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Understanding Ferrocement Construction
Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are
urged to contact VITA or similar organizations
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Patrice Matthews
and Suzanne Brooks handling typesetting and layout, and
as senior editor.
J.P. Hartog, the author of this paper, has worked over the
30 years in naval architecture.
Mr. Hartog is experienced in the
areas of boat building and design, and has extensive
ferrocement design and construction.
A native of Holland, he
received his degree in structural engineering form the
University in Delft.
He is presently employed by the Holland
Marine Design, located in San Francisco, California.
Edward Harper, one of the reviewers of this paper, is a
boat builder with experience in wood, fiberglass, and
He also lectures in naval architecture and ship building.
He is employed by he College of Fisheries, St. John's, New
The other reviewer, Louis Zapata, operates Expressions,
Inc., located in Washington, D.C.
Expressions is an association
of independent contractors doing rehab and add-on new
He received his B.S. in Physics from San Jose State College,
Jan Jose, California.
VITA is a private, nonprofit organizations that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING FERROCEMENT CONSTRUCTION
by VITA Volunteer J.P. Hartog
What is Ferrocement?
Ferrocement is a building material composed of a relatively
layer of concrete, covering such reinforcing material as steel
wire mesh. Because
the building techniques are simple enough to
be done by unskilled labor, ferrocement is an attractive
method in areas where labor costs are low. Sand, cement,
and water usually can be obtained locally, and the cost of
the reinforcing material (steel rods, mesh, pipe, chicken
or expanded metal) can be kept to a minimum.
There is no need for
the complicated formwork of reinforced cement concrete (RCC)
construction, or for the welding needed for steel
Virtually everything can be done by hand, and no expensive
Here are some additional advantages of ferrocement
Ferrocement can be shaped in any form.
It can be formed into sections
less than 25 mm (1 inch) thick and assembled over a light
material is very dense, but structures made from
it are light in weight.
It is also rot- and vermin-proof, impervious
to worms and borers, and watertight.
Ferrocement is more versatile than RCC and can be formed
simple or compound curves.
In contrast, RCC construction is cast
in sections and needs extensive and very solid formwork to
the weight of the concrete.
In Third World countries, ferrocement is almost always
competitive with steel, wood, or glass-fiber reinforced
plastic (FRP) construction, because steel and FRP are
and wood is becoming more and more scarce.
Because its use for
construction requires locally available materials and a
supply of hand labor, local jobs can be created.
What are the disadvantages of ferrocement? Structures made
can be punctured by forceful collision with pointed
hulls used in deep water are subject to this danger unless
designed. Because of
the danger that many lives may be
lost at sea, hulls for deep water should be constructed
direct, expert supervision.
If serious damage does occur, it may
be difficult in some countries to locate a skilled repair
In corrosive environments (for example, sea water) it is
observed that after several decades the reinforcing
However, this failure is almost always due to
incomplete coverage of the metal by mortar during
Special care must be used to cover it completely if the
porous or is applied by spraying.
It is nearly impossible to fasten objects to ferrocement
bolts or screws, because drills usually break against the
covered reinforcing material.
Fastening with nails or by welding
is not possible.
Although the ease of ferrocement construction encourages
to try it who have never built anything, the results of
effort can appear shoddy.
It has been observed that visitors to a
harbor can immediately identify the badly built boat hulls
ferrocement; the casual observer usually mistakes neat
hulls for another material.
Such perceptions often discourage
authorities from approving the use of ferrocement.
Ferrocement's features make it useful in a wide range of
including aqueducts, boats, buildings, bus shelters,
bridge decks, concrete road repair, factory-built homes,
water storage containers, irrigation structures, retaining
sculptures, and traffic-caution signboards.
In its final cured
stage, ferrocement is somewhat flexible and can be bent
without developing cracks.
Ferrocement can be used in such compound-curved
structures as domes, roofs, and ship hulls.
curvature adds to the strength, stiffness, and impact
of these structures, which can be built over a minimum of
forms. Round or
conical tanks, silos, and pontoons can also
be constructed very satisfactorily with thin-walled
The least desirable designs for ferrocement construction are
those that have large flat surfaces combined with angles of
degrees or less.
