Impact Test on Aggregates

The aggregates play an important role in the construction industry. There are many tests which are performed to check the quality of aggregates. Aggregates are a very important component of concrete, so the quality really matters when it comes to aggregates.

The property of a material to resist impact is known as toughness. Due to the movement of vehicles on the road, the aggregates are subjected to impact. It results in their breaking down into smaller pieces. Therefore, the aggregates should have sufficient toughness to resist their disintegration due to the impact. This distinctive property is measured by impact value test. The aggregate impact test value is a measure of resistance to sudden impact or shock, which may vary from its resistance to gradually applied compressive load.

The Procedure of Impact Test on Aggregates:

The following procedure is applied, to find out the impact value of aggregate:

  • Aggregates for the test sample can be decided by passing it through 12.5 mm sieve and retained on 10 mm sieve.
  • The sieved aggregates should be dried in an oven and then filled in a cylindrical steel cup and tamped with 25 strokes by temping rod.
  • The test sample is filled in 3 layers and each layer is tamped for 25 numbers of blows.
  • Metal hammer (weighing approx. 14 kg) is pre-arranged to drop with a free fall of 380mm. The test specimen is subjected to 15 numbers of blows each at not less than 1 second.
  • The crushed aggregate is removed from the test specimen and sieve it through the 2.36 mm IS sieve.
  • An impact value is measured as % of aggregates passed through the 2.36mm sieve (W2) to the total weight of the sample (W1).

Aggregate impact value (The value of aggregate impact test) = (W1/W2)*100
The value of aggregate impact test should not be more than 45% by weight of aggregates, used for concrete other than wearing surface. An impact value 35% is permissible for the bituminous macadam. Maximum permissible impact value for the water bound macadam is 40%. The price of aggregate impact testing machine is approximately Rs 4000 to 6000 in India.

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