The purpose of conducting an Impact Test on Aggregates is twofold. Firstly, it is performed to determine the impact value of road aggregates. This value is a measure of the strength or toughness of the aggregates and is calculated by subjecting them to a standard amount of impact. The impact value is then used to evaluate the durability and performance of the aggregates in road construction.
Secondly, the Impact Test is carried out to assess the suitability of the road aggregates for use in road construction based on their impact value. If the impact value is found to be low, it indicates that the aggregates are strong and durable and can withstand heavy traffic and loads without breaking or deteriorating quickly. On the other hand, if the impact value is high, it suggests that the aggregates are weak and brittle and may not be suitable for use in road construction.
In summary, the Impact Test on Aggregates is a crucial step in evaluating the quality and performance of road aggregates. The test helps determine the impact value of the aggregates and assess their suitability for use in road construction based on their strength and durability.
Aggregate Impact Value on Coarse Aggregates
Apparatus for Aggregate Impact Test
The IS: 2386 (Part IV) – 1963 apparatus includes several components for testing purposes. Firstly, there is a testing machine that weighs between 45 to 60 kg and features a metal base with a painted lower surface of at least 30 cm in diameter. This machine is designed to be supported on a level and plane concrete floor that is at least 45 cm thick. It should also have the necessary provisions for fixing its base.
Secondly, a cylindrical steel cup is included, which has an internal diameter of 102 mm, a depth of 50 mm, and a minimum thickness of 6.3 mm. This cup is an important component of the apparatus.
Thirdly, a metal hammer or tup is also part of the apparatus, weighing between 13.5 to 14.0 kg. The lower end of the hammer is cylindrical in shape, measuring 50 mm long and 100.0 mm in diameter, with a 2 mm chamfer at the lower edge. The hammer should be case-hardened, slide freely between vertical guides, and be concentric with the cup. Additionally, the free fall of the hammer should be within 380±5 mm.
Fourthly, the apparatus includes a cylindrical metal measure that has an internal diameter of 75 mm and a depth of 50 mm. This measure is used for measuring aggregates.
Fifthly, a tamping rod that is 10 mm in diameter and 230 mm long, rounded at one end, is also included.
Lastly, a balance of capacity not less than 500g, readable and accurate up to 0.1 g, is part of the apparatus.
Theory of Aggregate Impact Test
Toughness is the property of a material that enables it to resist impact. In the case of aggregates used for road construction, they are constantly subjected to impact due to the movement of vehicles. This impact can cause the aggregates to break down into smaller pieces, which can be detrimental to the overall quality of the road. Therefore, it is important for aggregates to have sufficient toughness to resist disintegration due to impact. The measure of this characteristic is called the aggregate impact value test.
The purpose of the aggregate impact value test is to determine the resistance of an aggregate to sudden impact or shock. It is important to note that this measure of resistance may differ from the resistance of the same aggregate to a gradually applied compressive load. The impact value test is crucial in ensuring the quality of road construction materials, as it provides a measure of the aggregate’s ability to withstand impact and maintain its integrity over time. This, in turn, can have a significant impact on the durability and safety of the road.
Procedure of Aggregate Impact Test
The given procedure is for testing the impact resistance of aggregates that are sized 10.0 mm to 12.5 mm. The first step is to dry the aggregates by heating them at 100-110° C for 4 hours and cooling them. The test material is then obtained by sieving the aggregates through 12.5 mm and 10.0 mm IS sieves, and collecting the aggregates that pass through the former but are retained on the latter.
To begin the test, the collected test material is poured into a measuring cylinder to fill about 1/3 of its depth. The material is then compacted by gently striking it 25 times with the rounded end of a tamping rod. This process is repeated for two more layers until the cylinder is full, after which the surplus aggregates are removed, and the net weight of the aggregates is determined to the nearest gram.
Next, the impact machine is prepared by bringing it to rest on a level plate, block or floor, without wedging or packing up, so that it is rigid and the hammer guide columns are vertical. The cup is then fixed firmly on the base of the machine, and the entire test sample is placed in it and compacted with 25 gentle strokes from the tamping rod.
The hammer is then raised to a height of 380 mm above the surface of the aggregate sample in the cup, and allowed to fall freely on the sample. This process is repeated 15 times, at intervals of not less than one second between successive falls. Afterward, the crushed aggregate is removed from the cup and sieved through a 2.36 mm IS sieve until no further significant amount passes in one minute.
The fraction passing through the sieve and the fraction retained in the sieve are weighed with an accuracy of 1 gm, and the aggregate impact value is computed. The mean of two observations, rounded to the nearest whole number, is reported as the Aggregate Impact Value.
Observations of Impact Test
| Observations | Sample 1 | Sample 2 |
| Total weight of dry sample (W1 gm) | ||
| Weight of portion passing 2.36 mm sieve (W2 gm) | ||
| Aggregate Impact Value (percent) = W2 / W1 X 100 |
Mean =
Result of Impact Test
Aggregate Impact Value =
Recommended Aggregate Impact Test Values
The Aggregate Impact Value (AIV) test is a standard test that is used to determine the hardness of aggregates. The AIV test measures the resistance of an aggregate to impact. During the test, a standard amount of aggregate is placed in a cylindrical measure and subjected to a standard amount of impact. The resulting impact value is used to classify the aggregate into different categories based on its hardness.
The AIV test is a useful tool for evaluating the quality of aggregates used in construction projects. The test helps to determine the suitability of the aggregate for use in different applications. The results of the test can be used to classify aggregates into different categories, such as high strength, medium strength, and low strength. Aggregates that have a high AIV value are generally considered to be harder and more durable than those with a low AIV value.
The classification of aggregates based on their AIV values can be used to select the appropriate aggregates for specific applications. For example, aggregates with a high AIV value are typically used in the construction of roads and pavements, while aggregates with a lower AIV value are often used in the production of concrete. The AIV test is a critical component of the quality control process for aggregates and helps to ensure that only the highest quality aggregates are used in construction projects.
| Aggregate Impact Value | Classification |
| <20% | Exceptionally Strong |
| 10 – 20% | Strong |
| 20-30% | Satisfactory for road surfacing |
| >35% | Weak for road surfacing |
The Indian Roads Congress has established specific limits for the percent aggregate impact value for various types of road construction. These limits serve as guidelines to ensure that the quality of the road construction meets certain standards.
The percent aggregate impact value refers to the percentage of aggregate material that breaks down under impact, such as from a hammer or other force. This value is an important indicator of the quality and durability of the road construction material.
The specified limits for percent aggregate impact value vary depending on the type of road construction. These limits have been set based on various factors such as traffic volume, weather conditions, and the type of material being used.
By adhering to these limits, road construction companies can ensure that their projects meet the necessary quality standards and are able to withstand the wear and tear of regular use. This ultimately leads to safer and more reliable roads for drivers and other road users.
| Sl No | Type of 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 |