Permeability loss in porous asphalt due to binder creep

After several years in service, porous asphalt looses permeability primarily due to clogging. This paper presents a phenomenon, postulated as binder creep, as another source of permeability loss in porous asphalt. To ascertain the binder creep phenomenon, samples were separately conditioned at 15 °C, 20 °C, 30 °C and 35 °C. Permeability measurements were carried out at regular intervals up to 60 days using a falling head water permeameter. Permeability reduction was quantified in terms of increased time of flow and reduced coefficient of permeability over time. The results showed that, permeability loss was more significant on samples conditioned at high temperatures. Statistical analyses showed that temperature and time have significant effect on permeability reduction of the porous mixes tested. Over an extended period of time and due to gravitational forces, it was postulated that binder creep in the mix had taken place which steadily disrupted air voids continuity and subsequently caused the mix permeability to reduce. Permeability reduction took place more rapidly during the first week but later tend to asymptote. This finding implies the need to specify a time frame within which porous asphalt permeability measurements must be made. A recommended practice would be to conduct the permeability test not exceeding 1 day after the porous asphalt samples were compacted to eliminate inaccuracies in permeability measurements due to interference by binder creep.