22.3. Aggregate tests

In order to decide the suitability of the aggregate for use in pavement construction, following tests are carried out:

• Crushing test
• Abrasion test
• Impact test
• Soundness test
• Shape test
• Specific gravity and water absorption test
• Bitumen adhesion test

This chunk presents an overview of various tests conducted to assess the performance properties of aggregates before they are used in pavement construction. Each test evaluates a different essential characteristic that aggregates must possess to ensure road durability, safety, and functionality.

Imagine preparing for an exam where you need to know a variety of subjects. Just like studying different topics prepares a student for all kinds of questions, these tests help ensure aggregates are ready for the pressures of roadway demands.

One of the models in which pavement material can fail is by crushing under compressive stress. A test is standardized by IS:2386 part-IV and used to determine the crushing strength of aggregates. The aggregate crushing value provides a relative measure of resistance to crushing under gradually applied crushing load. The test consists of subjecting the specimen of aggregate in standard mould to a compression test under standard load conditions. Dry aggregates passing through 12.5 mm sieves and retained 10 mm sieves are filled in a cylindrical measure of 11.5 mm diameter and 18 cm height in three layers. Each layer is tampered 25 times with a standard tamping rod. The test sample is weighed and placed in the test cylinder in three layers each layer being tampered again. The specimen is subjected to a compressive load of 40 tonnes gradually applied at the rate of 4 tonnes per minute. Then crushed aggregates are sieved through a 2.36 mm sieve and weight of passing material is expressed as percentage of the weight of the total sample, which is the aggregate crushing value.

The crushing test evaluates how well an aggregate can withstand compressive forces. During the test, aggregates are compressed under a certain weight. After crushing, the amount that passes through a sieve is measured, allowing engineers to calculate a percentage that indicates the material's strength. A low percentage generally means a stronger aggregate, while a high percentage indicates weakness.

Think of this as testing an eggshell. If you press down gently and it cracks easily, that tells you the shell is weak. Similarly, the crushing test reveals how well aggregates can hold up under stress.

Abrasion test is carried out to test the hardness property of aggregates and to decide whether they are suitable for different pavement construction works. Los Angeles abrasion test is a preferred one for carrying out the hardness property and has been standardized 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. The machine consists of a circular drum mounted on a horizontal axis enabling it to be rotated. An abrasive charge consisting of cast iron spherical balls is placed in the cylinder along with the aggregates. The quantity of aggregates depends on the gradation and usually ranges from 5-10 kg. After specific revolutions, the material is sieved, and the passing fraction is expressed as a percentage of the total weight of the sample. This value is called the Los Angeles abrasion value.

The abrasion test focuses on how well aggregates can resist wear and tear from traffic and environmental factors. The aggregates are placed in a rotating drum along with steel balls. As the drum turns, the aggregate rubs against the balls, simulating conditions similar to those they would face on the road. Afterward, the losses are measured to determine their hardness.

Consider how a shoe sole wears down after walking on rough surfaces. The abrasion test is similar; it assesses how the aggregates will hold up when subjected to the constant ‘walking’ of vehicles.

The aggregate impact test is carried out to evaluate the resistance to impact of aggregates. Aggregates passing 12.5 mm sieve and retained on 10 mm sieve is filled in a cylindrical steel cup. The material is filled in 3 layers where each layer is tamped for 25 blows. A metal hammer is arranged to drop on the sample from a height of 38.0 cm. The test specimen is subjected to 15 blows. The crushed aggregate is allowed to pass through a 2.36 mm IS sieve. The impact value is measured as percentage of aggregates passing sieve to the total weight of the sample.

The impact test evaluates how aggregates respond to sudden forces or shocks. The aggregates are subjected to a series of blows from a heavy hammer. The percentage of material that breaks down after these impacts reveals the aggregate's toughness. A lower percentage suggests a tougher, more durable material, while a higher percentage indicates it is more prone to breaking.

Think of it like testing a plate by dropping different objects on it. If it chips or cracks easily, it’s not very tough. This test shows how aggregates can handle sudden forces similar to how the plate reacts.

Soundness test is intended to study the resistance of aggregates to weathering action, by conducting accelerated weathering test cycles. Aggregates are subjected to cycles of alternate wetting in a saturated solution of either sodium sulphate or magnesium sulphate for 16-18 hours and then dried in an oven at 105-110°C to a constant weight. After five cycles, the loss in weight of aggregates is determined by sieving out all undersized particles and weighing. The loss in weight should not exceed 12 percent when tested with sodium sulphate and 18 percent with magnesium sulphate solution.

The soundness test examines how aggregates hold up against weathering and environmental factors, which can cause them to break down over time. Aggregates are alternately soaked and dried in salt solutions to simulate freezing and thawing cycles they might encounter in real-world conditions. The amount of weight lost after several cycles indicates their durability.

Imagine keeping ice cubes in a warmer room; they eventually melt away. This test is similar—by stressing aggregates through wet-dry cycles, it demonstrates whether they can withstand the 'weather' of roadway conditions.

The particle shape of the aggregate mass is determined by the percentage of flaky and elongated particles in it. Aggregates which are flaky or elongated are detrimental to higher workability and stability of mixes. The flakiness index is defined as the percentage by weight of aggregate particles whose least dimension is less than 0.6 times their mean size. The elongation index is defined as the percentage by weight of particles whose greatest dimension is 1.8 times their mean dimension.

Shape tests evaluate the physical form and dimensions of the aggregate particles. Flaky and elongated particles can negatively impact how well aggregates blend and compact, affecting the overall pavement's strength and performance. The flakiness index measures how many of these undesirable shapes are present compared to more favorable shapes.

Think about mixing marbles: if most are round but some are long and flat, the flat ones will not fit into the mix as well. Just like marbles, aggregate shapes affect how well they work together in pavement.

The specific gravity and water absorption of aggregates are important properties that are required for the design of concrete and bituminous mixes. Apparent specific gravity is computed on the basis of the net volume of aggregates excluding water-permeable voids. Bulk specific gravity is computed based on the total volume of aggregates including water-permeable voids. Water absorption measures the difference between the apparent and bulk specific gravities, representing the volume of voids.

Specific gravity tests measure how heavy the aggregate is for its size, which informs the necessary mix designs for concrete or bitumen. Water absorption relates to the voids in aggregates and indicates how much water they can take in, influencing how well they will perform when mixed. This is crucial for ensuring that aggregates maintain their integrity over time.

Consider a sponge: it's light but can absorb a lot of water. Similarly, aggregates with many voids can change how heavy or light the mixture feels, which influences how durable the final pavement product will be.

Bitumen adheres well to all normal types of road aggregates provided they are dry and free from dust. In the absence of water, there is practically no adhesion problem of bituminous construction. Adhesion problem occurs when the aggregate is wet and cold. Several laboratory tests are conducted to determine the adhesion of bitumen binder to an aggregate in the presence of water. The static immersion test is specified by IRC and is quite simple. The principle of the test is to immerse aggregate fully coated with binder in water maintained at 40°C temperature for 24 hours. IRC has specified that the maximum stripping value of aggregates should not exceed 5%.

The bitumen adhesion test checks how well bitumen sticks to aggregate when water is involved. If the aggregate is wet, it can prevent the bitumen from adhering properly, which is crucial for the strength and longevity of the pavement. The static immersion test assesses this by seeing how well the bitumen holds on to the aggregate after being submerged in water.

Imagine trying to paint a wet wall; the paint won’t stick properly. This test reveals whether bitumen can effectively ‘stick’ to aggregates, highlighting the importance of proper conditions for effective construction.

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