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Concrete is a strong and inexpensive construction material
when it is properly
prepared and used. This introduction explains the importance
of a good mixture
and describes the materials used in the mixture. Following
this are entries on:
Calculating amounts of materials for
Mixing concrete by machine or by hand
Testing concrete mixtures
Making forms for concrete
Placing concrete in forms
Making quick-setting concrete
Useful sources of information on concrete
Concrete is made by combining the proper proportions of
cement, water, fine
aggregate (sand), and coarse aggregate (gravel). A chemical
takes place between the water and cement, causing the
concrete to harden or set
rapidly at first, then more slowly over a long period of
Importance of a Good Mixture
After concrete has set, there is no simple non-destructive
test to find out how
strong it is. Therefore, the entire
responsibility for making concrete
as strong as a particular job
demands rests with the supervisor
and the people who prepare,
measure, and mix the ingredients,
place them in the forms, and watch
over the concrete while it hardens.
The most important factor in
making strong concrete is the
amount of water used. Beginners
are likely to use too much. In
general, the lower the ratio of
water to cement, the stronger the
concrete will be. <see figure 1>
The proper proportioning of all materials is essential. The
section on "Calculating
Amounts of Materials for Concrete" provides the
Aggregates: Gravel and Sand
To make strong concrete, the coarse aggregate (gravel) and
fine aggregate (sand)
must be the right size, have the right shape, and be
Coarse aggregate sizes can vary from 0.5cm (1/4") to 4
or 5cm (1 1/2" or 2") in
diameter. The maximum size depends on the nature of the
work. In general the
largest particles should not be more than one-fourth the
thickness of the smallest
dimension of the section. Sand can vary from sizes smaller
than 0.5cm down to,
but not including, silty material.
Very sharp, rough, or flat aggregate should not be used in
concrete. The best
aggregate is cubical material (from a rock crusher) or
rounded gravel (from a
stream bed or beach).
Proper grading means that there are not too many grains or
pebbles of any one
size. To visualize this, think of a large pile of stones all
5cm (2") in diameter.
There would be spaces between these stones where smaller
pebbles would fit. We
could add to the pile just enough smaller stones to fill the
largest spaces. Now
the spaces would be smaller yet, and even smaller pebbles
could fill these holes;
and so forth. Carried to an extreme, the pile would become
nearly solid rock, and
only a very small amount of cement would be needed to fill
the remaining spaces
and hold the concrete together. The resulting concrete would
be very dense,
strong, and economical.
It is extremely important that the aggregate and sand be
clean. Silt, clay, or bits
of organic matter will ruin concrete if too much is present.
A very simple test
for cleanliness makes use of a clear wide-mouth jar. Fill
the jar to a depth of
5cm (2") with the fine aggregate (sand) and then add
water until the jar is three
quarters full. Shake the mixture
vigorously for a minute. The last
few shakes should be sideways to
let the sand level off. Then let it
stand for three hours. If there is
silt in the sand, it will form a
distinct layer above the sand. If
the layer of very fine material is
more than 3mm (1/8") deep, the
concrete will be weak. <see figure 2>
If there is too much fine or silty material, another source
of sand should be
found. If this is impractical, it is possible to remove the
fine particles. This can
be done by putting the sand in a container like a drum.
Cover the sand with
water, stir or agitate vigorously, let it stand for a minute,
then pour off the
liquid. A few such treatments will remove most of the fine
and organic matter.
In very dry climates, the sand may be perfectly dry. Very
dry sand will pack into
a much smaller volume than sand that is moist. If 2 buckets
of water are added
to 20 buckets of bone dry sand, you can carry away about 27
buckets of damp
sand. If your sand is completely dry, add some water to it.
Another point to consider in selecting an aggregate is its
strength. About the
only simple test is to break some of the stones with a
hammer. If the effort
required to break the majority of stones is greater than the
effort required to
break a piece of concrete of about the same size, the
aggreage will make strong
concrete. If the stone breaks easily, the concrete made of
these stones will be no
stronger than the stones themselves.
The water used to prepare concrete must be clean, and free
of organic matter.
Water acceptable for drinking is preferable. Any clear,
fresh water is acceptable.
Salt water may be used if fresh water is not readily
available, but it will reduce
the strength of concrete about 15 percent.
If you must use dirty or muddy water, let the water settle
in a large pan or tank
to remove most of the dirt.
