--- title: "Effects of carbonation and permeability on durability" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:carbonation-permeability-durability" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/carbonation-permeability-durability site: BCT Engineering --- # Effects of carbonation and permeability on durability Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2072 Kartik, 6 marks - Video: [Effect of Permeability on Durability of Concrete](https://www.youtube.com/watch?v=Zk_5LsYQCYs) — Ahmed Emad - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/carbonation-permeability-durability - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question What are the effects of carbonation and permeability on concrete durability? ## Short answer Carbonation reduces alkalinity, depassivates steel and causes corrosion that cracks and spalls concrete. Permeability governs the ingress rate of CO₂, water and chlorides; low permeability slows carbonation and attack, greatly extending service life. Both are mitigated by low w/c ratio, adequate cover and proper curing. ## Answer #### Effect of carbonation on durability **Carbonation** is the reaction of atmospheric CO$_2$ with the calcium hydroxide of the cement paste: $$ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} $$ - 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 reduces alkalinity, so the steel passive layer breaks down and **corrosion** begins; rust expansion then cracks and spalls the cover. - Surface shrinkage cracks may form, giving access to more CO$_2$, water and chlorides. - It slightly increases surface hardness, but this does not offset the corrosion risk. #### Effect of permeability on durability - Permeability controls how quickly water, oxygen, CO$_2$, chloride and sulphate ions enter the concrete. - Low permeability (low w/c, good curing and compaction, supplementary cementitious materials) delays carbonation, corrosion, sulphate attack, freeze-thaw damage and leaching, and so extends service life. - High permeability makes the concrete deteriorate quickly. #### Link between them Carbonation depth increases with the permeability of the concrete to CO$_2$: dense concrete of w/c 0.45 carbonates a few mm in 50 years, while w/c 0.7 concrete may carbonate to 30 mm or more. Both effects are therefore controlled by the same measures: w/c not more than 0.45 to 0.55, minimum cement content, full compaction, curing for 7 to 10 days and adequate cover (IS 456 Tables 4, 5 and 16). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. 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) 2. Which factors increase the rate of carbonation? A. Low w/c and long curing B. Thick cover and coatings C. High w/c, low cement content, poor curing, cracks and high CO2 concentration D. Fully saturated concrete Answer: C. Permeable concrete lets CO2 in faster; prevention uses low w/c, curing and cover. (Key point: Factors for carbonation) 3. How does w/c affect carbonation depth over 50 years? A. Both carbonate to the same depth B. w/c 0.45 carbonates a few mm, while w/c 0.7 may reach 30 mm or more C. w/c 0.7 carbonates less than w/c 0.45 D. Carbonation depth is independent of permeability Answer: B. Carbonation depth is related to permeability to CO2. (Key point: Carbonation and permeability) 4. 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) ## 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 - [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× - [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 - [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 - [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 ## Related in other subjects - [Regression of permeability on compressive strength](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Aconcrete-permeability-regression-sums) — Probability and Statistics, chapter 5