Aggregates

Aggregates — Detailed Notes

Types • Classification (size, shape, texture, moisture) • IS:383 grading zones • Shape indices & formulas

Decision Flowchart — Choose Aggregate Type & Quality Checks

Figure — quick selection & QC flow for aggregates.

1) Types of Aggregates (by source)

• Natural aggregates: River/river-bed sand, crushed rock from quarries (granite, gneiss, limestone etc.).
• Manufactured aggregates: Manufactured sand (M-sand), crushed gravel/stone.
• By-products & industrial: Blast furnace slag, steel slag, copper slag, bottom ash.
• Recycled: Recycled Concrete Aggregate (RCA) from demolition.

Classification & permitted use per IS:383 (see grading & limits). 0

2) Classification — Size (Coarse & Fine)

Fine aggregate: Mostly passes 4.75 mm IS sieve (sands). Grading zones I–IV exist for fine aggregates (IS:383 table limits). 1

Coarse aggregate: Single-size common nominal sizes: 10, 12.5, 16, 20, 40, 63 mm. Graded coarse aggregates: 12.5/16/20/40 mm as per IS. 2

Key definitions (nominal max size, max size, single-sized)

• Maximum aggregate size: smallest sieve through which 100% of material passes.
• Nominal maximum size: smallest sieve size through which most of the material passes (some retained allowed).
• Single-size: material retained almost entirely on a given sieve (for e.g., 20 mm single-size).

3) Shape & Surface Texture

• Shape: Rounded (river pebbles), sub-angular, angular, flaky/elongated (undesirable in excess).
• Surface texture: Smooth (polished river rock) → lower bond; Rough/angular → better mechanical interlock, higher water demand.
• Effect on concrete: Angular/rough increases strength and interlock but increases water/paste demand; rounded gives better workability but less bond.

Control of shape/texture is critical for mix-design & workability. (See shape indices below.)

4) Moisture States & % Moisture

Aggregates exist in different moisture states — these affect batch water and must be corrected in mix calculations:

• Oven-dry (OD): All moisture removed. Used for laboratory measurements of water absorption.
• Air-dry (AD): Some internal moisture; surface appears dry.
• Saturated Surface Dry (SSD): Pores saturated but no free surface water — reference state for batching.
• Wet / Damp: Free surface water present — contributes extra water to the mix.

Percent moisture (by mass):
% Moisture = (Mass_wet - Mass_od) / Mass_od × 100

When batching, correct batch water as: Batch water = Target water − (Free surface moisture × Aggregate mass). SSD basis is recommended for aggregates. 3

5) Grading Zones for Fine Aggregates — IS:383 (Table for Grading Zone I–IV)

The grading limits for fine aggregates (percent passing by mass at each IS sieve) are given below (Table 9, IS:383). These are the commonly used grading zones for concrete mixes. 4

IS Sieve	Zone I (% passing)	Zone II	Zone III	Zone IV
10.0 mm	100	100	100	100
4.75 mm	90–100	90–100	90–100	95–100
2.36 mm	60–95	75–100	85–100	85–100
1.18 mm	30–70	55–90	75–100	90–100
600 μm	15–34	35–59	60–79	80–100
300 μm	5–20	8–30	12–40	15–50
150 μm	0–10	0–10	0–10	0–10

Note: special tolerances & recommendations are in the standard (e.g., crushed stone sands may have higher % on 150 μm). See IS:383 product manual and clause notes for tolerances. 5

6) Mean Dimension, Flakiness Index & Elongation Index

Mean dimension (Dm)

For a particle retained on a given sieve, mean dimension is the average of its three principal dimensions (length L, breadth B, thickness T):

Mean dimension (Dm) = (L + B + T) / 3

Flakiness Index (FI)

Definition: Percentage by mass of particles whose least dimension (thickness) is less than 0.6 × mean dimension (Dm). This test & definition are given in IS:2386 (Part 1). 6

Flakiness Index, FI (%) = (Mass of flaky particles / Total mass of sample tested) × 100

Procedure (brief): Take representative sample of specified size fractions (e.g., 10–20 mm), gauge each particle for least dimension using the IS gauge; separate flaky particles; weigh and compute percentage. See IS 2386 for full method. 7

Elongation Index (EI)

Definition: Percentage by mass of particles whose greatest dimension (length) is greater than 1.8 × mean dimension (Dm). (IS:2386). 8

Elongation Index, EI (%) = (Mass of elongated particles / Total mass of sample tested) × 100

Typical acceptance: Lower FI & EI are preferred. Project specifications usually limit combined flakiness+elongation (IS:383 requires combined check). 9

7) Angularity Number

Meaning: Angularity number quantifies the degree of angularity (absence of rounding) of coarse aggregates by measuring voids in compacted single-sized specimen. The method is standardized in IS:2386 (Part 1) / related methods. 10

Calculation (concept):

Angularity number = (Measured % voids in compacted sample) − 33

Interpretation: Rounded aggregates have about 33% voids; angularity number typically ranges 0–12 (higher → more angular). The exact test procedure (cylinder compaction and volumetric determinations) is given in the standard. 11

Why angularity matters

• More angular → better interlock & mechanical bond with paste, but higher water/paste demand and reduced workability.
• Used in bituminous mixes and where interlock is important (base/sub-base, certain concrete mixes).

8) Example: Flakiness & Elongation Calculation (Worked)

Sample of 10–20 mm coarse aggregate, total mass = 10 000 g.

• After sieving & gauging, flaky particles mass = 1200 g → FI = 1200/10000 × 100 = 12%.
• Elongated particles mass = 300 g → EI = 300/10000 × 100 = 3%.

IS:383 asks for combined checking (flakiness + elongation) during QC; refer clause 5.3. 12

9) Useful Formulas & Quick Diagrams

Percent passing (sieve analysis)

% Passing on a sieve = (Mass passing sieve / Total mass taken) × 100

Bulk density & voids (concept)

Voids (%) = (1 − Bulk density / Particle density) × 100

Diagram — Shape thresholds (schematic)

10) Practical QC & Specification Tips

• Always test new sources for grading, deleterious content, flakiness/elongation and mechanical properties (ACV/LA/ICV) — IS:383 recommends test frequencies & methods. 13
• Use SSD basis in batching or correct for free moisture; perform absorption tests (IS:2386 Part 3).
• For critical structural or wearing surfaces, control flakiness index and angularity as per project specs.
• Blend sizes to get desired packing & minimize paste demand (use Fuller-Thompson or other packing concepts).

References & Standards

• IS 383 (Coarse & Fine Aggregate for Concrete) — product manual & grading clauses (see Product Manual / Table 9 for grading zones). 14
• IS 2386 (Methods of test for aggregates) — Part 1 & Part 3 detail particle size & shape tests (flakiness, elongation, angularity, absorption). 15
• Aggregate testing & lab guides — practical procedure notes (example lab guides & civil-engineering resources). 16

If you want, I can convert this into a printable PDF, include IS clause extracts (exact clause numbers & table images), or create an editable canvas file for iterative edits.