Cement for concrete, if it is a U.S. brand, comes in 42.6kg
(94 pound) sacks, and
is 28.4 liters (exactly 1 cubic foot) in volume. It must be
kept perfectly dry prior
to use, or the chemical action will begin and the cement
will be ruined.
Mixing the materials, getting them in place rapidly, tamping
or compacting to a
dense mixture, and proper curing are important parts of the
These will be discussed in the sections on mixing and curing
Concrete reinforced with steel rods is used for structures
such as large buildings
and bridges. Proper design of reinforced concrete and
placement of steel reinforcing
is a complex procedure that requires the help of a trained
CALCULATING MATERIALS FOR CONCRETE
Three methods are given here for finding the correct
proportions of cement,
water, and aggregate for concrete:
A "Concrete Calculator" fold-out
Using water to estimate proportions
A "rule of thumb"
Using the Concrete Calculator
The amounts of materials needed for a concrete construction
job can be estimated
quickly and accurately with the "Concrete
Calculator" chart. The chart is given in
both English (Chart A) and metric (Chart B) units.
To use one of the charts, you must know:
o The area of
concrete needed in square meters or square feet.
o The thickness of
concrete needed in centimeters (inches).
o The kind of work
to be done (see below).
o The wetness of the
sand (see below).
To use the calculator, follow these steps
o Make a light
pencil mark on Scale 1, representing the area of concrete
needed. If the
volume is less than 400 liters or 15 cubic feet, multiply it by
factor (for example, 10); then, when you find the amounts of
chart says to use, divide them by the same factor to get the
o Make a similar
mark on Scale 2, the slanted scale indicating thickness.
o Draw a straight
line through the two marks intersecting Scale 3 to find the
volume of concrete
(If the shape of
the area is complex, measure it in sections, add up the
volumes of all the
parts and mark the total volume on Scale 3.)
o Mark the kind of
work on Scale 4. A line through the marks on Scales 3
and 4 to Scale 5
will give the amount of fine aggregate needed.
o Continue on a
zig-zag course as shown in the KEY to calculate the rest of
o Add 10 percent to
the amounts indicated by the chart to allow for wastage
o If the mix is too
wet or too stiff, see page 312 for instructions on adjusting
Materials can be measured in buckets. Most buckets are rated
by the number of
gallons they hold. To convert to liters, multiply gallons by
3.785. To convert to
cubic feet: 1 cubic foot = 7.5 gallons. A 4-gallon bucket
would hold 15.15 liters or
0.533 cubic feet.
The definitions used in the chart are:
Kind of Work:
"5" means "5-gallon paste" (5 gallons or
19 liters of water to one sack of
cement), for concrete subjected to severe wear, weather, or
weak acid and alkali
solutions. An example is the floor of a commercial dairy.
"6" means "6-gallon paste," for concrete
that is to be watertight or subjected to
moderate wear or weather. Examples: watertight basements,
tanks, storage tanks, structural beams and columns.
"7" means "7-gallon paste," for concrete
not subjected to wear, weather, or water.
Examples: Foundation walls, footings, and mass concrete
where water tightness
and abrasion resistance are not important.
Sand or rock screenings up to 0.5cm (1/4") in diameter.
It should be free from
fine dust, loam, clay, and organic matter or the concrete will
be weak. The
particles should vary in size.
Pebbles or broken rock from 0.5cm (1/4") up to 4 or 5cm
(1 1/2" or 2"). Nothing
larger than 2cm (3/4") should be used with a 5-gallon
Condition of Sand:
slightly damp but leaves very little water on the hands.
wet, leaves a little water on the hands.
wet, leaves a lot of water on the hands.
Gallons: Chart A is based on the U.S. gallon (0.835 Imperial
Using the Water Displacement Method
The "Concrete Calculator" chart assumes that the
aggregate is well graded. When
the aggregate is not well graded, an alternate method can be
used to find the
correct proportions for a concrete mixture. The advantage of
this method is that
only a small sample of the ungraded aggregate needs to be
divided into coarse
and fine particles.
Well-graded aggregate seldom occurs naturally. Some
"pre-mix" processing would
be needed to grade it.
Remember that when you make concrete, you are filling the
spaces in the
aggregate with cement mortar or paste. The amount of cement
paste needed can
be found by adding water to a known volume of aggregate. To
1. Divide a sample
of the aggregate into coarse and fine particles by sifting it
through a 0.5cm
2. Fill a pail with
the coarse aggregate (dry).
