Gas mileage - asphalt vs. concrete

[Mr. Furious]

I sent this question to Click & Clack, but they get so many great questions that I doubt it'll get answered.

A little while back, I was driving back from my parents' house on I-85. In Durham, I-85 switches from asphalt to concrete. My car has a computer that displays your current gas mileage (updated every couple of seconds). I had it on to see the impact my particular driving style had on mileage. Anyway, when I passed from asphalt to concrete, my mileage (according to the computer) dropped by 8-10 MPG. I was driving the same speed, and there wasn't a significant difference in hills or turns.

Can the road surface have such a significant impact on fuel economy?

[RichardM]

Speaking as an electrical engineer who is not an expert on roadway surfaces but is a member of SAE (Society of Automotive Engineers) and is a long time racer/rallyer, the road surface does play into fuel mileage. But it is not the type of surface but rather the shape of the surface. Concrete can be rougher and hence have more friction. But asphalt can have more small ups and downs, i.e. pot holes. The smoother the surface, the better the mileage.

[Delta-V]

8-10 mpg? No, it's not the road. Most likely it was a change in the slope of the road.

[PrincessLeia]

Slight hijack here:

Does anyone know why there are concrete highways in the US? I went down to Oregon last week and could notice the difference as soon as we crossed the border from BC into Washington and were driving on the I-5. My dad said the US builds concrete highways so that if the country were ever invaded, the army would be able to move the tanks around without destroying the roads as tanks are too heavy for asphalt. Is there any truth to this, or are there other reasons for building concrete roads?

[me, no really]

My dad is a civil engineer who works in road transport. He has told me (a while ago now) that the primary reason is that concrete resists water damage better. If you expect your road to need to carry a lot of water (low lying areas, prone to flooding, gets lots of rain etc) then it may be better built out of concrete - it will last longer. Having said that I know that asphalt technology has improved a lot since then and there are asphalt surfaces that are very good at draining water away from the surface, so maybe that will change. I don't know enough about current practice to be sure.

me

[Troberg]

My car has a computer that displays your current gas mileage (updated every couple of seconds).

Don't trust those numbers, they are a very rough estimate based on intake manifold pressure measurement. They can show more or less if the difference is large enough, but not how much. They should be considered more or less as "bells and whistles"-gadgets.

My dad is a civil engineer who works in road transport. He has told me (a while ago now) that the primary reason is that concrete resists water damage better. If you expect your road to need to carry a lot of water (low lying areas, prone to flooding, gets lots of rain etc) then it may be better built out of concrete - it will last longer. Having said that I know that asphalt technology has improved a lot since then and there are asphalt surfaces that are very good at draining water away from the surface, so maybe that will change. I don't know enough about current practice to be sure.

Not entirely correct. For the last few years, I've been working on road maintenance planning software, so I have some insight into these matters. Here goes:

Concrete is hard, asphalt (or more correctly asphalt concrete, as it contains stone filling in addition to the asphalt) is soft. This means that concrete will stand up to heavy loads better and will not deform even if heavy vehicles are parked on it. This is why it is used on airports, in garages, at traffic lights and on bus stops. Concrete is not always used in these examples, sometimes a harder asphalt is used instead. This is also why asphalt tend to have puddles when wet.

On the other hand, it's also more prone to frost damage, and repairs are more complicated compared to the relatively simple "add some binder, heat and pour on new asphalt" procedure for asphalt.

Concrete lasts longer, which makes it a good alternative on heavily trafficked roads where repairs cause problematic interruptions. The base life span for asphalt (modified by traffic, asphalt type and other factors) we use is 25 years, while the life span of concrete is 50 years. Of course, the price for that is the price, concrete cost a lot more to use. There are also local price variations, depending on the availability of asphalt/concrete and production facilities.

Asphalt also tends to give a slightly rougher surface as it wears down, compared to concrete (which is why water drains off it better). This is very important for traffic safety reasons, as it has a huge impact on braking distances and vehicle control, especially in bad weather. Concrete can give off a fine limestone dust, which in wet weather becomes a smooth sludge that is almost as slippery as ice. This could, theoretically, also give asphalt a slightly worse mileage, but given the huge increase in safety, it's a minor issue.

There are also other construction considerations, but I think I've covered the main ones.

Read this post two times and you're almost ready to work as highway engineers.

I can certainly believe that road surface affects mileage. As mentioned earlier, the smoothness of the road affects mileage, mostly because it affects the rolling resistance of the tires on the road.

Earlier this year I purchased new tires for my car, going from fairly worn, conventional touring tires, to a performance tire with an agressive tread. It is also 30 mm wider than the touring tire. The increase in rolling resistance due to the extra tread with and agressive tread has resulted in a reduction of fuel economy, from about 28.6 mpg to 23.5 mpg (ten-tankfull averages quoted, which averages out differences from one tank of gas to the next).

Your example sounds even more extreme, but is not out of the realm of possibility.

