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Ferrocement: Applications in Developing Countries – Part 2 of 3
| By pinoyfarmer | July 30, 2007 | 
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IV. Ferrocement for Food-Storage Facilities
The problem of food storage in the developing countries is emerging as a major subject of attention from technical assistance organizations. Increasing supplies of food grains, such as rice, wheat, and maize, resulting from the Green Revolution have caused an unprecedented need for grain storage in developing countries, yet most production areas are still unprepared to store this new abundance adequately. Figures in the order of 25 percent indicate how much grain is lost to inadequate harvest and inadequate storage facilities and practices. In addition to grain storage, facilities are urgently needed to protect all products sensitive to temperature, humidity, rain, wind, pest animals, bacteria, or fungi. Other typical products requiring storage are peas and beans; oil crops such as peanuts and soybeans; salt; drinking water; and related nonfood items such as fertilizers, pesticides, and cement. Major needs are small-scale silos, particularly for on-farm storage.
A particular advantage of ferrocement in building food-storage facilities in developing countries is its adaptability to an almost unlimited range of curved shapes and local conditions. Ferrocement silos require little maintenance, and they offer protection against rodents, birds, insects, water, and weather. Ferrocement is watertight, and, with appropriate sealants, it can also become airtight (see Appendices B and C). In an airtight ferrocement bin, respiration of grain, or similar products, quickly removes oxygen from the atmosphere inside and replaces it with carbon dioxide.* Any insects (adults, larvae, pupae, or eggs) or aerobic microorganisms present cannot survive to damage the stored product. No fumigation is needed. Hermetic storage of this kind is “particularly suitable in the tropics for the storage of dry grain.”
Methods developed for ferrocement boatbuilding can be applied to storage facilities to yield a structure of high quality. Ferrocement silos could be built in a factory, but they are particularly adaptable to on-site construction, an important consideration in remote areas without even vehicular access. As with other applications, silos require only simple artisan skills, performed by local labor with minimal supervision. In Thailand, workmen with experience acquired in the construction of a single silo have been able to supervise unskilled laborers. In many developing areas, building a food-storage facility with ferrocement is not very different from building a traditional one. In principle, the wire mesh is a substitute for bamboo or wattle mesh, and the cement mortar is a substitute for mud. Indeed, bamboo, instead of wire mesh, reinforcement might be technologically feasible if care is taken to avoid delamination caused by expansion of bamboo when it absorbs moisture.
Ferrocement can be considered for silos with curved walls, in sizes to hold 1-30 tons of grain or more. The base can be continuous with the walls, making a strong, monolithic construction (as in ferrocement boats) to prevent foundation failures and moisture damage from floods or a high watertable in the soil. Silos can be easily sealed against air or water vapor with a rubber- or bitumen-based paint.
Appendix B contains photographs of ferrocement silos developed and tested by a government research institute in Thailand for the special needs of a humid, tropical country. Bins with capacities of 4-10 tons all have sloped walls of the same height, with base and top diameters that vary in size. For larger capacity, the Thais built clusters of bins, though larger bins and connected side-by-side modifications are under consideration. Clusters have the advantage that only one bin at a time need be opened to withdraw the product while the low-oxygen atmosphere is maintained in the others. (See also Figure 1 1.)
In contrast to the above-ground Thai silo, an underground ferrocement storage unit has been developed in Ethiopia to replace the traditional unlined storage pits (see Appendix C). Also, it is reported that a similar underground storage system “has gained wide acceptance in Latin America where several millions of tons of produce are stored in these hermetic underground pits.
Where grain and similar food crops can be stored with confidence, banks can lend money for construction of silos, using the crop as mortgage collateral. Traditionally, farmers borrow money (often at high interest rates) to plant their crops, then sell the product at harvest time-when its price is lowest-to pay their debts. A silo that enables a farmer to store his harvest gives him the opportunity to sell in the off-season, usually 4-6 months after harvest, when prices may increase 40-100 percent. The farmer can also store bulk food and seed for his family’s needs without the loss he faces by using traditional methods.
Appendix D describes a well-developed industry in New Zealand, building water-storage tanks of ferrocement. Figures 8 and 15 show other food-storage applications.
V. Ferrocement for Food-Processing Equipment
During the panel’s deliberations on food-storage facilities, the more inclusive category of food-processing equipment as a whole emerged as an exciting possibility for ferrocement application. New to the panelists, the idea has not, to their knowledge, been studied at all in developing countries-and only slightly studied elsewhere. Though data are lacking, the subject is included in this report because the panel considers it a potentially profitable area for research.
