Aggregates in Concrete: Classification, Properties and Testing

Why Aggregates Matter in Concrete

Here’s a fact that surprises many students: aggregates make up 60–80% of the total volume of concrete. Cement paste is the binder, but aggregates are the bulk. They don’t just fill space — they significantly influence concrete’s strength, stiffness, durability, workability, shrinkage, and cost. Choosing the wrong aggregate can ruin an otherwise well-designed mix.

Poor aggregates can cause: excessive shrinkage (alkali-silica reaction from reactive silica in aggregates), weak concrete (soft, porous aggregates), poor workability (flaky, elongated particles), and cracking (unsound aggregates that expand on weathering). This is why IS 383 lays down detailed requirements for aggregates used in concrete.

Classification of Aggregates

Figure 1: Classification of Aggregates by Size, Origin, and Unit Weight

Fine Aggregate

Fine aggregate passes through a 4.75 mm IS sieve. Natural river sand is the traditional fine aggregate, but increasing environmental restrictions on river sand mining have made Manufactured Sand (M-Sand) — produced by crushing granite or basalt — increasingly popular. As per IS 383:2016, fine aggregate is classified into Zones I through IV based on fineness, with Zone II and III being ideal for concrete.

Coarse Aggregate

Coarse aggregate is retained on a 4.75 mm sieve. Maximum aggregate size (MAS) is chosen based on structural requirements — typically 20 mm for beams, columns, slabs and 40 mm for mass concrete. The MAS should not exceed 1/4 of the minimum dimension of the structural member, 1/3 the slab thickness, or 3/4 of the clear spacing between rebars.

Properties of Good Aggregates

Property	Requirement	Test
Specific Gravity	2.6–2.7 for normal weight	Pycnometer / Basket method
Water Absorption	< 2% (coarse), < 3% (fine)	IS 2386 Part III
Soundness	Loss < 12% (Na₂SO₄), <18% (MgSO₄)	IS 2386 Part V
Impact Value	< 30% (road), < 45% (concrete)	IS 2386 Part IV
Crushing Value	< 30% (road), < 45% (concrete)	IS 2386 Part IV
Abrasion (LA test)	< 30% (road), < 50% (concrete)	IS 2386 Part IV
Flakiness Index	< 25% (IS 383)	IS 2386 Part I
Elongation Index	< 15% (IS 383)	IS 2386 Part I

Shape and Texture of Aggregates

Shape significantly affects workability and strength. Rounded aggregates (river gravel) have low surface area per unit volume → better workability, less cement paste needed → but lower bond strength with cement paste. Angular, irregular aggregates (crushed stone) have higher surface area → better interlocking and bond → but need more water for same workability.

Flakiness Index (FI): Percentage (by mass) of aggregate particles whose least dimension is less than 0.6 times the mean sieve size. Flaky particles are weak and add to water demand. IS 383 limits FI to 25%.

Elongation Index (EI): Percentage of particles whose greatest dimension exceeds 1.8 times the mean sieve size. Elongated particles also reduce workability and strength. IS 383 limits EI to 15%.

Key Tests on Aggregates

1. Los Angeles Abrasion Test (IS 2386 Part IV)

Measures hardness/resistance to abrasion. Aggregate sample is placed in a steel drum with steel balls and rotated. The percentage of material finer than 1.7 mm after the test is the LA value. Lower = harder. Aggregate with LA value > 30% is not suitable for road wearing courses but can be used for concrete (limit < 50%).

2. Aggregate Impact Test (IS 2386 Part IV)

A sample of aggregate is subjected to 15 blows of a 14 kg hammer falling from 380 mm height. Aggregate Impact Value (AIV) = (mass passing 2.36 mm / total mass) × 100%. AIV < 30% for concrete, < 20% for wearing courses.

3. Aggregate Crushing Value Test (IS 2386 Part IV)

Aggregate sample is compressed under a load of 400 kN in a steel cylinder. ACV = % of material passing 2.36 mm sieve. ACV < 30% for general concrete, < 25% for high traffic road construction.

IS 383:2016 Requirements

IS 383:2016 is the governing standard for coarse and fine aggregates from natural sources for concrete. Key requirements: clay/silt content (washed through 75 μm sieve) < 3% for fine aggregate, < 1% for coarse aggregate; organic impurities must pass colorimetric test; sulphate content (as SO₃) < 0.4%; chloride content (as Cl) < 0.06% for reinforced concrete.

🎯 Exam Tips (RTMNU)

The 4.75 mm sieve is the dividing line between fine and coarse aggregate — state this clearly.
Know the three limits for max aggregate size (¼ member dimension, ⅓ slab thickness, ¾ rebar spacing) — all three are needed in one answer.
FI uses 0.6× mean size; EI uses 1.8× mean size — these numbers are common MCQ traps.
LA value limits: <30% for road, <50% for concrete — different limits for different applications.
For Impact and Crushing tests, the same limit of <30% applies for general concrete.
IS 383:2016 limits: silt in fine aggregate <3%, coarse aggregate <1%.

✅ Key Takeaways

Aggregates = 60–80% of concrete volume; quality matters enormously.
Fine aggregate: passes 4.75 mm sieve; Zone II/III ideal for concrete (IS 383).
Coarse aggregate: retained on 4.75 mm; max size governed by member dimensions and rebar spacing.
Key properties: specific gravity, absorption, soundness, hardness (LA, impact, crushing tests).
Shape: Angular > Rounded for strength; but rounded gives better workability.
FI < 25%, EI < 15% as per IS 383.

🔗 External Reference: IS 383:2016 – BIS Aggregate Specification

❓ FAQs

Q1. What is the maximum aggregate size allowed in an RCC beam of 200 mm width with 25 mm spacing between bars?

Maximum aggregate size = smallest of (1/4 × 200 = 50 mm) and (3/4 × 25 = 18.75 mm). So maximum aggregate size = 18.75 mm → use 16 mm or 20 mm nominal max size (rounded down to available standard size: 16 mm).

Q2. What is the difference between LA abrasion value and aggregate impact value?

LA Abrasion Value measures resistance to progressive abrasion/wearing action. Aggregate Impact Value measures resistance to sudden impact/shock loads. Both express the % of fines generated — lower value = stronger aggregate. LA test is more relevant for pavement aggregates.

Q3. Why is M-Sand becoming popular as a replacement for river sand?

Environmental regulations restrict river sand mining. M-Sand (Manufactured Sand from crushed granite/basalt) has controlled grading, lower clay/silt content, and consistent quality. However, it tends to be angular and may require more water; this is managed by using plasticizers.

Q4. What causes alkali-silica reaction in concrete?

Alkali-Silica Reaction (ASR) occurs when reactive amorphous silica in aggregates reacts with hydroxyl ions from alkalis (Na, K from cement) to form an expansive silica gel that absorbs water and causes cracking. Prevented by using non-reactive aggregates, low-alkali cement, or adding fly ash/GGBS.

Q5. What is the soundness test for aggregates?

Soundness test (IS 2386 Part V) checks if aggregates disintegrate under weathering. Aggregates are subjected to 5 cycles of immersion in Na₂SO₄ (or MgSO₄) solution and drying. Weight loss should be <12% for Na₂SO₄ cycles and <18% for MgSO₄ cycles.