Failure Criteria Example

Results from a mechanistic pavement analysis are compared to empirically derived failure models to determine pavement life (in terms of loads to failure) and the controlling failure mechanism. In this example, various thicknesses of HMA were plotted against the computed number of repetitions to failure for 150 mm (6 in.), 250 mm (10 in.), and 350 mm (14 in.) thick granular bases (see Figures 1 – 3). The controlling criterion in the design is the one requiring a greater thickness of surface material for a given traffic level. Keep in mind that these charts were developed holding the material properties constant; they are not to be used in actual design.

The pavement designs resulting from three traffic levels (the arrows in the graphs) are given in Table 1. In this table, the resulting thickness of asphalt concrete was rounded to the nearest 12.5 mm (0.5 in.), and the assumed minimum thickness of the HMA layer was 50 mm (2 in.). Note that the rutting criterion controlled the design in all cases except those involving the thicker two pavement sections at 2,000,000 load repetitions. The choice of which of the three equivalent pavement designs to use at any of the traffic levels would depend on a life-cycle cost analysis. Finally, for the 2 million load repetitions, the granular base thickness has less impact on HMA thickness.


Figure 1: Design chart of example pavement (150 mm (6 in.) base).


Figure 2: Design chart of example pavement (250 mm (10 in.) base).


Figure 3: Design chart of example pavement (350 mm (14 in.) base).


Table 1: Comparison of Example Designs
9,000 lb (40 kN) Wheel Loads Base Thickness HMA Thickness Controlling Criterion
in. mm in. mm
2 x 104 6 150 4.0 100 Rutting
10 250 2.5 60 Rutting
14 350 2.0* 50 Rutting
2 x 105 6 150 6.0 150 Rutting
10 250 5.0 125 Rutting
14 350 3.5 90 Rutting
2 x 106 6 150 8.5 210 Rutting
10 250 8.0 200 Fatigue
14 350 7.5 190 Fatigue
*Assumed minimum thickness




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