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This ferrocement concrete information can be used to make concrete biogas digesters and gas storage tanks. This information can also be used for the construction of water tanks, grain storage tanks, or for almost any construction project that needs strong concrete walls.

This section is adapted from the book, Ferrocement Water Tanks and Their Construction. The book describes methods of constructing water storage tanks from wire-reinforced concrete. These tanks are widely used in many parts of the world to collect and store water for housing, irrigation, and industrial purposes. The tanks are made by hand troweling a cement-rich concrete onto a mesh of wire reinforcement to form cylindrical (round) tanks with thin walls which vary in thickness from 3.0 to 10.0 centimeters (1.0 to 4.0 inches) depending on the size of the tank.

Part One

Although the word ferrocement is used here, this is not strictly true. These tanks should really be called wire-reinforced concrete tanks. The main difference is that in ferrocement there is a very dense layer of mesh reinforcing wire that has to have a minimum value of wire volume for each unit volume of concrete. The quantities of straight wire reinforcement used here fall far below this minimum value, but they provide enough strength for the tank sizes described here.

Still, wire-reinforced concrete is closer to ferrocement concrete than to ordinary steel-bar reinforced concrete. The wire distributes the load throughout the concrete, preventing the force of the load from concentrating in planes of weakness which would lead to early failure (Diagram 29, Figure 3). Straight wire is used because it is many times cheaper than the same weight of mesh wire.

This method of water tank construction is useful in rural areas for several reasons.

· The basic raw materials are available and are already being used for many familiar purposes.

· Except for the cement, which must be kept dry, the materials are not easily damaged.

· The skills needed to use the materials are often already known, and people can learn to make good tanks after only a few days of instruction.

· The users of the tank can contribute their labor to pay for part of the cost.

· The construction techniques are not complicated and do not demand the use of expensive and complicated machines or electricity.

· Trained supervision can be kept to a minimum.

· Leaks resulting from bad workmanship or damage are easy to repair, and only minimal maintenance is needed.

· The framework can be carried from site to site and used to build many separate tanks.

Wire-reinforced concrete is used for a wide variety of purposes, but its main advantage for water tanks (and biogas systems) is its ability to resist rusting and its low cost in comparison with other building materials. Galvanized iron (GI) tanks have been widely used in the past for water storage, but they are expensive and galvanized iron will rust and leak in five to ten years, even if they are carefully maintained. The life of reinforced concrete water tanks is expected to be more than 50 years.

The thin walls of ferrocement tanks are able to bend under load; this helps prevent the concentration of stresses that cause cracking. The wire reinforcement distributes the stresses throughout the concrete, increasing its ability to carry pulling and bending loads (tensile loads) without cracking. The thick walls of steel-bar reinforced tanks are not able to bend very much under load; they are more likely to crack than to bend.

Wire-reinforced concrete tanks have been proven in use over many years in all extremes of weather conditions and can be built confidently with capacities up to 125 cubic meters (4,400 cubic feet or 33,000 gallons). But the tanks should not be built off the ground unless expert advice is available.

Cost of construction: While the final price of the tanks will vary according to local conditions, it will depend on the following.

· Materials: the cost of sand, cement, straight wire, mesh wire, and formwork. The formwork can be made either for one usage with temporary local materials or of more permanent construction from steel sheeting and angle iron. Well-made forms will last for many years and the formwork cost for each tank will become increasingly smaller.

· Wages: the cost of wages for plasterers, laborers, and supervisors if the tank is not built totally by self-help.

· Transportation: the cost of transporting people and materials.

The quality of the water stored in the tank will depend initially on the quality of the water put into the tank. Diseases that are related to water and water use are causes of sickness and death to people who do not have protected or purified water supplies or efficient sanitation. Any measures that reduce the risk of these diseases will be of great value to water users.

The bacteria that cause some of the most serious illnesses, especially cholera, typhoid, and diarrhea in children, cannot live long outside of the human body. In stored water, most will die within one week, although cholera and typhoid germs can remain dangerous in stored water for up to four weeks.

Water in a tank must be protected from contamination. As shown in Diagram 28, a cover is usually necessary. The tank must not be allowed to become a breeding ground for malarial mosquitoes. Overflow pipes should be covered with window screen and so should the inlet. These precautions will prevent trash, insects, rats, and mice from falling into the tank.

To further improve the quality of the water, certain chemicals can be added. Both chlorine and iodine kill disease-causing bacteria and viruses in water. But unlike chlorine, iodine has no taste or smell, and iodine is medicinal in that it can prevent and cure goiter.

Chlorine, even though it is used all over the world to make water safe to drink, can increase the risk of some types of cancer; it is made inactive by organic matter in water, and its strength is reduced if the water is alkaline (base).

