208 Highway Engineering
Case (a) Case (b) Case (c) Case (d)
Figure 7.11Derivation of cumulative design traffic valuations.7.5 Pavement deterioration,
7.5.1 Flexible pavements
Experience has indicated that, for heavily trafficked roads, deterioration in
the form of cracking/deformation is most likely to be found in the surface of
the pavement rather than deeper down within its structure. A well-constructed
pavement will have an extended life span on condition that distress, seen in the
form of surface cracks and ruts, is taken care of before it starts to affect the
structural integrity of the highway (HD 26/01) (DoT, 2001).
There are four basic phases of structural deterioration for a flexible pavement
(HD 26/01):Phase 1
When a new/strengthened pavement is reaching stability, at which point its load
spreading ability is still improving.
Phase 2
Load spreading ability is quite even and the rate of structural deterioration can
be calculated with some confidence.Phase 3
At this stage structural deterioration becomes less predictable and strength may
decrease gradually or even rapidly. This is the ‘investigatory’ phase. A pavement
entering this phase should be monitored in order to ascertain what if any reme-
dial action is required to be carried out on it. (Residual life is defined as the
period of time before a pavement reaches this phase.)
Phase 4
Here the pavement has deteriorated to failure. Strengthening can only be
achieved by total reconstruction. This phase can last quite a number of years,
with maintenance becoming necessary with increasing frequency until the point
is reached where the costs associated with this treatment make reconstruction
the cheaper option.