Grouted Macadam: Material Characterisation for Pavement Design

Chapter 2 Review of “Traditional” Road Pavement Types and Design
SAMIs can be laid as continuous membranes like geotextiles or they can comprise
thin layers (≅ 10 mm) of a modified type of asphalt (Nunn and Potter, 1993).
However, this kind of solution is only effective in situations where no vertical
movement occurs between the edges of a crack.
Reinforcement of the asphalt overlay is generally made with geogrids (manufactured
with steel, glass fibres or polymers). The reinforcement can either be applied at the
interface between the cracked pavement (or base) and the new overlay or closer to
the surface, depending on the type of movement of the cracks (Nunn and Potter,
1993).
Increasing the thickness of the bituminous overlay is another method to delay the
reflection of the cracks at the surface of the new layer. In fact, the current practice to
minimise reflective cracking in UK is to lay thick, and therefore expensive,
bituminous surfacing making this type of construction less competitive (Ellis et al.,
1997). According to Thom (2003a), the inclusion of reinforcement in new overlays
generally reduces their required thickness by 25 to 70 mm, which represents a
considerable saving in the cost of the new material.
The use of polymer modified bituminous mixtures represents a way of reducing the
appearance of reflective cracks in the surface of composite pavements, due to the
increased flexibility of the material, associated with improved temperature
susceptibility. Softer bitumens can also be used although these may lead to
permanent deformation of the layer if not well chosen (Nunn and Potter, 1993).
A way of treating the existing concrete pavement or base is to deliberately introduce
cracks at a closer spacing than the naturally occurring thermal transverse cracking.
Thus, the magnitude of the thermal movements at each individual crack will be
reduced. Hence the tensile strains in the asphalt will also be reduced, minimising the
occurrence of reflective cracking. Any reflected cracks should be much finer and less
likely to lead to deterioration in the surface layer. Minimising the size of the surface
crack, by controlling its location and severity, gives a longer pavement life and
reduces future maintenance costs, both in terms of maintenance works and in the cost
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