Too much iodine, like too much chlorine, can be poisonous. Follow the directions for using iodine very carefully. The result will be a method of purifying water that is an improvement on the chlorine method. Iomech Ltd., a Canadian company, sells systems that add measured amounts of iodine to water systems in many developing countries, including Egypt and New Guinea.

Designing the tank: The great advantage of wire-reinforced concrete over standard reinforced concrete is its ability to resist shrinkage during the curing (drying) process, its resistance to severe cracking under tensile load, and the need for only one set of forms for construction. Pouring a thin shell of concrete between two closely spaced shutters, the usual method of ferrocement construction, is a highly skilled and difficult job.

The weakness in tension and the brittle type of failure occur because no matter how carefully the concrete is mixed and placed, there will always be planes of weakness between the edges of the different lumps that make up the concrete. In compression (top-down pressure), these planes of weakness are held together by the load; but under tensile loading (sideways pressure), they can open up beyond their elastic limit. The separate cracks then join together, causing the concrete to fail (Diagram 29, Figure 3).

In standard reinforced concrete, the reinforcing bars work to limit and control the tendency of the concrete to crack under tensile load according to the amount and distribution of the steel bars and the degree of loading. In wire-reinforced concrete under moderate tensile loads, the concrete contributes greatly to the tensile strength of the whole structure. This is because the wire, distributed throughout the concrete, will allow the load to be absorbed throughout the concrete and will prevent the buildup of too much stress in weak spots.


Figure 1: Catching and using the rain water from a roof

Secondary storage, fish pond, or irrigation

-Strainer (mesh sieve) to prevent rubbish from flowing into the tank with the water and to stop insects, flies, and rodents from flowing into the tank.

Figure 2: Protecting the tank from further contamination

-The concrete diagrams in this section are from the book, Ferrocement Water Tanks and Their Construction.

Foundation: The foundation of the tanks carry the weight of the tank and water down to the ground. The floor in smaller tanks is usually continuous with the walls. The floor slab carries the weight of the walls and the water directly onto the foundation (Diagram 29, Figure 5).

Walls: The vertical tensile stresses set up across a horizontal plane at the joint of the wall and the floor are nearly double the hoop tension stresses produced by the walls stretching outward under load. The tanks should be designed with vertical reinforcement both within the walls and between the walls and the floor to prevent cracking (Diagram 32, Figures 21 and 22).

The tanks described here have one layer of straight wire reinforcement and one layer of the more expensive woven mesh (chicken) wire. The mesh wire, which starts in the floor and goes into the walls, will provide the reinforcement that limits cracking between floor and walls.

The joint between walls and floor will not remain completely rigid. It is likely that very small cracks will open and that the joint will bend outwards and throw a greater part of the load onto the wire reinforcement (Diagram 30, Figure 9). These small cracks are unlikely to be serious. If the cracks become wide enough to allow water to reach the wire because there is not enough wire or the wires are too widely spaced--the wire will rust and eventual tank failure will occur. In most of the tanks a thick concrete layer is built up around the junction of the floor and the walls to strengthen the joint.

The chicken wire layer that is wrapped around the formwork (before the straight wire is added) will play an important part in preventing cracks from occurring as a result of shrinkage and loading. Although the mesh wire is expensive, it will contribute greatly to the strength of a water tank.

Floor: The concrete floor of a tank is usually built first to give the walls a solid foundation. There should be mesh wire running from the floor up into the walls to provide resistance against cracking. A bitumen (asphalt or tar) movement joint between the floor and the walls is usually provided to take expansion and settlement.

The concrete floor of large tanks (over 10.0 meters in diameter) must be cast in sections with bitumen-sealed expansion joints between the sections. This will allow the floor to expand and contract without cracking. The floors of the smaller tanks can be built as one continuous piece.

Roof: A roof provides a good cover against the loss of water from evaporation, which in hot, dry parts of the world, can exceed two meters per year. It prevents trash, insects, rats, and mice from getting in and it keeps the water cool. The extra stresses set up by the weight of a concrete roof are not great if the junction between the roof and the walls is curved. Sharp angles concentrate stresses and start cracks.

The greatest stresses set up in a roof of this kind are caused by its expansion in hot weather from the heat of the sun. These stresses can be over 20 times greater than the static load stresses due to the weight of the tank. For this reason, the roof and walls should be painted white to reflect the heat of the sun or be protected with a grass roof (Diagram 30, Figure 11) over the concrete roof. The roof may also be built from lightweight materials, such as galvanized iron, fastened to a conventional structure built over the tank (Diagram 30, Figure 12).



