Page 86 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

The correct characterisation of the input required, and the availability of unbiased, properly formulated and calibrated
transfer functions for different pavement materials are vitally important to the successful application of the method.
The models are developed in a research environment, but the resilient and strength properties of the available
materials must be characterised from available data as best as possible for each design project. Detail on material
characterisation for the SAMDM 1996 can be found in Theyse et al (1995 and 1996.) The 1996 material resilient
response characteristics (Young’s modulus and Poisson’s ratio) and damage models are currently generally used by
practitioners and are provided in the following sections. It is important to always clearly state how the inputs were
obtained, and provide justification for their use.

7.1.1 Hot Mix Asphalt Fatigue

(i) 1996 SAMDM
In the SAMDM, asphalt surfacing layers are only analysed for fatigue. It is assumed the cracks start at the bottom of
the layer and propagate up to the surface. The structural capacity, or fatigue life, determined represents surface
cracking over a defined area of the road. This area depends on the reliability assigned to the road category, for
example, 95% reliability for Category A implies 5% of the road area is cracked.

Rutting, or permanent deformation in an asphalt layer has typically been considered a function of the mix properties,
and has therefore not been considered in the structural analysis.

The resilient moduli for thin continuously and gap-graded asphalt surfacing layers of are shown in Table 27. These
layers are generally less than 50 mm in thickness.

Table 27. Elastic Moduli for Asphalt Materials
used in SAMDM 1996
Code Depth (d) Below Modulus
Surface (mm) (MPa)
1
AG ≤ 50 3000
2
BC ≤ 100 4000
100 < d ≤ 150 5000
150 < d ≤ 200 6000
200 < d ≤ 250 7000
Notes
1. Gap graded asphalt surfacing, as defined in TRH14 (1985)
2. Continuously graded hot mix asphalt, as defined in TRH14 (1985)

The Poisson’s Ratio is generally assumed between 0.4 and 0.44, with 0.44 the recommended value.

Asphalt layers are modelled as a bound layer, which
bends under the load application. This induces
cracks at the bottom of the layer, which propagate
up to the surface. The damage function uses the HMA Fatigue Transfer Functions
horizontal tensile strain at the bottom of the layer, It is generally understood that the 1996 SAMDM fatigue
which represents the resistance to the crack transfer functions for asphalt are not that reliable.
formation. This concept is illustrated in Figure 31,
where ε t represents the tensile strain. The transfer In South Africa, we generally use asphalt layers that are
functions for both thin (< 50 mm) surfacing layers less than 50 mm thick, and failure of the asphalt layer is
and thick (> 75 mm) asphalt bases are shown in not necessarily a terminal condition for the pavement.
Equation (18) in Table 28, with the constants The pavement can continue to carry traffic with the
applicable to the required reliability level or Road application of crack sealants to cracks, a seal to
Category. For thick asphalt bases, a shift factor to waterproof the layer, or patches to correct particularly
account for the propagation of cracks from the weak areas. For these reasons, in an analysis of the full
bottom of the layer to the surface is also used, and pavement system, the structural capacity of the asphalt
is given in Equation (19) in Table 28. layer is usually not considered in the critical layer
determination.

Both Shell (Huang, 1993) and the Asphalt Institute
(Austroads, 1992) have transfer functions for fatigue of
asphalt. It is appropriate to use these transfer functions
as an additional check for a design.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 75