Induction heating of electrically conductive porous asphalt ... - TU Delft

ARTICLE IN PRESS
6 Q. Liu et al. / Construction and Building Materials xxx (2010) xxx–xxx
Fig. 9. Induction heating image of a sample with 10% steel wool type 000.
wool results in a decrease of the maximum strength of porous asphalt
concrete. Steel wool type 000 offers a much weaker reinforcement
than steel fiber type 1. The maximum ITS of samples
containing steel wool type 000 happens at around 4% with a value
of 2.27 MPa, only slightly higher than the ITS of plain samples
without steel wool at a value of 2.06 MPa and much lower than
the maximum ITS of samples containing about 11% steel fibers at
a value of 2.62 MPa. It is very interesting to note that the ITS of
samples with optimum electrical conductivity containing 10% of
steel wool type 000 and 20% of steel fiber type 1 are 1.99 MPa
and 2.03 MPa, very close to each other. So, steel wool type 000
and steel fiber type 1 have the same effect on the ITS of porous asphalt
sample at their respective content for optimal electrical conductivity.
From the point of view of the electrical conductivity,
steel wool type 000 has better performance than steel fibers.
3.6. Induction heating of porous asphalt concrete
To study the induction heating of porous asphalt concrete, samples
containing steel wool type 000 were used. Fig. 9 shows an
induction heating image of the sample with 10% steel wool. All
samples have the similar images like this. On the top surface of
the sample, clusters of steel wool work as small heaters. This corresponds
to the shining dots on the surface of the sample in Fig. 9.
The image shows uniform temperature in the horizontal direction.
This is because the magnetic field is constant at the same distance
from the coil. The temperature of the sample decreases from top to
bottom. It was also found that samples had a higher heating rate
when they were closer to the coil of the induction machine. The
reason for this is that the magnetic field is stronger close to the
coil. Finding the optimum distance between the pavement and
the coil will be a topic for future study.
The temperatures of all the samples with different steel wool
volume contents were recorded and used to analyze the effect of
the steel wool content on the heating rate of porous asphalt samples
(Fig. 10). The plain sample without steel wool was almost not
heated in 3 min because of the very low heating speed. Below 10%
of steel wool in the mixture, the temperature reached increased
with the increase of the volume content of steel wool. The samples
could be heated, even if hey were not conductive, but the heating
rate was lower. The temperature cannot be increased any more
when the steel wool volume content is above 10%. This value coincides
with the minimum in the resistivity curve in Fig. 8. The minimum
resistivity will result in a maximum Joule heat in the
sample; hence the temperature will be also a maximum. Adding
steel wool above 10% does not improve the heating and will result
in a decrease of the indirect tensile strength, as shown in Fig. 8. So,
Temperature ºC
160
140
120
100
80
60
40
20
0
10% is the optimal volume content of this type of steel wool to heat
easily porous asphalt concrete.
4. Conclusions
30s
90s
150s
0 2 4 6 8 10 12 14 16
Steel wool volume content %
It was found that long steel wool type 000, is better than short
steel fiber type 1 to make porous asphalt concrete electrically conductive.
However, short steel fibers have better strength reinforcement
capacity than steel wool. The mechanisms of electrical
conductivity and induction heating are explained in this paper.
10% (by volume of bitumen) of steel wool type 000 is proposed
as an ideal content to make porous asphalt concrete conductive
and for a fast induction heating. Besides, this value gives an acceptable
indirect tensile strength to porous asphalt concrete. Adding
more bitumen for durability purpose can increase the electrical
conductivity of porous asphalt concrete, but it can also decrease
its indirect tensile strength. It is expected that the self-healing
capacity of asphalt concrete will be enhanced with the induction
heating, although this is a matter for future study.
5. Further research plan
60s
120s
180s
Fig. 10. Effect of steel wool type 000 volume content on the heating rate of porous
asphalt concrete.
It has been proved that the electrically conductive porous asphalt
concrete can be heated quickly with induction energy. However,
work needs to be done to detect the healing effect of this
porous asphalt concrete. The idea is to introduce some damage
Please cite this article in press as: Liu Q et al. Induction heating of electrically conductive porous asphalt concrete. Constr Build Mater (2010), doi:10.1016/
j.conbuildmat.2009.12.019