24
The mechanistic model output shows an interesting vertical strain distribution especially
in the HMA layer (0 to 175 mm [0 to 7 in]). In the pavement system with a granular base,
vertical strains at the top region (0 to 12.5 mm [0 to 0.5 in]) are tensile in nature. This is
probably due to the boundary conditions imposed by the modeling software. When a lower
Poisson’s ratio value was used for the HMA (e.g., 0.30), the vertical strains at the top region of
the HMA showed compressive responses instead of tensile. The remainder of the strain
distribution (granular case) suggests that the rest of the HMA is in compression with the lower
region (100 to 175 mm [4 to 7 in]) presenting a greater magnitude of compressive responses. In
the case of composite pavements, the highest compressive stresses develop in the middle of the
layer. This suggests that higher vertical deformations presented in the HMA are prone to occur
in this region (50 to 100 mm [2 to 4 in]).
As the stiffness of the base increases, the compressive strains in the unbound layers
(subbase and subgrade) noticeably decrease. The significant reduction
of vertical strains at top
of the subgrade—at a depth just below 600 mm (24 in)—suggests that rutting due to permanent
deformation of the subgrade is greatly minimized or even unlikely to occur.
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