Highway drainage systems are designed to remove surface water and groundwater without deteriorating the road structure; hence, their durability is crucial to sustain the performance of highways. Filtration performance of a drainage system consisting of aggregate and geotextiles is one key parameter that needs to be carefully assessed so that clogging of the system does not result in failure. Substituting the natural aggregate in drainage systems with recycled materials would result in preservation of natural resources and employment of stockpiled recycled material, creating a mutually beneficial solution. In this study, the long-term filtration performance of three different recycled materials from different industries (i.e., two steel slag, recycled asphalt pavement, recycled concrete aggregate) has been investigated when used as filter material in drainage systems. Furthermore, two natural aggregate materials as control soils were used to compare the performance of recycled materials. A nonwoven geotextile with a low apparent opening size that represents the least favorable scenario in terms of clogging
was selected as the filtration media. Long-term filtration experiments were carried out in the modified gradient ratio test setup and the hydraulic gradient ratio (GR) along with the permeability ratio (KR) were determined under different hydraulic gradients to evaluate the clogging performance. The results indicate that even under the most undesirable conditions, recycled materials exhibit a satisfactory filtration performance compared with natural aggregates, and they can be employed as aggregate material in highway drainage systems.