3. Fill the pail
with water. The amount of water used equals the amount of
and cement paste needed to fill the spaces.
4. Into another
pail, put an amount of fine aggregate equal to the volume of
water used in
5. Fill the pail
with enough water to bring the water level to the top of the
The volume of water used equals the volume of cement paste
needed to fill
the remaining spaces.
Add about 10
percent to this volume to allow for waste and to make the mix
6. To find the
correct ratios of materials, divide the volume of cement paste
needed into the
volumes of fine and coarse aggregates.
7. Add these two
ratios to get the ratio for ungraded aggregate. For example:
If you are using
a 19-liter (5-gallon) pail, and it takes 12.8 liters (3.4
gallons) of water
to fill the pail in Step 3, put 12.8 liters (3.4 gallons) of
fine aggregate in
the second pail (Step 4). If Step 5 takes 6.4 liters (1.7
water, this is the volume of cement paste needed. Divide this
volume into the
volumes of fine and coarse aggregates to get the ratios of
(coarse aggregate) = 3
(fine aggregate) = 2
The sum of the two ratios is 5, so the ratio of ingredients
in this case is 1:5, or
1 part cement paste to 5 parts ungraded aggregate, by
To find the ratio of water to cement, see "Kind of
Work" page 309. For directions
on adjusting a mixture that is either too wet or too stiff,
see page 312.
Using "Rule of Thumb" Proportions
For a variety of small concrete construction tasks and for
repair and patch-work,
the following simple "rule of thumb" can be used
as a simple guideline.
Use the ratio 1:2:3, by volume, to proportion the cement and
aggregate and use a
water-cement ratio of 6 gallons water to 1 sack of cement.
That is, for every
sack of cement (28.4 liters or 1 cubic foot) used, add 56.8
liters (2 cubic feet) of
fine aggregate and 85.2 liters (3 cubic feet) of coarse aggregate.
Add 28.7 liters
(6 gallons) of water for each sack of cement.
A home-made box of 28.4-liter (1-cubic foot) volume will
help in proportioning
the mixture. The volume of concrete produced by a one-sack
batch using the
proportions given above will be about 142 liters (5 cubic
The most common mistakes made by inexperienced persons are
using too much
cement, which increases the cost, and using too much water,
which produces weak
Concrete must be thoroughly mixed to yield the strongest
product. For machine
mixing, allow 5 to 6 minutes after all the materials are in
the drum. First, put
about 10 percent of the mixing water in the drum. Then add
water uniformly with
the dry materials, leaving another 10 percent to be added
after the dry materials
are in the drum.
Making a Mixing Boat or Floor
On many self-help projects, the amount of concrete needed
may be small or it
may be difficult to get a mechanical mixer. Concrete can be
mixed by hand; if a
few precautions are taken, it can be as strong as concrete
mixed in a machine.
Tools and Materials
Lumber, 2 pieces: 183cm x 91.5cm x 5cm (6'x 3' x 2")
Galvanized sheet metal: 183cm x 91.5cm (6'x 3')
Nails, Saw, Hammer
Concrete for a mixing floor: about 284 liters (10 cubic
feet) of concrete is needed
for a 244 cm (8') diameter mixing floor that is 5cm
(2") thick with a 10cm (4")
The first requirement for mixing by
hand is a mixing area that is both
dean and water-tight. This can be
a wood and metal mixing boat (see
Figure 3) or a simple round
concrete floor (see Figure 4).
The ends of the wood and metal
mixing boat are curved to make it
easier to empty. The raised edge of
the concrete mixing floor prevents
loss of water while the concrete is
The procedure is:
o Spread the fine
aggregate evenly over the mixing area.
o Spread the cement
evenly over the fine aggregate and mix these materials by
turning them with
a shovel until the color is uniform.
o Spread this
mixture out evenly and spread the coarse aggregate on it and
o Form a hollow in
the middle of the mixture and slowly add the correct
amount of water
and, again, mix thoroughly.
The mixture should
be placed in the forms within 20 minutes after it is completely
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When work is finished for the day, be sure to rinse concrete
from the mixing
area and the tools to keep them from rusting and to prevent
cement from caking
on them. Smooth shiny tools and boat surfaces make mixing
The tools will also last much longer. Try to keep from
getting wet concrete on
your skin because it is caustic. If you do, wash it off as
soon as possible.
