Coarse aggregate is one of the major constituents of concrete, occupying about 60–70% of the volume of hardened concrete. It provides the bulk strength, dimensional stability, and economy to concrete mixes. In construction, the two most commonly used sizes are 20 mm and 12 mm (also known as 12.5 mm) coarse aggregate.
Coarse aggregate is governed in India by IS 383 — Specification for Coarse and Fine Aggregates for Concrete, along with testing methods specified in IS 2386 (Parts 1–8).
Coarse aggregate refers to crushed stone or natural gravel particles that are retained on a 4.75 mm IS sieve and have a maximum size of 80 mm. It is the inert filler material in concrete that provides bulk strength.
Fine Aggregate: 0.075 mm to 4.75 mm (Sand)
Coarse Aggregate: 4.75 mm to 80 mm
| Type | Description | Common Use |
|---|---|---|
| Crushed Stone | Made by crushing granite, basalt, limestone | RCC concrete |
| Natural Gravel | Naturally rounded stones from riverbeds | PCC, where available |
| Recycled Aggregate | From crushed concrete waste | PCC, non-structural works |
| Lightweight Aggregate | Sintered fly ash, expanded clay | Lightweight concrete |
| Aggregate Size | Range | Typical Use |
|---|---|---|
| 40 mm | 20 to 40 mm | Mass concrete, foundations |
| 20 mm | 10 to 20 mm | RCC slabs, beams, columns (most common) |
| 12 mm (12.5 mm) | 4.75 to 12 mm | Thin slabs, congested reinforcement, plaster concrete |
| 10 mm | 4.75 to 10 mm | Tile bedding, screed |
| 6 mm | 2.36 to 6.3 mm | Fine concrete, repairs |
20 mm aggregate (passing through a 20 mm sieve and retained on a 10 mm sieve) is the most widely used coarse aggregate in residential and commercial construction in India.
12 mm aggregate is used where smaller-size aggregate is required — typically in thin sections, areas with congested reinforcement, or for surface concrete work.
| Parameter | 20 mm Aggregate | 12 mm Aggregate |
|---|---|---|
| Particle Size | 10 – 20 mm | 4.75 – 12 mm |
| Suitable Thickness of Section | > 100 mm | < 100 mm |
| Cement Demand | Lower | Higher |
| Water Demand | Lower | Higher |
| Workability | Lower (more harsh) | Better |
| Strength Contribution | Higher (less surface area) | Slightly lower |
| Use in Congested Reinforcement | Difficult | Better |
| Cost per m³ | Slightly lower | Slightly higher |
| Typical Use | Slabs, beams, columns | Thin sections, plaster concrete |
Aggregate is graded so that smaller particles fill the voids between larger ones, producing a denser, stronger concrete with less cement.
| IS Sieve Size | % Passing |
|---|---|
| 40 mm | 100 |
| 20 mm | 95 – 100 |
| 10 mm | 25 – 55 |
| 4.75 mm | 0 – 10 |
| IS Sieve Size | % Passing |
|---|---|
| 20 mm | 100 |
| 12.5 mm | 90 – 100 |
| 10 mm | 40 – 85 |
| 4.75 mm | 0 – 10 |
| Property | Typical Value | Test Standard |
|---|---|---|
| Specific Gravity | 2.6 – 2.8 | IS 2386 Part 3 |
| Bulk Density | 1450 – 1700 kg/m³ | IS 2386 Part 3 |
| Water Absorption | < 2% | IS 2386 Part 3 |
| Aggregate Crushing Value | < 30% (for general concrete) | IS 2386 Part 4 |
| Aggregate Impact Value | < 30% (for general concrete) | IS 2386 Part 4 |
| Aggregate Abrasion Value (Los Angeles) | < 30% | IS 2386 Part 4 |
| Flakiness Index | < 30% | IS 2386 Part 1 |
| Elongation Index | < 30% | IS 2386 Part 1 |
| Soundness (sulphate) | < 12% loss | IS 2386 Part 5 |
Pass aggregate through a set of standard sieves to verify grading limits per IS 383.
Measures the resistance of aggregate to crushing under gradually applied compressive load.
For general concrete: less than 30%; for wearing surfaces: less than 25%.
Measures the toughness of aggregate against sudden impact loads.
Determines aggregate density and porosity. Higher water absorption indicates more porous aggregate.
Aggregate is rotated in a steel drum with steel balls; abrasion value indicates wear resistance.
Measures shape characteristics. Flat or elongated particles weaken concrete; both should be less than 30%.
Sodium or magnesium sulphate is used to check resistance to weathering.
For mixed aggregate (60% 20 mm + 40% 12 mm):
| Standard | Description |
|---|---|
| IS 383 | Coarse and Fine Aggregates for Concrete — Specification |
| IS 2386 (Part 1) | Particle size and shape |
| IS 2386 (Part 2) | Deleterious materials |
| IS 2386 (Part 3) | Specific gravity, density, water absorption |
| IS 2386 (Part 4) | Mechanical properties |
| IS 2386 (Part 5) | Soundness |
| IS 2386 (Part 6) | Petrographic examination |
| IS 2386 (Part 7) | Alkali aggregate reactivity |
| IS 2386 (Part 8) | Bulk density and voids |
| IS 456 | Code of practice for plain and reinforced concrete |
| Material | Maximum Limit (IS 383) |
|---|---|
| Coal & Lignite | 1.0% |
| Clay Lumps | 1.0% |
| Materials Finer than 75 µ | 3.0% (uncrushed) / 1.5% (crushed) |
| Soft Fragments | 3.0% |
| Total Limit (all deleterious) | 5.0% |
| Type | Price per Cubic Foot (Rs) | Price per Tonne (Rs) |
|---|---|---|
| 20 mm Aggregate | 35 – 60 | 1,200 – 1,700 |
| 12 mm Aggregate | 40 – 65 | 1,300 – 1,800 |
| 10 mm Aggregate | 45 – 70 | 1,400 – 1,900 |
| 40 mm Aggregate | 30 – 50 | 1,000 – 1,500 |
Recycled aggregate is produced by crushing demolition concrete waste. It is increasingly used for sustainability and to reduce demand on natural stone resources.
IS 383:2016 now permits up to 25% replacement of natural aggregate with recycled aggregate for plain concrete and up to 20% for RCC (with restrictions).
Coarse aggregate is the structural skeleton of concrete. The choice between 20 mm and 12 mm depends on the thickness of the structural member, reinforcement spacing, and the strength grade required. For routine RCC work, a blend of both sizes gives the best workability and strength.
Always verify aggregate quality through sieve analysis, crushing value, water absorption, and visual inspection before use. Conformance to IS 383 and proper storage at site go a long way in producing durable, economical concrete.