Which of the following statements about impermeability is true?
Correct Answer: B. Only B (Impermeability is NOT a property of fresh concrete)
📚 Detailed Explanation: Impermeability is a Property of Hardened Concrete, Not Fresh
Why B (Only B) is correct: Impermeability — the resistance to water seeping through a concrete mass — can only be measured and is only relevant in hardened concrete. Fresh concrete is a fluid, plastic mass with no formed pore structure; the concept of impermeability does not apply to it. Statement A is therefore wrong; Statement B is correct.
Properties of Fresh Concrete vs. Hardened Concrete
| State | Key Properties | Examples |
|---|---|---|
| Fresh concrete | Rheological / workability properties | Workability, plasticity, consistency, cohesion, segregation resistance, bleeding tendency |
| Hardened concrete | Mechanical and durability properties | Compressive strength, tensile strength, impermeability, durability, creep, shrinkage, modulus of elasticity |
Impermeability of Hardened Concrete
| Factor | Effect on Impermeability |
|---|---|
| Higher w/c ratio | More capillary pores → lower impermeability (more permeable) |
| Larger aggregate size | More paste-aggregate interfaces and transition zones → lower impermeability |
| Proper aggregate grading (more fines) | Better void-filling → higher impermeability |
| More cement content | More hydration products fill pores → higher impermeability |
| Full compaction | No entrapped air voids → higher impermeability |
| Longer curing | More C-S-H gel fills capillaries → higher impermeability |
- Impermeability is a property of hardened concrete, not fresh concrete.
- Ordinary concrete is NOT perfectly impermeable — it always absorbs some water.
- Permeability increases with w/c ratio and aggregate size; decreases with cement content and curing.