Fibre and micro concrete
Fibre concrete (FC) is basically made of sand, cement, fibres and water. In the case of micro concrete (MC) fine aggregate is used instead of fibre. It is one of the newest building materials used in low-cost building. However, through intensive research and wide practical experience in many parts of the world, it has become a mature technology.
The types and characteristics of fibre concrete are extremely diverse, depending on the type and quantity of fibre used, the type and quantity of cement, sand and water, the methods of mixing, placing and curing, and - not least - on the skill of production, supervision and quality control.
In the 1960s fibre reinforced concretes, using steel fibre, glass fibre, polypropylene and some other synthetic fibres, were developed and research on them 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. This section, therefore, mainly deals with natural fibre concrete.
Depending on the available resources in different places, a wide range of natural fibres has been tested. These are essentially organic fibres, since the only practical example of a natural inorganic fibre is asbestos. The organic fibres are either of vegetable (cellulose base) or animal origin (protein base).
Vegetable fibres can be divided into four groups:
• Bast or stem fibres (eg jute, flax, hemp, kenaf)
• Leaf fibres (eg sisal, henequen, abaca)
• Fruit hair fibres (coir)
• Wood fibres (eg bamboo, reeds, bagasse).
Animal fibres include hair, wool, silk, etc., but are less recommended if not perfectly clean, as contaminants, such as grease, weaken the bond between fibre and matrix.
A variety of building elements can be made out of natural fibre concrete or micro concrete, but its most widespread application is in the production of Roman tiles and pantiles for roofing. After a few years of experimental work, large-scale applications in low-cost housing projects with FC sheets began in the late 1970s in several countries. However, the results of these field experiences with FC sheets were extremely diverse, ranging from "very satisfactory" to "complete failure" (leaking roofs, breakage of sheets, etc.), creating controversies and uncertainty about the viability of the new technology.
This situation led SKAT (Swiss Centre for Appropriate Technology) to undertake, together with a number of international experts, a systematic evaluation of production experiences in 19 developing countries, resulting in a state-of-the-art report on "FCR - Fibre Concrete Roofing" in 1986 (Bibl. 11.08). The main conclusions of the study were:
• Most failures in FCR production and application were due to the lack of know-how transfer, inadequate professional training, and consequently insufficient quality control.
• The present level of know-how is sufficiently advanced to ensure the provision of good quality and durable roofing, with a minimum life-span of 10 years or more.
• A square metre of FC sheets or tiles can be produced at a cost of 2 to 4 US$ (that is, 4 to 8 US$ for the FC roof including the supporting structure), which is cheaper than any comparable roofing material, but this cost benefit can be completely reversed, if certain minimum standards of production and installation are not observed.
• The fibre content of FCR is required primarily to hold together the wet mix during manufacture, to inhibit drying shrinkage cracking and to provide early strengths until the roof is installed. In normal portland cement matrices, the fibres decay within months or a few years on account of alkali attack. Hence, FCR must be installed and treated with the same care and precautions as for burnt clay materials or unreinforced concrete.
• The main advantage of the technology is that a cheaper, and thermally, acoustically and aesthetically more satisfactory substitute for galvanized corrugated iron (gci) sheeting can be manufactured locally on any desired scale (usually small or medium scale), with a relatively small capital investment and large job creating effect. Compared to asbestos cement (ac) one advantage is the absence of any health risk.
The FCR study also identified the need for a follow-up program to assist and advise potential and existing producers and users of FCR. So, in collaboration with ITDG, GATE and other AT organizations, a Roofing Advisory Service (RAS) was established in 1987, at SKAT, St. Gall. RAS issues manuals and periodicals and generally serves as a clearing house for information and technical assistance on all aspects of fibre and micro concrete roofing.
For a general understanding of the role played by the respective constituent materials, some of the main points are discussed here:
• The main purpose of reinforcing concrete with fibres is to improve its tensile strength and inhibit cracking. While steel and asbestos reinforcements fulfil this function over many years, natural fibres maintain their strength only for a relatively short period (quite often less than a year), on account of their tendency to decay in the alkaline matrix, especially in warm humid environments.
