Cement

A. Introduction to Cement

Cement is one of the most useful materials in wells construction. It can easily be mixed with sand and water to make mortar or with gravel, sand and water to make concrete. Both mortar and concrete are among the strongest and most durable materials used for all types of construction around the world. Mortar is normally used as the bonding agent between bricks or rocks while concrete is normally reinforced with steel bars and molded to the desired size and shape.

For well work, mortar or concrete is usually the best material for the lining, headwall, platform and cover of dug wells,and the platform and seal around the top three meters of casing in drilled wells.

Cement is available in almost every country in the world and sand and gravel are usually available locally. Occasionally it will be difficult to get cement for wells construction either because there are other higher priority demands for the cement or because it just costs too much. It is impossible here to say how or even whether cement can be obtained in such a circumstance.

Of the two cement compounds, mortar and concrete, concrete is the stronger. This is because the rock that makes up the gravel itself is stronger than the concrete and so contributes to its strength. Sometimes the two can be used interchangeably where lack of materials or working conditions demand it. Remember that concrete is the stronger product and should be used where possible.

NOTE: The rest of the discussion in this appendix will deal specifically with concrete. The same procedures can and should be followed if mortar is used instead.

B.Ingredients of Concrete

Concrete is made from cement, sand, gravel and water. These ingredients are combined in certain proportions to achieve the desired strength. The amount of water used to mix these 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. Sand and gravel, which are sometimes referred to as fine and coarse aggregate respectively, should be clean and properly graded. Cement and water form a paste which, when mixed, acts as a glue to bind the aggregates together in a strong hard mass.

1. Proportions:

NOTE: A 1:2:4 mix will go a little farther than the 1:2:3 mix and allows a little more room for using not the best sand or gravel than a 1:2.5:5 mix.

2. Choice of ingredients

When used, it should be dry, powdery and free of lumps. When storing cement try to avoid all possible contact with moisture. Store it away from exterior walls, off damp floors, and stacked close together to reduce air circulation. If it could be kept completely dry it could be stored indefinately. Even exposed to air it will gradually draw moisture, thus limiting even the covered storage time to between 6 months and 1 year depending on conditions.

- fine aggregate: Sand should range in 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. (You can sometimes scrape about 30 cm of fine surface sand off and find coarser, more suitable sand beneath it.)

- large aggregate: Within the recommended size limits mentioned later, the larger the gravel you use the stronger and more economical the concrete will be.

- one-fifth the minimum dimension of the member;

- three-fourths the clear space between reinforcing bars or between reinforcement and forms. (Optimum aggregate size in many situations is about 2.0 cm.)

The shape and surface texture of aggregates affect properties of freshly mixed concrete more than they affect hardened concrete. Rough textured or flat and elongated particles require more water to produce workable concrete than do rounded or cubical aggregates and more water reduces the final strength of the concrete.

It is extremely important to have the gravel and sand clean. Silt, clay, or bits of organic matter in even low concentrations will ruin concrete. A very simple test for cleanliness makes use of a clear wide-mouth jar. Fill the jar about half full of the sand and small aggregate to be tested, 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 that 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 done by putting the sand (and gravel 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 aggre-gate 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 and stirred in two buckets of water, you could carry away about 27 buckets of damp sand. If your sand is completely dry, add some water to it or else measure 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 it cure properly. 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 necessary to keep the surface damp continuously.

3. Making Quick-Setting Concrete

To produce quick-setting concrete with high initial strength, calcium chloride can be added to the mixture.

This will not affect the estimation of materials needed because the calcium chloride will be dissolved in the water used to mix the concrete.

Quick-setting cement is often useful for example, when repeated castings are needed from the same mold. A concrete mixture which contains calcium chloride as an accelerator will set about twice as fast as a mixture which 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 1 kg (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.

The best way to add the calcium chloride is to 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.

4. Estimating quantities of materials needed

1. Calculate the volume of concrete needed.

2. Multiply the volume of concrete needed by 3/2 (1.5) to get the total volume of dry loose material needed. The cement and sand do little to add to the volume of the concrete because they fill in the air spaces between the gravel.

3. Add 10% (1/10) for losses due to handling.

4. 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. (1:2:3=6)

5, Determine the amount of cement needed by multiplying the volume of dry material needed (from step 2) by the proportional amount of the total mix (amount cement needed) = 1/6 x (volume dry materials).

6. Divide by the unit volume per bag, 33.2 liters per 50 kg bag cement or 1 cubic foot per 94 lb. bag cement. When figuring the number of cement bags round up to nearest whole number.

