Remedial work during the third ‘investigatory’ phase is more economic than
total reconstruction at the end of its full design life. If replacement of the sur-
facing or overlaying on top of it is expedited at the start of this phase, the time
to failure can be greatly extended.
7.5.2 Rigid pavements
Cracking in rigid concrete slabs can be promoted by stresses generated at the
edge/corner of slabs. These can vary from narrower hairline cracks which often
appear while concrete is drying out, to ‘wide’ cracks (>0.15 cm) which result in
the effective loss of aggregate interlock, allowing water to enter its structure and
cause further deterioration. ‘Medium’ cracking greater than 0.5 mm will result
in partial loss of aggregate interlock.
Failure is defined as having occurred in an unreinforced concrete pavement if
one of the following defects is present (HD 26/01)(DoT, 2001):
A medium or wide crack crossing the bay of the concrete slab longitudinally
or transversely
A medium longitudinal and medium transverse crack intersecting, both
exceeding 200 mm in length and starting from the edge of the pavement
A wide corner crack, more than 200 mm in radius, centred on the corner.
7.6 Materials within flexible pavements,
7.6.1 Bitumen
Bitumen is produced artificially from crude oil within the petroleum refining
process. It is a basic constituent of the upper layers in pavement construction.
It can resist both deformation and changes in temperature. Its binding effect
eliminates the loss of material from the surface of the pavement and prevents
water penetrating the structure. Two basic types of bituminous binder exist:
Tar – obtained from the production of coal gas or the manufacture of coke
Bitumen – obtained from the oil refining process.
With the decreased availability of tar, bitumen is the most commonly used
binding/water resisting material for highway pavements.
The oil refining process involves petroleum crude being distilled, with various
hydrocarbons being driven off. The first stage, carried out at atmospheric pres-
sure, involves the crude being heated to approximately 250°C. Petrol is the most
volatile of these and is driven off first, followed by materials such as kerosene
and gas oil. The remaining material is then heated at reduced pressure to collect
the diesel and lubricating oils contained within it. At the conclusion of this stage
of the process a residue remains which can be treated to produce bitumen of
Highway Pavement Materials and Design 209