The world food problem is caused in part by poor distribution and protection of available foodstuffs in developing countries. Most foods, especially in a raw, unprocessed state, are highly perishable; they are irreversibly affected by temperature changes and, especially, by even trace amounts of biological and chemical contaminants. All these problems are most severe in tropical climates. So, in developing countries much of the food yield deteriorates soon after harvesting because processing plants to preserve food are lacking in rural areas. Excessive costs, the absence of a suitable construction material, and lack of skilled labor prohibit the manufacture of even simple, conventional processing equipment designed to store, convey, and process bulk quantities of complex natural raw materials and their derivatives.
That most food-processing equipment is large, heavy, and awkwardly shaped constrains regions where transportation is difficult and expensive. The general lack of foreign currency in developing countries makes it difficult to pay for this equipment, which is made of steel, copper, and other metals often available only from industrialized countries.
The use of ferrocement, even on a modest scale, could influence the creation or expansion of food industries in developing countries and contribute to the improved nutrition of the inhabitants. Where transportation is difficult, ferrocement equipment can be manufactured and erected on site, by local labor, and with easily transported ingredients. It requires little foreign currency, and is uniquely suited to the fabrication of large, heavy, awkwardly shaped shell structures. It can be as strong and structurally rigid as the structures it imitates.
Extensive preliminary laboratory research is needed, particularly into the interface between a ferrocement surface and the foodstuffs it touches. This surface must be made extremely smooth, dense, and hard (for example, by techniques of multiple trowelling during the setting period). The use of coatings should be explored, such as stainless-steel foil bonded to the surface.
In addition, research must answer these questions:
· What is ferrocement’s ability to meet local sanitary requirements? What methods can be used for cleaning and sterilizing?
· What are its pressure and thermal tolerances (heat transfer, thermal expansion)?
· What is the moisture-vapor transmission rate (particularly important for low-temperature applications)?
Nevertheless, a serious, large-scale effort is justifed to investigate the use of ferrocement to replace steel for the manufacture of at least some basic food-processing equipment, e.g., tanks (see Figure 12), vats, pipes, trays, drying tables, cold stores and freezing chambers,* ovens, waste-product sewage treatment facilities, butchering facilities,* and dairies.
VI. Ferrocement for Low- Cost Roofing
Rapid population growth and industrial development have created overwhelming demands on human settlements. A still greater burden will fall on cities, towns, and rural communities in the future. According to a recent U.N. estimate, the world population will double by the year 2000 to nearly 7 billion people,* while the world urban population will increase to more than 3 billion, or 51 percent of the total world population. The size of future housing requirements alone is staggering: during 1970-1980, Asia, Africa, and Latin America will need housing for 325 million people entering urban areas, at the rate of 90,000 people per day.** And these figures do not include the vast number of rural dwellings and new or modernized work places and public facilities that will be needed.
Developing countries already have acute housing shortages because of rapid population growth and, sometimes, disasters. Typical examples are Ceylon, 200,000 houses short; India, 11.9 million; Philippines, 3 million; Republic of (South) Korea, I million houses short in 1970 with demand continuing to grow at over 100,000 houses per year.
To these needs for basic housing to accommodate population increases and to improve housing quality must be added the periodic necessity to replace housing destroyed by natural disasters prevalent in the developing world. Earthquakes, typhoons, hurricanes, cyclones, floods, and fire take a vicious toll of tens of thousands of dwellings each year, as in Bangladesh, Peru, and Nicaragua.
Of the desperately needed new materials and construction methods, the most critical component is appropriate roofing. Under normal conditions in developing countries the roof of a dwelling structure constitutes the major expense, often as much as 60 percent of the total cost. For most people a long-lasting roof is too expensive. Yet, most roofs manufactured from cheaper local materials such as grass or reeds (thatch) or earth products (sand, mud, rock) are short-lived and dangerous in an earthquake, flood, or fire. Thatch is notorious for harboring vermin and insects. Furthermore, an adequate roof covering is often impractical because it needs a high-cost supporting structure. For instance, tiles make excellent roofs, but they are so heavy they require extensive supporting frames. In many regions, wooden supports decay rapidly, though the covering remains sound.
To satisfy their shelter needs, many developing countries expend scarce foreign exchange for galvanized iron and other metal roofings from Industrialized countries. However purchased, bulky metal sheets are expensive to transport within a country. In hot climates the heat absorption of metal roofs converts homes into ovens. Corrosion is also a problem, particularly where the metal is exposed to saltwater spray.
The previously described advantages of ferrocement for developing countries apply to roofing. Ferrocement appears to have decided advantages over several other roofing materials and could well play a major role in housing construction in developing countries. (See Figures 13-l S.)
Ferrocement roofing materials can be factory mass-produced in prefabricated form, a process best suited to the concentrated demand of urban areas. Though it might be more economical to mass-produce roofing in an urban factory and truck it to a rural area (should trucking be possible), ferrocement is also easily fabricated on site in rural areas, using local labor and materials.
