Structural solutions that are applied in constructing road pavement structures exposed to repeated traffic loads have typically been developed over a long time span. Empiricism has also played a big role when structural solutions have by time been adjusted to the local ambient conditions and available types of construction materials. Long-term feedback obtained from the actual performance of pavement structures has resulted in operative solutions even though the applied design approaches may have been somewhat vague from a theoretical point of view.
When various types of alternative earth construction materials have increasingly been taken into use in constructing road and field structures, the status quo described above has changed. The mechanical properties, especially the stiffness and strength of these alternative materials, may be quite different from those of the traditional construction materials such as sand, gravel, and crushed rock aggregates. Replacing one of the structural layers in a road or field structure with a material having fundamentally different mechanical properties may therefore change the overall performance of the whole structure. This means, of course, that the empirically calibrated design approaches are not valid as such anymore.
From the mechanistic pavement analysis, it is well known that under a wheel load acting on the road surface a large stiffness difference in between two structural layers on top of each other results in the development of tensile stresses at the bottom of the stiffer layer. In the case of an asphalt concrete layer resting on top of an unbound base course layer, this is one of the fundamental distress mechanisms against which the mechanistic design of a road pavement is normally made. Therefore, it is fairly evident that if we replace an unbound road pavement layer either with a very stiff material (e.g., a self-cementing or cement-stabilized layer of fly ash) or a material with very low stiffness (e.g., a layer of rubber shreds), tensile stresses tend to develop in places different from the traditional type of road structure. Correspondingly, the critical distress mechanisms that are decisive regarding the service life of the structure will change as well.
The paper provides some examples of analyzing the mechanical behavior of pavement structures in which traditional construction materials have partly been replaced by alternative construction materials in the way they have been suggested to be utilized in road infrastructure applications. The analysis reveals that, especially on soft subgrade soil areas, marked tensile stresses may develop into the alternative construction material layers that are much stiffer than the underlying ones. An evident conclusion from the analysis is also that there is an urgent need to develop a mechanistic design approach applicable for these non-traditional types of pavement structures. It is needed to ensure the so-called structural compatibility of the alternative construction materials, a novel concept introduced in the current paper.
