Combined Flakiness & Elongation Index
(IS: 2386 Part I)
| IS Sieve Size (mm) | FLAKINESS INDEX | ELONGATION INDEX | |||
|---|---|---|---|---|---|
| Passing | Retained | Wt. of Fraction (W1) | Wt. Passing Thickness Gauge (W2) | Wt. Retained Thickness Gauge (W3) (Non-Flaky) |
Wt. Retained Length Gauge (W4) |
| 63 | 50 | 0 | |||
| 50 | 40 | 0 | |||
| 40 | 31.5 | 0 | |||
| 31.5 | 25 | 0 | |||
| 25 | 20 | 0 | |||
| 20 | 16 | 0 | |||
| 16 | 12.5 | 0 | |||
| 12.5 | 10 | 0 | |||
| 10 | 6.3 | 0 | |||
| TOTAL | 0 | 0 | 0 | 0 | |
Flakiness Index % (W) = (Total W2 / Total W1) * 100
0.00 %
Elongation Index % (w) = (Total W4 / Total W3) * 100
0.00 %
Combined Flakiness & Elongation Index = (W + w)
0.00 %
Acceptance Limit: Max 30% (As per MORTH)
Technical Guide: Combined Flakiness & Elongation Index
The **Combined Flakiness and Elongation Index** is a crucial quality control test for coarse aggregates used in road construction (DBM, BC, WBM) and concrete mixes. It determines the shape of the aggregate particles.
1. Why is this test important?
Aggregates that are "flaky" (thin like a coin) or "elongated" (long like a needle) are mechanically weak. Under heavy traffic loads (rollers or vehicles), these particles tend to break or crush easily compared to cubical (blocky) aggregates. This crushing reduces the strength of the road pavement or concrete structure.
MORTH Specifications: For Bituminous Concrete (BC) and Dense Bituminous Macadam (DBM), the combined index must not exceed 30%. For WBM base layers, up to 35% is sometimes permitted.
2. Standard Definitions (IS 2386 Part 1)
- Flakiness Index: The percentage by weight of particles whose least dimension (thickness) is less than 0.6 times their mean dimension.
- Elongation Index: The percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension.
3. Testing Procedure
- Step A: Sieve the aggregate and separate into fractions (e.g., 20-16mm, 16-12.5mm). Weigh each fraction (W1).
- Step B (Flakiness): Try to pass each particle through the slot of the Thickness Gauge. The weight passing is W2. The weight retained is W3 (Non-Flaky).
- Step C (Elongation): Take the Non-Flaky portion (W3) and test it on the Length Gauge. The weight retained (that doesn't fit) is W4.
4. Engineering Implications
Using poor quality aggregate with high flakiness results in:
- Poor Interlocking: Flaky particles do not lock together well, leading to road deformation.
- Higher Bitumen Consumption: Flaky particles have more surface area, requiring more bitumen to coat them, increasing costs.
- Reduced Strength: Concrete made with flaky aggregates has lower compressive strength.