| Rainwater Reservoirs above Ground Structures for Roof Catchment |
|3. Material testing and mixing|
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.