Structural Number for recycled in-place asphalt 2

[bmoc]

Does anyone know the ASSHTO SN for crushed in-place asphalt?

I'm rehabilitating an existing parking lot in good shape, minor surface cracks only, by crushing 2" of existing pavement. 5" to 6" of existing agg. base will remain in place untouched, and overlaying with 2" of new HMA. Any thoughts on using just one layer in overlaying the existing crushed material?

Thanks.

 

[dicksewerrat]

So you are going to crush in place the wear course and put down 2" of new material? Are you going to pick up the old material and crush to some specification? or just smash it with a large heavy roller?

 

[VAD]

I am not sure why you would opt for recycling the existing asphalt pavement if your surface cracks are minor. Perhaps you are thinking that the crushed layer will mitigate the formation of cracks in thew new asphalt overlay. If it occurred once it will occur again after about an equivalent number of years.

You stand the chance of developing structural cracks in your new pavement since you may want to pave before the recycled crushed aggregate has been conditioned. However, if your loads are just cars then this may not occur.

Regarding the layer coefficient a value of around .14 is used for virgin crushed stone. This is still debatable. I would think that a somewhat smaller value would be appropriate for the recycled material. You should check Yyoder and Witckak pavement Design text to confirm the value suggested.

Dicksewerrat's question leads perhaps to provideing a layer coefficient value depending on the state of the material. The SN is obtained by multiplying the layer coefficient by the thickness of layer.

Good Luck

 

[Ron]

I have quite a bit of experience with using existing asphalt pulverized and mixed with existing base to create a new base (in-place base recycling). When asphalt emulsion is used as an additive, the layer coefficient (this is what you are asking...not the structural number) of the new base material is generally in the 0.16 to 0.20 range, depending on the existing materials. If asphalt emulsion is not used, and the recycling is dry, the layer coefficient can range from about 0.06 to 0.18 depending on the final gradation and particle interaction.

If you are just going to pulverize the existing asphalt and leave in place as a layer on top of the base, don't waste your time or money. That's an accident waiting to happen!

 

[dirtdoctor]

After seeing CPR projects on numerous light duty roads and some small commercial parking lots, I have come to the conclusion that the greatest drawback to a recycled base is grade control. I don't care how well the recycled base is shaped, cured out, or compacted, you always have difficulty in obtaining an even surface with only one surface course. We almost always would 'drag out' the paver to level low spots that develop in a finished CPR base. Poor rideability used to be the norm until we used a leveling course first. And the costs associated with mobilizing and using a CPR unit are very high, especially if your only mode of pavement distress is minor surface cracking. Nearly all of the time, you can buy a lot of paving fabric and hire a good fabric installation guy a whole lot cheaper.

 

[dirtdoctor]

OOPS, I forgot the main question:

As far as a layer coefficient, we usually use 110% to 130% of the layer coefficient for the virgin stone base beneath the in-place asphalt. Where I hail from, the stone base # is 0.18 for crushed aggregate base, so we use 0.20 to 0.24 for the CPR mixture, depending on the size and quality of the asphalt stone as well as the amount of new stone added to the blend.

 

[KRSServices]

Without having seen the actual needs assessment or the site, my comment would be why the option of grinding was chosen in the first place? The result of the operation will be smaller aggregate sizing and an abundance of manufactured fines.

If there are minor cracks, would it not be cheaper to perform some maintenance on the existing structure and extend it's life rather than removal of the existing and then overlaying with a new lift of ACP?

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