Q.22 Hollow pre-cast concrete blocks possess good thermal insulation because of the:
- Aoutside air near the block
- Bair entrapped within the blockCorrect
- Cresistance to efflorescence
- Dseepage resistance
Solution
Concept
Air trapped inside the hollow cores is a poor conductor of heat, so hollow blocks give good thermal insulation.
Step-by-step solution
1. Hollow blocks have empty cores (cavities) inside. 2. These cavities are filled with still air. 3. Still air has very low thermal conductivity. 4. So less heat passes through the block. 5. So the answer is (B).
Explanation
Hollow concrete blocks have air spaces inside them. Still air does not conduct heat well, so it acts as an insulating layer. This keeps buildings cooler in summer and warmer in winter.
Why the other options are wrong
A: Outside air is not part of the block and gives no insulation. C: Efflorescence resistance relates to salts, not heat. D: Seepage resistance relates to water, not heat.
Shortcut
Hollow means trapped air, and trapped air means insulation.
Common mistake
Thinking that moisture or salt resistance affects thermal insulation.
Correct answer: B) air entrapped within the block