In this article we will discuss about:- 1. Meaning of Ferro Cement 2. Materials Used in the Production of Ferro Cement 3. Construction 4. Properties 5. Applications 6. Advantages.
Meaning of Ferro Cement:
Essentially ferro cement is a form of reinforced concrete, but it differs from the conventional type of reinforced concrete. The reinforcement used in if consists of closely spaced, multiple layers of wire mesh or fine rods completely surrounded by cement mortar. Ferro cement is much thinner than reinforced concrete and the mesh may be formed in any shape without a conventional form. Then it can be plastered or mortared by any method.
In 1940, Mr. P.L. Nervi an Italian engineer and architect developed this material, known as Ferro cement. It consists of closely spaced wire meshes which are impregnated with rich cement mortar mix. Though these materials is of recent origin but have been used in many countries notably in China, New Zealand and U.K. There is a growing awareness of the advantages of this technique of construction all over the world.
It is well known that conventional reinforced concrete elements are heavy, brittle, develop cracks and cannot be repaired satisfactorily, if damaged. The reinforcement is also liable to be corroded. These disadvantages make the normal reinforced concrete un-suitable for certain type of works. Due to these shortcomings of normal reinforced concrete, Ferro cement can be considered as one of its substitute in many situations.
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In case of ferro cement, the mortar provides the mass and the wire mesh imparts tensile strength and ductility to the material. In terms of structural behaviour ferro cement exhibits very high tensile strength weight ratio and superior cracking performance. The distribution of small diameter wire mesh reinforcement over the entire surface provides a very high resistance against cracking. Also many other engineering properties such as toughness, fatigue-resistance, impermeability etc., are considerably improved by the use of ferro cement.
Materials Used in the Production of Ferro Cement:
In ferro cement productions following materials are used:
(i) Cement Mortar Matrix:
The ferro cement consists of a rich cement mortar matrix of 10 to 60 mm thickness with a reinforcement volume of 5 to 8% in the form of one or more layers of very thin mesh and a skeleton reinforcement consisting of mild steel bars or welded mesh.
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(ii) Cement:
The choice of cement depends on the service conditions. The cement should be fresh, of uniform consistency and free of lumps and foreign matter.
(iii) Sand or Fine Aggregate:
Sand is an inert material. It occupies 60.to 75% volume in the ferro cement mortar. To impart good properties to the mortar, sand should be hard, strong, non-porous and chemically inert. It should be free from silt, clay and other organic impurities. The maximum particle size should be 2.36 mm.
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The particles larger than 2.36 mm if present in sufficient quantity may cause the mortar porous if particles less than 1.18 mm are present in large quantity, will require more water for the required workability, affecting the strength and impermeability very adversely. Thus the sand should be of grading zones II and III with particles greater than 2.36 mm and smaller than 1.18 mm removed. Use of fine sand in ferro cement is not recommended.
(iv) Water/Cement:
As it governs the strength and workability of the mortar, it depends upon the maximum grain size, the fineness modulus and the grading of the sand. Generally water/cement ratio by mass may vary between 0.35 to 0.60. In order to reduce permeability, water/cement ratio should be kept below 0.40. The moisture content of the aggregate should be taken into account in the calculations of required water. The amount of water many be reduced by the use of appropriate admixtures.
(v) Cement/Sand Ratio:
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Cement sand ratio may be kept 1:2 or 1:3. The slump of fresh mortar should not exceed 50 mm for most works the strength of moist cured cubes should be about 350 MPa.
The matrix constitutes about 95% of the ferro cement and governs the behaviour of the final product. Thus proper selection of constituent materials, their mixing and placing is important. The total volume of cement and fines should be about 300 cm3 per litre of mortar. A change in the amount of cement must be accompanied by a corresponding change in the fines.
Thickness of Elements:
The thickness of ferro cement elements usually is of the order of 2 to 3 cms with 2 to 3 mm external cover to reinforcement.
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Reinforcement:
The reinforcement used in ferro cement is of two types:
(a) Skeleton Steel:
The skeleton steel frame is made conforming exactly to the geometry and shape of the structure. This is used for holding the wire mesh in position and shape of the structure. The diameter of the steel rods may vary from 3 to 8 mm and spaced at 70 to 100 mm apart. It may be tied reinforcement or welded wire fabric.
The welded wire fabric is made from 3 to 4 mm diameter wires welded at 80 to 100 mm centre to centre. These skeleton frames are used for cylindrical or other surfaces where these meshes can be bent easily. In case where higher stresses may occur, mild steel bars are used as skeleton steel.
The spacing of transverse and longitudinal steel bars depends upon the type and shape of the structure. In case of boat hulls a spacing of 75 to 100 mm is adequate, where as in water tanks, bins etc. the spacing may vary between 200 to 300 mm. The bars may be tied with the binding wires, they may also be welded. The reinforcement should be free from dust, loose rust, coating of paints, oil etc.
