|Journal of the Network of African Countries on Local Building Materials and Technologies - Volume 3, Number 2 (HABITAT, 1994, 42 p.)|
** By Baris Der-Petrossian UNCHS (Habitat). The production of this paper is partly based on UNCHS's earlier studies on the same subject.
The ideal roof cladding material should have low self weight and sufficient strength to support different types of loads including its weight and loads due to wind pressure or any other imposed force. It should totally exclude rain, prevent excessive heat gain or loss, have good resistance to fire and require little maintenance during its life span. Given these requirements, it is not surprising that only a few materials (such as galvanized-iron sheets, aluminium sheets, asbestos-cement sheets clay and concrete tiles) have become the established roof cladding materials. In most developing countries, these materials are often import-dependent and are invariably scarce or prohibitive in cost. Sometimes they are not available at all. Fired-clay tiles can normally be produced from indigenous resources but their suitability is restricted by the fact that not all clay types can be used for their production; moreover, they are energy-intensive and, like concrete roofing tile, their self-imposed weight is such that it requires a heavy roof structure to support the cladding.
Clearly, then, there is as yet no adequate solution to the problem of an appropriate low-cost roofing material: this remains a fundamental constraint in the delivery of low-income shelter. It is against this background that attempts have been made in many countries to develop a new technology-fibre-concrete roofing technology-which, in principle, otters the potential for improving the availability of low-cost roofing materials. Despite its potential advantages, fibre-concrete technology is relatively new and requires further development before it could be readily accessible for wide-scale adoption. One of the ways in which this development process could become a reality is through a continuous dissemination of technical information. This article is, thus, a contribution of UNCHS (Habitat) to the overall effort in information How regarding fibre-concrete technology. In principle. FCR tiles, in comparison to sheets, are less expensive, have less sell-imposed weight and, in general, have less intricate quality control requirements. For this reason, FCR tiles seem to have gained popularity as a potential low-cost roofing material.
Information in this article is based mainly on extensive work undertaken by (a) Mr. J.P. Parry of Intermediate Technology Workshop, Cradley. The United Kingdom, (h) Intermediate Technology Development Group, ITDG. Rugby, The United Kingdom, and (c) Swiss Centre for Appropriate Technology, SKAT, Saint Gallen, Switzerland, among others.
INPUTS REQUIRED FOR FIBRE-CONCRETE ROOFING
(a) Raw materials
The basic raw materials required for the production of FCR are cement, sand, fibre and water. Optionally, colorants in the form of standard chemical pigments, such as those already in use by the concrete industry, can be added. As described below, each of the main raw materials play a crucial role in the production process and the final properties of FCR depend as much on the appropriateness of each item as on production techniques.
(i) Cement: Ordinary Portland cement is the type used for FCR. In order to achieve the desired results in strength and durability, it is essential that the quality of cement measures up to the standards required for normal concrete and masonry practice.
(ii) Sand: Normally, any type of sand which is suitable for cement mortars can be used for FCR. Yet the quality of sand should not be taken for granted. The size of sand particles has a direct influence on the quality of FCR. Sands with pin-tide sizes which will pass through a sieve with 2 mm diameter holes hut, at the same time be retained by a sieve with 0.6 mm holes, are suitable. The absence of tine particles or the predominance of over-sized particles leads to mortars which tend to split when the wet mix is being placed on the moulds. Where the mortar mix contains coarse particles of sand, the matrix may become relatively stronger after selling, but will thereafter become permeable. Related to appropriate particle sizes, sand should be well graded, that is, contain a good range of particle sizes which, upon consolidation, compact to form a dense mass almost without voids. It is also desirable that the shape of sand particles be angular, rather than rounded, so as to improve the strength of the mortar. Finally, the sand should be clean.
(iii) Fibre: Popular roofing fibres can be classified into three main groups: mineral fibres - of which asbestos is the most popular, animal fibres, and vegetable fibres. Vegetable fibres are easily the most appropriate for the purpose of low-level technology production of FCR. The coir from coconut husk, stem fibre such as jute and leaf fibre such as sisal are the most common examples of vegetable fibres which have been used with success. In general, the selection of suitable fibres should aim at avoiding fibres which are:
- excessively still, oily or greasy:
- easily impregnated by chemicals which have adverse effects on cement, e.g. sugar; and
- susceptible to large dimensional changes from wet to dry state.
A simple test for the suitability of fibres involves chopping up a sample of the fibre and mixing it in a sand-cement mortar 100 times the weight of the fibre. The resulting concrete is allowed to set overnight. If the fibre pieces protruding from the concrete can be easily pulled out or if the concrete surrounding a particular location of fibre is discoloured or powdery, the fibre is unsuitable. The main function of fibre in the concrete is to resist segregation of the fresh mix during moulding and to prevent the formation of shrinkage cracks during the initial setting and curing stages.
