……… is used to construct very thin, hard and strong surface:
🔬 Understanding Thin Structural Elements
The question asks for a material specifically suited for constructing surfaces that are simultaneously very thin, hard, and strong. Conventional concrete requires a significant thickness (cover) to protect its large reinforcing bars. Therefore, a specialized material is needed to achieve these properties in a thin profile.
📝 Detailed Analysis of Materials
(c) Ferro–Cement
This is the correct answer. Ferrocement is a unique composite material made by plastering a rich cement mortar over multiple, closely spaced layers of fine wire mesh. The high concentration of reinforcement, distributed throughout the section, allows for the creation of very thin (often just a few centimeters) yet incredibly strong, hard, and ductile elements. It is the ideal material for applications like boat hulls, water tanks, and complex shell roofs where a thin, strong surface is required.
(a) Reinforced Cement Concrete (RCC)
This is incorrect. RCC uses large diameter steel bars as reinforcement. To protect these bars from corrosion, a significant thickness of concrete cover is required. This makes it impossible to construct "very thin" structural elements with RCC.
(b) Post–Stressed Concrete
This is incorrect. Post-stressing is a technique used to create strong, long-span structural members like beams and slabs by tensioning steel tendons after the concrete has hardened. It is a method for enhancing the performance of thick members, not for creating thin surfaces.
(d) None of these
This is incorrect as Ferro-Cement is the correct material.
📊 Comparison of Concrete Types
| Material | Typical Thickness | Reinforcement | Primary Use |
|---|---|---|---|
| Ferro-Cement | Very Thin (e.g., 2-5 cm) | Multiple layers of wire mesh | Thin shells, boats, tanks |
| Reinforced Concrete (RCC) | Thick (e.g., >10 cm) | Steel bars (rebars) | Beams, columns, slabs, foundations |
| Pre/Post-Stressed Concrete | Thick | High-tensile steel tendons | Bridges, long-span beams |
💡 Study Tips
- Thin = Ferro: The keyword in the question is "very thin." Immediately associate this requirement with Ferrocement.
- Think "Mesh, not Bars": Ferrocement uses fine wire mesh, allowing for thin sections. RCC uses thick bars, requiring thick sections for cover.
- Visualize the Application: Think of a canoe or a water tank. You need a material that is thin, strong, and can be shaped easily. That is the essence of ferrocement.