|Rainwater Reservoirs above Ground Structures for Roof Catchment (GTZ, 1989, 102 p.)|
|5. Construction of ferro-cement reservoirs|
5.1.1 Preparation of ground
Choose the tank capacity according to Chapter 2.4. Determine the location of the proposed tank taking all facts into consideration to achieve maximum catchment capability.
Prepare the ground. Remove all garbage and scraps from the area where you intend to work, as well as loose stones, bricks, and smaller rocks. Check whether the chosen area is level. If you don't have a straight timber board to extend the spirit level, make yourself a simple level instrument as shown in Fig. 5.1. Take three timber boards, straight and if possible edge-shot. Fit the boards together in a rectangular triangle. This means the two short sides have to be equally long (example: a = 1.42m, b = 2.00m). Mark the centre, exactly half of the long side of the triangle. Fix a plumb-line in the centre of the right angle. If you turn this up with the long side on the ground, the plumb must be on your centre mark. If this is the case, the ground is level.
The height of the wall depends on the size of the reinforcement mesh. If this exceeds 1.80 m, the wall should not be higher than 1.95 m.
On the levelled ground mark the foundation slab circle by dividing the size given in Table 6 by two. To mark out the circle use a string and two sticks as shown in Fig. 5.2.
Table 6 : Dimensions for ferro-cement tanks
Within the circle remove all topsoil (vegetable soil). If the depth of this layer is more than 100 mm, you have to refill with hardcore. Note no foundation should be made on topsoil. Refilling is also required if only part of the marked-out area has a deeper layer of topsoil. The refill must be compacted by pile-driving. It also helps to fill water on top of the compacted refill overnight and piledrive the next day. The same is required if the soil is very sandy. See also Chapter 5.2.
Note: The structure of the tank can crack if it is built on unstable ground. Any structure is as strong as its foundation and the ground founded on.
Table 7 : Schedule of material consumption
If the most suitable place chosen is on sloping ground, you first have to level this area. Dig out an area which is between 1.00 and 1.50 m larger than the diameter of the foundation slab. Make sure the area is really on level ground. Before digging the tank foundation, save the sloping ground from sliding (see Fig. 5.3).
This can be done by stepping the slope behind the proposed tank. The raiser can be made out of wooden poles rammed into the ground and horizontally saved by either round poles or boards fixed to the rammed poles. The wood should be treated. The cheapest way is to dip it in old engine oil and dry it in the sun before use. The steps should not be filled up with soil but with solid material like small rocks, bricks and similar objects. This is the place where leftover material like cement mortar should be poured in later. The soil above the steps should be saved by planting bushes. Proceed with the foundation as described. If the ground is rocky, remove the soil between the different rocks as much as possible, together with loose rocks. Fill up the gaps between the rocks with lean-mixed concrete, cover the area with empty cement bags and water them for at least three days, keeping the bags constantly wet.
5.1.2 Preparation of reinforcement
On a flat area near the proposed tank, mark another circle with the radius of the foundation, then cut off pieces of reinforcement mesh as shown in Fig. 5.4, laying them out on the circle as shown. Trim off the comers lying outside the circle and save the pieces. Reinforcement mesh can easily be cut in a number of ways. The most efficient is to place a hard stone or large hammer or mattock beneath the reinforcement wire and then cut it using an old chisel and hammer or a panga. Cut another piece of reinforcement mesh from the roll and lay it over the remaining part of the circle. Make sure the overlap between the two pieces is at least one full square. Again trim and save the pieces outside the circle. Tie the two pieces together with short pieces of binding wire, Fig. 5.5. If there is still one final portion of the circle not yet covered with reinforcement mesh, this should be filled by tying the remaining corner pieces (cut earlier). Again make sure all overlaps are one full square as a minimum. The floor reinforcement is now complete.
For other technical solutions for reinforcing the foundation slab on unstable ground, see Chapter 5.2.
5.1.3 Preparation of the floor
Mix a part of cement with one part of sand and two parts of gravel (aggregate dia. 20). Check the material as described in Chapter 3 and follow the advice given there. Mix the entire amount dry, that means without water, until the colour is uniform and you can trace the cement on each shovel of material. Now add water carefully to make the mixture workable. This means the entire mixture should become moist. If your mixture flows off the shovel or even separates into material and water, your concrete is far too wet. This will result in the cement penetrating the ground together with the water. At the same time your mixture contains less cement than required. In general it can be said that concrete which is easy to work with is too wet. Working with concrete requires strength and good care. Clean a wheelbarrow of all the leftovers of formerly used concrete and transport the ready con" crete to the tank floor area. On the well" levelled 100-mm-deep area, fill in a layer of 50 mm. Level it as shown in Fig. 5.6 and compact it. Remember if the concrete does not need any compaction it is too wet.
When the whole excavated foundation pit (floor) is covered with a well-compacted layer of 50 mm concrete, place the prepared reinforcement onto it. Mix the same amount of concrete as for the first layer. When you prepare the second layer keep a ring of 400 mm from the edge of the circle open, without concrete. Compact the concrete as you did for the first layer, see Fig. 5.7.
Take your prefabricated draining pipe (tap unit), consisting of two elbows, one tee, three pieces of 3/4 inch pipe, and the tap. Stuff some paper into the open end of the pipe to keep cement out and fix this part at that side of the floor where you want to tap water. Tie the pipe to the reinforcement to make sure it is fixed in position (see Fig. 5.8).
5.1.4 Preparing the wall reinforcement
Find the length of reinforcement mesh in Table 7 under BRC mesh. Cut the right amount of material from your roll and form into a cylinder with an overlap of two squares' minimum 300 mm. Tie the overlap together with binding wire as done with floor reinforcement (see Fig. 5.9).
At the bottom of the cylinder made out of reinforcement mesh, bend the vertical wire segments horizontally. Bend them alternately inwards and outwards, forming right angles. Move the cylinder onto the floor of the proposed tank and tie it to the floor reinforcement. Make sure the vertical mesh forms a cylinder in good circular shape. This is achieved if the vertical reinforcement is at the same distance to the external floor circle (Fig. 5.9).