• For many applications (eg roofing), this loss of strength is not necessarily a drawback. The fibres hold together the wet mix, inhibit cracking while it is being shaped and during drying, and give the product sufficient strength to survive transports, handling and installation.
• When the fibres lose their strength, the product is equivalent to an unreinforced concrete. However, by then the concrete will have attained its full strength, and since cracking had been inhibited in the early stages, it might be stronger than a similar product made without reinforcement.
• The same end-strength of the product can be achieved without fibre (MC). However, during manufacture and transport greater care is required.
• The fibre content is generally about 1 to 2 % by weight, never by volume, as fibre densities can vary greatly.
• Fibre concrete products have been produced with long fibres as well as with short (chopped) fibres, both methods having advantages and disadvantages.
• With properly aligned long fibres higher impact resistance and bending strengths are achieved. The method of working several layers of fibre into the concrete, such that each fibre is fully encased in the matrix, is, however, relatively difficult, and thus rarely done.
• In the short fibre method, the chopped fibres are mixed with the mortar, which is easy to handle as a homogeneous mass. Since the fibres are randomly distributed, they impart crack resistance in all directions. The length and quantity of the fibres is of importance, since too long and too many fibres tend to form clumps and balls, and insufficient fibres lead to excessive cracking.
• Extremely smooth and uniform fibres (eg some varieties of polypropylene) that can easily be pulled out, are ineffective. On the other hand, too good a bond of mortar to fibre will result in a sudden, brittle mode of failure, when the fibres fail in tension.
• If methods can be found to overcome the weakening and decay of natural fibres, a wide range of semi-structural applications of natural fibre concrete will be possible, eg hollow beams, stair treads, etc. Therefore, intensive research is being conducted on fibre durability (see BIBLIOGRAPHY).
• Since natural fibre decay is caused by the alkaline pore water in the concrete, it is necessary to reduce the alkalinity. This is achieved by using high alumina cement or replacing up to 50 % of the portland cement with a highly active pozzolana (eg rice husk ash or granulated blast furnace slag). Best results were obtained by adding ultra-fine silica fume (a by-product of the ferro-silicon and silicon metals industries), but this pozzolana is unlikely to be available in most developing countries.
• In order to seal the pore system of the concrete matrix several methods were tested (eg use of higher proportion of fines, lower water-cement ratio, etc.), and interesting results were achieved by adding small beads of wax to the fresh mortar. When the set concrete is heated (eg by the sun), the wax melts and fills the pore system, thus reducing absorption of water which causes fibre decay.
• A vital requirement is that the fibres are free from all impurities, such as grease which interferes with the fibre-mortar bond, and sugar (as on bagasse fibres) which retards the setting of cement.
• The cementitious matrix of the earlier specimens of the composite contained a large proportion of cement (2 parts cement: 1 part sand), which was why it was named "fibre cement". The new generation of mechanically compacted fibre reinforced composites contains only 1 part cement: 1 to 2 parts sand (depending on the quality of cement, therefore the name "fibre concrete" became more appropriate.
• For MC a proportion of 1 part cement, 2parts sand and 1 part aggregate is usually suitable.
• The proportion of cement needs to be higher if the sand is not well graded and if compaction cannot be done by a vibrating machine. For manual compaction by tamping the cement: sand ratio should be 1: 1.
• Ordinary portland cement of the standard quality available in most places is usually suitable. For the production of roofing components, slow setting qualities should be avoided, as they delay demoulding and thus require far more moulds and working space.
• For applications in which the improvement of fibre durability is essential (and slow setting causes no problems), the cement should be partially replaced by a pozzolana (eg rice husk ash). Since the qualities of cement, pozzolana and fibres differ greatly, the proportion of cement replacement should be determined by laboratory tests.
Sand and aggregate
• In order to obtain as small a proportion of voids, angular sand particles of good grain size distribution should be used. The small particles fill the gaps between the large ones, requiring less cement and resulting in a less permeable matrix.
• For FC products only sand between 0.06 and 2.0 mm is used.
• For MC products between 25 and 50% aggregate is used. The maximum grain size should not exceede two thirds of the product's thickness.
• The sand and aggregate should be of silicious origin or have similar characteristics. They should not contain minerals which may react chemically with the cement.
