Confinement Effect on Stiffness

It was found that the stiffness of unbound materials could vary with the stiffness of the asphalt layers. This could happen both when the variation in stiffness was due to temperature variations and when it was due to fatigue damage to the asphalt. For the granular layers the change in stiffness was the opposite of what would be expected due to the non-linearity of the material. To describe this stiffness variation of the unbound layers the following function is defined to calculate the effect of confinement:

where S_n is the bending stiffness for layer n,

and Sref and SF are constants.

If all layers above layer n are still fully bonded, the bending stiffness for layer n is calculated from:

If full slip has developed between two or more layers their combined stiffness is found from:

For partial slip between layers a linear interpolation is done between full and no slip.

Unbound layer stiffness is affected by the confinement effect through the following equation:

where E_ini  is the initial layer stiffness,

          S_n,ini is the initial bending stiffness for layer n

This equation implies that the unbound layer stiffness is only affected by the change in the stiffnesses of the layers above but not their absolute values.