AASHTO Pavement Design

[gsullivan]

I am performing an AASHTO pavement design utilizing Structural Number (SN). I am looking for a resource that provides a typical structural coefficient for pervious paving. Typical pavement coefficients are in the 0.4-0.44 range, but I can't find a coeff. for pervious pavement.

 

[Ron]

If you are talking about Pervious Concrete pavement, the reason you can't find such is that you are using flexible pavement design techniques and Pervious Concrete is a rigid material. You can treat is like concrete or you can treat it like a fractured concrete section if you want to use the flexible process. In that case, the layer coefficient would be like a very strong base material, say O.20 or so.

If you are talking about pervious asphalt pavement, then essentially that is like an open graded friction course where the layer coefficient is usually taken as 0.0 (not considered a structural layer). Open graded asphalt should have another layer of asphalt under it for strength and for base protection.

 

[oldestguy]

It is too bad the folks doing the analysis of the Ottawa Road test back then didn't publish how they determined the three factors they reported in the "Interim Guide for the Design of Flexible pavement Structures".

Apparently there was some "back calculation" from performance, regional factor, etc. to arrive at the SN.

I just looked through the 1961 publication and find no clear description on how they came up with a method that then can be used for other pavement types.

I have an article from "Concrete Construction" Magazine, March 1982 giving some recommended figures for various materials:

Blacktop.....0.40

Blackbase ....0.27

Crushed stone base .... 0.13

Portland Cement concrete... 0.50

Interesting stuff from a concrete group??
The figures apparently came from an Illinois DOT design manual.

I don't have the book any more, but Yoders' book on "Pavement Design" or similar name, had quite a bit on variations from the original Ottawa Raod test info.

So depending on other things, such as Marshall test, Hveem, etc. to compare pavement "strength", the original document is unclear. If you can do such a comparison, that's your best bet.

Or take the figure of 0.20 and realize that all the factors going into the design are quite rough anyhow.

 

[gsullivan]

This is a flexible pavement, pervious asphalt pavement. Structural coeff. of zero for that?

 

[eric1037]

I assume you are designing this type of pavement for stormwater management. I have performed several designs. Do internet searches on the following:

Porous asphalt
Pervious asphalt
Pervious pavement
Porous pavement

You really need to check out this reference:

http://www.epa.gov/ednnrmrl/publications/reports/epa600280135/epa600280135.htm

It has some of the most comprehensive design information even though it was published in 1980.

Also, Cahill & Associates are one of the leading experts in this technology.

Finally, check out the Asphalt Paving Institute. They have several publications that discussion design, construction & maintenance.

This is a very exciting technology that is showing promise.

Good Luck!

 

[Ron]

gsullivan...open graded asphalt doesn't handle repetitive tension loads very well. An asphalt pavement layer thickness is controlled by the tension in the bottom of the layer, to prevent cracking. When you have such low resistance to tensile strain, it usually isn't economical to provide an open graded section thick enough to withstand the loadings....after all, they don't charge less for that mix of asphalt. That's why it is usually done with an asphalt structural layer under it to provide strength, while the open graded friction course provides lateral drainage.

Pervious asphalt, in the same vein, is attempting to pass water through, which will saturate the base. Therefore the base has to be very strong for it to work at all.

 

[eric1037]

If you are trying to use this as a method of stormwater control and/or retention, we have had success using a open-graded, crushed material below the asphalt. Also, since the asphalt is open-graded, you have to use a polymer additive to the asphalt mix to get the tensile properties that Ron is referring to. Simple asphalt cement just doesn't encapusulate the aggregate.

 

[gsullivan]

So basically for my flexible pavement design per the AASHTO method (DARWin anaylysis), I can't plug in any structural coefficient for the pervious asphalt pavement; I am completely relying on the underlying aggregate for stability.

 

[eric1037]

gsullivan:

What is your purpose for using pervious asphalt. Is this a road or parking lot? Is it for storm water control or some other purpose.

My experience with this is for storm water control. If that is the case, you only put this type of material in one layer. Usually it is 2-1/2" thick and it isn't rolled very much.

Therefore, you need to have a relatively solid, thick base to place it on. There is some structural capacity in the asphalt concrete. Refer to the link that I provided above. You may also want to purchase some of the documents from the National Asphalt Pavement Association.

Here is a link:

http://www.hotmix.org/view_article.php?ID=533

This isn't designed like a normal pavement and it isn't just plug and chug. You have to think outside the box and do some research.