Background Information[edit | edit source]
Ferrocement is a term commonly used to describe a steel-and-mortar composite material. Essentially a form of reinforced concrete, it exhibits behavior so different from conventional reinforced concrete in performance, strength, and potential application that it must be classed as a completely separate material. It differs from conventional reinforced concrete in that its reinforcement consists of closely spaced, multiple layers of steel mesh completely impregnated with cement mortar. Ferrocement can be formed into sections less than 1 inch thick, with only a fraction of an inch of cover over the outermost mesh layer. Conventional concrete is cast into sections several inches thick with an inch or so of concrete cover over the outermost steel rods. Ferrocement reinforcing can be assembled over a light framework into the final desired shape and mortared directly in place, even upside down, with a thick mortar paste. Conventional concrete must be cast into forms.
These fairly simple differences lead to other, more remarkable differences. Thin panels of ferrocement can be designed to levels of strain or deformation, with complete structural integrity and water tightness, far beyond limits that render conventional concrete useless. Ease of fabrication makes it possible to form compound shapes with simple techniques; with inexpensive materials; and, if necessary, unskilled (but supervised) labor.
HISTORY OF FERROCEMENT
The most extensively used building medium in the world today is concrete and steel combined to make reinforced concrete; familiar uses are in high-rise buildings, highway bridges, and roadways. Yet, the first known example of reinforced concrete was a ferrocement boat. Joseph-Louis Lambot's original French patents on wire-reinforced boats were issued in 1847 not long after the development of portland cement. (See Figures 6, 7.) This was the birth of reinforced concrete, but subsequent development differed from Lambot's concept. The technology of the period could not accommodate the time and effort needed to make mesh of thousands of wires. Instead, large rods were used to make what is now called standard reinforced concrete, and the concept of ferrocement was almost forgotten for a hundred years. Reinforced concrete developed as the material familiar today in fairly massive structures for which formwork to hold the fresh concrete in the wide gaps between reinforcing rods and a fairly thick cover over the rods nearest the surface are required.
Reinforced concrete for boatbuilding reappeared briefly during the First World War, when a shortage of steel plates forced a search for other boatbuilding materials. The U.S. and U.K. governments, among others, commissioned shipbuilders to construct seagoing concrete ships and barges, some of which continued in use after the war. The same phenomenon occurred in the United States during the Second World War. However, the conventional use of large-diameter steel rods to reinforce the concrete required thick hulls, making the vessels less practical to operate than lighter wood or steel ships.
In the early 1940's, Pier Luigi Nervi resurrected the original ferrocement concept when he observed that reinforcing concrete with layers of wire mesh produced a material possessing the mechanical characteristics of an approximately homogenous material and capable of resisting high impact. Thin slabs of concrete reinforced in this manner proved to be flexible, elastic, and exceptionally strong. After the Second World War, Nervi demonstrated the utility of ferrocement as a boatbuilding material. His firm built the 1 65-ton motor sailer Irene with a ferrocement hull 1.4 inches (3.6 ems) thick, weighing 5 percent less than a comparable wood hull, and costing 40 percent less. The Irene proved entirely seaworthy, surviving two serious accidents. Other than simple replastering necessitated by the accidents, the hull required little maintenance.
Despite this evidence that ferrocement was an adequate and economical boatbuilding material, it gained wide acceptance only in the early 1960's in the United Kingdom, New Zealand, and Australia. In 1965, an American-owned ferrocement yacht built in New Zealand, the 53-foot Awahnee, circumnavigated the world without serious mishap, although it encountered 70-knot gales, collided with an iceberg, and was rammed by a steel-hulled yacht. Other ferrocement boats have shown similar practicality, and their number is steadily increasing.
Recent emphasis on ferrocement as a boatbuilding material has obscured Nervi's noteworthy applications to buildings. He built a small storehouse of ferrocement in 1947 (Figure 15). Later he covered the swimming pool at the Italian Naval Academy with a 50-foot vault and then the famous Turin Exhibition Hall-a roof system spanning 300 feet. In both ferrocement is one of the structural components; the ribs and outer surface are reinforced concrete (as in Figure 8).
Nervi's work and subsequent applications presage an application of ferrocement on land that may overshadow the fresh-water applications.