[Recklessmess]

I remember this experiment in my sophomore year high school physics class. We used one of those little red wagons and a brick (or bricks). Then we tested its stopping distance on several different surfaces. Our school was only a year old, so we had several fresh and flat surfaces to test. We used asphalt, concrete, gravel, sand, and I'm sure more I'm forgetting. I was surprised to find that the stopping distance on asphalt was much greater than that on concrete, proving to us that the resistance of the road material does materially alter the efficiency of the vehicle. There was other science stuff that we did, mathematical equations, but it's one of those cool high school experiments that I'll never forget. <sigh>

[DrRocket]

Originally Posted by PrincessLeia

Slight hijack here:

Does anyone know why there are concrete highways in the US? I went down to Oregon last week and could notice the difference as soon as we crossed the border from BC into Washington and were driving on the I-5. My dad said the US builds concrete highways so that if the country were ever invaded, the army would be able to move the tanks around without destroying the roads as tanks are too heavy for asphalt. Is there any truth to this, or are there other reasons for building concrete roads?

The interstate highway system was championed by President Eisenhower based on his experiences as an observer in the U.S. Army's 1919 Transcontinental Motor Convoy, and of his observations of the German autobahn system after the war. In his book "At Ease" he wrote "The old convoy had started me thinking about good, two-lane highways, but Germany had made me see the wisdom of broader ribbons across the land."

You'll see asphalt used on sections of the sytem, but the base is still concrete. The asphalt is used to get a bit more life out of the road before completely rebuilding the section.

[Troberg]

I can certainly believe that road surface affects mileage. As mentioned earlier, the smoothness of the road affects mileage, mostly because it affects the rolling resistance of the tires on the road.

The by far greatest resistance on a car moving at a constant speed is air resistance.

If the road goes really bad, mileage will drop, as the road requires slowdowns and accellerations, but as long as you can keep a constant speed, the smoothness of the surface is neglible.

My dad said the US builds concrete highways so that if the country were ever invaded, the army would be able to move the tanks around without destroying the roads as tanks are too heavy for asphalt.

It's not mainly the weight, as it's distributed over a large footpront, it's the tracks that tear up the asphalt. Remember that tanks turning cause the tracks to slide over the ground. Tracks are really damaging to any surface. As far as pure weight goes, a truck of similar weight is much worse, due to it's smaller footprint. As a rule of thumb, a single truck wears down the road about as much as 1000 cars due to the higher ground pressure.

Regarding the OP, could there have been a coincident change in the wind speed and/or wind direction when you switched over from asphalt to concrete? Did you change direction in a prevailng wind, or was the concrete road wider, and less protected by the adjacent terrain/foliage?

[Mr. Furious]

Originally Posted by mommyrex

Did you change direction in a prevailng wind, or was the concrete road wider, and less protected by the adjacent terrain/foliage?

The road did widen from 2 lanes to (IIRC) 5 when it switched from asphalt to concrete. The highway changed direction slightly (from WSW to W, if I'm eyeballing it correctly).

[chelle]

Did you change speed when the road changed? Because I have found that speed does have an affect on gas milage.

My parents recently bought a pop-up camper and we towed it from Missouri to Tennessee (with a 2000 Cherokee 4.0 automatic) on vacation a few months ago. We started out doing the speed limit and the gas milage dropped from around the 21 nomal average to 14. After that, we kept the speed down to about 5 below the speed limit and our gas milage went up to around 18.

In addition to that, I have a 67 mustang with a 302 and a 3 speed automatic that I use as my daily driver. My speedometer isn't accurate and for a while I was thinking that 72 on my speedometer was close to 65. During that period I was averaging around 13,5 - 14 mpg. Then I had my parents pace me one day and found out that 68 on the speedometer is actually closer to 65. So from then on, I kept my speed down and my gas milage went up to 15.5 - 16 mpg (still pathetic by most standards, but not bad for what it is).

[Delta-V]

Originally Posted by Troberg

The by far greatest resistance on a car moving at a constant speed is air resistance.

At speeds above 45 or so, that's true. Below that, not so true.

Originally Posted by PrincessLeia

My dad said the US builds concrete highways so that if the country were ever invaded, the army would be able to move the tanks around without destroying the roads as tanks are too heavy for asphalt. Is there any truth to this, or are there other reasons for building concrete roads?

Originally Posted by Troberg

It's not mainly the weight, as it's distributed over a large footpront, it's the tracks that tear up the asphalt. Remember that tanks turning cause the tracks to slide over the ground. Tracks are really damaging to any surface.

I've heard that as well, but I have never heard the claim supported. The US military almost always moves by rail wherever possible. It's a simple matter of economics. You'd need a tank transporter for every tracked vehicle, and US Army divisions don't have that many, nor do they need to under normal circumstances. A railcar can take one or two Abrams tanks (depending on capacity) and 2 of just about everything else, so few trains could transport an entire division.

Tracked vehicles aren't that reliable, the tracks wear out quickly, and the crews wear out even quicker. It's common practice to move tanks by truck transporter any time they're going to be moving more than a few miles (outside of combat, anyway). Even in Desert Storm, tracked vehicles were moved by truck to their assembly areas. So tracked vehicles on the roads wasn't a concern.

Here is what my BIL and sister say about the concrete vs. asphalt debate:

Concrete lasts longer, but takes a lot more to replace when it does fail.

Asphalt doesn't last as long, but is cheaper to replace.

Over the years, the cost difference is about a wash (i.e., no significant difference).

One of the reasons we have so few concrete modern roads here in the US is back when they were building the interstates, the concrete companies decided to not put rebars between sections (to save some money). So now we have that horrible thump-thump-thump of older concrete. From what my BIL says, they almost destroyed their own market with that short sighted decision.