Now use the last remaining concrete, filling in the space underneath the wall inside and outside the wall reinforcement. Stamp the concrete down carefully and firmly. After this work is finished, cover the floor and the edge outside with plastic, see Fig. 5.10, and keep it covered overnight. Do not work on the wall reinforcement. Next morning remove the cover and splash the concrete all over with water. This- must be repeated every three hours throughout the day and throughout the entire job, at least four times a day. Remember the strength of the concrete depends on good watering during the construction period. Evaporation of the moisture contained in the concrete should be avoided since it is needed for the setting process. The water you splash on the concrete will evaporate and at the same time keep the concrete moist.
5.1.5 Preparation of the tank roof reinforcement
Mark a circle in your preparation area with the radius of the tank reinforcement. Put an upright cement block in the centre about 450 mm high. There are different ways of cutting the reinforcement mesh. The roof has to be a dome shape, so the cement block marks the highest point. Make sure an overlap of at least two squares, about 300 mm, is given and tie the reinforcement well together. Whether this has been achieved can be proved by lifting up the dome. It should be well stabilized in one piece. Fig. 5.11 shows one way of cutting the reinforcement mesh to save material. After all triangles extending the circle are cut off, cover the dome of rein" forcement mesh with chicken wire and tie it well to the mesh. Overlapping of the chicken wire is important. 200 mm is required, Fig. 5.12.
5.1.6 Further procedure on the tank wall
First prepare some spreaders out of wood and fix them in the upper third part between the reinforcement mesh. This will prevent vibration and bending during the further work on the wall. The next step is to wrap the outside of the wall reinforcement mesh with one layer of chicken wire, from the uppermost horizontal wire to the concrete floor. Take the roll of chicken wire and push the top long edge over the free vertical mesh. While continuing to encircle the cylinder, keep pulling the chicken wire tight in both vertical and horizontal directions, Fig. 5.13. Try to do this job with three people, one carrying the roll and pulling the wire, one fixing it with prepared short pieces of binding wire on the mesh, and the third assisting him in pulling the chicken wire tightly into position. Overlap the two ends of the chicken wire by 200 mm and fasten it. The chicken wire has to cover the entire wall reinforcement. Depending on the size of the roll of wire and the height of the tank wall reinforcement, it might require three or more layers. It is imperative that each layer overlaps the other by at least 200 mm.
After the chicken wire wrapping is completed and all sags and bulges are tightened up, take a roll of 16-gauge binding wire and wrap it four times around the top horizontal. This helps to support the roof. Then proceed to spiral the binding wire down every 100 mm for the next 600 mm of wall height, pulling it very tight. After you have completed wrapping the next 600 mm downwards, reduce the intervals between wraps to 80 mm for the 600 mm or more, but make sure the bottom 700 mm have an interval of only 50 mm. At the very bottom do the same as at the top, and wrap the wire round four times. It is most important that the wrapping of reinforcement with binding wire be done by pulling it with force and fixing the ends very tightly. If your roll of wire ends before the wall reinforcement is wrapped, tie the end around a vertical reinforcement wire of the mesh, using pliers, Fig. 5.14.
Now the reinforcement of the wall is ready. It must be lined with cloth from the outside to hold the plaster which will be smeared on the reinforcement from the inside. The material you choose for this will be determined by availability. Clean sacks can be cut in straight pieces and sewn together. If you can get sugar sacks made of nylon, this will be best but jute sacks or sisal material are also suitable. The nylon sugar sacks have proved to be very good. Fix the cloth temporarily at the reinforcement, overlapping by about 50 mm. Then take a roll of sisal string and wrap the cloth from top to bottom at intervals of 50 mm in the same way you tied the gauge wire, but using a uniform interval all over, Fig. 5.15.
After the tank is entirely wrapped, make sure there is no gap where the reinforcement is visible. Prepare a scaffold to bridge the wall without touching it. Remember all the plaster has to be transported via this scaffold and during all this procedure the wall will be protected from vibrations. Inside there should be enough space between the scaffold and the wall to allow one man to plaster it. In case material for stable scaffolding is not available, fix two ladders together at the top as shown in Fig. 5.16. Outside they should be spread to prevent transmitting vibrations to the wall. Remove the stabilization spreading from inside. Now it may be necessary to adjust the wall to make it stand straight and in a good circle. The foreman climbs the ladder or scaffold and inspects the shape. Where it is not in good cylindrical form, adjustments are made by fastening lengths of binding wire to the reinforcement mesh. Pull the wire with just the right strength to remove any sags or bulges (see Fig. 5.16). The next step is the preparation and mixing of the plaster, see Chapter 3.
5.1.7 Plastering the tank from the inside
Before you start plastering, remember that the entire first coat of the tank must be plastered in one day. It is not possible to keep the unfinished plaster overnight. Where the one-day-old plaster is joined with the fresh plaster, leaking will occur. Make sure you have enough labour for mixing the plaster and carrying it to the plasterers. If necessary have two plasterers and start on two opposite sides of the tank wall. Push or smear the plaster onto the wires from the inside of the tank. Starting at the bottom, no plaster should leak through the outside cover cloth. Plaster in this way until you reach the top horizontal reinforcement wire. However, leave two squares empty at the top underneath the future roof for construction of the overflow, Fig. 5.17. After the plastering is finished, hang plastic sheets from the top to the bottom to cover the inside plaster. The plastic should overlap and be protected against blowing away by the wind. This can be achieved by fixing sisal strings at the bent formerly vertical wires of the reinforcement mesh, and weighting them with bricks or stones at the end above the floor. If the outside cover material is nylon, no further action need be taken. In the case of sisal, just splash water on it until you leave the site in the evening. This material must be kept wet to prevent the plaster from drying out. Don't forget to keep splashing water on the floor; the floor concrete should never become dry.
Only remove the plastic cover the day after you have plastered the inside of the tank. Start the second coat by throwing plaster evenly. The plaster can be a slightly wetter mixture than you used the day before, but it should not be shiny. Remember that the wall thickness when finished is only 50 mm, 30 mm of which are on the inside. Fill up the sags and level them with the bulges. Finish with a wooden float. Work with two plasterers and finish the job in one operation, see Fig. 5.18. After you have finished, cover the inside again with plastic like you did the day before.
