Aggregate Impact Test

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The aim of aggregate impact test is to determine the relative measure of the resistance of aggregate to sudden shock or impact in which in some aggregate differs from its resistance to a slowly applies compressive load.

The property of a material to resist impact is known as toughness. Due to movement of vehicles on the road, the aggregate are subjected to impact resulting in their breaking down into smaller pieces.

The aggregates should therefore have sufficient toughness to resist their disintegration due to impact. This characteristic is measured by impact value test.
The aggregate impact value is a measure of resistance to sudden impact or shock, which may differ from its resistance to gradually applied compressive load.

Aggregate Impact Value {AIV}

• AIV is the percentage of fines produced from the aggregate sample subjecting it to a standard amount of impact.

• The standard amount of impact is produced by a known weight, i.e. a steel cylinder, falling a set height, a prescribed number of times, onto an amount of aggregate of standard size and weight retained in a mould.

• Aggregate Impact Values, AIV’s, below 10 are regarded as strong, and AIV’s above 35 would normally be regarded as too weak for use in road surfaces.

• Aggregate Impact Values and Aggregate Crushing Values are often numerically very similar, and indicate similar aggregate strength properties.

Recommended Aggregate Impact Test Values

Classification of aggregate using Aggregate Impact value is as given below

• Exceptionally strong = <10%

• Strong – 10 to 20%

• Satisfactory for road surfacing – 20 to 30%

• Weak for road surfacing = >35%

Specified limits of percent aggregate impact value for different types of road construction by Indian Roads Congress is Given Below.

1. Wearing Course – Not more than 30%

2. Bituminous surface dressing – Not more than 30%

3. Penetration macadam – Not more than 30%

4. Bituminous carpet concrete – Not more than 30%

5. Cement concrete – Not more than 30%

6. Bitumen bound macadam base course – Not more than 35%

7. WBM base course with bitumen surfacing – Not more than 40%

8. Cement concrete base course – Not more than 45%

Procedure

To carry out the aggregate impact test, the followings are the procedures use:

1. Test sample are prepare as follows;

The sample is sieve through test sieves sizes of 14.00mm, 10.00mm and the material passing through sieve 14.00mm is collected while that of 10.00mm is retain.
The material collected is dry in the oven for about 2 hours (with the temperature not exceeding 100c) and is allow to cool for another hours.
The cylindrical measure is fill (each layer is about one third the weight of the measure) and each layer is tamp with 25 blows of the tamping rod. Each blow is given by allowing the tamping rod to fall freely from a height of about 50mm above the surface of the aggregate evenly distributed over the surface.
The net mass of the aggregate in measure is recorded.

2. The steel cup is fix firmly in position of the base of the machine and the whole test sample is place in it and is compacted by a single tamping of 25 strokes of the tamping rod as above

3. The height of the hammer is adjust to that its lower face is 380 + or – 5mm above the upper surface of the aggregate and then allow it to fall freely to the aggregate.

4. The aggregate is subjected to a total of 15 blows, each being deliver at an interval of not less than 1 sec.

5. The crushed aggregate is remove and pass through size 2.36mm

6. The fractions passing through and retain is weigh on the sieve.

7. The whole procedure is repeated using the second sample of the mass.