Seal Coating of Asphalt Streets 5

[SlideRuleEra]

I'm advising a local private community's management on a proposed project to seal coat not only the two small asphalt paved parking lots, but also the approximately 2 miles of paved asphalt streets. I used seal coat on a parking lot project many years ago and have a low opinion of its benefits, other than "looking nice", for a while anyway. I have not been able to find any meaningful recommendations on using seal coat on streets. Its use appears to be intended only for parking lots. Other informed opinions, either for or against using seal coat for this project will be appreciated.

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[ACtrafficengr]

If applied early enough, it may have some value as a preventive maintenance measure. If the road has more than hairline cracks, it's probably too late for a seal coat to do much good.

I found this in NCHRP 523:

Description: Fog seals are placed primarily to seal the pavement, inhibit raveling, enrich the hardened/oxidized asphalt, and provide some pavement edge-shoulder delineation

Fog seals are very light applications of a diluted asphalt emulsion placed directly on the pavement surface with no aggregate. Typical application rates range from 0.23 to 0.45 liters per m2 (0.05 to 0.10 gal per yd2).

Conditions Addressed:

Functional/Other
[ul]
[li]Longitudinal, transverse, and block cracking (M)[/li]
[li]Raveling/weathering (loose material must be[/li]
[li]removed)[/li]
[li]Asphalt aging, oxidation and hardening[/li]
[li]Moisture infiltration[/li]
[/ul]
Structural
Adds no structural benefit, but can help reduce moisture infiltration through fatigue cracks (if their severity is low)

Contraindications:
[ul]
[li]Structural failure (such as significant fatigue cracking)[/li]
[li]Flushing/bleeding (M)[/li]
[li]Friction loss (M-H)[/li]
[li]Thermal cracking (H)[/li]
[/ul]Expected Life 1 to 2 years when placed in a preventive maintenance mode.

Typical Costs $0.36 to $0.54 per m2 ($0.30 to $0.45 per yd2) of pavement surface area.

http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_523.pdf

 

[oldestguy]

My favorite subject for this expression:

"If you have money to spend on seal coating, place it in my pocket and we both will be better off". Seal coating will cause cracks.

I also once sent an e mail message to a Cathodic church council in a town where I lived. "Dear Council: I have a complaint. I see you tore out the whole parking lot and and replaced it recently. You have destroyed the best example that I have had available to developers and architects as to why you should never seal coat asphaltic pavements. Your frequent "seal coating" had caused the cracks to widen and also caused more cracks to develop so that it was very easy to convince the others how bad that is. Of course I do have other examples, so not all is lost"

My reasoning is this: The usual seal coat products are less elastic than the asphalt in the pavement. So, in cold weather the pavement tends to shrink and go into tension and stretches at the same time. The cover on top is more rigid and it does not stretch. The result: it tends to restrict the stitching of the pavement and causes cracks. At that Catholic church the cracks were as wide as an inch and were so numerous that it was a real hazard for persons walking.

If one must add some stuff on top, include some fine gravel to give a fraction wearing surface and a slight bit of more pavement thickness. Also if you want it black, to look new, buy the cheap0est stuff you can find, but plan on more cracks..

Take a look at typical pavement with parking stalls marked. You will see the cracks along-side the stripes, due to the less "stretchable" paint on the surface.

 

[oldestguy]

Pardon my misspelling, but you got the jist.

 

[Ron]

SRE....I'm not a fan of seal coats in general and absolutely abhor coal tar coatings on asphalt. They are a waste of money.

A slurry seal coat of asphalt has some protective value and can seal some cracks; however, the seal coat must be designed properly and applied properly.

All seal coats, even "microsurfacing" have limited effective lives, usually less than the hot mix they are protecting.

 

[oldestguy]

More effective in extending a pavement life is crack sealing with a rubberized product applied in routed cracks. In our area too many county highway depts use a typical penetration grade asphalt and these are not suitably stretchable. That use more or less appears to be a "make work" project in colder weather. In my experience, in Wisconsin, the asphaltic concrete pavements fail from the bottom up, progressing from the cracks, and covering the surface does nothing to extend life.