CHARACTERISTICS OF FERROCEMENT
Ferrocement is a high-quality structural material whose simple constituents and formation make it usable for many construction purposes in even the most underdeveloped societies. In no way an inferior product specifically for cheap uses, it is in some respects more sophisticated than prestressed concrete. Ferrocement usually uses a freestanding frame of wire mesh that is mortared in place on site. The wire mesh is formed into the desired shape (domes, simple curves, or compound curves). Supporting framework used to outline the shape can be wood, precast concrete, or a simple jig made from steel rods or pipes. These supports are usually very rudimentary and serve only to outline the shape for the layers of wire mesh to be added next. They can eventually be removed or left in place to become part of the final structure.
The economy of ferrocement construction, compared with steel, wood, or glass-fiber reinforced plastic (FRP), depends greatly on the product being built, but ferrocement is almost always competitive, particularly in tropical developing countries where steel is expensive, frequently drains foreign exchange reserves, and requires sophisticated facilities and skilled operators. FRP is much more costly, creates a fire hazard, requires advanced technology, sophisticated materials, and skilled labor; and its ingredients are sensitive to tropical temperatures. Wood is almost nonexistent in many arid or deltaic countries. Even heavily forested countries such as Indonesia, the Philippines, and Thailand foresee serious shortages due to growing demands of an increasing world population. Furthermore, in the tropics wood is subject to rot, insects, and teredos.
The relatively low unit cost of materials may be the greatest virtue of ferrocement. Worldwide, the costs of sand, cement, and wire mesh vary somewhat; but the greatest variable in construction costs is the unit cost of labor. In countries with high-cost labor, the economics of ferrocement often make it noncompetitive. But, according to UNIDO, experience has shown that where unskilled, low-cost labour is available and can be trained, and as long as a standard type of construction is adhered to, the efficiency of the labour will improve considerably, resulting in a reduced unit cost. Under these conditions, ferrocement compares more than favourably with other materials used in boatbuilding, such as timber, steel, aluminum or fibreglass, all of which have a higher unit material cost and require greater inputs of skilled labour.*
SUITABILITY TO DEVELOPING COUNTRIES
Although the increased interest in ferrocement for water and land use is fairly recent, successful examples of innovative applications, within a wide range of construction techniques and sophistication, already promise a major impact on developing countries for the following reasons:
- Ferrocement may be fabricated into almost any conceivable form to meet the particular requirements of the user. This is particularly pertinent where acceptance of new materials may be dependent on their ability to reproduce traditional designs.
- The basic raw materials for the construction of ferrocement-sand, cement, and reinforcing mesh-are readily available in most countries. Sand and cement are used in building and road construction, and mesh is used in agriculture (chicken netting) and housing construction (plastering lath).
- Except for highly stressed or critical structures such as deep-water vessels, adequate ferrocement construction does not demand stringent specifications. A wide range of meshes can be used; both hexagonal and square meshes have produced successful structures. The cement is of standard quality used in building construction. Special grades are unnecessary.
- Little new training is required for the laborers, providing a skilled supervisor is on hand. Cement construction techniques are widely known in developing countries, and indigenous construction workers often show a good aptitude for plastering. (See Figures 9, 10.)
- Transportation, logistics, and materials-handling are serious problems in developing countries, and ferrocement construction simplifies each one. Sand and water can usually be obtained in the region of the building site; and the quantity of cement normally required can be easily transported. Only the wire mesh may require transportation from distant production centers. Under extremely difficult conditions (such as in the roadless highlands of Nepal), wire mesh may be handloomed on site from reels of straight wire, a technique apparently already in use in rural areas of the People's Republic of China. (See Appendix A.) For simple, indigenous-type boat hulls and agricultural or construction uses, no well-developed or centralized building site is required (though it is an option for a builder). Construction can well be done on site at the riverbank, in the village, high in the mountains, or wherever needed.
- Ferrocement withstands severe abuse. Authenticated reports tell of boat hulls wrecked on reefs and successfully surviving savage poundings. Afterwards, the ferrocement was easily and rapidly repaired on site. Only simple tools are needed to repair any damage to the mesh and only cement and sand to make a fresh mortar. Such repairs are usually good for the remaining life of most ferrocement products, though the more stringent requirements of deep-water boats may dictate that the repair be reworked by skilled labor.
This report explores these advantages in land and water uses, and summarizes the basic material properties of ferrocement. Appendices contain descriptions of specific applications.