Now remove the cloth from the outside and start plastering as for the inside, but not more than 10 mm thick. Fill the sags and level them with the bulges, Fig. 5.19. Do not float. Allow the plaster to set (about 1-2 hours). After this job is finished, which can be done simultaneously with the inside plaster levelling, Fig. 5.18, cover the wall outside as well as inside again with plastic and keep covered for at least one night. Make sure the plastic cannot be blown away. Secure it with sisal string and stones as indicated above.
The following day remove the plastic from the outside first. Throw on a light coat of plaster about 10 mm thick. Then smooth it with a wooden float until the entire outside is a smooth plumb wall. During outside plastering, prepare the overflow. Choose the place carefully. Under no circumstances should the overflow be at the same side as the tap. It is advisable to locate the overflow at a side where a slope (even a slight one) directs the flow-off away from the tank Make sure the water does not flow towards the house. With a hacksaw, cut the horizontal wires of the reinforcement mesh. The size of the opening should be slightly larger than the downpipe filling the tank. Bend down the piece of wall and wrap with several pieces of chicken wire. Place a flat board under the overflow and use one or two posts to support it. Plaster the top and smooth it. With the plaster form a gutter-shape, round or squared, whichever is easier. Do not cut through the top wires. This reinforcement is needed to keep the circle together, Fig. 5.20.
Remove the plastic foil from the inside wall and prepare the nil. If possible, the nil coat should be applied on the wooden - float-finished plaster from inside on the same day you plaster. If this is not possible, the wall must be splashed with water before applying the nil. Mix pure cement with water until you have a thick soupy consistency. This is achieved by nearly equal parts of cement and water. Stir until smooth and free of lumps. This is done most easily by adding cement to the water, in small portions, while stirring constantly, not the other way around. The mixture is called nil. Use a steel rectangular trowel and smooth it very evenly onto the new plaster. If the nil is still too thin, add more cement. Leave a 100 mm strip around the bottom of the wall free of nil. These 100 mm should be marked first to make sure you always meet the right height, Fig. 5.21. After the nil is finalized, cover the wall again with plastic, clean the concrete floor of all leftover mortar, and splash with water to keep it wet. Next day, prepare the same 1:3 mixture as specified for plastering the floor. There should be no dust or loose material left on the concrete floor. The plaster is to slope towards the outflow. You should start on the opposite side with a thickness of about 40 mm and slope down towards the outflow, reducing the thickness to not less than 15 mm, Fig. 5.22. After you have finished the float, finish the plaster with a wooden float. Give the plaster a few hours before you start the next job. For this you need a clean, round glass bottle of about 100 mm in diameter. Cut a few straight timber boards for use as walkways on the still fresh floor plaster. Use a ladder to get onto the floor and the timber boards to step on. Start at that part of the floor which you plastered first. Prepare a very well mixed cement mortar of 1:3 with a minimum of water. This plaster is to be thrown into the corner between the wall and the floor up to the mark of the nil coat at the wall. Do this a few metres at a time. Then use the glass bottle to smooth it and at the same time to shape a round concave arch, Fig. 5.23.
After you have shaped the corner remove the rest of the plaster carefully from the floor. Working in steps of about 2 m, move round the tank until you reach the starting point. From here you start again, this time with the nil coat. If the plaster is still too wet, wait for a while. This is an important operation and has to be done very carefully, since this part of the tank sometimes leaks. After the corner all around the tank is neatly shaped and covered with nil coat, pour water onto the floor slab. The nil coat for the floor is not to be done before the roof is finished.
5.1.8 Preparing the roof reinforcement
Turn the prepared reinforcement of the roof over and start covering it from the inside with cloth. You must sew the cloth (sisal, jute or nylon, as for the wall) to the reinforcement. It is imperative that the cloth remains in the same shape as the dome when you turn the roof back over into the normal position, Fig. 5.24. Move the roof onto the tank and put it in position, then tie it in this position using the former vertical ends of the reinforcement mesh, as well as with binding wire using pliers. After the roof reinforcement is secured onto the wall, cut out a 450/450-mm manhole using a hacksaw. Trim off all excess wires from the roof reinforcement. Examine from inside the tank. Where cloth forms a sag, sew it back to the reinforcement. Support and prop up the roof from inside with poles, starting in the centre. Now mark the place (or places) where the downpipe from the roof will be fitted into the tank. This remains an opening. Start plastering and use the same mixture as you used for the wall, keeping it slightly wet. If the mixture falls through the wire it is too wet. After the roof is plastered with the first coat, cover the entire tank with plastic.
Next day place the second coat on the roof, smooth it with a wooden float. Prepare the manhole cover using the same size and shape as the opening. Cut a piece of reinforcement mesh slightly smaller than the manhole cover. Shape a mould in the ground or shutter with wood to form handles as shown in Fig. 5.26. Make sure the manhole cover does not become more than 30 mm thick. You can put a piece of chicken wire on top of the reinforcement mesh and fix the end of the handles on the bottom side of the reinforcement. Put the reinforcement into the mould and pour 1:3 concrete in it, lifting the reinforcement up slightly.
After two days you can remove the support poles from inside the tank and start plastering the dome from the inside. This plaster is mainly to cover the sags and the reinforcement and is not supposed to be entirely smooth. But it is important that all reinforcement still visible is covered by at least 10 mm plaster. Remember water in the tank will evaporate and condense inside the roof. The condensation creates corrosion of the reinforcement if it is not properly covered with cement mortar. While plastering cut a piece of galvanized gauze wire overlapping all sides of the overflow opening by 30 mm. Fix it in position with binding wire from inside, then plaster on top of the overlap, keeping the overflow opening uncovered, Fig. 5.27. If you need a scaffold inside the tank to reach the dome for plastering, make sure it does not damage the floor plaster. Always put straight timber boarding underneath the scaffold. After the roof plaster is finished, remove everything from inside the tank and clean the floor, using a broom -if available a wire broom. It is very important to clean the floor as well as possible. If there are marks or holes in the floor plaster, patch them with a 1:3 mixture. Next prepare a mixture like nil, but add one part fine sand (two parts cement, one part fine sand) and water. The consistency should be slightly stiffer than the nil coat you used for the wall. Plaster this mixture on the floor inside the tank using a rectangular steel trowel. After you have finished the last coat, set the floor under water. However, use the water carefully Do not pour a bucket of water on the floor from the manhole. The water will spoil the smooth surface. If you have a hose, put the end on the bottom of the floor and open the tap. If a hose is not available, take a timber board covered at one end with some cloth, and place this end carefully on the floor through the manhole. The upper end must lean on the edge of the manhole. Then take a bucket of water and pour it along the board slowly, so that it runs down to the bottom. Repeat this until the floor is covered with water. After the manhole cover has been inserted, the reservoir is ready. Downpipes should be connected but, before the crew leaves the site, water must be filled into the reservoir up to a level of at least 100 mm.
