Abstract:
Flexible pavements are typically designed to provide a good ride quality and to resist
rutting and fatigue cracking. The two types of distresses are mainly caused by wheel loads
and are accelerated by environmental factors. Although all the pavement layers (base, subbase
and road bed soil) contribute towards rutting and fatigue cracking, the contribution of the
asphaltic concrete (AC) layer alone could be very significant.
The pavement network in Pakistan is experiencing premature rutting problem. The
objective of this study is to identify the causes of failure of N – 5 (Nowshera – Peshawar).
Based on the objectives and severity of rutting, three sections of road were selected
comprising one good and two bad sections. Rutting of each section was measured. Two sites
were selected for investigation of base and subgrade. The cores and samples of base and
subgrade were subjected to various field and laboratory tests and material properties of each
pavement layer were determined.
Analysis was carried out to find the most significant material/ mix properties affecting
the rutting. Test results and analysis revealed that apart from excessive load and environmental
conditions, non standard and non uniform construction practices mainly of the top layer have
resulted in pavement failure. Subgrade, though weak, is adequately protected by a very strong
base course. Asphalt mix design followed was found to contain excessive sand and low coarse
aggregate percentages. Vast variation in asphalt content and material properties indicate, poor
quality control and bad construction practices. Good natural drainage and good quality base
course have ensured structural adequacy of the pavement despite top layer failure.
