TECHNICAL PAPER #64
 
                       UNDERSTANDING FERROCEMENT
                             CONSTRUCTION
 
                                  By
                              J.P. Hartog
 
                          Technical Reviewers
                             Edward Harper
                             Louis Zapata
 
                             Published By
 
                                 VITA
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                 Tel: 703/276-1800 . Fax 703/243-1865
                      Internet: pr-info@vita.org
 
 
                Understanding Ferrocement Construction
                          ISBN: 0-86619-284-0
              [C]1988, Volunteers in Technical Assistance
 
 
                                PREFACE
 
This paper is one of a series published by Volunteers in Technical
assistance to provide an introduction to specific state-of-the-art
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 situations.
They are not intended to provide construction or implementation
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 illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  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 Margaret
as senior editor.
 
J.P. Hartog, the author of this paper, has worked over the past
30 years in naval architecture.   Mr. Hartog is experienced in the
areas of boat building and design, and has extensive knowledge of
ferrocement design and construction.   A native of Holland, he
received his degree in structural engineering form the Technical
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 qualified
boat builder with experience in wood, fiberglass, and ferrocement.
He also lectures in naval architecture and ship building.
He is employed by he College of Fisheries, St. John's, New Foundland.
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 construction.
He received his B.S. in Physics from San Jose State College,
Jan Jose, California.
 
VITA is a private, nonprofit organizations that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
 
                UNDERSTANDING FERROCEMENT CONSTRUCTION
 
                     by VITA Volunteer J.P. Hartog
 
1. OVERVIEW
 
What is Ferrocement?
 
Ferrocement is a building material composed of a relatively thin
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 construction
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 wire,
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 construction.
Virtually everything can be done by hand, and no expensive machinery
is needed.
 
Here are some additional advantages of ferrocement construction.
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
framework.  The 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 into
simple or compound curves.  In contrast, RCC construction is cast
in sections and needs extensive and very solid formwork to support
the weight of the concrete.
 
In Third World countries, ferrocement is almost always economically
competitive with steel, wood, or glass-fiber reinforced
plastic (FRP) construction, because steel and FRP are expensive
and wood is becoming more and more scarce.   Because its use for
construction requires locally available materials and a large
supply of hand labor, local jobs can be created.
 
What are the disadvantages of ferrocement? Structures made of it
can be punctured by forceful collision with pointed objects.  Boat
hulls used in deep water are subject to this danger unless expertly
designed.  Because of the danger that many lives may be
lost at sea, hulls for deep water should be constructed under
direct, expert supervision.  If serious damage does occur, it may
be difficult in some countries to locate a skilled repair shop.
 
In corrosive environments (for example, sea water) it is often
observed that after several decades the reinforcing materials
become corroded.  However, this failure is almost always due to
incomplete coverage of the metal by mortar during construction.
Special care must be used to cover it completely if the mortar is
porous or is applied by spraying.
 
It is nearly impossible to fasten objects to ferrocement with
bolts or screws, because drills usually break against the lightly
covered reinforcing material.   Fastening with nails or by welding
is not possible.
 
Although the ease of ferrocement construction encourages people
to try it who have never built anything, the results of amateur
effort can appear shoddy.  It has been observed that visitors to a
harbor can immediately identify the badly built boat hulls as
ferrocement; the casual observer usually mistakes neat ferrocement
hulls for another material.  Such perceptions often discourage
authorities from approving the use of ferrocement.
 
Some Applications
 
Ferrocement's features make it useful in a wide range of applications,
including aqueducts, boats, buildings, bus shelters,
bridge decks, concrete road repair, factory-built homes, food and
water storage containers, irrigation structures, retaining walls,
sculptures, and traffic-caution signboards.   In its final cured
stage, ferrocement is somewhat flexible and can be bent slightly
without developing cracks.  Ferrocement can be used in such compound-curved
structures as domes, roofs, and ship hulls.   Compound
curvature adds to the strength, stiffness, and impact resistance
of these structures, which can be built over a minimum of internal
forms.  Round or conical tanks, silos, and pontoons can also
be constructed very satisfactorily with thin-walled ferrocement.
 
