Asphalt Pavement Distress Due To Bus Traffic

dmx · Feb 13, 2003

A project has landed in my lap as a result of some medical problems with the engineer that was doing the project. We are looking into pavement distress of asphalt pavement (rutting, cracking, shoving, etc.) at several high volume transit bus stop locations around a city in the northeast and need to make a recommendation on a solution to solve the problem. We have conducted several inspections of various bus stop locations to catalog the pavement distress and have researched a few options to correct the problem. These include complete reconstruction of the pavement section, addition of Superpave HMA or milling and placing of an Ultra-Thin Whitetopping on the existing asphalt. I have gotten extensive literature on both of these methods and understand the processes well. What I would like to know is if anyone has been faced with a similar problem, what solutions did you investigate, why was one selected and another discarded and what were the long term results of the implemented solutions. Other recommendations to correct the probelm would be helpfull as well.

Also, in general have you found that these types of pavement distresses are a result of an incorrect pavement design for the type and/or frequency of traffic that is using the road or is it more an issue of poor quality control during construction.

Thanks for your help.

jheidt2543 · Feb 13, 2003

This is one of the reasons concrete pavement is great! (no I don't work for the PCA). I had a similar problem, not on city streets, but in a parking lot that got lots of bus traffic. Everywhere the buses turned, they would break up the asphalt paving. The turns were made from a stop or very slow speed, it seems the combination of heavy load and turning wheels just ground the asphalt. I think the asphalt section was 4" thick on 10" of compacted stone base.

Our solution was to cut out the broken areas about 6'square and patch with a 6" thick section of concrete paving. This worked in this instance because the buses had to turn in virtually the same spot all the time.

Just a thought, it worked in this situation.

dicksewerrat · Feb 13, 2003

Call the Minnesota Road Research Board. They have been testing asphalt under all kinds of conditions. In Minnesota we have very hot, average cold and high traffic situations. you could also call the City of Minneapolis Aphalt Labs for some good information.

dmx · Feb 14, 2003

Sorry about the double post.

mike80 · Feb 17, 2003

I would agree with Jheidt2543. Here in Cincinnati, they are placing concrete pads everywhere the city buses stop. These are being installed anytime the pavement in the area is being replaced or overlaid. The pads are approximately the length of the bus, maybe a little longer.

Mike

samirram · Feb 17, 2003

No doubt that placing concrete pads is the best solution on parking areas of buses and trucks but is it so for traffic light signals stops,I think a well improved asphaltic layer should be placed there,our department is doing a special research on this specific problem and I will let you know when final results are reached.

ACtrafficengr · Feb 18, 2003

Have you checked to see if it is the asphalt that is failing, or the base and subbase? If the asphalt is relatively thin and the base is weak, the HV traffic may be pushing the asphalt down into the base. If that is the case, improve the drainage (always a good idea anyway), and check the base & subbase for excessive fines. You may mix a stabilizer into the base material such as portland cement, asphalt emulsion, foamed asphalt, or calcim chloride.

IRstuff · Feb 19, 2003

Orange County, CA also using concrete pads at bus stops.

Seems to be the prevailing solution... TTFN

Trussdoc · Feb 19, 2003

I'm not an asphalt or roadway engineer, but for my own curiosity: what effect does hot exhaust gas, oil/hydraulic leaks, and dripping fuel have on a surface on which buses park? It would seem concrete would be the ideal solution right off the bat.

Ron · Feb 21, 2003

You mentioned rutting, cracking and shoving....three distinctly different problems with three different causes.

Rutting is usually a base, subbase, or subgrade stability problem, but could be exacerbated by poor asphalt mix characteristics.

Cracking, depending on the pattern, is a result of overstressing or weathering. Overstress in the bottom of the asphalt shows as longitudinal, then transverse, then alligator cracking. Weathering cracks result from drying shrinkage cracks (usually a long-term block cracking pattern), freeze-thaw fracture, or wet-dry cycle fracture.

Shoving is a mix stability problem relative to the imposed loads, or can be caused by slippage of one or more asphalt layers slipping on top of the base because of poor bonding between asphalt and base (improper tack coat).

Having said all that....portland cement concrete works well and is very durable for this application. Make sure you transition the concrete-to-asphalt interface to prevent failure as the bus moves from one type of pavement structure to the other. For heavy loads such as airfields, truck terminals, bus terminals and the like, I use a taper on the concrete vertical face of about 15 degrees. This "pushes" the asphalt against the concrete every time it's loaded, rather than "shearing the joint".

KRSServices · Feb 24, 2003

I feel that I should add my two cents worth to the already valuable advice provided. Yes, over the years, i have found that the areas designated for high volume bus stops require more maintenance and repairs due to the symptoms you have described. In general, I have found the majority of problems areas were situated where the turning wheel movements occurred and to the sides closest to the curbs. In the majority of those cases the ACP was generally within spec but the base and subgrade were the culprits for failures. Quite often in those areas when tested for deflection, the results were poor. My deflection tests utilize an overloaded single axle. In repairing those sections particularly for major terminals, I have chosen to remove some of the subgrade and replace with 200 to 300 granular, 200 to 250 mm cement stabilized base then 100 mm ACP. Prior to placement of the ACP, the entire section was load tested (zero deflection). This structure, although heavy, has worked very well as a general specification. I have not replaced too many areas with concrete paving due to freezing issues so I cannot speak directly on that, other than what has already been noted. Good luck! KRS Services

http://www.krs-services.com

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