---
title: "Porosity, permeability, corrosion, carbonation"
subject: "Concrete Technology and Masonry Structure"
chapter: 6
key: "semester_5/concrete_technology_and_masonry_structure:porosity-permeability-corrosion-carbonation"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/porosity-permeability-corrosion-carbonation
site: BCT Engineering
---

# Porosity, permeability, corrosion, carbonation

Concrete Technology and Masonry Structure · Chapter 6: Concrete durability

- Asked in: 2082 Bhadra, 3+4 marks
- Video: [CONCRETE PERMEABILITY TEST IN LABORATORY](https://www.youtube.com/watch?v=vWs1BkTnscg) — Surabhi
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/porosity-permeability-corrosion-carbonation
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6

## Question

Clarify porosity and permeability with suitable illustration. Explain corrosion of steel and carbonation including its effect on concrete durability.

## Short answer

Porosity is the total void volume, while permeability reflects pore connectivity. Steel corrodes when carbonation (CO₂ + Ca(OH)₂ → CaCO₃ + H₂O) lowers pH below ~9, destroying its passive film, or when chlorides exceed the threshold. Carbonation depth follows d=K√t, and once it reaches the bar, rust expansion cracks the cover, reducing durability.

## Answer

#### Porosity
Porosity is the ratio of the volume of voids (gel pores, capillary pores, air voids) to the total volume of the material, expressed in percent. It affects strength and the amount of water that can be absorbed. Pore sizes: gel pores (1 to 3 nm), capillary pores (10 nm to 10 µm), air voids (above 50 µm).

#### Permeability
Permeability is the ease of fluid flow through the material under pressure ($q/A = K\,\Delta h/L$). It depends on the connection and size of the pores, not only on their volume.

```text
 same porosity            different permeability
 +--------------+         +--------------+
 | o  o   o  o  |  low    | o=o=o   o=o  |  high
 |  o   o   o   |  perm.  |  \\ ||  //   |  perm.
 | o   o   o  o |         | o=o=o=o==o   |
 +--------------+         +--------------+
 isolated pores            connected pores
```

#### Corrosion of steel
Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by **carbonation** reducing pH below about 9, or by **chlorides** above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery:

```text
   concrete (electrolyte, pore water)
     anode               cathode
   Fe -> Fe2+ + 2e-    O2 + 2H2O + 4e- -> 4OH-
 ===[ steel bar ]==============[ steel bar ]===
         \-- e- flow in the steel -->--/
        Fe2+ ------- ion flow ------- OH-
```

1. **Anode** (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$
2. **Cathode** (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$
3. The ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, which oxidises to $\text{Fe(OH)}_3$ and finally to hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust).

The rust has **2 to 6 times the volume** of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones.

#### Carbonation
**Carbonation** is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

$$
\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}
$$

- The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
- The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and **starts corrosion** once the carbonation front reaches the bar.
- The rate is highest at a relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
- It is detected with a **phenolphthalein** spray: the uncarbonated concrete (pH above 9.2) turns pink, while carbonated concrete stays colourless.
- It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
- Prevent it by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.

**Effect on durability:** carbonation and corrosion together shorten the service life. The life of the member is the initiation time (until the carbonation front reaches the bar) plus the propagation time (rusting, then cracking and spalling of the cover and loss of steel section).

## Check yourself

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

1. 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)

2. In the corrosion cell of steel in concrete, what happens at the anode and cathode?
   A. Anode: O2 is reduced to OH-; cathode: Fe is oxidised
   B. Both reactions dissolve iron as Fe2+
   C. Anode: Fe -> Fe2+ + 2e-; cathode: O2 + 2H2O + 4e- -> 4OH-
   D. Anode: Fe2+ is reduced to Fe; cathode: water is oxidised to O2
   Answer: C. Iron dissolves at the anode, giving electrons which are consumed with oxygen and water at the cathode. (Key point: Anode and cathode reactions)

3. What happens to the pH of the pore solution on carbonation and why does it matter?
   A. It falls from about 12.5 to 13.5 to about 8 to 9, destroying the passive film once the front reaches the bar
   B. It rises to above 14, protecting the steel
   C. It stays the same but strength drops
   D. It falls only at the steel surface because of chlorides
   Answer: A. Corrosion starts when the carbonation front reaches the reinforcement. (Key point: Carbonation pH drop)

4. In service life terms, what are the two stages of reinforcement corrosion?
   A. Initiation time (carbonation or chlorides reach the bar) and propagation time (rusting and damage)
   B. Curing time and setting time
   C. Mixing time and placing time
   D. Loading time and unloading time
   Answer: A. Carbonation and corrosion together shorten service life. (Key point: Service life model)

## Related in Concrete Technology and Masonry Structure

- [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
- [Permeability as root cause of poor durability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Apermeability-root-cause-durability) — Concrete Technology and Masonry Structure, chapter 6, asked 3×
- [Carbonation of concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acarbonation-phenomenon) — Concrete Technology and Masonry Structure, chapter 6
- [Causes of deterioration; effects of steel corrosion](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acauses-deterioration-corrosion-effects) — Concrete Technology and Masonry Structure, chapter 6, asked 2×
- [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

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