Highway Engineering

Pavement Maintenance 259

Example 9.1 Contd

(1) Estimate the life in millions of standard axles that the pavement has a
0.5 probability of achieving
(2) Estimate the life in millions of standard axles that the pavement has a
0.9 probability of achieving.

Solution

If the daily flow of commercial vehicles after 10 years is 950 vehicles, then,
assuming a growth rate of 3%, the initial flow on the opening day is esti-
mated using the following formula:

F 0 =Fn/(1 +r)n

where
F 0 =initial flow on opening day
Fn=flow after nyears
r =traffic growth rate

Substituting the above values into Equation 9.1, the following initial flow is
obtained:

F 0 =950/(1.03)^10

=706.89 commercial vehicles

Addis and Robinson (1983) have shown that the total number of commer-
cial vehicles Tn, using the slow lane over a given time span ncan be expressed
in terms of the initial daily flow F 0 , the growth rate ras follows:

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

Therefore, since:

F 0 =706.89

r =0.03

n = 10

Tn=(365 ¥706.89)/((1.03)^10 - 1)/0.03)

=2.96 million commercial vehicles

This figure multiplied by the historic vehicle damage factor gives the total
number of standard axles incident on the pavement since its opening:

Cumulative standard axles =2.96 ¥0.9 =2.66 million standard axles (msa)

From Fig. 9.4, it can be seen that a pavement with a bituminous roadbase
and a standard deflection of 50 ¥ 10 -^2 mm has a 0.5 probability of achieving

Contd