Short Answer:

The impact value of aggregate is a measure of its toughness, or resistance to sudden shocks or impacts. It shows how well the aggregate can handle loads that hit it quickly, such as from vehicles or machinery. A lower impact value means the aggregate is tougher and more suitable for road and pavement construction.

To determine the impact value, a standard test called the Aggregate Impact Value (AIV) test is used. In this test, aggregates are placed in a metal mould and hit with a hammer of fixed weight from a fixed height. The broken pieces are then measured to calculate the impact value as a percentage.

Detailed Explanation:

Impact value of aggregate

The impact value of aggregate is a crucial property in civil engineering, especially when selecting materials for roads, pavements, and other surfaces that experience heavy and fast-moving loads. It tells us how much resistance the aggregate has to sudden shocks, such as the impact of a tire or machine part. If the aggregate breaks easily under such force, it is considered weak and unsuitable for such applications.

This test is particularly important for materials used in surfaces that deal with dynamic loads, like highways, airport runways, or industrial floors. Aggregates that are too brittle can break under pressure, leading to early failure and costly repairs. That’s why the impact value test is used to ensure the material’s quality before use.

The Aggregate Impact Value (AIV) is expressed as a percentage of the crushed material (finer than 2.36 mm) to the total weight of the sample. A lower value indicates tougher material. For most road work, the value should be below 30%.

How it is determined

To find the impact value of an aggregate, a standard procedure is followed using a testing machine called the Aggregate Impact Testing Machine. The process includes the following steps:

Sample Preparation:
A dry sample of aggregate passing through a 12.5 mm sieve and retained on a 10 mm sieve is taken. The weight of the sample should be about 350 grams.
Filling the Mould:
The cylindrical mould is filled with the aggregate sample in three equal layers. Each layer is compacted using a tamping rod with about 25 strokes.
Applying Impact:
The filled mould is placed on the base of the testing machine. A hammer weighing 13.5 to 14 kg is dropped from a height of 38 cm on the top of the sample. This is repeated 15 times.
Sieving the Crushed Material:
After the test, the crushed sample is sieved through a 2.36 mm sieve. The weight of the fines (material that passes through the sieve) is measured.
Calculation:
The impact value is calculated using the formula:

Impact Value (%) = (Weight of fines / Total weight of sample) × 100

This test gives an idea of the aggregate’s behavior under sudden impact. Aggregates used in different construction works have specific recommended limits for impact value. For example, surface courses of pavements require more tough aggregates than base layers.

Conclusion:

The impact value of aggregate is an important property that determines how the material will behave under sudden or dynamic loads. It is measured through a standard test where a fixed weight impacts the sample, and the amount of crushed material is calculated. This helps engineers choose the right type of aggregate for roads and other structures that experience high impact forces.