Summary
Electrically conductive concrete is usually produced by mixing conductive materials into
the wet concrete to decrease its resistivity. A voltage applied across the concrete causes the
temperature of the material to increase by Joule heating. Although many applications of such a
material are possible, this study focused on its use in highway anti-icing. A conductive concrete
overlay could be used to heat the roadway surface, thus preventing an ice-road surface bond
without the use of chemical deicers. This technique is particularly valuable for highway sections
in which the surrounding environment is sensitive to an increase in salt. In this study, the effect
of welded wire fabric on the heating ability of conductive concrete was examined. The height of
the fabric and the placement of electrodes were varied to find the optimal arrangement for
surface heating.
Introduction
To fully remove snow and ice from highways, both mechanical and chemical methods are
generally required. Although snow plows can move the bulk of the mass off the road surface, a
bond exists between the pavement and an underlying layer of ice that is difficult to remove by
plow alone. Chemical deicers or salts such as NaCl (most common), CaCl, and MgCl break this
bond by lowering the temperature at which water freezes, melting some of the ice and allowing
plows and traffic to move the remaining slush off the road. These salts are effective and
inexpensive, which are very appealing qualities to departments of transportation. However,
excessive use of salt can have devastating effects on the adjacent environment, particularly lakes
and ponds (Figure 1). In the Upper and Lower Cascade Lakes in Keene, NY for example,
chloride levels are 100 to 150 times greater than that of a comparable lake (Langen et al, 2006).
Figure 1. Section of Route 73 adjacent to Upper Cascade Lake in Keene, NY.
Road salt damages plants by disturbing the water balance in the soil, leading to
dehydration. The excess sodium cations introduced to the soil by heavy use of NaCl reduce soil
fertility by forcing out other essential ions such as calcium and magnesium and can promote
erosion. Chloride can bind with toxic heavy metals, forming complexes that are more easily
absorbed by plants. Sufficient levels of chloride concentration in lakes and ponds can lead to
meromixis, a state in which the body of water remains stratified and does not “turn over” in the
spring and fall. This condition leads to a concentration of nutrients and an absence of oxygen in
the bottom portion of the lake. (Langen et al, 2006)
One possible solution to this problem is the use of electrically conductive concrete
instead of deicing chemicals on roads near chemically sensitive areas. This material is produced
by mixing conductive aggregate or fibers into concrete. The relatively small amount of
conductive material renders the entire sample conductive by the connections between each piece,
which form a current-carrying network. When a voltage is applied across the concrete, the
resistivity of the sample causes it to warm through Joule heating. The heat emitted by the
concrete would fill the same role as the salt, breaking the bond between ice and pavement that
prevents a clear, safe highway. The concrete can also be used as an anti-icer if it begins to heat
before the storm event, thus melting all precipitation as it hits the road.
Previous Research
A substantial body of research has been developed in recent years in electrically
conductive concrete aimed particularly at its possible deicing and anti-icing applications (Yehia,
Tuan, et al, 2000; Tuan, 2004; Green and Janoyan, 2005; Vincent and Janoyan, 2005). The
optimal proportions of steel shavings and steel reinforcing fibers has been studied extensively
and found to be 15 – 20% shavings and 1.5% fiber by volume, optimized for electrical
conductivity (heating performance) and strength (Yehia, Tuan, et al, 2000). These proportions
created a concrete that sufficiently lowered the resistivity of the material (less than 10 Ω∙m)
while maintaining workability, surface finish, and necessary mechanical properties with a
heating efficiency of approximately 88%. This study also found that the use of shavings or
fibers alone did not create sufficiently conductive concrete. The Roca Spur Bridge in Nebraska
was completed in 2002 with a conductive concrete overlay (Tuan, 2004b). The bridge
demonstrated the effectiveness of conductive concrete in preventing snow and ice accumulation
on a bridge surface (Figure 2).
Figure 2. Roca Spur Bridge in use during February 2004.
Image credit: http://www.cement.org/tech/cct_con_design_conductive.asp
Further research has been done to investigate the use of materials in conductive concrete
that are less expensive and easier to implement. A comparison of steel fiber reinforced concrete,
wire mesh, and nickel wires found that mesh cut in the middle, suspended in fiber reinforced
concrete, warmed the surface best (Vincent and Janoyan, 2005). It was also found that the
samples because hottest where the separate sides of mesh were closest.
