--- title: "Water, permeability and durability; corrosion remedy" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:water-permeability-durability-corrosion" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/water-permeability-durability-corrosion site: BCT Engineering --- # Water, permeability and durability; corrosion remedy Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2078 Bhadra, 6 marks - Video: [How to control Corrosion of steel embedded in concrete](https://www.youtube.com/watch?v=DQH2pD_lLeg) — Highway Guru - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/water-permeability-durability-corrosion - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question Discuss the relation between water and permeability on concrete durability. Write the corrosion of steel and its remedial measure. ## Short answer Water carries aggressive ions; excess mixing water creates interconnected capillary pores, increasing permeability and reducing durability. Steel corrosion occurs when chlorides or carbonation break the passive film, forming rust that expands and cracks concrete. Remedial measures include low w/c, adequate cover, PPC, epoxy coatings, and patch repair. ## Answer #### Water, permeability and durability Water is the vehicle for almost all deterioration processes: it carries aggressive ions (chlorides, sulphates), enables carbonation and corrosion, and freezes in pores. The mixing water decides the pore system: water in excess of that needed for hydration (w/c above about 0.38) is left as **capillary pores**, which become interconnected, so **permeability increases rapidly with w/c** (e.g. between w/c 0.4 and 0.7 it may rise by several hundred times). Higher permeability means faster ingress and less durable concrete. Hence IS 456 (Table 5) limits w/c to 0.40 to 0.55 by exposure, and requires full compaction and curing. #### Corrosion of steel Steel is protected by a passive film in the alkaline concrete (pH above 12.5). Chlorides or carbonation break this film down. With moisture and oxygen, an electrochemical cell forms: anode $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$, cathode $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$. The product is rust of 2 to 6 times the volume of the steel, which cracks and spalls the cover and reduces the bar area. #### Remedial measures - Prevent: low w/c, enough cement, adequate cover (Table 16), limited chloride, PPC, coatings, epoxy-coated or galvanised bars, inhibitors, and crack control. - Repair: remove the cracked cover, clean the bars to bright metal, apply an anti-corrosive primer, patch with polymer-modified mortar or micro-concrete, and apply a protective coating, or use cathodic protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is water called the vehicle of deterioration? A. It always protects steel B. It carries aggressive ions, enables carbonation and corrosion, and freezes in pores C. It makes concrete denser D. It only matters during mixing Answer: B. Almost all deterioration processes need water. (Key point: Role of water) 2. Why does permeability rise rapidly with w/c above about 0.38? A. Excess water makes extra gel pores that are sealed B. More water increases cement content C. Higher w/c reduces air voids D. Excess water beyond hydration needs leaves capillary pores that become interconnected Answer: D. Between w/c 0.4 and 0.7 permeability may rise several hundred times. (Key point: w/c and capillary pores) 3. 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) 4. What is the correct repair sequence for corroded reinforcement? A. Remove cracked cover, clean bars to bright metal, apply anti-corrosive primer, patch with polymer-modified mortar, apply protective coating B. Paint over the rust and plaster the surface C. Add more water to the patch D. Cut the bars out and leave them unprotected Answer: A. Removing rust and unsound concrete and protecting the bar prevents recurrence. 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