(b) Wire Mesh:
The wire mesh consists of galvanized wire of 0.5 to 1.5 mm diameter spaced at 6 to 20 mm centre to centre. The wire mesh may be of the shape as square woven wire mesh or welded wire mesh, expanded metal or Hexagonal wire mesh etc. Generally square woven meshes consisting of 1.0 to 1.5 mm diameter wires spaced about 12 mm are preferable. The minimum yield strength of plain wires used in fabric should be 415 MPa and 500 MPa for deformed wires. The steel content may vary between 300 kg to 500 kg per cubic metre of mortar.
Construction of Ferro Cement:
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
4. Curing
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
D. Guniting
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.
D. Guniting:
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.
Properties of Ferro Cement:
1. Compressive strength. The behaviour of thin ferro cement element under compression primarily is controlled by the properties of cement mortar matrix. Its compressive strength varies from 27.5 to 60 MPa.
2. Tensile strength. The tensile strength of ferro cement depends mainly on the volume of reinforcement in the direction of force and the tensile strength of the mesh. The ultimate tensile strength is 34.5 MPa and allowable tensile stress is taken as 10.0 MPa.
The tensile behaviour may be divided into three groups:
1. Pre cracking stage.
2. Post cracking stage.
3. Post yielding stage.
A ferro cement member subjected to increasing tensile stresses behaves like a liner elastic material till the development of first crack in the matrix. Once the cracks are developed, the material enters the stage, of multiple cracking and this stage continues upto the point where wire mesh starts to yield.
In this stage numbers of cracks go on increasing with the increase in tensile stress without any significant increase in the width of the crack. With the yield of reinforcement, the mortar enters the stage of crack widening. At this stage the number of cracks remains constant, but the width of crack go on increasing. The behaviour mainly is controlled by the reinforcement bars.
Fatigue Strength:
The fatigue behaviour of ferro cement flexural elements is governed by the tensile fatigue properties of mesh like reinforced and pre stressed concrete beams. The fatigue strength of ferro cement is poor under cyclic loading.
Impact Strength:
The impact strength of ferro cement has been found to increase linearly with the increase of specific surface (volume fraction) and ultimate strength of mesh reinforcement. Further for the same reinforcement fraction, the element having welded wire mesh reinforcement showed highest impact strength while chicken mesh reinforced section showed lowest impact strength. The impact strength of woven mesh reinforced ferro cement is found higher than chicken wire mesh and lower than welded wire mesh reinforced elements.
Applications of Ferro Cement:
Due to the very high percentage of well distributed and continuously running steel reinforcement, the ferro cement behaves like steel plates. Its cracking resistance, ductility, impact and fatigue resistances are higher than that of normal concrete. The impermeability of ferro cement product is far superior than ordinary R.C.C. products.
Due to these properties the ferro cement can be used for the following purposes:
1. Ferro cement can be used successfully for casting domestic overhead tanks. These tanks being light and flexible can be transported and hoisted without difficulty. The inlet and out let connections also can be done easily with the help of modem adhesives like “m seal”. These tanks will be cheaper than any other type of material tanks.
2. These tank units can also be modified into silos for storing grains in villages. These tanks will help in preserving grains from moisture effect and rodents.
3. Similar ferro cement tanks can be used in villages as gas holding units in ‘Gobbar gas’ plants. With some modifications ferro cement tanks can also be used as septic tanks.
4. Due to the favourable properties of ferro cement, this material has been widely used for boat building in U.K., U.S. and New Zealand. It has been reported that 14 m long ferro cement boat weighs only 10% more than the wooden boats. Ferro-cement boats are found 300% cheaper than fibre reinforced concrete boats, 200% cheaper than steel boats and 35% cheaper than timber boats.
5. The cost of ferro cement is only about 10% of the cost of cast iron. Thus the use of ferro cement manhole covers is becoming very popular, where these manhole covers are not subjected to heavy vehicular traffic.
6. Ferro cement is becoming more popular material for pre-fabricated roof units. The folded plates of ferro cement being light can be advantageously used as prefabricated roof units. A 3 cm thick ferro cement folded plate with two layers of chicken wire mesh can be used safely over a span of 3.5 m. It can also be used for prefabricated channel units for roof construction.
7. Ferro cement being a light material, considerable reduction in self-weight of structure and foundation cost can be reduced to a great extent. A 30% reduction in dead weight on supporting structure, 15% saving in steel consumption and 10% saving in roof cost has been observed in USSR by the use of ferro cement.
8. Ferro cement is found most suitable material for the production of pressure pipes. It is much lighter than normal RCC pipes.
9. Ferro cement also is found suitable for casting curved benches for parks, gardens, and open cinema theatre. It can also be used to cast tree guards. They can be cast in two parts to facilitate their removal at a later data.
Advantages of Ferro Cement:
Following are the advantages of the ferro cement:
1. The construction technique of ferro cement is simple. It does not require skilled labour.
2. Complete or partial elimination of form work is possible.
3. Ferro cement construction is easily amenable to repairs in case of local damage due to abnormal loads as impact load.
Summary of composition and properties of commonly used ferro cement are shown in Table 23.4 below:
