Highway Engineering

Within this text, an empirical overlay design method for flexible pavements
as detailed in LR833 is presented. Other methods such as component analy-
sis (Asphalt Institute, 1983) and analytically based processes (Claessen &
Ditmarsch, 1977) are also available but are not addressed here.
The information in LR833 is based on the results of full-scale highway experi-
ments where the surface has been overlaid with rolled asphalt at a time when,
some sections of the road were approaching failure.
The LR833 method is based on assigning a representative deflection to a
section of highway – termed the 85th percentile value (85% of the deflections
found are less than this value). This value is then used in association with the
family of curves as given in Fig. 9.4 to deliver a predicted life. If this is less than
required, a bituminous overlay can be considered. Any residual life derived will
be the period of time remaining before the application of an overlay or some
other form of remedial action will be necessary.
The overlay design procedure is contained within a number of charts, each
specifying the thickness of rolled asphalt overlay required to strengthen a pave-
ment of given deflection so that the extended life required by the designer can
be achieved. Charts for four different roadbases are contained in LR833:

Granular roadbases whose aggregates have a natural cementing action

Non-cementing granular roadbases

Cement-bound roadbases

Bituminous roadbases.

For a given roadbase material, different overlay thicknesses will be derived
depending on the probability assigned to the assessment of the remaining life
of an existing pavement and to the design of the extension of its life by over-
laying. Therefore, LR833 supplies design charts for two different levels of prob-
ability, 0.5 and 0.9, for each roadbase material.
An illustration of the design chart for bituminous roadbases with a 0.5 prob-
ability of achieving their desired life is given in Fig. 9.5.

Pavement Maintenance 261

Example 9.2 – Overlay design using LR833

Taking the pavement detailed in Example 9.1. If a further 15 years of service

is required from the pavement and the vehicle damage factor is expected to

be 1.05 over this time span, estimate the thickness of bituminous overlay

required to provide sufficient strengthening.

The future growth rate for traffic is set at 4%. Assume a 0.5 probability

level.

Solution

Assuming that the present flow is 950 vehicles per day, using Equation 9.2,

the estimation of cumulative traffic over the next 15 years is:

Tn= 365 F 0 /((1 +r)n-1)/r)

Contd