Usually in large construction works, whether the available aggregate is suitable or not for the work is determined by the impact value test of aggregate. The impact value test of aggregate gives a relative measure of the resistance of an aggregate to sudden shock or impact.
Steps by step procedure for Impact value test of aggregate
- The test sample consists of aggregate are of size passing through 12.5mm and retained on 10mm I.S. Sieve.
- Sample of aggregate shall be dried in an oven for a period of 4 hours at a temperature of 100 to 110-degree Celsius and then cooled.
- Then the aggregate is filled into the standard cylindrical measure in three layers approx. of equal depth.
- Each layer is tamped 25 times with the tamping rod as a straight edge. The weight of the sample contained in the cylinder measure is taken (A). The whole sample is filled into a cylindrical steel cup firmly fixed on the base of the machine.
- A hammer weighing about 14 kg. is raised to a height of 380mm above the upper surface of the aggregate in the cup and allowed to fall freely on 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 1 second.
- Then the crushed aggregate is removed from the cup and the whole of it is sieved on 2.36mm I.S. sieve.
Calculation Procedure of Impact Value Test
|Observations||Sample 1||Sample 2|
|A = weight of oven dry sample taken in mould.|
|B = weight of fraction passing 2.36mm sieve|
|Aggregate Impact Value (AIV) =B/A*100||C||D|
Then take the, Mean = C + D / 2
Therefore, The Impact Value of Aggregate =
- The aggregate crushing value should not be more than 45% for aggregate used for concrete other than wearing surfaces.
- Also, 30% for concrete used for wearing surfaces such as runway, roads and airfield pavements.