Aggregate makes up over nine-tenths of a bituminous pavement, so the road is only as good as the stone in it. But “good stone” is not one property — it is seven, and each one guards against a different way the pavement can fail.
This post covers all seven, explains what damages the aggregate in each case, and shows which test checks it.
1. Strength
- Aggregates in the top layers are subjected to stress action from traffic wheel load, wear and tear, and crushing.
- For a high quality pavement, aggregates should possess high resistance to crushing and withstand the stresses due to traffic wheel load.
Strength here means resistance to being crushed under a steadily applied load — the slow, heavy squeeze of a wheel passing overhead. It is checked by the crushing test.
2. Hardness
- Aggregates in the surface course are subjected to constant rubbing or abrasion from moving traffic.
- They should be hard enough to resist the abrasive action caused by traffic movement.
- The abrasive action is severe when steel tyred vehicles move over aggregates exposed at the top surface.
Hardness is resistance to rubbing. It is checked by the Los Angeles abrasion test.
Note the mention of steel tyred vehicles. A rubber tyre is soft and deforms around the stone; a steel rim grinds directly against it. That is why the abrasive demand is far higher where steel tyred traffic runs.
3. Toughness
Resistance of the aggregates to impact is termed toughness.
Aggregates in the pavement should resist the effect caused by steel tyred wheels jumping from one particle to another at different levels, which causes severe impact.
Toughness is checked by the aggregate impact test.
Strength, Hardness and Toughness — Three Different Things
Students routinely confuse these three, so it is worth separating them clearly by the type of loading each resists:
| Property | Resists | Nature of the Load | Test |
|---|---|---|---|
| Strength | Crushing | Gradually applied compressive load | Crushing test |
| Hardness | Abrasion | Continuous rubbing | Los Angeles abrasion test |
| Toughness | Impact | Sudden blows | Aggregate impact test |
Slow squeeze, steady rub, sudden blow. Three different attacks, three different properties, three different tests. A stone can be excellent at one and poor at another.
4. Shape of Aggregates
- Flaky and elongated particles have less strength and durability than cubical, angular or rounded particles of the same aggregate.
- Hence too flaky and too elongated aggregates should be avoided as far as possible.
Read that first point carefully — it says the same aggregate. Two pieces of identical rock behave differently purely because of their shape.
Why? A flat or long sliver has a weak axis. Load it across that axis and it snaps easily, whereas a cubical piece of the same rock has no such weakness in any direction. Flaky particles also lie flat during compaction and fracture under the roller.
Shape is checked by the shape test, which measures the flakiness index, elongation index and angularity number.
5. Adhesion with Bitumen
- Aggregates used in bituminous pavements should have less affinity with water compared with bituminous materials.
- Otherwise the bituminous coating on the aggregate will be stripped off in the presence of water.
This is the most subtle of the seven properties, and the most destructive when it goes wrong.
The problem is a competition. Both water and bitumen want to stick to the stone surface. If the aggregate prefers water, then water arriving at the pavement will push the bitumen off the stone — a failure called stripping.
Once stripping begins, the mix loses its binder where it matters most. The aggregate particles come loose, the surface ravels, and the pavement disintegrates. Crucially, none of this depends on the aggregate being weak. Perfectly strong stone will produce a failed road if bitumen will not stay attached to it.
Adhesion is checked by the bitumen adhesion test, giving the stripping value.
6. Durability
The property of aggregates to withstand adverse action of weather is called soundness.
- Aggregates are subjected to the physical and chemical action of rain and bottom water, the impurities in that water, and the action of the atmosphere.
- It is therefore desirable that road aggregates be sound enough to withstand weathering action.
Note the two words paired here — durability is the requirement, and soundness is the name of the property. Examiners often ask for one and expect the other.
Note also that the attack is both physical and chemical. Physically, water entering pores freezes or crystallises and forces the stone apart. Chemically, impurities dissolved in the water attack the minerals themselves.
Durability is checked by the soundness test, an accelerated weathering test.
7. Freedom from Deleterious Particles
Aggregates used in bituminous mixes or Portland cement concrete mixes usually require the aggregate to be clean, tough and durable, and free from excess amounts of:
- Flat or elongated pieces
- Dust
- Clay balls
- Other objectionable material
The reason dust and clay matter so much connects back to property 5. A dust coating sits between the stone and the bitumen, so the binder never touches the aggregate at all — it sticks to the dust, and the dust washes away. Clay is worse still, because it actively attracts water.
