Extending the Life of Porous Asphalt

Earlier this month, I described some of the advantages that porous asphalt has over regular asphalt in terms of reducing flooding, reducing water contamination, and increasing groundwater recharge.

In spite of the advantages, porous asphalt has some drawbacks, including a shorter life than regular asphalt.  Dense, graded asphalt concrete roads can survive up to 18 years, whereas porous asphalt roads generally last between 10-12 years.  The wear-and-tear experienced by porous asphalt roads is generally in the form of ravelling.  Ravelling is the process by which stones are ripped from the surface of the roadway, and this eventually leads to serious problems, such as potholes.  Ravelling also increases noise pollution.¹

Regular asphalt concrete is a self-healing material because cracks in the material tend to close on their own when the material is allowed to rest.  Asphalt beams stressed to the point of failure can recover 90% of their original resistance when allowed to rest under pressure.  Heat is an important component of this process, and this can be observed in the field, where cracks that form during the winter disappear during the summer.²

The self-healing property of asphalt is due to the asphalt-binder, which is semi-liquid at warm temperatures.  The binder coating separate rock pieces can flow together and re-bind the material.  Due to the increased void spaces, this self-healing property is reduced in porous pavement.  Scientists have been working on improving the self-healing properties of porous pavement to extend its life.²

In the laboratory, the self-healing properties of porous asphalt have been shown to improve with the addition of steel-wool.  Theoretically, the steel fibers would aid in heating up the finished asphalt, via induction energy, and thus encouraging self-healing.  This study indicated that the temperature due to induction heating increased as a greater volume of steel-wool was added to the mixture.  Furthermore, shorter mixing times were associated with greater inductive heating, as it was associated with longer steel-fiber lengths in the finished mixture.³

In a separate study, steel-wool mixed porous asphalt beams were subjected to a four point bending fatigue resistance test.  The self-healing properties of the steel-treated beams was compatible to high-quality, non-porous asphalt.¹

1. Liu Q, Schlangen E, Van Bochove G, Van de Ven M, Van Montfort J. 2012. Evaluation of the induction healing effect of porous asphalt concrete through four point bending fatigue test. Construction and Building Materials. 29.1. p403.

2. Garcia A, Liu Q, Schlangen E. 2011 Induction healing of asphalt mastic and porous asphalt concrete. Construction and Building Materials. 25.9. p3746.

3. Garcia A, Schlangen E, Van Bochove G, Van de Ven M. 2012. Optimization of composition and mixing process of a self-healing porous asphalt. Construction and Building Materials. 30.1. p59.