| Appropriate building materials |
|Fundamental information on building materials|
Ferrocement is principally the same as reinforced concrete (RCC), but has the following differences:
• Its thickness rarely exceeds 25 mm, while RCC components are seldom less than 100 mm.
• A rich portland cement mortar is used, without any coarse aggregate as in RCC.
• Compared with RCC, ferrocement has a greater percentage of reinforcement, comprising closely spaced small diameter wires and wire mesh, distributed uniformly throughout the cross-section.
• Its tensile-strength-to-weight ratio is higher than RCC, and its cracking behaviour superior.
• Ferrocement can be constructed without formwork for almost any shape.
Ferrocement is a relatively new material, which was first used in France, in the middle of the 19th century, for the construction of a rowing boat. Its use in building construction began in the middle of the 20th century in Italy. Although its application in a large number of fields has rapidly increased all over the world, the state-of-the-art of Ferrocement is still in its infancy, as its long-term performance is still not known.
• The essential ingredients of the mortar which represents about 95 % of ferrocement are portland cement, sand, water, and in some cases an admixture.
• Most locally available, standard cement types are suitable, but should be fresh, of uniform consistency and without lumps or foreign matter. Special cement types are needed for special uses, eg sulphate-resistant cement in structures exposed to sulphates (as in seawater).
• Only clean, inert sand should be used, which is free from organic matter and deleterious substances, and relatively free from silt and clay. Particle sizes should not exceed 2 mm and uniform grading is desirable to obtain a high-density workable mix. Lightweight sands (eg volcanic ash, pumice, inert alkali-resistant plastics) can also be used, if high strengths are not required.
• Fresh drinking water is the most suitable. It should be free from organic matter, oil, chlorides, acids and other impurities. Seawater should not be used.
• Admixtures can be used for water reduction, thus increasing strength and reducing permeability (by adding so-called "superplasticizers"); for waterproofing; for increased durability (eg by adding up to 30 % fly ash); or for reduced reaction between mortar and galvanized reinforcements (by adding chromium trioxide in quantities of about 300 parts per million by weight of mortar).
• The recommended mix proportions are: sand/cement ratio of 1.5 to 2.5, and water/cement ratio of 0.35 to 0.5, all quantities determined by weight. For watertightness (as in water- or liquid-retaining structures) the water/cement ratio should not exceed 0.4. Great care should be exercised in choosing and proportioning the constituent materials, especially with a view to reducing the water requirement, as excessive water weakens the ferrocement.
• The reinforcing mesh (with mesh openings of 6 to 25 mm) may be of different kinds, the main requirement being flexibility. It should be clean and free from dust, grease, paint, loose rust and other substances.
• Galvanizing, like welding, reduces the tensile strength, and the zinc coating may react with the alkaline environment to produce hydrogen bubbles on the mesh. This can be prevented by adding chromium trioxide to the mortar.
• The volume of reinforcement is between 4 and 8 % in both directions, ie between 300 and 600 kg/m3; the corresponding specific surface of reinforcement ranges between 2 and 4 cm2/cm3 in both directions.
• Hexagonal wire mesh, commonly called chicken wire mesh, is the cheapest and easiest to use, and available almost everywhere. It is very flexible and can be used in very thin sections, but is not structurally as efficient as meshes with square openings, because the wires are not oriented in the principal (maximum) stress directions.
• Square welded wire mesh is much stiffer than chicken wire mesh and provides increased resistance to cracking. However, inadequate welding produces weak spots.
• Square woven wire mesh has similar characteristics as welded mesh, but is a little more flexible and easy to work with than welded mesh. Most designers recommend square woven mesh of 1 mm (19 gauge) or 1.6 mm (16 gauge) diameter wires spaced 13 mm (0.5 in) apart.
• Expanded metal lath, which is formed by slitting thin gauge sheets and expanding them in the direction perpendicular to the slits, has about the same strength as welded mesh, but is stiffer and hence provides better impact resistance and better crack control. It cannot be used to make components with sharp curves.