A workable mix should be smooth and plastic--neither so wet
that it will run nor
so stiff that it will crumble.
If the mix is too wet, add small amounts of sand and gravel
in the proper
proportion, until the mix is workable.
If the mix is too stiff, add small amounts of water and
cement, maintaining the
proper water-cement ratio, until the mix is workable.
Note the amounts of materials added so that you will have
the correct proportions
for subsequent batches.
If a concrete mix is too stiff, it will be difficult to
place in the forms. If it is
not stiff enough, the mix probably does not have enough
aggregate, which is
A "slump cone" is a simple device for testing a
concrete mixture to see that it
has the right proportion of materials.
Tools and Materials
Heavy galvanized iron sheet: 35.5cm x 63.5cm (14 1/8" x
Iron strap: 3mm x 2.5cm x 7.5cm (1/8" x 1" x
3") 4 pieces
16 Iron rivets: 3mm in diameter and 6mm long
Wooden dowel: 16mm in diameter and 61cm long
To perform the test:
o Dampen the slump
cone and set it on a flat, moist, non-absorbent surface.
Stand on the clips
at the bottom of the cone to hold it down.
o Fill the cone in
three layers approximately equal in volume. Because the
diameter at the
bottom of the cone is large, the first layer should fill the
cone to about
one-fourth its height. <see figures 5 to 7>
o Stroke each layer
with the wooden
dowel. <see figure 8>
o After the top
layer has been
stroked with the
smooth the surface
concrete so the
cone is filled
o Carefully lift the
cone off the
o Place the empty
of the cone and
the height of the
is the slump.
Suggested slumps for various types of construction are:
o Reinforced walls
and footings: 5cm to 13cm (2" to 5")
o Unreinforced walls
and footings: 2.5cm to 10cm (1" to 4")
o Thin reinforced
walls, columns and slabs: 7.5cm to 15cm (3' to 6')
walkways, culverts, drainage structures, and heavy mass
concrete: 2.5cm to
7.5cm (1' to 3')
Correcting the Mixture
If the slump is not within the desired range, or if the mixture
is obviously either
too fluid or too stiff, the proportions of the mixture must
be changed. To make
the mixture more fluid and increase the slump, increase the
proportion of water
and cement without changing the water-cement ratio. To make
the mixture stiffer
and decrease the slump, increase the proportion of the
changing the fine aggregate-coarse aggregate ratio. Do not
add just water to
make the mix more fluid; this will weaken the concrete.
MAKING FORMS FOR CONCRETE
Fresh concrete is heavy and plastic. Forms for holding it in
place until it hardens
must be well braced and should have a smooth inside surface.
Cracks, knots, or
other imperfections in the forms may be permanently
reproduced in the concrete
Wood is commonly used for forms, because of its light weight
and strength. Since
cracks between boards can mar the concrete surface, plywood,
which has a special
high-density overlay surface, is often used. The finish on
plywood provides a
smooth casting surface and makes it easier to remove the
forms for reuse.
If unsurfaced wood is used for forms, oil or grease the
inside surface to make
removal of the forms easier and to prevent the wood from
drawing too much
water from the concrete. Do not oil or grease the wood if
the concrete surface
will be painted or stuccoed.
Forms for flat work, such as pavements, may be 5cm x 10cm
(2" x 4") or 5cm x
15cm (2" x 6") lumber, the size depending on the
thickness of the slab. Stakes
spaced 122cm (4') apart hold the forms in place.
Figures 9 and 10 show forms for straight-wall construction.
To prevent the forms
from bulging, opposite studs should be tied together with
10- to 12-gauge wire,
which should be twisted to draw the form walls tight against
blocks. (The blocks are removed as the concrete is placed.)