This is why the bitumen adhesion topic notes that bitumen adheres well to all normal road aggregates provided they are dry and free from dust.
All Seven Properties and Their Tests
| Property of Aggregate | Test |
|---|---|
| Crushing strength | Crushing test |
| Hardness | Los Angeles abrasion test |
| Toughness | Aggregate impact test |
| Durability | Soundness test — accelerated durability test |
| Shape factors | Shape test |
| Specific gravity and porosity | Specific gravity and water absorption test |
| Adhesion to bitumen | Stripping value of aggregate |
This table is worth memorising as a unit. Almost every question on aggregate testing either asks for a property and expects the test, or gives the test and expects the property.
A Way to Remember the Seven
Group them by what threatens the aggregate:
- Threats from traffic: strength (crushing), hardness (abrasion), toughness (impact)
- Threats from the material itself: shape, freedom from deleterious particles
- Threats from water and weather: adhesion with bitumen, durability
Three from above, two from within, two from outside.
Quick Revision Notes
- Strength = resistance to crushing under traffic wheel load; needed especially in top layers.
- Hardness = resistance to abrasion; needed in the surface course; abrasion is severe under steel tyred vehicles.
- Toughness = resistance to impact, caused by steel tyred wheels jumping between particles at different levels.
- Flaky and elongated particles have less strength and durability than cubical, angular or rounded particles of the same aggregate.
- Adhesion: aggregates must have less affinity with water than with bitumen, or the coating will be stripped off.
- Soundness is the property of withstanding adverse weather action; the requirement is called durability.
- Weathering attack is both physical and chemical, from rain, bottom water, impurities and the atmosphere.
- Aggregates must be clean, tough and durable, free from excess flat or elongated pieces, dust, clay balls and other objectionable material.
Mistakes Students Commonly Make
- Confusing hardness and toughness. Hardness resists abrasion (rubbing); toughness resists impact (blows).
- Matching hardness with the impact test. Hardness goes with the Los Angeles abrasion test.
- Calling durability by the test name. The property is soundness; the test is the soundness test.
- Saying flaky particles are weak because they come from weak rock. They are weak because of their shape, even in identical rock.
- Describing stripping as a failure of the bitumen. It is a failure of adhesion — the aggregate preferring water to bitumen.
- Treating dust as a minor impurity. It prevents the bitumen from ever touching the stone.
- Listing fewer than seven properties, most often omitting freedom from deleterious particles.
Conclusion
Seven properties, seven ways a pavement can fail, seven tests to prevent it. Traffic attacks the stone by crushing, rubbing and striking it — hence strength, hardness and toughness. The stone can betray itself through poor shape or contamination — hence the shape test and cleanliness requirements. And water attacks in two ways, by stripping the binder and by weathering the rock — hence adhesion and soundness. Learn the property-to-test table as a single block and the whole of aggregate testing falls into place.
Frequently Asked Questions
What are the desirable properties of aggregates?
Strength, hardness, toughness, good shape, adhesion with bitumen, durability, and freedom from deleterious particles.
What is the difference between hardness and toughness?
Hardness is resistance to abrasion, the constant rubbing caused by moving traffic. Toughness is resistance to impact, caused by steel tyred wheels jumping from one particle to another at different levels.
What is toughness of an aggregate?
Resistance of the aggregate to impact.
What is soundness?
The property of aggregates to withstand the adverse action of weather. It is what the durability requirement measures.
Why should flaky and elongated aggregates be avoided?
Because they have less strength and durability than cubical, angular or rounded particles of the same aggregate, so they weaken the mix regardless of the quality of the parent rock.
What is stripping?
The removal of the bituminous coating from the aggregate in the presence of water, which happens when the aggregate has greater affinity for water than for bitumen.
Why must aggregates be free from dust?
Because a dust coating prevents the bitumen from adhering to the stone surface itself, so the binder sticks to the dust rather than to the aggregate.
Which test checks the hardness of aggregates?
The Los Angeles abrasion test.
Which test checks adhesion to bitumen?
The bitumen adhesion test, which gives the stripping value of the aggregate.
Which test checks durability?
The soundness test, an accelerated durability test that simulates weathering action.