The least desirable designs for ferrocement construction are
those that have large flat surfaces combined with angles of 90
degrees or less.  However, non-bearing walls, partitions, dock
floats and septic tanks, with or without internal or external
stiffening, have been successfully constructed.   Large, flat-bottomed
barges can also be built with ferrocement in combination
with precast RCC frames and girders.
 
History
 
The practice of mixing burnt lime with water to make cement can
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 into
use.  Portland 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 metal
reinforcing inside concrete.   The Chinese had long used cement in
combination with bamboo-rod reinforcing for building boats.  The
use of ferrocement as a boat-building material was demonstrated
by the Italian engineer and architect Pier Luigi Nervi in 1945,
when his firm built the 150-metric ton motor sailer Irene.  The
hull was only 35 mm thick, and was reinforced with three layers
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 percent
less.  The Irene proved to be a seaworthy vessel, with very
little maintenance, and survived two serious accidents that required
only simple repairs.
 
By the early 1960s, ferrocement had gained wider acceptance as a
construction material, especially in boat building.   After 1970,
production slowed because of the rising costs of materials and,
especially, labor.  Ferrocement construction, however, continues
to offer unlimited possibilities for uses both on water and land
in places where labor costs are low.
 
2. TECHNOLOGY
 
Ferrocement is a form of RCC made from mortar and layers of thinly
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 (see
Figure 1 and Table 1).  Mortar is the term applied to the mixture

ufc1x3.gif (486x486)


of cement, sand, and water before it solidifies into concrete.
 
The main steps in ferrocement construction are assembly of forms
(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 vertically.
Two layers of 19 gage, 11-mm opening, square mesh on each side.
Total weight, about 44 kg/[m.sup.2] (9 pounds/square foot), of which 18%
is steel.
 
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 side.
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 vertically.  Each
side covered with one layer of 19 gage, 11-mm opening, welded
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.
 
                         Table 1
               FORCES ON FERROCEMENT STRUCTURES
 
Compression    Tends to press together or make more compact.
 
Crushing       Presses between two opposing forces so as to
               break, squeeze together, or put out of shape.
 
Flexing        Bends or curves without breaking; perhaps under
               its own weight.
 
Impact         Hits with force, collision, or violent contact.
 
Shear          Forces two contacting layers to slide upon each
               other in opposite directions parallel to the plane
               of their contact.
 
Tension        Tends to cause extension or increase in length.
 
 
2.1 FORMWORK
 
Forms can either be removable or can be incorporated into the
finished product.  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 one
structure of a kind is to be made.
 
Wooden-Frame Method
 
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 stapled
to them.  The other layers of mesh and rods are then solidly
tied to the inside layers and to each other, and the entire form
is checked for smoothness before applying mortar.   After the
structure has cured, it can be lifted off the form, which may be
used again.
 
The advantage of the open wooden-frame method is that small
structures can be built with simple woodworking hand tools.  Disadvantages
are that it requires a large quantity of wood, that it
must be done carefully in order to get a good finish on the interior,
and that the wood is some times difficult to remove and may
not be reusable.  This method is in common use for making small
boats.
 
Pipe-Frame Method
 
Steel water pipe (schedule 40ST material, about 27 mm outside
diameter, 21 mm inside diameter; nominal 3/4-inch diameter) takes
the place of wooden frames.  The pipes are incorporated into the
ferrocement structure and act as transverse stiffeners.   The longitudinal
rods are positioned and tied to the pipes.   The inner
layers of mesh are tied to the rods and worked into position over
the pipes.
 
For more complex structures, construction of the pipe frame can
require welding and pipe-bending equipment (which can be as simple
as two 35-mm diameter fixed pins in a solid mounting).   Temporary
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 leave a
void.
 
Trussed-Frame or Webbed-Frame Method
 
Instead of pipes, trussed or webbed frames made of reinforced
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 together,
saving time and effort and reducing the amount of wood framing
needed.
 
2.2 REINFORCING MATERIALS
 
Many different kinds of reinforcing steel can be used.   The material
must be flexible; the tighter the curves of the structure,
the more flexible the reinforcing material must be.   Chicken wire
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 such
high performance structures as deep-water marine hulls.   Wire mesh
can be woven on site from coils of straight wire, using a hand
loom adapted for the purpose.
 
For adequate crack-resistance, stiffness, and strength, a minimum
of 30 pounds of steel to one cubic foot of ferrocement is recommended.
This and other properties of ferrocement are shown in
Table 2.
 