(iv) Water: The production of good quality FCR requires good quality water, preferably standard potable water. In most circumstances, rain water can be used, but if it is collected from roofs, care should be taken to avoid those which are excessively contaminated with debris. Apart from the four basic raw materials, there are two additives as yet not widely used, but nonetheless of importance to FCR technology. The first comprises water-proofing agents which minimize permeability. These are normally in the form of already prepared pore filling materials or water repellent materials. The second is pigments for colouring the FCR products. These could be obtained as commercial products in very finely ground form ready to mix with the concrete.
b) Capital items
FCR is feasible at several scales of technology but in practice its popularity in most developing countries is predominantly with small-scale labour intensive technologies. At this scale of technology the main capital requirements are as follows:
(i) basic masonry equipment, i.e., spades, measuring pans, and a trowel;
(ii) a moulding table which could be locally fabricated (it is essential that it be geometrically straight and flat at the top);
(iii) a small vibrating machine for screening which can be charged by a 6 or 12 volt car battery. The vibrating machine could be the principal piece of equipment which together with a set of moulds and accessories the production can start;
(iv) a set of moulds the size of roofing tiles or sheets which can be locally fabricated from fibre concrete, asbestos cement or glass fibre and mounted on wooded frames;
(v) a curing tank which can be cast in concrete on site, and a shed for production and curing of the products.
THE PRODUCTION PROCESS
At the small-scale level, the production of FCR is a relatively simple process. The first step is similar to the process of mortar preparation as in basic masonry practice. After preparing the fibre and chopping it up to lengths of about 15 to 25 mm and sieving the sand to requisite sizes, the basic ingredients (sand, cement, water and fibre) are mixed into a concrete. The ratio of mix is normally 1 part of cement to 3 parts of sand. Then, 1 per cent of the weight of this total mix is accounted for by the fibre content. Only a small amount of water is required - just enough to produce a plastic or workable state. Whereas sand and cement can be measured by volume, it is always desirable to measure fibre by weight because of the possible variations in quantity if measured by volume.
One of the most demanding and skill-intensive aspects of FCR production is the transformation of fresh concrete matrix into tiles or sheets. This involves spreading a quantity of the fibre concrete on a polythene sheet which is placed on a vibrator for compaction. Thereafter, the sheet of concrete is lifted and casted on a mould. A good compaction is one of the critical factors in producing good-quality tiles. Figure 1 shows the percentage of voids in a concrete versus the compressive strength.
Transferring the freshly prepared mix from the polythene sheet onto the die or sheet mould seems an easy task, but it is a delicate aspect of the operation - a manual process requiring precision. The next stage is the normal concrete curing process which is done by transferring the set of FCR tiles off the moulds into a curing tank for at least seven days.
Figure 1. Good compaction is required to improve the compressive strength of concrete
QUALITY CONTROL AND TESTING PROCEDURES
A basic check on quality control of FCR products should cover the following:
(a) Quality of the basic raw materials: cement, sand, fibre and water;
(b) Proportion of the raw materials for the concrete mix:
- Too much cement means unnecessary additional cost and may imply inadequate quantity of sand which leads to shrinkage cracks, while too little cement with excess sand may lead to a brittle and porous product. Figure 2 shows the effect of cement content and the quality of sand on the compressive strength of mortar.
- Excessive quantities of fibre could create lumps, thus farming a porous and weak product; similarly, insufficient fibre leads to a fragile product with little tensile strength which may even break when separated from the mould;
- Excessive water will produce a deformed product, while too little water could cause unsatisfactory setting with traces of air bubbles in the under surface of the finished product.
Figure 3 shows a typical graph which indicates the relationship between water content and strength of concrete.
(c) Adequate homogeneity in the mixture of the concrete improves the quality of the products;
(d) Good compaction of the fresh concrete mix ensures a product of the right thickness and improves its durability and strengths;
(e) The sizes and the shape of the moulds should be exactly the same for uniform production. The process of moulding the fibre-concrete into sheets or tiles requires exactitude, without which the final product may not be suitable for use in construction;
(f) Curing should not be taken for granted, since inadequate curing decreases the strength of the FCR product. A setting time of fresh concrete for one or two days under shed and under relatively humid conditions is also required. The minimum of fourteen days of curing in water should be followed.
