--- title: "Causes of deterioration; effects of steel corrosion" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:causes-deterioration-corrosion-effects" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/causes-deterioration-corrosion-effects site: BCT Engineering --- # Causes of deterioration; effects of steel corrosion Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2075 Ashwin, 4+3 marks; 2071 Chaitra, 6 marks (asked 2 times) - Video: [Deterioration of Concrete Structures: Part 1](https://www.youtube.com/watch?v=wjH6OtBbbW8) — Curious Civil Engineer - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/causes-deterioration-corrosion-effects - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question Explain the physical and chemical causes of concrete deterioration. What are the effects of corrosion of steel in concrete? ## Short answer Physical causes include freeze-thaw, thermal effects, shrinkage, abrasion, erosion and salt crystallization. Chemical causes are sulphate and acid attack, alkali-aggregate reaction, chloride ingress, carbonation and leaching. Corrosion leads to expansive rust, cracking, spalling, loss of steel section and bond. ## Answer #### Physical causes Physical causes act by mechanical, thermal or moisture processes without changing the chemical composition of cement paste. | Cause | Mechanism | Effect and control | |---|---|---| | **Freezing and thawing** | Water in pores freezes, expanding by 9%, causing hydraulic pressure; repeated cycles | Scaling, cracking, D-cracking. Use air entrainment (4 to 7%), low w/c, sound aggregate | | **Thermal effects** (temperature cycle, fire) | Differential expansion/contraction; above 300 °C paste loses water, above 500 °C Ca(OH)2 decomposes | Cracking, spalling, loss of strength. Joints, cover, insulation | | **Shrinkage and moisture movement** | Drying shrinkage restrained by steel/supports | Cracks, which open the way for aggressive agents. Low w/c, curing, joints | | **Abrasion** | Wear by traffic, sliding or rolling | Surface loss in pavements, floors. Hard aggregates, high strength, surface treatments | | **Erosion and cavitation** | High velocity water with silt, or vapour bubbles collapsing | Pitting of spillways. Smooth surface, high-strength concrete | | **Salt crystallisation** | Salt solution crystallises in pores on drying | Pressure causing scaling and efflorescence. Dense concrete, membranes | | **Wetting and drying** | Moisture gradients cause cyclic stress | Cracking and surface deterioration | All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them. #### Chemical causes Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete. | Cause | Reaction | Effect and control | |---|---|---| | **Sulphate attack** | Sulphates (soil, groundwater, sea) react with $\text{Ca(OH)}_2$ and C3A to form gypsum and ettringite | Expansion, cracking, softening. Use sulphate-resistant cement (IS 12330), PPC, low w/c, dense concrete (IS 456 Table 4) | | **Alkali-aggregate (silica) reaction** | Alkalis of cement react with reactive silica of aggregate to form an expansive gel which swells with moisture | Map cracking, pop-outs. Use non-reactive aggregates, low-alkali cement, fly ash | | **Acid attack** | Acids dissolve $\text{Ca(OH)}_2$ and C-S-H into soluble salts | Loss of material, softening. Dense concrete, protective coatings | | **Chloride attack** | Chlorides destroy the passive film of steel | Corrosion of reinforcement. Limit chloride (IS 456 Table 7, 0.6 kg/m$^3$ for RCC) | | **Carbonation** | $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$ | Lowers pH, then corrosion. Low w/c, cover | | **Leaching and efflorescence** | Soft water dissolves $\text{Ca(OH)}_2$ and carries it out | Loss of strength, white deposits | | **Sea water** | Combined action of sulphate, chloride, crystallisation, abrasion | Use PPC/slag, w/c 0.45 or less, cover | | **Biological** | Bacteria produce sulphuric acid in sewers | Surface erosion | The rate is controlled by permeability: the denser the concrete, the slower the attack. #### Effects of corrosion of steel in concrete 1. Expansive rust creates tensile stress; **cracks run along the bars**, followed by **spalling** of the cover and **delamination**. 2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member. 3. **Loss of bond** between steel and concrete, so anchorage fails. 4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process). 5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does freezing and thawing damage concrete? A. Ice contracts and pulls the paste apart B. Frozen water reacts with C3A to form ettringite C. Freezing dissolves Ca(OH)2 which leaches out D. Water in pores expands about 9% on freezing, creating hydraulic pressure; repeated cycles crack and scale it Answer: D. The 9% expansion of freezing pore water causes pressure; control by air entrainment (4 to 7%) and low w/c. (Key point: Freeze-thaw mechanism) 2. Why does drying shrinkage cause cracks? A. Concrete expands on drying and crushes B. Shrinkage only occurs when concrete freezes C. Shrinkage is restrained by steel or supports, producing tensile stress; the cracks let aggressive agents in D. Shrinkage is caused by sulphates Answer: C. Restraint turns shrinkage into tension; control by low w/c, curing and joints. (Key point: Shrinkage cracking) 3. What is the mechanism of sulphate attack? A. Sulphates dissolve silica to form a gel B. Sulphates lower pH and corrode steel directly C. Sulphates react with Ca(OH)2 and C3A to form gypsum and ettringite, which expand and crack the concrete D. Sulphates leach Ca(OH)2 as white deposits only Answer: C. The expansive products cause cracking and softening; use sulphate-resisting cement, PPC and low w/c. (Key point: Sulphate attack) 4. Which is the correct sequence/set of effects of corrosion on an RC member? A. Increase in bar area and bond, so strength increases B. Cracks along bars, spalling and delamination, loss of bar area and bond, loss of serviceability C. Only surface staining with no structural effect D. Uniform shrinkage of the whole member Answer: B. Rust expansion cracks and spalls cover; pitting reduces steel area, bond is lost and serviceability fails. (Key point: Effects of corrosion) ## Related in Concrete Technology and Masonry Structure - [Physical and chemical causes of deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aphysical-chemical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 5× - [Chemical causes of concrete deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Achemical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 5× - [Physical causes of concrete deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aphysical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 6× - [Mechanical and physical causes of deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amechanical-physical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 6× - [Physical properties of OPC and concrete behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-physical-properties) — Concrete Technology and Masonry Structure, chapter 1 ## Related in other subjects - [Causes of deterioration of stone](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acauses-deterioration-stone) — Civil Engineering Materials, chapter 2, asked 4× - [Physical and chemical weathering](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-and-chemical-weathering) — Engineering Geology I, chapter 6, asked 5× - [Physical and chemical disintegration](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aphysical-chemical-disintegration) — Soil Mechanics, chapter 1 - [Chemical versus physical weathering; factors](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Achemical-versus-physical-weathering-factors) — Engineering Geology I, chapter 6