However, non-bearing walls, partitions, dock
floats and septic tanks, with or without internal or
stiffening, have been successfully constructed.
barges can also be built with ferrocement in combination
with precast RCC frames and girders.
The practice of mixing burnt lime with water to make cement
be traced to antiquity.
The Romans were the first to use concrete
as a construction material.
They made a hard-setting concrete by
adding crushed volcanic powder (pozzolan) to the
mixture. In the
nineteenth century, modern hydraulic (Portland) cements came
cements set hard, and can withstand loads up to 420
kilograms per square centimeter.
In the 1840s, Joseph Louis Lambot of France began to put
reinforcing inside concrete.
The Chinese had long used cement in
combination with bamboo-rod reinforcing for building
use of ferrocement as a boat-building material was
by the Italian engineer and architect Pier Luigi Nervi in
when his firm built the 150-metric ton motor sailer
hull was only 35 mm thick, and was reinforced with three
of 6-mm (one-quarter inch) rods.
Four layers of mesh were used on
each side of the rods.
The hull weighed five percent less than a
comparable wooden hull, and the price (at that time) was 40
less. The Irene
proved to be a seaworthy vessel, with very
little maintenance, and survived two serious accidents that
only simple repairs.
By the early 1960s, ferrocement had gained wider acceptance
construction material, especially in boat building.
production slowed because of the rising costs of materials
Ferrocement construction, however, continues
to offer unlimited possibilities for uses both on water and
in places where labor costs are low.
Ferrocement is a form of RCC made from mortar and layers of
spaced steel rods or wires.
Layers behave together as a composite,
in which the concrete absorbs most of the compression and
the steel reinforcing absorbs the tensile and shear stresses
Figure 1 and Table 1).
Mortar is the term applied to the mixture
of cement, sand, and water before it solidifies into
The main steps in ferrocement construction are assembly of
(if used), assembly of reinforcing materials, application of
mortar, curing, and finishing and painting.
A. 5/8-inch (15-mm) slab.
Two layers of 4.5-mm to 5-mm mild steel
rods are spaced at 75-mm intervals horizontally and
Two layers of 19 gage, 11-mm opening, square mesh on each
Total weight, about 44 kg/[m.sup.2] (9 pounds/square foot),
of which 18%
B. 5/8-inch slab.
Four layers of expanded metal, 9-mm opening;
one layer of gage 22, 12-mm opening, chicken wire on each
Total weight, about 44 kg/[m.sup.2], of which 20% is steel.
C. 1-inch (25-mm) slab.
Two layers of 6-mm (1/4-inch) mild steel
rods spaced at 75-mm intervals horizontally and
side covered with one layer of 19 gage, 11-mm opening,
mesh. Then each side
covered with two layers of 18 gage, 25-mm
opening, chicken wire.
Total weight, about 70 kg/[m.sup.2], (14.3
pounds/square foot) of which 18% is steel.
ON FERROCEMENT STRUCTURES
to press together or make more compact.
Presses between two opposing forces so as to
squeeze together, or put out of shape.
or curves without breaking; perhaps under
with force, collision, or violent contact.
two contacting layers to slide upon each
in opposite directions parallel to the plane
to cause extension or increase in length.
Forms can either be removable or can be incorporated into
They should be strong enough to support themselves
and the weight of the steel and concrete structure before
the mortar has set.
Wooden frames are removable; if the work is
done with care, they can be collapsed for reuse if more than
structure of a kind is to be made.
Spaced, thin, narrow boards (battens) are nailed over fairly
widely-spaced wooden transverse forms or frames.
The first inside
layers of mesh are positioned over the battens and tied or
to them. The other
layers of mesh and rods are then solidly
tied to the inside layers and to each other, and the entire
is checked for smoothness before applying mortar.
structure has cured, it can be lifted off the form, which
The advantage of the open wooden-frame method is that small
structures can be built with simple woodworking hand
are that it requires a large quantity of wood, that it
must be done carefully in order to get a good finish on the
and that the wood is some times difficult to remove and may
not be reusable.
