The history of Ferrocement is traced back to the 1940s were an Italian engineer used it for the first time in the construction of fishing boards. Later the ACI included it in its codes in the 1980s. 40 years after it was first introduced.
The use of Ferrocement ensures enhanced elasticity, flexibility strength, and resistance to cracking. Ferro-cement is a system of construction using a reinforced mixture of lime or cement, sand, and water called mortar. A framework of metal mesh, woven, expanded metal, or metal-fibers, and closely spaced thin steel rods such as rebar is used to apply the prepared mixture/mortar.
The rebar material in general used is iron or steel rods. The utmost importance is the thickness of the mesh or reinforcing material used. The diameter of reinforcement should not be more than 0.5 mm to 1 mm. Ferro cement is different from Ferro concrete as aggregates are not used and the mortar is a rich mix of sand and cement.
The ratio of cement to sand is 1:5 to 2:5. That is 1 portion of cement and 5 portions of sand or 2 portions of cement and 5 portions of sand. Mortar is hydraulic cement mortar. Water content ranges from 0.35 kg water per kg of cement to 0.6 kg water per kg of cement.
The cement type used is Ordinary Portland cement. Sand occupies 60% to 75% of volume. Admixtures used are plasticizers. Plasticizers are water content reducing agents without compromising the strength of the mortar. Air entrainment, use of plasticizers, and less water ensure the strength of mortar.
Multiple layers of mesh are used to achieve the required density of steel with reinforcing rods in between for support. Generally, a double layer of mesh on both sides with rods in between is used for creating a framework for pouring mortar. The diameter of rods used is generally between 3 mm to 8 mm. The mesh used can be square mesh, expanded metal mesh, hexagonal wire mesh, improved hexagonal wire mesh.
Usage of mesh reduces the risk of cracking and provides tensile strength and flexibility. Mortar is then applied on both sides of the mesh which is generally known as an armature. The thickness of the steel skeleton varies from 6mm to 20 mm based on the loading requirements. Steel bars used are generally of grade FE500 and are placed at 75 mm to 120 mm c/c. The reinforcing bars should be free from, rust and impurities.
Sand used is confirming to Zone l or zone ll and shall be free from impurities.
Water used shall be free from salts and other organic impurities. pH should be 7 or greater.
PROPERTIES OF FERROCEMENT:
- As the steel percent is more, tensile strength and cracking resistance is enhanced.
- It also ensures enhanced punching and shear resistance.
- Highly flexible
- Water cement ratio is less, this forms impermissible density enhancing durability.
TECHNIQUES FOR MANUFACTURING:
Skeletal armature method:
In this method, several layers of wire mesh is attached to either side of the rods skeleton to achieve the desired shape and then it is welded to hold in position following which, until the surplus mortar appears on the opposite side, the mortar is applied from one side by forcing it to penetrate through the mesh layers This excess quantity is then pressed back, and therefore, the remaining mortar is struck off to offer a clean finish.
The skeletal steel is placed at the middle of the section in both directions. It simply acts as spacer rods and doesn’t contribute to the strength but adds to the dead weight of the structure. The diameter of the steel bars of the skeletal steel is determined by the size of the structure. It has to be tied in sequence, cut a specified length to have a development length, and bent into a specific profile.
Closed mold method
In the closed mold method, the layers of wire mesh are either tied or stapled together and held together in position against the closed mold’s surface. The mortar is then applied from one side. Though it isn’t mandatory to separate the mold from the ferrocement structure, if needed, is often removed using release agents, and other treatment agents. The advantage of using the closed method is plastering needs to be done on one side only and it also eliminates the use of bars and rods, thus, only a mesh is required.
Integral mold method:
The integral part of the entire structure as the name suggests is mold which is to be made. The core as the integral mold is known to consist of a semi-rigid framework having few layers of mesh or a rigid foam of insulating materials like polyurethane or polystyrene. Mortar is applied from both sides. The quality and durability can be enhanced by increasing adding more layers of mesh and applying more mortar. The utmost care is to be taken that the bond between the core and the added layer is built to ensure its usage as a single integral structural unit.
Open mold method:
It is partially similar to the closed mold method. Initially, the mortar is applied to the open mold through one side of the mesh layers and rods. A lattice of wooden strips is called mold. The mold is coated with a release agent or polyethylene sheeting.
It is used to cover the mold entirely, thus, a close but non-rigid and transparent mold would be formed. The open mold method advances the easy removal of the mold and allows the observation and repair of any glitches during the process of mortar application.
The availability of machinery for mixing, nature, and application of ferrocement, handling and placing, the skill, and the overall cost of labor are some factors that affect the construction of ferrocement.
DISADVANTAGES OF FERROCEMENT:
- Low shear strength
- Low ductility
- Susceptibility to stress rupture failure
- Collision with pointed objects can cause failure
- Fastening of ferrocement structure with bolt, screw, weld, etc. Is difficult.
- Tying rods and mesh together is tedious and time-consuming.
APPLICATIONS OF FERROCEMENT:
- Marine applications
- Water supply and sanitation
- Residential buildings
- Rural energy
- Wind tunnels
- Mobile homes
- Silos and bins.