highway questions 2

[rb1957]

I looked for a highways forum, figured this'd be close enough.

A couple questions ...
1) why, when resurfacing a highway, do they skip over overpasses ? They've "munched" the road surface on both sides of the overpass, but left the overpass decking. This noticed on the 401 in Toronto.

2) why do they use concrete as the top surface on highway on ground level, but bitumen for overpasses ? This noticed on the 407, north of Toronto.

another day in paradise, or is paradise one day closer ?

 

[Ron]

rb1957...
bridge decks are reinforced concrete. Milling into those can intersect the reinforcing steel...not good. Putting a thin overlay after milling will not last as the stresses at the interface are quite high from traffic loading.

Milling the ground supported pavement is usually a no-brainer....usually not heavily reinforced if at all and not detrimental if impinged. When asphalt is the surfacing, milling and overlaying is a common remediation.

 

[cvg]

different types of pavement, both rigid and flexible are used where appropriate and economical, and usually based on local preferences. The local preference is different in Toronto than it is in Arizona. You can find a lot more information in the link below

http://www.pavementinteractive.org/category/pavement/

 

[MotorCity]

When the wearing surface of a bridge needs to be replaced, it is milled. One of the reasons asphalt is used is that the operator of the milling machine can visually see when he hits concrete and knows when to stop milling. This is useful because often the thickness of the wearing surface is unknown or varies. Its kind of similar to the practice of dyeing paint of subsequent coatings. When you apply a second coat, its a different color than the first coat so that you can areas you missed.

 

[rb1957]

thx MC ... makes sense, and very practical.

why do you think they'd leave the overpass wearing surface unmilled when they've milled the ground supported wearing surface ?

another day in paradise, or is paradise one day closer ?

 

[MotorCity]

I would guess because the contractor is waiting for approval of the local DOT to use the milling equipment on the bridge due to its weight.

 

[cvg]

around here, most local roads are asphalt and bridge decks are usually (but not always) concrete without blacktop. freeways are nearly allways concrete with blacktop rubberized asphalt for sound reduction. the bridges are only milled if they have a blacktop.

attached photo shows a photo with black topped and un-black topped bridge decks, side by side. you can see that the black top is peeling where it was placed over the concrete deck in a thin layer.

 

[IRstuff]

I think that re-surfacing bridges is not as cut and dry as regular roadways; if you mill off an extra 1/4 inch every time, the bridge potentially becomes weaker over time, while roadways can tolerate cracks better.

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[JoeS11]

concrete and asphalt behave completely differently so their use depends on the situation. Bridges are almost always concrete but can have asphalt surface for ease of maintenance. Concrete is 4x more expensive and takes 50x longer to cure. Asphalt expands and contracts and concrete only shrinks and hardens. Gotta take into consideration the materials behavior as a designer to make the best choice about which material to use.

 

[sixaparrat]

Also bear in mind that the surface often fails due to deflection of the subgrade, which will be less (usually) with a concrete subgrade such as a structure. So maybe no remedials required?

Cheers,
Chris

 

[BridgeEI]

They skip over the bridges usually for a couple of reasons. The asphalt adds additional dead load to the bridge and some aren't designed for that weight. An asphalt overlay will trap water beteeen the asphalt and concrete deck which usually deteriorates the deck at a much faster rate. In my area we don't do asphalt overlays for the 2nd reason, it's a huge maintenance nightmare.

 

[bridgebuster]

It's possible that the bridge deck has a waterproofing membrane below the black top. In some environments this is an economical way of preserving the bridge deck.

Here in NY, we sometimes pave bridges when the deck is still in good structural condition but the wearing surface has become "polished" over time.