5.2.1 General advice
In Chapter 5.1, detailed advice was given on the entire procedure of building a ferro-cement structure. This and the following chapter will only mention the differences in the technique employed. This means there will be no advice on curing the plaster and concrete, including the repeated covering with plastic sheets and splashing with water. The general plastering technique will be named but not explained in detail. Mixing the plaster, testing, sifting the sand etc. are explained in Chapter 3, and remain the same for all types of ferro-cement tanks and waterproof plastering of reinforced bricktanks. The advantages or disadvantages of the various tank structures should first be established by consulting Chapter 2.
5.2.2 The mould
The mould is an expensive capital investment which, if carefully handled, is good for ten to fifteen structures. Since it involves the bending of corrugated galvanized sheets as well as welding, it should be manufactured by a well-equipped.workshop experienced in this type of work.
The mould is constructed in six sections. This is necessary to reduce the weight and make it easy to handle, especially when it has to be removed from inside the tank. The sections are fabricated from rolled sheets of corrugated iron, joined together by mild steel angles (30 × 30 × 3 mm). These steel angles are fixed by spot welding to the corrugated sheets. Three of the sections have overlaps, being 30 mm wider on each side than one sixth of the circle, Fig. 5.29. The other three sections are exactly one sixth of the circumference. The example given in Fig. 5.29 shows a diameter of 4.00 m with a height of 1.85 m. This makes a tank of 20 m³ capacity. The sections are bolted together with timber spacers. These spacers are to be made after all the welding is done and the mould is to be assembled for testing. The timber spacers are made to cover up unequal parts of the mould and should therefore be made specifically for each joint. They also serve to ease the dismantling of the mould after the wall has been made. The overlapping of the mould sections are to prevent mortar passing through during plastering. Test assembly should be performed on an even and horizontal base, preferably a concrete floor. After all holes for bolts are drilled, the mould should be bolted together. The single parts of the assembled mould including the timber spacers must be marked clearly, so that after dismantling, reassembly is always done in the same way. It is best to number the parts and paint the numbers on the inside with oil paint. Do not forget to number the spacers as well. The spacers should be made of hardwood and oiled afterwards. If carefully handled they can last for some time, but are easily replaceable, Figs 5.28, 5.29.
5.2.3 Foundation slab
Different soil conditions require different foundations. The foundation slab can be constructed as described in Chapter 5.1. If welded reinforcement mesh is not available, it can be substituted by one layer of fencing mesh combined with one layer of chicken wire. If reinforcement rods are available, any size above 4 mm diameter can be used to produce a mesh as shown in Fig. 5.30 by tying crossed rods with binding wire. The squares should not exceed 150 mm. If only reinforcement bars of 10 mm or more diameter are available, the squares can be enlarged up to 200 mm, but in this case one layer of chicken wire should be fixed to the self-made mesh.
If you intend to cover the reservoir with a concrete slab, there should be a pier in the centre of the tank to support it. This requires double reinforcement in the centre of the floor slab. The concrete slab can only be constructed if sufficient reinforcement is available. If this is not the case, the roof can be constructed as a ferro-cement dome, as described in Chapter 5.1. If you intend to choose this type of roof there is no need to double-reinforce the centre of the floor slab, since a pier to support the roof is not needed.
5.2.4 Foundation on unstable ground
In general it must be said that any foundation on unstable ground remains a ask. Sometimes it is advisable to choose another place where ground conditions are more favourable. But, as explained already for reservoirs, there are not many suitable places if the best catchment position is to be achieved. In the case of unstable soil the best and usually cheapest method is to exchange the soil. After marking the area of the tank foundation, extend the radius for another 500 mm and excavate all soil which is not stable, even if you only reach the stable soil at a depth of 1.00 m. If, for instance, even at this depth stable soil occurs on one side but the other side is still unstable, continue to excavate until your entire ground appears to be stable enough for the structure. Now find backfilling material. This can be old bricks and gravel. Natural hardcore is preferred. Fill this material in layers of not more than 300 mm and compact mechanically by pile-driving. Do not backfill more than half a metre per day and fill the compaction with water overnight. Next morning first pile-drive the work from the day before. After the backfill is finished, keep compacting for one week, filling up with water once a day and recompacting the following day. Then construct a concrete slab foundation as described in Chapter 5.1.
Another method to be applied whenever there is uncertainty about the soil conditions is the ring foundation. Mark the external diameter of the slab and then mark another circle reducing the radius by 500 mm. Between the external and internal circle dig out a trench of 400 mm depth, Fig. 5.31. Do the same in the centre in a square or circular shape of 600 mm (in case you need a pier). The bottom of the trench must be levelled and recompacted. The same applies to the area inside the circle. The foundation and slab area is now ready for reinforcement. See Chapter 6 for reinforcement of foundation. Follow the advice given there for the construction of the slab too. Make sure it will be level and smooth, otherwise there will be problems in assembling the mould. Before concreting the slab, do not forget to place the water tap unit as shown in Fig. 5.8.
5.2.5 Assembling the mould and placing the reinforcement
The mould is in six sections, each marked to ensure that they are assembled in the correct order. Tapered wooden spacers are used between the frames to make it easier to remove the sections (Fig. 5.28).
Assemble the mould loosely near the base and check that all the pieces are present and fit properly. This should be done before the mould is required.
Find the centre of the slab and mark a circle on the slab the same size as the mould to ensure the mould is centred on the base.
Take the first section A-B and place it on the line. Take the second section B-C, and place it on the line with the overlap outside the first section and the letters matching.
Place the wooden spacers between the metal flanges and bolt them together loosely.
Do not tighten until all sections are assembled.
Bolt the remaining sections together in the correct order (B-C, C-D, etc.) until the mould is loosely assembled.
Using the circle on the slab as a guide, move the sections until the mould is circular and in the centre of the slab.
