Structural properties for pavement design

[eddiegr]

I have been tasked to develop a pavement section for a fire station in the islands. The fire station consists of a long driveway subject to a 70,000 lb fire apparatus and another area for staff parking lot. I am proposing two pavement sections to lower the costs. I am using AASHTO 1993 for the design. The structural number required is 2.30 based on converted ESAL's and the CBR of the subgrade. This is where I get stocked. I need to assign a structural number for the base and asphalt (there is not local pavement design manual available). I contacted local suppliers and they agreed to send me the local asphalt mix and gradation for the base course. Is there a formula or chart I can use to estimate the Sn for each layer based on asphalt properties and gradation?

 

[Ron]

You are looking for the layer coefficient for the material, not the structural number. The layer SN is the layer coefficient X the layer thickness. In general, for design, the layer coefficient used for dense graded asphalt mixes will be in the 0.42 to 0.44 range. For base materials, look at correlation tables in Yoder and Witczak. I think there are some correlation tables in the AASHTO Manual (1993) as well....I'm not in my office right now so working from memory.

For most base materials with reasonable strength (CBR 80 or so), the layer coefficient will be in the 0.15 to 0.20 range. In my area, crushed limerock has a layer coefficient of 0.18. For soil cement, a typical layer coefficient will be in the 0.15 range if the strength is upwards of 250 to 300 psi.

Don't forget to consider the influence of groundwater on the material. This occurs in base and subgrade layers....

SN=a₁D₁ + a₂D₂m₂ + a₃D₃m₃
Where a₁=asphalt layer coefficient, D₁=asphalt thickness.....and so on