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South African Pavement Engineering Manual
Chapter 10: Pavement Design
Figure 31. Critical Parameter and Location for Asphalt Layers
Table 28. Transfer Function and Constants for Asphalt Layers
log ε t
α �1− β � (18)
N = 10
f
= Fatigue life
Where N f
α, β = Constants, values shown below
ε t = Horizontal tensile strain at bottom of asphalt layer
Thin Surfacing (< 50 mm)
Reliability Continuously Graded Gap-graded
Level
(Category) α β α β
95% (A) 17.40 3.40 15.79 3.705
90% (B) 17.46 3.41 15.85 3.719
80% (C) 17.54 3.42 16.93 3.736
50% (D) 17.71 3.46 16.09 3.774
Thick Asphalt Bases (> 75 mm)
Reliability Asphalt Stiffness (MPa)
Level 1000 2000 3000 5000 8000
(Category) α β α β α β α β α β
95% (A) 16.44 3.378 16.09 3.357 15.78 3.334 15.52 3.317 15.086 3.227
90% (B) 16.81 3.453 16.43 3.428 16.11 3.403 15.73 3.362 15.296 3.272
80% (C) 17.25 3.543 16.71 3.487 16.26 3.435 15.83 3.383 15.390 3.291
50% (D) 17.87 3.671 17.17 3.583 16.68 3.524 16.10 3.441 15.650 3.346
Shift Factor for Crack Propagation
If thickness of layer < 25 mm SF = 1 (19)
If thickness of layer ≥ 25 mm SF = 0.0489 • t – 0.2225
Where t = Layer thickness in mm
(ii) SAPDM
The revised SAMDM, known as SAPDM (South African Pavement Design Method), will include updated models for
asphalt layers. These models will include analysis methods for both fatigue and permanent deformation.
At the time of writing this chapter, field testing is being performed, to obtain data with which to calibrate the
potential models. Thereafter, the models most suited to South African conditions will be selected. For more on the
progress on this research, see www.sapdm.co.za.
7.1.2 Unbound Granular Base and Subbase Layer Permanent Deformation
(i) 1996 SAMDM
In the 1996 SAMDM, unbound granular layers are assumed to accumulate permanent deformation, from shear
deformation, in the layer. The resilient properties for unbound granular base and subbase layers are given in
Table 29. The suggested ranges are shown, along with the values used in the development of TRH4 in brackets.
Section 7: Structural Capacity Estimation: Flexible Pavements
Page 76