Figure 4: Curved walls are stronger than flat walls

Figure 5A: Foundation of small tank

Figure 5B: Walls and base joined

Figure 6A: Foundation of Large open tank -

Figure 6B: Walls and floor separate



Tank fittings: Most tanks will have pipes of some sort to take water in and out. The outlet pipe for smaller tanks often goes through the bottom of the walls, the overflow pipe through the top of the walls, and the water enters through the roof. With larger tanks, the inlet and outlet pipes should both go through the floor. This reduces the risk of cracking and leakage around the pipes which could happen if the pipes were cut into the walls. The overflow pipe should still go through the top of the walls. In all cases where the outlet pipe passes through the floor, a wire screen of some sort is recommended to prevent the outlet pipe from being blocked by trash.

The opening of the outlet pipe is usually placed 10.0 centimeters (4.0 inches) or so above the level of the tank floor to prevent dirt that has settled to the bottom of the tank from being used. The hole for the overflow pipe is cut out while the concrete is still green (half dry). The overflow pipe is then set in the hole. The open end of the pipe should be covered with a layer of window screen to stop mosquitoes and other insects from entering and breeding in the tank.

On large tanks used for watering animals (Diagram 30, Figure 13), the outlet is often provided with an adjustable pipe section fitted inside the tank. If the water trough is damaged by the animals or if the ball valve in the trough sticks open, only a little water will be lost. The outlet to the tank should be fitted with a sand filter to screen the water before it is used in order to remove most organic matter and bacteria. This sand filter must be kept permanently wet if it is to work correctly. The sand should be cleaned or replaced from time to time (Diagram 30, Figure 14).

Reinforcing wire: There are many different types of steel reinforcing mesh that can be used to make reinforced concrete. These generally consist of thin wires, either woven or welded into a mesh, but the main requirement is that the wire must be easy to use and, if necessary, flexible enough to be bent around sharp corners. The wire is tied and held firmly in place so that the concrete can be troweled over it.

Chicken wire can be expensive to buy. It can cost over ten times as much as an equal weight of straight wire. For these reasons, straight wire is used for the second wire layer. Because reinforcement is needed in the vertical direction, a single layer of mesh wire is wrapped onto the formwork before the straight wire. To reduce the cost of buying ready-made mesh wire, handlooms can be adapted to weave mesh wire from straight wire.

The straight and mesh wire can be galvanized iron or steel, but under no circumstance should aluminum painted wire be used. Aluminum can react with cement to give a very poor bond between the wire and the concrete.

Cement, sand, and water: The cement that is used to make the concrete can be an ordinary Type One or Type Two Portland cement (to BS-12 or similar specification). Lower strength cement has been used in the past with some success, but it cannot be recommended. Sometimes lime is mixed with the cement in the ratio of one bag of lime to five bags of cement to improve the workability of the concrete and to reduce shrinkage cracks. The cement should always be kept dry until it is used.

The main requirement for the sand is that it be free from organic and chemical impurities that would weaken the concrete. Most clean sands are usable. If the quality of the sand is in doubt (such as salt contamination), it should be washed with clean water. A silica sand is probably best, but sands consisting of other hard minerals can be used. Moderately coarse sand, although it makes the concrete more difficult to work, will resist shrinkage and cracking better than a fine grain sand. If the sand has a high dirt content, the concrete will be weak. There should be a mixture of all grain sizes without too many of any one particle size.


Figure 9: How the loads are taken

Figure 11: Roofs for tanks less than 5m diameter

Figure 12: Roof for larger tank

Figure 13: Ball valves for cattle watering

Figure 14: Slow sand filter in tank over outlet

The water must be fresh and free of chemicals in solution or suspended dirt and organic material. Clean water is a must for a strong, long-lasting concrete. Salt water should never be used.

The concrete mix: Making a strong concrete mix from cement, sand, and water is one of the most important stages in building with concrete. The concrete must be prepared with the correct proportions of materials. It must be well mixed and workable enough to be troweled by hand onto the formwork and between the wires to form a dense, compact layer. It must also be dried slowly in order for the concrete to achieve its full potential strength and durability.

Ratio of cement and sand by volume: Increasing the proportion of cement in the concrete will increase the concrete's strength and make it more workable. Extra cement will also increase the danger of shrinkage and cracking which may cancel out the value of the increase in strength.

The concrete mixture preferred here uses a cement to sand volume ratio of 1:3 (one part by volume cement to three parts by volume sand). Measuring boxes or buckets should be part of the construction equipment and should be used to achieve the correct proportions. Measuring the materials on a shovel does not give reliable results.

Ratio of water and cement plus sand by weight: A dry, low water concrete mix will be stronger than a wet, high water mix made with the same proportions of cement and sand. Dry mixes are difficult to work with on formwork. They are more likely to contain air pockets and be poorly bonded to the reinforcing wire. Wet mixes are easy to trowel by hand, but the finished concrete will not be as watertight or airtight as a carefully made and used dry mix nor will the wet mix be as strong or as long lasting as a dry mix can be.