Freer in concept and makeup than most conventional roofing, ferrocement can be shaped into domes, vaults, extruded shapes, flat surfaces, or free-form areas.
Before ferrocement can be used widely for roofing, research and experiments will be required to determine the shapes and types of roofing members to be manufactured, and to explore designs and methods for anchoring and bolting these various shapes to supporting walls.
After this research and experimentation is completed, on-thejob training centers may be required to introduce the new material and its new building techniques. Preferably, these centers should also offer programs dealing with other ferrocement applications.
Research efforts to find ferrocement modifications that prove less expensive or easier to manufacture are highly recommended. Possibly, for example, ferrocement can be sandwiched on two sides of a core of foam concrete or other lightweight material to make a less expensive and lightweight, yet still structurally strong, material.
VII. Ferrocement Materials Technology
A working definition of ferrocement is “a thin shell of highly reinforced portland cement mortar.” Generally, ferrocement shells range from 1/2 inch to 2 inches in thickness, and the reinforcement consists of layers of steel mesh, usually with steel reinforcing bars sandwiched midway between. The resulting shell or panel of mesh is impregnated with a very rich (high ratio of cement to sand) portland cement mortar. (Other hydraulic cements may also be used.)
Specifications of ferrocement technology range widely according to use-from oceangoing vessels in which human lives are totally dependent on the material, to small, expendable household items. Although this chapter deals with ferrocement materials science in general, in practice the quality of the ferrocement used must be matched with the end use of the product.
REINFORCING MESH
Many different kinds of reinforcing mesh will produce successful ferrocement structures. (See Figures 16, 17.) A general requirement is flexibility. Shapes with tight curves need more flexible meshes. Chicken wire, the cheapest and easiest to use, is adequate for the structural requirements of most boats in developing countries and for all uses on land. It is not the most recommended mesh for high-performance structures, such as deep-water marine hulls.
The wire mesh could be woven on site from coils of straight wire, giving a local engineer greater opportunity to adapt the mesh size and wire diameter to any given job. Because wire coils are less bulky than mesh, this method might also save considerably on transportation costs (both ocean shipping and internal trucking costs). With less wire surface exposed to air, this method may, under corrosive tropical conditions, reduce deterioration during storage. A simple handloom could be adapted for weaving the wire into mesh.
For most purposes, the mesh need not be welded. Nongalvanized wire is excellent, though it will rust if stored in the open too long. Standard galvanized meshes (galvanized after weaving) are adequate.
Figure 17. Types of reinforcing mesh commonly used for ferrocement (R.B. Williamson, University of California, Berkeley)
CEMENT, SAND, AND WATER
The quality of cement used is not too critical. Ordinary Type 1 or 2 portland cement is adequate; grades for more specific purposes are unnecessary even for boatbuilding. Grading the sand is seldom important, except to improve mortar workability. Current experience indicates that volcanic sands and beach sands are adequate, but sand should not have an excess of fine particles. Experiments need to be made in using coral sand as a substitute for regular sand, which is not readily available in some areas. Organic debris and silt that will not bond to the mortar reduce the strength of the ferrocement and should be washed out. Water containing these impurities should also be filtered and purified; otherwise, water quality is not critical in general practice.
CONSTRUCTION
The three major problem areas in ferrocement construction are mortar mixing, mortar application, and curing. The mortar must be dense and compact. A trained supervisor can teach the mixer operator to judge mortar quality from the way it tumbles or rolls off the mixer blades. A general mixis 1 part cement, 2 parts sand. Water is added to give the required pastelike consistency (roughly 0.4 parts water by weight). A horizontal, paddle-bladed mixer is recommended for highest-quality mixing; it is critical for deep-water boats. For land uses, experience shows that hand-mixing is also satisfactory. Determining the cement-to-water ratio can be done with adequate accuracy by observing the mortar’s consistency. Sand normally does not have a fixed moisture content; even in the same sandpile, the bottom layers tend to be more wet than the upper ones.
Fingers and trowels are used for mortar placement in the mesh structure. Mortar guns are not recommended because the heavier parts of the mortar (i.e., sand) tend to separate out. A certain amount of vibration helps to produce complete mortar penetration of the mesh and assure good compaction. An orbital sander (a simple power tool used widely in woodworking) with a metal plate substituted for the sandpaper pad has been found to provide the correct amount of vibration; the vibration is localized, so already-placed mortar is not shaken out of the mesh. It is also possible to create enough vibration by using a piece of wood with a handle attached, though this not recommended for building deep-water boats.
