Disruption of air voids continuity based on permeability loss due to mortar creep

Abstract

Mortar creep takes place when the asphalt mortar continuously migrates downwards due to gravitational forces and can significantly disrupt air voids continuity in porous asphalt samples. This study was an extension to a previous work that ascertained the existence of the binder creep phenomenon as reflected from the continual permeability loss especially on samples conditioned at elevated temperatures. Nonetheless, in this paper, the terminology "mortar creep" was adopted instead of "binder creep". This is because, in an asphalt mixture, the aggregates are glued together not by the binder in isolation, but by the mortar; which is comprised of asphalt binder, fine aggregates and filler. The variables investigated included aggregate gradation, binder type, bitumen content and conditioning temperature. The mixes were prepared using conventional bitumen (60/70 pen. grade) and modified asphalt binder (PG76) at three levels of binder content at 0.5% increment. Permeability loss was continuously monitored over an extended period up to 120 days using a simple falling head water permeameter. Over the test period, the samples were separately conditioned at 15 °C, 20°C, 30°C and 35°C. The results showed that all factors significantly affect the occurrence of mortar creep in porous asphalt prepared in the laboratory, especially for the specimens conditioned at the highest temperature. Permeability loss was more significant on specimens' prepared using conventional binder at a higher bitumen content.