• Skeletal steel, which generally supports the wire mesh and determines the shape of the ferrocement structure, can be smooth or deformed wires of diameters as small as possible (generally not more than 5 mm) in order to maintain a homogenous reinforcement structure (without differential stresses). Alternatively, skeletal frameworks with timber or bamboo have been used, but with limited success.
• Fibres, in the form of short steel wires or other fibrous materials, can be added to the mortar mix to control cracking and increase the impact resistance.
• The first step is to prepare the skeletal framework onto which the wire mesh is fixed with a thin tie wire (or in some cases, by welding). A minimum of two layers of wire mesh is required, and depending on the design, up to 12 layers have been used (with a maximum of 5 layers per cm of thickness).
• The sand, cement and additives are carefully proportioned by weighing, mixed dry and then with water. Hand mixing is usually satisfactory, but mechanical mixing produces more uniform mixes, reduces manual effort and saves time. The mix must be workable, but as dry as possible, for greater final strength and to ensure that it retains its form and position between application and hardening.
• After checking the stability of the framework and wire mesh reinforcement, the mortar is applied either by hand or with a trowel, and thoroughly worked into the mesh to close all voids. This can be done in a single application, that is, finishing both sides before initial set takes place. For this two people are needed to work simultaneously on both sides.
• Thicker structures can be done in two stages, that is, plastering to half thickness from one side, allowing it to cure for two weeks, after which the other surface is completed.
• Compaction is achieved by beating the mortar with a trowel or flat piece of wood.
• Care must be taken not to leave any reinforcement exposed on the surface, the minimum mortar cover is 1.5 mm.
• Each stage of plastering should be done without interruption, preferably in dry weather or under cover, and protected from the sun and wind. As in concrete construction, ferrocement should be moist cured for at least 14 days.
• Boat construction (one of the most successful uses. especially in China).
• Embankment protection, irrigation canals, drainage systems.
• Silos (above ground or underground) for storage of grain and other foodstuffs.
• Water storage tanks, with capacities up to 150 m3.
• Septic tanks and aqua privies, and even complete service modules with washing and toilet facilities.
• Pipes, gutters, toilet bowls, washbasins, and the like.
• Walls, roofs and other building components, or complete building, either in situ or in the form of precast elements.
• Furniture, such as cupboards, tables and beds, etc. and various items for children's playgrounds.
Some Applications of Ferrocement
Furniture, sanitary units, roofing elements at the Structural Engineering Research Centre, Madras, India.
Latrine squatting slab, washing basin, toilet flush cistern and water tank (made of 5 square elements, assembled on site) at the Housing & Building Research Institute, Dhaka, Bangladesh.
• The materials required to produce ferrocement are readily available in most countries.
• It can take almost any shape and is adaptable to almost any traditional design.
• Where timber is scarce and expensive, ferrocement is a useful substitute.
• As a roofing material, ferrocement is a climatically and environmentally more appropriate and cheaper alternative, to galvanized iron and asbestos cement sheeting.
• The manufacture of ferrocement components requires no special equipment, is labour intensive and easily learnt by unskilled workers.
• Compared with reinforced concrete, ferrocement is cheaper, requires no formwork, is lighter, and has a ten times greater specific surface of reinforcement, achieving much higher crack resistance.
• Ferrocement is not attacked by biological agents, such as insects, vermin and fungus.
• Ferrocement is still a relatively new material, therefore its long-term performance is not sufficiently known.
• Although the manual work in producing ferrocement components requires no special skills, the structural design, calculation of required reinforcements and determination of the type and correct proportions of constituent materials requires considerable know-how and experience.
• Galvanized meshes can cause gas formation on the wires and thus reduce bond strength.
• The excessive use of ferrocement for buildings can create unhealthy living conditions, as the high percentage of reinforcement has deleterious electromagnetic effects.
• Research on the condition of older ferrocement structures.
• Development of simple construction guidelines and rules of thumb which can be applied without special technical knowledge.
• Galvanized mesh can be immersed in water for 24 hours and then dried for 12 hours, in order to allow the salts used during galvanizing to come to the surface. The residue can then be brushed off.
• Problems with galvanized mesh can be reduced by adding chromium trioxide to the mixing water.
• Complete enclosure of dwelling units with ferrocement components (ie for floor, walls and roof) should be avoided.