--- title: "Aggregate sizes and importance of grading" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:aggregate-size-grading-importance" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-size-grading-importance site: BCT Engineering --- # Aggregate sizes and importance of grading Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2078 Bhadra, 6 marks - Video: [Lecture-8 Concrete Technology|| Aggregates](https://www.youtube.com/watch?v=8Z1vuxlQSxM) — A Civil Structures - 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-size-grading-importance - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question Define nominal size, single size and maximum size of aggregate with example. Why is grading of aggregate important with regard to the properties of concrete? ## Short answer Nominal size is the sieve designating the aggregate (e.g., 20 mm graded passing 40 mm and 20 mm limits). Single size has most particles between two adjacent sieves (e.g., 20 mm single size passing 40 mm, retained on10 mm). Maximum size is the smallest sieve with100% passing. Grading reduces voids, saving paste, and improves workability, strength, density, and economy. ## Answer #### Definitions (IS 383, IS 2386) - **Nominal size**: the size by which the aggregate is designated, i.e. the sieve size through which most of it passes. A "20 mm nominal size graded aggregate" (IS 383 Table 7) has 100% passing 40 mm, 95-100% passing 20 mm, 25-55% passing 10 mm and 0-10% passing 4.75 mm. - **Single size aggregate**: most particles pass one sieve and are retained on the next smaller one. E.g. 20 mm single size: 100% passes 40 mm, 85-100% passes 20 mm, 0-20% passes 10 mm and 0-5% passes 4.75 mm. It is used for making up a grading by blending. - **Maximum size**: the smallest sieve on which practically no material is retained, i.e. the sieve through which 100% of the aggregate passes. E.g. an aggregate with 100% passing 40 mm and 95% passing 20 mm has a maximum size of 40 mm and a nominal size of 20 mm. #### Importance of grading Grading is the distribution of particle sizes (found by sieve analysis, IS 2386 Part 1). - A well-graded aggregate has the fewest voids, so less cement paste is needed to fill them: **economy**. - Better **workability**, with less segregation and bleeding. - Higher **density and strength** and lower permeability, so more durable concrete. - Less shrinkage, because less paste and water are required. - Poor grading (too much of one size, or gap-graded) gives harsh mixes, high water demand, honeycombing and higher cement use. - Grading uniformity is checked by the fineness modulus and the grading limits of IS 383 Tables 7 and 9. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the nominal size of an aggregate? A. The sieve through which 100% passes B. The size designating the aggregate, i.e. the sieve through which most of it passes C. The largest particle in the sample D. The size retained on the 4.75 mm sieve Answer: B. Nominal size is the designation size, e.g. 20 mm graded aggregate with 85-100% passing 20 mm. (Key point: Nominal size) 2. A single size aggregate is one in which: A. All sizes are present in proportion B. Most particles pass one sieve and are retained on the next smaller one C. Only particles above 40 mm are present D. Sand and gravel are mixed Answer: B. Single size aggregate lies mostly between two adjacent sieves and is blended to make a grading. (Key point: Single size aggregate) 3. An aggregate has 100% passing 40 mm and 90% passing 20 mm. Its maximum and nominal sizes are: A. 20 mm and 40 mm B. 40 mm and 20 mm C. 40 mm and 40 mm D. 20 mm and 20 mm Answer: B. Maximum size is the sieve through which 100% passes (40 mm); nominal size is 20 mm. (Key point: Maximum size) 4. Why does a well-graded aggregate improve concrete? A. It has most voids, so more paste is used B. It has least voids, so less paste fills voids, giving economy, workability and strength C. It increases shrinkage D. It removes the need for w/c control Answer: B. Least voids means less paste and water, better workability and density, and less shrinkage. 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