The impact value of aggregate is calculated for understanding which one of the following characteristic property of aggregate?
Correct Answer: B. Resistance to sudden dynamic load
🧱 Detailed Explanation: The Aggregate Impact Value (AIV) Test
The Aggregate Impact Value (AIV) test, specified in IS 2386 (Part IV): 1963, evaluates an aggregate’s toughness — its ability to absorb energy and resist fracture when subjected to sudden, repeated impact loading. This distinguishes it from the ACV test (gradual static load) and the LA test (continuous abrasion and impact in a rotating drum).
Why B (Resistance to sudden dynamic load) is correct:
In the AIV test, a 13.5 kg steel hammer is raised to a height of 380 mm and dropped 15 times onto a sample of 10–12.5 mm aggregate in a steel cup. The repeated sudden impact blows simulate the dynamic loads aggregates experience under traffic — particularly heavy commercial vehicles on road pavements. The fraction crushed through a 2.36 mm sieve gives the AIV%.
In the AIV test, a 13.5 kg steel hammer is raised to a height of 380 mm and dropped 15 times onto a sample of 10–12.5 mm aggregate in a steel cup. The repeated sudden impact blows simulate the dynamic loads aggregates experience under traffic — particularly heavy commercial vehicles on road pavements. The fraction crushed through a 2.36 mm sieve gives the AIV%.
Specific gravity (A) is measured by water immersion; abrasion (C) by the LA rotating drum; porosity (D) by water absorption. None involve a hammer-drop apparatus.
Static Load vs. Dynamic Load Tests (IS 2386 Part IV)
| Test | Load Type | Property | Apparatus |
|---|---|---|---|
| Aggregate Crushing Value (ACV) | Gradual static (40 t over 10 min) | Crushing strength | Steel cylinder + press |
| Aggregate Impact Value (AIV) | Sudden dynamic (15 hammer drops) | Toughness | Drop-hammer apparatus |
| Los Angeles Abrasion | Continuous abrasion + impact | Abrasion resistance | Rotating drum + steel balls |
AIV Acceptance Limits
| Application | Maximum AIV |
|---|---|
| Wearing course of road pavement | 30% |
| Bituminous macadam / runway concrete | 30% |
| General building concrete (non-structural) | 45% |
Key Concepts for Students
- AIV measures toughness (energy absorption capacity), NOT crushing strength or hardness — though all three diminish together in weak aggregates.
- Lower AIV = better aggregate: the less material passes the 2.36 mm sieve after 15 blows, the tougher the aggregate.
- The AIV uses the same sieve sizes as the ACV: 12.5 mm and 10 mm for sample preparation, 2.36 mm for post-test sieving. Only the loading mechanism differs.
- AIV is especially important for aggregates in road pavements and runways, which experience frequent sudden impact from vehicle wheel loads.