What is the importance of air void content in bituminous pavements?

The air void content of bituminous materials is an important control parameter for the
quality of bitumen being laid and compacted. If the air void content is too high, it allows
for intrusion of air and water. Moreover, it also increases the rate of hardening of binders
which produce premature embrittlement of pavements. In addition, too high a void content
will also lead to differential compaction subject to traffic loads and result in formation of
ruts and grooves along the wheel track.
However, a minimum amount of air void should be maintained to avoid instability during
compaction process and to provide space for bitumen flow in long-term consolidation
under traffic loads. A sufficient amount of air voids should be designed to make room for
expansion of binder in summer and compaction by road traffic as suggested by National
Association of Australian State Road Authorities (1968), otherwise bleeding and loss of
stability may occur and the pavement will deform readily under severe loads.