14.1.2. Mechanisms of Weathering

• Oxidation:
  – Reaction of bitumen with atmospheric oxygen.
  – Increases stiffness and reduces ductility.

Oxidation occurs when bitumen reacts with oxygen in the atmosphere. This reaction causes the bitumen to become stiffer, meaning it can't bend or flex as easily as it originally could. Additionally, the ductility or the ability to stretch without breaking decreases, which can lead to cracking or failure under stress.

Think of oxidation like an apple turning brown after being cut and exposed to air. The longer it's exposed, the stiffer and less appealing it becomes, similar to how oxidized bitumen becomes less flexible and more brittle.

• Ultraviolet (UV) Radiation:
  – UV rays break chemical bonds in bitumen.
  – Leads to surface hardening and brittleness.

Ultraviolet light from the sun can break down the chemical structures in bitumen, which makes it harder on the surface. This hardening makes the bitumen more brittle, increasing the risk of cracking, particularly in sunny environments.

Consider how a rubber band left out in the sun becomes brittle and eventually breaks. UV radiation affects bitumen in a similar way, degrading its flexibility and strength over time.

• Thermal Effects:
  – High temperatures accelerate oxidation.
  – Freeze-thaw cycles cause thermal cracking.

Heat speeds up oxidation in bitumen, hastening the degradation process. Additionally, the cycling between freezing and thawing causes the bitumen to expand and contract, leading to cracks forming, a problem particularly in regions with significant temperature changes.

Picture how an ice cube expands and then shrinks as it melts and refreezes, causing cracks in ice. Similarly, as temperatures fluctuate, bituminous materials expand and contract, creating weakness and cracks.

• Moisture Intrusion:
  – Promotes stripping of binder from aggregates.
  – Accelerates deterioration through hydrolysis and emulsification.

Moisture can seep into the structure of bituminous materials, which can lead to the binder separating from the aggregates that hold the pavements together. This process is known as 'stripping', and moisture can also help break down the chemical bonds further, worsening the material's condition through hydrolysis (reaction with water) and emulsification (mixing of water and bitumen).

Think of how a sponge absorbs water. As the sponge holds more water, it can break apart. Similarly, moisture infiltrating the pavement can cause it to break down, leading to structural failure.

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