Abstract:
Moisture Damage is one of the prime reasons of early failures in flexible pavements. It weakens the pavement structure progressively by removing the bitumen film from aggregate surface causing loss of adhesion between aggregate surface and bitumen and cohesion inside the bitumen. This experimental study was executed to scrutinize the moisture susceptibility of three selected wearing course gradations. These gradations include National Highways Authority Class-B (NHA-B) which is commonly used in Pakistan, Superpave-2 (SP-2) and Asphalt Institute Manual Series-2 (MS-2). To check moisture sensitivity and durability of these gradations Retained Marshall Stability (RMS), Modified Lottman test and Hamburg Wheel Tracking Device (HWTD) test were performed. The materials used in this study include aggregate from Margalla quarry and two binders of same penetration grade 60/70 from refineries Attock Refinery Limited (ARL) and Pak Arab Refinery Limited (Parco) which produce bitumen through distillation and air blown technique respectively. Marshall Mix design of MS-2 was used to determine the optimum binder content of each mix combination which was then used to prepare Marshall and gyratory samples for the performance testing. The results of all the tests show that mixtures prepared with ARL 60/70 binder performed well than that of Parco 60/70. All the mixes passed the minimum criteria for RMS and Tensile Strength Ratio (TSR) but the aggregate gradation given in Asphalt Institute Manual Series-2 provided better resistance to moisture damage because of its dense structure and more particles passing from #200 sieve. Whereas in HWTD NHA-B performed well because of coarser gradation and its ability to sustain heavy load of the steel wheel of wheel tracker followed by SP-2 while MS-2 gradation failed this test. It is because of its finer gradation and inability to carry more load. SP-2 performed better than NHA-B in RMS and TSR but showed high rut depth as compared to NHA-B. The mixes with high values of unconditioned Indirect Tensile Strength (ITS) provided better resistance to permanent deformation. The correlation of RMS and TSR showed that mixes with high RMS exhibited high TSR values. Mixes prepared with finer gradation showed high TSR values and higher rutting.
