It seems absurd to me to keep paving the same roads over every year with the same tar. As Einstein once said"The definition of insanity is trying the same thing and expecting a different result." We know the asphalt won’t last long, but we install it anyway. Concrete lasts much longer and is sometimes used on major highways because it needs less maintenance. Why isn’t it used more? Also, are there other alternatives besides asphalt and concrete?
If your town is paving the same roads every year, they’re doing something wrong. I’ve been in my house for 10 years and they repaved once. They patch here and there, which you can’t do easily in concrete. When they do patch, you can drive over it almost immediately, another thing you can’t do with concrete. Many of the patches are due to utility work, which would also have to be done with a concrete road, and be more difficult to make driveable again.
Concrete also tends to heave, which makes the road a bit uneven.
Cost. Considering how many thousands/millions of miles of road need paving and whether it’s affordable is a major factor. More exactly, it’s a ratio of cost/other drawbacks to benefits. Concrete lasts longer than asphalt but even concrete needs replacing eventually and when it does it’s a major p.i.t.a. How long/often a major road can remain closed for repairs is another problem, especially because it’s the more traveled roads that wear out the fastest.
I believe that at one point some futurologists thought roads might be paved with steel, but evidently that never worked out.
Considering what roads have to endure they are pretty durable. Here in Ningland the roads annually transform into something resembling Swiss Cheese because ground under them is soft and falls apart and washes away. No practical road material can stand up to the conditions here. Go out west and you’ll see roads that last much longer because of the more consistent and dryer weather.
Another factor is that road building and repairs are important to our economy to keep people working. I’m not saying it’s a great system, but infrastructure improvements do tend to employ a lot of people at the lower end of the economy creating a trickle up effect.
Concrete’s up-front costs are much higher- there’s a lot more prep work involved with getting the substrate soil properly compacted and stabilized, and then there’s the work involved with setting up the forms themselves, the work of actually testing the concrete to make sure it is within specs, and the final work of actually pouring and finishing the concrete itself.
By comparison asphalt requires somewhat less prep work as far as the soil is concerned, requires no forms, and can be laid and finished by machine. Plus, I think the per linear foot cost is less for asphalt as well.
The tradeoff is that the concrete road as a whole will likely last you 50 years with no major maintenance (potholes and buckling excepted), while asphalt does degrade and wear out in much less time- probably 15 years or so, and requires grinding level and resurfacing (laying another thin layer of asphalt or some kind of sealer).
Another thing to remember is that when the concrete road does manage to degrade to the point of needing to be re-done, it’s often easiest and cheapest to resurface that road with asphalt over the concrete, as a old, beat up concrete surface is still probably better than prepared soil as far as substrates for asphalt are concerned. Jackhammering up old concrete and re-pouring it is probably the most expensive option available.
Indeed it has to do with cost. Roads in western Europe, like to Autobahn, are considerably thicker than American roads, and thusly last much longer in good condition - evidently, good roads are a priority for Germany, and the citizens there pay accordingly. I don’t think we’d be willing to pay for good roads even if presented with that option - we want the cheapest (thinnest) asphalt we can get away with, which is why we are always patching.
Asphalt can be recycled. This summer, my daughter and I took a trip thru the mountains, and encountered a huge lane-wide contraption that appears to rip-up the old asphalt, then re-process the material (with additional fresh tar) and lay down a fresh layer - all in one pass. Not so with concrete.
Oh, and we are pretty sure Einstein said nothing of the sort.
The local city engineers say that concrete costs three times as much as asphalt.
And as others have said, its much harder to patch.
Hmm, I just changed jobs, and I now work for an equipment supplier to the paving business.
First off, it is not the “same old tar”, There is a lot of inertia, but consider that if the old road lasted 10 years, then experimenting with something new takes 12 years to find out if it is 20% better, and it is very hard to simulate diurnal heat cycling, traffic stress, seasonal stress, weather, etc.
The oil business is driving some of the change. Asphalt Cement (“tar”) started out as a waste product of refining oil, so the paving was finding a use for something very cheap. But as oil prices increase, it becomes worth “cracking” the heavy hydrocarbons to make more gasoline or diesel, and the price of heavy crude increases to be closer to light crude, and the price of asphalt goes up. A lot. It is a big problem for poorer states…well, pretty much every state but Texas and Alaska. Texas gets more oil revenue the higher the price of asphalt …same with Alaska, but they are also limited by the length of the paving season rather than the price of Asphalt.
So Asphalt based paving is becoming more expensive, and Portland cement is becoming more competitive. Portland Cement generally requires re-bar, and forms, and extensive finishing work, so there are a whole lot of reasons it doesn’t dominate. It does tend to be pretty long wearing.
The mechanical flexibility of asphalt paving still has a lot of advantages, though. So there are many initiatives to keep using it, but to somehow reduce the amount of oil it takes to make it. These are:
-Rubberized Asphalt. Old tires are ground up an mixed with the asphalt, replacing around 20%. This yields a much more durable paving, and significantly reduces road noise. It also provides a way to recycle some problematic waste. (old tires) Arizona and California have really embraced rubberized asphalt. Colder climate states are experimenting with it. If it becomes very popular, the value of old tires will increase, and much of the economic incentive will be eliminated. But really, it is the “new big thing” in the asphalt paving biz.
