Aggregate Impact Value Test Experiment

Objective

To estimate the aggregate impact value of given sample.

IS Standards used:IS: 2386(Part-4)-1963

Need and scope:The ‘Aggregate Impact Value’ gives a relative measure of the resistance of an aggregate to sudden shock or impact, which in some aggregates differs from its resistance to a slow compressive load. The property of a material to resist impact is known as toughness. Due to movement of vehicles on the road the aggregates are subjected to impact resulting in their breaking down into smaller pieces. The aggregates should therefore have sufficient toughness to resist their disintegration due to impact. This Characteristic is measured by impact value test. The aggregate impact value is a measure of resistance to sudden impact or shock, which may differ from its resistance to gradually applied compressive load.

Apparatus Required:

1. Impact Testing Machine: Impact testing machine conforming to IS: 2386 (Part IV) - 1963, as shown in Figure 1.
2. Metal Cylinder: cylindrical metal measure of 75 mm dia. and 50 mm depth.
3. Test Sieves: IS Sieves of the sizes and apertures appropriate to the specification of the material to be tested, (Sieves of sizes–12.5 mm, 10 mm and 2.36 mm).
4. A tamping rod of 10mm circular cross section and 230mm length, rounded at one end and Oven.
5. Balance: A balance of capacity not less than 500 g, readable and accurate to 0.1g.
6. Oven: A well-ventilated oven thermostatically controlled to maintain a temperature of 100 to 110°C.

Preparation of test sample:

1. The test sample shall consist of aggregate the whole of which passes a 12.5 mm IS Sieve and is retained on a 10 mm IS Sieve. The aggregate comprising the test sample shall be dried in an oven for a period of four hours at a temperature of 100 to 110 °C and cooled.
2. The measure shall be filled about one-third full with the aggregate and tamped with 25 strokes of the rounded end of the tamping rod.
3. 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 the tamping rod as a straight-edge.
4. The net weight of aggregate in the measure shall be determined to the nearest gram (Weight A) and this weight of aggregate shall be used for the duplicate test on the same material.

Experimental Procedure:

1. The impact machine shall rest without wedging or packing upon the level plate, block or floor, so that it is rigid and the hammer guide columns are vertical.
2. The cup shall be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by a single tamping of 25 strokes of the tamping rod.
3. The hammer shall be raised until its lower face is 380 mm 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.
4. The crushed aggregate shall then be removed from the cup and the whole of it sieved on the 2.36 mm IS Sieve until no further significant amount passes in one minute.
5. The fraction passing the sieve shall be weighed to an accuracy of 0.1 g (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 gram, the result shall be discarded and a fresh test made.
6. Two tests shall be conducted.

Calculations:The ratio of the weight of fines formed to the total sample weight in each test shall be expressed as a percentage, the result being recorded to the first decimal place:

Aggregate impact value = B/A X 100

Where, B = weight of fraction passing 2.36 mm IS Sieve, and A = weight of oven-dried sample

Observations and Calculation:

The mean of the two results shall be reported to the nearest whole number as the aggregate impact value of the tested material.

Average Aggregate impact value of aggregate sample 1 (%) =

Average Aggregate impact value of aggregate sample 2 (%) =

General remarks:

Viva Questions:

1. How aggregate impact value is expressed?
2. How does toughness differ from compressive strength?
3. Aggregate impact value material A is 20 and that of B is 45. Which one is better surface course and why?
4. Which test simulates the field conditions better, aggregate crushing value test or impact value test?