 

[SlideRuleEra]

Many thanks, guys. ACtrafficengr, I have downloaded the report from the link you provided and will be going through it. Your taking time to summarize its contents concerning seal coat will be a big help.

Ron and oldestguy, I value the experience that both of you have in addressing this question. In the presentation that I am preparing for the community's management may even quote oldestguy's statement: "Asphaltic concrete pavements fail from the bottom up. Covering the surface (with seal coat) does nothing to extend life."

I believe that is what is going on here. The pavement is not that old, but has continuing problems because of our constant high water table. Appears that a Contractor has come along with a "sales job" to convinced management that seal coat is a miracle cure all the problems with the pavement. I'll be trying my best to get them to cancel the previously awarded contract for seal coating (and to walk management through cancellation fee negotiations, if necessary). Sure to be an uphill struggle.
Thanks again.

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[Ron]

SRE...everything you post is worth a star!! Your historical information on your website is one of the most valuable resources of this website. Thank you.

I've attached a paper I wrote about 20 years ago on design of pavements in high groundwater areas. Hopefully it will help in some way. Convincing a homeowner's association is always difficult, particularly when salesmen sound better lying than we do telling the truth!

 

[ACtrafficengr]

A lot of people seem to use seal coats a more for aesthetics than function, I think. Make it look nice and black and new-looking and shiny.

Back in my technology transfer days with the Cornell Local Roads Program, I worked for a guy who had studied sealers. He sealed our parking lot with several different products, and watched their performance over time. Years and years after the black had worn off, the sealed surfaces were notably less porous than the control. The thing was you'd only notice when it was wet, and if you knew what to look for. The person that originally requested the study moved on, so he never published his results.

It never occurred to me to ask what happens with sealer buildup from habitual users, other than filling in the texture and reducing traction.

 

[bridgebuster]

oldestguy - I'll have to send you a picture of the parking lot at my Cathodic church, which has been seal coated a few times. It'll probably make up for the example you lost.

 

[oldestguy]

Not sure if this applies to the Slide Rule sites, but in Wisconsin and other northern sta3tes, the use of salt for ice control is another factor to consider. The freezing point of the salt water is lower than 32 degrees. This water, in the cracks is one of the prime factors in deterioration of the lower zone of an asphalt concrete mat. It is easy to get a new freeze thaw cycle in the pavement layers each day when temperatures are still somewhat below freezing. I attribute this as the reason the deterioration of the bonds is concentrated in the under sides where the salty water sits If one takes the time to dig into the deteriorating pavements, he likely will see the sections of "intact" pavement as dish shaped, thin at the cracks and better farther in. Top may appear in good shape, but not so underneath.

As to Ron's statements about high ground water, my observations show this is also important in freezing areas, easily noticed as contributing to frost heaving in winter. That thawed ground is the situation that controls the Wisconsin pavement design procedures, tied into the Ottawa Road test results as developed by AASHTO. I was lucky enough to be one of the two authors of that Wisconsin method back in the early 50's. Still in use by the way.

AC traffic: a question: Was Jim Spencer there then?

 

[ACtrafficengr]

oldestguy, he had moved on long before I started. He retired, I believe as university provost, while was there. That was 10 years ago.

Funny story: When he retired, he dropped off a lot of materials from his library, including some MUTCDs from the '50s and '60s. Shortly afterwards, I was asked about NY road signs from the period. I sent them a few pages, and received a nice thank you phone call from a woman with a lovely Quebecois accent, saying they were using it for props for the Hanks/DeCaprio movie "Catch Me if You Can."

 

[oldestguy]

AC Thanks. One of Jim's first jobs was coaching us CE's in summer survey camp 1947. A very practical guy who helped me with a Master's project later, dealing with underground drainage of highway subgrades and getting fieldwork cooperation of Thompkins county.

 

[Ron]

ACtrafficengr....call Lynne Irwin...he probably has everything that was ever studied in the Cornell Local Roads program.

 

[oldestguy]

AC and BigH:

In checking out Jim Spencer on Internet I see when he was an instructor a summer survey camp he had yet to graduate.

Here is a half hour interview of him taken a year ago.

http://ecommons.library.cornell.edu/bitstream/1813/30917/11/streaming_30917_2.html