Pavement Design concept

Over the years a gradual transition took place moving from empirical pavement design methods to rational design concepts using mechanistic-empirical calibrated performance models.

The philosophy of the analytical approach to pavement design is that the structure should be treated in the same way as other civil engineering structures. This contrasts with the traditional method of designing pavements which is based on experience and the use of a test (the CBR) on the subgrade. It is because of the complexities of structural behaviour and material properties that empirical procedures have endured for so long in pavement engineering. However, with the knowledge now available from research, a procedure similar to that outlined above can be applied to asphalt and rigid pavements.

Mechanistic-Empirical pavement (re-)Design

The procedure of an analytical pavement approach may be summarised as follows:

1. Specify the loading.
2. Estimate the size of components.
3. Consider the materials available.
4. Carry out a structure analysis using theoretical principles.
5. Compare critical stresses, strains or deflections with allowable values.
6. Make adjustments to material or geometry until a satisfactory design is achieved.
7. Consider the economic feasibility of the result.

Conversely, for a pavement of known thickness constructed on a subgrade of identifiable characteristics, it is possible to determine the loads that the pavement can safely carry. This method of evaluating the load-bearing capacity is known as the ‘reverse-design method’ as is used for the evaluation of pavements.

Following the load input into the model, the stresses and strains are calculated at the design positions. For flexible pavements these are at the bottom of the bituminous layer (fatigue cracking), the top of the subgrade (rutting) and in a cement bound base at the bottom of this layer (reflective cracking).

For concrete pavements the edge-loading position is critical. Stresses and strains are calculated at the edge position using Westergaard incorporating temperature induced stresses and the measured load transfer. By means of fatigue relationships the (residual) allowable number of standard axles and thus the residual pavement lives are calculated. The assessment process also corrects the results for seasonal variations (e.g. flexible material/concrete temperature, subgrade variations etc).

Implementation of calibrated design criteria into Pavers ® allows the pavement designer to access the full advantages of the layered elastic design method, including treatment of wander, and quickly produce designs for complex load mixes and layered structures that are consistent with the original design concept.