The ties should be spaced about 76cm (2 1/2') vertically on
the studs. When the
forms are removed, clip the wires close to the concrete and
punch them back. Pit
holes caused by punching back the wires should be pointed up
Forms should be easy to fill with concrete and easy to
remove once the concrete
has hardened. Screws or double headed nails which can be
taken out easily can be
a great help in removing wooden forms without damaging the
Forms are sometimes made of other materials. For example,
metal forming is more
economical for repeated work, such as curbs, slip forming
for monolithic concrete
tanks or silos, and reinforced concrete floors for
The finest natural finish on a concrete surface can be
obtained by casting on
polyethylene. Sometimes polyethylene forms are used for
decorative work, or a
kraft paper with a polyethylene film surface is used as form
PLACING CONCRETE IN FORMS
To make strong structures, it is important to place fresh
concrete in the forms
correctly. The wet concrete mix should not be handled
roughly when it is being
carried to the forms and put in the forms. It is very easy,
through joggling or
throwing, to separate the fine aggregate from the coarse
aggregate. Do not let
concrete drop freely for a distance greater than 90 to 120cm
(3' to 4'). Concrete
is strongest when the various sizes of aggregates and cement
paste are well
mixed. The concrete mix should be firmly tamped into place
with a thin iron rod
(about 2cm or 3/4" in diameter), a wooden pole, or a
When the forms are filled, the hard work is done, but the
process is not finished.
The concrete must be protected until it reaches the required
strength. It starts to
harden almost immediately once the water is added, but the
hardening action may
not be complete for several years.
The early stage of curing is extremely critical. Special
steps should be taken to
keep the concrete wet. In temperate climates, the mixture
should be kept wet for
at least 7 days; in tropical and subtropical climates, it
should be kept wet for at
least 11 days. Once concrete dries, it will stop hardening;
after this happens, rewetting
will NOT re-start the hardening process.
Newly-laid concrete should be protected from the sun and
from drying wind.
Large areas such as floors or walls that are exposed to the
sun or wind should be
protected with some sort of covering. Protective covers
often used are: canvas,
empty cement bags, burlap, palm leaves, straw, and wet sand.
The covering should
also be kept wet so that it will not absorb water from the
Concrete is strong enough for light loads after 7 days. In
most cases, forms can
be removed from standing structures like bridges and walls
after 4 or 5 days, but
if they are left in place they will help to keep the
concrete from drying out. In
small ground-supported structures such as street drains, the
forms can be removed
within 6 hours of completion provided this is done
carefully. Plans will usually say
if forms should be left in place longer.
Concrete is usually expected to reach the strength for which
it was designed
after 28 days. Concrete that is moist cured for a month is
about twice as strong
as concrete that cures in the open air.
Quick-setting concrete is often useful; for example, when
repeated castings are
needed from the same mold. A concrete mixture that contains
calcium chloride as
an accelerator will set about twice as fast as a mixture
that does not. The mixed
batch must be put into the forms faster, but since
quick-setting batches are
usually small, this is not a problem. Calcium chloride does
not lessen the strength
of fully-cured concrete.
No more than 1kg (2 pounds) of calcium chloride should be
used per sack of
cement. It should be used only if it is in its original
containers, which should be
moisture-proof bags or sacks or air-tight steel drums.
To add the calcium chloride, mix up a solution containing
1/2kg per liter (1 pound
per quart) of water. Use this solution as part of the mixing
water at a ratio of 2
liters (2 quarts) per sack of cement (42.6kg or 94 pounds).
Solid (dry) calcium
chloride must never be added to the concrete mix; only use
it in solution.
John Bickford, Connecticut; Robert D. Cremer, New York;
Kenneth D. Hahn,
California; R. B. Heckler, Florida
A Building Guide for Self-Help Projects, Accra, Ghana:
Department of Social
Welfare and Community Development, 1961.
Design and Control of Concrete Mixtures, Chicago: Portland
Use of Concrete on the Farm, Farmers' Bulletin No. 2203,
Washington, D.C.: U.S.
Department of Agriculture, 1965.
Other Useful Publications:
Basics of Concrete, Ideas and Methods Exchange No. 49,
Washington, D.C.: U.S.
Department of Housing and Urban Development, Division of
Concrete Technology: Student Manual, Albany, New York:
Hobbs, Wesley. Making Quality Concrete for Agricultural and
University, Addis Ababa, Ethiopia: Haile Sellassie
Useful sources of information on concrete, including
Portland Cement Institute
18 Kew Road
Johannesburg, South Africa
Instituto del Cemento Portland Argentino
San Martin 1137
Buenos Aires, Argentina
Cement and Concrete Association of Australia
147-151 Walker Street
North Sydney, Australia, N.S.W.
Associacao Brasileria de Cimento Portland
Caixa Postal 30886
Sao Paulo, Brazil
Cement and Concrete Association
52 Grosvenor Gardens
London, S.W. 1, England
The Concrete Association of India
P.O. Box 138
Bombay 1, India
Portland Cement Association
33 West Grand Avenue
Chicago, Illinois 60610 USA
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