This study aimed to continue investigating methods of creating electrically conductive
concrete using materials and methods that departments of transportation already use, namely
welded wire fabric and steel fibers. Although steel shavings can be used to create conductive
concrete with excellent mechanical and electrical properties, this material is difficult to use in
bulk quantities due to the cleaning necessary to remove contaminants that might compromise the
properties of the final product (Green and Janoyan, 2005). Welded wire fabric is commonly
used by many departments of transportation to reinforce roadway surfaces. Thus, understanding
its effect on the heating ability of conductive concrete is vital if this technology is to be accepted
by departments of transportation for use on highways.
Materials
Four 15x15x2” resistivity/heating samples were tested, incorporating two welded wire
fabric heights and two concrete mixes. In one set of samples, the 12x15” sections of 4x4W4xW4 mesh were placed above the thinnest layer of concrete possible, about ½” above the
base of the mold. The higher one was placed approximately in the center of the mold, at about
1”. The mesh was originally covered in a layer of rust, which was removed using wire brushes.
In both sets of samples, the mesh was positioned so it was not in direct electrical contact with the
electrodes (Figure 3). One concrete mix contained 0.25% intact Dramix 80 steel fibers by
volume, while the other contained 0.5% fibers cut in half (Appendix A). These low percentages
were used due to the poor workability and finish of concrete incorporating large amounts of steel
fiber.
Figure 3. Mesh placement (gray) in relation to electrodes (black).
Three 6x12” compression cylinders were poured for each mix. Concrete molds were
constructed from ¾” plywood, connected with 1 ¼” drywall screws. Steel strips (3/16” x 3/4”)
were used to create the electrodes. They were drilled with ¼” holes every inch to facilitate
bonding with the concrete. On each sample, four 10” and four 4” electrodes were attached to the
concrete by connecting the steel to the side of the mold with ¼ x 5 ½” bolts while curing (Figure
4). The bolts were used as electrical contacts during testing. Multiple electrodes were used to
allow for investigating the effects of electrode placement. Thermistors were installed using ¼”
wooden dowels to record the temperature of the concrete at the surface and at 1” depth at nine
locations, as well as ambient temperature (Figure 4).
Figure 4. Concrete mold with electrodes and thermistor temperature probes. The steel bars bolted to the sides of the
mold are electrodes. Inside the mold, each dowel is attached to one or two thermistors.
Procedure
Sample Constuction
Concrete samples were poured in the structures lab at Clarkson University on June 29,
2006. Both mixes were prepared by first wetting the 1/3 yd3 drum and adding the dry
components, then adding the premixed water and admixture. The drum was not sufficiently
dampened before mixing the full-length fiber sample, which resulted in a large amount of dry
material adhering to the sides of the drum. A scoop and rod were used to dislodge the material
during a ten minute period of short bursts of mixing and resting. The more evenly distributed
sample was then mixed for a final three minutes. The slump was initially found to be 8”, which
was very different from the expected 1-3” range. Upon closer inspection, the concrete appeared
to be not yet uniformly mixed. The concrete was therefore mixed further by hand for
approximately five minutes and retested with a slump of 3”.
To avoid the problems of the previous mix, the half-length fiber sample was mixed in two
standard cycles of three minutes mixing, three minutes rest, and three minutes final mixing.
When poured out for a final hand mixing, the concrete appeared to have a much higher slump
than expected. The concrete was allowed to sit, covered, for fifteen minutes to allow it to set.
After this time the concrete still did not appear sufficiently stiff, so the true water/cement ratio of
the mix was examined and found to be .39, higher than the design .35. More cement was added
to bring this ratio to its proper value. The slump was tested and initially found to be 8”. After
waiting five minutes for further setting of the recently introduced cement, the slump was found
to be 5.5”. The compression cylinders were poured in accordance with AASHTO T 126-93, in
three layers that were rodded 25 times each.
It was observed that the half-length fibers allowed for a better initial surface finish than
the full-length fibers. Forms in which the half-length fibers were used were much easier to strike
off. In both mixes the heating and resistance molds were difficult to strike off due to the large
number of probes. After pouring, all samples were placed in the curing room, where they were
kept damp by an automatic watering system.
