The construction of ferro cement may be divided into four phases as:
1. Fabrication of skeleton frame system
2. Fixing of bars and mesh
3. Application of mortar and
The quality of mortar and its application is the most critical phase.
Mortar can be applied by the following four methods:
A. Hand plastering
B. Semi mechanised process
C. By centrifuging
In ferro cement construction no form work is required as in conventional reinforced concrete. So it is more suitable for structures with curved surfaces as shells and other free form shapes.
First the skeleton frame is made using small diameter steel rods bent to the required shape, generally cylindrical in shape. Usually this frame provides rigidity to the whole structure before impregnation or plastering. Placing of mortar is known impregnation of mesh with matrix.
The required numbers of wire mesh layers are fixed on both sides of the skeleton frame. First external mesh layers are fixed and tied to the frame bars. The mesh should be fixed by staggering the hold positions in such a way that the effective hold size is reduced. Between two mesh layers space of 1 to 3 mm should be left. Whenever two pieces of the mesh are to be joined, a minimum overlap of 80 mm should be provided and tied at an interval of 80 to 100 mm centre to centre.
A. Placing or Impregnation Operation by Hand:
It is the most critical operation in ferro cement casting. If the mortar impregnation is not proper, the quality of the structure will not be good and it will not give desired performance.
A sufficient quantity of mortar is dashed from outside through the layers against a G-1 sheet held on the other side. The flexible G-1 sheet is moved around and the mortar is dashed from the outside. The process is continued till the whole structure is built up. During process of putting the mortar, it should be ensured that no voids are left in the body of the structure.
It can be ensured by using a wooden hammer of about 100 mm diameter with 150 mm long wooden handle. The mild hammer blows are given over the temporarily held form to remove the voids. This will give sufficient vibrations for compacting the mortar. The whole thickness is build up gradually in two or three consecutive dashing of mortar and then both internal and external surfaces are made smooth.
Shells and boat hulls like structures are built by the technique known as two operation mortar impregnation. In this system, first the outside mesh is plastered and the inner layer is left exposed. The excess mortar is scrapped by trowel and wire brushes. The mortar is left for setting till it attains sufficient strength for carrying the load from the inside during the application of a second layer of mortar. Before applying the second layer, fine cement slurry is sprayed over the entire inner surface.
In structures where many layers are used as reinforcement and thickness is more than 20 mm, in such cases it is desirable to do the casting in three layers. The middle or core layer is applied first covering the skeleton steel and one layer of wire mesh. This core provides a firm surface for mortar application on its top and bottom. The core is cured at least for 3 days before the other two layers of mortar are applied. For getting the good bond, between old and new mortar cement slurry should be sprayed over the middle layer.
For thin cylindrical units of about 1 m diameter, 6 mm diameter steel rods at a spacing of 15 cm be used for making a cage of cylindrical shape and then woven or chicken mesh can be tied to the mesh and impregnated or plastered. In such type of construction, the use of chicken mesh is not advisable as it is very flexible and plastering over it may not be satisfactory.
In this method the control of thickness is difficult and the minimum thickness of the section works out to be more than 20 mm. The greater thickness not only makes it uneconomical, but also some technical advantages are lost. The strength obtained by hand plastering or impregnation is lower compared to other methods due to poor compaction of mortar by this method.
The units cast by this method may be used for pipes, storage structures and gas holder units etc. This method of casting is suitable of units of shapes for which making of mould is difficult. This method can also be applied for making cylindrical shaped units of size approximately 60 cms in diameter or above.
B. Semi Mechanised Process:
A semi mechanised process for making ferro cement cylindrical units has been developed by Structural Engineering Research Centre (SERC) Roorkee. In this process a central cylindrical mould is used. Over this central mould one layer of wire mesh is wound. Over this layer a 4 mm diameter wire is tied at a spacing of 150 mm in both directions. One layer of chicken mesh is wound over this wire layer. This forms the complete wire mesh system of reinforcement.
Now the cement sand mix prepared is impregnated or plastered layer by layer. Due to the tightly wound mesh around the form work, the thickness of the unit is reduced. With this system, units upto one cm thickness can be cast containing two layers of wire mesh in that thickness i.e. within 1 cm thickness. This system is called semi mechanised as the mould can be rotated to facilitate dashing of mortar.
Advantage of Semi Mechanised Method:
Following advantages have been observed of this method:
1. In this method better compaction can be obtained by means of a straight edge pressed against the inner mould.
2. The uniformity of thickness is better in this method than hand plastering.
3. The wire mesh can be wound tightly over the mould and also can be tightened during the casting operation. This helps in avoiding unevenness of thickness and looseness in the mesh.
4. This system does not need any sophisticated equipment and electricity.
5. Local, un-skilled people can handle this process.
6. This process can be adopted easily in rural areas.
7. The cylindrical units of size upto 1.0 m or above can be cast by this process.
C. Centrifuging Method:
For the fabrication of concrete cylindrical units, generally centrifuging process is adopted. The first crack strength of ferro cement has been observed higher in comparison of normal reinforced concrete. Thus the pipe thickness can be reduced, resulting in lesser dead weight. In the existing centrifuging process, the mild steel reinforcement cage has been replaced by wire mesh layers cage. The trial casting at SERC Roorkee has shown that this method can be adopted for casting ferro cement units. Due to good compaction, ferro cement pipes cast by centrifuging process can be used as high pressure pipes.
The process of guiniting can be adopted for applying the mortar to the wire mesh system. This process seems to be suitable for mass production of ferro cement prefabricated units. A continuous process of layer guniting with an interval of about, an hour will yield good results. If the process is applied properly by an experienced gun man can produce good compacted and uniform surface.