Aggregates make up over 90% of the volume of road base courses and asphalt mixtures. Because aggregates bear the direct stresses transmitted by traffic loads, their mechanical properties determine the road's durability and lifespan.
This article explains three crucial aggregate tests required by public works departments (such as JKR) for road base materials: Aggregate Crushing Value, Aggregate Impact Value, and the Flakiness Index.
1. Aggregate Crushing Value (ACV)
ACV evaluates the aggregate's resistance to crushing under a gradually applied compressive load.
- Test Method: A standard mass of aggregate is placed in a steel cylinder and subjected to a 400 kN compressive load over 10 minutes. The crushed material is sieved through a 2.36mm sieve. The weight of the passing material is expressed as a percentage of the total sample weight: $\text{ACV (%)} = \frac{\text{Weight of Fines (<2.36mm)}}{\text{Total Weight}} \times 100$
- Significance: Lower ACV percentages represent stronger aggregates. For road wearing courses, JKR specifications typically require an ACV of less than 30% to prevent aggregate degradation under heavy wheel loads.
2. Aggregate Impact Value (AIV)
AIV measures aggregate toughness—its resistance to sudden shock or impact, which mimics the pounding action of vehicle tires.
- Test Method: Aggregates are placed in a small metal cup and subjected to 15 blows from a 14kg hammer falling freely from a height of 380mm. The percentage of crushed material passing the 2.36mm sieve is calculated: $\text{AIV (%)} = \frac{\text{Weight of Fines (<2.36mm)}}{\text{Total Weight}} \times 100$
- Significance: Like ACV, a lower AIV signifies a tougher aggregate. While ACV and AIV are correlated, aggregates can sometimes pass ACV but fail AIV, especially if they have micro-fissures from blasting.
3. Flakiness Index (FI)
The shape of the aggregate particles is just as important as their strength. The Flakiness Index measures the percentage of particles whose thickness is less than 0.6 times their mean dimension.
- Test Method: Aggregates are sorted using a standard thickness gauge containing slot-like openings of various sizes.
- Significance: Flat, flaky aggregates align horizontally during compaction, leaving excessive air voids and creating shear planes. Under roller and traffic loads, flaky particles fracture easily, changing the mix gradation and leading to early pavement failure. JKR specifications generally require a Flakiness Index below 25-30% for road base aggregates.