Figure 2. The effect of the cement content and the quality of sand on the compressive strength of a mortar
Figure 3. The relationship between water-cement ratio and the compressive strength of concrete
STRENGTH AND DURABILITY TEST
In undertaking strength tests for FCR, one has to make a distinction between two types of loads that the material will be subjected to when used in construction. The first comprises of permanent or self-imposed load of FCR sheets or tiles - which is relatively high. The other set of loads is classified as external, and refers to wind pressure and possibly the weight of people walking on the roof, even though as with asbestos-cement roofing sheets, one would expect adherence to certain codes of practice which would not subject the roof cladding to human loads. Wind loads on the other hand should not be underestimated. For example, a 145-km/h wind could result in an uplift pressure of as high as 90 kg/Sqm. Thus the strength of FCR products and their tightness with supporting structure should be reasonably high to withstand such pressures. In the absence of standardized test procedures for strength of FCR, several field projects have based their approaches on stipulated standards for asbestos-cement roofing sheets or ordinary concrete tiles. For instance, a testing rate of about one per cent for a batch of FCR products is recommended based on the British Standards Specification BS: 690 (asbestos cement roofing). Similarly, FCR products should be tested for strength at between two and three weeks. However, it is advisable to soak the sheets or tiles in water for 24 hours prior to strength tests, as recommended for comparable materials.
A typical strength and durability test would consider the following:
(a) Impact testing: A 5 kg weight is dropped from a height of about one meter onto a sheet or tile. A crack indicates a weak and unsatisfactory product.
(b) Bending tests: In order to determine the capacity of FCR to withstand bending stresses, loads should be placed centrally as line loads at the midspan of the FCR product, rather than uniformly or partially distributing such loads. A minimum of 50 kg load is recommended for placing at the mid-span for a non-destructive test. In placing the line loads, the bearings at the supports should be about 50 to 75 mm wide: the width of the loaded area should be about 225 mm. It is desirable to soak all the tiles or sheets in water for 24 hours prior to loading.
(c) Porosity test: This is probably the single most important test because the function of a roof is basically to keep out the rain. In the absence of standardized test procedures, international standards for porosity test on asbestos-cement roofing sheets are recommended as a possible useful test on fibre concrete roofing products. In this test, the sheet is laid flat and a head of water is maintained at 20 mm above the peak of the corrugations. After 24 hours, there may be dampness but a good quality product should not have moisture on the under side. Some ease studies have tried modification of this lest procedure but whatever the approach, the principle should ensure that a relatively large area of the sheet or tile is tested at a time.
A. COMPARISON BETWEEN FCR SHEETS AND TILES
In principle. FCR sheets are relatively more costly to produce than the tiles because for the same area of roof coverage, the sheets tend to consume more raw material inputs. A cement mortar of one part cement to three parts sand is a typical mix for FCR tiles, while a mix of 1:1 is normal for the sheets. The table below and the accompanying figure 4 give a brief comparison between sheets and tiles.
For a rafter-spacing of about 1.5 in (see figure 6), sheets are normally supported on 75 × 50 mm purlins spaced at 850 mm on centres and tiles rest on 50 × 25 mm battens at 400 mm on centres.
The volume of timber used for the purlins is therefore less for the tiles than for the sheets by about 25 per cent.
A metre long FCR sheet weighs approximately 20 kg while a tile 0.5 metre long weighs 1.62 kg. For this reason, the FCR sheets are cumbersome and delicate to handle in the production process. The large size of the sheet is a disadvantage in quality control for a rather small-scale manual production technology and it is a more demanding task to lay the sheets over the roof structure as well. If a single sheet has to rest on three purlins and if they are not placed accurately in a straight line, a single sheet (A) will not be supported correctly in the centre and could break. Tins degree of accuracy will not be so important if two separate sheets (B) are installed. Figure 5 shows these configurations.
Figure 6 gives details of a roof structure for FCR tile cladding.
USE OF FCR PRODUCTS IN CONSTRUCTION
The viability of FCR technology not only depends on the quality of the end product but also on the manner in which it is used in actual construction practice. Good quality FCR products may be wrongly applied in construction to the extent that the entire roofing component fails. Several field experiences suggest that any apparent unpopularity or non-viability of FCR may be more a result of inappropriate construction practice than of poor quality production. In the absence of an established code of practice for FCR, due consideration should be given to the following construction details:
Table 1. Summary of basic production data for comparison between FCR sheets and tiles
Sheets 1000 × 780 mm
Tiles 500 × 250 mm
Figure 4. Timber support cross members
Figure 5. Accuracy of purlin in supporting FCR sheets is important
A typical roof structure for FCR pantiles
The relative weakness of FCR in terms of porosity and low-strength can be minimized if a desirable pitch is used in construction. A minimum of 22.5 degrees is recommended, but this may have to go up to 30 degrees in areas where torrential and heavy rains are prevalent.
