14.5.1. Surface Defects

• Ravelling: Loss of aggregate particles due to weak binder.

Ravelling occurs when the surface of a pavement starts to lose its aggregate particles. This usually happens because the binder, which is the glue that holds the aggregates together, is weakened. When the binder is not strong enough to keep the aggregates in place, they can be easily dislodged by traffic or environmental factors. Over time, this leads to an uneven surface and can cause safety hazards.

Think of ravelling like the peeling paint on a wooden fence. At first, the paint keeps the wood intact and protected. But as the paint deteriorates and loses its grip, parts of the wood start to flake away, leaving it exposed. Similarly, when the bituminous binder weakens, it can no longer hold the aggregate, leading to a rough and damaged surface.

• Cracking: Due to thermal stresses, oxidation, or fatigue.

Cracking in pavements often results from various factors including thermal stresses, oxidation, and fatigue from repeated traffic loads. Thermal stresses occur when temperatures fluctuate greatly, causing the pavement to expand and contract. Oxidation is a chemical process where the binder reacts with oxygen in the air, leading to hardening and brittleness. Fatigue is caused by repeated loading from vehicles, which eventually leads to the formation of cracks.

Imagine a rubber band that you stretch repeatedly. Over time, it becomes increasingly stiff and eventually, it snaps. In the same way, pavements undergo cycles of expansion and contraction due to temperature changes, which can lead to cracks as the material loses its flexibility and resilience.

• Bleeding: Caused by low viscosity binder in hot weather.

Bleeding occurs when excess binder rises to the surface of the pavement, often observed in hot weather conditions. This happens because the binder becomes less viscous (thinner) when heated, leading it to flow out from the body of the pavement. This can create a slick surface, reducing friction and making the pavement slippery, which poses risks for vehicles.

Think of it like a slice of pizza that has too much grease on top. When you heat the pizza, the cheese and oil can ooze out, making the surface slippery and less enjoyable to eat. Similarly, when the pavement’s binder flows out due to heat, it compromises the surface quality and safety.

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