Aggregate Impact Value test

AIM:

To determine the aggregate impact value of coarse aggregates as per IS: 2386 (Part IV) – 1963.

APPARATUS:

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i) Impact testing machine conforming to IS: 2386 (Part IV) – 1963
ii) IS Sieves of sizes – 12.5mm, 10mm and 2.36mm
iii) A cylindrical metal measure of 75mm dia. and 50mm depth
iv) A tamping rod of 10mm circular cross section and 230mm length, rounded at one end
v) Oven

PREPARATION OF SAMPLE:

i) The test sample should conform to the following grading:
                                               – Passing through 12.5mm IS Sieve 100%
                                               – Retention on 10mm IS Sieve 100%
ii) The sample should be oven-dried for 4hrs. at a temperature of 100 to 110oC and cooled.
iii) The measure should be about one-third full with the prepared aggregates and tamped with 25 strokes of the tamping rod. A further similar quantity of aggregates should be added and a further tamping of 25 strokes given. The measure should finally be filled to overflow, tamped 25 times and the surplus aggregates struck off, using a tamping rod as a straight edge. The net weight of the aggregates in the measure should be determined to the nearest gram (Weight ‘A’).

PROCEDURE:

i) The cup of the impact testing machine should be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by 25 strokes of the tamping rod.
ii) The hammer should be raised to 380mm above the upper surface of the aggregates in the cup and allowed to fall freely onto the aggregates. The test sample should be
subjected to a total of 15 such blows, each being delivered at an interval of not less than one second.

RESULTS:

i) The sample should be removed and sieved through a 2.36mm IS Sieve. The fraction passing through should be weighed (Weight ‘B’). The fraction retained on the sieve
should also be weighed (Weight ‘C’) and if the total weight (B+C) is less than the initial weight (A) by more than one gram, the result should be discarded and a fresh test done.
ii) The ratio of the weight of the fines formed to the total
sample weight should be expressed as a percentage.
Aggregate impact value = B/A x 100%
iii) Two such tests should be carried out and the mean of the results should be reported.
A sample proforma for the record of the test results is given in the below table.

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Recommended Aggregate Impact Value :

Various agencies have specified the maximum permissible aggregate Impact values for the different types of pavements. IRC has specified the following values: 

  1. The maximum allowable aggregate Impact value for water bound Macadam; Sub-Base coarse 50%
  2. Where as cement concrete used in base course is 45%.
  3. WBM base course with Bitumen surface in should be 40%.
  4. Bituminous Macadam base course should have A.I.V of 35%.
  5. All the surface courses should possess an A.I.V below 30%.

DISCUSSION:

Aggregate Impact Value test determines the Aggregate Impact Value (AIV) of aggregates which provides a relative measure of the resistance of an aggregate to sudden shock or impact.

Resistance of the aggregates to impact is termed as toughness. Aggregates used in the pavement should be able to resist the effect caused by the jumping of the steel tyred wheels from one particle to another at different levels causes severe impact on the aggregates.

The aggregate Impact value indicates a relative measure of the resistance of aggregate to a sudden shock or an Impact, which in some aggregates differs from its resistance to a slope compressive load in crushing test. A modified Impact test is also often carried out in the case of soft aggregates to find the wet Impact value after soaking the test sample.

Lower the aggregate crushing value greater will be the “resistance to to sudden shock or impact (toughness)”.

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