--- title: "Desirable properties of road aggregates" subject: "Transportation Engineering I" chapter: 6 key: "semester_6/transportation_engineering_i:aggregate-desirable-properties" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/aggregate-desirable-properties site: BCT Engineering --- # Desirable properties of road aggregates Transportation Engineering I · Chapter 6: Highway Materials - Asked in: 2078 Chaitra, 3 marks; 2075 Bhadra; 2074 Bhadra, 8 marks; 2073 Bhadra, 4 marks; 2071 Bhadra (asked 5 times) - Video: [Unit:6 | Highway Material | Desirable properties of Road Aggregate | Los Angele's Test | Abrasion](https://www.youtube.com/watch?v=qhdIILY56AE) — Prashant YT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/aggregate-desirable-properties - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-6 ## Question Enumerate the desirable properties of road aggregates (to be used in different types of pavement construction). ## Short answer Desirable properties include strength (crushing value ≤30%), hardness (LA abrasion ≤30-40%), toughness (impact value ≤30%), durability (soundness loss ≤12%), angular cubical shape (flakiness+elongation ≤30-35%), rough texture (polished stone value ≥55), good bitumen adhesion, low water absorption (≤2%), proper grading, cleanliness, specific gravity 2.5-3.0, and local availability. ## Answer Road aggregates form the main part of a pavement (about 90-95% by weight), so they must have the following properties (IRC, NRS 2070 and IS:383). 1. **Strength** - resistance to crushing under wheel loads, particularly in the base and surface (aggregate crushing value not more than 30% for the wearing course). 2. **Hardness** - resistance to wear and abrasion by traffic (LA abrasion value below 30-40%). 3. **Toughness** - resistance to impact, in the surface and base (aggregate impact value below 30% for wearing courses; 45% for lower layers). 4. **Durability / soundness** - resistance to weathering, frost and chemical action (soundness loss not more than 12% with sodium sulphate). 5. **Shape** - angular, cubical particles that interlock; flaky and elongated particles are avoided (combined flakiness and elongation index below 30-35%). 6. **Surface texture and skid resistance** - a rough texture for good skid resistance (polished stone value at least 55 to 60 for the wearing course). 7. **Affinity with bitumen (adhesion)** - hydrophobic aggregate that resists stripping in the presence of water (basic rocks such as limestone and basalt are better than siliceous ones such as quartzite, granite or siliceous gravel). 8. **Low water absorption and porosity** - not more than 2% (porous aggregates absorb bitumen and are weak). 9. **Good grading** - proper particle size distribution for dense packing. 10. **Cleanliness** - free from clay, silt, organic matter and weak materials. 11. **Specific gravity** - sufficient (2.5-3.0) and uniform. 12. **Economy and availability** near the site. #### Different pavement types - **Flexible pavement surface** - hard, tough, skid resistant, well shaped, with good bitumen adhesion. - **Base and sub-base** - strength, grading and drainage. - **Rigid pavement** - good strength, low alkali reactivity, low thermal expansion and cleanliness. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which property is the resistance to impact? A. Strength B. Toughness C. Hardness D. Shape Answer: B. Toughness is resistance to impact, measured by the aggregate impact value; hardness is wear resistance. (Key point: Strength and toughness) 2. Which particle shape is desirable? A. Flaky and elongated B. Angular, cubical C. Rounded smooth D. Weak and thin Answer: B. Angular cubical particles interlock well. (Key point: Aggregate shape) 3. Which type of aggregate has good affinity with bitumen? A. Hydrophobic, such as limestone, basalt or granite B. Hydrophilic, such as quartzite or siliceous gravel C. Clayey aggregate D. Porous aggregate Answer: A. Hydrophobic aggregates resist stripping in water. (Key point: Bitumen affinity) 4. Which property gives skid resistance on wearing course? A. Rough surface texture and PSV of at least 55 to 60 B. High water absorption C. Smooth polished surface D. Low specific gravity Answer: A. Rough texture resists polishing, and PSV measures this. 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