Asphalt Millings Structural Coefficient for Pavement Design 3

[Baldie]

Does anybody have any experience or opinions about an appropriate structural coefficient to use in pavement design for asphalt millings placed instead of an aggregate base course (ABC) layer beneath asphalt pavement?

The millings will not have any emulsion, rejuvenators, or additives of any kind such as would be used for Cold In Place Recycled Asphalt (CIPR). It will simply be milled from existing pavement, stockpiled, and then compacted as a base course layer. My gut feel is that this material would be at least as good as ABC which is normally taken as having a coefficient of 0.12 in my area of practice. I have done some research and found that CIPR coefficients are usually taken as 0.25 to 0.35, but this will not be CIPR. Any studies or references would be helpful.

The actual structural coefficient will be a function of the placement density which in turn will depend at least partially (or perhaps significantly) on the placement temperature and gradation, binder, etc. of the existing asphalt pavement that will be milled. The state DOT has a laboratory procedure to establish a maximum wet density which is essentially a one point modified Proctor. Any thoughts on what percentage of maximum should be specified would be appreciated as well.

Thanks in advance for any suggestions.

 

[VAD]

Hello Baldie:

I would not use the milled material immediately below the asphalt pavement. I would prfer to have crushed aggregate that could be properly compacted and provide uniformity than the milled material that would not allow proper grain to grain contact of its constituent aggregate materials.
The milled material can be best used to improve the subgrade or bottomlayer of the granular base course.

 

[kmerl]

I feel vad is correct. Using milled materials directly below flexable pavement is probably not a good idea. Although no additives will be used the problem with flexable pavement is it was flexable when placed and thereby would not consitiute rigidity when milled and replaced.

The other problem is milling operations are inconsistancy. Depending on the materials being milled and method of removal the resultant materials can vary greatly, i.e. a standard mateterial is difficult to come by.

If you really want to push the issue then I would suggested having the materials milled and do a full round of mechanical testing, this could include grain size, CBR testing, and treat it just like a soil. Also if the use is going to be more industrial I woudl suggest hands down use the DGA.

Keithe J. Merl

 

[Focht3]

Why not blend in some asphalt and use it as "black base"? Gradation is certainly an issue - it would have to be screened first - but the material could be useful. And the additional asphalt could improve the stiffness of the layer -

Could it be used just below the wearing course? Yes - but with a lot of QA/QC before and during construction. And the owner would have to accept the risk that the pavement might have to be resurfaced more quickly than a "traditional" section.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[jimbo]

I have used recycled asphalt on several sites carrying bus and light truck traffic and autos. It was broken into pieces not to exceed 12", piled and reused as base. It is much better than aggregate because of the water-resistant nature of the material. The asphalt pavement recycled was of unknown heritage, but not milled.

The new asphalt pavement lasted very well. I had and have no concern about placing a new surface on the recycled material. It was well compacted before placing the wearing surface, of 3" minimum thickness.

 

[kmerl]

jimbo..
Did you complete any testing on the asphalt materials while being placed. I would be interested to see the denistyies and viod ratios acheived during placement. Also how thick was the recycled asphalt placed?

I have used recycled concrete ,locally call 2RC, and have had great success with stability. In fact those materials compacted so well that we acheived a resonable amount of hydrophobic reaction.

Again, I think the proposed loading conditions may dictate the ability to use recycled asphalt as base. I also think Focht3 may be on to something, if the sub soils are generally weak, but I would think a mixing operation may be too costly, compared to purchasing manufactured materials.

Oh and back to the main question, I would use a Modified Proctor value of 95% (ASTM D1557) for materials under pavement.

Pleasure as always...

Keithe J. Merl

 

[jimbo]

kmerl
"Did you complete any testing on the asphalt materials while being placed?" NO

"Also how thick was the recycled asphalt placed?"
I DON'T REMEMBER. IT WAS A LONG TIME AGO.

I had designed/speced/oversaw the construction of over 10 pavements per year for many years. Testing was not a priority.

Also, I had worked for a site work/asphalt contractor for 3 years. I was not lacking in experience.

Usually the spec in that area was 8" crushed agg and 3" hot mix surface. The recycled asphalt was placed on the ground (subbase) and agg was added to obtain 8" thickness.

