Construction
                               <see image>
 

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                          Concrete Construction
 
OVERVIEW
 
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:
 
  o   Calculating amounts of materials for concrete
  o   Mixing concrete by machine or by hand
  o   Testing concrete mixtures
  o   Making forms for concrete
  o   Placing concrete in forms
  o   Curing concrete
  o   Making quick-setting concrete
  o   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 reaction, hydration,
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 time.
 
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>

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The proper proportioning of all materials is essential. The section on "Calculating
Amounts of Materials for Concrete" provides the necessary information.
 
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 properly graded.
 
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>

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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.
 
Water
 
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 construction process.
These will be discussed in the sections on mixing and curing concrete.
 
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 engineer.
 
CALCULATING MATERIALS FOR CONCRETE
 
Three methods are given here for finding the correct proportions of cement,
water, and aggregate for concrete:
 
     o   A "Concrete Calculator" fold-out chart
     o   Using water to estimate proportions
     o   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.

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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
   a convenient factor (for example, 10); then, when you find the amounts of
   materials the chart says to use, divide them by the same factor to get the
   actual amounts needed.
 
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 needed.
 
   (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
   the materials.
 
o  Add 10 percent to the amounts indicated by the chart to allow for wastage
   and miscalculation.
 
o  If the mix is too wet or too stiff, see page 312 for instructions on adjusting
   it.
 
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.
 
                                    NOTES
 
 
                                    NOTES                               
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, driveways, septic
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.
 
Fine Aggregate:
 
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.
 
Coarse Aggregate:
 
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 paste.
 
Condition of Sand:
 
    Dry: feels slightly damp but leaves very little water on the hands.
    Average: feels wet, leaves a little water on the hands.
    Wet: dripping wet, leaves a lot of water on the hands.
 
Gallons: Chart A is based on the U.S. gallon (0.835 Imperial Gallon).
 
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 do this:
 
1.  Divide a sample of the aggregate into coarse and fine particles by sifting it
    through a 0.5cm (1/4") screen.
 
2.  Fill a pail with the coarse aggregate (dry).
 
3.  Fill the pail with water. The amount of water used equals the amount of
    fine aggregate 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 Step 3.
 
5.  Fill the pail with enough water to bring the water level to the top of the
    fine aggregate. 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
    more workable.
 
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
    gallons) of 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
    materials:
 
        19 liters (coarse aggregate)  = 3
        -----------------------------
         6.4 liters (cement paste)
 
        12.8 liters (fine aggregate)  = 2
        ----------------------------
         6.4 liters (cement paste)
 
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 volume.
 
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 feet).
 
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.
 
MIXING CONCRETE
 
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
 
Or:
 
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")
high rim
Shovel
 
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

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concrete floor (see Figure 4).

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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
being mixed.
 
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
   mix thoroughly again.
 
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
mixed.
 
 
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 surprisingly easier.
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
uneconomical.
 
Slump Tests
 
Slump Cone
 
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 25 1/2")
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
 
Testing
 
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>

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o  Stroke each layer 25 times
   with the wooden dowel. <see figure 8>

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o  After the top layer has been
   stroked with the dowel,
   smooth the surface of the
   concrete so the cone is filled
   exactly.
 
o  Carefully lift the cone off the
   concrete.
 
o  Place the empty cone along-side
   the concrete. Measure the
   difference between the height
   of the cone and the height of the
   concrete. This difference
   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')
 
o  Pavements, 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 aggregates without
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
surface.
 
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

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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 wooden spacer
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 with mortar.
 
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 concrete.
 
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 multistory buildings.
 
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 finer.
 
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 shovel.
 
CURING CONCRETE
 
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.
 
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
 
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.
 
Sources:
 
VITA Volunteers:
 
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 Cement Association
 
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 International Affairs
 
Concrete Technology: Student Manual, Albany, New York: Delmar Publishers
 
Hobbs, Wesley. Making Quality Concrete for Agricultural and Home Structures,
University, Addis Ababa, Ethiopia: Haile Sellassie
 
Useful sources of information on concrete, including how-to-do-it manuals:
 
Portland Cement Institute
18 Kew Road
Richmond
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