A compromise between wet and dry mixes must be made. Experience has shown that a 1:3 cement to sand by volume ratio and a 0.5:1 water to cement plus sand by weight ratio will work. If, in order to make the mix more workable, extra water has to be added to give a water to cement plus sand weight ratio of 0.6:1, then a better grade of sand or a larger proportion of cement should be used. (Other makers of ferrocement structures use a 1:2 cement to sand by volume and a 0.4:1 water to cement plus sand by weight ratio.)

Under most conditions the workability is usually controlled by eye during mixing. (If someone is skilled in using a funnel to make what is called a slump test, they should use it to determine the workability of the mix.) If the water to cement plus sand by weight ratio is not to exceed 0.5:1, the mixing must be carefully supervised.

The amount of water to be added to the dry mix of cement and sand is affected by the presence of moisture in the sand, which can vary greatly between the top and bottom layers of the sand pile. The wetter the sand is to begin with, the less additional water is needed. One or two small-scale trial mixes using a sample of completely dry sand and a measured water to cement plus sand weight ratio will allow the people who will be mixing the concrete to see and learn the feel of a good mix.


Figure 15: Mixing the concrete by hand

Figure 16: A tool for straightening and tightening wires that are bent and slack

Figure 17: Trowelling the concrete onto the formwork - 1. concrete onto hawk

Figure 17: Trowelling the concrete onto the formwork - 2. concrete onto float

Figure 17: Trowelling the concrete onto the formwork - 3. concrete onto formwork

Formwork: Formwork is needed to support the walls while the concrete that has been troweled on hardens and sets. Well-made formwork is expensive but will, with care, last for years. Its initial cost will be spread out over many construction projects. Experience shows that well-made formwork makes construction work almost foolproof. The chicken wire and the straight wire are wound easily and quickly around the cylindrical forms and the concrete is then troweled over the wire layers. The formwork must be rigid enough to hold the weight of the concrete as it is being applied and dried without moving. If the formwork moves during the setting period, the concrete is likely to crack and be very weak.

The formwork can be made from circular corrugated iron (GI) sheets. The "hills and valleys" of the corrugations provide a mark for the tank builders to help them wind the reinforcing wire on at the correct spacing, although the straight wire will tend to bundle together in the valleys of the corrugations.

The main advantage of the corrugated sheets, besides durability, low cost, and light weight, is that they allow for an accurate measurement of the final wall thickness when the corrugations on both the inside and the outside faces of the tanks are filled with concrete. In this way the total wall thickness is easily achieved.

Tools: The necessary tools should be collected in advance to make a standard kit for tank construction. It is possible to use local tools and equipment if they are available, but it is always safer to make sure that all necessary tools are on hand by bringing the standard tools. The concrete can be mixed either by hand or with a concrete mixer. Hand mixing is hard work and if incomplete will result in a lumpy concrete that is difficult to trowel onto a tank wall. Hand mixing can manage drier mixes better than concrete mixers, because the drums of concrete mixers tend to do a poor job at mixing low-water mixes. Treading the concrete under foot on the mixing slab is one of the best ways of working a dry mix. Steel (plasterer's) floats are much easier to use than floats made of wood. Because overworking wet concrete, especially with a steel float, can cause the concrete layer to drop off, the concrete should be applied quickly and carefully. The surface of the first layer (there are two per side) must be roughed or brushed when it has hardened a little, to give the second layer something to hold on to.

List of tools and equipment: Four plasterer's steel hand floats, four hand hawks 30 x 30 cm with 4.0 cm diameter handles, four troweling boards 75 x 75 cm, two wire brushes for cleaning formwork, two oiling brushes, one brush or scratching tool for concrete, one hacksaw and spare blades, one woodsaw, one set of wrenches, one crowbar--1.0 meter long one wire snip for mesh wire one bolt cutter for straight wire, two cold chisels for cutting green concrete, one two-kilogram sledgehammer, one gauging box for sand and cement 50 x 50 x 40 cm or other container to hold 100 liters of sand, one sieve 5.0 mm maximum openings for sand, four flat-ended shovels for mixing, one spirit level, one 50 meter long cloth or plastic tape measure, one string line, one axe for trimming wood, two pickaxes for digging, two mattocks for ground leveling, one wire tension tool, one set of formwork sections, one wheelbarrow for carrying concrete, water containers, and plastic sheeting to cover the drying concrete.

Part Two

Summary of methods: This section contains a detailed description of standard tank construction practices that have been used successfully for over 25 years in many countries all over the world. What is outlined here is a design for a small ten cubic meter capacity water tank. The book, Ferrocement Water Tanks and Their Construction, from which this information was taken, has information for building large 150 cubic meter capacity open tanks and many other different water tank designs, sizes, and adaptations of the basic design described here. VITA is another good source of information on ferrocement construction.

