Gradation design of porous asphalt mixture (PAM) for low-strength application in wet environment

Porous asphalt mixture (PAM) is potentially suitable for application in wet environment given its excellent performance in drainage due to the typical open-graded design, whilst ravelling and clogging are the two major issues directly related to the functional service life of PAM pavements. With the increasing up-build of non-motorised transport facility in Singapore which is a tropical country with frequent thunderstorms during monsoon periods, it deserves research into designing well-performing PAM for low-strength pavements such as pedestrian and cycling pathways. In this research, two areas are researched on designing appropriate PAM: firstly, relationship between gradation design and permeability performance is established and 7% content of fine aggregates is selected to achieve adequate coefficient of permeability for Singapore; secondly, four aggregate gradations with different packing structures created by the coarse aggregates were designed, and the performance was evaluated in the aspect of permeability, mechanical strength, resistance to ravelling and resistance to clogging. Three testing scenarios were applied in ravelling and clogging tests, which were unconditioned, ageing-conditioned and moisture-conditioned. It is found that moisture exposure is a more severe condition than ageing on PAM’s performance. On the whole, PAM with high content of intermediate size aggregates (i.e. 6.3–4.75 mm) in the coarse fraction and low content (i.e. lower than 7%) of fine fraction is suggested for low-strength PAM pavement in Singapore.