                         Table 2
            SOME PROPERTIES OF A FLAT FERROCEMENT SLAB
                   Slab size =  one square meter.
 
Note: 1 inch = 25 mm, 1 foot = 305 mm, 1 pound avoirdupois =
0.45 kg.
 
                                 Minimal            Minimal
Thickness, Volume,     Weight,   recommended        recommended
mm         [m.sup.3]   kg         Wt. of steel,      reinforcing
                                 kg                 surface, [m.sup.3]
 
   15        0.015       40            7                 3
  25         0.025        70          12                 5
  35         0.035       100          17                 7
 
 
The adhesion between the mortar and the steel is of utmost importance
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 square
inches per cubic inch of mortar (Table 2).
 
Because the maximal tensile or shearing stresses (Table 1) occur
at the surfaces of the ferrocement slab, the mesh layers should
be positioned as close to the surface as possible.   At the same
time, the steel must be completely covered to protect it from
corrosion (Figure 1).  In thin-walled ferrocement , small-diameter

ufc1x3.gif (600x600)


wires are used in the outer layers and the lowest possible cement-to-water
ratio is used, in order to give the greatest protection
against corrosion.
 
To prevent cracking, the mortar layer covering the mesh should be
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 impact
resistance after the mesh and rods have been tied together with
wire ties.
 
 
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 water to
prevent the formation of gas bubbles along the galvanized surfaces.
The bubbles would adversely affect the bond between mortar
and steel.
 
Instead of the conventional mesh-and-rods design, several layers
of expanded metal have been used with considerable success.  The
layers of expanded metal are a little more difficult to form over
compound curvatures, but they have sufficient adhesive surface,
impact-resistance, and stiffness.
 
A minimum of two layers of 3/8 inch (9 mm opening) expanded metal,
or equivalent weight in mesh or chicken wire, is used on each
side.
 
                         Table 3
         COMMON TYPES OF METALLIC MESH FOR REINFORCEMENT
 
 
Name                            Opening,     Wire       Weight,
                                mm           gage no.   kg/[m.sup.2]
 
Galvanized, expanded metal        9          --             1.85
Square, welded mesh              12          19            1.15
Stucco wire                     25           20            0.49
Chicken wire                     25          18            0.93
Chicken wire                     12          22            0.62
 
 
Two layers of rods are used, usually spaced at intervals no
greater than 100 mm both horizontally and vertically (Figure 1).
For continuous strength, the mesh sections should be tied with a
minimum overlap of 100 mm and the rods should have a minimum
overlap of 40 times their diameter (a 250-mm overlap for 6-mm
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, well-graded
sharp sand, clean water and, optionally, small amounts of additives
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; 10%
to 15% should pass through a #100 mesh sieve (opening 0.149 mm),
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 causes
rust in the reinforcing.  Up to 15% of the cement may be replaced
by plasticizing and air-entraining agents, for example, pozzolan,
diatomaceous earth, or fly ash.   The ratio of water to cement
should be 0.45:1 by weight if the sand is perfectly dry; otherwise
it should be 0.40:1.
 
In some circumstances the use of a high-early strength Portland
cement is advantageous, for example in production-line work,
where it is desirable to remove the structures from the forms as
soon as possible, or in cold climates to reduce the period needed
for protection against low temperatures.   Type III Portland cement,
which is used primarily for mass production by commercial
ferrocement builders, fulfills these requirements.   However, its
alkaline (salt-water) resistance is low.   Type V Portland cement,
although slower setting than Type III, is preferred for ferrocement
construction because of its high resistance to sulfate and
to alkaline solutions.
 
The chemical reaction between the cement and water (called hydration)
in the mortar mix makes the mortar set hard.   The hardening
(and strengthening) of the mortar is rapid at first.   It reaches
near-maximum strength by the time curing is complete, usually up
to 30 days.  The mortar must be kept moist during application and
curing.
 
