Appendix D[edit | edit source]
New Zealand Ferrocement Tanks and Utility Buildings
Perhaps the greatest development in farm water storage in New Zealand has been the introduction of ferrocement tanks, which retain most of the advantages of earlier tanks with few of their limitations. The cost of smaller sizes is comparable to that of other tanks, but the storage cost per gallon drops off rapidly when larger sizes are used. Paralleling this consideration is the continuing economy offered by the indefinite life of the ferrocement tank.
In most parts of the country, ferrocement tanks are available as stock items in sizes ranging from 200 to 5,000 gallons, Thus, the factory can deliver a tank ready for pipe connections directly to the prepared base. If required, tanks larger than 5,000 gallons can be constructed on site by the same system used at the factory.
The widespread availability of ferrocement tanks and the versatility of the material provide the farmer with economic water-storage facilities involving only minimum site work. Permanent materials are used throughout, and, since all work can be controlled in the factory, most manufacturers confidently offer a 25-year guarantee on their products.
Factory-produced tanks are designed for convenient handling with simple equipment. Small tanks are loaded on the truck, and unloaded by a truck-mounted hoist. Usually, tanks over 1,000 gallons (4,500 litres) are winched onto the truck.
Site preparation is a simple matter, usually calling for no more than removal of vegetation and trimming the soil roughly level. If the tank is to be placed on rock, or if it is desirable to provide a concrete base, a layer of sand must be spread under the tank. This prevents point contact, which would generate high local stresses and probably result in cracking.
The tank is then ready for pipe connections. Generally, standard pipe fittings are built in during manufacture, but special items can be provided by arrangement. If necessary, additional items may be installed on site by chipping a hole and plastering the fitting in place.
Perhaps the most obvious adaptation from water-storage tanks is to tanks of other forms, such as sheep or cattle troughs and septic tanks. Septic tanks are constructed with earthenware fittings and are supplied ready for installation. They are manufactured in various forms, with the actual details determined somewhat by requirements of local governing bodies. (See Figure 5.)
Impermeability is an important characteristic of ferrocement in its use for water retention. Since impermeability promotes hygiene, this material is frequently used where hygiene is of prime importance. Most tank producers have a range of killing sheds, dairies, and freezing chambers-all constructed of ferrocement. (See Figure 4.)
By the simple process of placing a window or door frame against the inside former before plastering, the water tank is transformed into a tool shed, site office, pump room, small laboratory, or any similar structure. When required, plumbing and electrical circuits can be embedded in plaster.
Many manufacturers have developed additional features for special circumstances. Instead of using a circular former, as for tanks, the details may be modified slightly so that the office or pump room is square or rectangular. Freezing chambers are constructed of two layers of plaster separated by insulation and vapor barriers. Usually, the freezing equipment is mounted on the roof. Toilet rooms, shower rooms, and laundries are available with all plumbing fixtures in place, so that on site it is necessary to connect only the water supply and drains.
A further advantage of small ferrocement buildings is that relocation at a later date is no more of a problem than the initial delivery from the factory.
TECHNICAL DETAILS
Tanks are constructed by applying two or three layers of plaster against an inside former until the required thickness has been built up. The reinforcing is placed at the stage appropriate to ensure correct location within the wall.
The water pressure in a loaded tank generates hoop stresses in the tank walls. The resulting tension is resisted by a continuous spiral of reinforcing wire, usually No. 8 s.w.g. The spacing of the wire is determined by the diameter and depth of the tank.
Some manufacturers prefer a woven mesh of No. 14 s.w.g. and I 1/2-in or 2-in mesh; others use a chain netting. In some instances a light welded-steel fabric is incorporated in the lower section of the walls to accommodate additional stresses that develop during handling.
The tank floor, which may range in thickness from 2 1/2 in (6.2 cm) for a small tank to 4 in (10 cm) for the largest, is reinforced with a welded grid of steel. A typical reinforcing is 3/8-in diameter rods welded into a grid at 8-in or 10-in centers (10 mm at 20-25 cm). Loops of light steel project into the wall section, and additional handling loops protrude from the edge of the floor for lifting or dragging the completed tank.
The manufacturing sequence varies from one factory to another. In some cases the floor is cast first; in others it is cast after the walls. Some manufacturers place 1/2 in (12 mm) of plaster against the former, position the steel, and then continue plastering up to a total thickness of 1 in to 1 1/4 (25 to 30 mm). At other plants the reinforcing is placed directly against the inside former, and the main body of plaster is applied. The final layer of plaster is applied from the inside after removing the form.
In all cases a strong concrete coving is provided between the wall and floor to seal and strengthen the joint.
The plaster may be applied either manually or pneumatically.
Most tanks are provided with a flat or conical roof 1 1/2 in to 2 in (38 to 50 mm) thick. The roof may incorporate a separate small header tank to provide constant pressure (Figure D-1). If a roof is not needed, the upper edge of the tank wall is thickened to give added strength.
Finally, the tank is given a cement wash inside and is painted outside with a cement-based paint or other suitable surface coating. A little water is placed in the tank, which is kept in the factory yard for some time before delivery to allow the humid atmosphere to cure the cement fully.
