Building a 10 cubic meter ferrocement water tank

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Ferrocement

Ferrocement is a sand-cement-water mixture applied to a a wire reinforcing matrix. Our tank has a wall 5 centimeters (2 inches) thick reinforced with a matrix of steel reinforcing rods and 5 layers of chicken wire.

Since the wall of the tank is relatively thin, it is of utmost importance to make it with concrete of the highest quality and strength.



The following will help to assure a durable concrete:

1. Use washed, screened sand free of soil and organic matter. Do not use sea sand unless it has been thoroughly washed with fresh water to remove the salt.
2. Mix 4 parts of sand to 1 part (by volume) of Portland cement. Mix the dry ingredients thoroughly before adding water. The preferred method is to use a cement mixer. If done by hand, the mixing should be done on a clean surface to avoid contamination of the mix with soil. A piece of 1/2-inch plywood, 4 feet by 4 feet, makes a good mixing surface. Alternatively, the day before one could pour a thin, flat mixing slab directly on the ground.
3. Use clean, fresh water. Dirty or salty water will weaken the concrete.
4. Add only enough water to provide a stiff, plastic mix. Adding too much water at this point can yield a weak concrete and one that will crack as it dries.
5. Prepare only as much mix as can be used within about 1 hour. If the mix becomes too hard to use, throw it away or use it for some other purpose. DO NOT ADD WATER TO SOFTEN THE MIX AND DO NOT ADD OLD MIX TO NEW MIX! This will result in a much weaker and less durable concrete.
6. Keep the hardening concrete continuously moist for at least 15 days. Spray the surface with water frequently throughout the day to keep it from drying out. The hardening of concrete is a chemical process that requires water. Drying before this curing process is complete will result in a weaker product.

Why a capsule shape? The capsule shape of our tank provides a strong tank despite the thin walls. It is less likely to develop cracks that will leak than a cylindrical or rectangular tank with a flat bottom. The water pressure tends to act at right angles to the surfaces of the tank, pushing downward on a horizontal bottom and outward on the vertical walls. This often can break the bond between the floor and the walls, resulting in a leak. In a tank with a rounded bottom, these forces are evenly distributed along the curved surface, making cracking far less likely. The curved top is also very strong and will easily support the weight of several people.

Framework To facilitate the construction of the tank, it is made in two steps. The bottom and sides are completed first, and then the top is added. The basic framework of the tank is made from 10 mm (3/8 inch) steel reinforcing rods (rebar). 
 

Make 3 rings 2 meters (79 inches) in diameter. Cut pieces of rebar 643 cm (253 inches) long and mark 15 cm (6 inches) from one end for overlap when the ends are tied together. To bend the rebar into a circle, nail three pieces of wood together as shown in the photograph and using this as a form of anvil gently hammer the rod into the desired curve as shown. Before tying the ends together, measure the diameter at various points around the ring and make the necessary adjustments to make it round.

Make one ring 70 cm (28 inches) in diameter. Cut a piece of rebar 235 cm (93 inches) long, and mark 15 cm (6 inches) at one end for overlap.

Cut four pieces of rebar 714 cm (281 inches) long, and make a mark 200 cm (79 inches) from each end. Using one of the large rings just made as a guide, bend the rebar between the two marks into a semicircle to form four U-shaped pieces. Cut eight pieces of rebar 340 cm (134 inches) long, and make a mark 200 cm (79 inches) from one end. Using one of the U-shaped pieces as a guide, bend the short end of each rod into a quarter-circle to form eight J-shaped pieces.

Using the U-shaped rebars as a guide, excavate a hemispherical hole for the base of the tank. Dig a sloping trench from the center of the hole to one side to accommodate the drain pipe

Cross the four U-shaped rebars in the center of the bottom of the hole, arranged so that the vertical parts are equally spaced around the rings. Tie the bottom large ring at the marks made previously 2 meters from the top ends. Tie the second ring about mid-way up, and tie the top ring about 20 cm (8 inches) from the top. Center the small ring at the bottom. Securely tie the rebars at every point where they cross.
Tie the J-shaped rods mid-way between each of the vertical rods.
Between each of the vertical 10-mm (3/8-inch) rods, tie a 6-mm (1/4-inch) rebar. These thinner rods can easily be bent by hand to conform to the J shape of the adjacent rebars. Cut the lower ends so that they extend slightly beyond the small ring in the center. Again securely tie all points where the rods cross. Leave one of the vertical bars untied for the moment to facilitate entering and leaving the tank.

