Before ACP overlay: to mill or not to mill 2

[Drumchaser]

I have heard from others that many somewhat recent ACP overlays, that were not milled before hand, are falling apart.

Are others out there experiencing this? And if so was a chip seal placed before the overlay?

Have you had more success/longevity with overlays when milling was performed?


Thanks for your comments.

 

[Ron]

DC...I was hoping you'd get some comment on this. It's always good to find a variety of perspectives since there are so many variables in all this stuff....for instance one area might use a different viscosity/penetration grade of AC that might contribute where another might perform nicely....would be good to know if what I've seen in this area is an anomaly or not.

 

[Drumchaser]

Hey Ron

Yes...still hoping to get some comments...and yes, many variables. We almost always place a one course surface treatment before any new mix is placed...this could be why we have not seen an over abundance of problems with non-milled surfaces. The 300+ ACP livens up the OCST...adds a good bite.

I am always eager to hear about others problems and triumphs relating to pavements. Personally I can only think of one instance of delaminating in the past few years. Am aware of several short-term failures due to a variety of reasons.

If others are enjoying excellent results on overlays by planing before hand, I would sure be interested. As I stated earlier planing is cheap and provides a benefit to owner and contractor. This would be significant news. Thought we would have had some input/direction from the fed's on this if in fact a major benefit.

Thanks for your comments.

 

[Ron]

DC...I believe you're right. When we have put a crack relief layer (single or double chip seal), the bond is fine and the performance of the overlay is good.

The only advantage to milling is that it is cheaper than a chip seal.

Mill and overlay is one of the remediation strategies put forth by the Feds from the SHRP program.

 

[Drumchaser]

Ron...we are speaking of flexible pavements, correct?

When you use the term crack relief layer, it sounds as if you are describing reflective cracking caused by underlying concrete pavement.

The chip seal(s) we use is more for water barrier than anything.

Our experience has shown if no OCST is applied,
the chances are slim for long-term performance.

Caution should be used when milling, if too much is taken off ....before you can get the new on...it "can" get ugly in a big way.

What SHRP doc. are you referencing? and does it imply that the milled surface is preffered for an overlay?

What are the PG binders used in your area? for surface course ACP?

 

[Ron]

We use a chip seal as a crack relief layer over old flexible sections. We don't do much with asphalt over concrete....even a chip seal wouldn't stop that type of reflection.

Is your OCST as course as a chip seal or is it finer?

We use typical dense graded mixes (Check the Florida DOT website for their quirky designations....Type S1, Type S3 are the most common...S1 has 3/4" top size aggregate and S3 uses 1/2" top size.)

Here is a document from the Feds that shows a survey done with 41 agencies (State DOT's) responding...out of 41 agencies, 38 use mill and overlay.

 

[Drumchaser]

Thanks.

If a given (flexible pavt.) section of (ACP) rdwy. is cracked our goal would be to remove the cracked layer(s) by miiling, place OCST, and place new (layer(s) on top.

I think the nomenclature for our one course surface treatment (OCST) is the same as others chip seal.

A typical OCST aggr. for use would be 3/8" (20-40 %ret)

Rigid pavt.s w/ACP on top.
We have had some luck using a layer of crack attenuating ACP mix combined with a heavy (ac w/% rubber content) OCST, to eliminate reflectiive cracking. Have also used some fiberglass grid.

 

[Roadwork]

May want to look into fabric/poly interlayers. I was first introduced to it when I moved to Maryland three years ago, and although haven't used it myself yet, I've heard that it has produced very good results on airfields taxiways here. I have also seen it being used on County roads in Southern Maryland. Also, one report I read stated that they have been doing alot of testing on it out in California with excellent results (10-20 year life spans out of a single overlay).

 

[Drumchaser]

Roadwork

we have uaed the fiberglass grid with good results so far. Have heard others horror stories though. It is tough to place/phase.

 

[ACtrafficengr]

I haven't read this yet, but it sounded relevant to this discussion. Most NCHRP publications are pretty decent:

NCHRP Synthesis 388: Pre-Overlay Treatment of Existing Pavements

http://gulliver.trb.org/news/blurb_...r+-+04-21-2009&utm_content=Customer&utm_term=

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[Drumchaser]

Thanks AC
good stuff

After performing an Evelyn Wood on the NCHRP doc. it appears that the preferred based pretreatment is based on an extensive evaluation of the existing condition of the section in question.

One of our major problems is loss of fines and raveling. Due to our use of open graded mix designs an underseal (chip seal) is most effective in protecting sub layers.

While I would agree a milled surface would inherently hold and grip a new mat, I still maintain (minus a chip seal)a properly tacked sub layer provides a well bonded overlay.

 

[Ron]

Drumchaser...I agree that the condition of the existing surface is significant. Milling does not mitigate cracking as well as a crack relief layer such as a chip seal, but each has its place.