• Fine particles of silt and clay should be reduced as far as possible, as clay interferes with the bond between sand and cement.
• The correct proportion of sand must be determined by sample tests. Too much sand will result in a brittle, porous product. Too little sand means a wastage of the far more expensive cement and a greater tendency to develop cracks on setting.
• In order to safeguard against corrosion of the steel reinforcements, clean drinkable water is used to prepare concrete mixes. In fibre concrete, impurities, such as salts, do not necessarily affect the fibres, and satisfactory results have already been achieved with brackish water. But it is always recommended to use the cleanest available water.
• A correct water to cement ratio is vital for the quality of the product. The tendency is to use too much water because it makes working with the mix easier. Excessive water gradually evaporates, leaving pores which weaken the product and increase its permeability. The correct water to cement ratio is 0.5-0.65 by weight.
• Admixtures may be useful to accelerate or retard setting, or to improve the workability of the fresh mix, but are likely to be expensive and difficult to get. Generally, no additives are needed for FC/MC products, except in cases where fibre durability requires improvement and waterproofing is vital.
• As discussed above (see Fibres), fibre decay can be retarded by reducing the alkalinity of the cement matrix. This is achieved by adding a suitable pozzolana, such as rice husk ash, fly ash or granulated blast furnace slag.
• Reducing the permeability of the product also retards fibre decay. An interesting method (also discussed above) is to add small beads of wax to the fresh mix. In the hardened concrete, the wax melts on heating, forming an impervious film in and around the voids (Bibl. 11.07).
• A variety of other waterproofing agents is also available, and their selection should be governed by availability, cost and effectiveness.
• The colour of FC/MC products can be changed as desired by adding a pigment (in powder form) to the fresh mix, approximately 10 % by volume of the cement for red pigments, but considerably more for other colours. However, pigments are usually more expensive than cement and constitute a significant cost increase in the end product (Bibl. 11.15).
Hydraulic press and drag mould, for the production of corrugated fibre-cement roofing sheets, reinforced with coir fibre or wood wool. In this method, developed at the Central Building Research Institute, Roorkee, India, the cast sheets are kept pressed in the form during the setting period (4 hours), after which they are demoulded and cured in vertical stacks (Photo: K. Mukerji).
• Corrugated roofing sheets and tiles.
• Flat tiles for floors and paving.
• Light wall panels and cladding elements.
• Render for masonry or concrete walls.
• Door and window jambs, window sills, sunshades, pipes.
• Most other non-structural uses.
• A large variety of cheap, locally available natural fibres (even agricultural by-products) can be used.
• If correctly manufactured and applied, FC/MC products can be the cheapest, locally produced durable material.
• The technology is adaptable to any scale of production, right down to one-man production units, as in the case of small-scale pantile production.
• The thermal and acoustical performance of FC/MC roofing is superior to that of gci sheets.
• The alkalinity of the concrete matrix prevents the fibres from being attacked by fungi and bacteria.
• In many developing countries, the limited availability and high price of cement can make FC/MC an inappropriate alternative to other locally produced materials.
• Good quality FC/MC products can only be made by well-trained workers, with great care in all stages of production and with regular and thorough quality control. Without these, failure is almost certain.
• The introduction of this relatively new material faces great reluctance and mistrust, on account of past negative experiences or lack of information.
• Incorrect handling, transportation and installation of FC/MC products can easily develop cracks or break, becoming weak or useless before beginning its service life.
• In areas of limited supplies, the local production and distribution of cement should receive special attention and support, as without the availability of sufficient, standard priced, good quality cement, the FC/MC technology is not viable.
• Know-how transfer in the form of training courses and technical assistance by experienced practitioners is an essential requirement at the outset of every FCR/MCR project (Information available through RAS at SKAT, St. Gall).
• Problems of damage during handling, transports and installation can be reduced by making smaller products. Roofing sheets should not be longer than 1 m, and they should be transported (eg in trucks) standing vertically and tied securely, rather than lying, to avoid breakage.
• FC/MC roofs must be treated like clay tile roofs, and moving on them should not be done without crawling boards.
• The more successful FC/MC applications there are in a country, the greater will be the acceptance of the new technology.
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