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|Section 5: Construction techniques|
The material which is used at almost all stages is concrete For this matter, an extensive description of concrete in construction work will be included for convenience.
Concrete is a strong, durable and inexpensive construction material when properly prepared. After concrete has set, there is no simple non-destructive test to evaluate how strong it is. Therefore, the entire responsibility for making concrete a strong material in accordance with specifications rests with the supervisor on the fob 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. Beginners are likely to have too much. See the entry on a slump cone for further details.
The proper proportion of all the materials, designed for the application, is essential. The concrete calculator will help give the proper proportions and amounts, for your job.
Properly graded, clean, sharp aggregate and sand is required to make good concrete. When we glue two pieces of paper together, we spread the glue evenly and in a thin layer, and press firmly to eleminate air holes. In concrete, the cement is the glue, and the sand and aggregate the material being joined.
By properly graded we mean that there are not too many of any one size grains or pebbles. Visualize this by thinking of a large pile of stone all 1 1/2" 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 voids. Now the voids 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 stick it together. The resulting concrete would be very dense and strong.
Sharp aggregate and sand is desirable. Smooth, rounded stones and sand can make fairly good concrete, but sharp, fragmented particles work better because the cement as a glue can get a better grip on a rough stone with sharp edges.
It is extremely important to have the aggregate and sand clean. Silt, clay, bits of organic matter will ruin concrete if there is very much present. A very simple test for cleanliness makes use of a clear wide-mouth jar. Fill the jar about half full of the finer material available, the sand and small aggregate, and cover with water. Shake the mixture vigorously, and then allow it to stand for three hours. In almost every case there will be a distinct line dividing the fine sand suitable for concrete and what which is too fine. If the very fine material amounts to more than 10% of the suitable material, then the concrete made from it will be weak.
This means that other fine material should be sought, or the available material should be washed to remove the material that is too fine. This can be cone by putting the sand (and fine aggregate if necessary) in some container such as a drum. Cover the aggregate with water, stir thoroughly, and let stand for a minute, and pour off the liquid. One or two such treatments will remove most of the very fine material and organic matter.
Another point to consider in the selection of 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 aggregate stones is greater than the effort required to break a similar sized piece of concrete, then the aggregate will make strong concrete. If the stone breaks easily, then you can expect that the concrete made of these stones will only be as strong as the stones themselves.
In very dry climates several precautions must be taken. If the sand is perfectly dry, it packs into a smaller space. If you put 20 buckets of bone dry sand in a pile, stirred in two buckets of water you could carry away about 27 buckets of damp sand. The chart does not take this extremely dry sand into account. If your sand is completely dry, add some water to it or else do your measurements by weight instead of volume. The surface of the curing concrete should be kept damp. This is because water evaporating from the surface will remove some of the water needed to make a proper cure. Cover the concrete with building paper, burlap, straw, or anything that will hold moisture and keep the direct sun and wind from the concrete surface. Keep the concrete moist by sprinkling as often as necessary; this may be as often as three times per day. After the first week of curing, it is not so necessary to keep the surface damp continuously.
Mixing the materials and getting them in place quickly, tamping and spading to a dense mixture is important. This is covered on the entry on mixing.
Reinforcing concrete will allow much greater loads to be carried. Design of reinforced concrete structures can become too complicated for a person without special training, if they are large or must carry high loads.
Use the alignment chart as follows. Make a light pencil mark on the left-most scale representing the area of concrete needed. Make a similar mark on the slanted thickness scale. Draw a straight line through these marks intersecting the third scale. This is the volume of your concrete. If your project has a complex shape, add up the volumes of all the parts before proceeding.
Now mark the total volume of concrete on the third (volume) scale, and the kind of work on the fourth. (See definitions.) A line through these two points will give the amount of fine aggregate needed. Continue on a zig-zag course as shown in the KEY to calculate the coarse aggregate, sacks of cement, and water.
It may be necessary to make slight adjustments to the mix, depending upon the type of aggregate used. The final mixture should be wet enough and workable enough to go into the forms fairly easily, requiring light spading or tamping to produce a dense mixture. Too much moisture produces a weak cement. The figures in the alignment chart do not allow for waste which may run as high as 10%.
All materials can be measured in "buckets" instead of cubic feet. The nomograph will still give the correct proportions. The total amount of concrete produced, however, will depend upon the size of the bucket used as the measure. Most buckets are rated by the number of gallons they can hold. To convert to cubic feet, then, you must know that one cubic foot equals 7.5 gallons. A four gallon bucket would hold 0.533 cubic feet. Incidentally, one cement sack holds exactly one cubic foot, so "buckets" can also be substituted for "sacks" on the chart.
Similarly, if your volume of concrete needed is less than 15 cubic feet, you can multiply this by some convenient factor (say 10) and then divide the amounts of materials the chart says to use by the same factor to get the actual amounts needed.
Definitions used in the chart are given on the fold-out page.
HAND MIXING CONCRETE:
Proper mixing of ingredients is necessary to get the highest strength concrete. Hand mixed concrete made with these tools and directions can be as strong as machine mixed concrete.
TOOLS AND MATERIALS
Lumber - 2 pieces 6' x 3' x 2"
Galvanized sheet metal - 6' x 3'
Or concrete for making a mixing floor. (About 10 cubic feet of concrete are needed for an 8' diameter mixing floor made 2" thick with 4" high rim.)
