Asphalt Concrete Pavement Design

[theCorkster]

I'm working on a project in California and have designed the AC pavement sections using the Caltrans method. I've also calculated the structure sections using the AASHTO methodology. In order to design the AASHTO sections, I've used the empirical relationship recommended by AASHTO to convert resistance value to resilient modulus. Overall, the AASHTO method produces comparable to slight thicker AC sections, but yields significantly thinner baserock sections.

The AASTHO method pavement section thickness is slightly affected by using a low value of the drainage coefficient, m. However, I've also read that it's not really appropriate to use a low value for m (say 0.4) for poor drainage conditions, as compared to using a value of 1.0 for average drainage conditions.

I know that the Caltrans resistance value used in the design represents a saturated condition, and therefore the worst-case condition. However, the AASHTO baserock section varies as much as 75 percent less than the Caltrans section.

Any similar experiences between the two design methods?

Does anyone know if AASHTO recommend a minimum baserock section regardless of calculation, and if so, what is that thickness?

Also, does AASHTO recommend using a geosynthetic separation fabric between the base and soil subgrade?

Thanks

 

[fel3]

It's been about 12 years since I did an AASHTO pavement design calculation and I don't remember all of the details. The calculation was for a federal prison in California, where I'm based. The Federal Bureau of Prisons required AASHTO for this project, but I also did a Caltrans calculation for comparison. My notes are stored away and I can't get to them easily, but as I recall my results were similar to yours in that Caltrans had thicker baserock and thinner AC than AASHTO. Since its easier to add AC than baserock to as existing pavement section (e.g. for future maintenance), I used the Caltrans design and justified it to the FBOP.

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