Originally Posted by TheDuke
I would like to have someone "smarter" than me, explain the reason for concrete roads in the first place? In the end they all get paved over anyway, then all the broken joints come up through the asphalt.
I don't claim to be smarter than anybody, but I used to be an engineer and actually designed roads and parking areas as one small part of my job.
Concrete can be reinforced with steel reinforcing bars and is a "rigid" paving surface.
Asphalt pavement cannot be reinforced with steel reinforcing bars, and is a "flexible" paving surface.
Macadam is similar to asphalt, but uses coal tar as a binder instead of petroleum-based asphalt, so it's less damaged by leaking petroleum products that act as a solvent for asphalt.
Anyway, when you build a road out of asphalt or macadam, you've got to have good soil conditions. The load-bearing course is actually the graded crushed stone aggregate underlayer (i.e. gravel to the layman), not
the asphalt. The asphalt is just a binder course to keep the aggregate from going anywhere. The load of a vehicle is only spread out over the vehicle's "footprint," literally where the rubber meets the road.
When you build a road out of concrete, you can do it in poorer soil conditions because the concrete itself is load-bearing and spreads the load out over the entire slab, not just over the "footprint" of the vehicle. The aggregate underlayer is less to provide load-bearing and more to provide drainage.
All road surfaces wear over time, due to traffic and weather. In areas where you have cold winters and a definable frost line, the road surface will heave with the onset of winter as subsurface water freezes and expands. In the deep south, especially in coastal areas, you have a different problem, in that the road surface will subside as water is squeezed out from under the road and the soil consolidates and compacts.
Concrete road surfaces chip at the edges, and when water penetrates into the reinforcing steel, it rusts and expands (iron oxide is a larger molecule than iron), causing a condition known as "spalling" where the expanding rebar can literally pop the concrete off from over it. There are only two fixes, complete replacement of the concrete and the less expensive overlaying the concrete with asphalt.
Asphalt road surfaces can split in curves because asphalt cannot resist sideways forces, only vertical ones, and a vehicle's centrifugal force imparts a sideways force toward the road's edge. Asphalt pavement develops waves at the approaches to intersections for the same reason, because when a vehicle brakes to a stop or accelerates from a stop, it applies sideways forces in the direction of the road.
Fixing asphalt properly requires complete removal of the asphalt and regrading of the aggregate underlayer. However, just as in overlaying concrete with asphalt, highway departments have the twin constraints of budget and public dissatisfaction with long repair times, so they take the quick-fix option of removing most
of the asphalt and putting down a new asphalt layer, without doing anything to the load-bearing gravel course under it.
In both cases, concrete and asphalt, the fix is temporary and less effective than a whole new road surface. A new road surface should last about 10 years before it needs to be repaired, but a repaired road surface can only be expected to last 5 years before it needs to be repaired again.
Everything else I know about pavement design is the actual number-crunching, that I learned in college and continuing education refreshers over the span of a few decades; I don't think you want to read about crunching numbers, so I'll leave it at that.