--- title: "Aggregate size; well-graded from grading curve" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:aggregate-size-well-graded" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-size-well-graded site: BCT Engineering --- # Aggregate size; well-graded from grading curve Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2067 Magh (old course), 3+2 marks - Video: [Particle Size Distribution Curve](https://www.youtube.com/watch?v=cYsAU8PkiAE) — Elementary Engineering - 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-well-graded - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question Write short notes on the size of aggregate used in concrete construction. How is it determined that aggregate is well graded or not from the grading curve? ## Short answer Coarse aggregates sizes include 40,20,12.5,10 mm; max size limited to1/4 member thickness and5 mm less than bar spacing. Fine aggregate passes4.75 mm. A well-graded aggregate shows a smooth S‑shaped grading curve wholly within IS 383 upper and lower limits; a flat step indicates gap grading, a steep curve uniform grading. ## Answer #### Size of aggregate Aggregates are classified by size (IS 383): - **Fine aggregate**: mostly passes the 4.75 mm sieve (down to about 150 micron). - **Coarse aggregate**: mostly retained on the 4.75 mm sieve. - Standard coarse sizes: 40 mm, 20 mm, 12.5 mm and 10 mm, either graded or single size. - The **maximum size** is chosen according to the thickness of the member, the spacing of the bars and the method of placing: 20 mm for ordinary RCC, 40 mm and above for mass concrete and foundations, and 10-12.5 mm for thin sections or pumped mixes. - A larger size gives less surface area, so less water and cement are needed, but too large a size reduces strength because of weaker bond, and it is also limited by cover and bar spacing. For high-strength concrete, a smaller maximum size (10-20 mm) is better. - Size is determined by sieve analysis using the IS sieves 80, 40, 20, 10, 4.75 mm, 2.36 mm ... 150 micron. #### Judging well-graded from the grading curve Plot the sieve size on a log x-axis against the percentage passing on the y-axis. ```text % pass 100| ____.----- upper limit | _.-' ..--- curve of sample | .-' ..--' 50| .' .-' lower limit |.'-' 0+-----------------------> sieve size (log) ``` - The aggregate is **well graded** if the curve is smooth and continuous (S-shaped) and lies **within the upper and lower limits** given in IS 383 (Table 7 for graded coarse aggregate and Zones I-IV for sand). - A curve with a flat portion (step) shows **gap grading**; a steep curve shows a single-size (uniformly graded) material. - A curve lying above the upper limit means the aggregate is too fine, and one below the lower limit means it is too coarse. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Coarse aggregate is defined as aggregate: A. Passing 4.75 mm and retained on 150 micron B. Retained on 4.75 mm sieve C. Passing 80 mm only D. Retained on 150 micron Answer: B. Coarse aggregate is retained on the 4.75 mm sieve; fine passes 4.75 mm. (Key point: Fine vs coarse) 2. What is the effect of a very large maximum aggregate size? A. Reduces surface area, so less water and cement, but too large reduces strength due to weak bond B. Always increases strength C. Increases water demand D. Allows closer bar spacing Answer: A. Larger size lowers surface area, but excessive size weakens bond and is limited by cover and bar spacing. (Key point: Selecting maximum size) 3. A grading curve indicates a well-graded aggregate when it is: A. Flat with steps B. Very steep C. Smooth, continuous and within IS 383 upper and lower limits D. Above the upper limit Answer: C. Smooth S-shaped curve inside the envelope limits indicates good grading. (Key point: Well-graded curve) 4. A flat portion in a grading curve shows: A. Gap grading B. Well grading C. Too fine material D. Single size material Answer: A. A flat step means a missing size range, i.e. gap grading; a steep curve means single size. 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