Technical Presentation of Self-standing Wire Mesh Cisterns Built in Semiarid Brazil

Transcription

1 15th International Rainwater Catchment Systems Conference 2011 Worldwide Multi Objective Rainwater Harvesting and Utilization March 28 April 4, 2011 Taiwan, R.O.C. Technical Presentation of Self-standing Wire Mesh Cisterns Built in Semiarid Brazil Harald Schistek Johann Gnadlinger IRPAA Regional Institute for Appropriate Small-Scale Agriculture Avenida das Nações,04 CEP Juazeiro - Bahia, Brazil Abstract The paper gives a description of the technical aspects of a w ire mesh cistern developed by a non-governmental organization in the rural area of semiarid Brazil. For the success of rainw ater catchment systems for household use, it is essential to have available technologies that bring together simplicity of construction, high resistance and low cost: three factors that seem difficult to get under one roof. The self standing w ire mesh cistern, w hich uses a ferro-cement technology, seems to correspond to these three requirements. The main advantage is that it is not necessary to use a mould during the construction process, because the mesh is selfstanding. Further, the construction has low input of materials and is very simple: on a concrete base is placed a self-standing w ire mesh, cylindrical in shape, already the diameter of the future tank. To enable the application of mortar, the mesh is w rapped w ith a nylon shade screen. The application of four thin layers of mortar provides to the w all the necessary resistance. The roof consists of similarly made segments. Normally the tanks have a storage capacity of 16 cubic meters, but the volume can easily be enlarged or reduced by a simple addition or subtraction of the mesh length. Guarantee of this technology against leaks and the low cost recommends its use in governmental and non-governmental programs. Self-standing w ire mesh cisterns are

2 currently successfully being built in rural communities of semiarid Brazil, but also in Haiti, Mexico and India. The cisterns (of 10 cubic meters) constructed in the quakestricken region near Port-of-Prince in Haiti resisted the earthquake of KEYWORDS: w ater resources, social technologies, rainw ater harvesting INTRODUCTION The Brazilian Semi-arid Tropics are a region extending over one million square kilometers and peopled by about 21 million inhabitants, almost 50 percent in the rural area. The collection of rainw ater is an indispensable element for w ater security, especially in the part of the region w ith crystalline rock base, w hich covers about 80% of the area and has limited and often brackish groundw ater resources. For human use, a rainw ater tank needs to be w ater proof and closed, so that there are neither leaks nor evaporation losses. Over the years, after several attempts and experiments w ith materials like bricks, stones, synthetic materials, cylindrical mortar reservoirs has proven to be the most resistent ones. In these, the ferro-cement technology stands out for its high strength and reduced use of materials. - The traditional ferro-cement technology employs several types of thin w ire mesh tied w ith w ire to a structure of vertical steel bars that supports the w eight of the mortar. The construction of this structure is slow and uses more steel than necessary for the needs of the reservoir. - As a simplification of the traditional ferro-cement technology, the so-called mesh and w ire tank w as developed, keeping, how ever, the basic principle of the ferrocement that ensures strength and security against leakage. In this case, vertical steel bars are not used, but a mould of flexible steel sheets, removable, around w hich is w ound the mesh and around the mesh, the galvanized w ire. The w all support, w hile the mortar is fresh, is maintained by a mould, w hich is removed after the application of the tw o outer layers of mortar. - The challenge for the new technology, described in this paper, w as to eliminate the mould, w ithout losing the simplicity and security, that the ferro-cement offers, and the one piece w all, w ithout seams or combination of single elements. A product of the steel industry, w idely used to separate outdoor spaces, such as homes, parks, etc., offered itself as an ideal solution: fence w ire mesh, tw o meters in height, w ith a 15

3 cm x 5 cm mesh of galvanized w ire, 3 mm in diameter, w ith a tensile strength of 65/70 kgf / mm², slightly higher than the 3.4 mm w ire used in construction of tanks w ith moulds w ith a strength of 50 kgf / mm², and w ith the same storage volume. The screen is supplied in rolls 25 meters in length. The application of the mortar in four layers imitates the principle of composite materials such as plyw ood sheets or armored glass. METHODOLOGY The construction follow s the basic procedures of construction of tanks w ith a mould: at the construction site the layer that contains organic matter - normally around 20 cm is removed and the soil is compacted. Then tw o layers are placed, each three centimeters thick, one of crushed gravel and another of w ashed coarse sand, both compacted, w hich serve as foundation. Within the next layer, the underlayment, the w ire mesh is placed (Photo 1). One must be careful to exactly join the part of concrete placed on the outside of w ire mesh cylinder w ith the inner part (Photo 2). The cylinder of the mesh needs to be leveled properly. Photo 1. Placing the self -standing wire mesh Photo 2. Joining the concrete at the mesh base The next day, placing the first layer of mortar to stabilize the mesh and allow the implementation of the structural and sealing layers, found a surprising solution. The challenge w as to find a w ay that allow s the first layer of mortar to stick to the metal mesh. In the original ferro-cement technology several layers of fine w ire mesh are applied and during the application of the mortar, another person holds on the other side a thin piece of plyw ood, so that no mortar falls into the void. But this w as not the

