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 4x4-

W4xW4 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 half-

length 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 Upper Electrodes Lower Electrodes

Half Upper, Half Lower 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 mixCylinder 1 8751 7310Cylinder 2 9154 7640Cylinder 3 8238 7431Mean 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 32.5 Ω∙mLower mesh 27.3 Ω∙m

Half length fibers

Upper mesh 27.9 Ω∙mLower mesh 16.8 Ω∙m

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 daysFull length

fibersUpper mesh 65.9 46.6 72.9Lower mesh 52.2 104.6 114.9

Half length fibers

Upper mesh 25.2 34.1 51.0Lower 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.