Chemistry of Bitumin Road Degeneration

[charles65]

TL;DR

Bitumin decays over time by ultraviolet light and slightly acid water. What is the chemistry?

All our bitumin roads decay over time by ultraviolet light and slightly acid water. What is the chemistry of this? There is a lot of stuff on the internet about change in physical degeneration but not chemical degeneration? What is a suitable reference?

 

[Borek]

What is the chemistry of this?

Not sure about the role of water, but - taking into account chemical composition and UV in the presence of air - cracking and oxidation are main culprits.

 

[Astronuc]

Summary:: Bitumin decays over time by ultraviolet light and slightly acid water. What is the chemistry?

All our bitumin roads decay over time by ultraviolet light and slightly acid water. What is the chemistry of this? There is a lot of stuff on the internet about change in physical degeneration but not chemical degeneration? What is a suitable reference?

One might find terms such as ageing, weathering or more broadly 'environmental degradation' with respect to bitumen, or more generally pavements, and even more broadly civil structures.

Consider - https://theconstructor.org/transportation/durability-bituminous-pavements/16209/

The 'degradation' process usually refers to deleterious effects that are physical/chemical and that diminish a material's ability to perform as intended/designed in its service environment. Therein lies the challenge to the designer and the engineers and managers responsible for maintenance.

It is not only the bitumen (or cement) but the aggregate as well, and the interface between the aggregate and bitumen. UV light serves to 'activate' the bitumen, while chemical species, e.g., O₂ or Cl^-, will chemically interact with the bitumen compounds to change the chemical nature of the bitumen.

 

[Baluncore]

Pure bitumen subjected to UV in a clean environment will be cracked, progressively losing the volatile products.

I would expect the exposed bitumen pavement to melt and flow, or be cracked into smaller molecules that melt and flow. The rock aggregate will provide some shade, while dust and sand blown across the surface will stick or be embedded, to form a protective layer. Tree roots will do more damage than UV.

 

[chemisttree]

These days, bitumen is a very different product than it was in years past. Bitumen or asphalt bottoms are what is left after the crude oil refining process. Refiners sort of consider it to be a waste product. As the price of gasoline rises, they try harder to extract something useful. This has resulted in an asphalt product that has less aliphatic content resulting in a stiffer material less suitable for roads from the low temperature perspective. It becomes brittle at low temperature and cracks under load. To counteract this, the asphalt divisions of the various refiners blend non-reactive thermoplastics (polyethylene, for example), reactive thermoplastics (epoxy-substituted acrylics, for example) and rubber products to reestablish low temperature flexibility and toughness. Think of it, they refine out all of the aliphatic content of the asphalt... then they add high price polymers to counteract the effect!

In addition to modifying the physical properties of asphalt by adding flexibilizers like rubber, antioxidants and UV stabilizers are added as well. Most of these either react with UV or acids as well. When these are deactivated, the various oxidative, hydrolysis (reacting with water!) and UV degradation processes can occur both on the asphalt and the added polymer modifiers.

You can get an idea of the science involved here.