Technology

General

Of the roofing options available in most developing countries, galvanized corrugated iron (gci) sheeting is by far the most widely used "modern" material, mainly due to its ease of handling and large span, requiring less supporting structure. The disadvantages, however, are that it is an imported material in most developing countries; its thermal performance is very unsatisfactory (extremely hot during the day, cold at night, causing condensation problems); heavy rainfall causes serious noise problems; and the often poorly galvanized sheets tend to rust through within 2 or 3 years.

Asbestos cement (ac) sheets are also extremely popular in many countries for similar reasons as gci, and also on account of their better thermal performance and fire resistance. However, they are brittle and diffilcult to transport, the fibres or the whole sheets have to be imported in many countries, and the serious health risks of mining and processing asbestos are leading to a steady decline of the ac industry.

A promising alternative has been found in fibre reinforced concrete roofing (FCR) and more recently in micro concrete roofig (MCR). These are roofing elements basically made of sand, cement and water, and in the case of FCR, with the addition of natural or synthetic fibres for reinforcement.

The main advantages of FCR and MCR are:

+ they can be produced locally in any developing country, where cement is available at sufficiently low cost;
+ the technology is adaptable to any scale of production, including one-man production units;
+ with a proper training course in the production and installation of FCR and MCR, virtually anyone (even unskilled workers) can learn the techniques;
+ the thermal and acoustic performance of FCR and MCR is superior to that of gci and ac sheets;
+ compared with burnt clay tiles, FCR and MCR require less timber for the supporting structure, can cost less to buy and can be equally durable;
+ compared with thatch roofs, FCR and MCR are more durable and eliminate the fire risk.

There are, however, some problems of FCR and MCR, such as:

- the limited availability and high price of cement in some developing countries;
- especially in dry areas with limited water supplies, the large amount of clean water required for preparing and curing the roofing elements
- the need for good training of producers and users of FCR and MCR, and strict quality control, without which failures are almost certain;
- the need for great care in handling, transporting and installing the roofing elements to avoid cracks and breakage;
- the difficulty of introducing this relatively new roofing system, where potential users do not know the advantages, or have heard of past negative experiences (which were mainly because of insufficient training of the producers and inadequate construction of the roof substructures);
- the fact that the roof is generally not strong enough to be walked on.

FIGURE

Development of FCR and MCR

The most well-known fibre reinforced concrete was asbestos cement, which was invented in 1899. In the 1960s fibre reinforced concretes, using steel, glass and synthetic fibres were developed and research is still underway. However, these can generally be considered inappropriate for applications in developing countries, due to the high costs and limited supplies of such fibres. Therefore, the fibres referred to in FCR are mainly natural flores.

In the mid- 1970s, FCR developments focussed on the production of sheets of about one metre square, since the aim was to substitute gci and ac sheets. However, the FCR sheets, which were produced with simple, locally made equipment and without any mechanization, had several disadvantages, for instance:

- high cement consumption (about 15 kg per m²), similar to that of asbestos cement;
- on account of their large size and weight, difficulty to handle and cure in water tanks, and to transport and install without breakage;
- the need for very accurately constructed supporting structures to avoid differential stresses and breakage of sheets.

On the basis of a research and development projector FCR sheets, funded until 1981 by the U.K. Government through the Intermediate
Technology Development Group (ITDG), the Intermediate Technology Workshops (ITW) of J.P.M.Pany & Associates Ltd., Cradley Heath, U.K., succeeded in 1983 in developing a new pantile system, which requires only 5 kg of cement per m² (by means of vibration compaction), is easier to manufacture, transport and install, and is less sensitive to errors. This is the basis of the technology dealt with here.

Research and development continued both in the field and laboratory, where the tendency of the fibres to decay m the alkaline matrix, especially in warm humid environments, was one of the main issues. Fibre decay is not a serious problem in roof tile production - as explained below - but ways were found, especially by the careful selection and preparation of the raw materials, to produce roof tiles without fibre reinforcement - this was called MCR.

Procedures

As indicated above, FCR and MCR technology requires good training and practical experience to achieve satisfactory results. The information given on this folder must therefore be regarded as a brie introduction to the technology and not as an instruction manual. The reader is advised to refer to some of the publications listed under Select Bibliography for further details, but when embarking on FCR or MCR production, advice should be sought from the Roofing Advisory Service (c/o Swiss Center for Appropriate Technology, Tigerbergstr. 2, CH - 9000 St. Gall, Switzerland), from where details of experienced equipment suppliers and users of FCR and MCR can be obtained.

Materials, Proportioning and Mixing

Cement

· Ordinary Portland cement of the standard quality available in most places is usually suitable. Slow setting qualities should be avoided as they delay demoulding and thus require far more moulds and working space.
· About 0.4 kg of cement is needed for a 6 mm thick pantile of 50 x 25 cm, corresponding to a cement: send ratio of 1 :3 by weight or volume (because their densities are roughly the same). Using too much cement means additional cost, but too little cement will produce a brittle and porous tile.
· Partial replacement of the cement by a pozzolana (eg rico husk ash. crushed burntclay, fly ash) to increase the durability of the fibres is possible, but not recommended, as it causes slow setting.