Clearing the site and preparing the foundation: The site chosen for the tank should be cleared of plants, loose soil, and any rocks that could cut through the floor of the tank. For small tanks it is often only necessary to clear the site and put down a 15 cm (6.0 inch) layer of sand and gravel. Larger tanks need a ring of concrete, separate from the tank floor, to support the walls. The ring is prepared by digging a trench under the line of the walls and backfilling it with concrete (Diagram 29, Figure 6).

Collecting the materials: The sand, gravel, and water should be collected before construction starts. If this is done by the people who will be using the tank, they will be able to contribute a large proportion of the total cost of the construction. With experience, a trained builder with five helpers can build a tank in about three days.

The sand and gravel should be stored in piles next to the concrete mixing slab. The slab is built by troweling a layer of concrete onto a layer of gravel 2.0 meters by 2.0 meters square. The slab is finished with a small wall to prevent the spillage of concrete slurry during mixing (Diagram 30, Figure 15). For smaller tanks, it may be cheaper to include a wooden mixing platform as part of the basic equipment. Mixing the concrete on the bare ground will contaminate and weaken the concrete with dirt.

Make sure that enough sand is collected for each tank before construction begins. The cement, mesh wire, and straight wire should be collected and stored under cover. The formwork and any other materials or tools should be collected and checked. All underground pipes should be installed before the concrete is laid. Although approximate quantities of materials needed for each tank are known, it is always a good idea to have extra materials on hand to allow for wastage.

Design: The tanks have been designed for construction by relatively unskilled workers. The tanks have a diameter of 2.5 meters, a height of 2.0 meters, and a capacity of approximately 10.0 cubic meters. The final wall thickness will be about 4.0 centimeters (1.5 inches). The tanks are built on site and should not be moved. (Portable tank construction is described in the book, Ferrocement Water Tanks and Their Construction.)

Casting the foundation: The foundations of small tanks are below and separate from the tank floor. They are made by casting a 7.5 cm (3.0 inch) thick slab of concrete 2.8 meters in diameter onto a 10.0 cm (4.0 inch) thick layer of sand and gravel. The concrete is prepared from a by volume mix of 1:2:4 of cement to sand to gravel. It should be kept wet in such a way that drying takes seven days. The ring trench of large tanks is filled with concrete after the floor slab has been cast.

Into the concrete foundation is cast a 1.0 meter length of 20 mm diameter steel water pipe with a tap on the outside end. The pipe is curved (with 45 percent joints) so that it projects 10.0 cm (4.0 inches) above the floor of the tank. A piece of wire is threaded through the pipe to act as a "pull-through" after the tank is completed (Diagram 32, Figure 20).

Mixing the concrete for the floors and walls: The cement and sand are gauged either onto the mixing slab or into a cement mixer in the by volume ratio of 1:3. It is difficult to judge these volumes accurately by shoveling. A gauging box should be made to measure the sand. If the box is 50 x 50 x 40 cm, then one box of sand mixed with a 50 kilogram bag of cement will give a concrete volume mix of 1:3 (cement to sand).

The dry cement and sand on the mixing slab are turned by hand (if a cement mixer is not used) several times into piles from one side of the slab to the other until they are completely mixed together. A hole is then made in the center of the pile and water is added into the mix until the desired consistency is achieved (0.5:1 water to cement plus sand by weight. Add only a small amount of water at a time. It is very difficult to make a wet mix drier by adding more cement and sand. The workability of a dry mix can be increased by treading it with one's feet to break up any dry lumps. (Warning! If lime is used in the mix, it can burn!)

It is usually easier to build the floor of a tank before the walls are built. The floor is made of two 1.0 cm (0.5 inch) thick layers of concrete with one layer of chicken wire between the two concrete layers. This mesh (chicken) wire extends 1.5 meters from the floor into the walls if the floor is built first. The mesh wire extends 1.5 meters from the walls into the floor if the walls are built first. The inside corners between the floor and the walls should be built up with extra concrete. This is called "coving."

Take care that the concrete does not block the outlet pipe. Before the concrete on the floor has stiffened, make a shallow depression in the middle; this will allow the tank to be easily cleaned in the future. The dirt can be brushed into the valley and cupped out (Diagram 32, Figure 22). After the concrete has dried, the inside of the tank is painted with a thick adhesive cement or asphalt layer to seal the tank.

Formwork: The 2.0 meter high formwork is made from standard galvanized iron (GI) roofing, 0.6 mm thick with 7.5 cm corrugations. The formwork is rolled into a cylinder with a radius of 1.25 meters. Steel angle iron (40 x 40 x 5.0 mm) is bolted vertically on the inside face at the ends of each set of four sheets. This allows the metal sheets to form a circle when they are bolted together. Between the ends of each section is placed a wedge which is pulled out to allow the formwork to be dismantled (Diagram 32, Figures 18 and 19).

