 |  | Rainwater Reservoirs above Ground Structures for Roof Catchment (GTZ, 1989, 102 p.) | ||
 |  | 3. Material testing and mixing | ||
|  | (introduction...) | ||
| | 3.1 Sand | ||
| | 3.2 Water | ||
| | 3.3 Cement | ||
| | 3.4 Reinforcement | ||
| | 3.5 Mixing cement plaster | ||
| | 3.6 Curing | ||
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Most of this chapter has been taken from Laurie F. Childers publication on ferro-cement tanks, published by UNICEF Regional Office, Nairobi, Kenya, in 1985. Since the technique introduced is the same as the author has used for waterproof plastering of reinforced bricktanks, it is recommended for all types of ferro-cement tanks and the plastering of bricktanks.
Let us start with some general remarks. The technique for preparing waterproof cement plaster is delicate, not in theory but in practice, because it is influenced by the site conditions. Therefore, it is imperative to first create favourable site conditions for good quality work. This means, for instance, that the place where the mortar is to be mixed is clean, flat, smooth and large enough. If possible, some flat metal sheets should be laid on the ground. If the working ground is only a soil surface, it must be swept like Africans clean their courts. If the soil surface is not hard, it is possible to prepare an area with stamped clay. The working area must be clean before the mixing of the sand and cement can start. Likewise, it is necessary for all material to be on site before the preparation of plaster is started. All material not only means the sand, cement and water for the entire job, but also all curing agents and tools. Tools have to be clean and free of old mortar; this also applies for all buckets to be used. If it is obvious that some sort of scaffold is needed, this has to be there and tried out before the work starts. It must always be remembered that once the plastering of a ferro-cement structure has started, there must be no break until the first coat is finished. The same applies for waterproof plastering of a bricktank. Curing the plaster on the structure is as essential as the right mixture. Organizing the job is also important to achieve smooth hand-in-hand working.
Ignoring this important advice means risking a lot of money and almost certainly creating cracks and leakages. Although smaller faults can be repaired, it must be remembered that a leakage occurs after the reservoir has been filled with water. To repair it means draining the water and usually wasting it. Experience shows that negligence is often found if contractors are employed. The reason lies in the time factor where profit is expected. It is therefore better to stop the work entirely and employ another contractor than to allow an ill-prepared job to start. It should also be realized that leaking or cracking tanks give the whole technology a bad reputation. This is especially importent in all those places with no prior experience. Leaking reservoirs can discredit rainwater harvesting. Every finalized cistern construction must immediately be filled with water at least 10 cm high, irrespective of whether it is a bricktank or a ferro-cement structure. This water serves as a long-term curing agent and will keep the plaster moist.
The sand to be used for ferro-cement is the same as is used for waterproof cement plaster applied to bricktanks. The sand has to be clean and well graded. This means having grains of many sizes, but 90% should still pass through a mosquito wire sieve. Sand must be clean, because like water it may have some impurities that weaken the cement bond, such as clay, silt and organic matter. Dirty sand can be washed by repeatedly rinsing with water. This should be done on a well-prepared sloping ground by pouring water on the sand while turning it with shovels. The dirt must run off, otherwise the effect is minimized. If sand has to be washed afterwards, it is too wet for ferro-cement plaster and should be given time to dry partly while turning it with shovels three times a day. There are two easy field tests for determining if the sand needs to be washed:
1. Rub a moist handful of sand between your palms. Suitable sand will leave hands only slightly dirty.
2. Fill a clear glass container 100 mm high with sand. Then fill with water. Shake the glass vigorously, place it on level ground and leave undisturbed for one hour. The sand settles immediately and any silt and clay settle as a dense layer on top. This layer is of another colour than the sand, often darker, and should not be more than 6% of the entire thickness of sand (Fig. 3.1). If you have had 100 mm sand, a 6-mm upper layer of silt or clay is acceptable; if it is more, the sand has to be washed.
If sand is not taken at the site but supplied, so-called river sand is likely to meet the quality demanded. Sand has to be stored close to the mixing area on cleared ground. Before the sand can be used for mixing, it has to be sifted. For this purpose a special sieve has to be made out of galvanized gauze wire supported by chicken wire (see Fig. 3.2). The frame of the sieve must be of boards about 100 mm high so that all material which does not pass the sieve remains on top and cannot fall onto the sifted sand. The sieve is to be used by shaking it. Two men hold the sieve, while another shovels sand onto it, not more than three or four shovel fuls at a time. The two men then shake the sieve backwards and forwards. The clean material will fall through the sieve. The rest has to be put aside in such a way that it cannot accidentally be mixed with either of the two piles of sand, the sifted or the unsifted one. Since shaking the sieve is hard work, it has been observed that people start to make mistakes after some time. It should therefore be made clear in advance that the sifting crews are to be changed. Sifting sand and preparing the mixture are just as important as plastering. The final product, the ferro-cement tank or the plaster of the bricktank, depends very much on the care taken by the staff preparing the mixture and in charge of the material.
