Highway Engineering

9.5 Overlay design for bituminous roads,

The deflection incident on a flexible pavement can be lessened and its structural

strength enhanced by adding a bituminous overlay. The reduction in deflection

is influenced by three factors:


 The thickness of the overlay


 The deflection of the existing pavement


 The elastic properties of the overlay material.

The first two factors in particular have been found to be the major determinants

of overlay thickness. Subgrade strength and the original thickness of the pave-

ment were not found to be major influential factors.

260 Highway Engineering

Example 9.1 Contd

a life of approximately 3.8 msa. If the probability is increased to 0.9, the life

reduces to just under 3.0 msa.

Therefore, the pavement is approximately 70% towards the design life it

has 0.5 probability of attaining and approximately 90% towards the design

life it has 0.9 probability of achieving.

0.1 1 10 40

0

20

40

60

80

100

120

Cumulative standard axles (msa)

Standard deflection

(mm ¥ 10 -2)

140

Probability of

achieving

life

0.25

0.50

0.75

0.90

Figure 9.4Relationship between standard deflection and life for pavements with a bituminous
roadbase (source: LR833). (Crown copyright 1978)