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

ARTICLE IN PRESS
4 Q. Liu et al. / Construction and Building Materials xxx (2010) xxx–xxx
heating samples were the same as the ones used to measure the electrical resistivity.
The cylindrical samples cut from Gyratory specimens were used to avoid the
problem of binder concentration on the surfaces of samples and to get a higher
heating efficiency for that thinner samples mean less temperature difference between
the top and the bottom of the samples. The distance between the coil of
the induction machine and the top surface of the heating sample was about
32 mm. Each sample studied was heated for 3 min and its temperature change
was measured with a 640 480 pixel, full color infrared camera.
3. Results and discussions
3.1. Effect of conductive fiber volume content on the electrical
resistivity of porous asphalt concrete
The effect of steel fiber (type 1) volume content on the resistivity
of porous asphalt concrete samples is shown in Fig. 3. The resistivity
of the samples shows three stages: high resistivity stage,
with fiber volume content less than 10%; transit stage, with fiber
volume content from 10% to 20%; and low resistivity stage, with fiber
volume content higher than 20%. Samples with less than 10% of
fibers exhibited insulating behavior, with resistances higher than
10 9 X m. During the transit stage (between 10% and 20% of fibers),
the electrical resistivity of the sample suffered a sharp decrease
from 10 9 X m to 4.7 10 4 X m. It was discovered that, for the steel
fiber type 1, 20% fibers are needed to make the porous asphalt concrete
electrically conductive.
In addition, when drying the sample after cutting, it was found
that plain samples, without steel fibers, glued a little bit to the
plate bellow, showing drainage of binder in the sample. However,
samples with steel fibers did not show this phenomenon, which
shows that the steel fibers could prevent the drainage of binder.
3.2. Effect of conductive fiber volume content on the indirect tensile
strength of porous asphalt concrete
The effect of steel fibers volume content on the indirect tensile
strength (ITS) of porous asphalt concrete is shown in Fig. 4. The ITS
of the plain porous asphalt concrete studied was 2.06 MPa. The ITS
of porous asphalt concrete increased with the volume of steel fibers
until a maximum of 2.62 MPa with a steel fibers content of
11%. Adding more fibers to the mixture results in a decrease of
the ITS, because too many fibers reduce the mastic film thickness,
causing a bad adhesion between the asphalt components. Finally,
samples with 20% of steel fibers (the one with highest electrical
1,00E+10
ITS MPa
2.80
2.60
2.40
2.20
2.00
1.80
1.60
0 5 10 15 20 25
Fiber volume content %
Fig. 4. Effect of steel fiber type 1 volume content on the ITS of porous asphalt
concrete.
conductivity), had a maximum tensile strength of 2.03 MPa, close
to that of the plain porous asphalt concrete.
3.3. Electrically conductive mechanism in porous asphalt concrete
Broken porous asphalt samples after indirect tensile test were
observed under the microscope to check the fibers distribution.
The conductive mechanism was explained according to the percolation
theory [15]. Fiber distribution in samples with different contents
of steel fibers is shown in Fig. 5. When 5% of steel fibers is
added to the mixture, they are uniformly distributed and do not
contact each other, having a similar resistivity to that of an asphalt
concrete sample without fibers. When more steel fibers are added
to the mixture, they start contacting each other, which causes to a
gradual decrease in the resistivity. If the fiber volume content
reaches more than 10% (percolation threshold), the first conductive
paths are formed in the sample. This corresponds to a sharp decrease
of resistivity. Beyond the percolation threshold, the conductive
network develops and spreads gradually in three dimensions
with the increase of the volume content of the steel fibers. When
the fiber volume content is more than 20%, steel fibers contact each
other in all directions and many conductive networks and passages
are formed, corresponding to a very low value of resistivity at
which adding more steel fibers does not reduce the resistivity of
sample any more.
3.4. Effect of bitumen content on the electrical resistivity of porous
asphalt concrete
Resistivity Ω.m
1,00E+08
1,00E+06
1,00E+04
0 5 10 15 20 25
Fiber volume content %
Fig. 3. Effect of steel fiber type 1 volume content on the resistivity of porous asphalt
concrete.
When fibers are added to the mixture, the thickness film of
mastic around the aggregates is reduced. This can have a negative
effect on the mechanical properties of asphalt concrete pavements
so, to ensure the durability of this porous asphalt, bitumen content
needs to be increased. In this section, higher bitumen contents
were used to check how the electrical conductivity of porous asphalt
concrete was affected. The fiber content was fixed at 20% in
the mixture (for optimal electrical conductivity purpose) and the
bitumen content was gradually increased from 4.5% to 5.4%. The effect
of bitumen content on the electrical resistivity of porous asphalt
concrete samples is shown in Fig. 6. An increase in the
bitumen content causes a reduction in the electrical resistivity of
porous asphalt concrete. Furthermore, ITS reductions from
2.03 MPa to 1.40 MPa were also observed with the increase on
bitumen content.
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