-Recycled asphalt. Old paving is ground up and mixed into virgin asphalt. The asphalt changes chemically over time, so there are limits to how well this works, but again, it uses up a lot of what is otherwise a problematic waste stream.
-Graded aggregates. By adding smaller rocks to fill space between the bigger rocks, and smaller yet to fill space between those, and so on down to coarse sand, you can use less asphalt and have more durable product. It is much stiffer to apply, so takes different equipment, and the aggregates have to be sorted out and added in the right proportion, so there is a lot of hassle and so forth.
-Anti-stripping additive. One failure mode for asphalt is that the bond between the rocks and the asphalt fails. This is called “stripping”, and how bad it happens seems to depend on the pH of the aggregate. There are additives that are supposed to help prevent this. How effective these are is an open question. When a roadway fails, it is very difficult to tell if the anti-stripping additive was actually used in the mix as it was supposed to be. It is possible to tell by smell when the hot load is delivered that there is some additive, but not how much, nor what kind exactly, and the chemical analysis is expensive and only provides results well after the road has been laid. The research seems to indicate that it does, work, and the science says it does, but nobody is really sure that it works in real roads with real traffic, and real weather.
Asphalt is the most recycled product, nearly twice as much as paper, glass, aluminum and plastics combined.
"Almost hell now, Pennsylvania,
Blue ridge mountains, Conestoga river,
Roads are crap there, worse than New Jersey,
Last paved '73, bumps for you and me,
Bumpy roads, take me home,
To the place I belong,
Pennsylvania, land of potholes,
Take me home, bumpy roads."
The answer is because government authorities look at things on a short term budgetary basis. Economically you can build a road or highway that will last for a long period of time with minimal maintenance, as such the net present value of the costs of building this road over a 20 year period will be significantly less than the alternative of paying less upfront for an inferior road, but which will require substantially more maintenance and resurfacing costs over that same 20 years.
Many elected officials that are responsible for approving such expenditures are more concerned about the current budget than the 20 year NPV of a project because it won’t help them to get re-elected when they paid 3 times as much upfront costs for a road project, even though it will cost less over the long term. Also, many government authorities require departments to accept the low bid on projects like road building.
As a result, this is why much of our countries infrastructure is shit.
Politics is also a factor. People are always complaining about government spending. So politicians prefer to spend a million dollars patching a road in a way that will last for five years than spend three million dollars doing a quality job that will last for five decades.
eta: ninja’d
Had a neighbor back in the 80’s who was a chemical engineer, she and her husband owned an engineering firm specializing in runway composition. She said the composite concrete they make runways out of is a far superior product to what roads are made out of and if used in road building we’d see much better results in longevity and ride quality… but at a cost almost 50 times higher (in 1980’s money).
Also, with the up-front cost, that’s money you could be spending on other budgetary needs, such as utility support, education, law enforcement and fire protection, etc. If you have a small budget to begin with, the discussion of how much money you’ll save over 10 or 20 or 50 years is purely academic when you can’t afford the down payment to begin with.
Regarding the durability of concrete vs the flexibility and affordability of asphalt, one thing I have seen road workers do is to lay down concrete curbs and then pour the asphalt in between. As far as I can tell, aside from looking a bit nicer (and providing a solid running surface for joggers who’d rather not be running in the mud or in the traffic lane), these provide a bit of protection for the asphalt, preventing chunks of it from breaking free along the edges of the road where it meets the damp soil on the right-of-way.
And because I can’t seem to contribute to a discussion without throwing in some free tangential history trivia, it used to be common for aircraft carriers, even ones with armored protection for the interior spaces, to feature wooden flight decks. While these were much more vulnerable to damage (taking up literally the entire top of the ship, and being the single most important part of it to boot), they were also much easier to repair, and provided some substantial weight and space savings (lighter flight deck means less structural supports required belowdecks to hold it up, which means more space in the hangars). They also helped to lower the ship’s center of gravity, making it much more stable.
As an aside, runways are also not built quite the same way as roads. They are built substantially thicker, since they not only deal with heavier vehicles than your typical highway does, but they also have to be able to stand the stress of having a 747 drop out of the sky and land on them. It’s typical for the ends of the runway (called the Overrun) to be much more lightly built, since aircraft are not expected to land on that portion, and they provide a safety margin for planes coming to a stop at the end of their landing roll. Similarly, taxiways and parking ramps are also much more lightly built than the runway, if still substantially more durable than many paved surfaces designed primarily for automobiles.
And remember, the street past your house probably has all kinds of things underneath: drinking water lines, sanitary sewer lines, storm water drainage, possibly natural gas lines, fiber optic or other cable, even electricity or phone lines in many places.
Any time anything major goes wrong with one of those, the street needs to get dug up and then replaced. Not a huge deal with asphalt – backhoe can easily take up the existing pavement and patching is just as easy (can even cold patch it for a small hole), but both jobs are way harder with concrete. Looking at the number of patches on my street, my guess is that long-term, asphalt might be much cheaper, even needing quicker replacement, than concrete with expensive removal and patching.
To touch on something others have not (so far): when I learmned to drive, the Virginia driver’s manual claimed asphalt offers better performance in bad weather.
Offhand Googling does not seem to support that, so I don’t have a cite, but they told us asphalt has a better coefficient of friction than concrete when both are wet.
Per Heinlein: “The answer to any question starting with “Why don’t they…” is almost always, “money”.”