Porous Asphalt Frost Depth

[trey25624]

Does anyone have any good advice or experience with the effect of frost depth on porous asphalt. I'm designing a porous asphalt parking lot and everything I read is conflicting. Some literature says freezing / heaving of soils is not really an issue, others say a certain percentage of the storage volume should be below the frost line. I'm in Michigan where our frost depth is around 3.5' feet, so I can't possibly imagine designing a pavement section that is at least that thick. Any thoughts?

 

[rconner]

I noticed there had been no response to your inquiry. I have no experience in and am not aware of specific research in this field; however, I have heard at least anectdotically over the years that “pavements” in general might actually increase depth of frost penetration (I guess the presumption is that some insulating? snow is typically removed from most pavements, whereas snow might remain in sometimes heavy layers for weeks or months in other areas etc.)

Your question turned around the other way might be equally interesting i.e. what would be the effect on the porous pavement due to re-freeze of rain or surface sun-thawed water perched above frozen soil layers below the “porous” pavement?? (I ask the inverted question, in that the same day I saw your inquiry I happened to see a distant family member with a “sqeegee” out applying some kind of nasty black “sealer” over a section of his old asphaltic parking lot of his business – when I asked him why he was doing this, his response was to minimize freezing damage! While he appeared more concerned about some present porosity or than with runoff or retention of water etc., in some confused state I clearly couldn’t give him any advice either.)

In any case, your local water authority e.g. will likely be some familiar with “frost depth”, at least below traditional pavements or elsewhere (and while there are probably more applicable references for pavement design I believe there is a geographical map of “extreme frost penetration” contained in D100-05 AWWA Standard for Welded Carbon Steel Tanks for Water Storage – though I wouod warn it was probably developed before the alleged onslaught of “global warming”). You ask an interesting question, and like you I will await more learned responses.

 

[Horaz]

I do not have a specific answer as well, and have been reading the same thing. Some saying it is not important and other giving some guidance on Frost Depth. Have you seen the UNH web site?

http://www.unh.edu/erg/cstev/

some pretty detailed information based on actual installations/testing in a similar climate too.

They have an equation Total system thickness >= 0.65 * Depth max frost, Eg. 32" thick system if frost depth is 48".

I'd be interested in knowing what different design criteria you are using. Some things say use a worst case CN = 98 and others say calc using the potential max. retention. and what infiltration rate.

Hope this helps some, I am currently researching it for a potential project as well, luckily I am in a very sandy area which seems very suitable to it.

 

[trey25624]

rconner and Horaz thanks for your input. The UNH website provided seems to be a reasonable assumption for the 65%. I had seen another website that had a similar equation, although it required 65% of the required volume be stored beneath the frost line. That seems excessive. But for the UNH equation for a 3.5' frost depth I would need a 28" pavement section. Still pretty thick.

Ultimately the review agencies will have the final say, and I can guarntee like most review agencies they will take the cautious route and have us over design it. Since cost obviously is an issue I need to reduce that bed thickness as much as possible. So I have to be able to feel confident in my design so that I know it will function properly, and also confident so I can argue for the smaller bed thickness with the agency.

I think this is an important aspect of using porous pavement, so anyone who understands the mechanics behind frost heaving you input would be appreciated. I have read the void space in the stone bed reduces the effects of frost heaving, but I need some more information as to why that is if thats the case.