Ferro cement is a type of thin wall reinforced concrete commonly constructed from hydraulic cement, sand, and water, in which a mesh of wire is used as reinforcement. It is known for several unique properties that make it suitable for various applications, particularly in boat building, water tanks, and simple housing structures.

Understanding Ferro Cement Properties

Let's examine the characteristics of ferro cement to determine which of the given options is *not* one of its properties.

• Impervious nature: Ferro cement, when properly constructed with a rich cement mix and adequate compaction, is known for being relatively watertight or impervious. The dense matrix and multiple layers of fine mesh help in achieving this property, making it suitable for water-retaining structures.
• Capacity to resist shock: Ferro cement exhibits good impact resistance. The distributed reinforcement (wire mesh) throughout the matrix helps in absorbing energy from impacts and preventing sudden brittle failure, unlike traditional concrete which might crack and shatter more easily.
• No need of formwork: While some simple shapes can be constructed with minimal or no formwork, especially for curved surfaces or thin shells, it is not universally true that ferro cement *never* requires formwork. Complex shapes, flat panels, or larger structures often still benefit from or require some formwork, though it's generally less elaborate than for traditional reinforced concrete. However, compared to traditional methods, the need for rigid, extensive formwork is significantly reduced or eliminated in many typical applications.
• Strength per unit mass is low: Ferro cement is characterized by a high strength-to-weight ratio. Due to its thin sections and the efficient distribution of reinforcement, it achieves significant strength while being relatively lightweight compared to traditional concrete sections of equivalent strength. Therefore, stating that its strength per unit mass is low is incorrect. It is actually known for its high strength per unit mass.

Analyzing the Options

Based on the typical properties of ferro cement:

• Option 1: Impervious nature - This is generally a property of well-made ferro cement.
• Option 2: Capacity to resist shock - This is a known property due to the wire mesh reinforcement.
• Option 3: No need of formwork - While not absolutely 'no need' in all cases, the requirement is significantly less compared to conventional concrete, making it a distinguishing feature often highlighted.
• Option 4: Strength per unit mass is low - This statement is contrary to the actual property of ferro cement, which is known for its high strength per unit mass.

Therefore, the statement that is *not* a property of ferro cement is "Strength per unit mass is low".

Conclusion

Ferro cement is valued for its durability, impact resistance, and good strength-to-weight ratio. The option stating that its strength per unit mass is low is inaccurate. The material is known for providing considerable strength despite being relatively lightweight compared to conventional reinforced concrete.

The final answer is “Strength per unit mass is low”.

Ferro Cement Properties Revision Table

Additional Information on Ferro Cement

Ferro cement differs from traditional reinforced concrete in several ways:

• Reinforcement: Uses multiple layers of small diameter wire mesh (like chicken wire or welded mesh) uniformly distributed, rather than larger diameter reinforcing bars (rebar) concentrated in certain areas.
• Aggregate: Uses fine aggregate (sand) and no coarse aggregate (gravel).
• Thickness: Typically much thinner sections, often less than 1 inch (2.5 cm) thick.
• Construction Method: Often built by applying mortar onto the mesh armature, sometimes by hand or spray.

These characteristics contribute to its unique properties, including its flexibility, durability, and ability to be molded into complex shapes.