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South African Pavement Engineering Manual
Chapter 10: Pavement Design

concrete inlays constructed by labour intensive methods have a low riding quality, this is rectified by the construction
of an asphalt overlay.

Where the noise levels on concrete pavements are an issue, particularly in urban areas, the surface can be treated to
reduce the noise. Re-texturing or asphalt overlays are an appropriate surface treatment in this situation.

Composite pavements have some special construction requirements, for example, the vertical longitudinal joint
between a concrete and asphalt pavement. Chapter 12: 3.12.7.1 discusses some special requirements.

Decision makers should note that creating a composite pavement by overlaying concrete with asphalt results in a
new pavement system, requiring a different set of deterioration models and intervention criteria in pavement
management systems. Traditional flexible and concrete models do not necessarily apply.

5.3.2.5 Rehabilitation Design Methods
The structural capacity of uniform sections requiring structural improvement may be increased by an overlay, partial
or full reconstruction. Many methods of designing these rehabilitation options are available, and are discussed in
Sections 7, 8 and 9.

The most appropriate methods should be selected for the design. The input information required for a structural
design should be available if the process outlined in this chapter and Chapter 6: 7 is followed, and a uniform section
report produced.

5.4 Economic Assessment

The selection of the final pavement design is based on the life cycle economic assessment of a number of alternative
designs. The purpose of the structural design method is therefore not the selection of the final design, but to
provide the designer with a number of design alternatives with the required structural capacity. The details of the
economic analyses for new and rehabilitation design are slightly different, refer to TRH4 and TRH12 for more
information. Although TRH4 and TRH12 refer to flexible pavements, the same economic principles apply for concrete
or concrete block pavements.

The basic principle of the economic assessment is the same for new and rehabilitation design. The cost comparison
of alternative pavement designs, for a specific design case, is based on the Present Worth of Cost (PWOC), Net
Present Value (NPV) and Internal Rate of Return (IRR) of the initial construction and anticipated maintenance and
rehabilitation costs. The cost comparison allows the selection of the final design on economic considerations, but
should not override all other considerations. Financial affordability should also be considered. The availability of
funds for the initial construction, and the availability of maintenance funds must be considered, as these could
influence the final design decision.

The PWOC of the alternative designs is calculated from Equation (16), and the NPV from Equation (17). The IRR is
obtained by setting the NPV in Equation (17) equal to zero, and solving for r, the real discount rate.

m
M i S
PWOC = C + �� � − (16)
n
(1 + r) i (1 + r) p
i=1
m
M i S
NPV = −C − �� � + (17)
n
(1 + r) i (1 + r) p
i−1
where PWOC = Present Worth of Cost
NPV = Net Present Value
C = Construction cost in terms of current cost
S = Salvage value in terms of current cost
th
M i = Cost of the i maintenance or rehabilitation action in terms of current cost
m = Total number of maintenance and rehabilitation actions
th
n i = Number of years from the present to the i maintenance or rehabilitation action
p = Analysis period
= Real discount rate
r

Items that are assumed to be equal for different design alternatives, such as the salvage value of the pavement at
the end of the analysis period and the road user cost, are usually omitted from the economic assessment. An

Section 5: Pavement Investigation and Design Process
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