Raised dimples in AC paving 4

[rlward]

I have a very large area paved with 2" AC w/RAP over 4" recycled base. The area was slurry sealed shortly after the AC was placed. It has developed many (well over 100), 6" diameter raised dimples in the AC Paving with hieghts of approximately 1/2". The dimples are random throughout the area. Cores reveal a rock in the center of the dimple directly under the AC. Tests indicate the AC and baserock material is within specifications, and the compaction is ok.
The sub base is compacted native soil made up of sandy soils and the water table is down approximately 75 feet.
The project is located in the
Central San Joaquin Valley of California and has been in use for about 12 months.
The dimples were first noticed in July, 2010.
I am guessing some kind of action is pushing the rock up, causing upheaval, but cannot identify the cause.
Has anybody experienced this?

 

[Ron]

Any chance of a reactive, expansive aggregate?

 

[oldestguy]

Reminds me of blisters under vinyl sheet flooring laid on concrete that had some it internal "SALTS" move toward the surface durng drying, before the flooring sheet was placed.. Later those higher concentrations of salts exhibited an attraction (osmosis effect) for moisture and significant pressure developed. Any chance those rocks were high in some salts? In our case the pores of the concrete were similar to an osmotic membrane pore sizes.

This can be demonstrated by placing a salty solution on one side of a barrier made of concrete, and having clean water on the other side. The clean water moves through the concrete to the salty side with significant pressure developed in the salt solution.

Could this apply to the question? Don't know. You don't need free water for this, since moisture in the air will do it..

 

[Drumchaser]

What is the 4" recycled base?

Why was the new ACP slurry sealed so quickly?

I have seen the blister effect, but not to this magnatude.

 

[MiketheEngineer]

That is a strange one - do you have Olympic size moles??

 

[rlward]

Drumchaser,
Recycled base mostly concrete, but there is some Lt Wt aggregate also.

City of Fresno is more like, I don't care what your recomendations are, the specs say to seal it.

Do you know what the root cause of the blisters were on the project you have seen?

 

[Drumchaser]

I'm not sure if "oldestguy" is correct, but it sure sounds good/plauseable.

Our blisters were on top of thin ACP layer (C.A.M.- high % AC content mix design). This upper layer had been placed on an old section of conc pavt. with a few inches of existing ACP on top of it. All were dismayed at the phenomenon. It did not last long. We placed an additional surface ACP course (as planned) and to my knowledge have not had issues with this roadway to date.

City of Fresno must be worried about skid. It is my understanding slurry seals are great for skid, but lousy on permeability.

 

[rlward]

Oldestguy,
First, you cannot be the oldest guy, this year I was told I was older than god. Last year, just older than dirt.

Anyway, back to the subject, we got back some test results today of the rock under the dimples. Soluable Sulfate was as high as 706 and Soluable Chloride was 64. A Little higher than expected. Seems this may be the culprit. Thanks

 

[Hoaokapohaku]

By word of mouth, I recently heard about an investigation by the Civil Engineering Dept. at the University of Hawaii into a similar situation. In that case, a base of recycled material containing both crushed concrete and RAP supported the new pavement. Initially, it was thought that some foreign material in the crushed concrete was causing a reaction and "dimpling." After no culprit was found in the recycled concrete, the RAP was investigated. Eventually, they found that aluminum present in the RAP was reacting with the crushed concrete, producing a gas that in turn caused eruptions of the gas through the new pavement. I don't remember exactly what in the crushed concrete was reacting with the aluminum. The source of the aluminum in the RAP was aluminum temporary striping tape present in the RAP. Apparently, new asphalt surface had been placed over previously placed pavement that had been striped with aluminum temporary marking tape. Later, the pavement was milled and the aluminum tape was shreded and dispersed in the RAP.

In your situation, if the RAP in the mix contained aluminum, and the aluminum in the mix were to be in contact with the recycled concrete base, a similar reaction might occur.

 

[Hoaokapohaku]

Still don't remember what in the concrete was reacting with the aluminum, but my foggy memory just informed me that the gas produced that caused the eruptions was hydrogen.

 

[Drumchaser]

Hoaok.....

Yep.

In my region, aluminum tape is not allowed to be used in work contracts, however maintenance forces routinely utilize it on spot patches and rarely remove it after permenant markings are in place.

 

[Hoaokapohaku]

Drum...Aluminum tape is allowed here, but only for temporary striping. It must be removed when the permanent striping is placed or before a subsequent lift is placed. That's been the spec for a long time, but 15 to 20 years ago it wasn't enforced so there is a lot of that aluminum tape paved over in old paving projects.