NOTE: This calculation, even with the 10% addition for handling losses, rarely leaves any extra concrete, particularly for small jobs requiring less than 5 hand mixed bags of cement.

Here is an example:

- the volume of the lining and headwall would be the volume of the 20.8m high cylinder with a 0.7m radius minus the volume of the 20.8m high cylinder with a 0.6m radius.

- V = [P(.7)2(20.8)-[ P(.6)2(20.8)]

= [(3.1416)(.49)(20.8)] - [(3.1416)(.36)(20.8)]

= 32.0-23.5

= 8.5m3


FIG. IV-1. CROSS SECTION OF TYPICAL WELL FOR CALCULATION OF CONCRETE NEEDS

- the volume of the platform would be the volume of the .08m high cylinder with a 2m radius minus the volume of the .08m high cylinder with a .7m radius.

- V = [(3.1416)(2)2(.08)] - [(3.1416)(.7)2(.08)]

[(3.1416)(4)(.08)] - [(3.1416)(.49)(.08)]

= 1 - 0.1 = 0.9 m3

Following the steps outlined above the volume of materials necessary to construct the well would be computed as follows:

1. total volume = 8.5 + .9 = 9.4 m

2. (9.4)(1.5) = 14.1 m dry material estimated

3. 14.1 x 1.1 = 15.5 m dry material necessary because of losses in transport.

4. 1:2:4 cement:sand:gravel 1+2+4=7

5. 15.5 x 1/7 = 2.2 m cement

15.5 x 2/7 = 4.4 m sand

15.5 x 4.7 = 8.9 m gravel

6. 2.2 m cement = 2,200 liters (1.) of cement

2,200 1. cement 33.2 1. per 50 kg bag cement = 66.26 bags of cement

67 bags of cement will be needed.

C. Construction with Concrete

1. Outline of Concrete Work

2. Interior Well Forms

a. Introduction

An interior well form or mold is circular with a smooth exterior surface which will form the inside surface of the lining. This form can be used either on the surface with other forms to make lining rings or in the well to form a lining that is poured in place. (See Fig. IV-2.)


FIG. IV-2. INTERIOR LINING FORM

Freshly mixed 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.

Forms should be easy to fill with concrete and easy to remove once the concrete has hardened. Be sure that the fasteners used to hold the form together are both accessible and easy to unfasten.

NOTE: When using nails to hold a wood form together, do not drive them all the way in. Leave them sticking up just enough so that they can easily be pulled when necessary to remove the form.

b. Materials for Forms

The following materials are used to construct interior forms:

By using boards as wide as possible, form construction will be easier and quicker. It will also reduce the number of lines on the concrete surface that form at the junction of two boards. Plywood is excellent, especially if it has a special high density overlay surface. This allows for a smoother concrete finish, easier form removal and less wear on the forms.

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.

3. Concrete Reinforcement

Reinforcing concrete will allow much greater loads to be carried. Design of reinforced concrete structures that are large or must carry high loads can become too complicated for a person without special training.

Concrete alone has great compression strength but little tension strength. Concrete is very difficult to squeeze (compression), but breaks relatively easily when stretched (put in tension). Reinforcing steel has exactly the opposite properties; it is strong in tension and weak in compression. Combining the two results in a material (reinforced concrete) which is strong in both compression and tension and therefore useful in a large number of situations.

Concrete is best reinforced with specially made steel rods which can be imbedded in the concrete. Bamboo has also been used to reinforce concrete with some success although it is liable to deteriorate in time.

- at least 2.5 cm from the form everywhere.

- in a plane approximately two-thirds of the way into the thickness of the pour from the side which will have a weight or force pressing on the concrete. (See Fig. IV-3.)


FIG. IV-3. REINFORCING ROD PLACEMENT (ALSO SEE FIGS. 8-3 AND 8-5 FOR REROD PLACEMENT IN LINING)

- in a grid so that there is never more than 3 times the final concrete thickness between adjacent rods.

- no closer than 3 cm to a parallel rod.

Rod size

Overlap

6 mm

12 cm = 120 mm

8 mm

16 cm = 160

10 mm

20 cm = 200

12 mm

24 cm = 240


FIG. IV-4. REROD OVERLAP

4. Mixing Concrete BY Machine or By Hand

a. Mixing by Machine

Concrete must be thoroughly mixed to yield the strongest product. For machine mix, allow 5 or 6 minutes after all the materials are in the drum. First, put about 10% of the mixing water in the drum. Then add water uniformly with the dry _ materials, leaving another 10% to be added after the dry materials are in the drum.

b. Mixing by Hand

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.

