Porous asphalt shows
excellent performance in both noise reduction and water drainage. Although
porous asphalt has these great qualities, its service life is much shorter
(sometimes only half) compared to dense graded asphalt roads. Ravelling, which
is the loss of aggregate particles from the surface layer, is the main damage
mechanism of porous asphalt surface wearing courses. In this research, an
induction healing approach (namely, activating the healing process of asphalt
concrete through induction heating) was developed to enhance the durability of
the porous asphalt roads. Steel fibers are added to a porous asphalt mixture to
make it electrically conductive and suitable for induction heating. When micro
cracks are expected to occur in the asphalt mastic of the pavement, the
temperature of the mastic can be increased locally by induction heating of the
steel fibers so that porous asphalt concrete can repair itself and close the
cracks through the high temperature healing of the bitumen (diffusion and
flow). The closure of micro cracks will prevent the formation of macro cracks.
In such a way, ravelling can be avoided or delayed in the end.
To make asphalt mastic and porous asphalt concrete electrically conductive and suitable for induction heating, steel (wool) fibers were incorporated into them. The electrical conductivity and induction heating speed of asphalt mastic and porous asphalt concrete were first studied in this research. Asphalt mastic and porous asphalt concrete with steel fibers can be heated with induction energy. There is an optimal volume content of steel fiber in asphalt mastic or porous asphalt concrete to obtain the highest induction heating speed. Adding more steel fiber above this optimal volume content does not increase the induction heating speed anymore. Furthermore, the highest induction heating speed corresponds to the minimum electrical resistivity. However, porous asphalt concrete does not need to be fully conductive for induction heating. Every single steel wool is a heating unit. Nonconductive samples with steel fiber can still be heated with induction heating, but at a low heating speed. The diameter, length and content of steel wool fiber are important for the conductivity and heating speed of asphalt concrete matrix. It is proven that induction heating does not cause extra ageing to bitumen. Addition of steel wool also reduces the binder drainage problem in porous asphalt concrete.
The mechanical properties of porous asphalt concrete with steel wool fiber were studied in this research. Adding moderate percentage steel wool to porous asphalt concrete reinforce it by increasing its particle loss resistance, indirect tensile strength and fracture energy, water damage resistance, stiffness and fatigue resistance. The steel wool was optimized to obtain the best particle loss resistance in porous asphalt concrete. 8% steel wool type 00 (volume fraction of bitumen) was considered as the optimal content.
The healing potential of porous asphalt concrete with steel wool fiber was also evaluated in this research with both cylinder and beam samples. Damaged porous asphalt concrete with steel wool fiber can greatly restore its stiffness, strength and fatigue life with induction heating, which proves that the healing capacity of porous asphalt concrete with steel wool fiber is enhanced by induction heating. The optimal induction heating temperature is 85 ºC for porous asphalt concrete to obtain the best healing rate. Reheating does not decrease the healing rate of porous asphalt concrete, which means that heating can be repeated when cracks appear again.
To apply the induction healing technology in real porous asphalt road, a trial section was constructed on Dutch motorway A58 in December 2010. This trial section survived the past two winters perfectly. Experiments were done on the cores drilled from the trial section and the results coincided with those on the laboratory made samples. The field cores showed good particle loss resistance, high strength, good fatigue resistance and high induction healing capacity. Based on the laboratory experiments and field experiences, induction healing can be a very good approach to enhance the durability of porous asphalt pavement.
Finally, some recommendations are given for further research. Steel fiber, mixing technology and induction generator need to be optimized. Modeling work is necessary to fully understand the mechanisms involved in induction healing.
To make asphalt mastic and porous asphalt concrete electrically conductive and suitable for induction heating, steel (wool) fibers were incorporated into them. The electrical conductivity and induction heating speed of asphalt mastic and porous asphalt concrete were first studied in this research. Asphalt mastic and porous asphalt concrete with steel fibers can be heated with induction energy. There is an optimal volume content of steel fiber in asphalt mastic or porous asphalt concrete to obtain the highest induction heating speed. Adding more steel fiber above this optimal volume content does not increase the induction heating speed anymore. Furthermore, the highest induction heating speed corresponds to the minimum electrical resistivity. However, porous asphalt concrete does not need to be fully conductive for induction heating. Every single steel wool is a heating unit. Nonconductive samples with steel fiber can still be heated with induction heating, but at a low heating speed. The diameter, length and content of steel wool fiber are important for the conductivity and heating speed of asphalt concrete matrix. It is proven that induction heating does not cause extra ageing to bitumen. Addition of steel wool also reduces the binder drainage problem in porous asphalt concrete.
The mechanical properties of porous asphalt concrete with steel wool fiber were studied in this research. Adding moderate percentage steel wool to porous asphalt concrete reinforce it by increasing its particle loss resistance, indirect tensile strength and fracture energy, water damage resistance, stiffness and fatigue resistance. The steel wool was optimized to obtain the best particle loss resistance in porous asphalt concrete. 8% steel wool type 00 (volume fraction of bitumen) was considered as the optimal content.
The healing potential of porous asphalt concrete with steel wool fiber was also evaluated in this research with both cylinder and beam samples. Damaged porous asphalt concrete with steel wool fiber can greatly restore its stiffness, strength and fatigue life with induction heating, which proves that the healing capacity of porous asphalt concrete with steel wool fiber is enhanced by induction heating. The optimal induction heating temperature is 85 ºC for porous asphalt concrete to obtain the best healing rate. Reheating does not decrease the healing rate of porous asphalt concrete, which means that heating can be repeated when cracks appear again.
To apply the induction healing technology in real porous asphalt road, a trial section was constructed on Dutch motorway A58 in December 2010. This trial section survived the past two winters perfectly. Experiments were done on the cores drilled from the trial section and the results coincided with those on the laboratory made samples. The field cores showed good particle loss resistance, high strength, good fatigue resistance and high induction healing capacity. Based on the laboratory experiments and field experiences, induction healing can be a very good approach to enhance the durability of porous asphalt pavement.
Finally, some recommendations are given for further research. Steel fiber, mixing technology and induction generator need to be optimized. Modeling work is necessary to fully understand the mechanisms involved in induction healing.