This method is in common use for making small
Steel water pipe (schedule 40ST material, about 27 mm
diameter, 21 mm inside diameter; nominal 3/4-inch diameter)
the place of wooden frames.
The pipes are incorporated into the
ferrocement structure and act as transverse stiffeners.
rods are positioned and tied to the pipes.
layers of mesh are tied to the rods and worked into position
For more complex structures, construction of the pipe frame
require welding and pipe-bending equipment (which can be as
as two 35-mm diameter fixed pins in a solid mounting).
reinforcing should be welded in because the pipe frames are
very floppy. A
disadvantage of the pipes is that unless filled
with a thin mortar, they can rust out from the inside and
Trussed-Frame or Webbed-Frame Method
Instead of pipes, trussed or webbed frames made of
bars and rods can be used.
The frames are covered with steel
mesh. An advantage
of this and the pipe-frame method is that
adjoining parts of the structure can often be constructed
saving time and effort and reducing the amount of wood
2.2 REINFORCING MATERIALS
Many different kinds of reinforcing steel can be used.
must be flexible; the tighter the curves of the structure,
the more flexible the reinforcing material must be.
may be the cheapest and easiest to use.
It is adequate for most
boats and for all uses on land, but is not recommended for
high performance structures as deep-water marine hulls.
can be woven on site from coils of straight wire, using a
loom adapted for the purpose.
For adequate crack-resistance, stiffness, and strength, a
of 30 pounds of steel to one cubic foot of ferrocement is
This and other properties of ferrocement are shown in
PROPERTIES OF A FLAT FERROCEMENT SLAB
Slab size = one square meter.
Note: 1 inch = 25 mm, 1 foot = 305 mm, 1 pound avoirdupois =
Wt. of steel,
The adhesion between the mortar and the steel is of utmost
in ferrocement construction.
The specific reinforcing surface
(the contact surface area of the rods, mesh, and/or expanded
metal per unit volume of mortar) should be at least five
inches per cubic inch of mortar (Table 2).
Because the maximal tensile or shearing stresses (Table 1)
at the surfaces of the ferrocement slab, the mesh layers
be positioned as close to the surface as possible.
At the same
time, the steel must be completely covered to protect it
corrosion (Figure 1).
In thin-walled ferrocement
wires are used in the outer layers and the lowest possible
ratio is used, in order to give the greatest protection
To prevent cracking, the mortar layer covering the mesh
not more than 2 mm (3/32 inch) thick.
Rods are used to space the
mesh, hold it in place, and to give added stiffness and
resistance after the mesh and rods have been tied together
If galvanized rods or mesh are used, a very small amount of chromium
trioxide ([Cr.sub.2][O.sub.3]) should be added to the mortar
prevent the formation of gas bubbles along the galvanized
The bubbles would adversely affect the bond between mortar
Instead of the conventional mesh-and-rods design, several
of expanded metal have been used with considerable
layers of expanded metal are a little more difficult to form
compound curvatures, but they have sufficient adhesive
impact-resistance, and stiffness.
A minimum of two layers of 3/8 inch (9 mm opening) expanded
or equivalent weight in mesh or chicken wire, is used on
OF METALLIC MESH FOR REINFORCEMENT
Galvanized, expanded metal
Square, welded mesh
Two layers of rods are used, usually spaced at intervals no
greater than 100 mm both horizontally and vertically (Figure
For continuous strength, the mesh sections should be tied
minimum overlap of 100 mm and the rods should have a minimum
overlap of 40 times their diameter (a 250-mm overlap for
rods). Extra rods
and mesh may be needed in certain areas; for
example, at the stems and keels of boats.
2.3 APPLYING MORTAR
Mortar is made from a good grade of Portland cement,
sharp sand, clean water and, optionally, small amounts of
to achieve an earlier setting strength or for plasticizing.
A rich mortar is used in ferrocement construction.
The ratio of cement to sand should be 1:2 by weight.
The sand used in the mortar should be clean, dry, and sharp;
to 15% should pass through a #100 mesh sieve (opening 0.149
and 100% through a #8 sieve (opening 2.38 mm).
Only fresh water
should be used for mixing.
Although salt water does not affect
the ultimate strength, it should be avoided, because it
rust in the reinforcing.