The formwork is erected after the concrete floor has hardened. The bolts passing through the angle iron and wood wedges are tightened to provide a rigid cylindrical form. The formwork is cleaned free from concrete and dirt. It is then oiled to prevent the new concrete walls from sticking to it. Mesh wire is then wrapped tightly around the formwork to a single thickness and tied with short pieces of straight wire.

The 1.5 meters of mesh wire coming out of the concrete floor are overlapped with and tied tightly with straight wire to the wall's mesh wire. The mesh wire has a 50 mm mesh and is made from 1.0 mm wire (Diagram 32, Figure 21). Other makers of ferrocement structures use chicken wire with mesh openings as small as 15 mm. The smaller the mesh opening, the stronger and more expensive the tank will be.

To complete the reinforcement, the straight wire, 2.5 mm in diameter, is then wound tightly around the tank from the base up, at the following spacings:

· two wires in each corrugation for the first eight,

· one wire in each corrugation to the top, and

· two wires in the top corrugation.


Figure 18: Standard formwork for 10 cubic meter tank - top view of format

Figure 18: Standard formwork for 10 cubic meter tank - cross section of form

Figure 19: Tank foundation

Figure 20: Assembling the formwork

Figure 21: Erecting formwork and winding on reinforcing wire

Figure 22: The completed tank

About 200 meters of wire will be needed (weight 78.0 kg). The chicken wire must be wound tight enough to hold the straight wire out of the valleys in the corrugations. If the straight wire is bent, it can be straightened by making and using a special tool (Diagram 31, Figure 16). A wrench or a pair of pliers may be able to do the same job.

When building ferrocement biogas digesters, use two wires in all corrugations and two layers of mesh wire in the end pieces. Because digesters lie on their sides, the stress on the concrete may be greater.

The concrete is applied by hand to the wire covered walls with a plasterer's steel float in layers not greater than 1.0 cm (½ inch) thick because thicker layers tend to fall off. The concrete which has already been prepared at the mixing slab is carried to the side of the tank and emptied onto a square board (A). This board prevents dirt from becoming mixed with the concrete and catches any concrete that falls off of the wall during troweling. The concrete is scooped off this board using a plasterer's float (B) and a builder's hawk (C). The concrete is then transferred back onto the face side of the float and pressed into the formwork (Diagram 31, Figure 17).

The concrete is troweled onto the formwork from the base of the tank upwards to fill the corrugations and just barely cover the reinforcing wire. The walls are built up in this way in horizontal sections around the tank. The square board is kept under the section being worked on. Some experts say that vibrating the float as the concrete is pushed into the formwork will help the concrete penetrate the wire mesh so that there will be no air pockets.

When the first layer of concrete has hardened sufficiently, the surface is brushed rough or scratched so that it will be easier for the second layer to stick to the first layer. The 1.0 cm (3/8 inch) thick second layer, which provides the outside surface, is finished with a smooth surface. It must be well bonded to the first layer, which should be only a day or so old when the second layer is applied.

Concrete must be applied quickly. If it is more than half an hour old, it should be used to make a floor slab or concrete path. The concrete quickly becomes unworkable without adding an excessive amount of water that would weaken the final product. In hot climates the concrete pile should be covered with wet sacks or plastic to prevent rapid drying.

In order to make tanks more watertight and airtight, an asphalt seal can be used between the two layers of concrete. The seal is not necessary; it is extra protection. The asphalt seal (bitumen emulsion) is applied in two layers. The first layer is mixed with water--one part asphalt, three parts water. The second layer contains no water. After the asphalt seal has dried, the second layer of concrete can be applied.

After the second layer of concrete has had a day or two to dry, the formwork is removed by removing the holding bolts and pulling out the wooden wedges which will leave the formwork free to be pulled away from the inside of the concrete tank. The sections are then lifted clear of the tank, cleaned of all concrete, and stored for later use.


Figure 31: Constructing the roof

Figure 32A: Making the access hatch

Figure 32B: Making the access hatch

The exposed corrugated concrete on the inside of the tank is brushed rough, and two 1.0 cm (3/8 inch) layers of concrete are troweled on a day or two apart to fill up the corrugations and cover up any exposed reinforcing wires. The methods for applying the concrete on the inside are the same as those used on the outside of the tank.

If the tank has to be left overnight with a layer incomplete, the top edge of the concrete should be at the same height all around the tank. On the next day, the joint should be brushed rough and coated with a cement and water mix to make a strong bond before applying fresh concrete. It is best to complete any one layer in the same day. If that cannot be done, finish a complete band around the tank. Concrete joints will then only occur on a horizontal line. Vertical joints are more likely to crack when the concrete dries.

