Objective:
(i) To assess the impact value of the road aggregates;
(ii) To evaluate their appropriateness in road construction on the foundation of impact value.
Apparatus:
The apparatus as per IS: 2386 (Part IV) – 1963 includes the following:
(i) A testing machine with the weight range of 45 to 60 kg and consisting of a metal bottom with a tinted low exterior at least minimum of 30 cm in width. It is held up on base level and plane concrete surface of at least 45 cm density. The device must also have requirements for setting up and fixing its base.
(ii) A cylindrical steel vessel of inner diameter 102 mm, height 50 mm and thickness of at least 6.3mm.
(iii) A metal tup or a hammer with tentative weight of 13.5 to 14.0 kg of the lower end. It should be cylindrical in shape with length of 50mm, 100.0 mm width, 2 mm chamfer at lower base and case must be hard and stiff. The hammer must be able to slide easily amid perpendicular guides and be solidified with the cup. The freefalling of the hammer has to be inside 380±5 mm.
(iv) A cylindrical metal measure with the interior width of 75 mm and height of 50 mm.
For measurement of the aggregates:
(v) Tamping rod 10 mm in width and 230 mm in length, curved at one end.
(vi) A equilibrium of ability minimum of 500g, clear and precise up to 0.1 g.
Theory
The characteristic any kind of material to oppose the impact is called toughness. Because of the movement of the automobiles on the road the aggregates are exposed to the impact leading in their breakdown into many smaller pieces. The aggregates must thus have adequate rigidity to resist their breakdown due to excessive impact. This property is calculated by impact value test. The aggregate impact value is a calculation of the resistance to abrupt impact, bang or shock, which may vary from its resistance to steadily, applied compressive weight.
Procedure:
The test specimen includes the aggregates of size ranging from 10.0 mm 12.5 mm. Aggregates can be dehydrated by warming them at temperatue of 100-110° C for a timeframe of 4 hours and spread to cool down.
- The material has to be sieved with help of 12.5 mm and 10.0mm IS sieves. The aggregates transiting through the 12.5mm sieve and are held on the 10.0mm sieve incorporates the test substances.
- Aggregates have to be poured to approximately fill about just 1/3 rd height of measuring cylindrical container.
- Compress the given material by delivering 25 moderate blows with the curved end of the tamping rod.
- Increase two additional levels in analogous approach, so that the cylindrical container is full to the brim.
- Strike off the additional aggregates.
- Assess the final mass of the aggregates to the closest gram(W).
- Bring the impact device to off mode by neither wedging or packing up on the level plate, slab or ground level, so that it is inflexible and the hammer guide columns are in a perpendicular position.
- Position the cup rigidly in place on the bottom of the device and put the whole of the test specimen in it and compress by providing 25 mild strokes with the tamping rod.
- Lift the hammer unless its lower face is 380 mm on top of the facade of aggregate specimen in the cup and permit its freefall on the aggregate specimen. Once again give 15 such gentle blows at a gap of minimum of one second between the consecutive falls.
- Detach the compressed aggregate from the cup and filter it with the help of 2.36 mm IS sieves unless no additional considerable amount goes through in one minute. Measure the weight fraction passing through the sieve to a preciseness of 1 gm. Also, measure the weight of fraction absorbed in the sieve.
- Calculate the aggregate impact value. The mean of two observations, curved to closest whole number is indicated as the Aggregate Impact Value.
Observation
| Sample 1 | Sample 2 | |
| Aggregate weight of dry sample ( W1 gm) | ||
| Weight of portion passing 2.36 mm sieve (W2gm) | ||
| Total Impact Value (percent) = W2 / W1 X 100 |
Mean =
Result: aggregate impact value-
Recommended value
Division of the aggregates applying the Aggregate Impact Value as shown below:
Classification of aggregates using Aggregate Impact Value is as given below:
| Aggregate Impact Value | Classification |
| <20% | Extremely strong |
| 10 – 20% | Strong |
| 20-30% | Acceptable for road surfacing |
| >35% | Not suitable and weak for road surfacing |
Given limits of percent aggregate impact value for various kinds of road construction by Indian Roads Congress is shown below:
| N.o | Variety of the pavement | Aggregate impact value not more than |
| 1. | Wearing Course | 30 |
| a) | Bituminous surface dressing | |
| b) | Penetration macadam | |
| c) | Bituminous carpet concrete | |
| d) | Cement concrete | |
| 2. | Bitumen bound macadam base course | 35 |
| 3. | WBM base course with bitumen surfacing | 40 |
| 4 | Cement concrete base course | 45 |