Up to 15% of the cement may be replaced
by plasticizing and air-entraining agents, for example,
diatomaceous earth, or fly ash.
The ratio of water to cement
should be 0.45:1 by weight if the sand is perfectly dry;
it should be 0.40:1.
In some circumstances the use of a high-early strength
cement is advantageous, for example in production-line work,
where it is desirable to remove the structures from the
soon as possible, or in cold climates to reduce the period
for protection against low temperatures.
Type III Portland cement,
which is used primarily for mass production by commercial
ferrocement builders, fulfills these requirements.
alkaline (salt-water) resistance is low.
Type V Portland cement,
although slower setting than Type III, is preferred for
construction because of its high resistance to sulfate and
to alkaline solutions.
The chemical reaction between the cement and water (called
in the mortar mix makes the mortar set hard.
(and strengthening) of the mortar is rapid at first.
near-maximum strength by the time curing is complete,
to 30 days. The
mortar must be kept moist during application and
The temperature during application and curing influences the
ultimate strength of the structure.
At freezing temperatures
(0 [degrees]C) or below, growing ice crystals will destroy
the bond between
sand and cement, causing the structure to fail.
boiling point, the early hardening will occur too fast.
process also produces some heat.
However, in thin-walled
ferrocement structures the heating effect is
mortar will generally achieve a compression strength of
pounds per square inch (310 kg/[cm.sup.2]) in 28 gays when
is 15 [degrees]C (60 [degrees]F), in 23 days at 21
[degrees]C (70 [degrees]F), and in 18
days at 26 [degrees]C (80 [degrees]F).
It was stated earlier that for most ferrocement construction
water-cement ratio of 0.40:1 should be used for a workable
and high strength.
This ratio assumes that the sand in the mix is
completely dry before the water is added.
As this is hardly ever
the case, allowance should be made for the water already
in the sand; the volume or weight of the water to be added
should then be adjusted.
This can be done by taking two identical
samples of the sand, weighing one sample on site, and drying
other one in an oven.
The weight difference between the two samples
shows the amount of water already in the mix.
should be subtracted from the amount of water to be added to
same volume of cement-sand mix as used in the sample.
The best test of a mortar mixture is to try it on a model
of the structure that is to be built.
Use the same rods and mesh
arrangement with the mortar that will be used in the
Another, less accurate, method is the widely-used
"slump test". A
sheet metal cone about 450 mm (18 inches) high is filled
several layers of mortar and rods.
The last layer or mortar is
trowelled flat and the cone is set base down on a flat,
surface. Then the
cone is carefully lifted, leaving the contents
difference between the height of the metal cone
and the height of the wet contents is called the slump; it
the relative water content of the mortar.
A good dry mix,
as used for ferrocement, should show not more than 65 mm
inches) of slump.
More would indicate excessive wetness and could
result in shrinkage and cracks.
Compromises are sometimes necessary in the composition of
mortars. A high
cement-to-sand ratio makes a strong, rich
mortar, which is more workable, produces a better finish,
far less permeable to water than a weaker mortar with a
However, a rich mixture shrinks more than a
weaker mortar, causing hair cracks and sometimes large
For important projects, test panels should be made and,
curing, can be laboratory tested to determine crushing,
tensile, shear, and flexing strengths, as well as impact
resistance (Table 1).
In general, a mortar made with a cement-to-sand
ratio of approximately 1:2 and a water-to-cement ratio of
0.40:1 will produce the least amount of shrinkage and a
For large structures and where the distance from the mixing
to the construction site is considerable, it may be
to pump the mortar to the construction area.
A special plasterer's
pump is used to transport the mortar through pipes to the
work site. For
better flow through the pipes, the water to cement
ratio should be slightly higher than normal, with a slump of
mm or more. A
disadvantage of this method is that incomplete
mixing or separation of the cement and sand during travel
clog the pipes. They
must then be taken apart, cleaned out, and
reassembled, resulting in a substantial loss of time and
The available mortar guns have not been successfully used
the heavier parts of the cement-sand mix tend to separate at
After checking the reinforcing for smoothness (and pounding
flat spots, retying loose mesh, etc.), the structure is
mortar. All loose
rust should be wire-brushed off; oily and dirty
surfaces should be sprayed with a hydrochloric acid (HCl;
protect skin and eyes) solution and, after cleaning,
with fresh water.