Tighten all bolts.
Place paper at the bottom of the mould to prevent oil from spilling on the concrete. Using shutter oil (or old engine oil) paint the outside of the mould. If oil does get on the concrete it should be cleaned off. Make sure the mould is oily, but that no oil runs off.
Ensure the mould is in the right position. Check this by measuring the distance from the mould to the edge of the concrete slab. Do not start reinforcing until the mould is assembled and stable.
5.2.6 Placing wire reinforcement
Wrap the chicken mesh around the bottom of the mould once with approximately SO mm tucked under the mould to the inside, Fig. 5.32.
When you have reached your starting point, pull the chicken mesh, cut it and tighten it, overlapping by 100 mm, then start again above the first circle, overlapping the already fixed mesh by about 100 mm. If the height of the tank requires a third circle of wire mesh, do the same again, overlapping the second circle by 100 mm.
Wrap the chicken mesh around the top of the mould. The top layer should overlap the mould by approximately 100 mm and the remainder is folded over the top of the mould.
Wrap 8-gauge fencing wire around the outside of the chicken mesh, starting at the bottom. The corrugations are used to keep the spacing even. The wire should be wrapped with two wires in each corrugation for the first eight corrugations from the bottom, and then once per corrugation up to the top three, which again should have two strands each.
Use the thin tie wire to fasten the chicken wire to the fencing wire to prevent it from slipping, Fig. 5.33.
5.2.7 External plastering of tank was
Also see Chapter 5.1 and general advice for material and mixing in Chapter 3.
Mix the first batch of mortar (1 :3 cement: sand, as stiff as possible). Make the first batch small, about one wheelbarrowful. Although the method of application is similar to plastering a wall, progress will be slow at the beginning. Make sure the mortar is stiff and not too wet. If it is too wet it will slip and leave cracks in the plaster which should not happen at all.
Start plastering at the bottom and work up. Try to work evenly around the tank. But remember there will be another external coat so if the plaster is not very even and smooth this can be put right with the second coat.
Apply the first coat thinly. Make sure all the corrugated iron is covered but some of the wire can still be left showing. You will experience that the more plaster you try to apply for the first coat, the more difficult it is to prevent the plaster from slipping. The right technique can only be learned by experience.
Leave a hole at the top for the overflow, about 100 mm below the top of the mould. Ensure the mortar remains damp by covering the finished parts with plastic. This is especially important in bright sunlight and on hot summer days.
The second coat can be applied as soon as the first coat is stiff enough, and so it is possible to do both on the same day. However if there is any doubt it is better to leave it until the following day.
Remember that the mortar will require wetting and covering when finished.
Mix the mortar in the same way as the first coat (1 :3 cement: sand; -stiff).
Plaster up from the bottom uniformly.
Plaster as thinly as possible but be sure all the wires are covered. Smooth the surface using a wooden float. This is the outside
finish of the tank, so try to make a smooth surface.
Keep plaster damp while you work -do not let it dry out. When the second layer is completed, dampen with water and cover with plastic immediately: Leave to cure for at least one full day.
5.2.8 Removal of mould
Fig. 5.34 shows the ferro-cement wall with the mould still fixed. After completing the second layer of plaster, the tank should be left at least one full day before removing the mould.
Remove all bolts.
Remove all the timber spacers.
Starting at joint A-A, pull the edge of the inner panel A-B.
If the panel sticks, lever off gently using the flange of the adjacent panel. Do not knock the shutters as the plaster will still be soft and can be easily damaged.
Remove panel A-B completely.
Repeat with panels C-D and E-F.
Remove panels B-C, D-E and F-A.
Inspect plaster inside and outside for damage.
Wet inside and outside of plaster.
5.2.9 Internal plastering of the tank wall
Before you start this work remove all leftover oil visible on the plaster. This must be done very carefully, washing away with lots of water.
It is possible to plaster inside the wall of the tank as soon as the mould is removed but, as before, remember that the plaster must be applied in one continuous layer. If you are not sure there is enough time for this, start the next morning. But do not forget to cover the wall. Access to the inside should be via a scaffold or ladders as shown in Fig. 5.16. Under no circumstances should the wall be exposed to any load. At this stage even vertical load should be avoided.
Clean the overflow hole of mortar and put the pipe in position. The overflow should start about 100 mm below the top, since the reinforcement at the top should not be cut.
Bend up the wire that was left inside at the bottom of the mould. Plaster only up to 100 or 150 mm above the floor slab.
Plaster inside the tank in the same way as the outside, using a 1:3 mortar mix, as stiff as possible, and applying the plaster as thinly as possible. The plaster should just be thick enough to fill the corrugations. Cover the wire completely and provide a smooth finish using a wooden float, Fig. 5.35.
The chicken wire pulled underneath the mould should be tied in the corner. Prepare a mortar fillet in the corner between wall and floor as shown in Fig. 5.36 or apply the technique as shown in Fig. 5.23.
Using additional mortar, ensure the top of the wall is smooth and level. Trim the high points if necessary, but do not cut off the chicken wire bent inwards.
Make sure that all the plaster is damp and cover immediately with the plastic sheets as soon as it is finished.
Prepare the nil coat and apply it as described in Chapter 5.1.7.
5.2.10 Screeding the tank floor
This layer acts as a final finish and seal to the tank
Roughen the bottom of the wall to ensure a good bond between the screed and the wall, cleaning it of all dust.
Wet the mortar filet and the base slab.
Mix mortar using a dry mix of 1:3 cement: sand.
Apply a thin screed, approximately 15 mm thick over the floor slab, starting from the fillet and working inwards. Finish the surface of screed by 'shining', i.e. dusting the surface of the screed with cement and using a steel float to produce a very smooth surface.
Dampen walls and floor and cover the tank completely with the plastic sheet.
Alternatively apply the technique described in Chapter 5.1.
5.2.11 Roof support pier
Leave floor screed to set for at least two days, covered in water, before starting the next stage.
Mark the centre of the tank.
Build a brick pier 230 mm wide in the centre up to the level of the top of the tank wall.
Plaster the pier with 1:3 mortar (cement: sand) to produce a smooth finish.
Dampen all walls and the floor, and cover the tank with PVC sheeting.
