Aggregate Mechanical Testing & Morphological Controls
Pavement Engineering Handbook
Chapter 2.02: Aggregate Mechanical Testing & Shape Morphology
1. Mechanical Failure Assays: Crushing, Impact, and Abrasion Resistance
Mineral stone aggregates form the structural skeleton of flexible and rigid pavements, bearing over **90 to 95%** of the transferred vehicular stress. Under the **IS 2386 (Part 4)** protocol, their mechanical endurance is quantified through three critical indices:
- Aggregate Crushing Value (ACV): Evaluates the material's resistance to progressive crushing under a steady, compressive load. A graded sample is subjected to a $40\text{ tonnes}$ compressive force over $10\text{ minutes}$. The percentage of crushed material passing an **IS 2.36 mm sieve** yields the ACV percentage.
- Aggregate Impact Value (AIV): Quantifies structural toughness against sudden dynamic shock loads. The sample is subjected to 15 blows from a $14\text{ kg}$ hammer falling freely from a calibrated height of $380\text{ mm}$. A lower weight percentage passing the 2.36 mm sieve indicates a tougher stone matrix.
- Los Angeles Abrasion Value (LAAV): Simulates the continuous frictional wearing action of moving vehicle tires and abrasive dust. Aggregates are mixed with standard abrasive steel charges inside a rotating horizontal drum for 500 to 1000 revolutions. Fines passing an **IS 1.70 mm sieve** determine the LAAV percentage.
2. Morphological Property Frameworks: Flakiness & Elongation Gauges
The geometric shape of aggregate particles directly governs the structural packing efficiency, void ratios, and workability of hot-mix asphalt. Particle shapes are classified using **IS 2386 (Part 1)** thickness and length gauges:
- Flakiness Index (FI): The weight percentage of particles whose least dimension (thickness) is less than **0.6 times** their mean sieve size dimension:
$$t_{\text{limit}} = 0.6 \cdot d_{\text{mean}}$$
- Elongation Index (EI): The weight percentage of particles whose greatest dimension (length) is greater than **1.8 times** their mean sieve size dimension:
$$l_{\text{limit}} = 1.8 \cdot d_{\text{mean}}$$
QA/QC Red Flag: Excessively flaky and elongated particles fracture easily under field rollers during compaction and disrupt the locking mechanism of the stone matrix, leading to premature ravelling.
3. Durability & Hydrophobic Stripping Integrity
To prevent moisture-induced damage, aggregates must resist weathering and maintain a strong bond with the bituminous binder:
- Soundness Test (IS 2386 Part 5): Quantifies resistance to cyclic weathering. Samples are immersed in sodium sulfate or magnesium sulfate solutions for 5 cycles to simulate freeze-thaw/wet-dry actions.
- Bitumen Stripping Value (IS 6241): Evaluates the chemical affinity between the aggregate surface and the bitumen film. Pre-coated stone aggregates are immersed in water at **40°C** for 24 hours. The unstripped surface area must remain above **95%** to pass quality standards.
4. MoRTH Source Property Compliance Thresholds Matrix
**MoRTH Section 500 (Revision 5)** specifies maximum permissible mechanical criteria limits across various pavement structural layer mixes:
| Pavement Layer Type | Max LAAV Limit | Max AIV Limit | Max Combined FI + EI Limit |
|---|---|---|---|
| Granular Sub-Base (GSB) | 40% | 30% | Not Explicitly Tracked |
| Wet Mix Macadam (WMM) | 30% | 23% | 35% |
| Dense Bituminous Macadam (DBM) | 35% | 27% | 35% |
| Bituminous Concrete (BC) Wearing Course | 30% | 24% | 35% |
๐ฌ Interactive Combined Flakiness & Elongation Index Auditor
Input laboratory raw weight mass fractions to evaluate geometric compliance for sub-base, base, or asphalt concrete mix designs.
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