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.   Near the
boiling point, the early hardening will occur too fast.   The hydration
process also produces some heat.   However, in thin-walled
ferrocement structures the heating effect is negligible.  The
mortar will generally achieve a compression strength of 4,400
pounds per square inch (310 kg/[cm.sup.2]) in 28 gays when the temperature
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 a
water-cement ratio of 0.40:1 should be used for a workable mix
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 contained
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 the
other one in an oven.  The weight difference between the two samples
shows the amount of water already in the mix.   That weight
should be subtracted from the amount of water to be added to the
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 section
of the structure that is to be built.   Use the same rods and mesh
arrangement with the mortar that will be used in the structure.
Another, less accurate, method is the widely-used "slump test".  A
sheet metal cone about 450 mm (18 inches) high is filled with
several layers of mortar and rods.   The last layer or mortar is
trowelled flat and the cone is set base down on a flat, horizontal
surface.  Then the cone is carefully lifted, leaving the contents
behind.  The difference between the height of the metal cone
and the height of the wet contents is called the slump; it measures
the relative water content of the mortar.   A good dry mix,
as used for ferrocement, should show not more than 65 mm (2-1/2
inches) of slump.  More would indicate excessive wetness and could
result in shrinkage and cracks.
 
Compromises are sometimes necessary in the composition of ferrocement
mortars.  A high cement-to-sand ratio makes a strong, rich
mortar, which is more workable, produces a better finish, and is
far less permeable to water than a weaker mortar with a lower
cement-to-sand ratio.  However, a rich mixture shrinks more than a
weaker mortar, causing hair cracks and sometimes large cracks as
well.
 
For important projects, test panels should be made and, after
curing, can be laboratory tested to determine crushing, compression,
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 workable
mix.
 
For large structures and where the distance from the mixing site
to the construction site is considerable, it may be advantageous
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 75
mm or more.  A disadvantage of this method is that incomplete
mixing or separation of the cement and sand during travel can
clog the pipes.  They must then be taken apart, cleaned out, and
reassembled, resulting in a substantial loss of time and labor.
The available mortar guns have not been successfully used because
the heavier parts of the cement-sand mix tend to separate at the
hose nozzles.
 
After checking the reinforcing for smoothness (and pounding out
flat spots, retying loose mesh, etc.), the structure is ready for
mortar.  All loose rust should be wire-brushed off; oily and dirty
surfaces should be sprayed with a hydrochloric acid (HCl; danger:
protect skin and eyes) solution and, after cleaning, neutralized
with fresh water.
 
All the mortar should be applied at one time at an even temperature;
it should be shaded from direct sunlight and winds, and
protected from frost.  A few simple tools are needed:  buckets or
shallow containers to carry the mortar; steel and wooden floats;
soft brooms for erasing float marks; and long flexible boards for
finishing long, curved surfaces.
 
The stiff mortar is pushed with hand pressure through the reinforcing.
As this is done, great care must be taken to avoid leaving
air pockets, which can occur in back of the rods or the expanded
metal.  In places where penetration is very difficult, a
pencil vibrator or an orbital sander with a metal plate substituted
for the sandpaper pad can be used to ensure complete covering
of the reinforcing by the mortar.   Localized vibration can
also be created by using a piece of wood with a handle attached.
 
Air pockets can be located after curing by tapping the structure
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 rods
until it appears on the other side, where the other workers finish
it off smoothly with approximately 2 mm of mortar protruding
beyond the mesh.  The same finishing is then done on the opposite
side.
 
It is of the utmost importance that none of the work that has
been completed be allowed to dry out while the workers are completing
another part of the structure.   In direct sunlight or
during hot weather, moistened gunny sacks or other coarsely woven
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 put
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 conventional
tilting-drum mixer, because of the stiffness of the
mortar used for ferrocement construction.
 
2.4 CURING
 
Curing reduces shrinkage and increases strength and water tightness.
There are two types of curing:   wet curing and steam curing.
 
The ideal method of wet curing is to immerse the structure completely
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 purpose.
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 steam-cleaning
plant or boiler) under this tent, close to (or under) the structure.
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 (82 [degrees]C).
This temperature should be held for at least four hours, after
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 24
hours, compared with a minimum 14 days for wet curing.   However,
steam curing may result in a less durable, more porous structure,
especially if it is done by an inexperienced person.
 
