Which of the following is a limitation of lightweight concrete compared to conventional concrete?
Correct Answer: A. Increased permeability
📚 Detailed Explanation: Increased Permeability is the Key Limitation of Lightweight Concrete
Why A (Increased permeability) is correct: Lightweight concrete achieves its low unit weight through porous lightweight aggregates (expanded clay, pumice, expanded shale) or deliberately introduced voids (aerated/foamed concrete). This inherent porosity creates continuous capillary pathways through which water, chlorides, CO2, and other aggressive agents can penetrate, making permeability the primary durability limitation versus conventional dense concrete.
Lightweight Concrete: Advantages vs. Limitation
| Property | vs. Conventional Concrete | Advantage or Limitation? |
|---|---|---|
| A. Permeability | Higher (more porous structure) | LIMITATION — reduced durability; more chemical attack risk |
| B. Density | Lower (300–2000 kg/m³ vs. 2400) | Advantage — reduces dead load; enables longer spans |
| C. Thermal insulation | Better (lower thermal conductivity) | Advantage — reduces heating/cooling energy |
| D. Fire resistance | Better (lower conductivity slows heat transmission) | Advantage — slower temperature rise; less spalling |
| Compressive strength | Generally lower | Limitation — requires careful structural design |
Types of Lightweight Concrete and Their Permeability
| Type | Density (kg/m³) | Permeability Level | Application |
|---|---|---|---|
| Aerated / Foamed (AAC) | 300–800 | Very high — porous foam structure | Non-structural walls; insulation |
| No-fines concrete | 1600–2000 | High — deliberate void structure | Drainage layers; walls |
| Lightweight aggregate (structural) | 1400–2000 | Moderate to high — porous aggregate | Bridges; offshore; upper floors |
- Increased permeability is the primary limitation of lightweight concrete.
- Higher porosity allows water, chlorides, and CO2 to penetrate more easily, reducing durability.
- Reduced density, better thermal insulation, and better fire resistance are all advantages.