22.3.3. Impact test

The aggregate impact test is carried out to evaluate the resistance to impact of aggregates.

The impact test assesses how well aggregates can withstand sudden forces or impacts. This is essential because aggregates in pavement must endure not only steady loads but also occasional shock loads from vehicles. This test provides insight into the durability and performance of the aggregates under such conditions.

Think of a bouncy ball being dropped on a hard surface. If the ball can withstand the impact without cracking or breaking, it indicates that it is tough. Similarly, aggregates need to be strong enough to handle the heavy weight and sudden impacts from vehicles driving over them.

Aggregates passing 12.5mm sieve and retained on 10mm sieve is filled in a cylindrical steel cup of internal diameter 10.2mm and depth 5 cm which is attached to a metal base of impact testing machine. The material is filled in 3 layers where each layer is tamped for 25 number of blows.

In this step, the selected aggregate samples are precisely measured and placed into a testing cup in layers. Each layer is compacted to mimic the density found in actual pavement conditions. This procedural step is vital to ensure consistency and reliability in the test results.

Imagine making a layered cake. Each layer needs to be pressed down firmly to make sure the cake holds together when it's finished. Similarly, tamping each layer of aggregates ensures they compact tightly, which reflects how they would behave in actual road conditions.

A metal hammer of weight 13.5 to 14 Kg is arranged to drop with a free fall of 38.0 cm by vertical guides and the test specimen is subjected to 15 number of blows.

In this phase, a heavy metal hammer is dropped onto the aggregate sample from a fixed height to simulate the impact forces the aggregate might experience in real-world conditions. The number of blows is standardized to ensure uniformity in results.

Consider you are using a hammer to drive a nail into wood. Each hit of the hammer exerts a force that pushes the nail deeper. In the same way, the falling hammer in the test impacts the aggregates to see how well they hold up against that force.

The crushed aggregate is allowed to pass through 2.36 mm IS sieve. The impact value is measured as percentage of aggregates passing sieve (W2) to the total weight of the sample (W1).

After the impacts, the debris falls through a specific sieve. By measuring how much weight of the aggregates passed through the sieve, we can calculate the impact value. This percentage indicates how much the aggregate was shattered under the impact; a lower value signifies stronger aggregates.

Think of a container filled with marbles. When you shake the container, some marbles might break, and smaller pieces will fall through a sieve. By measuring those pieces, you can tell how strong the marbles were. This is similar to what happens in the impact test with aggregates.

Aggregates to be used for wearing course, the impact value shouldn’t exceed 30 percent. For bituminous macadam the maximum permissible value is 35 percent. For Waterbound macadam base courses the maximum permissible value defined by IRC is 40 percent.

The impact values obtained from testing help determine the suitability of aggregates for various types of pavement. For instance, aggregates used in the surface layer (wearing course) must be particularly strong, so an impact value above 30% indicates they may not be suitable.

Consider a sports shoe versus a casual shoe. The sports shoe is designed to handle more stress and impacts, so it has stricter standards for durability than casual shoes. Likewise, aggregates used for roads that face heavier vehicle traffic need to meet strict impact value guidelines.

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