Asphalt Millings Structural Coefficient for Pavement Design
Asphalt Millings Structural Coefficient for Pavement Design
(OP)
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.
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.





RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
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
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
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
RE: Asphalt Millings Structural Coefficient for Pavement Design
"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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
Have you had a chance to revisit the pavement recently to see how it is currently holding up?
Keithe J. Merl
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
RE: Asphalt Millings Structural Coefficient for Pavement Design
"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".
RE: Asphalt Millings Structural Coefficient for Pavement Design
Keithe J. Merl
RE: Asphalt Millings Structural Coefficient for Pavement Design
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
RE: Asphalt Millings Structural Coefficient for Pavement Design
Do you mean structural coefficient of 0.11 using the AASHTO design method?
RE: Asphalt Millings Structural Coefficient for Pavement Design
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?
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.
**Need Professional Development Hours / Continuing Education ??
**Try this course...
http://pdhonline.org/cgi-bin/quiz/courses/courselist.cg...
RE: Asphalt Millings Structural Coefficient for Pavement Design
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.