Autobiographical note: Those of you who actually know me will get the reference.
Around 1997, in the US alone, 70 billion tons of asphalt were used annually. That number has probably increased since then. It should be noted that asphalt is not only used in paving, but also for things like roofing, adhesives and batteries. The popularity of asphalt is due to its waterproofing and binding properties — in roads, the ability to bind various sands and stones together to create a smooth, paved surface — and it is derived from petroleum processing. It’s basically the very last usable byproduct of the processing. According to the Chemical Engineering News, asphalt is composed mainly (around 80%) of carbon; the rest is made up of hydrogen (10%), sulfur (up to 6%), small amounts of oxygen and nitrogen, and trace amounts of iron, nickel, and vanadium.
“Ideally, asphalt used for paving roads should remain viscoelastic in all weather conditions. However, many asphalt roads soften in summer and suffer from rutting, or permanent deformation, as it is also called. At low temperatures, neutral molecules in asphalt arrange themselves into more organized structural forms. As a result, the material hardens, becomes brittle, and cracks under the stress of heavy traffic loads. This is known as thermal and fatigue cracking.
Asphalts also lose their plasticity and therefore harden and crack or crumble when they lose their more volatile lower molecular weight constituents or when these constituents are oxidized. This process is known as aging. Moisture from rain and other sources can also invade and damage asphalts, particularly aged or oxidized asphalts because they have a larger number of polar constituents to attract water molecules.”*
So that explains why half of Kentucky is always under construction.
*Quoted material can be found in this article.