Which of the following is a limitation of lightweight concrete compared to conventional concrete?
Correct Answer: A. Increased permeability
📚 Detailed Explanation: Increased Permeability is a Limitation of Lightweight Concrete
Why A (Increased permeability) is correct: Lightweight concrete achieves its low density through porous lightweight aggregates or introduced voids. This porous structure, while reducing weight, also increases the permeability of the concrete, making it more susceptible to water ingress, chemical attack, and reduced durability. Permeability is the primary durability limitation of lightweight concrete compared to conventional dense concrete.
Advantages vs. Limitations of Lightweight Concrete
| Property | Lightweight Concrete | Advantage or Limitation? |
|---|---|---|
| Density | 300–2000 kg/m³ (vs. 2400 for conventional) | Advantage: reduces dead load; enables longer spans |
| Thermal insulation | Much better than conventional (lower thermal conductivity) | Advantage: reduced heating/cooling costs |
| Fire resistance | Better than conventional (lower thermal conductivity slows heat transmission) | Advantage: lower spalling risk; better FRL |
| Permeability | Higher than conventional concrete due to porous aggregate and void structure | LIMITATION: lower durability; more water ingress |
| Compressive strength | Generally lower (structural LWC: 17–55 MPa; non-structural: <17 MPa) | Limitation: not suitable for very high loads without special design |
| Sensitivity to water content | Porous aggregates absorb water variably; mix consistency harder to control | Limitation: requires careful mix design and quality control |
Types of Lightweight Concrete
| Type | Density (kg/m³) | Method | Application |
|---|---|---|---|
| Non-structural (insulating) | 300–800 | Aerated/foamed; no coarse aggregate | Roof insulation; partition walls (AAC blocks) |
| Moderate strength | 800–1400 | Lightweight aggregate (expanded clay/shale) | Floor screeds; infill panels |
| Structural LWC | 1400–2000 | Structural lightweight aggregate; full mix design | Long-span bridges; offshore platforms; upper floors |
- Increased permeability is the primary durability limitation of lightweight concrete.
- Reduced density, enhanced thermal insulation, and higher fire resistance are advantages, not limitations.
- Lightweight concrete is suitable where weight reduction is critical but durability requirements must be carefully considered.