Good sand for construction should never be taken from
Correct Answer: C. sea beds
📚 Detailed Explanation: Why Sea Sand is Unsuitable for Construction
The source of sand significantly affects its suitability for concrete. While river sand and quarry dust are commonly used after appropriate processing, sea bed sand poses specific hazards that make it generally unsuitable for reinforced concrete without extensive treatment.
Why C (sea beds) is correct: Sea sand contains two major problems: (1) Salts (predominantly sodium chloride, NaCl) — chloride ions penetrate concrete and initiate corrosion of steel reinforcement, causing the well-known reinforcement corrosion-expansion-cracking failure in coastal structures. (2) Shell fragments — the calcium carbonate shells and irregular flat pieces introduce flakiness and weakness. IS 456:2000 strictly limits chloride content in concrete mix water and aggregates. Sea sand, with its high salt load, cannot meet these limits unless very thoroughly washed multiple times — not practically feasible on most construction sites.
Sand Sources: Suitability for Construction
| Source | Main Issue | Suitability |
|---|---|---|
| River bed | May contain silt — wash and test | Excellent after silt removal |
| Quarry (crushed stone dust) | May be angular — adjust water | Good with grading check |
| Sea bed | High chloride (salt) + shells | Not suitable for RCC |
| Desert sand | Too round, too fine, silica dust | Generally not suitable |
Key Concepts for Students
- Chloride limit in fine aggregate: IS 456 limits chloride ion content to 0.06% by mass for reinforced concrete.
- Sea sand can be used in mass concrete (gravity dams, plain concrete) if thoroughly washed — but never in reinforced or prestressed concrete.
- River sand is preferred because moving water naturally washes away silt and salt, leaving clean rounded particles.