2.5 FINISHING AND PAINTING
 
After curing, the surface is rubbed down with abrasive (carbide)
stone to achieve a smooth finish, and then rinsed thoroughly with
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 with a
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 resin,
and one or more coats of epoxy paint applied as a finish.   In the
author's experience, after sealing one side of the ferrocement
slab it is best to wait as long as possible before sealing the
other side.  Due to continuous hydration and curing, the untreated
surfaces will show a white powder for a long time.   Even after
careful removal of this powder and rinsing, it will take years
before paint will form a good bond with the untreated surface.
 
If boats will be left continuously in salt water, an anti-fouling
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 upon the
diesel fuel), the insides of the tanks should be sprayed with a
polysulfide compound.  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.   Ferrocement
tanks intended for water storage should be given a cement wash
inside and stored with a little water inside them.
 
Underground ferrocement grain silos in Ethiopia are waterproofed
with bitumen.  After curing, the surface is cleaned with a wire
brush, and a coat of bitumen emulsion (diluted 1 volume of emulsion
to 1 volume of water) is scrubbed into the surface.   After it
dries, a cement-emulsion mixture (1 volume of water to 1 volume
of cement to 10 volumes of emulsion) is brushed on.
 
2.6 EXAMPLES OF CONSTRUCTION FROM THAILAND
 
Example 1:  Storage Silos
 
Food and water storage silos are constructed in Thailand using
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.   Hoops of
reinforcing rod are positioned horizontally and are wired to the
pipes. One layer of wire mesh is placed on the outside of the
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 tested
by filling them with water for one week.   Leaking indicates cracks
or weak sections.
 
Example 2:  Irrigation Channels
 
Ferrocement has been successfully used for farm irrigation and
water-control structures, including water tanks, hydraulic gates,
pipes, irrigation channels, and channel linings.   Structures are
thinner and lighter than RCC and can be prefabricated or built on
site.  The use of forms is optional.  Typical drop channels measured
600 by 1000 mm.  Thickness was 30 mm.  Two layers of galvanized
hexagonal mesh (gage 21 with 19-mm mesh opening) were
used, one layer on each side of a framework of 6-mm mild steel
rods, placed 250 mm apart both horizontally and vertically.  The
mesh was then tied to the rods with wire.
 
For a channel section, a mold of 2-mm mild steel was used.  The
mild steel rods were 5 mm in diameter, each side covered with one
layer of galvanized hexagonal wire mesh, gage 21, 19-mm mesh
opening.  The edges of the mesh overlapped 100 mm.  All fabricated
structures were cured for 20 days.   It was found that the channel
sections could be made in larger units than RCC, thus reducing
the number of joints.
 
3. SUMMARY
 
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
   obtained locally;
 
o  Only a few simple hand tools are needed to build uncomplicated
   structures;
 
o  Repairs are usually easy and inexpensive;
 
o  No upkeep is necessary;
 
o  Structures are rot-, insect-, and rat-proof, and non-flammable;
 
o  Structures are highly waterproof, and give off no odors in a
   moist environment;
 
o  Structures have unobstructed interior room; and
 
o  Structures are strong and have good impact resistance.
 
The main disadvantage of ferrocement for smaller structures and
boats is its high density (2400 kg/[m.sup.3], 150 pounds/cubic foot).
Density is not a problem, however, for larger structures (for
example, large domes, tanks, and boats over 12 m long).   Large,
internally-unsupported domes and curved roofs have been built
that could not have been constructed with other materials without
elaborate ribs, trusses, and tie rods.
 
The large amount of labor required for ferrocement construction
is a disadvantage in countries where the cost of unskilled or
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.
 
BIBLIOGRAPHY
 
International Ferrocement Information Center, Proceedings of the
Second International Symposium on Ferrocement, 14-16 January
1985, Bangkok, Thailand. Bangkok:   author, 1985.
 
Journal of Ferrocement (quarterly).   International Ferrocement
Information Center, GPO Box 2754, Bangkok 10501, Thailand.
 
Narayan, J.P., V.V.N. Murty, and P. Nimityongskul, "Ferrocement
Farm Irrigation Structures."   Journal of Ferrocement, vol. 20,
pages 11-22, 1990.
 
Paramasivam, P., and T.F. Fwa, "Ferrocement Overlay for Concrete
Pavement Resurfacing."  Journal of Ferrocement, vol. 20, pages 23-29,
1990.
 
Romualdi, James P. (ed.), Ferrocement:   Applications in Developing
Countries. Washington, D.C.:   National Academy Press, 1973.
 
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