5.2.12 Roof slab shutter
Leave the pier and wall for at least a full day before starting work. Making the roof shuttering may damage the walls if it is done too soon after plastering.
Check the inside diameter at the top of the tank.
Mark a circle on the ground with the diameter of the inside of the tank
Cut the sheets of shutter ply in half, lengthwise, to make strips 600 mm wide.
Place the strips of ply over the circle to cover it as efficiently as possible and mark their positions.
Construct the supporting structure of poles and rafters inside the tank, ensuring that the top surface of the ply will be level with the top of the wall. Nail the rafters to the vertical props.
Starting at the side opposite the manhole, lay the sheets of shutter ply on the rafters and cut to shape. Nail the ply to the rafters.
Make an opening in the shutter ply at the manhole, as all the inside timber has to be removed through it later. The opening should be 650 × 650 mm.
Fill all gaps in the shutter or between shutter and wall with paper or plastic.
Lay a damp proof course on top of the tank wall.
Cut hardboard into strips 300 mm wide, or use hardboard from the slab.
Tie the hardboard around the top of the tank to form an edge shutter so that this edge is 100 mm above the wall.
Form an edge shutter for the manhole. This should be 75 mm deep and slightly larger than the hole in the shutter (650 × 650 mm) so that it can rest on the shutter.
Paint the shutter with shutter oil or old engine oil.
5.2.13 Roof reinforcement
On the preparation area cut a piece of reinforcement mesh to the size of the circle. If this is not available use 6-mm reinforcement mild steel and tie together in the same way as the floor reinforcement.
Place the reinforcement in the shutter and either support it with 20-mm plywood blocks or distribute about four wheelbarrows of concrete in the shutter and place the reinforcement on top of this concrete. Ensure that the reinforcement is 20 mm above shutter level.
Double the reinforcement in the centre above the pier which will support the roof.
5.2.14 Concreting roof slab
The concrete used must be of good quality and care should be taken in selecting the sand and stone. See Chapter 3.
Start at the side furthest from the manhole and work evenly towards the manhole opening.
Use a straight piece of timber, the edge shuttering and manhole shutter as guides to form a smooth surface as the concrete is laid (Fig 5.37)
5.2.15 Removing shuttering
Do not remove the shutter until seven days after the roof slab was cast.
Remove the shutter as gently as possible, starting at the middle.
Remove the shutter from next to the walls last and do not lever or strike the tank walls. Clean all shuttering material and remove all nails.
Save the shuttering for reuse.
After you have removed all shutter material, start to clean the floor carefully. There should be no dust left. If there is any damage in the floor, repair with mortar 1: 1 sand and cement using only sifted sand. Wet the floor for one day or night and apply the nil coat. Since the nil of the wall only goes down to the mortar fillet, this must also be covered with nil. After this job is done, the main structure is finalized. Prepare the manhole cover according to Fig. 5.26.
Remember the tank now has to be filled with water at least to a level of 100 mm to keep it wet. If it is possible the entire structure of the reservoir should be sprayed with water twice a day for at least one week. If this cannot be assured, it should be covered entirely by plastic sheeting to avoid drying off.
5.2.16 Alternative roof structure
If there is not enough reinforcement mesh or rods to be used for making a mesh on the site, it is possible to prepare a roof of ferro-cement by applying the technique described in Chapter 5.1. This type of dome roof does not need a pier to support it. Special care has to be taken when the dome reinforcement is placed on the wall. The bottom circle line must have a ring wire tightly fixed to the reinforcement.
After the reinforcement dome has been placed in position on top of the wall, the ring reinforcement of the wall must be tied to the ring reinforcement of the dome using binding wire. The chicken wire projecting beyond the top of the wall should be neatly fixed to the dome reinforcement.
The ferro-cement technique here remains the same as described in Chapters 5.1 and 5.2. The difference in this technique lies in the method of shuttering applied. In principle it is a made-on-site mould. By means of shuttering, therefore, it can be used in cases which are unsuitable for other techniques, see Chapter 2.2. In addition this technique uses fencing mesh which is also adequate for the technique explained in Chapter 5.2.
Choose the tank size according to the advice given in Chapter 2.4.
Choose the most suitable location of the tank according to the general advice given in Chapter 2.4.
Prepare the ground and dig the foundation. The different techniques required for the different soil conditions are explained in Chapters 5.1 and 5.2.
Don't forget to place the water tap unit, see Chapter 5.1 and Fig. 5.8.
5.3.1 Preparation of the mould or erecting the shutter
Keep the concrete covered with plastic for two days until the slab has set. This means keeping the slab wet. Mark the centre point of the tank and a circle corresponding to the inner diameter of the tank, see schedule Chapter 5.1.
This circle is needed for positioning the framework. Prepare the supporter for the shutter as shown in Fig. 5.38. Using a spirit level make sure the upright rafters have strong support and are fixed in a vertical position. The base timber cross on the slab should be fixed in the centre to avoid dislocation. This can be done either with steel nails hammered into predrilled holes, or by putting pieces of 10-mm or 20-mm reinforcement steel into the still fresh concrete after the slab is poured or during this process. It is imperative that the timber structure to support the shutter be immovable. After you have prepared the four major supporters at right angles to each other, the four vertical rafters must form a square on the slab. All sides should be the same length. Now form an octagon by preparing intermediate supporters. Ensure that the octagon is equilateral, meaning all sides on the ground have the same length. Start fixing the corrugated roof sheets. These sheets must overlap each other by at least one corrugation. They should be fixed using short nails at the top and the bottom. Fix the sheet about 20 mm above the concrete slab. Keep the last sheet unnailed and use this opening to go inside and check the supporting structure. If necessary fix additional spreading stabilization, see Fig. 5.39.
5.3.2 Fixing reinforcement
After the shutter is closed and the last roof sheet closely fixed, tie soft wire tightly around the galvanized sheets at least in the upper third and bottom third parts. Now the mould must be well stabilized and really fixed in position. If this is not the case, improve the support structure. Do not start wrapping the mould with mesh wire before stability is achieved. Once this is done pull out all nails or as many as you can still pull. Then roll wire mesh around the mould as shown in Fig. 5.40. The wire mesh must extend under the sheet at least 150 mm to the inside of the tank, as shown in Fig. 5.32. It also must extend about 200 mm over the top of the sheets. Overlapping must be 200 mm. It will be difficult to push the mesh wire underneath the mould, so the 20-mm space must be kept between the slab and the sheets. Where the mesh is hindered by the vertical supporting structure inside the mould, cut the wire vertically in such a way that both of the cut sides can pass the supporting timber. If the fencing mesh is not tight to the mould, which is likely, use soft wire again to tie it round tightly. The next step is to increase the reinforcement by a layer of chicken wire, overlapping by 200 mm, Fig. 5.41. The chicken wire should be tied loosely to the mesh wire.
