Impact test on aggregate (IS:5640-1970)

 Objective - To determine the impact value of given road aggregate.

Apparatus & materials required - 

1. Impact testing machine.
2. A cylindrical metal having internal diameter 75 mm and depth 50 mm.
3. Tamping rod of 10 mm diameter and 230 mm long.
4. IS sieve of sizes 12.5 mm, 10 mm and 2.36 mm.
5. Balance
6. Thermostatic oven
7. Aggregate sample

Theory - The aggregate impact value indicates a relative measure of the resistance of an aggregate to impact. The aggregate impact value is expressed as the percentages of fines formed in terms of the total weight of the sample.

    Due to traffic loads, the road aggregates are subjected to impact and and there is possibility of aggregates breaking into smaller pieces. The road aggregate should be tough enough to resist fracture under impact. A test designed to evaluate the toughness of aggregate is called aggregate impact test.

                                                      Aggregate impact testing machine

Procedure - 

1. The given road aggregate is sieved through 12.5 mm and 10 mm sieves and the aggregates passed through 12.5 mm sieve and retained on 10 mm sieve is taken as sample.
2. Aggregate sample is dried in oven for four hours at a temperature of 100 to 110 degree celsius and cooled.
3. The cylindrical measure is filled with the test aggregates in three layers. each layer is about one third of the height of the measure. Each layer is tamped 25 times with the tamping rod before putting the next layer.
4. The weight of the aggregate in the measure is determined and taken as W1.
5. The entire sample is then transferred to the steel cup of the testing machine. The cup has internal diameter of 102 mm and depth 50 mm and rigidly fixed with base plate.
6. the aggregate in the steel cup is then tamped 25 times.
7. The hammer of the machine is raised until its lower face is 380 mm above the upper surface of the aggregates in the cup and is allowed to fall freely on the aggregates.
8. 15 such blows are given to the sample. the time interval between two blows should not be less than one second.
9. The sample is then removed from the cup and sieved through 2.36 mm sieve.
10. The fraction passing through the 2.36 mm sieve is weighed and taken as W2.
11. The above test is repeated at least three times to get the average value.

Observations - 

Result -

     The average aggregate impact value of given sample is ___ .

Precautions - 

1. The test sample should be dried for four hours at 100 to 110 degree celsius.
2. Care should be taken while removing aggregates after the test to avoid any loss.
3. Weights of aggregates should be taken accurately.

Note :- Aggregate is considered to be exceptionally strong if aggregate impact value is less than 10%, strong if between 10-20% and satisfactory for road surfacing if the value lies between 20-30%. The aggregate which are to be used in surface course, the aggregate impact value should not be more than 30% and for the base course it should not exceed from 40%.

Reference :- Transportation engineering by A. K. Upadhayay.