporting loads of more than
100 psf. On a hilly site, they can be used to cost-effectively
build garage floors over full basements (see "Installing
Precast Garage Slabs," JLC, June 2002).
Although there are several different proprietary ways to
produce hollow-core planks, most are cast on 300- to
600-foot-long beds, then saw-cut to length (see Figure 1). To
reduce cost and weight, tubes are used to create voids in the
continuous plank during casting; for strength, the plank is
reinforced with welded wire fabric and prestressed with
1/2-inch strands of low-relaxation #7 wire. When the individual
planks are cut to length, they are slightly cambered, which
helps them resist their design load.
Figure 1. Hollow-core planks are slip-cast
on long beds, tensioned with wire strand, then cut to length
after the concrete has cured (top). The planks are cambered but
will flatten out when loaded — with a 3-inch topping
slab, in the case shown here (middle). A crane is needed to
maneuver the 8-foot-wide planks on the job site
The panel fabricator for the project I followed, J.P. Carrara
& Sons of Middlebury, Vt., uses Dynaspan slip-forming
machinery to produce hollow-core planks in a limited number of
widths and in 6-, 8-, and 10-inch thicknesses; this home was
built using the 8-inch-thick version. Most of the planks were
roughly 8 feet wide; the longest plank measured about 33 feet
long. Carrara estimates that lead times for hollow-core planks
average three to four weeks, and that the installed cost on a
small residential project runs between $15 and $22 per square
Hollow-core planks can be used with both block and poured
foundations; typically they're supported by a ledge formed at
the top of the wall or by pockets formed in the middle, with
embedded and grouted rebar tying the planks and the foundation
This project had a full basement and a conventional
cast-in-place foundation. The top of the 12-inch-wide
foundation wall provided bearing for the ends of the
first-floor planks and the base of the precast wall panels
Figure 2. Both the floor and the wall
panels bear on the 12-inch-wide poured concrete foundation,
with welded plates fastening the floor panels to the foundation
walls. Rebar projecting from the bottom of the wall panels
slides into grouted holes drilled into the top of the
When the foundation was poured, lengths of 3/8-inch steel angle
with 8-inch-long anchors extending at an angle into the
concrete were embedded at predetermined locations along the
inside top edge of the foundation. These plates matched up with
similar ones embedded in the hollow-core planks by the
fabricator. Once the planks were installed, the plates were
welded together, creating a strong connection between the
foundation and the floor system (Figure 3).
Figure 3. When there is an opening in the
floor system, planks can be supported by steel beams (too) or
by special steel brackets supplied by the fabricator (middle);
plastic shims are used to help level the planks. Where the
planks bear on the foundation walls, steel plates cast into
both the planks and the foundation are welded together
To accommodate openings in the floor system for a masonry
fireplace and stairwell, some of the planks bore on structural
steel instead of on the foundation wall. Where the ends of
planks met each other over a steel header, workers cut slots
into the cores, lay in rebar, and filled the cores with grout
to tie the planks together.
Next, workers installed backer rod in all of the keyways
between the planks, then filled keyways and joints with
4,000-psi nonshrink grout (Figure 4). After the building shell
was completed, a 3-inch-thick topping slab with embedded
radiant tubing placed on 2 inches of rigid foam was installed,
completing the first level's finished floor.
Figure 4. Keyways between planks are
fitted with backer rod (top), then filled with high-strength
nonshrink cement grout (bottom).
Precast concrete walls are less common in residential
construction than hollow-core plank decks, and they require
longer lead times — eight to 10 weeks at the Carrara
precast plant — and considerable coordination between the
designer, the GC, and the panel fabricator.
The architect on this project, Jim Sanford (also of
Middlebury), had already collaborated with the Carrara plant on
several precast projects and so was familiar with the
connection details that would be needed between the concrete
panels and the wood framing and millwork used to finish the
house. Working from Sanford's architectural plans, the
fabricator's engineering team prepared shop drawings for each
Panel design. A few different types of precast wall
systems are available, including solid-wall and double-wall
panels. In special casting beds at its plant, Carrara forms
solid-wall precast panels for uninsulated walls and insulated
sandwich panels for enclosing conditioned living space (Figure
Figure 5. Precast concrete wall panels are
formed on a flat casting bed (top). Sandwich panels consist of
inner and outer skins of concrete separated by a layer of rigid
foam (bottom left); fiber-composite connectors (bottom right)
inserted through prepunched holes in the foam during casting
hold the inner and outer wythes together.
The company's insulated sandwich panels can be either
loadbearing or nonbearing, and can be built to various
thicknesses to achieve different R-values. The ones used on
this project were 11 inches thick and had a 3-inch-thick
rigid-foam core sandwiched between inner and outer skins of
reinforced concrete. According to an ASHRAE 90.1-2001
envelope-performance study for this project, the thermal mass
of the concrete combined with the 3 inches of rigid foam makes
these 11-inch walls perform like R-32 framed walls.
The 3-inch outer skin and 5-inch inner wythes of concrete are
reinforced with welded wire fabric and #4 and #5 rebar grids.
Special fiber-composite connectors hold the inner and outer
concrete wythes together without creating a thermal bridge
between the inner and outer skins.
Pressure-treated door and window bucks and top plates,
reinforced welding plates, structural inserts, and other
details are specified in the fabricator's shop drawings, which
workers at the plant refer to as they assemble the form.
Depending on the desired exterior finish, other materials like
form liners of brick veneer can also be added. On this house,
an exposed aggregate finish was specified, so the forms were
sprayed with a retardant before any concrete was placed.
The wall panels are cast from self-consolidating concrete (SCC)
with a 3,500-psi release strength and a 5,000-psi 28-day
strength. High-performance SCC concrete is designed to fill
formwork and flow around complicated reinforcement without
mechanical vibration. Admixtures such as fly ash and silica
fume (byproducts of the coal-fired generation of electricity)
give the concrete extra strength.
Since the interior of the precast panels in this project would
remain exposed, the casting was given a smooth steel-trowel
finish. After curing for a day, the forms were removed and the
exteriors washed to expose the aggregate. The tallest panels
measured a little over 12 feet high, and the longest about 26
Installation. Whereas hollow-core planks can be laid
flat and stacked on top of each other in a flatbed trailer for
transport to the site, large wall panels must be propped at an
angle on racks to prevent damage. A delivery might consist of
only one or two wall panels.
Once the panels for this project arrived on site, a large crane
lifted them directly off the truck and placed them into
position. While a gas-driven vacuum lift can be used to lift
the floor planks, the wall panels were hoisted into place with
the help of lifting rings bolted to inserts cast into the
concrete at the plant.
Most of the time, precast wall panels are set directly on
footings. However, these panels — along with the edges of
the concrete-plank floor system — bear directly on top of
the foundation walls.
Panels are cast with #5 rebar pins projecting 8 inches from the
bottom; when they're installed, these pins slide into 1
1/2-inch-diameter holes in the top of the foundation wall. The
holes are carefully laid out and cored after the panels have
been cast. During installation, the holes are filled with
nonshrink grout; small plastic shims are used to support and
level the panels. Later, any gaps between the top of the
foundation wall and the bottom of each panel are sealed with
caulk (Figure 6).
Figure 6. Rebar pins at the base of each
wall panel slide into holes bored into the top of the
foundation wall (top); the holes are filled with grout before
installation, and plastic shims are used to level the panels.
Braces lock the panels into position until the framing is
After each of the wall panels has been positioned, leveled, and
plumbed with the help of the crane, the installation crew locks
it into place with adjustable steel braces bolted to threaded
inserts cast into the panel and drilled into the concrete deck.
At intersections and corners, wall sections are fastened to
each other by welding together steel plates cast into the
panels (Figure 7). Any gaps between the panels at corners are
filled with caulk.
Figure 7. After assembly, the metal corner
connector plates (top photos) are welded together (middle).
Once the panels are in place (bottom), regular framing and door
and window installation can begin.
Once the panels are installed and braced, conventional framing
can begin. On this project, the second-story floor system and
roof framing were simply nailed to wood plates connected to
pressure-treated bucks embedded in the wall panels during
fabrication. PT stock was used wherever the framing came in
contact with the concrete.
Cost and Performance
Comparing the cost of building with precast concrete panels
with that of conventional framing is difficult. Subtrades
— such as electrical and hvac — may incur
additional expenses working around some of the limitations
presented by this type of construction. For example, the
hollow-core plank floor system has cavities through which
electrical and mechanicals can be routed, while the wall panels
do not (though chases and outlet boxes can be cast into wall
panels for extra cost).
On the other hand, all the insulation and the finished surfaces
for both interior and exterior walls can be included in the
cost of fabricating wall panels, which Carrara estimates runs
between $20 to $75 per square foot of wall area, depending on
the size and complexity of the panel.
A recently completed energy analysis indicated that this house
outperforms a comparable Energy Star home by more than 50
percent, thanks in part to the thermal mass of the walls and
the house's low air-infiltration rate. But with rising fuel
costs, the viability of precast concrete construction may be
more dependent on the proximity of a precast plant in your
area, since multiple truckloads will be required to deliver
wall panels to a job site.
Andrew Wormer is an associate editor at