Roof: The tank can have a galvanized iron sheet roof supported by angle-iron, or a reinforced concrete roof can be made. A ferrocement roof is built onto a tank by laying concrete onto shaped formwork which is supported from below (Diagram 32, Figures 31 and 32). The roof is reinforced with two layers of mesh wire which are tied onto mesh wire coming out of the top of the walls. A prefabricated angle-iron frame is set into the wire mesh to provide formwork for an access hatch into the tank.

This is how both ends should be built when making digesters, though of course only one access hatch will be needed.

Concrete is troweled onto the roof in 1.0 cm (3/8 inch) layers with two layers of mesh wire in the middle and allowed to slowly dry over a seven day period. When the concrete is strong enough, the roof and access hatch formwork is removed and two 1.0 cm (3/8 inch) layers of concrete are troweled onto the bottom of the tank roof and the access hatch. Do not add a concrete roof to a concrete tank until the walls are strong enough to hold it; that will be at least several days after the walls have been finished.

Applying concrete to a tank may look difficult at first, but most people can learn within a few hours. The secret is to apply well-mixed concrete quickly and firmly with the float and work it smoothly over the wall. The surface finish is not of greatest importance on water tanks. What is important is that the layers be of uniform thickness throughout, with no gaps or weak spots.

Drying the tank: After the concrete has been applied, the tank should be covered with black plastic or wet sacks. If the drying concrete is exposed to hot sunlight or wind, it will dry too fast and the final strength will be greatly reduced. Loss of moisture at too fast a rate enlarges the shrinkage cracks that form in the drying concrete and can lead to tank failure.

In very hot weather, tanks must be covered anytime work is stopped. In milder weather, the tanks can be left uncovered except at night. When finished, keep the tanks covered for seven days or more to allow the concrete to slowly dry. The concrete should be sprayed with water as often as three times a day for the seven days. The concrete will take at least one month to reach its full strength, and for the first few days it will be soft enough to allow holes to be cut in it for pipes.

Slow drying is absolutely necessary for strong, waterproof tanks. It is one of the most important construction steps. It is also one of the most difficult things to guarantee in the field.

Filling the tank with water: Concrete shrinks as it dries. If the tank is filled rapidly with water, the concrete will not have enough time to expand again as it reabsorbs water. There is a great risk of severe cracking or even tank failure. An empty tank (or biogas digester), especially a new one, should always be filled over a several day period. It should be left for a week or so with only a shallow depth of water before complete filling is begun.

Materials needed for a ten cubic meter water tank (roof not included): 600 kg of cement, 200 meters of 2.5 mm diameter straight wire, 20 meters of 50 mm mesh and 1.0 mm wide chicken wire, water pipes, water tap, gate valves, overflow pipe, one cubic meter of sand, and 0.5 cubic meter of gravel.

The ferrocement information described above can be used to make water tanks, concrete biogas digesters, the water tanks for concrete biogas storage tanks, or just about anything that can be made out of concrete, including boats.

Remember that whatever is built should be built on the ground where it is to be used and that it will be hard to construct if it is too small to be worked on from the inside.

In this ferrocement design, gravel is not used except in foundations which are not really part of the ferrocement structure. The foundations are beds upon which the floor of the ferrocement tanks are built. Thin steel reinforcing rods can be added to the mesh and straight wire reinforcement, but they are not necessary in small tank (ten cubic meter capacity or less) construction. Ferrocement walls are thin, usually only 2.0 cm (3/4 inch) to 10.0 cm (4.0 inches) thick, but they are very strong.

If these directions are adapted to design a concrete biogas digester, two layers of mesh wire should be used at both ends of the digester. Digester walls can use the straight wire and mesh wire combination for digesters of ten cubic meters capacity or less. Digesters with capacities between ten and 100 cubic meters should probably use two layers of mesh wire. Building digesters with capacities larger than 100 cubic meters will require a more complicated level of ferrocement techniques than those described here. VITA should be able to help anyone interested in more information on different ferrocement techniques.

If it becomes too complicated to make totally round ferrocement biogas digesters, try making the digesters with flat floors and round walls and roofs. The sides of the floor should tilt down a few degrees towards the center line of the digesters. This tilt, in addition to the few degrees tilt down from inlet to outlet (necessary to the biogas digestion process), will make the digesters easier to clean than completely level floors would be.

Keeping the walls and roof curved will make for stronger digesters. The walls and floors should be connected with overlapping mesh wire. Concrete coving should be used on the insides of the tanks where floor meets wall.

A few ideas on making concrete digesters watertight and airtight:

1) Make a cement paste, replacing five to ten percent of the water with poly vinyl acetate latex paint. The combination can also be used as a paint to coat the inside or outside of a digester or gas tank.

