--- title: "Physical and chemical causes of deterioration" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:physical-chemical-causes-deterioration" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/physical-chemical-causes-deterioration site: BCT Engineering --- # Physical and chemical causes of deterioration Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2078 Kartik, 6 marks; 2071 Shrawan, 6 marks; 2070 Chaitra, 6 marks; 2068 Chaitra, 4 marks; 2065 Shrawan (old course), 5 marks (asked 5 times) - Video: [Top 8 types of concrete deterioration and their prevention || concrete defects || #civilogy](https://www.youtube.com/watch?v=ozieevP7I5I) — civilogy - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/physical-chemical-causes-deterioration - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question Write down (explain in brief) the physical and chemical causes of concrete deterioration. ## Short answer Physical causes: freezing-thawing, thermal effects, shrinkage, abrasion, erosion, salt crystallization, wetting-drying; all increase permeability. Chemical causes: sulphate attack, alkali-aggregate reaction, acid attack, chloride corrosion, carbonation, leaching, sea water, biological attack. Control by dense concrete, low w/c, proper cement, and curing. ## 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. ## 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. What happens to cement paste at high temperature in a fire? A. It gains strength because more hydration occurs B. Above 300 °C paste loses water and above 500 °C Ca(OH)2 decomposes, causing cracking, spalling and strength loss C. Only the aggregate is affected D. Ca(OH)2 forms above 500 °C Answer: B. Dehydration and decomposition of Ca(OH)2 weaken the paste. (Key point: Thermal and fire effects) 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. What causes alkali-silica reaction? A. Chlorides react with aggregate to form rust B. CO2 reacts with Ca(OH)2 in aggregate C. Sulphates react with C3A in cement D. Cement alkalis react with reactive silica in aggregate to form an expansive gel that swells with moisture Answer: D. Gel swelling causes map cracking and pop-outs; use non-reactive aggregate, low-alkali cement or fly ash. 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