Aggregate Impact Value IS: 2386 (PART-4)

The aggregate impact value indicates a relative measure of the resistance of an aggregate to a sudden shock or an impact, which differs from its resistance to a slow gradually increasing compressive load.

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Why to test Aggregate Impact Value?

Toughness is the property of a material to resist impact. Due to traffic loads the road stone are subjected to the pounding action of impact and there is possibility of breaking into smaller pieces. The road stone should therefore be tough enough to resists fracture under impact. A test designed to evaluate the toughness of stones i.e. the resistance of the stones to fracture under repeated impacts may be called an impact test for road stones.

Apparatus

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Preparation of the Test Sample

• The test sample shall consist of aggregate the whole of which passes a 12.5-mm IS Sieve and is retained on a 10-mm IS Sieve.
• The aggregate comprising the test sample shall be dried in an oven for a period of four hours at a temperature of 100 to 110°C and cooled.
• The measure shall be filled about one-third full with the aggregate and tamped with 25 strokes of the rounded end of the tamping rod. further similar quantity of aggregate shall be added and a further tamping of 25 strokes given.
• The measure shall finally be filled to overflowing, tamped 25 times and the surplus aggregate struck off, using the tamping rod as a straight-edge.
• The net weight of aggregate in the measure shall be determined to the nearest gram (Weight WA) and this weight of aggregate shall be used for the duplicate test on the same material.

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Test Procedure

• The impact machine shall rest without wedging or packing upon the level plate, block or floor, so that it is rigid and the hammer guide columns are vertical.
• The cup shall be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by a single tamping of 25 strokes of the tamping rod.
• The hammer shall be raised until its lower face is 380 mm above the upper surface of the aggregate in the cup, and allowed to fall freely on to the aggregate. The test sample shall be subjected to a total of 15 such blows each being delivered at an interval of not less than one second.
• The crushed aggregate shall then be removed from the cup and the whole of it sieved on the 2.36-mm IS Sieve until no further significant amount passes in one minute.
• The fraction passing the sieve shall be weighed to an accuracy of 0.1 g (Weight W_B).
• The fraction retained on the sieve shall also be weighed (Weight W_C)
• If the total weight (W_B+W_C) is less than the initial weight (Weight W_A) by more than one gram, the result shall be discarded and a fresh test made.
• Two tests shall be made.

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Calculate Aggregate Impact Value

The ratio of the weight of fines formed to the total sample weight in each test shall he expressed as a percentage, the result being recorded to the first decimal place:

Aggregate Impact Value = $\frac{W_B}{W_A}\times 100$

Where,

• W_A is Weight of oven-dried sample
• W_B is Weight of fraction passing through 2.36-mm IS Sieve

Reporting of Results

The mean of the two results shall be reported to the nearest whole number as the aggregate impact value of the tested material.

Maximum Permissible Aggregates Values for the different types of pavements

Sr.	Type of Pavements	Maximum Aggregates Value % (IRC Recommendations)
1	Granular Sub Base (G.S.B.)	40%
2	Base course W.B.M (Water Bound Macadam)	30%
3	Base course W.M.M. (Wet Mix Macadam)	30%
4	Base course Crusher Run Macadam	30%
5	Base course / Binder course B.M. (Bituminous Macadam)	30%
5	Base course / Binder course D.B.M. (Dense Bituminous Macadam)	27%
6	Surface course / Wearing course S.D.B.C. (Semi Dense Bituminous Concrete)	27%
7	Surface course / Wearing course B.C. (Bituminous Concrete)	24%
8	Cement Concrete Pavement (Wearing Surface)	30%
9	Cement Concrete Pavement (Other than Wearing Surface)	30%