There was a fascinating series of articles in The New Yorker a few months ago, about some of the many causes and effects of global warming and other environmental issues (part one is here). One of the causes (well, part of one of the causes – I’m simplifying dramatically) is the growing number of roads on the planet, which are paved with black asphalt. That got me wondering: why do we use concrete for sidewalks and asphalt for roadways? Why don’t we use the same material for both?
I have no doubt that there is a reason, I just don’t know what it is.
If you have ever driven the interstates in Pennsylvania, you’ll find that they DO, and HAVE used concrete in interstate roadways. You’ll find it even more down south, where the advantages are better. Concrete vs asphalt have different advantages & disadvantages. Concrete is a longer lasting, harder material. But it is installed in preformed sections that must be trucked in, and the roadway has a tendency to buckle at the seams of these sections. This is more of problem in the Northern states, where frost heave quickly makes the road relatively unpleasant (before eventually becoming undriveable) - one of the reasons PA’s roads seem to be so bad is that on concrete road sections the drive is ker-bump-ker-bump-ker-bump the whole way. I believe maintenance on Concrete roadways is often harder as well. Asphalt is cheaper initially, and while it is affected by frost too (witness all the potholes), it is easier to fix back to a relatively smooth driving experience.
Hopefully a civil engineer will chime in with a more detailed discussion of the differences, but note that their certainly are concrete roads (they’re fairly common out here in Texas). And even asphalt roads may just be a layer of asphalt over concrete, and not asphalt all the way through.
Yes, we have concrete roads as well. Very steep roads are often done in concrete since it can be scored to improve traction.
Though durable, concrete is more expensive and slower to install. Therefore it is used mainly for high-traffic highway projects like interstates, or only when its necessary economically feasible.
Regarding preformed sections, cite? Only bridge beams are precast. Road surfacing is typically done using a large machine called a slip form paver. Many concrete trucks are used to feed the massive machine which forms the concrete, places it, and inserts junction bars at preset intervals. Google “Gomaco” to see more of these units.
Never seen a slip form paver, but I’ve observed crews placing rebar by hand and pouring a series of slabs. Agree that doing it off-site would make no sense at all.
To a point, asphalt is a “living” surface. Able to expand and contract without the requirement for expansion joints. Expansion joints are required in concrete roadways, so they are never all that smooth, and the the joints allow water in, which can freeze and cause cracking.
Of course asphalt can also develop cracks, and if overloaded may even develop ripples or even wrinkles in extreame cases. It is easier and cheaper to patch or resurface though.
At least in the desert southwest, I’m pretty sure it depends on the amount of truck traffic the road will bear. If there is lots of heavy truck traffic, portland cement will be cheaper in the long run. For roads with lighter loading, asphalt MacAdam will win out.
There are a lot of factors in the paving equation. One of the primary considerations is the amount of traffic. An interstate highway carries a huge amount of traffic, particularly truck traffic. The demands of that kind of traffic are extraordinary. It is expensive to build interstates because the bed has to be prepared and a hard, heavy, durable surface has to be installed. What many don’t realize is that the concrete for an interstate highway is of a specific consistency and is about a foot thick.
One legally loaded semi-truck will put as much wear and tear on a road surface as 900 automobiles (that’s why they are taxed so heavily). Contrast that with a suburban or country road with little truck traffic. A few inches of asphalt that is much cheaper will suffice.
If the concrete on an interstate starts to get rough or bumpy a layer of asphalt can be put over it. The base remains in place but it looks like an asphalt road.
My father was a county engineer in rural Iowa for most of his working life, and as such I’ve had a lot of road construction trivia soaked into my brain. Let me see how much I can regurgitate here.
A two-lane concrete roadway in rural territory, properly constructed, costs somewhere around $1 million / mile to construct (keep in mind this is rural areas, with very few, if any, bridges, culverts, or overpasses). With proper maintenence, it will have usable life of around 30-40 years. However, construction time can take several months (generally an entire summer) during which time the roadway is essentially unusable for all but local traffic (people who live along that road, for example). Also, concrete roadways generally require about 2 weeks to cure completely, during which time the road should not be driven on at all.
A two-lane asphalt roadway costs roughly 1/4 of what a concrete roadway will cost. Asphalt roads also have a shorter curing time (generally a few hours instead of weeks). However the lifespan is also about 1/4 of the concrete roadway.
A lot of concrete roadways, towards the end of their design lifespan, will receive what is known as an asphalt overlay. This process puts down several inches (anywhere from 2-12) of asphalt over the exisiting concrete surface. This can add a few years to the life of the road quickly and cheaply; however, often these overlays deteriorate quickly (within 4-5 years).
My father was a big proponent of concrete roadways over asphalt ones, his reasoning being that it’s less disruptive to repave a road every 40 years as opposed to asphalt every 10-12.
The link I provided would let you see the machines-they were also featured on one of the cable TV ‘Modern Marvels’ Big Machines segments. The short read is that on a prepared elevation, the unit intakes concrete direct from the mixer(s), places and finishes the mix one or more lane width at a time such that after cure, you cut control joints and it’s time for line paint.
On many of the projects around here, I’ve noted that after cure, a profiler passes over the finished roadway knocking off tiny high spots and producing a road surface which is very nice and smooth to drive on.
I do this for a living for airfields, and heartily will stand by Spartydog’s comments.
When you consider that you have fifty tons of aircraft slamming onto particular spots of a runway for a landing, you learn the difference between flexible and concrete pavements.
If you want a great example of failure of flexible (asphalt) pavements, take a look at the New Jersey Turnpike at the northern exits. Where I-95 ‘turns into’ the NJT at exit 18W, you’ll note at least 6" ruts. This is more than failure for a runway.
Asphalt = flexible. Concrete = solid pieces on a base course. Both have their advantages through cost and weight bearing. It’s a matter of economics. I put the first 1000’ of runway in concrete and the rest in asphalt.
Ah, this is the sort of thing I was looking for (why one material over another). Thank you!
Most of the streets I see around here (Seattle) and that I remember from growing up in California were concrete, overlayed with asphalt – even some new roads were immediately overlayed with asphalt. I can understand adding a layer of asphalt later in the road’s life to get more years out of it, but why add asphalt right from the get-go? Is it just for aesthetics, so it matches the rest of the city? Or would it cause a structural problem, at the seam or something?
Maybe this is a dumb question, but I’ll ask anyway: why is asphalt black? Is there any way to intentionally lighten its color (other than painting over it – and I know it fades over the years)?