Highway Engineering

 Dense bitumen macadam/hot rolled asphalt (DBM/HRA)


 Dense bitumen macadam with 50 penetration grade bitumen (DBM50)


 Heavy duty macadam (HDM)


 High modulus base with 35 penetration grade bitumen (HMB35).

The total thickness of a fully flexible pavement depends on the type of road-

base used. A DBM/HRA base is the least stiff and therefore requires the great-

est pavement thickness. As the stiffness progressively increases from DBM50 up

to HMB35, a reduced pavement thickness provides an equivalent level of struc-

tural strength and integrity.

Similarly for flexible composite materials, the thickness of the pavement

decreases as the strength of the CBM material used becomes greater. The design

thicknesses for the asphalt (bituminous) layers within a flexible composite

pavement are given in Fig. 8.8. The design thickness of the lower base varies

from 150 mm to 250 mm depending mainly on the type of cement bound

macadam used and the incident design traffic. A simplified diagram of the

required thicknesses for the lower base of a flexible composite pavement is given

in Fig. 8.8.

Structural Design of Pavement Thickness 239

Design traffic in left hand lane (msa)

1 10 100 1000

150

200

250

300

350

400

450

Design thickness of combined

asphalt layers (mm)

DBM50

DBM/HRA

HDM

HMB35

Figure 8.7Design thickness for flexible pavements – recipe-based specification for roadbase (HD

26/01) (DoT, 2001).