Tips for Working With Concrete
Choice of Ingredients:
Concrete is made from cement, sand, gravel and water. These ingredients are commonly combined in a 1:2:3 proportion to achieve maximum strength (1 part cement, 2 parts sand, 3 parts gravel). The amount of water used to mix the ingredients is by far the most important factor in determining the final strength of the concrete: Use the least amount of water that will still give you a workable mix. Generally between 20 and 30 L of water is mixed with each 50 kg bag of cement (see "Mixing Concrete" section below).|
- Cement: The most common used cement is Portland. It should be dry, powdery and free of lumps. When storing cement try to avoid all possible contact with moisture. Store away from exterior walls, off damp floors, and stacked close together to reduce air circulation.
- Water: In general, water fit for drinking is suitable for mixing concrete. Impurities in the water may affect concrete, setting time, strength, shrinkage or promote corrosion of reinforcement.
- Sand: Sand should range is size from less than .25 mm to 6.3 mm. Sand from sea shores, dunes or river banks is usually too fine for normal mixes. However, you can sometimes scrape about 30 cm of fine surface sand off and find coarser, more suitable sand beneath.
- Gravel: Optimum gravel size in most situations is about 2 cm. Finer gravel may be used to fill the annular between the borehole and the well casing.
|It is extremely important to have clean sand and gravel. Even small amounts of silt, clay or bits of organic matter will ruin concrete.|
To see if sand is good for making concrete, fill a wide-mouthed jar half full of the sand. Cover with water, shake the mixture vigorously, and then allow it to stand for three hours. There will be a distinct line dividing sand suitable for concrete and that which is too fine. If more than 10% of the sand is too fine, concrete made from it will be weak. Other sand must be found or silt and clay washed out of the available sand. To wash sand, put it in a drum, cover it with water, stir thoroughly, 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.
Estimating Quantities of material needed.
- Calculate the volume of concrete needed.
- Estimate the total volume of dry material by multiplying the required volume of concrete by 1.65 to get the total volume of dry loose material needed (this includes 10% extra to compensate for losses).
- Add the numbers in the volumetric proportion that you will use to get a relative total. This will allow you later to compute fractions of the total needed for each ingredient. (i.e. 1:2:4 = 7).
- Determine the required volume of cement, sand and gravel by multiplying the total volume of dry material (Step 2) by each components fraction of the total mix volume (Step 3) i.e. the total amount cement needed = volume of dry materials * 1/7.
- Calculate the number of bags of concrete by dividing the required volume of cement by the unit volume per bag of cement (0.0332 m3 per 50 kg bag of cement or 1 ft3 per 94 lb bag).
For example, for a 2 m x 2 m x 10 cm thick pump pad:
- Required volume of concrete = 0.40 m3
- Estimated volume of dry material = 0.4 x 1.65 = 0.66 m3
- Mix totals = 1+2+4 = 7 (1:2:4 cement:sand:gravel)
- Ingredient Volumes: 0.66 x 1/7 = .094 m3 cement
0.66 x 2/7 = .188 m3 sand
0.66 x 4/7 = .378 m3 gravel
- # Bags of cement: 0.094 m3 cement / .0332 m3 per 50 Kg bag
= 2.83 bags of cement (use three bags)
Concrete can be made much stronger by reinforcing it with steel rods (rebar or rerod) that are embedded in the concrete. Reinforced concrete should be at least 7.5 cm thick. Rerod should take up 0.5% to 1% of the cross-sectional area. The rebar should be placed within the concrete form and be located at least 2 cm from the edge of the form. It should be placed in a grid pattern so that there is never more than 3 times the final concrete thickness between adjacent rods. (With a final thickness of 10 cm use a grid spacing of 25 cm). All intersections where rods cross should be tied with wire. The proper space from the bottom of the pour in a slab is one third the height of the final thickness. It can be achieved by setting the rod grid on a few small stones before the concrete is poured or simply pulling the rebar grid a couple of centimeters up into the concrete after some concrete has been spread over the whole pour.
Concrete must be thoroughly mixed to yield the strongest product.
A. Mixing by Machine: Add about 10% of the mixing water in the drum. Then gradually add water uniformly with the dry materials leaving another 10% to be added after the dry materials are in the drum. Allow 5 or 6 minutes after all the materials are in the drum.
B. Mixing by Hand: The mixing area must be both clean and water tight. Use the following procedure:
- Spread the sand evenly over the mixing area.
- Spread cement evenly over the sand and combine until the colour is uniform.
- Spread the mixture out evenly and add the gravel on it and mix it thoroughly again. All dry materials should be thoroughly mixed before water is added.
- Shape dry mix into a pile and form a hollow bowl in the centre. Pour some of the water into the bowl, gradually mixing in the dry mixture until all the water is adsorbed. Re-form the pile and bowl, add and mix more water. Repeat until concrete is ready to pour.
A workable mix should be smooth and plastic (not wet and runny or dry and crumbly). If the mix is too wet, add small amounts of sand and gravel (in the proper proportions) until the mix is workable. If the mix is too stiff, add small amounts of water and cement until the mix is workable. Note the amounts of materials added for future batches.
Once the concrete is poured into the form, its surface should be worked to and even finish. Where the surface will later be walked on it should be kept somewhat rough to prevent people from slipping when it gets wet. This texture can be achieved by finishing with a wooden float (trowel) or by lightly brushing the surface.
After the forms are filled the concrete must be cured until it reaches the required strength.
Curing involves keeping the concrete damp so that the chemical reaction that causes the concrete to harden will continue for as long as necessary. Once the concrete dries the chemical hardening will cease and cannot be reactivated. The best way to keep the concrete wet in very hot countries is to plug to drainage channel soak-away pit and then fill the concrete pad and drainage channel with water. Water can be added as needed to keep the concrete covered.
Another easy way to keep the concrete from hardening too quickly is to cover the exposed surface with a damp
protective cover. The covers can be canvas, empty concrete bags, burlap, palm leaves, or straw. The covering should be kept damp so that it will not absorb water from the concrete.
It is a good idea to place many thorn branches over the pad area and appoint local people to watch over the pad to ensure that people do not walk on it during the curing process.
The concrete forms can be removed in 3 to 6 hours if no load is on the structure. The pad will take 4-7 days to harden completely if it is going to be moist cured. If possible, wait this long before finishing installation of the pump.|
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