Every stone in a road surface is hammered thousands of times a day — by truck tyres, braking loads, and the shock of wheels hitting small surface irregularities. An aggregate that is strong under a slow squeeze can still shatter under sudden blows. The aggregate impact value (AIV) test measures exactly that: the toughness of aggregate, its resistance to sudden impact or shock.
What the Test Measures
The property being measured is toughness — the ability of a material to resist fracture under repeated impact. This is different from crushing strength (resistance to a gradually applied load, measured by the aggregate crushing value test). A rock can score well in one and poorly in the other, which is why specifications ask for both.
The Standards
- IS 2386 (Part IV) – 1963 — Methods of test for aggregates for concrete; Part 4 covers mechanical properties including the impact value
- BS 812-112 — Testing aggregates; method for determination of aggregate impact value (AIV)
The apparatus dimensions, hammer weight, drop height, and number of blows below are the same in both.
Apparatus
The impact testing machine, as specified in IS 2386 (Part IV), consists of:
| Item | Specification |
|---|---|
| Testing machine | Total weight 45–60 kg; metal base 22–30 kg with a plane lower surface ≥ 300 mm diameter; fixed on a level concrete floor or block ≥ 450 mm thick |
| Cylindrical steel cup | 102 mm internal diameter × 50 mm deep, wall ≥ 6.3 mm thick, inner surface case-hardened |
| Metal hammer (tup) | 13.5 to 14.0 kg; lower end cylindrical, 100 mm diameter × 50 mm long, 2 mm chamfer at the lower edge, case-hardened; slides freely in vertical guides |
| Free fall of hammer | 380 ± 5 mm onto the specimen |
| Cylindrical measure | 75 mm internal diameter × 50 mm deep, for measuring the sample |
| Tamping rod | 10 mm diameter × 230 mm long, rounded at one end |
| Balance | Capacity ≥ 500 g, accurate to 0.1 g |
| IS sieves | 12.5 mm, 10 mm, and 2.36 mm |
Test Procedure
- Prepare the sample. Sieve the aggregate through 12.5 mm and 10 mm IS sieves. The test material is the fraction passing 12.5 mm and retained on 10 mm. Dry it in an oven at 100–110 °C for about 4 hours and let it cool.
- Fill the measure. Pour the aggregate into the cylindrical measure to about one-third depth, compact with 25 gentle blows of the tamping rod, add a second layer the same way, then a third layer until the measure is full.
- Transfer to the cup. Empty the measure into the steel cup of the impact machine and tamp the surface 25 times with the tamping rod.
- Apply the blows. Raise the hammer to 380 mm above the top of the aggregate and let it fall freely. Repeat for a total of 15 blows, at intervals of about one second.
- Collect the crushed material. Remove the cup and sieve the entire crushed sample through the 2.36 mm IS sieve.
- Weigh. Record the total weight of the sample (W₁) and the weight of the fraction passing the 2.36 mm sieve (W₂), both to 0.1 g.
Calculation
Aggregate Impact Value (AIV) = (W₂ / W₁) × 100
- W₁ = total weight of the dry sample (g)
- W₂ = weight of fines passing the 2.36 mm sieve (g)
The result is reported as a percentage, to the nearest whole number.
Worked Example
A quarry sample is tested for a wearing-course mix:
- Total weight of dry sample, W₁ = 512.4 g
- Weight of material passing 2.36 mm sieve, W₂ = 118.6 g
AIV = (118.6 / 512.4) × 100 = 23.1%
A second test on the same sample gives 22.4%. Mean = 22.8% ≈ 23%.
Interpretation: 23% is below the 30% maximum for wearing courses, so this aggregate is acceptable for the surface layer (see the limits table below).
Limits: Where Can the Aggregate Be Used?
The Indian Roads Congress recommends these maximum aggregate impact values for different pavement layers:
| Pavement layer / use | Maximum AIV |
|---|---|
| Wearing / surfacing course (bituminous carpet, surface dressing, cement concrete surface) | 30% |
| Bituminous macadam, base / binder course | 35% |
| Water-bound macadam base course | 40% |
| Sub-base course | 50% |
| Cement concrete base course (as per ISI) | 45% |
AIV is also used to classify stone by toughness:
| AIV | Classification |
|---|---|
| Less than 10% | Exceptionally strong |
| 10 – 20% | Strong |
| 20 – 30% | Satisfactory for road surfacing |
| Greater than 35% | Weak for road surfacing |
Common Mistakes in the Impact Test
- Testing damp aggregate. Moisture cushions the blows and changes the result. Dry the sample properly and let it cool before testing.
- Sloppy tamping. Uneven or rushed tamping leaves voids, so the hammer energy goes into rearranging particles instead of breaking them — the value comes out low and flattering.
- Reusing crushed material. Once a sample has been hit, its weak particles are already gone. Always prepare a fresh sample for the second determination.
- A worn hammer or bent guides. The hammer must fall freely and concentrically. Friction in the guides robs energy and quietly invalidates months of results.
Aggregate Impact Value vs Crushing Value
The two tests are often confused because they use the same sample size (12.5–10 mm) and the same 2.36 mm sieve. The difference is what they measure and how:
| Impact value (AIV) | Crushing value (ACV) | |
|---|---|---|
| Property | Toughness — resistance to sudden impact | Strength — resistance to gradual crushing load |
| Loading | 15 hammer blows, 14 kg from 380 mm | Steady load of 40 tonnes over 10 minutes |
| Standard | IS 2386 Part IV / BS 812-112 | IS 2386 Part IV / BS 812-110 |
| Typical wearing-course limit | 30% | 30% |
Run both on any new aggregate source. Together they catch the two common failure modes of road stone: shattering under traffic shock and crushing under heavy wheel loads.
Frequently Asked Questions
What is the aggregate impact value test?
A laboratory test that measures the toughness of aggregate — its resistance to sudden impact — by dropping a 13.5–14 kg hammer 15 times from 380 mm onto a sample and measuring the percentage of fines produced.
What is the formula for aggregate impact value?
AIV = (W₂ / W₁) × 100, where W₁ is the total weight of the dry sample and W₂ is the weight of material passing the 2.36 mm sieve after the blows.
What is the maximum aggregate impact value for a wearing course?
30%, as recommended by the Indian Roads Congress. For bituminous macadam the limit is 35%, and for water-bound macadam base it is 40%.
What is the difference between AIV and ACV?
AIV measures toughness under repeated impact (hammer blows); ACV measures crushing strength under a gradually applied 40-tonne load. Both are required by most road specifications.
Why is the sample limited to 12.5–10 mm aggregate?
A single narrow size fraction removes the effect of grading, so the result reflects the stone’s own toughness rather than the mix of sizes in the sample.
Final Thoughts
The impact test takes less than an hour in the lab, but it answers a question that matters for the next twenty years of a road’s life: will this stone survive traffic, or will it grind itself into dust under the wheels? Test every new quarry source, test it dry, and hold wearing-course stone to the 30% line. Pavements are too expensive to build twice.
Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.
Mohsin Raza Adil
BSc Civil Engineering Technologist (NTC-registered), Pakistan. I write practical, field-tested guides on construction materials, soil testing and site engineering — the way it is actually done on site.
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