Mechanistic-Empirical Analysis

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Figure 8.  HMA and Rigid Base Repetitions to Fatigue Failure 

 

The number of load repetitions to HMA fatigue failure is much greater in a pavement 

with a cement-bound base (e.g., soil cement) than in pavement with a granular base.  Table 9 

shows an infinite number of load repetitions for the HMA on CTB, lean mix, RCC, and PCC 

base courses; this is because when any of these bases are used, the strain at the bottom of the 

HMA becomes very small (CTB case) or compressive in nature (lean mix, RCC, and PCC cases) 

and the flexible layer is highly unlikely to fail due to fatigue cracking.  

 

It can be observed that for composite pavements where the rigid base is a soil cement, 

CTB, or lean mix, the base is the layer that controls the design in terms of fatigue, as it would 

fail earlier than the HMA layer.  In the case of RCC and PCC fatigue evaluation, the repetitions 

were determined to be infinite because the stress ratio (SR) term after a load was applied for 

RCC and PCC were 0.17 and 0.16, respectively.  The fatigue behavior of RCC was assumed to 

be similar to that of conventional PCC as recommended by the American Concrete Institute 

(ACI) (Delatte, 2004).   

 

Permanent Deformation (Rutting) Prediction 

 

The modeling of rutting in the HMA layer uses the relationship from the proposed 

MEPDG (NCHRP, 2004), as shown in Equation 20, to obtain the accumulated plastic strain.  

This strain results from the sum of various plastic strain deformations inside the asphalt layer, 

which can be used to determine the rut depth after a specific number of load repetitions.  To 

compute the rut depth, the HMA layer is divided into sub-layers according to the criterion 

described in the MEDPG, and plastic strains are computed at various points located at different 

depths from the surface.   

 

Figure 9 shows the results obtained for the rutting in the HMA layer in terms of rut depth 

versus the type of base used.  The results suggest that as the stiffness of the base increases, the 

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rut depth in the HMA layer increases as well.  This was an expected outcome because the high 

rigidity of the base does not allow any significant vertical deformation to occur, thus the HMA 

layer absorbs all the vertical strains and deforms itself as illustrated (exaggerated for illustration 

purposes) in Figure 10.  The 12.5 mm (0.5 in) rut depth shown in Figure 9 represents the 

allowable value used by the Asphalt Institute and Huang (2004).   

 

The HMA rutting results show that for 50,000,000 18-kip load repetitions, the typical 

flexible pavement constructed with a granular base was the only structure that met the 12.5 mm 

(0.5 in) rut depth criterion.  All of the composite pavement structures presented greater (up to 21 

mm [0.83in]) degrees of permanent deformation due to the high number of load repetitions.  It is 

noted, however, that the computed rut depth for all the structures (both flexible and composite) 

assumed no rehabilitation operations at any time during the 50,000,000 load applications.  

Therefore, if a functional rehabilitation is applied at any time during the service life of the 

pavement, part of the permanently deformed HMA would be replaced.  In addition, the use of 

premium mixes, such as SMA, may also help reduce the rutting progression.  Finally, it is also 

important to note that the model has not been validated and calibrated to the local conditions. 

 

 

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