(b) Rafters and purlins
In general, errors in the erection of a roof structure can be tolerated with the use of metal base sheets but not with stiff materials such as fibre concrete. The brittleness of FCR makes it susceptible to cracks when the roof structure is not properly mounted. Sometimes, the purlins and ratters are made from limber which is green or not seasoned, so that subsequent dimensional changes could lead to straining the FCR and, eventually, to cracks. As shown in figure 5, incorrect placing of timber, with either rafters or purlins sagging, could lead to cracks. The FCR sheets are more vulnerable to cracks than the tiles.
(c) Fixing gutters for rain collection (see figure 7).
(d) Ridge details
It is not necessary to cement the joints. If cemented joints are preferred, use a mixture of one part cement to three of sand. Finish the joints neatly.
(e) Fixing details
Poor fixing of FCR cladding onto the roof structure can reduce the strength of an otherwise sound roof, especially in areas where high wind speeds are recorded. In FCR, fixing is done in two ways:
(i) a nib is precast on the underside of the tile and through this a loop of wire or short length of a strong string is embedded;
(ii) the FCR tiles or sheets are drilled and nailed to the roof structure. The precast nib system seems advantageous but its stability depends largely on the strength of the string or wire and how securely it is fastened. Moreover, the nib is likely to form a loose bondage to the tile and may break off with time. The alternative of screwing and nailing of tiles or sheets to the roof structure is likely to cause cracking; in addition, the holes drilled in the cladding need to be sealed against rain seepage. Figure 9 shows some types of built-in fixings.
Figure 7. Gutter details a) Correct fitting - Fascia fitted in line with the wall, so that rain water will run into the gutter
Figure 7. Gutter details b) Wrong fitting - Fascia fitted at right angles to the roof, therefore rain water will miss the gutter
Figure 8. Section showing the ridge fixing details
(f) Overlap details
Errors in the end overlaps could occur through the moulding process during production. But even where perfection has been achieved in production, errors in construction could still mean that the end overlaps of FCR are the most vulnerable part of the roof construction. When the fitting is wrongly done one sheet or tile puts excess strain on the other, and this leads to cracks in the material. Sometimes, the overlapping details ignore the direction of the prevailing wind such that the gap between two tiles at the overlap is directly facing the wind and rain thrust. Figure 10 illustrates the importance of overlap details.
One sheet has been damaged by the other due to strain at a poor fitting overlap. The underlap roll of the next sheet, (1) made incorrectly (too high) prevents the overlapping sheet from resting on the purlin causing a gap (2). Then, when the hook holt fixing the sheet (3) is tightened, it bends the left hand sheet, which releases the strain by cracking along the top of the roll.
SCALES OF PRODUCTION FOR FCR TILES
So far, the popularly adopted scale of operation for FCR tiles is based on small-scale manual production technology. However, the range of production scales is not necessarily restricted to the cottage scale. There are at least three basic scales of production:
(a) minimum or cottage scale, which entails an output of approximately 50 tiles per day, entailing a single person operation:
(b) small-scale commercial production, which entails an output of 100 to 200 tiles per day with up to three persons involved in the operation; and
(c) industrial-scale production of up to 400 tiles per day with more than three persons involved. Despite being classified as industrial, the latter production technology is basically the same as the small-scale processes, except for the introduction of basic mechanical handling equipment to increase labour productivity.
Cost efficiency in building materials production is usually, determined by both (he production cost of the material and the cost of transporting the material to the point of use. Sometimes, for certain materials and in circumstances where the local infrastructure for transportation is underdeveloped, the cost of transportation could even be in excess of the production cost. At the small-scale level. FCR production has one important advantage. Production can lake place close to the point of use, so that cost of transportation of the finished item can be almost eliminated. In addition, the simplicity of the technology makes it easy to acquire the requisite skills in a relatively short period of time. The minimal investment requirement is an added advantage.
Figure 9. Alternative built-in fixings
Figure 10. Fixing of sheets
For most building materials, long-term durability is an important criterion for their wide-scale acceptance on the market. While the basic scientific principles justify the durability of FCR, the material has been in use only for about 25 years. Thus, it may have to pass the test of time before it achieves wide-scale adoption. Moreover, like several other low-cost innovative building materials, there are still gaps to be filled in the development cycle - notably, formulation of standards, promotion of quality control measures, effective-processes for technology transfer and, most of all, mechanisms for technology adaptation or improvement within the context of low-cost application of the material.
Note: The readers are invited to refer also to Journal Volume 1 No. 3 and Volume 2 No. 3 in which some other information and case-studies on FCR are included.
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