Sand

· Usually any type of clean sand that is suitable for cement mortars can be used for FCR and MCR, but in order to minimize the amount of voids, angular sand particles of good grain size distribution between 0.125 mm and 2.0 mm is ideal. The small particles fill the gaps between the large ones, needing less cement and resulting in a less permeable mix. Aggregates up to 4.0 mm may be used in MCR elements.
· Fine particles of silt and clay should be reduced as far as possible, as clay interferes with the bond between sand and cement.
· One pantile needs about 1.2 kg of sand, but the right amount must be found by sample tests. Too much sand makes a brittle, porous product; too little sand means a wastage of cement and a greater tendency to develop cracks on drying.

Fibres

· Natural fibres are likely to decay in the alkaline matrix within less clan a year, especially in warm humid areas. In FCR this loss of strength is not necessarily a drawback. The fibres are required to hold together the wet mix, inhibit cracking while it is being shaped and during setting, and give the product sufficient strength to survive transports, handling and installation. When the fibres lose their strength, the product is equivalent to unreinforced concrete. However, by then the concrete will have attained its full strength, and since cracking had been prevented in the early stages, it can be stronger than a similar product made without fibres.
· The fibre content ranges between 0.5 and 1% by weight, never by volume, as fibre densities can vary greatly.
· Sisal is the most common natural fibre used, but satisfactory results have also been achieved with other fibres, such as jute, flax, hemp, coir and banana fibre, as long as they are clean.
· In the early stages of development, long fibres were used. These gave high impact resistance and bending strengths, but making such FCR elements is cliff cult and thus rarely done.
· The fibres are now normally chopped to lengths of 12 to 25 mm and thoroughly mixed with the dry cement and sand before adding water. Since the fibres are randomly distributed, they impart crack resistance in all directions. The length and quantity of fibres is important, since too long and too many fibres tend to form clumps and balls, and insufficient fibres can cause excessive cracking, if the other ingredients are not of the right type or incorrectly proportioned.

Additives

· Generally no additives are needed for FCR and MCR, except perhaps a pigment to make a more attractively coloured product.

Water

· Tests have shown that concrete mixes prepared with brackish water are capable of producing satisfactory FCR and MCR elements, because they contain no steel reinforcements, which could corrode. However, it is always recommended to use the cleanest available water, preferably of drinking water quality, and this is essential when wire loops (for fixing on roofs) are inserted into the tile.
· Experience is needed to determine the correct amount of water, which should be just enough to make the mortar mix workable. Mixes with too little water are hard to work with and mould without cracking. Cement needs a certain amount of water to hydrate: insufficient water leaves some cement unhydrated ( without bonding effect), while excessive water gradually evaporates, leaving pores which weaken the product and increase permeability.
· Water is also needed to cure the tiles for about two weeks. The amount of water needed for this is often underestimated and can cause serious problems where water is scarce.

Moulding and Curing

· For these operations a screeding machine and a set of moulds are required. These are described in the section on Equipment.
· The wet mix is trowelled onto a polythene interface sheet on the screeding machine and, under vibration, smoothed with a trowel to the same level as the surrounding steel frame. At a predetermined spot at the top end of the pantile, a matchbox-size nib is formed, into which a wire loop is inserted for better fixing to the roof.
· The steel frame is lifted off the screeding surface and the plastic sheet slowly pulled over the setting mould, ensuring correct aligning of the tile edge to achieve uniform curvature.
· The mould with the fresh tile is then placed on a stack of moulds for initial setting and curing (24 hours), after which the tiles should be demoulded and cured for 2 weeks in water tanks.
· After curing, the hard tiles are then allowed to harden for another 2 or 3 weeks, before they can be used for installation on the roof.
· Since curing under water has frequently led to an unsightly efflorescence on the tile surfaces, some producers place the tiles on a wet gravel bed (such that the water does not reach the tiles), and cover them with black plastic sheets. This method, called "vapour curing", is a kind of autoclaving using solar energy. The tile quality and appearance is improved, while the setting and curing time is greatly reduced and a considerable amount of water is saved.

Roof Design and Installation of Tiles

· The main criteria for FCR and MCR roof construction are:

- minimum pitch of 22° in moderate climates, 30° in areas with severe driving rains;
- although straight and parallel rafters and battens are always recommended, pantiles tolerate slight inaccuracies (which are less acceptable for Roman tiles and must be avoided in the case of large sheets); pantiles may even be laid on a carefully constructed pole timber or bamboo structure (gaps between pantiles, eg on kitchen roofs, are often preferred, as smoke and hot air can escape easily and thus improve indoor comfort);
- the timber connections and fixing of tiles onto the battens must take into consideration that the uplift forces (suction) in windy areas can be much higher than the wind pressure and weight of tiles (special wire loops and fixing bolts have proved effective);
- only experienced craftsmen with special training in this technique should be entrusted with the roof construction and cladding.