Grouted Macadam: Material Characterisation for Pavement Design

Chapter 2 Review of “Traditional” Road Pavement Types and Design
permanent deformation (by the development of rutting at the surface of the
pavement) and the cracking of the bituminous layers. To avoid the appearance of
such distresses, the design criteria used in the analytical methods are the maximum
tensile strain in the underside of the asphalt (εt or εr) and the maximum compressive
subgrade strain (εz), as was illustrated in Figure 2.14.
The development of a rut is the result of the accumulation of permanent deformation
throughout the pavement structure. Within the asphalt layers this can be minimised
by an adequate mixture design and by a good compaction of all layers. According to
Brown and Brunton (1985) if the vertical strain in the subgrade is kept below a
certain level, experience has shown that excessive rutting will not occur, unless poor
mix design or inadequate compaction are involved.
Cracking of the asphalt layer arises from repeated tensile strain, the maximum of
which occurs at the bottom of the layer, as shown in Figure 2.14. The crack, once
initiated, propagates upwards causing gradual weakening of the structure (Brown and
Brunton, 1985). More recently, a different concept has been studied and discussed
among researchers, where a crack may initiate at the surface and propagate
downwards, especially in pavements with thick bound layers (Freitas et al., 2003).
The design procedure would consist of proportioning the pavement structure so that,
for the chosen materials, it would not present values of strain higher than the critical
levels stipulated for the design life. The analytical methods have been used with
computer programs that determine the stresses and strains in the various layers,
according to the mechanical properties of the materials used, i.e. stiffness modulus
and Poisson’s ratio. Different response models have been used, e.g., semi-infinite
half-space † and layered analytical models * , based on linear elastic theory and
isotropic layers (AMADEUS, 2000). For new materials, those properties may be
determined by laboratory tests, adjusted in some cases for loading rate, temperature,
confinement and/or age.
† Usually associated with Boussinesq’s equations, using the method of equivalent thicknesses to
transform the layered system into a semi-infinite linear isotropic half-space.
* Generally based on Burmister’s work, using a multi-layered, linear elastic pavement in which the
layers are treated as being horizontally infinite and resting on a semi-infinite subgrade.
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