--- title: "Aggregate shape, grading and alkali-aggregate reaction" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:aggregate-shape-grading-aar" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-shape-grading-aar site: BCT Engineering --- # Aggregate shape, grading and alkali-aggregate reaction Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2069 Chaitra, 2+2+1 marks - Video: [Concrete Technology | Chapter- Aggregate | Video 005](https://www.youtube.com/watch?v=au-ahL__1fU) — Silverline Techno Management Services - 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-shape-grading-aar - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question How can the shape of aggregate affect the properties of hardened concrete? How does the grading of aggregate affect the water requirement of the mix? Also explain the effects of Alkali-Aggregate reaction. ## Short answer Angular, rough aggregate improves bond and strength; flaky/elongated particles reduce strength and durability. Well-graded aggregate requires less water due to fewer voids and lower surface area. Alkali-aggregate reaction forms expansive gel causing map cracking, expansion and strength loss; controlled by non-reactive aggregate, low-alkali cement and pozzolans. ## Answer #### Effect of aggregate shape on hardened concrete - **Rounded** particles pack well, but have weaker bond with paste, so slightly lower flexural strength. - **Angular, rough** aggregate has better interlock and bond: higher compressive and flexural strength and a better abrasion resistance, but needs more water and paste. - **Flaky and elongated** particles lie flat, trap bleed water beneath them, and cause weak planes, lower strength and lower durability (higher permeability). - Shape also affects shrinkage and creep indirectly via paste content. #### Effect of grading on water requirement - A **well-graded** aggregate has fewer voids and a smaller surface area for a given workability, so **less water and paste** is required. - Too many fines (fine grading or excess sand) increase the surface area and water demand; too coarse or gap-graded mixes cause harshness and bleeding. - Hence good grading lowers w/c ratio for the same slump and improves strength. #### Alkali-aggregate reaction (AAR) It is the chemical reaction between alkalis (Na$_2$O, K$_2$O) from cement and reactive silica (opal, chert, strained quartz) or certain carbonates in the aggregate, in the presence of moisture. - It forms an expansive alkali-silica gel that absorbs water and swells, producing **cracks (map cracking), expansion, pop-outs and loss of strength and durability**, appearing after several years. - Control: use non-reactive aggregate, low-alkali cement (below 0.6% Na$_2$O equivalent), pozzolans (fly ash, slag) and keep concrete dry. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Compared with rounded aggregate, angular rough aggregate gives: A. Better interlock and bond, higher strength, but more water and paste demand B. Lower strength and less water demand C. No change D. Weaker bond Answer: A. Angular rough particles bond and interlock better but need more water and paste. (Key point: Angular aggregate) 2. Why are flaky and elongated particles harmful? A. They improve packing B. They lie flat, trap bleed water beneath and create weak planes C. They reduce permeability D. They increase bond Answer: B. Bleed water gathers beneath them, causing weak planes, lower strength and durability. (Key point: Flaky particles) 3. How does good grading affect water requirement? A. Raises it B. No effect C. Lowers it, since voids and surface area are smaller D. Raises w/c ratio Answer: C. Fewer voids and smaller surface area need less water and paste, lowering w/c for same slump. (Key point: Grading and water) 4. Alkali-aggregate reaction occurs between: A. Cement alkalis and reactive silica in aggregate in the presence of moisture, forming expansive gel B. Sulphates and steel C. Sand and water only D. Gypsum and aggregate Answer: A. Alkali-silica gel swells, causing map cracking and pop-outs. 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