| Figure 5. Horizontal Stresses Analysis with Varying Base Stiffness
Second, in the base layer of the typical flexible pavement structure (depths of 175 to 375
mm [7 to 15 in]) the stresses are small because of its low modulus. In the case of composite
pavements, higher tensile stresses develop at the bottom of rigid base layer. The magnitude of
these stresses increments as the stiffness of the base increases. Consequently, the tensile stress at
the bottom of the rigid layer criteria become critical and is the one used to predict fatigue life.
The horizontal strains output obtained from the mechanistic modeling (Figure 6) are
consistent with the results from the horizontal stresses. In this case, it can be observed that the
tensile strain at the bottom of the HMA, which is the most commonly used point of interest when
investigating flexural fatigue damage, is significantly larger in the granular base case than when
a rigid base was used. This suggests that the chance of having fatigue failure in the HMA when
using a granular base is much higher than that with any composite pavement structure.
Furthermore, the tensile strain at the bottom of the HMA only occurs for granular, soil cement,
and CTB bases; when lean mix, RCC, and PCC are used as bases, the strains become
compressive in nature. Thus the likelihood of fatigue cracking is greatly minimized. This
phenomenon was also noted in previous publications (NCHRP, 2004; Donald, 2003).
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