Chapter 17. New construction materials for developing countries*

* Contributed by the ILO.

RECENT ADVANCES IN materials technology have led to the development of new products in all groups of materials, including metals, polymers, ceramics and composites. The significant developments in materials technology have been brought about for a number of reasons. First, concern about a possible shortage of raw materials resulted in a re-examination of materials policy in many countries. Thus, research and development in new materials was encouraged. Second, the dramatic increase in the price of oil and energy costs in the 1970s prompted a reassessment of energy use in materials production. The worldwide production of materials for manufacturing and construction industries alone requires an equivalent energy of 109 metric tonnes of oil or 15 per cent of worldwide expenditure of energy. The amount of energy utilisation depends on the material to be produced. For example, it takes 3 × 10¹⁰ joules of energy to produce one cubic metre of Portland cement, but six and 29 times as much to produce the same volume of polystyrene plastic and stainless steel respectively.¹ In materials development efforts, a shift towards the production of “low energy materials” has recently been observed. In general, inorganic materials (e.g. ceramics and Portland cement) have received a great deal of attention.

This chapter deals with the use of new materials in rural construction which is of considerable importance in developing countries.

I. NATURAL FIBRE REINFORCED CONCRETE FOR RURAL CONSTRUCTION

Portland cement has been of great value for construction in developing countries. It is inexpensive in relation to its potential, easy to transport and usable with simple tools and cheap and readily available aggregates. However, it has its disadvantages. First, cement-based materials containing no reinforcements are brittle and weak in flexure (blending).² Second, they cannot be applied easily as plaster on mud walls. The use of reinforcements in the form of asbestos, conventional steel and wire mesh (ferrocement) raises the costs considerably thus making the products unfeasible for the rural poor.

It has been discovered that small quantities of natural fibres such as sisal or elephant grass in cement mortar result in a material of high flexural strength. The fact that the fibres themselves have tensile strength lower than that of cement, and that the resulting Fibre Reinforced Concrete (FRC) retains its flexural strength even after the fibres are dissolved by alkali attack from the free lime in cement mortar, can be explained as follows. Cement contains low flexural strength because during setting (transition from wet paste to solid state) it tends to crack due to shrinkage. The action of the fibres inhibits the formation of cracks during the crucial setting period as the material is held firmly at the time of shrinkage. The result is a material with considerably higher flexural impact (ability to withstand knocks), and tensile (resistance to pulling) strength.³ Production of large and thin concrete sheets was also made possible with the addition of 1 to 2 per cent chopped low-modulus fibres like sisal, coir and elephant grass. Other incidental advantages of FRC are: their lower density since the fibres act as fillers thus reducing the total weight, improvement in their thermal insulation, and sound absorption characteristics.

Types of Natural Fibres in Use

1. Sisal fibre⁴. Sisal fibre comes from an algave plant grown in most tropical countries under a range of soil and climatic conditions. It is grown on a large scale in East Africa, Angola, Brazil, Haiti and Madagascar. It has a declining export market due to the development of substitutes like synthetic fibres. At 1979 prices, sisal fibre cost US$500 per metric tonne.

Sisal FRC has properties which make it suitable for grain and water-storage, cladding walls and as roofing materials.

Roofing tiles of about 6 millimetres thickness can be produced with the addition of 1 per cent (by weight) of long fibres aligned along the tile using a 1:1 mortar, and by adding 0.5 per cent (by weight) of 25 mm fibres to the mix. These tiles are tough enough to be nailed and are practically unbreakable.⁵ The cost of such tiles in Kenya is US$0.50 per square metre (at 1979 prices) with the weight of about 10 kilograms per square metre.

Corrugated roofing sheets can also be made from sisal-reinforced concrete sheets of 1 square metre made from very rich mortar (two parts of cement with one part of sand) containing 0.5 per cent (by weight) of chopped fibres and a laminated sheet of two layers of long fibres each aligned in two perpendicular directions. These sheets are 9 to 10 mm thick and weigh 21 kg.⁶ They require a load well in excess of 200 kg to produce multiple cracking. Thinner sheets of 16 kg m^-2 weight can be produced if lower strength is tolerated. Sisal-reinforced corrugated roofing sheets are tough, have high impact strength, and can be nailed. At 1979 prices, the cost in Kenya is about US$3.00 per square metre compared with US$2.00 for corrugated iron, and US$3.00 for asbestos.