Prepare 10-mm mild steel reinforcement bars bending them at both ends after cutting them to the right length, as shown in Fig. 5.41. The bent ends are to be added to the height of the mould. These bars are loosely fixed in position by using soft wire and binding them to the fencing mesh. The vertical bars should be equidistant from each other, between 1.00 and 1.5 m. After this is achieved, start wrapping 16-gauge fencing wire around the outside, starting at the bottom. This is the same procedure as shown in Fig. 5.14 and spacing should be accordingly. Wrap the wire three times around the bottom, then coil the wire upwards with a 10-mm spacing for the first 600 mm above ground. From there you can increase the distance to 20 mm until you reach the upper 300 mm, when you should again narrow the distance to 10 mm and apply it again three times at the top.
5.3.3 Plastering outside
After the reinforcement is finished, prepare for plastering. For this work, see Chapters 3 and 5.1. The first plastering coat is done one day, then the structure is covered with plastic foil. On the second day, the second coat is be plastered, Fig. 5.42. The second coat is float-finished using a wooden float. After the second coat is finished, the structure must be covered again with plastic sheeting to keep the plaster damp. Only after a period of about 12 hours (overnight) can the plastic be removed and the mould be dismantled. For this purpose a scaffold or two ladders joined together as shown in Fig. 5.16 must provide access to the inside without damaging the wall. Dismantling of the shutter must be carried out with great care, avoiding unnecessary hammering and anything which can affect the structure.
5.3.4. Inside plastering
For inside plastering again see the advice given in Chapters 3 and 5.1. The wire mesh which was pushed underneath the mould provides reinforcement for the corner between the wall and the floor, about 100150 mm up the wail and inside the floor. Use a glass bottle as described in Chapter 5.1 and as shown in Fig. 5.24. After this is done or before you execute the curved corner, apply the nil coat to the wall.
5.3.5 Floor finish
Floor finish is to be performed as explained in Chapter 5.1. The construction can be done before shuttering the roof or after the roof is finished and the shutter removed. The advantage of doing the job before shuttering is the easy access for the material that has to be brought into the tank. The disadvantage is that the floor finish cannot be protected against damage during the work process. To apply the floor finish after the roof shuttering is dismantled means transporting all mortar through the manhole. If this is considered a reasonable procedure, which depends on the situation and the resources on site, then do it. It is the better solution. Otherwise, careful repair of the floor finish must be performed and the nil coat on the floor should always be applied after the shuttering is removed, see Fig. 5.44.
5.3.6 Roof shuttering
Figs. 5.45 and 5.46 show different stages of the roof shutter based on a central pole. The platform for the pole should be 250 mm higher than the tank wall. The central pole is fixed in position by a cross of timber boards supported at the wall. It might be necessary to fix a second cross as for moulding the shutter, in such a way that the ends of the timber boards (or rather rafters) form an octagon where they are supported at the wall. If this is achieved, segments of chipboard or blockboard should be cut as shown in Fig. 5.46. These trapezoidal pieces of timber are to be supported against the supporting understructure. A manhole of 450 × 450 mm must be made by nailing a frame of this size onto the shutter boards at the desired place. Reinforcement of the roof can be performed with the rest of the reinforcement rods and fencing wire. This should be done in such a way that the reinforcement rods fixed with binding wire to the vertical steel bars of the wall will meet in the centre of the roof, where they are to be tied together. This reinforcement should be covered and tied to the fencing mesh. The extended mesh from the wall must be tied to the roof mesh.
5.3.7 The roof concrete
After roof shuttering has been finished, the entire opening of the tank covered and gaps filled with paper to ensure that concrete cannot pass through, the roof can be concreted. The concrete should under no circumstances be wet and the mixture remains one part cement, two parts sand, one part ballast.
The roof structure described in Chapter 5.2 can also be used for this type of tank. But remember it requires a centre pier and the double reinforcement of the centre of the tank floor. This means the decision should be made before the construction work starts. It is also possible to use the roof structure of the tank described under 5.1 but this requires welded reinforcement mesh or reinforcement rods as described in this chapter.
Repairing a ferro-cement tank is easy but should not encourage slipshod work. If the structure is finished and cured as described, then no leaks are likely to occur. Small leaks which create only a wet stain need not be attended to, since they will close after some time. Only leaks where water flows out have to be repaired.
The major problem is not the repair work as such, but the fact that leaks usually cannot be identified until the reservoir is filled. As indicated earlier, the value of the water which is then wasted should be considered. As mentioned, curing after the structure is finished as well as while it is still under construction is just as important as the quality of craftsmanship and material. But, to make it entirely clear, no reservoir -and this applies to the bricktank as well -should ever become entirely dry. Only cement structures which never dry off remain waterproof in the long term. This means that, after the curing period ends, a reservoir has to be flied with water. If as is the case in Botswana there are long periods without rain, or even a drought, a reservoir cannot be kept without water until rain eventually occurs. Thus it is advisable to fill the cistern with borehole water. This will not only preserve the structure, making curing unnecessary, but also furnish immediate evidence of any leaks. If there is no rainfall some days after the structure is finished, and the necessary amount of borehole water is not available, a minimum filling of 100 mm is a must. Since reservoirs might be built in remote areas with almost no access to water and the likelihood of rainfall doubtful, the minimum fill must be provided. In addition, especially in hot arid climates, the structure must be covered on all sides with plastic foil in such a way that the moisture of the mortar cannot evaporate. This means the plastic sheets must be tied up with overlaps so that the wind cannot blow them away. If a leak appears when the tank is eventually filled with rainwater, it is more appropriate to consume the water than drain it to repair the leak. Two different methods of sealing an already filled reservoir from the outside can be tried, provided the material needed is available. However, there is no guarantee that these methods, or rather tricks, will seal the tank, but since they do not affect professional repair, they might be tried if large amounts of water are at risk.