The first requirement for mixing by hand is a mixing area which is both clean and watertight. This can be a wood and metal mixing trough (Fig. IV-5) or a simple round concrete floor (Fig. IV-6).


FIG. IV-5. MIXING TROUGH


FIG. IV-6. MIXING PLATFORM

Use the following procedure:

1) Spread the fine aggregate evenly over the mixing area.

2) Spread the cement evenly over the fine aggregate and mix these materials by turning them with a shovel until the color is uniform.

3) Spread this mixture out evenly and spread the coarse aggregate on it and mix thoroughly again. All dry materials should be thoroughly mixed before water is added.

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 mixing 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.

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, thus making it an uneconomical use of cement.

5. Pouring Concrete into Form

To make strong concrete 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 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 the concrete drop freely for a distance greater than 90 to 120 cm. Concrete is strongest when the various sizes of aggregates and cement paste are well mixed.

Properly proportioned concrete will have to be worked into place in the form. Concrete that would on its own flow out to completely fill in a form would be too wet and therefore weak.

When pouring concrete structures that are over 120cm high, leave holes in the forms at intervals of less than 120cm through which concrete can be poured and which can later be covered to permit pouring above that level. Alternatively, a slide could be used through which concrete could flow down to the bottom of the form without separating.

Any "u"-shaped trough wide enough to facilitate pouring concrete into it, narrow enough to fit inside the form, and long enough so that the concrete can slide down the chute without separating will work.

As the concrete is being placed it should be compacted so that no air holes, which would leave weak spots in the concrete, are left. This can be done by tamping the concrete with some long thin tool or vibrating the concrete in one of several ways. Tamping can be accomplished with a thin (2 cm) iron rod, a wooden pole or a shovel.

On large commercial jobs concrete is compacted with a special vibrator usually powered by an air compressor which is submerged in the concrete immediately after it is poured. The concrete will be compacted to some extent as it is moved into its final position in the form. However, special attention must be paid to the edges of the pour to make sure that the concrete has completely filled in the form up to the edges. If the forms are strong enough they can be struck with a hammer on the outside to vibrate the concrete just enough to allow it to settle completely in against the forms. Too much tamping can force most of the large aggregate toward the bottom of the pour, thus reducing the overall strength of the concrete.

6. Finishing

Once the concrete is poured into the forms, its surface should be worked to an even finish. The smoothness of the finish will depend on what the surface will be used for. Where more concrete or mortar will later be placed on this pour, the area should be left relatively rough to facilitate bonding. Where the surface will later be walked on, as for example the cover of a well on which a pump will be mounted, it should be somewhat rough to prevent people from slipping on the concrete when its surface is wet. This somewhat rough texture can be achieved by finishing with a wooden float or by also lightly brushing the surface to give it a texture. A very smooth finish can be made with a metal trowel. Overfinishing (repeated finishing) can lead to powdering and erosion of the surface.

7. Curing Concrete

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 is necessary to achieve the desired strength. Once the concrete is allowed to dry the chemical hardening action will gradually taper off and cease.

The early stage of curing is extremely critical. Special steps should be taken to keep the concrete wet. Once the concrete dries it will stop hardening; after this happens it cannot be re-wetted in the field to re-start the hardening process.

Covering the exposed concrete surfaces is usually easier than continuously sprinkling or frequently dousing the concrete with water which would otherwise be necessary to prevent the concrete surface from becoming dry to the touch. Protective covers often used include canvas, empty cement bags, burlap, plastic, 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. Where concrete structures are being cast on the ground, the forms can be removed as soon as the concrete sets enough to hold its own shape (3 to 6 hours) if there is no load on the structure and measures are taken to ensure proper curing.

The concrete's final strength will result in part from how long it is moist cured. As can be seen from the graph, concrete will eventually reach about 60% of its design strength if not moist cured at all, 80% if moist cured for 3 days, and almost 100% if moist cured for 7 days. If concrete is kept moist it will continue to harden indefinitely.

You will also notice from the graph that even though the concrete may be cured for 7 days, at that point it will only have gained about 60% of the strength it will ultimately have and that it will be another 3 weeks before it reaches 90% of its ultimate strength. Practically,this means that when pouring a concrete ring which will later be put in the well even after curing has stopped, the ring should be left alone for at least another week (and preferably longer) before it is installed in the well. It will during that time harden to reach about 75% of its final strength.


GRAPH: STRENGTH OF CONCRETE AFTER DAYS OF MOIST-CURING