What would be the implications of a metal highway design? [closed]

Question

I'm an engineering enthusiast but I don't yet have a formal education in engineering and I'm hoping you guys can tell me about the implications of using a metal road design in terms of durability and cost.

My (vague) idea is (for 2 lanes of road) a set of 3 rails laid down as a vertical layout for a length of road, similar to railroad tracks, which supply power for a self-deploying set if metal road panels. The horizontal panels are designed to drain water and grooved for tire traction. The way the panels self deploy isn't really important, it's just a mechanism that rides on the three rails and lays down a load of the metal panels.

The self deployment isn't really important it's just to lower the construction cost and explain the 3 rail design I had in mind.

I'm purposely being vague when I say metal, expecting answers to provide the kind of metal that would be optimal because I don't know.

It's just that we spend billions across the globe replacing roads, and while metal would obviously cost a lot more than cement, I'm really curious about whether the world would save money by building very durable roads which need to be replaced less often and which for the most part might be replaced systematically the same way they're deployed, requiring only a small fraction of the maintenance of a traditional road replacement.

Obviously the real maintenance cost of this imaginary design is unknown, my question is purely about the difference in cost for materials and the durability so I can have some basis for speculation about the viability of said design.

Answer 1

Asphalt, which is essentially crushed aggregate bound with bitumen has a lot of advantages for road building.

• It is porous and so can drain by allowing water to pass through it as well as running off the surface.
• Because it is made up of particles of crushed masonry it has good friction characteristics in all directions (ie you get the same grip turning as you do braking) which isn't greatly affected by wear. ie the 'grip' is largely inherent in the material rather than depending on a particular surface coating or treatment.
• Asphalt is pretty easy to lay as it can be continuously poured onto a prepared subsurface either by specialist plant or just by shoveling out out of the back of a flatbed truck.
• In road building it is the foundation layers of the road which are the expensive bit and even then the cost is largely associated with the planning, design, surveying and infrastructure (utilities, signage, bridges, culverts, drainage etc) rather than the materials.
• It can be repaired reasonably easily by planing off the top layer and relaying or even adding a dressing layer over the old surface.
• properly made repairs should be more or less seamless and permanent.
• it is possible to lay lower grade temporary surfaces during major works quickly and cheaply.
• serviceable temporary repairs for potholes etc can be made very cheaply
• Trenches for utilities etc are easy to dig and fill in.
• Because the binder is relatively soft small defects can self-repair, especially in hot weather.
• Expansion joints are not required.
• Asphalt can be laid over an uneven surface and doesn't require precise leveling as it doesn't have discrete joints.
• Deterioration of asphalt roads over time is inconvenient but it is unlikely to lead to catastrophic failures in contrast to eg steel railway lines.
• Asphalt isn't subject to much in the way of corrosion or environmental degradation.

There are some very obvious potential problems with a road with a steel deck

• The surface would need to be textured to provide grip and water drainage. This would be ground away fairly rapidly by dirt and grit carried onto the road and the action of tyres and so would need renewal.
• Joints between sections would need to be well aligned to provide an adequate surface.
• How would they be joined ? Steel structures are generally either bolted or welded but both are expensive on this sort of scale compared to just raking out hot asphalt.
• You would still need to prepare a stable substructure.
• The construction would need to accommodate thermal expansion and expansion joints in bridges etc add a lot of noise and vibration when driven over.
• even mild steel is moderately expensive and requires a lot of energy to produce.
• Most structural steels need protection from corrosion by painting, coatings etc which require regular maintenance when exposed to the elements. This will be seriously exacerbated in regions where roads need to be salted in the winter.

For a real world comparison we can look at the relative cost per km of railway track vs multi-lane roads. This puts rail at around 6 times more expensive per km than roads. Note also that a large proportion of the cost of these projects is associated with structures such as bridges and signs/signalling as well as the cost of the land itself, rather than the actual road surface.

See also this document for estimates of civil engineering materials by weight. Note that it puts asphalt as 94 dollars per ton and structural steel at 2586 dollars per ton. This doesn't necessarily tell you a lot about the total costs of what would necessarily by very different approaches but illustrates that asphalt is a very inexpensive bulk material.

It is also worth considering that even in the case of steel bridges the road surface is almost invariably asphalt or concrete except where the structure is temporary or take very low traffic.