FERRO CEMENT

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INTRODUCTION

Italian engineer Pier Liugie Nervi is credited with inventing Ferro cement in the 1940’s.
The properties of Ferro cement, such as strength, watertightness, toughness, light weight, durability, fire resistance & environmental stability are hard to match with any other thin constructional material

DEFINITION

Ferro cement 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 material.”

FERRO CEMENT

A composite of steel & cementitious material is an ingenious invention of the mid 19th century which has a good potential for use in a wide variety of applications. It is basically a scaled down variation of concrete – as we know it today.
The large size reinforcing steel bars are replaced with wires or meshes, while the coarse aggregate is removed completely from the cementitious matrix. The resultant composite – called Ferro cement lends itself to casting in thin sections – sometimes less than 

Materials used in Ferro cement

Cement mortar mix.
Skeleton steel.
Steel mesh reinforcement or Fibre-reinforced polymeric meshes.
Admixtures.

CEMENT MORTAR MIX

OPC and fine aggregate matrix is used.
The matrix constitutes 95% of the composite and governs its behavior.
FA (sand) occupies 60 to 75% of the volume of the mortar.
Plasticizers and other admixtures are used.

MIX PROPORTIONS
Sand: cement ratio (by mass) 1.5 : 2.5
Water: cement ratio (by mass) 0.35 :0.60


Steel mesh reinforcement

Consists of galvanized steel wires of diameter 0.5 to 1.5 mm, spaced at 6 to 20mm centre to center. Available as woven/interlocking mesh and welded mesh. Welded wire mesh has hexagonal or rectangular openings. Expanded-metal lath is also used.


ADVANTAGES OF FERRO CEMENT

High ductility
High resistance to cracking width 
Ability to undergo large deflection
Improved impact resistance and toughness
Good fire resistance
Good impermeability
Low strength to weight ratio
Low maintenance costs


CONSTRUCTION METHODS

Skeletal Armature Method
Closed Mould Method
Integral Mould Method
Open Mould Method


Skeletal Armature Method

In this method the skeleton steel is welded to desired shape, on either sides of which are tied several layers of stretched meshes. This is strong enough, so that mortar can be filled 
in by pressing for one side and temporarily supporting from the other side.

Closed Mould Method
Several layers of meshes are tied together against the surface of mould which holds them in position while mortar is being filled in

Integral Mould Method

Using minimum reinforcement any integral mould is first to be considered to act as a framework. On this mould layers of meshes are fixed on either sides and plastering is done on both the sides


Conclusion 

Ferro cement being a labour intensive and a material saving technique, has never been able to compete with reinforced cement concrete. However, innovative structures in different parts of the world have clearly indicated the unique, unmatched properties of this material. 
The application of Ferro cement to the dome structure has made it possible to construct a light but strong, durable weather resistant shell with a weight reduction to almost 1/10th of the conventional material