Kind of work
- "5" means "5 gallon paste" which is concrete subjected to severe wear, weather, or weak acid and alkali solutions Examples would he the floor of a commercial dairy
- "6" means "6 gallon paste" for concrete to he watertight or subjected to moderate wear and weather Examples: watertight basements, driveways, septic tanks, storage tanks, structural beams and columns
- ''7" means ''7 gallon paste's for concrete not subjected to wear, weather, or water
Examples Foundation walls, footings, mass concrete, etc. where water tightness and abrasion resistance are not important
- Sand or rack screenings up to one quarter inch in diameter Should he free from fine dust, loam, clay and vegetable matter or the concrete will have low strength Particles should vary in size, not all fin. or coarse.
- Pebbles or broken rock from 1/4" up to 1-1/2". Nothing coarser than 3/4" should he used for a 5 gallon paste
Condition of Sand
- Dry-feels slightly damp hut leaves very little water an the hands
Average-feels wet; leaves a little water on the hands
Wet-dripping wet, leaves quit. a bit of water on the hands
- The chart is hosed on the U S. Gallon (This is 0. 835 of on- Imperial Gallon )
-Designed by John Bickford from data furnished by the Portland Cement Association of Chicago, Illinois, U. S. A
DESIGNER BY JOHN BICKFORD FOR VOLUNTEERS FOR INTERNATIONAL TECHNICAL ASSISTANCE INC. 9\28\62
On many self-help projects the amount of concrete needed may be small or it may be difficult to obtain a mechanical mixer. Under these circumstances hand mixing of the concrete will be necessary and, if a few precautions are taken, the quality of concrete can be made equivalent to that from a mechanical mixer.
The first requirement is a watertight and clean base upon which the mixing can be done. This can be a wood and metal mixing boat (Fig. 40A) or a simple round floor made of concrete (Fig. 40B).
The ends of the wood and metal mixing boat are curved to make emptying easier. The raised edge of the concrete mixing floor serves to prevent loss of water form the concrete.
The procedure for mixing is similar to that for mechanical mixers in that the dry materials should be mixed first. As a minimum it is recommended that the pile of stone, sand, and cement be turned completely once. It should be completely turned a second time while the water is being added. Then it should be turned a third time, Anything less than this will not adequately mix all materials. When this last step is completed the mix can be placed as usual.
Correctly placing the fresh concrete in the forms or shuttering is important in making strong structures. The wet concrete mix should not be handled roughly either in carrying to the shuttering or putting into the shuttering. In either case it is very easy, through joggling or throwing, to separate the fine from the coarse material. We have said before that the strongest concrete comes when the various sizes of aggregate and cement are well mixed together. The concrete mix should be firmly tamped into place with a thin (3/4") iron rod.
Be sure to rinse concrete from the mixing boat and tools when finished each day with the work. This will prevent rusting and caking of cement on them for smooth shinny tool and boat surfaces make mixing surprisingly easier, and the tools will last much longer. Also try to keep wet concrete off your skin, for the material is somewhat caustic.
When the shuttering is full the hard work is done, but the process is not finished. The shuttering must be removed and the concrete protected until adequate strength is attained. The hardening action of cement begins almost immediately after the water is added, but the action may not be fully completed for several years.
Concrete reaches the strength used in the designing after 28 days and is strong enough for light loading after 7 days. In most cases the shuttering can be removed from standing structures such as bridges or walls after 4 to 5 days. In small ground supported structures such as street drains it is possible to remove the shuttering within 6 hours of completion provided this is done carefully. Special conditions, usually specified on the plans, may require leaving the shuttering in place for a much longer time.
During the early stages of hardening or curing the cement in the concrete continues to need moisture. If there is insufficient water available the cement is unable to complete its job of gluing the aggregate together. Because of this, it is recommended that new concrete be protected from drying winds and the sun, and that the surface of the new concrete be kept damp. For cement floors or open construction a covering of banana or palm leaves will be adequate, but these should be given a sprinkling of water at least once and perhaps twice each day for a period of not less than one week.
TESTING THE STRENGTH OF CONCRETE
The Slump Cone
The following tools and materials are needed:
Heavy galvanized iron
Strap iron - 4 pieces 1/8" x 3" x 1"
16 iron rivets 1/8" diameter x 1/4" long
Wooden Dowel 24" long, 5/8" diameter
In making reinforced concrete, it is important to have just enough water to make the concrete settle finely into the shuttering (forms) and around the reinforcing when it is thoroughly tamped.
The easiest way is to look at the mix and at the way the workmen place the wet concrete. If the mix appears soupy and the aggregate shows up clearly in the mix, then it is too wet. At the same time it will be noticed that the workmen dump the mix into the shuttering and do very little tamping because. if they do any amount of tamping, large amount of water will immediately appear on the surface. The workmen will soon complain if the mix is too dry.
A more accurate method of making a decision on the proper amount of water is to use the slump test. This test requires a small cone made of fairly strong metal and open at both ends. Dimensions of the cone and tamping rod are shown in the sketch. Once this simple equipment is available the slump test becomes very easy. The steps to follow are listed below.
1. Set the slump cone on a smooth clean surface and stand on the hold-down clips at the bottom of the cone.
2. Have someone fill the cone to 1/4 of its height and tamp this layer 25 times.
3. Fill the cone to 1/2 its height and tamp this layer 25 times.
Avoid tamping the first layer again.
4. Fill the cone to 3/4 its height and tamp 25 times. Avoid tamping the previous layers.
5. Complete filling of the cone and tamp this layer 25 times.
6. Step off the hold-down clips and lift the cone vertically and very carefully off the concrete.
Since this process will have taken only a few minutes the concrete will still be very soft when the cone is removed and the top will fall to some extent while the sides bulge out. This is called the slump. Obviously, if the mix is too wet the concrete will lose its shape completely and become just a soft pile. A good mix, as far as the water-cement ratio is concerned will slump about 3" to 4" when the cone form is removed. It is well to keep in mind that dirty or muddy water can cause as much trouble as aggregate with excessive fine materials. Use clean or settled water.