Rapid setting cement
If it is possible to obtain a few kgs of this cement, make a simple test before you buy it. Take a clean tin, pour in some clean water, and add some cement slowly, stirring it with a stick. When the water/cement mixture becomes plastic, take some of it in your hand and mould it. This cement must become very warm in your hand and by the time the heat disappears it must harden instantly. If this test fails to produce the required result, you have either got the wrong cement or the material is overdated, or has become moist, losing its property, and cannot be used. If the test is successful, buy the amount you estimate you will need. At the tank leak start to enlarge the hole where the water is running out. The hole should at least be 10 mm large and just touching the reinforcement. While you enlarge the hole, more water will flow out. Therefore you should make all preparations in advance, having all tools and material at hand. The best way to do the job is with two people. One prepares the hole for sealing, the other prepares the material. As you have experienced during the test, there is a very short time span during which the cement is still plastic. This is the right time when the cement has to be forced into the hole. Experience has shown that the best tools for this job are a screwdriver used as a chisel, a light hammer, and a piece of timber a little bit smaller than the hole. With the chisel shape the hole like a swallow-tail, if possible, pressing the cement into the hole at the right moment using your thumb. Take the timber piece and the hammer at the last moment before the cement sets and press the plug in by hammering the timber. This trick is likely to work if the leak is not big and some tests are first made to find out the cement's exact setting time. A:ll that is required for this repair work is fast action.
This is a water-clear viscous liquid. It can be tried to seal leaks if the water flows out without pressure. Prepare the base for the job using sandpaper to achieve a slightly rough surface of the cement wall. Open the bottle or container with sodium silicate only when you have completed preparations, since this chemical hardens on contact with the air. Use a spatula or a thin metal sheet to apply it, using the same technique as described for the nil coat. The sodium silicate should cover an area slightly larger than the leak. This method has often been used with much success.
Even if the two methods for sealing the tank from the outside remain unsuccessful, they have not done any harm to the structure and do not influence the professional repair work required for ferro-cement structures, since the rapid setting cement or the sodium silicate can be chipped off together with the-original cement mortar.
The repair work requires the same attention to the quality of material and mixing as the construction work. If the sand which was used for construction is not available, or if the source of sand is unknown since the repair work only becomes necessary years later, material must be tested in accordance with Chapter 3. It is again important to sift the sand through gauze wire mesh (mosquito wire mesh). Mixture should be three parts sand with one part cement. There is a considerable risk of creating a new leak since different mixtures result in different expansion of this material. This might lead to the patch cracking away and leaking at the edges. It has therefore also to be stressed that the original construction work should follow strictly the advice given. It can sometimes be observed that the amount of cement is increased to be on the safe side. This is unnecessary and can result in repair problems since after some years there will be nobody who remembers the original mixture. Repair of ferro-cement tanks should not be done using other than normal Portland cement. We can only warn against the use of e.g. rapid setting cement, since the property of this material differs greatly from the original Portland cement, as shown in the example above. Different expansion also creates problems likely to result in new leaks if the mortar for repair is wet. Although the usually small patches can be filled with mortar containing more water than the original material, this seal will not last since the material will shrink and by doing so will most likely result in another leak. Small leaks should be opened by using a chisel and hammer, chipping the mortar around the leak, down to the chicken wire. This must be done from inside the reservoir. It can happen that the leak inside the tank is not visible at the wall. In this case hammer a nail through the wall from the outside. This has to be done rather carefully, knocking the nail with short hammer movements. Use a thin 2 1/2 inch nail. Where the nail appears inside is not necessarily the point of leakage. In all those cases where this method has to be used, the surrounding area with a diameter of 150 mm should be chipped away. This job must be done carefully so that no cracks develop on the outside, since this part of the wall will remain untouched. The nail should be pulled out again.
Preparations to be made for the patch differ depending on whether the tank is in use, meaning water is purposely drained, or there is a minumum of water left and/or the leak occurs in an upper part of the wall where the ferro-cement is rather dry. If the area of repair is dry' it is necessary to wet it by splashing it with water. This procedure has to be repeated several times from inside and outside until an area of at least half a square metre is really damp. Only then should the moist mortar be filled in with a trowel and the surface smoothed (trowel finish). The mortar must be stiff or it will drop at the top, creating a new leak. If a nail was used to find the location, the hole should be filled from the outside at the same time. It is important to protect the outside repair area of the tank from the sun. If possible a piece of plastic should be tied to the patch from the outside. If this is not possible the outside area has to be splashed with a lot of water every two hours, and the inside two times a day. This curing must be continued for weeks, unless the reservoir can be filled, although applying the nil coat on the patch should be done the next day, Fig. 5.48.
Larger repairs require the same technique in principle. Because of the nature of the structure, such repairs are seldom and become necessary either immediately after the tank is filled for the first time (in which case they are the result of shoddy workmanship), or they are created by external force, for example if a car bumps the tank. If major damage has to be repaired, the area should be cleared of all material inside and outside of the wall by careful chipping and without damage to the reinforcement. If the reinforcement was damaged by external force, a larger area has to be cleared to make it possible to patch the reinforcement. If this is welded reinforcement mesh and is bent inside without a break, do not try to rebend. This will further damage the wall. If it is broken it can be rebent into position and tightly tied together using binding wire and pliers. The area then has to be cleared of all damaged chicken wire and a patch of two layers must be tied to the reinforcement mesh. If the external force has damaged the coiled fence wire, this should be rebent too. A serious problem occurs only if this damage is close to the bottom. However, in these cases too a repair should be attempted, since there is a good chance that the tank can still be used. Before plastering can start, the surrounding area must be splashed with water as described above.
Repair of large holes where all material has been removed from inside and outside must be performed using the same technique as originally used. That means, if the structure has been built on a mould, it is necessary to shutter the hole from the inside and to apply two coats of plaster from the outside. Cure each coat or cover the area with plastic. If the tank was built without a mould, the plastering to the damage should be done from inside using a shutter outside. Apply the nil coat after one day. Curing the patch for a long period is unnecessary if the reservoir can be filled above the repaired part again after two or three days. After all, this is the best way. Again if small leaks appear they usually close over, Fig. 5.50.