Working from the inside of the tank and starting at the center, wind a 6-mm (1/4-inch) rebar around in a spiral spaced about 10 cm (4 inches) between turns. Tie each cross point securely. Raising the rebar frame about 5 cm (2 inches) above the bottom of the hole will facilitate tying the rebar at this point and pouring the base of the tank later on. Place small stones or pieces of brick at 3-4 points under the rebar.



Continue winding the spiral of 6-mm rebar up the vertical walls, overlapping each new bar where it joins the previous one by about 15 cm (6 inches). On the bottom half of the wall, the spacing between adjacent turns is about 10 cm (4 inches), and on the top half the spacing is about 20 cm (8 inches). Tie every point where the bars cross.

















   

Adding the chicken wire

  Cover the rebar frame with five layers of chicken wire, three outside and two inside. Alternate layers of chicken wire should be run at right angles to one another and placed in such a way that the wires lie over one another as little as possible so as to leave the smallest possible "holes" through the multiple layers. 

Stretch the chicken wire tightly over the frame so as to not leave bulges and "bubbles." Tie the five layers together tightly with short loops of wire. Extend the chicken wire from the ground level to the top ring, leaving the top 20 cm (8 inches) of the rebar frame uncovered to facilitate tying on the top later on.

Pouring the base  Before pouring the concrete for the base, place the drain pipe in the center. (See the drawing at the top of the page.) A coupling is placed so that its top edge is about 5 cm (2 inches) above the rebar, making it flush with the top surface of the concrete when the bottom is poured. A short nipple extends downward to an elbow, which connects to a horizontal drain pipe (sloped slightly downward). Another nipple, 20 cm (8 inches) long, is connected, hand tight only, to the top of the coupling. This nipple serves to keep dirt and concrete waste from entering the drain during construction. When the tank is in use, it serves to draw the water off above any sediment that might have settled in the tank. It is removed for cleaning the tank. Fill the trench for the drain pipe with stones to conform with the surface of the bowl-shaped hole.

The base is a hemispherical bowl about 10 cm (4 inches) thick. Begin pouring in the center around the drain pipe. Work the point of a trowel up and down to force the mix to fill any voids in the irregular surface of the hole and around the pipe and the rebar. (The rebar should already be suspended 5 cm above the surface of the hole.) Trowel the surface of the base to a depth of 5 cm (2 inches) above the rebar.

Continue the base up the sides to just above the bottom of the chicken wire, embedding the bottom 2-4 cm (1 inch or so) of the chicken wire in the concrete. 

















Applying the cement mix  

The cement mix is applied in four steps:

1. Load the chicken wire from the outside of the tank
2. Apply the finish coat to the outside
3. Apply the inside coat to the chicken wire
4. Apply the finish coat to the inside

 

Trowel the mix onto the chicken wire with lots of pressure and a vibrating motion to force the mix into the spaces. To keep the mix from passing through the wire and falling to the floor, back up the application with a short board. Fastening a handle to the board makes the job easier.

The cement mix should pass through the chicken wire and about 1 cm into the tank, leaving the inside surface rough. The outside surface should be left somewhat rough also to facilitate the bonding of the second coat. The entire thickness of the first coat should be about 2 cm (3/4 inch).

Apply the finish coat to the outside within one day after applying the first coat. If it is not possible to apply the finish coat the next day, use an acryllic bonding agent to bond the second coat to the first. Remember to keep the hardened concrete moist by frequently spraying it with water throughout the day. 

The second coat should be no less than 1 cm (1/2 inch) in thickness. Too thin a coat tends to dry out before the concrete has properly cured and bonded to the first coat. The result will be a separation of the second coat from the first as seen in the photograph on the left. 

End the second coat 5-6 cm (about 2 inches) below the top of the first coat, so that the joint between the bottom part of the tank and the top will not line up. That is, the layers will overlap when the top is completed. 

Apply the two coats on the inside of the tank in the same manner as on the outside. Be sure to use lots of pressure and a vibrating motion when applying the first coat to force the mix to fill all the voids in the outside layer. Apply a bonding agent according to the instructions on the container if more than one day lapses between application of the layers. Again remember to keep the hardened concrete continually moist by spraying it frequently with water. 
















