Why is freeway paving sometimes concrete and sometimes asphalt?

There are several factors for using concrete versus asphalt.

1) Cost: Concrete is much more expensive than asphalt when initially installed. However, concrete will last much longer than asphalt.

2) Traffic loads: The only concrete roadways (other than bridges) that I can think of are major highways and interstates here in Alabama. What do major highways and interstates have that local roads don't? Semi-trucks. If you notice, a road in a suburb may only be paved once ever 20 years. But interstate will have to be paved ever 5 years. This is due to constant loading and unloading of the roadway (think about a wheel of a truck or car slightly compressing the roadway and then it expanding once the wheel is gone). Then think about how many times this happens each day on the interstate. This wears on asphalt very heavily, not as much on concrete.

3) Application: If there is a lot of heavy truck traffic, then the road is likely to be concrete (this is also the reason that the majority of airplane runways are concrete). Also, on bridges, they are usually concrete, due to the constant expansion and contraction of the bridge.

In the future, I think that you will see a lot more asphalt than concrete. Asphalt design is constantly improving. Asphalt is also easy to recycle and reuse (at least moreso than traditional concrete). The new Interstate corridor running from Birmingham, AL to Memphis, TN (I-22) is almost complete, the majority of this roadway is asphalt.

Asphalt Vs Concrete Roads

Concrete Vs Asphalt Roads

The total lifetime cost for concrete and asphalt are about the same. Concrete costs more to install and last longer. Asphalt is cheaper and requires more frequent replacement. So the choice of concrete or asphalt probably has more to do with availability of highway funds and politics than anything else.

If concrete is repaired, it can not have a thin layer of concrete put on top. You will see repairs where a large chunk of concrete is broken out, the hole cut square, new rebar laid in and concrete poured to fill the hole.

Asphalt can be applied in a relatively thin (2" or so) layer on top of existing concrete or asphalt, so if the road surface is damaged, first the top will be ground off, then obvious cracks will be patched, then a new layer applied, using the concrete as subbase, to give a smooth driving surface.

Road surface or pavement (American English) is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past, gravel road surfaces, cobblestone and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Road surfaces are frequently marked to guide traffic. Today, permeable paving methods are beginning to be used for low-impact roadways and walkways.

I always assumed it was due to the cost of getting the materials. Places with lots of local concrete resources will find it cheaper to buy than asphalt, and vice versa. There's probably more to it than that, though.

As for what they first put down on a new road, I've always just figured that it was whatever was cheapest at the time.

I know they can put asphalt over concrete, but I don't think they put concrete over asphalt.

its a matter of what kind of climate,load factor,cost is involved

ashphalt does well in hot climates and is easy to repair cracks and potholes concrete lasts longer but can fetuige alot easier in extereme climates ashphalt is good at most things but the weakness is load it tends to deform as big things go over it concrete is better at high loads in most cases and ashphalt is alot cheaper to lay than concrete but concrete is costlier and hold up better its a matter of what the transportation department wants most of the time