Q. Write short notes on Impact Test?

Impact test for ballast shall be carried out using Impact testing machine.

The test sample shall be prepared out of ballast so as to conform to following grading:

Passing 12.5mm IS sieve 100% and Retention 10mm IS sieve 100%.

The sample shall be oven dried for 4 Hours at a temperature of 100 - 110°C and cooled.

The measure shall be filled about one-third full with the prepared aggregate and tamped with 25 strokes of the tamping rod. A 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 and tamping rod as a straight edge. The net weight of the aggregate in the measure shall be determined to the nearest gin. (weight 'A')

The cup of impact testing machine shall be fixed firmly in the 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.

The hammer shall be raised 380mm 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 sample shall be removed and sieved through 2.36mm IS sieve. The fraction passing through shall be weighed. (Weight "B") The fraction retained on the sieve shall also be weighed (Weight C) and if the total weight (B+C) is less than the initial weight (Weight "A") by more than one gm, the result shall be discarded and a fresh test made.

The ratio of the weight of the fines formed to the total sample weight shall be expressed as a percentage. Aggregate Impact Value = B/A X 100.

Two such tests shall be carried out and the mean of the results shall be reported to the nearest whole number as the Aggregate Impact Value of the tested material.