After seven days the samples were removed from the molds and replaced in the curing
room. One 4” electrode was found to have not bonded to the concrete on the sample using the
half length fiber mix with the lower mesh height. The following day the samples were relocated
to the testing location. In the process, a 4” and a 10” electrode was removed from the sample
using the half length mix with the upper mesh height. Later, another 4” electrode was observed
to be loose on the same sample. The protruding ends of the thermistor supports were removed
with a hacksaw, which provoked a 4” electrode on the full length upper mesh location sample to
detach. The samples were covered with wet burlap. However, after three days the burlap was
removed and silver-filled conductive epoxy used to reattach the electrodes to the samples.
Sample Testing
All thermistors were tested prior to concrete pouring and found to be functioning
properly. Initial tests showed the thermistors to be severely damaged by the curing process, as
they gave disparate and unreasonable temperature readings whether or not there was a voltage
across the sample (Appendix B, Figures 1, 2), although they were still able to respond to extreme
external heating (Appendix B, Figure 3). New surface temperature probes were constructed and
attached to samples using duct tape followed by a layer of caulk (Figure 5). The duct tape served
as a waterproof layer, while the caulk provided thermal insulation. These samples were tested by
using a BK Precision 1651A power supply to apply 12 VDC across the sample and measuring
temperatures at the surface as recorded by the thermistors. Data for the heating tests was
collected automatically using an HP (Agilent) 3852A data acquisition system.
Figure 5. Sample with partially attached new thermistors. The plywood visible on the sides of the sample is being
used as a clamp for the electrodes as the epoxy cures.
The resististance values of the samples were found using the four wire method. The
power supply was used to apply 12 V across the sample, verified by a multimeter (true voltage
for all tests was 12.3 VDC). A multimeter was also used to measure the current passing through
the sample.
All samples were tested for resistivity and heating ability multiple times, using different
electrode arrangements. The arrangements tested were using all electrodes, using all top
electrodes, using all bottom electrodes, using half top and half bottom electrodes, using the
diagonal of 4” electrodes on top, and using the diagonal on the bottom (Figure 6). In the halflength fiber samples, not all tests were able to be performed since many of the detached
electrodes did not sufficiently bond to the concrete through epoxy.
All Electrodes
Half Upper, Half Lower
Upper Electrodes
Lower Electrodes
Upper Diagonal
Lower Diagonal
Figure 6. Electrode arrangements used during testing.
Results
The compressive strengths of the concrete are suitable for highway applications (Table
1). Departments of transportation generally require a minimum strength of 3000 psi for concrete
used in road overlays (Green and Janoyan, 2005). As expected, the mix containing full-length
fibers had a higher strength than the half-length mix. Without considering the effect of the steel
fibers, the projected strength for these mixes was 6000 psi from the water/cement ratio of 0.35.
Higher values as were observed were expected due to the reinforcing effect of the fibers.
Full-length mix Half-length mix
Cylinder 1
8751
7310
Cylinder 2
9154
7640
Cylinder 3
8238
7431
Mean
8714
7460
Table 1. Compressive strengths of concrete mixes in psi.
The resistivity data suggest that none of the samples would be able to function as
highway anti-icers or deicers (Table 2). In all samples and in all trials the resistivity was lowest
when all electrodes received the applied voltage and highest when only one electrode on each
side received the applied voltage (Appendix C). This observation is not surprising, since more
electrodes imply a greater number of current paths. While low, these values were still not low
enough to fall below 10 Ω∙m, the value determined by Yehia and Tuan (2000) to be the
maximum resistivity for functional deicing or anti-icing conductive concrete. The median
resistivity of each sample also increased with time in almost all cases (Table 3). This suggests
that the heating ability of the concrete would become still worse over time. An increase in the
resistivity of conductive concrete over time was also observed by Yehia and Tuan, which they
explained as resulting from the progression of hydration and predicted would eventually reach a
constant value (2000). It was observed that during resistivity tests the ammeter readings began at
a high current and eventually decreased to a steady value. This behavior suggests that the
effective resistivity of the concrete increases as current flows through it, although it eventually
stabilizes.
Full length
fibers
Upper mesh
Half length
fibers
Upper mesh
Lower mesh
Ω∙m
27.3 Ω∙m
27.9 Ω∙m
16.8 Ω∙m
32.5
Lower mesh
Table 2. Lowest resistivity measurements of all samples. The resistivity values given are the mean of all resistivity
measurements using the all electrode arrangement.
