A type of thin reinforced concrete construction where the matrix of cement mortar is reinforced with several layers of continuous and relatively small diameter wire-meshes is Ferrocement. Mortar provides the compact mass and the wire-mesh provides ductility and tensile strength to the material.
In terms of structural behaviour, ferrocement exhibits very high tensile strength to weight ratio and superior cracking performance. The distribution of a small diameter wire mesh reinforcement over the entire surface, and sometimes over the entire volume of the matrix, provides very high resistance against cracking. Moreover, other main engineering properties, such as toughness, fatigue resistance, impermeability etc. are considerably improved. For the achievement of a stiff reinforcing cage, sometimes conventional bars in a skeleton form are added to thin wire-meshes.
Materials Used in Ferrocement
1. Cement mortar matrix
~ The ferrocement composite is a rich cement mortar matrix of 10 to 60 mm thickness with a reinforcement volume of 5% – 8% in the form of one or more layers of very thin wire mesh and a skeleton reinforcement consisting of their welded mesh or mild steel bars.
~ Normally, Portland cement and the fine aggregate (sand) mix is used in ferrocement. The matrix constituents about 95 % of the ferrocement and govern the behaviour of the final product. Plasticizers and admixtures may be added for achieving improved workability, water reduction for the increase in strength and reduction in permeability, waterproofing and increase in durability.
~ The mix proportions in terms of sand-cement ratio (by mass) normally recommended are 1.5 to 2.5. The water-cement ratio, by mass, may vary between 0.35 and 0.6; it must be kept below 0.4 to reduce permeability.
~ The reinforcement used in ferrocement is of the following two types:
ii. Wire mesh
~ The skeleton steel frame is made conforming exactly to the geometry and shape of the structure and is used for holding the wire-meshes in position and shape of the structure.
~ The skeleton steel consists of relatively large diameter (about 3 to 8 mm) steel rods and which are spaced at 70 to 100 mm. It may be tied-reinforcement or welded wire fabric.
~ The wire mesh consisting of galvanized wire of diameter 0.5 to 1.5 mm spaced at 6 to 20 mm centre-to-centre is formed by welding, twisting or weaving.
~ The hexagonal wire mesh is cheaper but structurally less efficient than the
mesh with square opening.
~ The rectangular meshes have better rigidity when placed or tied over the
~ The welded wire meshes have a higher Young’s modulus.
~ Wooven wire meshes are flexible and easy to work with then welded one.
Construction in ferrocement
The construction in ferrocement may be divided into the following four phases:
1. Fabrication of skeleton framing system.
2. Fixing of bars and mesh
3. Application of mortar
i. Its cracking resistance, ductility, impact and fatigue resistance are higher than those of concrete.
ii. It has a high tensile strength to weight ratio and also has superior cracking behaviour. Tensile strength of ferrocement depends mainly on the volume of reinforcement in the direction of the force and the tensile strength of the mesh.
iii. It has a nearly equal amount of tensile and compressive strength i.e. 27 MPa. The width of cracks at the failure of ferrocement can be as small as 0.05 mm.
iv. The ferrocement beams show poor resistance to fatigue under cyclic loading. Impact tests on ferrocement slabs show that the impact resistance increases almost linearly with the increase in *specific surface (volume traction) and ultimate strength of mesh reinforcement.
v. The impermeability of ferrocement products is far superior to that of ordinary reinforced concrete products.
Some advantages are:
i. Structures constructed using ferrocement are thin and light.
ii. The construction technique is simple and hence highly skilled labour is not required.
iii. Partial or complete elimination of framework is possible.
iv. The local damage due to abnormal loads (such as impact) can be easily repaired by ferrocement construction.
v. It can be used for the manufacture of precast units.
The ferrocement is used in the construction of the following:
i. Light and watertight structure.
ii. Portable structures such as mobile homes.
iii. Water tanks.
iv. Silos and bins.
v. Boat hulls.
vi. Biogas holders.
viii. Folded plates and shell roofs.
ix. Floor units.
x. Service core units.
xi. Wind tunnels.
xii. Modular housing.
xiv. Swimming pools.
xv. A permanent form of concrete columns.