Shotcrete homes challenge wood-frameconstructionBy John A. Koski

A New England companyis producing affordable single-family homes usingwire-reinforced insulatedshotcrete construction

The first shotcrete homes

w e re built shortly after the

s h o t c rete process was in-

vented in 1910. Because of

s h o t c rete’s versatility (it can be ap-

plied to vertical, overhead, and hori-

zontal surfaces) many of those early

homes, as well as those produced in

later years and today, are intere s t i n g

and unique.

In the 1960s, a new shotcrete con-

s t ruction panel system was devel-

oped. Referred to as welded wire

sandwich panels (WWSP), the sys-

tem had a core of polyurethane or

p o l y s t y rene insulation sandwiched

between two layers of welded wire

fabric. The panels, which usually

m e a s u re about 4x8 feet, were ere c t e d

and joined on the jobsite, and cov-

e red with a minimum of 11⁄2 inches of

s h o t c rete on both sides.

Over the years, WWSP systems

have been used to construct a variety

of stru c t u res including hotels, schools,

prisons, warehouses, sea walls, and

highway sound-barrier walls. On the

residential side, construction of

WWSP homes in quantity has been

limited, due in large part to cost. In

most areas of the country, lumber

supplies have consistently been inex-

pensive and plentiful. This has helped

make traditional wood-frame con-

s t ruction the system of choice for

most homebuyers. As a result, WWSP

homes have remained largely a cu-

riosity in residential construction. The

times, however, are changing.

Within the last 18 months, govern-

ment regulations on timber harvest-

ing, sparked by environmental con-

cerns, have caused lumber prices to

s o a r. According to the National As-

sociation of Homebuilders, lumber

costs for a typical 2,000-square - f o o t

home have gone up over $5,000

since October 1992. That price in-

c rease has opened the door for alter-

native methods of home constru c-

tion, such as WWSP.

Finding a marketOne company hoping to profit fro m

the rising costs of lumber is Maine

Built Stru c t u res, a division of Shotcre t e

Systems International, Lincolnville,

Maine. Curre n t l y, the company has

five WWSP homes under constru c t i o n

in Rhode Island and expects to com-

plete an additional 36 homes there be-

f o re the end of this year. In 1994, the

company plans to build 200 WWSP

homes by expanding its operations to

other parts of New England and into

eastern Canada.

A c c o rding to re s e a rch by Rhode

A typical WWSP home nearing completion.

Island’s State-Wide Multiple Listing

Service, the median sales price of ex-

isting single-family homes in Rhode

Island in 1992 was $118,000. New

homes are considerably more expen-

sive. How economical is WWSP con-

s t ruction in this market? Maine Built

S t ru c t u res estimates it can build a

1 , 9 2 0 - s q u a re-foot, 3-bedroom, single-

family WWSP home with a full base-

ment and one-car attached garage for

$129,000. Because of the high cost of

land in Rhode Island, about half of

that amount is for the lot on which

the home is built. When operations

expand to other areas, where land is

less expensive, the company expects

to sell the homes

for under $100,000

with lot.

Panel compo-sition

The WWSP sys-

tem used by

Maine Built Stru c-

t u res consists of

4x16-foot panels

with a core of

modified expand-

ed polystyre n e .

Panels of other

sizes can be facto-

r y - c u s t o m i z e d .

The polystyre n e

c o re provides in-

sulation and is sandwiched be-

tween welded wire mesh. The mesh

uses 11-gauge wire spaced to cre a t e

2x2-inch grid openings. Galvanized

t russ wires pierce the polystyrene at

an angle and are welded to the wire

mesh on both sides of the panel.

The truss wires serve the dual pur-

pose of combining with the wire

mesh to give the panel stru c t u r a l

s t rength and to hold the mesh

about 3⁄4 inch from the surface of the

p o l y s t y rene core during shotcre t-

i n g .

The wire mesh is made from re c y-

cled steel. The polystyrene panels also

use recycled materials. Although the

company hasn’t actively marketed

this helps-save-the-environment as-

pect of its homes, Alan Lilly, senior

consultant for the company, says it

may do so in the future. The system’s

use of recycled materials hasn’t gone

unnoticed, however. For example, Lil-

ly says the Massachusetts Institute of

Technology recently asked the com-

pany to make a presentation at its En-

v i ronmental Expo Forum on its use of

the panels.

Panel characteristicsThe standard thickness of the poly-

s t y rene core is 21⁄2 inches, with the

panels developing an insulating value

of R-14 when 11⁄2 inches of shotcrete is

applied to each side of the panel. Pan-

els also are available with a core thick-

ness of 4 inches. When 2 inches of

s h o t c rete are applied to each side of

the 4-inch panels, they have an insu-

lating value of R-22.

In addition to its insulating value,

WWSP walls have excellent thermal

mass characteristics. Because the

homes have at least 3 inches of shot-

c rete in their walls, they absorb and

retain heat in winter and help keep

the house cool in summer.

Panel erectionPanel erection on the jobsite is re l-

atively easy because each 4x1 6 - f o o t ,

21⁄2-inch-thick WWSP panel weighs

less than 80 pounds. Electrical con-

duit is installed by running flexible

conduit sections between the mesh

and the panel core. To prevent their

f reezing in winter, water and sewer

lines are usually placed in the

homes’ conventional wood-frame

interior walls.

Window and door openings are

made using wire cutters to cut the

w i re mesh and a re c i p rocating saw to

cut the polystyrene. Adjoining pan-

els are fastened to one another using

strips of wire mesh laid over both

sides of the joint area and secure d

with wire ties or a pneumatic fasten-

ing tool. According to Lilly, the foun-

dations and walls for a home can be

e rected by a crew of four workers in

a matter of hours.

Once the panels are in place, dry-

mix shotcrete is applied. The shot-

c rete makes the panels rigid and pro-

duces a monolithic stru c t u re without

A WWSP home with the sandwich panels in place and ready for shotcreting.

Worker applying shotcrete to a WWSP home.

c o n s t ruction joints. The final exterior

finish usually consists of 8-inch bevel

aluminum or vinyl siding on the

back and sides of the home, with

s h o t c rete on the front of the home

t roweled to a stucco-like finish. Inte-

rior finishes consist of plaster over

the shotcrete, wood paneling, or tra-

ditional drywall.

Other WWSP benefitsPanels with a 21⁄2-inch styro f o a m

c o re and 11⁄2 inches of shotcrete on

each side have a fire rating of 11⁄2

hours. A 2-inch application of shot-

c rete on each side provides a 2-hour

f i re rating, and with 31⁄8 inches of

s h o t c rete on each side, WWSP panels

have a 4-hour fire rating. Accord i n g

to the company, the modified, ex-

panded polystyrene core is non-com-

bustible and melts, rather than

burns, when exposed to fire. In addi-

tion, the layered composition of the

panels is highly resistant to sound

transmission, vermin, and intru d e r

e n t r y, and there is no off-gassing that

might create health pro b l e m s .

Earthquakes and hurricanesLast year, two WWSP homes in

California survived earthquakes mea-

suring 6.6 and 6.9 on the Richter scale.

Both homes were within 70 miles of

the earthquakes’ epicenters and had

no structural damage.

When Hurricane Andrew devastat-

ed parts of Florida in 1991, survivors

included 14 WWSP homes. Damage

to the homes, built by Habitat for Hu-

manity volunteers, was mostly limit-

ed to windows and wood-frame

roofs. Conventional wood-frame

s t ru c t u res across the street from some

of the WWSP homes were leveled by

A n d rew’s high winds. In addition, as

reported in the April 1990 issue of

C o n c rete Construction, 200 WWSP

homes in Puerto Rico had no stru c t u r-

al damage after being hammered by

Hurricane Hugo’s 195 mile-per- h o u r

w i n d s .

P U B L I C ATION #C930798

Copyright © 1993, The Aberdeen Gro u p

All rights re s e r v e d