13 days
17 days
22 days
Upper mesh
65.9
46.6
72.9
Full length
fibers
Lower mesh
52.2
104.6
114.9
Upper mesh
25.2
34.1
51.0
Half length
fibers
Lower mesh
19.4
29.2
45.9
Table 3. Median resistivity values, in Ω∙m, of all possible arrangements of electrodes for each sample at different
times after curing.
Heating tests indicate that applying a voltage across the samples does not heat them
considerably (Figure 7). The surface thermistors appear to register a delayed, muted version of
the temperature variations recorded by the ambient probe. In no trial did the temperature in one
location vary by more than 1˚ F (Appendix B, Figures 4 - 24). These observations suggest that
the samples cannot function as anti-icers or deicers.
Figure 7. Temperature variations of surface and ambient probes during heating test.
Although all loose rust was removed from the welded wire reinforcement before
placement in the samples, a fine layer that could not be removed by wire brush remained.
Although very thin, this layer was found to effectively electrically insulate the mesh. When
electrodes were connected to the slightly rusted area, generally no measurable current passed
through the wire fabric, although in certain contact locations the resistance could be found to be
2.04 MΩ. However, when electrodes contacted the unrusted areas where the mesh had been cut,
the resistance was approximately 1 Ω. These observations suggest that any effect the mesh has
on the resistivity of the sample is only through contact with the cut ends of the fabric, a very
small surface area. Since it is unlikely that departments of transportation use welded wire fabric
completely devoid of rust, similar resistance values are likely to be observed in a large-scale
application.
Despite the small surface area of usable contacts on the mesh, the results suggest that the
fabric did decrease the resistivity of the concrete. The electric resistivity of concrete containing
2% steel fibers has been found to be 0.54 M Ω∙m (Yehia, Tuan, et al, 2000). Although resistivity
measurements of the fiber reinforced concrete alone were not taken in this study, it is reasonable
to assume that the concrete mixes used in this study would have higher resistivity values due to
the lower concentration of steel fibers. However, all observed resistivity values observed were
much lower than this value, suggesting that the addition of welded wire fabric substantially
decreased the resistivity of the sample (Appendix C).
It is important to note that previous research has shown samples with higher resistivity
values exhibit greater heating. In one trial a conductive concrete sample having a resistivity of
approximately 20 Ω∙m experienced a temperature increase of 21.3˚ F over 5000 seconds in one
location (Green and Janoyan, 2005). In another experiment, a conductive concrete sample
containing steel fibers was found to have a resistivity of 41.0 Ω∙m but increased in temperature
by 2.5˚ F over 6000 seconds (Vincent and Janoyan, 2005). These previous results suggest that
resistivity is not the only important factor in determining the heating ability of conductive
concrete. The latter experiment particularly suggests that the absence of heating of the samples
in this study is not due simply to the high resistivity values of the concrete, since similar
resistivity values were found for these samples. Although the embedded mesh decreases the
overall resistivity, the extremely low resistivity paths that it produces do not contribute to Joule
heating. The lowered resistivity is a false encouragement, since the increased current cannot
noticeably contribute to heating.
To attempt to determine the cause of the mass thermistor failure, a probe was constructed
and placed in a cylinder of dirt and water for a week. The thermistor’s room temperature
resistance was then measured using the data acquisition system and found to have decreased
from 53 kΩ to 45 kΩ, suggesting that the thermistors failed due to water damage. This
thermistor also exhibited the same trend as observed in the embedded probes of steadily
decreasing in resistance as measurements were made.
Conclusions
These results suggest that electrically conductive concrete composed of 0.25 - 0.5% steel
fiber and welded wire mesh isolated from electrodes is not effective for deicing or anti-icing
applications. Using more area on the sides of the concrete as electrodes decreases the resistivity,
although this does not increase its heating capability. The data suggest the inability of the
samples to sufficiently increase in temperature is due to the low resistivity of the embedded
mesh, which lowers the resistivity of the sample without allowing the current passing through it
to perform Joule heating. If reinforcing mesh is to be used with electrically conductive concrete,
the mesh may require complete electrical isolation for the concrete to heat properly.