The framework for the top  Make two rings 70 cm (28 inches) in diameter of 10 mm (3/8 inch) rebar. Cut 2 pieces of rebar 235 cm (93 inches) long, and mark 15 cm (6 inches) at one end for overlap. Bend them into rings as before. Make one ring 150 cm (59 inches) in diameter. Cut a piece of rebar 486 cm (191 inches) long, and mark 15 cm (6 inches) at one end for overlap. As before check the diameters of the rings in various directions, and make the necessary adjustments for roundness. Tie the overlapping ends to form the rings. 
 
Make 5 rings 2 meters (79 inches) in diameter. Cut 5 pieces of rebar 628 cm (247 inches) long. The ends of the rebar should just meet, without overlap.

One of the large rings will be kept intact but without tying the ends of the rebar. Two of the large rings will be cut in half to yield 4 semicircles, and 2 of the rings will be cut in quarters to yield 8 quarter-circle arcs.

Tie one end of one of the semicircular arcs to one of the vertical 10 mm rebars.

Bend the end of the vertical rebar to conform to the arc. Tie the opposite end of the arc to its corresponding vertical rebar. Do likewise to the other 3 semicircular arcs, and temporarily tie them together at their centers.

Center one of the 2 small rings over the top of the arcs, and tie it on. Do likewise to the middle-sized ring. Tie the largest ring just above the tops of the vertical rebars. Adjust the overlap to make the ring fit, and tie the overlapping ends of the ring.

Tie on the 8 short arcs (quarter circles), and in between each tie on arcs of 6-mm (1/4-inch) rebar. 

Wind a spiral of 6-mm (1/4-inch) rebar around the top, and tie each cross point.

Cut the semicircular arcs in the middle, and bend up all of the ends to a vertical position.
 

 Tie the second small ring about 10 cm (4 inches) above the first, and cut off the ends of the protruding rebar. 

Finishing the top

Because of the double curve of the top, the chicken wire must be applied in relatively small pieces. Overlap the chicken wire on the bottom part of the tank, and overlap each adjacent piece of chicken wire by at least 10 cm (4 inches). Application of chicken wire to the inside and later finishing of the top can be facilitated by building a temporary scaffold inside the tank. 

Apply the cement mix just as you did for the bottom part of the tank, two layers outside and two layers inside. Before completing the second layer, punch a hole in the first and insert a 1-inch by 8-inch galvanized nipple in the top and mortar it in with the second layer. 

Fabricate a man-hole cover, slightly larger in diameter than the finished outside lip of the man hole. Apply the cement mix just as you did for the tank itself.  






When the tank is complete, paint the inside with an acryllic/cement waterproof sealer. Allow the concrete to cure for at least two weeks, keeping it continually moist, before beginning to fill the tank.














Materials List

Item	Quantity
Reinforcing rod, 10 mm x 9 m (3/8 in x 30 ft)	17
Reinforcing rod, 6 mm x 9 m (1/4 in x 30 ft)	28
Chicken wire, 22 gauge, 1", 4' x 150'	2.5 rolls
Tie wire, 16 gauge	9 kg (20 lb)
Cement	12 bags
Sand	3 m3 (100 ft3)
Pipe, galvanized iron , 1" x 10'	1
Nipple, galvanized iron, 1" x 8"	1
Nipple, galvanized iron, 1" x 2"	1
Coupling, galvanized iron, 1"	1
Elbow, galvanized iron, 1"	1
Teflon tape	1 roll
Cement/acrylic sealer	20 kg (45 lbs)

Labor requirements The work requires at least two people, and occasionally a third person can be helpful. No special skills are required, although it helps if at least one of the people has participated in the construction of such a tank. The labor requirement in the following table are based on the construction of the tank depicted in the photographs.

Task	Man hours
Cutting, bending, and tying rebar for the bottom part	48
Fitting and tying chicken wire for the bottom part	38
Installing drain pipe	6
Pouring the base	16
Appling cement mix to the bottom part	102
Cutting, bending, and tying rebar for the top part	22
Fitting and tying chicken wire for the top part	48
Building scaffold inside	12
Applying cement mix to the top part	88
Fabricating man-hole cover	16
Painting sealer on the inside	9
Scaffold removal and general clean-up	8
Total	413

I gratefully acknowledge the help of the builders of the tank depicted in the photographs, Don Reynaldo Obando, experienced in construction with ferrocement, and his assistant, Eliomar Salgado.

Phil A. Arneson El Ocotal Municipio de Yuscarán Departamento de El Paraíso Honduras