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Ferro-cement OR Ferrocement

According to ACI (American concrete Institute), Ferrocement is a type of thin wall reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small size wire mesh. The mesh may be made of metallic or other suitable materials.
  • Ferrocement differs from conventional concrete in that there is a higher ratio of steel to cement mortar.
  • Ferrocement has many properties of steel and yet it will not rust. Although it looks and feels like concrete it can flex without cracking.
  • By altering the steel/cement ratio we actually produce a material, which is superior to either steel or cement seperately.

History of ferro-cement

  • In 1940s, an Italian engineer Pier Luigi Nervi tested and presented in his paper, a new structural elements, an extremely thin plate of concrete made layers of small diameter wire mess and cement mortar with sand is used.
  • In 1980, ACI gives standards of use and mixes  of ferro-cement.

Properties of Ferro-cement

  • High tensile strength and stiffness.
  • Better impact and punching shear resistance.
  • Undergo large deformation before cracking or high deflection
  • Low weight
  • Low water cement ratio produces impermeable structures.
  • It is very durable, cheap and versatile material.

Material used in Ferrocement

  1. Cement Mortar
  2. Reinforcement ( skeleton steel/wire mesh)

1) Cement Mortar

  • Ordinary portland cement and fine aggregate matrix is used.
  • Particle size sand should not more than 2.36mm and less than 1.18mm.
  • More than 2.36mm particle can make the mortar porous and particles less than 1.18mm requires more water to impermeable structure.
  • Sand occupies 60% to 75% of the volume of the mortar.

2) Skeleton Steel

  • The skeleton steel frame is made conforming exactly to shape of the structure and the geometry.
  • The steel rods is spaced at 70 to 100mm apart and diameter may vary from 3mm to 8mm.
  • This is used for holding the wire mesh in shape of the structure and position.
  • In case where higher stresses may occur, the mild steel bars are used.

3) Wire Mesh

  • The wire mesh consists of galvanized wire spaced at 6mm to 20mm centre to centre and 0.5 to 1.5mm diameter.
  • The wire mesh may be of the shape as welded wire mesh or square woven wire mesh, Hexagonal wire or mesh expanded metal etc.
  • The yield strength of plain wires used in fabric should not be more than 415Mpa and 500Mpa for deformed bars.
  • The steel content may vary between 300kg to 500kg per cubic metre of mortar.

Techniques of Manufacturing

  1. Hand plastering
  2. Semi mechanised process
  3. By centrifuging
  4. Guniting

1) Hand plastering

  • In this method the control thichkness is difficult and the minimum thickness of section that can be casted works out to be 2cms.
  • The strength obtained by this system will be lower compared to other methods.
  • Units can be cast by this process and used in situations where for other improved methods do not exist.
  • These units can be used for pipes, storage structures and gasholder unit.

2) Semi mechanised process

  • This system is termed as semi mechanised because the mould can be rotated to facilitate dashing of mortar.
  • The uniformity of this system is also beter than the earlier system. 
  • Better compaction can be obtained by means of a straight edge pressed against the inner mould in this method
  • Local, un-skilled people can handle this process.

3) Centrifuging method

  • Centrifuging process is adopted for the fabrication of concrete cylindrical units.
  • Because of good compaction the ferrocement pipe cast by the method of centrifuging can be used as pressure pipes.

4) Guniting

  • The process of geniting can be adopted for applying the mortar to the wire mesh system.
  • This appears to be suitable process or mass production of ferrocement prefabricated units.
  • If the process applies with experienced gunman, it can give compacted and uniform surface.

Advantages of Ferro-cement

  • High ductility.
  • High resistance to cracking width
  • Ability to undergo large deflection
  • Improved impact resistance and toughness
  • Good in fire resistance
  • Good impermeability
  • Low strength to weight ratio
  • Low maintenance costs
  • Constant curing is needed for a period of 7 days to avoid any shrinkage cracks.

Disadvantages of Ferro-cement

  • Difficult to do welding
  • Binding mesh and rod along can be time consuming
  • Number of labour will be higher
  • There is excessive shrinkage due to higher cement content

Applications of Ferro-cement

  • Ferro-cement can be used for casting domestic overhead tanks. These tanks will be cheaper than any other type of material tanks.
  • Ferro-cement tanks can be used in villages as gas holding.
  • Ferro-cement used for made manhole covers
  • Pre-fabricated roof units.
  • For the production of pressure pipes
  • For casting curved benches for gardens, parks and open cinema theatre

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