Grouted Macadam: Material Characterisation for Pavement Design

Chapter 2 Review of “Traditional” Road Pavement Types and Design
1990). The former mechanism is typically associated with excessive permanent
deformation in the subgrade and represents failure of the pavement, while the latter is
confined to the bituminous layers and can be corrected by the replacement of the top
layers only.
Figure 2.3 – Permanent deformation under the wheel track
(Adapted from Whiteoak, 1990)
In most thick flexible pavements constructed in the U.K. rutting is associated mainly
with permanent deformation within the bituminous layers. It usually occurs at high
service temperature and is a result of the cumulative load applications by heavy
vehicles. This can occur under moving or stationary traffic, and particularly under
high shearing stresses. The primary factor influencing plastic deformations is the mix
composition, namely, the voids content, the bitumen type and content and the
aggregate interlock but, for a given composition, the behaviour will be governed by
the viscosity of the bitumen (Whiteoak, 1990). According to Taherkhani and
Collop (2005), in the UK, traditional Hot Rolled Asphalt (HRA), although still used
for surfacings, is rapidly being replaced by Stone Mastic Asphalt (SMA), since this
new mixture appears to offer better rut resistance.
In dense graded mixtures the voids content may be an important factor to be
controlled. Sousa (1994) has found that when the voids content drops below 2 to 3%,
the binder acts as a lubricant between the aggregates and reduces point-to-point
contact pressures. Without the aggregate skeleton resisting the shear stresses, which
appear near the edge of the tyres, the mixture rapidly develops large permanent shear
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