22.3.2. Abrasion test

Abrasion test is carried out to test the hardness property of aggregates and to decide whether they are suitable for different pavement construction works.

The abrasion test is essential for assessing how well aggregates can withstand wear and tear that occurs when vehicles drive over them. It specifically measures the hardness of the aggregates, which is a key property that ensures that they do not break down or wear away easily under the stresses of traffic and weather conditions. By determining this property, engineers can decide if the chosen aggregates are appropriate for different types of pavement, ultimately contributing to the longevity and safety of the road.

Imagine walking on a sandy beach versus a rocky path. The sand is soft and wears away easily underfoot, while rocks can withstand more pressure without breaking apart. Similar to this, the abrasion test helps identify if the aggregates will hold up under the constant pressure and motion of vehicles.

Los Angeles abrasion test is a preferred one for carrying out the hardness property and has been standardised in India (IS:2386 part-IV). The principle of Los Angeles abrasion test is to find the percentage wear due to relative rubbing action between the aggregate and steel balls used as abrasive charge.

In the Los Angeles abrasion test, aggregates are placed in a rotating drum along with steel balls that act as abrasives. As the drum rotates, the steel balls rub against the aggregates, mimicking the wear action that occurs on road surfaces. After a specified number of rotations, the aggregates are removed and sieved to determine how much of them was worn away. The percentage of weight lost is calculated to express the durability of the materials. This test is standardised in India to ensure consistency and reliability in results.

Think of a washing machine tumble cycle, where clothes rub against one another. Over time, this rubbing results in frayed edges and wear on the fabric. Similar to this, the Los Angeles abrasion test simulates how aggregates will wear over time under the constant rubbing of tires on roads.

Los Angeles machine consists of a circular drum of internal diameter 700 mm and length 520 mm mounted on horizontal axis enabling it to be rotated. An abrasive charge consisting of cast iron spherical balls of 48 mm diameters and weight 340-445 g is placed in the cylinder along with the aggregates.

The Los Angeles machine is designed to facilitate the abrasion test efficiently. It features a large drum that can rotate, creating the necessary action to simulate wear on the aggregates. The steel balls, with their specified weight and size, are essential as they provide a consistent abrasive action against the aggregates. Knowing the size and weight of these materials helps engineers to standardize the test and compare results across different tests and conditions.

Consider a blender where various ingredients are mixed to achieve a smoother consistency. The rotating blades help to chop and mix everything uniformly. In this case, the rotating drum and steel balls serve similarly, working together to uniformly evaluate the hardness of the aggregates.

The quantity of aggregates to be used depends upon the gradation and usually ranges from 5-10 kg. The cylinder is then locked and rotated at the speed of 30-33 rpm for a total of 500 -1000 revolutions depending upon the gradation of aggregates. After specified revolutions, the material is sieved through 1.7 mm sieve and passed fraction is expressed as percentage total weight of the sample. This value is called Los Angeles abrasion value.

To conduct the test, a specific amount of aggregates (5-10 kg) is used based on their size distribution. The drum rotates at a controlled speed for a designated number of revolutions to ensure that the wear simulated is consistent. After the test, the aggregates are sieved to separate the worn material, allowing for the measurement of abrasion by comparing the weight of what was lost. This measurement is crucial as it indicates how durable the aggregates would be in real-world applications.

Imagine counting how many puzzle pieces are lost after repeatedly tossing the box around. After each toss (like each rotation of the drum), you check how many pieces remain intact versus how many have fallen out. The same process in the abrasion test helps quantify the wear of the aggregates, which is vital for selecting materials that will endure across time.

A maximum value of 40 percent is allowed for WBM base course in Indian conditions. For bituminous concrete, a maximum value of 35 is specified.

Once the abrasion test is complete and results are obtained, those values are compared against established criteria to decide if the aggregates are acceptable for use. In India, a maximum abrasion value of 40% is allowed for Water Bound Macadam (WBM) base courses, while for bituminous concrete, it should be below 35%. These criteria ensure that the aggregates can adequately support the structural integrity of the pavement.

Think of quality checks in a factory. Items produced must meet certain standards before they can be sold; otherwise, they could fail prematurely. Similarly, aggregates need to pass these abrasion tests to be deemed suitable for road construction.

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