Aggregates (Fine & Coarse)

Aggregates are inert granular materials — sand, gravel, crushed stone — that occupy 60-75% of concrete volume and provide most of the structural strength. The Indian Standard IS 383:2016 specifies coarse and fine aggregate requirements; IS 2386 covers test methods. Aggregates are classified by particle size: fine aggregate (sand) ≤ 4.75 mm, coarse aggregate > 4.75 mm. Coarse aggregate is further classified by maximum size — 10 mm, 12.5 mm, 20 mm (most common), 40 mm (mass concrete only). Aggregates contribute to concrete via three mechanisms: load transfer through grain-to-grain contact, dimensional stability (low thermal expansion), and economy (cheaper than cement).

Key aggregate properties: (1) Grading (particle-size distribution) — well-graded aggregate has fewer voids, requires less cement paste; gap-graded has more voids and is unsuitable for normal concrete. IS 383 specifies grading limits (Table 7) for fine aggregate (Zones I-IV) and coarse aggregate (Tables 8-9). (2) Specific gravity — typical 2.6-2.8 for granitic, basaltic, limestone aggregates; affects concrete density and yield. (3) Bulk density — 1500-1750 kg/m³ for coarse aggregate; 1400-1600 for sand. (4) Water absorption — 0.5-2% for granite/basalt; 5-12% for limestone (more porous); high absorption requires moisture correction in mix design. (5) Soundness — resistance to weathering; tested per IS 2386 Part 5.

Indian aggregate sources: (a) Crushed stone (granite, basalt, limestone) — primary coarse aggregate from quarries. (b) Natural river sand — historically dominant fine aggregate but increasingly restricted by environmental regulations (river bed mining bans). (c) M-sand (manufactured sand from crushed stone) — rapidly displacing river sand for fine aggregate, especially in tier-1 and tier-2 cities. (d) Recycled aggregate from construction-and-demolition (C&D) waste — emerging as sustainable alternative; IS 383:2016 permits up to 25% replacement in non-structural concrete. The most-overlooked aggregate-related issue: river sand and M-sand have different particle shapes (rounded vs angular) — affecting workability and water demand. Mix designs must be re-trialled when switching aggregate sources, never substituted equivalent quantities.