Asphalt Deformation Question 1

[MJB315]

Hi All,

What do we know about an asphalt roadway's ability to support short-term and long-term loads? I am looking for some sort of published data that shows:

*A metal plate (or other test setup) being supported on an asphalt roadway (asphalt + base, any composition).
*How much load / bearing stress was supported in that test. (40 psi? 120 psi? 1 psi?)
*How much the roadway deformed (short + long term, if available)
*How variations in temperature affected the load / deformation results.

We have a client that is exporing putting a piece of equipment temporarily on an asphalt roadway. We are trying to get a feel for how that roadway could behave immediately / over time.

The bearing stress we would place would be low (400psf sustained gravity, possibly an additional 1500-2000psf under heavy wind loads).

Note that we have compared these stresses to a typical HS-20 truck (bearing stress ~11.5ksf) and have that as a data point. I'm just lunging and trying to see if we can get some additional information.

MJB

 

[Ron]

You are looking for the rheological properties of the asphalt. There have been numerous studies on this.

One relationship is the Marshall Stability of the asphalt. This was a commonly used design method; prior to the advent of the SuperPave program by the US Gov't.

You can determine the properties you want from falling weight deflectometer testing as well.

Unless the temperatures are quite high, most structural mixes of asphalt are not greatly affected by long term loading. This depends, of course, on the mix design parameters as well as the load. Further, it depends on the age of the asphalt as well. It is common for asphalt, particularly finer mixes with higher asphalt cement content, to exhibit "tenderness" or excessive deformation under load right after placement, and sometimes for months afterward.

The loads you are proposing are relatively light for a competent pavement section and neglecting the potential tenderness of new asphalt, I would not expect an issue with excessive deformation.

 

[beej67]

Echo what Ron says, but the most important issue is that not all asphalt is the same. I was an asphalt mix design technician in a former life, and did some of the early test work on SuperPave vs Marshal. The answer to your question is going to vary widely between mix some gypsy paver laid down in a parking lot that's probably 80% RAP, and mix that's on a modern highway. Asphalt isn't like concrete, its properties vary widely based on aggregate gradation and AC content.

The easy answer, though, is that if your "piece of equipment" is bearing over a wide area (many feet square) you probably don't have much to worry about. If it's bearing on a narrow footprint (foot or less) you have much more to worry about. Pretend the asphalt is a bunch of marbles encased in molasses, and try to visualize what sort of loads would cause ruts and which wouldn't. That visualization is grossly exaggerated, but not too far off the basic concept.

If you want to skip the science and just be safe, get a big steel plate and put your "piece of equipment" on that, and you're probably going to be in good shape.

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