Finally, certain conditions for adequately curing the mortar are essential. The warmth and humidity of most tropical regions is conducive to the rapid curing of ferrocement, but ferrocement must not be exposed to excessive drying action of the elements. It should be kept moist at least 7 days and protected from the sun and wind, both of which reduce the concrete’s strength by drying out surface moisture.
NOTE FOR ARCHITECTS AND ENGINEERS
As in the case of conventional reinforced concrete, the mechanical properties of ferrocement depend to a large extent on the properties of the cementitious matrix and the reinforcing steel. The apparent tensile properties of ferrocement represent a significant departure from that of ordinary reinforced concrete in that the dispersed reinforcement changes the observed cracking pattern. At a microscopic level the cementitious matrix is responding in the same way, but at the macroscopic level the first tension cracks generally appear at stress levels higher than for unreinforced mortar.
The setting of portland cement is the basic reaction in the fabrication of ferrocement. This setting process is identical to that of hardening conventional concrete, but special precautions must be taken if high levels of performance are expected. To produce an impermeable thin shell, for example, the mortar must have a low water-to-cement ratio. A proper moist-cure period is also imperative. Both of these ideals are readily appreciated by engineers and architects, but it may take special attention to achieve them in the field.
Figure 20. The three stages of typical stress-strain curve for ferrocement (Walkus, I.R. [Lodz Technical Univ., Poland], and T.G. Kowalsky [Hong Kong Univ.], “Ferrocement: A survey.” Concrete [London]. Vol. 5, No. 1, Feb. 1971)
Figure 20 shows a typical stress-strain curve for ferrocement. In stage I the material behaves in a linearly elastic manner with both the reinforcement and the matrix deforming elastically. Then, as the load increases, the cementitious matrix cracks, and stage II begins where there is a change of slope in the stress-strain curve. It has been shown that the stress at the first crack can be increased by increasing the surface area of the steel exposed to the cement, by decreasing the diameter of the wire, by increasing the volume of reinforcement. These cracks are very fine and can be seen only by special lighting effects or microscopic investigation. For most purposes, the materials are unchanged by loading into this region, which constitutes ferrocement’s practical working limit. Finally, stage III corresponds to the latter stages of deformation where the full load is being carried by the reinforcement. The stress limit of stage III can be predicted by considering the maximum load-carrying capacity of the steel reinforcement alone.
To put the mechanical properties into perspective, it is important to keep in mind that there is a transition from the characteristic behavior of ferrocement to that of conventional reinforced concrete and that much of the use of ferrocement in developing countries probably will fall on or near this transition. One of the important objectives in the future development of ferrocement will be a rational design system to cover the response of the structure to normal conditions, as well as the ultimate behavior of the structure. Engineering research is needed in this area.
The influence of the water-cement ratio on porosity has a great effect on the shrinkage, strength, and permeability of the final product. However, the practical upper limit of water-cement ratio for ferrocement depends on the acceptable value of permeability, since it is clear from Figure 21 that ferrocement made from mortar with a water-cement ratio of more than about 0.6 has a very high permeability.
Figure 21. Relationship between permeability and water-to-cement ratio (weight basis) for mature portland cement pastes (cement hydrated 93%). (R.B. Williamson, University of California, Berkeley)
The primary requirement for making waterproof mortar is tight control of the water/cement ratio, with the workability obtained by the gradation and quantity of sand as well as by the optional use of certain admixtures. This is also the prescription for making high-quality conventional concrete. Ferrocement is not as forgiving of careless practices as conventional concrete, and in the field it demands new degrees of control, compared to the simplicity of poured-concrete techniques.
Applying the mortar and ensuring that it penetrates the layers of mesh without leaving air pockets-a problem in ferrocement construction-is a particularly severe problem in boatbuilding.
Because ferrocement reinforcing has a somewhat different purpose from that of conventional reinforced concrete, these two considerations apply:
1. Adequate cover to protect the steel from corrosion is necessary because in almost every application of ferrocement, the durability and resistance to environmental effects depend on the thin mortar cover over the steel mesh and its ability to protect the easily corroded steel mesh.
2. It is desirable to have the mesh as near the surfaces as possible.
In a thin shell of ferrocement these considerations conflict; therefore, it is necessary to use a mesh of high-specific surface area (small-diameter wires) in the outer layers, and to use the lowest possible water-cement ratio to achieve the lowest permeability and greatest protection from reinforcement corrosion.
NOTE. Seawater places extra demands on ferrocement. Boats for marine use must be plastered with a cement resistant to sulfate attack. The surface should also be coated with paint or another sealant to further decrease saltwater penetration.
Related Posts:
Ferrocement: Applications in Developing Countries – Part 1
Ferrocement: Applications in Developing Countries – Part 3
Source: Ferrocement: Applications in Developing Countries (BOSTID, 1973, 89 p.)
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