Dense graded asphalt concrete road was popular in the past and is still used in most of the cases in our country, Nepal. However, dense graded asphalt concrete faces major drawbacks with respect to the water drainage and noise production while driving. This has urged the development of porous asphalt concrete The water can easily drain through the porous structure and the noise is mostly absorbed by the hollow portion;but the service life of porous asphalt concrete is much less than that of dense graded asphalt concrete. The porous asphalt concrete has been greatly affected by the raveling; the asphalt binder is degraded by the environmental factors, especially due to UV-radiation from the sun, which causes the loss in its binding ability .
It is well known fact that asphalt concert heals by itself, but it is slow process at ambient temperature and again with a condition that there should be no traffic circulation on the road. This property has initiated intense research on this subject matter and has lead to the invention of revolutionary technique of prolonging life of porous asphalt concrete within a short period of time, i.e. introduction of self-healing concrete by induction heating by Erik Schlangen at all of Delft Technical University, Netherland; which is basically normal porous asphalt concrete with addition of steel (wool) fiber in methodical proportion
The addition of the steel (wool) fiber in the mastic asphalt and porous concrete is to make it electrically conductive and suitable for induction heating. When the steel fibers are heated by induction heating, heat gets transferred from the steel fiber with high temperature to the asphalt concrete with low temperature. The binder material, then, gets melted, if the binder has been separated from the aggregates. Then the melted binder will flow into its original position again binding the aggregates together. The mere process of diffusion and flow takes place here. This prevents the formation of macro-cracks.
Steel (wool) fiber
The steel wool fiber plays instrumental role in induction heating and, eventually in the healing process.The asphalt mastic and porous concrete is made conductive and suitable for induction heating by the addition of steel fiber. The 8% of the steel (wool) fiber type II (volume content of bitumen) is expected to give best result, which is the optimum content of the steel (wool) fiber, based on experimental analysis. Increasing the steel fiber content above the optimum volume does not increase the induction heating speed and healing process. Furthermore, the highest induction heating corresponds to the minimum electrical resistivity. The diameter, length and content of steel (wool) fiber are. also, important for the conductivity and healing speed of asphalt concrete. Additionally, the steel(wool) fiber reinforces the asphalt concrete. It increases the particle loss resistance, indirect tensile strength, fracture energy, stiffness and fatigue resistance. This also reduces the binder drainage problem to great extent.
The trial porous asphalt concrete road with steel fiber was constructed on Dutch motorway A58. After, two years the field core showed good particle loss resistance,high strength, good fatigue resistance, and high induction healing capacity.