2) Sodium silicate can be mixed with cement for patching gas, digester slurry, or water leaks. The sodium silicate and cement solution will harden in the mixing bucket within two to three minutes. To make a large area airtight, extra water can be used when making the mixture.

3) Paint the concrete with a chlorinated rubber-based coal tar or bitumen paint in order to make sure the concrete is completely airtight and watertight.

4) Wet new concrete three times a day for seven days. Cover the concrete with moist sacks and make sure the cover is kept wet. It is very important not to let the sun dry the concrete too quickly. Slow drying, called curing, gives concrete much of its durability, strength, and helps make it watertight and airtight.

There is a Chinese method of sealing concrete digesters against leaks that does not cost very much, does not kill biogas producing bacteria, and does not harm the fertilizer value of the sludge. It is the paraffin/wax method. The following description of the method is adapted from the book Chinese Biogas Digester by Charles H. Nakagawa of the U.S. Peace Corps and Q. L. Honquilada of The Philippine Rural Reconstruction Movement.

The tools needed to apply the paraffin/wax are: a blow torch, a large paint brush, an old pot or tin can to melt the wax in, a small bowl or can with a wide mouth to hold the melted wax while it is being applied, and a steel brush.

The sealing of the inside surface of a concrete digester should be done at least two weeks after the concrete work has been completed.

The steps involved in paraffin/wax application are as follows:

1) Inspect the inside of the digester carefully.

1.1) Check for cracks or weak sections that make a "hollow" sound when tapped. If there are problems, make repairs.

1.2) Check for dampness by spreading cement powder on the concrete to see if it "wets" the cement. If a wetting or leak rate is fast, concrete repair is necessary.

1.3) Check for water on the floor of the digester. If there is any, it should be dried up and the floor should be checked again on the next day. If the water returns, there is a leak that has to be repaired.

Use the dry cement powder method to determine where the leak is coming from.

2) If there are no damaged areas in the digester, clean the walls and floor of loose concrete and sand particles with a steel brush. Wax sticks to rough surfaces much better than it sticks to smooth surfaces.

3) Use colored wax if possible. Colored wax makes it easy to see which sections have been painted with wax.

4) Melt the wax and carefully add kerosene to the melted wax in a volume ratio of 3.0 parts kerosene to every 10.0 parts melted wax. This will lengthen the hardening time of the wax and make it easier to work with. Be careful when adding the kerosene to the melted wax over open fires--it may burst into flame!

5) Place the melted wax and kerosene mixture in a smaller, wide-mouth container and allow it to cool off a little. Very hot wax does not stick to concrete very well. The hot wax drips off the paint brush, wasting much of the wax and may also ruin the brush hairs. A good sign of a workable temperature for wax is when it starts to develop a thin skin or film on the surface.

6) Apply the melted wax with a paint brush.

6.1) Place the bowl of melted wax below the work area to collect any drippings or fill the bottom of the digester with water. Any dripping wax will float in the water and can be easily recovered.

6.2) Apply with up-strokes and by "splashing" the wax on the surface using the brush.

6.3) Paint from the bottom up, working towards the top.

6.4) When finishing a small area, use left-right-left and diagonal strokes. This will guarantee that most of the concrete is covered with a layer of wax.

6.5) Place extra wax around joints and corners.

7) When the inside of the digester has been completely covered with wax, smooth it and force it into the concrete with heat. Use a blow torch operating on a medium-high flame. Using a flow torch will change the wax finish into a smooth, continuous layer of wax. This wax layer should effectively seal the digester against gas and water leaks.

8) When using the blow torch, do not let the flame remain on one section too long. This will burn away all of the wax and the high heat may crack the concrete, Constant left-right-left and circular motions work best.

9) After blow torching, there will be many very small holes visible. These areas will need "touch-up" patching with another layer of wax and blow torch application. Inspect again and repeat if necessary.

After a concrete digester has been in operation producing gas for a few weeks, it would be a good idea to check the walls and roof for leaks. Leaks below the slurry level should be easy to spot. For biogas leaks above that level, pour very soapy water on the concrete. If gas is leaking, fine bubbles will appear in the soapy water. A coat of adhesive cement should be applied to any area that fails the soap test. After the adhesive cement has dried, the area should be tested again for leaks.

Using concrete culverts to make digesters or water tanks for gas storage tanks can cause more problems than they are worth. Concrete culverts often sweat water under normal conditions. They are made strong enough to hold water underground, not aboveground. They are not made airtight or watertight. They often crack if used aboveground. If concrete culverts are used, at least one layer of cement or asphalt sealant will be needed. The least expensive method for making concrete digesters and gas storage tanks is to use the ferrocement method.