figure 3.1
figure
3.2
Clay, silt, salt, mica or organic matter in the water will weaken concrete and ferro-cement, as will certain invisible chemicals. Water that is fit for drinking is usually fit for mixing cement mortar. The quality of an unknown water can be tested by comparing it with water known to be good. This is best done by using the known water (such as drinking water), making three cakes of cement paste, each approximately 20 mm thick and 60 mm in diameter. At the same time make three identical cakes using the unknown water, and compare setting and hardening times of the two. types. The cement paste is prepared like the nil coat, adding cement in a small container with water while constantly stirring the mixture. Cement should not be added fast to avoid clotting or the development of lumps. To achieve equal test cakes, a glass can be used for shaping. All cakes must be of equal size and shape. A chart should be prepared to record the test results (see Fig. 3.3). Mark on your chart that the sample has set when you can no longer make an indentation with your finger tip. Mark the samples with A for drinking water mixture and B for unknown quality. Test for hardening by marking whether or not you can scratch the sample with your fingernail (see Fig. 3.3). The samples must be stored in the shade. If the chart shows that both samples are nearly equal, the water of unknown quality can be used for mixing cement plaster.
figure
3.3
Cement bonds and hardens in the presence of water. Therefore careful storage is imperative to avoid moisture reaching cement before use. The bags of cement should be stored in a closely packed pile, no more than ten bags high. The pile should be on a raised platform in a room with little air circulation. In rooms with open windows and doors, the pile should be covered with plastic sheeting. The same applies if the cement has to be stored outdoors. The platform must be made in such a way that moisture from the ground cannot affect the cement and the plastic sheeting has to be tied so that the wind cannot blow it away and rainwater does not affect the cement. As bagged cement ages and absorbs moisture from the air, it becomes lumpy. If lumpy cement is to be used, its proportion should be increased by half and bigger lumps be removed before mixing.
The cement bond is easily broken by forces which pull it apart-tensile stresses. Thus it is necessary to use a material like steel inside the concrete or plaster for large water tanks. The weight of the water will stretch the tank walls. Barbed wire or weld mesh is heavy enough to withstand the stress and hold the tank together. (Straight wire can be used in place of barbed wire, but the barbs help grasp the plaster, and the two twisting wires are stronger than a single wire.) Chicken wire helps hold the plaster together between the stronger wire.
Although the soil helps support the weight of the water, even the ground hemispherical tanks will stretch when full. Hard rocky soils provide better support. Loose or sandy soils should have more reinforcement (barbed wire) in the tanks.
Upright water tanks receive most tensile stresses in the bottom third of the wall and in the joint between the floor and wall. Extra reinforcement wires in the wall and joint and thickening the plaster at the joint prevent cracking at these points of stress.
The correct method of measuring the different aggregates of concrete or cement mortar is to weigh them. But this is not possible at most sites. The common way is to measure the volume. Although this is not a precise method, it is efficient enough if performed carefully. Measuring by the shovelful is not acceptable since this is too inaccurate. Measuring must be done with buckets or wooden boxes, all of equal size. A 1:3 mixture means three measurements of sifted sand to one measurement of cement. These two dry components have to be mixed (see Fig. 3.4) by shovelling a pile of sand with the required amount of cement added from one side of the mixing platform to the other and then back. This procedure has to be repeated 4-6 times until the dry mixture is of equal colour. Before adding water, prepare another pile of dry mixture. A second pile of dry mixture should always be ready before water is added to the first pile. This gives a certain guarantee that there will be no interruption of the supply of mortar for the plasterers.
Water must be added very carefully. It is appropriate to make a test of a small amount first and let the plasterer try to work it. The mortar for ferro-cement must be moist, not wet. If you can take it in your hand and shape a ball without water running through your fingers, this indicates the right consistency. Water should never be visible in the mixture and the mortar should not look shiny. For waterproof plaster on bricktanks, the mortar is slightly wetter, but here, as with ferro-cement, the same problem occurs if too much water is added. In this case the mortar slides on the underground where it settles and horizontal cracks appear. The cracks indicate that the mortar is no longer homogeneous. Work should stop until proper mortar is supplied. The content of water in the mixture is a most sensitive issue. It is called water-cement ratio. For easy understanding it should be realized that where water is, no other material can be. But since water will eventually run off, it will contain cement. The structure will be weakened if too much water is added. It can be said that only 10% more water than necessary to make the plaster workable will reduce the strength of the plaster by 15%. If 50% more water is added, the plaster will lose 50% of its strength. The same applies for concrete, although it can be observed everywhere that concrete is considered to be good and workable if it runs out of the wheelbarrow. This consistency is wrong and creates a weaker concrete.
figure
3.4
Therefore it must be realized that as little water as possible should be used for mixing, but the use of water should be generous for curing. It is not commonly known that cement plaster, ferro-cement and concrete have to be kept wet for at least 28 days, never being allowed to dry since the process of hardening will stop as soon as the mortar/ concrete dries, If, as in our case, waterproof plaster has to be achieved, the material must be kept wet for one year. But even after a year cement plaster should not be allowed to dry off. Remember curing is as important as material testing and mixing the right composition of mortar or concrete. The chart gives an indication of how important curing is.
The cured cement plaster achieved the following hardening results:
|
After 3 days |
20% of final hardness |
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After 7 days |
45% of final hardness |
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After 28 days |
60% of final hardness |
|
After 3 months |
85% of final hardness |
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After 6 months |
95% of final hardness |
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After 1 year |
100% of final hardness |
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