EVALUATION OF RUT RESISTANT BITUMINOUS MIXTURES IN PAKISTAN

Abstract

Bituminous mixtures used in the construction of Asphalt Concrete (AC) pavements have to resist load fracture (fatigue cracking) and permanent deformation (rutting). In Pakistan, predominant failure mode of AC pavements is rutting caused by permanent shear deformation. Shear deformation rapidly develops in AC pavements in hot climatic regions with high bitumen content and low air voids (e.g. due to over compaction during construction or faulty mixture design), specifically in the presence of slow moving, overloaded trucks with high tire pressures. As the pavement is compacted under traffic, air voids drop to dangerously low levels and the binder starts acting as a lubricant between the aggregates and reduces the aggregate friction or point to point contact pressure. This causes hydrostatic condition within the mixture where the load is no longer being carried by the aggregate matrix, but rather the bitumen. This results in plastic flow of bituminous mixture with constancy of volume and is manifested as wheel path rutting accompanied by upheavals next to the wheel paths. Marshall Method is the laboratory test procedure used for designing bituminous mixtures in Pakistan. Marshall Method is applicable to the design of bituminous mixtures containing aggregates up to 1-inch maximum size only. However, in Pakistan the standard Marshall Method (ASTM D 1559) is used for designing AC base course bituminous mixtures that contain large stone aggregates up to 2-inch maximum size. Large stone mixes, necessitate use of a bigger size specimen and different set of criterion. However, in spite of the above, a few highway sections in Pakistan did not develop premature and/or excessive wheel path rutting, though all of the above mentioned ingredients for shear deformation were present. This research study is aimed at identifying common factors (if any) in bituminous mixtures that gave good rut performance in Pakistan designed using the standard Marshall Method applicable for designing mixtures containing coarse aggregates up to 1-inch maximum size, though these performing mixtures contained coarse aggregates up to 1.5 to 2-inch size, and were subjected to very heavy axle loads and high tire pressures, and hot climatic conditions.