Aggregate Impact Test

During the construction process of pavement layers, compaction by heavy rollers, and also due to movement of heavy wheel loads of traffic the road aggregates is subjected to impact.

The aggregate impact test is carried out to evaluate the resistance to the impact of aggregates to fracture under repeated impacts; the test has been standardized by the Bureau of Indian Standards (BIS).

The aggregate impact machine consists of a metal base a cylindrical steel cup of internal dia 102 mm and depth 50 mm in which the aggregate specimen is placed. A cylindrical metal hammer of weight 13.5 to 14.0 kg having a free fall from a height 380 mm is arranged to drop through vertical guides.

Aggregates specimen passing 12.5 mm sieve and retained on 10 mm sieve is filled the cylindrical measure in three layers by tamping each layer by 25 blows by tamping rod. The hammer is raised to a height of 380 mm above the upper surface of the aggregate in the cup and is allowed to fall freely on the specimen. After subjecting the test specimen to 15 blows the crushed aggregate is sieved on a 2.36 mm sieve. The aggregate impact value is expressed as the % of the fines passing 2.36 mm sieve formed in terms of the total weight of the sample.