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Original:Ferrocement Applications in Developing Countries 11

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Ferrocement Applications in Developing Countries (BOSTID, 1973, 89 p.)[edit]

Appendix B[edit]

Ferrocement Food-Storage Silos in Thailand

A family of cheap, airtight bins made of ferrocement, the Thai silos are sized to hold 4-10 tons of grain, other foodstuffs (e.g., peanuts, soybeans), salt, fertilizer, pesticide, cement, or 2,000-5,000 gallons of drinking water. The silos were developed over the past 4 years by a government corporation in Thailand* to fit requirements of the developing world. The designs are versatile; the storage units can be built on extremely adverse sites: where the water table is at the soil surface or in remote areas where even vehicular access is impossible. These bins require no maintenance, are easily padlocked against thieves, and protect against water, rodents, birds, insects, aerobic microorganisms, weather, and serious loss of seed germinability.

These properties are made possible by using the material and methods of ferrocement boatbuilding. This construction produces a high-quality product that can be built by local labor with minimal supervision.

The materials needed are cheap and readily available in developing countries: standard grades of cement, a wide range of wire meshes (e.g., chicken netting), and sand. (See Table B-1.)

TABLE B-1. Cost Record for first Experimental Thailo, Thailand, 1969

Quantity

Thai Cost (in US 0

Labor

Skilled

63 man-hours (a)

Unskilled

135 man-hours (a)

20

Total labor costs


Materials

Cement

1.000 kg

25

Sand

1.725 kg

3

Aggregate

965 kg

3.5

Mortar plasticizer (b)

2 kg

2

Sealant for the base (b)

5 kg

2

Paint

3/4 kg

7.5

Steel

Chicken wire

2 rolls

18.5

No. 2 rod

80 m

2.5

Water pipe (b)

32 m

13

Total materials costs


77

TOTAL

121


(a) Labor figures refer to initial experiment and can be drastically decreased in practice. Construction of several Thailos at the same time also reduces labor costs because time spent waiting for sections to cure can be used productively on adjacent bins.
(b) Used to date because of availability in Thailand but may not be necessary for adequate performance of completed Thailo.

Source: R.B.L. Smith, et. al., Thai. J. Agr. Sci. 4 (July 1971): 143-155

As previously noted, the ferrocement bins are watertight and airtight. Respiration of grain, or similar product, quickly removes oxygen from the atmosphere in the bin, so that any insect (adults, larvae, pupae, or eggs) or aerobic microorganisms present cannot survive to damage the stored product. Thus, no fumigation is needed.

Thailos are easy to use. They are filled through a hatch in the top and emptied through another at ground level. The sides are sloped for firm support of a ladder (Figure 11), and the fairly low height (7 feet, or a 5-step ladder) of the entrance hatch simplifies manual filling from sacks or buckets.

As with any type of silo, it is important to dry the grain before loading; otherwise molding may occur. Tests to investigate the feasibility of on-site drying, using a fan and small engine (designed by the Tropical Products Institute*) to force warm air through the product while it is in the Thailo have been conducted successfully in Thailand.

TECHNICAL DETAILS OF THAILO CONSTRUCTION (See Figures B-1-B-3.)

The base of the Thailo is saucer shaped and, where necessary, is built on an earth pile to raise it above the water table. This gives a strong, easily constructed structure that can resist foundation failure. It consists of two layers of 5-cm-thick concrete (1 cement: 1½ sand: 2 aggregate) with mesh reinforcement and an asphalt seal between as added protection at building sites subject to flooding. The base may be easily modified to suite different ground conditions.

The walls slope inward to a central entrance hatch at the top. This truncated cone shape gives a very rigid structure, both during and after construction, and it eliminates the need for a roof structure. The walls are reinforced with 2-m-long poles (water pipe or bamboo), reinforcing rods, and one layer of wire mesh on internal and external faces. The mortar is hand-mixed and is applied as a thick paste using trowels and fingers. No formwork is required to support the mortar. The wall mortar consists of 1 part of standard cement, 1.75 parts sand with the optional addition of a plasticizer to improve workability. Water/cement ratio is approximately 0.3, and with only enough water to hydrate the cement, no voids due to excess moisture are left in the ferrocement, which becomes impermeable.

The top may be cast on site or precast and erected before cementing the walls. It consists essentially of a ferrocement lid with circles of rubber to make airtight seals. An inner lid of aluminum (trashcan lid) with a polystrene lining to insulate against heat and to prevent moisture condensation can be also used.

Controlling the water content of the mix and curing for several days under moistened sacking to avoid direct exposure to the drying effects of sunlight and wind are paramount construction considerations. On completion, the bin may be tested by filling it with water for 1 week. This is an excellent quality-control test because water is considerably more heavy than products likely to be stored and any cracks or weak sections, caused by poor workmanship, can be readily seen as leaks.