Neither I not the geotech people were confident of the testing method. Some were horrified by what I was doing.
We all survived and so did the pavement. :-D

 

[kmerl]

jimbo,
Have you had a chance to revisit the pavement recently to see how it is currently holding up?

Keithe J. Merl

 

[Focht3]

Asphalt reuse standards vary widely by the controlling agency and/or owner. Unfortunately, too few of the reuse projects have been documented in a manner that provides confidence to those who have not used the material. We can't all have the same experiences; that's why technical papers (and [green]Eng-Tips[/green]!) are so important. (I wish I had seen those projects, [blue]jimbo[/blue] -)

In Texas, TexDOT has final say in all roadways paved with some State money. Their specs don't allow the use of big chunks in reuse projects. A lot of asphalt ends up as daily cover in landfills...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Baldie]

Thanks for all of your opinions.

I still think that these millings, 2" max. particle size, would tend to act like a weak flexible asphalt pavement layer below the new stronger asphalt pavement if good compaction is achieved on the millings. I have seen contractors place millings to improve haul roads. After many heavy loads the millings look like normal asphalt pavement - at least on the surface. As Focht3 discusses, there was no documentation.

It appears that this would make a great research topic that could be put to good practical use.

Does anybody have an opinion about or experience with using Falling Weight Deflectometer to establish a structural coefficient of a test section? (Perhaps this should be a new thread.) I have doubts about the suitability of lab tests designed for soils to be applied to asphalt millings, although that would certainly be a place to start.

 

[BigH]

Looks like one of our contractors wants to use milled aspalt for subbase. Jimbo - if you could contact me at bohica@indiatimes.com - perhaps I could pick your brain on some details. Will post later. Thanks.

 

[jimbo]

kmerl
"Have you had a chance to revisit the pavement recently to see how it is currently holding up?"

NO- Last time I saw it was when it was 9 years old. It had a few small cracks. I have not seen it since.

Any light pavement around here which lasts 10 years is "good".

 

[kmerl]

Sounds like it held up well.... I would also be interested in millings for beneficial reuse.... thanks for the reply..



Keithe J. Merl

 

[Focht3]

FWD might be a very good way to evaluate this material. But I'm unsure about how one would go about this kind of analysis -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Laser28]

I have used millings as a base for asphalt concrete pavement in Northern Ontario, for roads carrying logging trucks. When aggregate sources are few and far between, you tend to make use of every resource.

We used a conservative structural coefficient for the millings of 11. After reconstruction the road was tested with an FWD, and the design was verified. We have had no problems with this method of construction.

 

[Baldie]

Laser 28,

Do you mean structural coefficient of 0.11 using the AASHTO design method?

 

[Laser28]

Sorry!

Yes, I meant a structural coefficient of 0.11 using the AASHTO design methodology. I would be interested in hearing what others use as coefficients for other recycled materials (rubblized concrete, for example) and for expanded asphalt. As the modulus of expanded asphalt increases over the first few months, does one use the target modulus, the initial modulus or some sort of factored modulus. I tend to use about 85 to 90 percent of the ultimate or targetted modulus for expanded asphalt, considering the fatigue consumption in the first year.

Comments?

 

[Ron]

FWD would work for evaluation and to develop an in-place structural coefficient, but it would be expensive. Backcalculation techniques are reasonably good now.

Milled asphalt doesn't always have an ideal gradation, but can be supplemented with soil to create an excellent base material. I would suggest running several CBR laboratory tests to check the stability, then you could correlate for a more accurate structural coefficient. That would give definitive results without a great deal of expense.

I think you will find the structural coefficient will approach 0.15 to 0.18, depending on gradation and compaction. If you mix a small amount of emulsion in there (1-2 percent), you'll see the structural coefficient go way up.

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[SirAl]

Good topic for discussion. Our firm has applied rubblized and crushed ACP into the design/construction of new road structures. I have found that the material appears to compact very well and forms a very good bridge over adverse subgrade. We have found that it is difficult to measure compaction of this material in the field and therefore rely pretty much upon proof rolling as a qualitative measure of placement acceptance.

In design, I personally would maintain the recycled material as subbase or as a subgrade in a subcut-replacement application. In this way I would be more inclined treat it as subgrade (and apply a subgrade CBR or other subgrade strength parameter) rather than treat it as a component of the actual pavement structure with an assigned structural coefficient. This may not be the best or optimal approach but it is the method I prefer at this time. Some 'gut feel' is definitely required.