Cladding of mud-brick walls by sisal-reinforced mortar considerably increases the flexural rigidity and strength of such walls. Cladding is done by laying long sisal strands between the bricks, and by adding chopped fibres to the mortar. At 1979 prices the material cost of sisal cement-clad walls in Kenya (assuming soil is freely available) is US$1.00 per square metre of wall compared to US$4.00 per square metre for a typical concrete-block wall.

Other potential uses of sisal-reinforced concrete are in the construction of food and water storage containers and earthquake-resistant houses. Sisal FRC can also be used for well-lining, pipes and gutters, and as gas-holders for biogas plants.

2. Elephant grass fibre⁷. Table 17.1 below compares the qualities of elephant grass fibre with asbestos for use in roofing material. These are results of tests performed at the University of Zambia. The table shows that the elephant grass-reinforced sheets have good potential as roofing material for rural housing. However, these sheets are not as strong as the asbestos ones, which means that more expensive supporting roof frames might be necessary if the standards of asbestos and cement are desired.

II. FRC APPLICATIONS IN DEVELOPING COUNTRIES⁸

The Intermediate Technology Development Group (ITDG) introduced the FRC technology in a number of developing countries. At present, FRC roofing materials are produced and used in the Dominican Republic. El Salvador, Honduras, Malawi, Sri Lanka, United Kingdom (on a very small scale) and Zimbabwe.

The use of FRC instead of conventional roofing materials has the following economic advantages:

- costs are between one-third and one-half of asbestos and cement roofing sheets requiring similar roof structure;

- although slightly more expensive than locally-produced traditional clay tiles, FRC requires a much cheaper and simpler roof structure;

- costs are about three-quarters of the cost of corrugated iron sheets, and require similar or slightly more expensive roof structure;

- costs are about the same as those of the medium-quality traditional thatch (where materials are bought) but a similar roof structure is required.

In the final analysis, the economic advantages of FRC would depend on its durability in relation to the conventional roofing sheets. In a salt-laden atmosphere, a minimum life of three to four years would make it more economical to use FRC than corrugated iron sheets or medium-quality thatching. Compared to asbestos, FRC would require a five-to-ten year durability. For traditional clay, many factors such as fuel supply and rising cost of timber for roof structure make a straightforward evaluation difficult.

It is still too early to establish the practical life of FRC. However, natural exposure tests of materials have shown that even after four years no signs of deterioration due to age can be detected. This shows that FRC can be a viable alternative to conventional materials.

Table 17.1. Properties of roofing sheets of varying combinations

Source: “Elephant grass fibres as reinforcements for roofing sheets”. Appropriate Technology, Vol. 6, No. 1, London, May, 1979.

The durability of conventional roofing materials is illustrated in Table 17.2 below.

Table 17.2 Durability of traditional roofing products

III. CONCLUSION

The appropriateness of FRC for construction in developing countries lies in the ease with which it can be produced labour-intensively, at a low cost and on a small scale in virtually any rural or urban setting. For many countries, it could serve as a good substitute for imported asbestos and corrugated iron sheets as roofing material.

However, specialised training is required for the manufacture, handling and use of FRC building materials. Provision of training should therefore be of primary concern in the setting up of local FRC production capacity in developing countries.

NOTES AND REFERENCES

1. See J.D. Birchall and A. Kelly: “New inorganic materials”, in Scientific American, Vol. 248, No. 5, New York, May, 1983.

2. The flexural strength of cement is less than 5 megapascals and its tensile strength is 35 megapascals.

3. The addition of one metre long grade 3L sisal aligned in beams consisting of 1:3 mortar of water to cement ratio 0.5 increases the flexural strength by more than a factor of three and its impact strength by over seven.

4. See D.G. Swift: “Sisal-Cement composites as low-cost construction materials” in Appropriate Technology, Vol. 6, No. 3, London, November, 1979.

5. They are better secured by wire loops moulded into the tiles, and will bear the weight of a man across 400 metre span.

6. Non-reinforced concrete corrugated sheets of the same size are 13 mm thick and 30 kg weight.

7. See “Elephant grass fibres as reinforcement in roofing sheets” in Appropriate Technology, Vol. 6, No. 1, London, May, 1979.

8. J.P. Perry: “Development and testing of roof cladding material made from fibre reinforced cement” in Appropriate Technology, Vol. 8, No. 2, London, September, 1981.