Know anything about construction and composition of roads?

Question

I don't anything, but i was speculating and wondering about them(I'm dull) Is there a lower more permanent layer, what considerations are made for thermal expansion and contraction? Pros and cons of concrete vs blacktop(or other)? A cross sectional picture would be awesome.

Answer 1

Boy, where to start?

All except dirt roads have a "base" or "road base" under them consisting of gravel or other mineral soil (not clay or silt). It serves many purposes - it distributes the load to a wide area of underlying native soil (forces from a point load bear within a 45-degree angle so a 2-foot-thick base spread a load to a 4' x 4' area). For an interstate, that can be 3 or 4 feet of gravel top by as much as a foot of concrete.

And, in freezeing climates, the base separates and therefore protects the pavements from freeze-thaw movement of wet ground beneath. In extremely cold climates with wet, fine-grained soils beneath, as much as 15 or 20 feet of gravel can be needed. The Alaskan Highway through northern BC and Yukon is an example.

Asphaltic versus portland-cement concrete (they are both concretes but are often called blacktop/macadam/tarmac versus concrete): Portland cemernt is a stronger and more versatile binder - for instance, you can use rounded gravels. Asphaltic concrete requires crushed stone with its angular pieces with greater surface area. Also, asphaltic concrete is senstive to creep at very high temperatures.

Both AC and PCC are available in many, many forms. Strength, temp resistance, salt tolerance, cure times, etc are by selected as needed. And the aggregate (the gravel inside) is important. The concrete mix can't be stronger than the aggregate used to make it.

PCC is rarely used without steel reinforcement. They make a great composite material because the concrete is very strong in compression while the steel is great in tension. That's why you see workers threading together an intricate cage of steel before pouring a concrete roadbed nad especially before pouring a wall, column or bridge.

Thermal expansion/contraction - yes, many roadways are poured with expansion gaps - they cause rhythmic bumps as you drive along. AC is often poured continuously. The cracks that naturally form get bigger in the cold when the pavement contracts.

Hope that helps a bit. Engineers spend a few semsters studying concrete design and some people spend their whole careers on it.