All the mortar should be applied at one time at an even
it should be shaded from direct sunlight and winds, and
protected from frost.
A few simple tools are needed:
shallow containers to carry the mortar; steel and wooden
soft brooms for erasing float marks; and long flexible
finishing long, curved surfaces.
The stiff mortar is pushed with hand pressure through the
As this is done, great care must be taken to avoid leaving
air pockets, which can occur in back of the rods or the
metal. In places
where penetration is very difficult, a
pencil vibrator or an orbital sander with a metal plate
for the sandpaper pad can be used to ensure complete
of the reinforcing by the mortar.
Localized vibration can
also be created by using a piece of wood with a handle
Air pockets can be located after curing by tapping the
with a hammer. These
places should be drilled out and filled with
a cement and water grout, or an epoxy compound.
Workers on one
side of the structure push the mortar through the mesh and
until it appears on the other side, where the other workers
it off smoothly with approximately 2 mm of mortar protruding
beyond the mesh. The
same finishing is then done on the opposite
It is of the utmost importance that none of the work that
been completed be allowed to dry out while the workers are
another part of the structure.
In direct sunlight or
during hot weather, moistened gunny sacks or other coarsely
cloth should cover completed areas.
If the work cannot be finished
in one operation, the finished work should be kept moist,
and a bond of thick cement grout or epoxy compound should be
on between the old and the new work.
Several polyvinyl- acetate
bonding products are also available.
If a concrete mixer is available,
a paddle-wheel type is greatly preferred over the
tilting-drum mixer, because of the stiffness of the
mortar used for ferrocement construction.
Curing reduces shrinkage and increases strength and water
There are two types of curing:
wet curing and steam curing.
The ideal method of wet curing is to immerse the structure
in water for a time that depends on the temperature of
the water. However,
immersion is not possible in most circumstances.
The accepted alternative is to cover the structure,
after all the mortar has been applied, with gunny sacks, tar
paper, or other fabrics, which are kept moist continuously.
Sprinklers or soaker hoses can also be used for this
This procedure must be carried out for at least 14
days. It is
desirable not to let the temperature fall below 68
[degrees]F (20 [degrees]C)
during the curing process.
Steam curing provides a moist atmosphere as well as a higher
temperature. It is
necessary to build a polyethylene tent over
the structure and move a steam-producing engine (a
plant or boiler) under this tent, close to (or under) the
No steam should be applied before the initial mortar set
has taken place.
After that, wet steam, at atmospheric pressure
only, should be applied slowly for approximately three hours
until the temperature inside the tent reaches 180 [degrees]F
This temperature should be held for at least four hours,
which it can be allowed to fall slowly.
The advantage of steam
curing is that the mortar achieves its 28-day strength in 12
hours, and the structure can be moved and worked on within
hours, compared with a minimum 14 days for wet curing.
steam curing may result in a less durable, more porous
especially if it is done by an inexperienced person.
2.5 FINISHING AND PAINTING
After curing, the surface is rubbed down with abrasive
stone to achieve a smooth finish, and then rinsed thoroughly
fresh water. Because
well-made ferrocement is impermeable (waterproof),
there should be no need for painting.
However, if painting
is desired, the structure should first be scrubbed with a 5%
to 10% solution of hydrochloric acid (HCl; protect eyes and
skin), flushed with clean, fresh water, and scrubbed again
weak solution of caustic soda (NaOH; protect eyes and skin),
after which it must be rinsed again.
The ferrocement can then be sealed with a coat of epoxy
and one or more coats of epoxy paint applied as a finish.
author's experience, after sealing one side of the
slab it is best to wait as long as possible before sealing
other side. Due to
continuous hydration and curing, the untreated
surfaces will show a white powder for a long time.
careful removal of this powder and rinsing, it will take
before paint will form a good bond with the untreated
If boats will be left continuously in salt water, an
paint should be applied below the water line.
