---
title: "Permeability as root cause of poor durability"
subject: "Concrete Technology and Masonry Structure"
chapter: 6
key: "semester_5/concrete_technology_and_masonry_structure:permeability-root-cause-durability"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/permeability-root-cause-durability
site: BCT Engineering
---

# Permeability as root cause of poor durability

Concrete Technology and Masonry Structure · Chapter 6: Concrete durability

- Asked in: 2081 Bhadra, 1+3 marks; 2080 Bhadra, 3 marks; 2080 Baisakh, 3+2 marks (asked 3 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/permeability-root-cause-durability
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6

## Question

Define permeability. "Permeability of concrete is often referred to as the root cause for lack of durability." Do you agree? Explain, and show with a schematic representation materials having similar porosity but different (low or high) permeability.

## Short answer

Permeability is the ease of fluid flow through concrete under pressure. It is the root cause of poor durability because most deterioration agents enter via connected pores. Materials with similar porosity can have low permeability with isolated pores or high permeability with connected pores.

## Answer

#### Definition
**Permeability** is the ease with which a fluid (water, gas, dissolved ions) can flow through concrete under a pressure difference. By Darcy's law for saturated flow,

$$
\frac{q}{A} = K\,\frac{\Delta h}{L}
$$

where $K$ is the coefficient of permeability (m/s), $\Delta h$ is the head and $L$ the thickness. Dense concrete has $K$ of $10^{-12}$ to $10^{-10}$ m/s.

#### Agree: permeability is the root cause of lack of durability
Almost every cause of deterioration needs a fluid to move into the concrete:
- Water carries sulphates, chlorides and acids inside (chemical attack, corrosion).
- Oxygen and CO$_2$ gas penetration cause corrosion and carbonation.
- Water in pores freezes and causes freeze-thaw damage; it also supports alkali-silica reaction and leaching.

If the permeability is low, these agents enter very slowly, so the concrete lasts longer. Permeability depends on w/c, curing, compaction, cement and microcracks. Hence IS 456 limits the w/c ratio and requires a minimum cement content (Table 5) and proper curing (Cl. 13.5).

#### Porosity and permeability are different
Porosity is the total volume of voids; permeability depends on whether the pores are **connected** and how large they are.

```text
 Low permeability           High permeability
 (isolated pores)           (connected pores)
 +----------------+         +----------------+
 | o   o    o   o |         | o==o==o   o=o  |
 |   o   o   o    |         |  \\  ||  //   |
 | o    o    o  o |         | o=o=o=o===o    |
 +----------------+         +----------------+
 same porosity (volume of pores), flow differs
```

For example, lightweight foamed concrete has high porosity but low permeability if its pores are discrete. A paste with continuous capillary pores (w/c above 0.7) has similar porosity but is highly permeable.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is permeability defined and expressed?
   A. Total volume of voids divided by total volume
   B. Ability to resist compressive load
   C. Rate of heat flow through concrete
   D. Ease of fluid flow through concrete under pressure, q/A = K(Δh/L) by Darcy's law
   Answer: D. Permeability is flow ease under pressure with coefficient K (m/s); the void volume ratio is porosity. (Key point: Permeability definition)

2. Why is permeability called the root cause of lack of durability?
   A. Because permeable concrete is always stronger
   B. Almost all deterioration needs water, gases or ions to move into the concrete, and low permeability slows this
   C. Because it only affects appearance
   D. Because it matters only for freezing
   Answer: B. Chemical attack, corrosion, carbonation and freeze-thaw all depend on ingress of fluid. (Key point: Permeability as root cause)

3. Two concretes have the same porosity but different permeability. What explains this?
   A. Pore connectivity and size: isolated pores give low permeability, connected pores give high permeability
   B. The one with higher permeability has more total pore volume
   C. Permeability depends only on aggregate colour
   D. The two must have the same permeability if porosity is equal
   Answer: A. Porosity is total void volume; flow depends on whether the pores are connected and how big they are. (Key point: Porosity vs permeability)

## Related in Concrete Technology and Masonry Structure

- [Porosity, permeability, corrosion, carbonation](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aporosity-permeability-corrosion-carbonation) — Concrete Technology and Masonry Structure, chapter 6
- [Effects of carbonation and permeability on durability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acarbonation-permeability-durability) — Concrete Technology and Masonry Structure, chapter 6
- [Water, permeability and durability; corrosion remedy](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awater-permeability-durability-corrosion) — Concrete Technology and Masonry Structure, chapter 6
- [w/c ratio, porosity and Powers' concept](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awc-ratio-porosity-powers) — Concrete Technology and Masonry Structure, chapter 4
- [Effect of w/c ratio on durability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awc-ratio-durability) — Concrete Technology and Masonry Structure, chapter 6
- [Capillary porosity of hydrated cement (28 days)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acapillary-porosity-28-days) — Concrete Technology and Masonry Structure, chapter 4, asked 2×

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