Aggregate Impact Value Test: Procedure, Formula and Limits

The aggregate impact value (AIV) measures how well stone stands up to sudden shocks. Road stone gets hammered: wheels drop into joints, loaded trucks bounce over patches, and compaction rollers vibrate on it during construction. A weak stone shatters into fines, and those fines change the gradation of a base or mix. The impact test reproduces that pounding on a small sample in a few minutes. It is used widely in India and the UK and appears in many highway specifications.

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Quick facts

Standards: IS 2386 (Part 4); BS 812-112 in the UK.
Sample size fraction: passing 12.5 mm, retained on 10 mm.
Hammer: 13.5 to 14.0 kg, dropped 380 mm, 15 blows.
Fines sieve: 2.36 mm.
Result: percent fines; lower means tougher stone.

The equipment

An impact machine weighing 45 to 60 kg, bolted to a solid floor or block so it cannot rock.
A case-hardened steel cup, 102 mm across and 50 mm deep.
A cylindrical measure, 75 mm across and 50 mm deep, used to set the sample volume.
A 10 mm tamping rod, 230 mm long with a rounded end.
Sieves of 12.5, 10 and 2.36 mm, a balance reading to 0.1 g and a drying oven at 100 to 110 degrees C.

Step-by-step procedure

Prepare the stone. Sieve out the 12.5 to 10 mm fraction and dry it in the oven for four hours, then let it cool.
Fill the measure. Add stone in three roughly equal layers, rodding each with 25 strokes, strike off the top level and weigh the stone (A). Use that same mass for the repeat test.
Load the cup. Move the sample into the steel cup in one go and compact it with 25 more strokes.
Strike it. Raise the hammer so its face is 380 mm above the stone and let it fall freely. Do this 15 times, at least one second apart.
Sieve. Empty the cup, sieve the whole sample on 2.36 mm and weigh the fines that pass (B).
Repeat with a second sample and average the two results.

Formula and worked example

AIV = (B ÷ A) × 100

Made-up example: the measure holds 340 g of stone. After 15 blows, 61 g passes the 2.36 mm sieve. AIV = 61 ÷ 340 × 100 ≈ 17.9%. A second sample gives 18.6%. The mean, 18.3%, is reported as 18.

Common specification limits

IS 383, concrete wearing surfaces (roads, runways, pavements): 30% maximum.
IS 383, other concrete: 45% maximum. For concrete of grade M65 and above, 22% maximum.
MoRTH bituminous concrete wearing course: 24% maximum.

Base and sub-base layers usually get looser limits than surface courses, because the stone there takes less direct punishment. Always use the limit in the contract you are working under.

Tips for good results

Keep the machine on a solid, level base. A machine that rocks absorbs part of each blow and gives a falsely low value.
Check the drop height before each test; a short drop flatters the stone.
Use oven-dry stone as the method requires. Some specifications also ask for a soaked-condition version for weaker stone, and soaked results should not be compared with dry ones.

How it relates to other toughness tests

The impact value is close in purpose to the aggregate crushing value and the Los Angeles abrasion test. The LA test tumbles stone with steel balls, so it combines impact and grinding. The micro-Deval test adds water and is better at catching stone that breaks down when wet. A full aggregate approval often uses one toughness test plus a soundness or durability test.

Questions people ask

What is aggregate impact value?

It is the percentage of fines produced when a standard sample of stone is struck by a falling hammer. A lower aggregate impact value means tougher stone that resists sudden blows, such as heavy wheels hitting a pothole edge.

How is the aggregate impact value test done?

Oven-dried stone passing a 12.5 mm sieve and retained on a 10 mm sieve is tamped into a 102 mm steel cup. A hammer weighing 13.5 to 14.0 kg is dropped 380 mm onto it 15 times. The crushed sample is sieved on 2.36 mm, and the fines are weighed.

What is the formula for AIV?

AIV = (mass of fines passing 2.36 mm / total mass of the sample) x 100. Run two tests and report the mean to the nearest whole number.

What is a good aggregate impact value?

Lower is better. IS 383 allows up to 30% for aggregate in concrete wearing surfaces and 45% for other concrete. India's MoRTH specification sets a 24% maximum for bituminous concrete wearing course aggregate.

What is the difference between impact value and crushing value?

The impact test hits the stone with repeated sudden blows. The crushing value test squeezes it under a slowly applied 40-tonne load. Some stones behave differently under the two, so specifications may call for either one.

Is the impact test used in the United States?

It is mainly a British and Indian test. US agencies more often rely on the Los Angeles abrasion test and the micro-Deval test to judge aggregate toughness and durability.

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