Concrete Paving Over Existing Pavement?

[triplemultiplex]

I observed three locations on I-90 in South Dakota and Minnesota over the weekend where new concrete pavement is being laid directly* on top of the existing asphalt and/or concrete surface.  This is not something I have seen before.  Typically, I'm used to seeing the old pavement torn out, the base is prepped and then the new pavement, be it concrete or asphalt, is then put in.  Paving asphalt on old concrete is old hat, but concrete on top of concrete?  Hmm.
(* There is some sort of construction fabric between the old and new pavement.)

I'd appreciate any insight anyone has on this practice.

Right away, I can see obvious cost savings and accelerated project timelines, but I wonder about its long term prognosis.
I am aware that it has become common to put down a layer of asphalt and then slip-form concrete on top, so might the practice I observed be a natural extension of that?

Bridges and overpasses did create interruptions to the construction sites I saw.  Old pavement was removed under overpasses so as to not impinge on vertical clearance requirements thus creating a long, subtle 'dip' in the carriageway.  Old pavement was also removed approaching any bridges so the new concrete will match the bridge deck.  In both of these places I see potential for a bad bump in the future as the concrete basically gets half as thick.

In two of the three construction zones, the open carriageway had likely already received the same treatment and the ride was the same as if were new concrete on a traditional rebuild.  If this type of repaving holds together long enough, there could be a great benefit for getting freeways fixed faster and cheaper.

[silverback1065]

This happens a lot, typically they either mill the concrete and then resurface it with asphalt, or they rubblize the concrete then resurface it with asphalt. Milling and resurfacing has issues where you get "reflective cracking" where the cracks that happen in the concrete below appear in the asphalt, or at the concrete joints. You usually don't get this when you rubblize it.

[SignGeek101]

I've seen it; asphalting over old cracked concrete. Happened on a surface street close to my house, but have never seen it on a highway.

[Big John]

He is talking about concrete being overlayed OVER asphalt.  I have seen newer projects where a new bed was made with concrete over an asphalt base, so I think the same principle would apply.

[cjk374]

This happens a lot, typically they either mill the concrete and then resurface it with asphalt, or they rubblize the concrete then resurface it with asphalt. Milling and resurfacing has issues where you get "reflective cracking" where the cracks that happen in the concrete below appear in the asphalt, or at the concrete joints. You usually don't get this when you rubblize it.

I've seen it; asphalting over old cracked concrete. Happened on a surface street close to my house, but have never seen it on a highway.

I don't think y'all understood what the OP is saying.  He is saying that a new slab of concrete is being poured (slip formed) over old concrete or old asphalt. I agree with 3MX in his concern over the longevity of the new concrete over older pavement.

A couple of questions:

1.  How bad was the  of the older pavement?

2.  Was there any other preparation being performed on the older surface?

3.  Were they installing dowel bars into "baskets" on top of the cloth that you saw?

[triplemultiplex]

1.  How bad was the  of the older pavement?

Bad, but not totally cracked up.  Concrete was at a point in it's life where you typically see an asphalt overlay to smooth out the ride.

2.  Was there any other preparation being performed on the older surface?

Old asphalt had been milled level where the rutting was getting bad.  I saw some joint repair in the older concrete portions, but for the most part, crews were simply unrolling construction fabric ahead of the slip-form paver.

3.  Were they installing dowel bars into "baskets" on top of the cloth that you saw?

I don't think so.  I recall seeing the dowels lined up out ahead of the fabric like one would normally see for slip-form paving.
Only one of these construction zones had an active paving operation when I passed through (the other two were observed on a Sunday).

This is the project's MnDOT website.
http://www.dot.state.mn.us/d7/projects/I90preserve/
I found the phrase "unbonded concrete overlay" in there.
That yields some informative returns via Google.
http://www.concreteconstruction.net/pavement/bonded-versus-unbonded-concrete-overlays.aspx

[johndoe]

It's known as "whitetopping".

[cjk374]

I have never seen or heard of white topping before. Very fascinating.

[noelbotevera]

I have never seen or heard of white topping before. Very fascinating.

I believe you can see it in PA. I-81 has an uneven lanes section just north of exit 52A-B (US 11) to exit 59 (PA 581). The existing road was still there before that segment started construction. Then, sometime in June-July 2015, they started on laying down new concrete, leaving the existing pavement under the new pavement. The left lane was completed, while the other is starting on its new set o' pavement.

[cjk374]

Won't white topping affect bridge clearances? 

[Big John]

Won't white topping affect bridge clearances? 

Asphalt overlays ("blacktopping") also does that.  Bridges are supposed to be designed to be initially 6" over the clearance standard to accommodate future overlays of the roadway below it.

[cjk374]

Won't white topping affect bridge clearances? 

Asphalt overlays ("blacktopping") also does that.  Bridges are supposed to be designed to be initially 6" over the clearance standard to accommodate future overlays of the roadway below it.

True. But most asphalt overlay jobs involve milling some of the old surface, therefore making up the difference.

[davewiecking]

Won't white topping affect bridge clearances? 

OP addressed the clearance issue in paragraph 4.

[cjk374]

Won't white topping affect bridge clearances? 

OP addressed the clearance issue in paragraph 4.

Ooops...my bad! It has been a day or two since I read that. 

[SD Mapman]

I observed three locations on I-90 in South Dakota and Minnesota over the weekend where new concrete pavement is being laid directly* on top of the existing asphalt and/or concrete surface.  This is not something I have seen before.  Typically, I'm used to seeing the old pavement torn out, the base is prepped and then the new pavement, be it concrete or asphalt, is then put in.  Paving asphalt on old concrete is old hat, but concrete on top of concrete?  Hmm.
(* There is some sort of construction fabric between the old and new pavement.)

I'd appreciate any insight anyone has on this practice.

Right away, I can see obvious cost savings and accelerated project timelines, but I wonder about its long term prognosis.
I am aware that it has become common to put down a layer of asphalt and then slip-form concrete on top, so might the practice I observed be a natural extension of that?

Bridges and overpasses did create interruptions to the construction sites I saw.  Old pavement was removed under overpasses so as to not impinge on vertical clearance requirements thus creating a long, subtle 'dip' in the carriageway.  Old pavement was also removed approaching any bridges so the new concrete will match the bridge deck.  In both of these places I see potential for a bad bump in the future as the concrete basically gets half as thick.

In two of the three construction zones, the open carriageway had likely already received the same treatment and the ride was the same as if were new concrete on a traditional rebuild.  If this type of repaving holds together long enough, there could be a great benefit for getting freeways fixed faster and cheaper.

Well I guess we'll see about duration. However, where they're doing it in SD is in Jones County, which is a county SDDOT really doesn't care about.

I don't think you'd see this around Sioux Falls or Rapid City.

[peterj920]

Wisconsin is currently reconstructing I-41 by asphalt paving the surface first, then laying concrete on top.  They said that the asphalt is more flexible than concrete and can better absorb frost and the spring thaw.  They're hoping that the asphalt underneath will better protect the top layer of concrete from those conditions. 

[cjk374]

Wisconsin is currently reconstructing I-41 by asphalt paving the surface first, then laying concrete on top.  They said that the asphalt is more flexible than concrete and can better absorb frost and the spring thaw.  They're hoping that the asphalt underneath will better protect the top layer of concrete from those conditions. 

This procedure was performed on I-20 in LA between Grambling & Ruston in the mid 90s. The slab looks just as new now as it did when they first opened it.