Most actual pavement design methods are based on the same principles. Linear elastic calculations are used to determine the stresses and strains in the pavement layers, under a reference traffic load, and then the calculated stresses and strains are compared with maximum allowable values, depending on the nature and characteristics of the pavement materials. Most usual design criteria are the following:
• For bituminous layers: a fatigue criterion, often limiting the tensile strain at the bottom of the bituminous layer to prevent upward cracking in thin bituminous layers. For thicker asphalt layers, where the cracking may be from the top-downwards due to aging, traffic and thermal effects, fatigue relationships will need correlating to actual cracking performance if this calculation route is selected;
• For cement-treated layers: a fatigue criterion, limiting the maximum tensile stress at the bottom of the treated layer; and
• For subgrades; a criterion limiting the vertical elastic strain at the top of the layer, to avoid risks of rutting.
Thus, pavement design is based on elastic calculations and on the application of a limited number of design criteria (mostly fatigue criteria). Calculations are generally performed with constant material properties (corresponding to the initial characteristics of the materials after construction) and a design traffic, defined by an equivalent number, NE, of standard axle loads (ESALs). For thicker asphalt pavements and pavements containing cement-treated layers, unless there has been a specific calibration between the causes of pavement deterioration, such as ageing and deterioration of materials and climatic effects (temperature and moisture variations, frost), then these factors will need assessment before the safety of the road pavement and embankment can be secured. The effect, either way, is likely to increases the road cost by over-design of the structure.
Design against frost is generally based on the evaluation of the frost sensitivity of the subgrade (by frost heave tests, swelling tests, or on the basis of empirical classifications). When the subgrade is sensitive to frost, a thermal propagation model is often used to determine the thickness of protective material needed to reduce penetration of frost into the subgrade. However, swelling of frost-sensitive soils during freezing, or loss of bearing capacity during the thaw period are generally not taken into account, except in some countries (some examples are given in the Annex).
