--- title: "Requirements of aggregates; need of grading" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:aggregate-requirements-grading" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-requirements-grading site: BCT Engineering --- # Requirements of aggregates; need of grading Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2075 Ashwin, 4+2 marks - Video: [Gradation of Aggregate CONCRETE TECHNOLOGY MODULE 1.3.9](https://www.youtube.com/watch?v=08DthLekRtU) — Career UnderConstruction - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-requirements-grading - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question Explain the basic requirements of coarse and fine aggregates in concrete which are to be used in the construction field. Why is there a need for grading of aggregate? ## Short answer Aggregates must be clean, hard, strong, durable, and free from coatings, organic matter, and deleterious salts. Coarse aggregate max size ≤1/4 member thickness, crushing value ≤45%, flakiness index ≤40%; sand in grading zones I–IV. Grading minimizes voids, yields dense packing, reduces cement paste, and enhances workability, strength, and durability. ## Answer #### Requirements of coarse and fine aggregates (IS 383:2016) **Common to both** - Clean, hard, strong, durable and chemically inert; free from clay, silt, organic matter, coal, mica, salts and shale (deleterious materials within the limits of IS 383). - Free from alkali-reactive and unsound material (soundness loss in the sodium sulphate test within 12% for coarse and 10% for fine aggregate). - Proper grading and shape. **Coarse aggregate** - Crushed stone or gravel; nominal size of 10, 20 or 40 mm chosen to suit the section (IS 456 cl. 5.3.3: not more than 1/4 of the minimum thickness of the member, and 5 mm less than the minimum clear bar spacing or cover). - Crushing value at most 45% and impact value at most 45% (30% for wearing surfaces); abrasion value at most 50% (30% for wearing surfaces). - Flakiness and elongation within limits (combined index not more than 40%); angular, rough texture preferred; water absorption generally below 2%. - Specific gravity 2.5-3.0. **Fine aggregate** - Natural sand, crushed stone sand or crushed gravel sand passing the 4.75 mm sieve, in grading Zones I-IV (IS 383 Table 9). - Fineness modulus 2.0-3.5; organic impurities checked by the colour test (the colour should not be darker than the standard); bulking must be allowed for. #### Need for grading Grading is the particle size distribution. It is needed so that the smaller particles fill the voids between the larger ones, giving the fewest voids and a dense, strong, workable and economical concrete that needs the minimum cement paste and water. It prevents segregation, harshness and honeycombing, and ensures uniform quality. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a requirement common to both coarse and fine aggregates? A. Must contain some clay for binding B. Must be clean, hard, strong, durable and chemically inert, free from deleterious matter C. Must be rounded and smooth only D. Must pass 4.75 mm sieve Answer: B. Aggregates must be clean, hard, durable and free from clay, silt, organic matter and other deleterious material. (Key point: General aggregate requirement) 2. According to IS 456, nominal maximum size of coarse aggregate should be not more than: A. 1/4 of minimum member thickness B. Half of the member thickness C. Equal to the bar spacing D. Twice the cover Answer: A. IS 456 limits it to 1/4 of the minimum thickness and 5 mm less than clear bar spacing. (Key point: Maximum coarse size limit) 3. Fine aggregate is described as: A. Retained on 4.75 mm sieve, FM 5-7 B. Passing 4.75 mm sieve, in zones I-IV with FM 2.0-3.5 C. Passing 40 mm, any grading D. Only natural river sand Answer: B. Fine aggregate passes 4.75 mm, falls in zones I-IV and has FM 2.0-3.5. (Key point: Fine aggregate requirement) 4. Why is grading needed? A. To make small particles fill voids between large ones, minimising paste and water B. To increase cement content C. To make mixes harsh D. To increase segregation Answer: A. Filling voids gives dense, strong, workable and economical concrete. 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