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

ARTICLE IN PRESS
2 Q. Liu et al. / Construction and Building Materials xxx (2010) xxx–xxx
recently Sherif et al. [9] and Tuan [10] made conductive concrete
containing steel fibers and shavings for bridge deck deicing.
Electrically conductive asphalt concrete is a relatively new
functional material developed to achieve electrical conductivity.
To reach this property, conductive filler or fibers should be added
to the mixture. Wu et al. [11] made the first electrically conductive
mixture by adding conductive carbon fibers, carbon black or graphite
to the asphalt concrete. They demonstrated that the conductivity
is proportional to the volume of conductive filler or fibers
added. An excess of conductive particles can cause the degradation
of the asphalt concrete properties, such as the strength or the
workability of fresh materials. Finally, they showed that adding
conductive fibers to the mixture is much more effective to increase
the conductivity than adding conductive filler. In addition, García
et al. [12] show how the conductivity is proportional to the volume
of aggregates in the mixture and how there is an optimum volume
of electrically conductive particles for each mixture. These authors
explain the conductive mechanism by means of the percolation
theory. With a low volume of conductive particles in the mixture,
the resistivity of asphalt concrete is that of a non conductive material,
but when the volume of conductive particles is above the percolation
threshold, the resistivity drops and the sample becomes
conductive.
It is theoretically feasible to repair cracks in asphalt concrete
before they become too big to be healed. The use of fibers to
reinforce binder has long been in practice. In this research, conductive
steel fibers are added to the asphalt mixture and induction
heating is used to increase the temperature locally, just
enough to increase the asphalt healing rates and repair microcracks
or the bond between aggregates and binder. The main
objective of using conductive fibers is to increase the conductivity
of binder so that micro-cracks can be healed via induction
heating. Induction heating in the asphalt industry is a novel technique
originally developed by García et al. [12]. In this method,
the power supply sends alternating current through the coil, generating
an alternating electromagnetic field. When the conductive
asphalt specimen is placed under the coil, this electromagnetic
field induces currents flowing along the conductive loops formed
by steel fibers. The induced current dissipates heat by the Joule
effect. This method can be repeated if damage returns. Traditionally,
in conductive roads, heat was generated due to the electrical
resistance in the conductive particles when connected to a power
source, but in this occasion, authors are trying to make porous
asphalt concrete appropriate for induction heating and the subsequent
healing of cracks. This method has the advantage of having
high volumetric heating rates. Another advantage of using fibers
is that they will probably improve the ageing behavior and the
resistance to water damage of bitumen by achieving thicker binder
film and by preventing drain-off of the mastic. This may improve
significantly the raveling resistance of porous asphalt
concrete.
This will be the first time that this induction heating technique
is applied to porous asphalt concrete. The objectives of this research
are:
To study the effect of steel fibers volume content on the electrical
conductivity of porous asphalt concrete.
To investigate the effect of the steel fibers volume content on the
indirect tensile strength of porous asphalt concrete.
To analyze the effect of steel wool volume content on the heating
rate of porous asphalt concrete.
To explore the relationship between electrical conductivity and
induction heating in porous asphalt concrete.
To optimize the type and volume content of steel fibers to obtain
a good electrical conductivity and a high induction heating rate
in porous asphalt concrete.
2. Experimental method
2.1. Materials
The aggregates used to make porous asphalt concrete specimens were quarry
material (Bestone, Bremanger Quarry, Norway) (size between 2.0 and 22.4 mm
and density 2770 kg/m 3 ), crushed sand (size between 0.063 and 2 mm and density
2688 kg/m 3 ), and filler type Wigro 60 K (size < 0.063 mm and density 2638 kg/m 3 ).
Finally, the bitumen used was 70/100 pen, obtained from Kuwait Petroleum, with
density 1032 kg/m 3 .
Besides, two different types of electrically conductive fibers were mixed in the
asphalt. The first one was steel fibers (called steel fiber type 1), with a diameter between
0.0296 mm and 0.1911 mm. The second one was steel wool type 000 with
diameters between 0.00635 mm and 0.00889 mm. These fibers had to be chopped
by hand, always by the same operator. Both types of fibers had an approximate density
of 7.8 g/cm 3 . These fibers had an electrical resistivity of 7 10 7 X cm. To find
the size distribution of the conductive fillers, more than 100 fibers of each type
were checked by taking photographs under the optical microscope and measuring
their length with an image processing program, obtaining the distribution shown in
Fig. 1a and b.
2.2. Porous asphalt design
Porous asphalt PA 0/16, the most commonly used mixture in the Netherlands,
was used in this research. The mixture composition was fixed based on the Dutch
Standard, RAW 2005 (Table 1) and Gyratory compactor was used to compact the
mixture. These specimens have a diameter of 100 mm and a variable thickness between
77.40 and 84.22 mm.
The maximum theoretical density of the mixture was 2.569 g/cm 3 . It was calculated
as the total weight divided by the total volume of all the materials before compaction.
After mixing, the maximum theoretical density of the loose mixture was
measured using an ultrapycnometer 1000 (America, Quantachrome Instruments),
a
Percentage %
b
Percentage %
70
60
50
40
30
20
10
0
25
20
15
10
5
0
0 0.125 0.25 0.5 1.0 2.0
Size of steel fiber mm
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Size of steel wool mm
Fig. 1. Size distribution of steel fiber type 1 (a) and steel wool type 000 (b).
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