The aim and purpose of aggregate impact test is to determine the impact value of the given aggregate which is to be used in road works.
Toughness is the property of a material to resist impact. Due to traffic loads the road stones are subjected to the pounding action or impact and there is possibility of stones breaking into smaller pieces.
The road stones should therefore be tough enough to resist fracture under impact.
A test designed to evaluate the toughness of stones i.e. the resistance of the stones to fracture under repeated impacts may be an impact test for road aggregate.
The aggregate impact value indicates a relative measure of the resistance of an aggregate to a sudden shock or an impact, which in some aggregate differs from its resistance to a slow compressive load.
The method of aggregate test specifies the procedure for determining the aggregate impact value of coarse aggregate.
Aggregate impact value is used to classify the stones in respect of their toughness property as indicated below:
<10% Exceptionally strong
10-20% Strong
20-30% Satisfactory for road surfacing
35% Weak for road surfacing
The aggregate impact value should not normally exceed 30% for aggregate to be used in wearing course of pavements.
Maximum permissible value is 35% for bituminous macadam and 40% for water bound macadam base courses.
Purpose of Aggregate Impact Test
For deciding the suitability of soft aggregates in road base course construction, this test has been commonly used.
A modified impact test is also often carried out in the case of soft aggregates to find the wet impact value after soaking the test samples.
The recommendations is given in the below table.
|
Condition of sample |
Maximum aggregate impact value, percent | |
| Sub-base and base | Surface course | |
| Dry | 50 | 32 |
| Wet | 60 | 39 |
Below is list of necessary Apparatus:
- Aggregate impact apparatus
- IS sieves (12.5 mm, 10.0 mm and 2.36 mm)
- Cylindrical measure and cylindrical cup
- Weighing balance
- Tamping rod
Aggregate Impact Test Procedure
Take clean and dry aggregate and sieve on IS 12.5 mm and 10.00 mm sieve.
Collect the aggregate passing IS 12.5mm sieve and retained on IS 10.0 mm Sieve.
Find the weight of empty cylindrical measure. Let the weight be ‘a’ g.
Fill the aggregate in the cylindrical measure in three layers, tamping each layer 25 times with the rounded end of the tamping rod.
Roll the tamping rod over aggregate surface and remove excess aggregate, if any.
Find the weight of the cylindrical measure with aggregate. Let the weight be ‘b’ g. Thus the weight of aggregate = W1 = ( b-a )
Transfer all the aggregate from the cylindrical measure to the test cylinder in one layer and tamp the layer 25 times with the rounded end of the tamping rod.
Fix the test cylinder firmly to the base of the impact tester.
Adjust the height of fall of the plunger to 380+ 5mm and set the blow counter to zero.
Lift the plunger gently and allow it to drop. This is one blow. Give 15 such blows.
Take out the test cylinder and sieve the crushed material on IS 2.36 mm sieve.
Find the weight of material passing the sieve.
Let weight be W2 grams.
Find the weight of aggregate retained on this sieve.
Let the weight be W3 grams.
Then,
Aggregate impact value = W2 / W1 * 100 %
And percentage of dust = W3 / W1* 100 %
Record in Tabular Column
| Weight of empty cylindrical measure = ‘a’ g. | |
| Weight of cylindrical measure + Aggregate = ‘b’ g | |
| Initial weight of aggregate = (b-a)= W1 g | |
| Weight of aggregate passing IS 2.3 mm sieve after test = W2 g | |
| Weight of aggregate retained on IS 2.36 mm sieve after test = W3 g | |
| Aggregate impact value = W2 / W1 * 100 % | |
| Percentage of dust = W3 / W1* 100 % |
Aggregate Impact Test Result
The Impact value of given aggregate sample is = ………….. %
Desirable value of Aggregate Impact Test
Various agencies have specified the maximum permissible aggregate impact values for the different types of pavements, some of the recommended aggregate impact values are given below:
|
S. No |
Types of pavement material/layer | Aggregate impact value, maximum ( %) |
|
1 |
Water bound macadam (WBM), sub-base course |
50 |
|
2 |
Cement concrete, base course (as per ISI) |
45 |
|
3 |
(i) WBM base course with bitumen surfacing
(ii)Built-up spray grout, base course |
40 |
|
4 |
Bituminous macadam, base course |
35 |
|
5 |
(i) WBM, surfacing course
(ii) Built-up spray grout, surfacing course (iii) Bituminous penetration macadam (iv) Bituminous macadam, binder course (v) Bituminous surface dressing (vi) Bituminous carpet (vii) Bituminous/Asphalt concrete (viii) Cement concrete, surface course |
30 |