4 solution. It w as necessary to find a solution that holds the mortar w ith ease, w ithout more construction costs. It could not be organic matter because of the thin layer of mortar small amounts of moisture w ould penetrate and break dow n the organic matter, causing sw elling and destruction of the concrete. A cheapest solution w ould bags used for packing onions. They have a capacity of 20 kg, and are made of w eaved polypropylene yarn. Other possibilities that can be used is nylon shade screen (50% shaded) or a w ire mesh (No. 20) w ith mesh size of 5 mm x 5 mm. In each case, the mesh is very thin and does not let the mortar pass through to the other side at the time of application and is strong enough to be w el adjusted and stretched around the mesh. If one uses onion bags, 25 bags are needed, w ashed w ell in advance, tied w ith a synthetic ribbon, every five centimeters (Photo 3). With a flexible nylon spatula the first layer of mortar is applied to stabilize the w ire mesh (Photo 4). The bags are not placed as a structural element, but rather as a simple support to facilitate the implementation of the first layer of mortar. Photo 3. Tying t he onion bag with ribbons Photo 4. Application of the first layer of mortar In order to prevent the elastic movement of the mesh, caused by the application of the mortar, the w ire mesh cylinder should be stabilized in place w ith w ire, stretched to the outside, and w ooden stakes placed from the inside. The next day, apply the second outer layer and on the follow ing days the tw o inner layers, all these w ith a thickness of about one centimeter. The composition of the mortar is one part cement, tw o parts coarse sand (w ashed, w ithout clay) and one part medium sand, kind used for plastering w alls. Tw o important observations must be made: - The application of layers needs to be done all at once, w ithout interruption;

5 - After each step, the tank must be covered by a canvas tarp and the w all must be w ell w atered before the next stage. This is to allow the mortar to cure and prevent premature drying. The roof is formed by 17 plate segments, w ith a thickness of one inch, also using w ire mesh. Photo 5 show s frame segments of the plates, and Photo 6 show s the application of the second layer of mortar w hen the first layer is still fresh. Photo 5. Frame segments of the plates Photo 6. Putting the second layer of mortar Remember that the roof plates should be manufactured as a first step before starting the construction of the tank, because the concrete needs time to cure, in order to avoid breaking of the plates w hen they are placed. The plates are supported in the center of the tank w ith a beam (Photo 7), placed in position in the form of a sloping roof. Photo 7. Beam wit h wooden disk supports the plates Photo 8. The tank ready with the plates The uniting of the plates w ith the tank w all w ith mortar (Photo 8.), follow s the pouring of the floor, three inches in thickness, a l w ith a mortar of one part cement and three

6 parts sand. At the final step, on the inside w alls and the floor one coat of cement slurry should be applied. RESULTS AND DISCUSSION The results of the construction of the w ire mesh cisterns show that this technology combines safety and simplicity of construction. The cost turned out to be the low est in comparison w ith various models of tanks currently used. The follow ing table show s the demonstration of inputs and prices. Item Unit Quantity Price/unit Price Crushed gravel m³ 1.20 R$ 40,00 R$ 50,00 Coarse sand m³ 2.20 R$ 40,00 R$ 88,00 Medium sand m³ 0.35 R$ 20,00 R$ 7,00 Cement 50 kg bag 14.5 R$ 20,00 R$ 290,00 15 x 5 cm, 3 mm w ire, 2 m height, galvanized self-standing w ire mesh Linear m 20 R$ 22,00 R$ 440,00 Plastic tw ine Kg 1 R$ 10,00 R$ Nylon screen (or onion bags) 10 m Unit 2 (25) R$ 20,00 (0,80) R$ 40,00 Bricklayer days 4 R$ 60,00 R$ 240,00 Total R$ 1155,00 CONCLUSIONS AND SUGGESTIONS 1. The safety of this technology against leaks and its low cost, recommend its use in governmental and non-governmental programs, such as the P1MC Program of One Million Cisterns of ASA a Netw ork of NGOs of Brazilian Semiarid Region; 2. The simplicity of construction, especially for brick layers, makes the technology suitable for popular tank building programs; 3. The costs of the cistern compared w ith cisterns built in other countries seems to be high, but this is partly because of the over-valuation of the Brazilian currency (1 Brazilian Real = US$ 0,60 on January 2011, and 0,42 on January 2009). 4. Self-standing w ire mesh

7 cisterns are currently successfully being built in rural communities of semiarid Brazil, but also in Haiti, Mexico and India. The cisterns (of 10 cubic meters) constructed in the quake-stricken region near Port-of-Prince in Haiti resisted the earthquake of 2010 (Photo 9). This is very surprising, because almost all the houses at the same place collapsed the same happening to traditional subsoil reservoirs. REFERENCES CHINDAPRSAIRT, P., Study and development of low-cost rainwater tank, Khon Kaen University, Thailand, 1986 GNADLINGER, J., Technical Presentation of Various Types of Cisterns Built in the Rural Communities of the Semiarid Region of Brazil, 9 th IRCS Conference, Petrolina, Barzil, 1999 LENGEN, J., von, Manual do arquiteto descalço, Rio de Janeiro, Casa do Sonho, 2002 NISSEN-PETERSEN, E., How to build the underground watertank with dome. ASAC consultants LDT, Kitui, Kenya, 1992 SANTOS, E. de O.; SANT OS D. N.; BRITO, L. T. de L.; SILVA, A. DE S.; Experiência Brasileira sobre Captação, Armazenamento, Gestão e Qualidade da Água de Chuva para Consumo Humano em Comunidades Rurais do Haiti, 5th Brazilian Rainwater Catchment and Management Symposium, Belo Horizonte, Brazil, 2007 SCHISTEK, H., A Construção de Cisternas de Tela e Arame, Editora Fonte Viva, 1998 SHARMA, PC, An Gopalratnam, VS, Ferrociment water tank, Ferrociment International Information Center, Asian Insitute of Technology, Bangkok, Thailand, 1980 THOMAS, T.H. & MARTINSON, D. B., Roofwater Harvesting: A Handbook for Practitioners, Delft, Netherlands, 2007 WATT, SB, Ferrociment Water Tanks and Their Construction, IT Publications, London, 1978