Bond Breaker When Placing Concrete over Asphalt?

[stuus]

Hello everyone,

I am reconstructing an airport pavement and the surface part of the section is 16" of PCC wich is being placed on top of 4" AC. Normally I would not see a problem with this but I have heard rumours of a law suit at the airport in Portland, OR. I believe that design involved placing concrete over asphalt as well (although I am not sure) and excessive cracking of the concrete was deemed to be the result of the concrete bonding to the underlying asphalt layer. Does anyone know more about this case?

Can anyone confirm similar experiences where bonding between PCC & AC have been the cause for excessive cracking in the PCC? If that is the case, what would be a good fix for the problem? (smoother & more dense AC mix?)

I know that the FAA previously have not allowed us to have an aggregate "interlayer" (such as sand) sandwiched between two pavement layers.

Any comments or experiences are appreciated.

 

[Ron]

The primary problem created with concrete bonding to weathered asphalt (and it will), is that you then have a continuous restraint system that will exacerbate drying shrinkage cracks.

There are several ways you can handle this. One is to use a SofCut saw and space your control joints closer than usual, with sawing taking place very soon after placement, and the depth of the joint deeper than usual to force early cracking in the joint.

Another way (and this is preferable to others) is to break up the asphalt using a reclaimer, which will break the asphalt layer into smaller aggregate-size pieces and in the process mix it with the underlying soil to create a very good subbase for the concrete pavement. This process is relatively inexpensive (a few dollars per square yard to break and mix). You can then place the concrete as anticipated with normal joint spacing.

 

[stuus]

Thanks Ron for your input. I see by your answer that I must clarify something. The asphalt section is new pavement and part of the project. It is not old and weathered. It is scheduled to be placed shortly before the concrete.

 

[csd72]

What about a sand blinding layer between the two?

I am at a loss why you have used asphalt followed by concrete, perhaps you should post this question on the civils page as concrete is only one aspect of this issue.

 

[Ron]

Why is the asphalt layer being used? I've been involved in both rigid and flexible pavement design for many years, but I don't see the benefit of the asphalt layer. Please explain the pavement section.

 

[conceng]

We have successfully used a heavy coating of white pigmented curing compound. This provides two functions. It keeps the asphalt cooler when paving on sunny days and acts as a bond breaker. This won't work as well for aggregate interlock on milled or aged pavements though.


Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com