Page 27 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Initial phase. During the initial phase, some bedding-in occurs. The rate at which any particular type of
distress increases may be high. The absolute value of the distress is, however, still well below the limits normally
indicating a terminal condition.
• Primary phase. The rate at which the distress increases normally reduces fairly rapidly to an almost constant
rate during the primary phase. Reliable service is expected during the primary phase as long as the appropriate
routine, preventative maintenance is done. Premature failure may, however, occur due to poor construction, a
lack of maintenance, extreme overloading or unexpected deterioration of the materials used in the pavement.
• Accelerated distress phase. During this phase, the rate of increase in distress becomes unstable. A terminal
condition may be reached if the response time of the Pavement Management System (PMS) is too long, and
reactive maintenance and rehabilitation is not done in time.
• Secondary phase. If reactive maintenance or rehabilitation is done in time during the accelerated distress
phase, a secondary stable condition may be entered, extending the life of the facility beyond the initial structural
design life.

The common flexible pavement types used in South Africa are pavements with the following base layers:
• Unbound granular
• Lightly cemented
• Bitumen stabilized
• Hot mix asphalt

These base layers are supported by granular or lightly cemented subbase layers. In addition to crushed stone, high
quality natural gravel may be used in base layers on roads carrying light traffic. Figure 11 shows the general long-
term distress behaviour of these pavement types. Flexible pavements generally deteriorate gradually over time.
Pavement structures with lightly cemented bases initially show little distress, but deteriorate rapidly once distress
initiates. Remedial action on pavements with lightly cemented bases is therefore urgent, once signs of distress are
noticed.

In addition to the changes in distress, the elastic response of pavements with lightly cemented layers also changes
with time, as illustrated in Figure 12. The behaviour of lightly cemented layers is discussed in more detail in 7.1.3.

The change in stiffness of these material types over time is shown in Figure 13. Asphalt layers tend to lose stiffness
with time, as the layer experiences fatigue cracking and deteriorates with time. Cemented layers, and to a lesser
extent BSM layers, show a sharp increase in stiffness in the first year, during which the layer cures, resulting in
increased stiffness. Thereafter, the layer gradually decreases in stiffness over time. Granular layers generally do not
show much change in stiffness over time, as long as the layer does not get wet and the support conditions remain
the same.

Drainage
Equivalent Granular State Drainage is an extremely important
consideration for pavements! Water is
The equivalent granular state is when a lightly the primary cause of premature
cemented layer has cracked or weakened to the failure, accelerated distress and
extent that the effective stiffness is similar to that of
an unbound granular layer. The “cracked” state does reduced structural capacity.
not imply the material has reached the consistency of All aspects of drainage are
a granular material, or that it has necessarily visibly comprehensively covered in SANRAL’s
cracked into smaller, granular like pieces. The cracks Drainage Manual and not repeated in
are generally micro-cracks that are not that visible, SAPEM. Download the Drainage
but result in a loss of stiffness. Manual from www.nra.co.za.

Section 3: Design Considerations
Page 16