AC Parking Lot - Grind/Deep Lift/Overlay 1

[roscoe]

I am supervising a job which has as part of the scope of work the re-surfacing of an old AC parking lot. The existing parking lot is approximately 4" of AC over about 6" of base rock. The subgrade soil profile is thick, firm adobe clay. There is a lot of aligator cracking in the existing pavement, which has been further exacerbated due to large number of heavy trucks going over it during adjacent construction activities we are doing. The specs call for a 2" grind throughout the parking lot, then a 2" AC overlay with 3/8" AC. We are doing deep lift sections on some areas of the parking lot, but there is not enough money in the budget to do it on all of the bad areas. We are also using fabric under the overlay.

I was wondering if anyone can comment on past experiences on the performance of overlays in general and more specifically over areas where the underlying existing AC has extensive cracking already. Also, any tips you have to assist me during inspection of the grind and overlay would be appreciated. Thanks!

 

[Ron]

Overlays over extensive cracking will not perform well! Have you considered a mix-in-place recycling process? This can be done by pulverizing and mixing the existing AC surface and base together while injecting an asphalt emulsion to create a new base material, which can then receive a new surface of AC. In this process, you can likely thicken the base and reduce the thickness of the AC layer, thus giving a more cost effective result and a better design response as it is more appropriate to design, construct, and remediate pavements with a "bottom-up" approach, rather than a "top-down" approach.

 

[ishvaaag]

2 in of new uncracked thing seems won't be enought where 4 inches before were not.

 

[roscoe]

Gents, thank you for your comments. I talked to the Contractor, his concern regarding the mix-in-place recycle method is that the existing AC in the severely cracked areas will not pulverize into small enough aggregate and it will take a lot of work to remove the large pieces of AC from the recycled/mixed base.

As an alternative solution, he has suggested eliminatiing the 2" grind and doing a levelling course along with a geotech fabric/grid over the existing AC. Then we would do a 2" or 3" AC overlay over that.

 

[jheidt2543]

I think you should reconsider Ron's comments above. He's right, the existing cracks will telegraph through in a short period of time. We all know that budgets drive a lot of projects but, why spend money on a solution that is bound to fail?

In my experiance 4" of asphalt over 6" of base is an unusual design. I'm more use to seeing 10"-12" of base and 3"-4" of asphalt, especially if there is truck traffic. A lot has to do with the subgrade but, the compacted base is cheaper than the asphalt and able to support more load. I think you would be better off stipping the existing asphalt, recrushing it for base, add some new base and end up with the 10"-12" thickness. Then put the asphalt down. The life cycle cost of this would be a better investment.

That's my two cents.

 

[Ron]

roscoe...your contractor is wrong! The material can be pulverized to as small a size as you want. Most of the time we have to be careful about getting the material too fine.... not having large chunks! Your application is a good one for mix-in-place and will stop the reflective cracking issue.

 

[roscoe]

Due to cost constraints, we have decided to proceed as follows:

1) Eliminate the 2" grind and 2" overlay

2) With the cost savings from elimination of the grind and overlay, we are going to be able to deep lift almost all of the badly cracked areas.

3) Then we will do a seal coat over the entire area.

I think this is the best solution given my cost constraints. Thanks again for your input!

 

[ACtrafficengr]

I suspect you may be dealing with a dirty base course. that can often lead to alligater cracks, especially if you have freeze/thaw conditions.

I suggest doing gradation and sand equivalent tests on your existing base course. If it has to many fines, cement stabilization or emulsion stabilization may help. Then again, grinding up the asphalt and mixing it in may help, if you make sure not to grind it into fines.

 

[BobSchrei]

Roscoe. I know you have made a decision but I wanted to add to the thread. Ron made some good points about in-place recycling. I can add some more. The 10-inches of pavement could be simply pulverized in place as Ron mentioned. A parking lot may not need an emulsion additive. Dry pulverization with moisture conditioning for comapaction would result in a deep well compacted base course with some AC from the old topping that we have found to be beneficial in the base's performance. This method would eliminate the cracks. Large chunks would be a potential in badly alligatored pavement as your contractor said but most of them tend to settle in the bottom of the mat which is not a concern(it simulates a large aggregate anyway that generally does not compromise the base.). Large pieces at the surface could simply be removed by a laborer as they appear or pounded in with a padfoot roller(padfoots perform well in deeper mats >6&quot. The topping could remain the 2" of HMA thought there may be a materials balance issue if you must match grades. Please note that the dry pulverization method would be more expensive than the 2" mill out but not by much. Remember that you are not hauling anything away in trucks. In fact, I can dry pulverize, shape and compact for anywhere form $1.00/sy to $2.50/sy depending on the scope of work. Your job, if wide open and with decent quantity(5,000+ sy), would be in the $1.50 range(ne Ohio union scale). Check out

http://www.arra.org

for the Asphalt Recycling and Reclaiming Association website.

 

[roscoe]

Thanks for your comments and the link to ARRA. We are already set to do deep lifting this Friday, and seal coat next week. I look forward to trying the recycle-in-place method on a future project. best regards.