For storage of diesel
fuel in ferrocement tanks (not recommended because of the
adverse effect of the alkaline action of the ferrocement
diesel fuel), the insides of the tanks should be sprayed
Several kinds of epoxy resins and compounds
are also available for the protection of bare metal, bonding
cement to any other material, filling in voids, etc.
tanks intended for water storage should be given a cement
inside and stored with a little water inside them.
Underground ferrocement grain silos in Ethiopia are
with bitumen. After
curing, the surface is cleaned with a wire
brush, and a coat of bitumen emulsion (diluted 1 volume of
to 1 volume of water) is scrubbed into the surface.
dries, a cement-emulsion mixture (1 volume of water to 1
of cement to 10 volumes of emulsion) is brushed on.
2.6 EXAMPLES OF CONSTRUCTION FROM THAILAND
Example 1: Storage
Food and water storage silos are constructed in Thailand
ferrocement with pipes or bamboo struts.
The base of the cone-shaped
silo is constructed first.
Then mesh from the base is
worked into the water pipe- or bamboo-framed walls.
reinforcing rod are positioned horizontally and are wired to
pipes. One layer of wire mesh is placed on the outside of
frame, and one on the inside.
Mesh, rods, and pipe are then fastened
together with short lengths of wire threaded through the
wall and twisted with pliers.
The water tightness of ferrocement grain storage bins is
by filling them with water for one week.
Leaking indicates cracks
or weak sections.
Ferrocement has been successfully used for farm irrigation
water-control structures, including water tanks, hydraulic
pipes, irrigation channels, and channel linings.
thinner and lighter than RCC and can be prefabricated or
site. The use of
forms is optional. Typical drop
600 by 1000 mm.
Thickness was 30 mm. Two layers
hexagonal mesh (gage 21 with 19-mm mesh opening) were
used, one layer on each side of a framework of 6-mm mild
rods, placed 250 mm apart both horizontally and
mesh was then tied to the rods with wire.
For a channel section, a mold of 2-mm mild steel was
mild steel rods were 5 mm in diameter, each side covered
layer of galvanized hexagonal wire mesh, gage 21, 19-mm mesh
opening. The edges
of the mesh overlapped 100 mm. All
structures were cured for 20 days.
It was found that the channel
sections could be made in larger units than RCC, thus
the number of joints.
The advantages of ferrocement construction are as follows:
o It is highly
versatile and can be formed into almost any
shape for a wide
range of uses;
o Its simple
techniques require a minimum of skilled labor;
o The materials are
relatively inexpensive, and can usually be
o Only a few simple
hand tools are needed to build uncomplicated
o Repairs are
usually easy and inexpensive;
o No upkeep is
o Structures are
rot-, insect-, and rat-proof, and non-flammable;
o Structures are
highly waterproof, and give off no odors in a
o Structures have
unobstructed interior room; and
o Structures are
strong and have good impact resistance.
The main disadvantage of ferrocement for smaller structures
boats is its high density (2400 kg/[m.sup.3], 150 pounds/cubic
Density is not a problem, however, for larger structures
example, large domes, tanks, and boats over 12 m long).
internally-unsupported domes and curved roofs have been
that could not have been constructed with other materials
elaborate ribs, trusses, and tie rods.
The large amount of labor required for ferrocement
is a disadvantage in countries where the cost of unskilled
semi-skilled labor is high.
Tying the rods and mesh together is
especially tedious and time consuming.
It is not possible to nail, screw, or weld to ferrocement.
International Ferrocement Information Center, Proceedings of
Second International Symposium on Ferrocement, 14-16 January
1985, Bangkok, Thailand. Bangkok:
Journal of Ferrocement (quarterly).
Information Center, GPO Box 2754, Bangkok 10501, Thailand.
Narayan, J.P., V.V.N. Murty, and P. Nimityongskul,
Farm Irrigation Structures."
Journal of Ferrocement, vol. 20,
pages 11-22, 1990.
Paramasivam, P., and T.F. Fwa, "Ferrocement Overlay for
Journal of Ferrocement, vol. 20, pages 23-29,
Romualdi, James P. (ed.), Ferrocement:
Applications in Developing
Countries. Washington, D.C.:
National Academy Press, 1973.