Electro-thermal evaluation of electrically conductive asphalt pavement containing recycled metal fiber

Abstract

Fast deicing of pavements is essential for several reasons: ensuring safety on the airfields, bridge decks, and highways, and enabling accessibility to winter resorts. These pavements include conductive materials to create a continuous network of electricity throughout the pavement. This study introduces the use of high-carbon recycled metal fiber (RMF) and carbon black modified binder (CBMB) to design an electrically conductive asphalt pavement (ECAP) with the capability of ice and snow melting. For this purpose, four ECAP samples with varying percentages of RMF and two types of binders, namely virgin PG 58 –22 and CBMB, were constructed and tested at the full-scale pavement testing site of Urmia University in northwest Iran. Subsequently, twelve cores were taken from the field samples, and laboratory experiments were conducted on them. Results show that the best electro-thermal performance was observed in the field sample containing 6% RMF with CBMB. The consumed energy calculated for heating the cores was lower than in recent studies; 1623 W/m² was estimated to raise the ECAP temperature from −20 to 0 °C in just 128 s, based on the cores’ electro-thermal experiments. The ECAP’s electro-thermal performance exhibited promising results for protecting pavements during extreme snow events.

Introduction

The importance of the transportation network is evident in the economy and tourism of any country. Both the quantity and quality of routes can be used to gauge the power of a country. Inadequate maintenance of highways and airports leads to significant pavement deterioration and damage. As a result, strengthening and maintaining these infrastructures should be a top priority. One of the transportation and environmental engineering challenges is utilizing these infrastructures during winter. Winter ice and snow accumulate on sloping roads, bridge decks, airport runways, and other transportation infrastructure, causing severe issues such as accidents and traffic delays. Reducing tire and pavement adhesion on roads and airports causes delays and economic damage and can be risky. Canceling flights and closing routes are the simplest and least proactive management methods for preventing these hazards. Traditional approaches negatively affect the sustainability and utilization of these infrastructures, as well as the environment. Research has shown that these methods cause severe damage to the surrounding soil and road pavement. For example, spraying sand, salt, or other chemicals to deice roads can destroy vegetation around the roads¹. In addition, studies show that a decrease in frictional resistance between tire and pavement increases traffic accidents². Xiao et al. examined the relationship between skid resistance and traffic accidents. They showed that when the skid resistance value of the road surface increased from 35 to 48%, traffic accidents decreased by 60%. Moreover, statistics indicate that bad weather is the primary cause of 10–15% of all traffic accidents³. These challenges have led scientists to propose methods tailored to various regions with differing potentials. So far, methods involving electrical resistance⁴, combustion⁵, geothermal⁶, and solar energy^7–9 have been studied as potential heat sources for melting ice and snow on transportation routes. These studies also investigated the environmental impact, stability, and resilience of the network. Findings point to high costs and limited accessibility of these newer technologies. Nevertheless, the approach can be described as a smart solution that enhances transportation safety, speed, convenience, and especially sustainability and environmental friendliness, all while remaining cost-effective and affordable^10,11. In order to achieve a smart method that reduces costs while enhancing the sustainability and resiliency of the road infrastructure, this research introduces a novel pavement which incorporates innovative waste materials with high electro-thermal performance. The significance of recycled metal fiber (RMF) and carbon black modified binder (CBMB) lies in suitable properties for electrically conductive asphalt pavement (ECAP), such as high carbon content, advantageous shapes and widespread availability^10,12–14. Therefore, the aim of this research is to present these waste and accessible materials as alternative additives for ECAP and to evaluate their efficiency through field sample testing.

Background

Electrically conductive cement concrete (ECON) and electrically conductive asphalt concrete (ECAC) systems have received more attention in recent years than other pavement deicing methods because of their environmental compatibility and stability. A conductive mixture has been designed to heat pavements using an electrical current by adding conductive materials to the mixture. Several studies have been conducted to improve the accessibility and cost-effectiveness of such systems. In recent years, extensive research on incorporating conductive materials into concrete pavement has been undertaken, leading to its implementation in a section of the Des-Moines airport and the Iowa State University parking lot in Iowa, USA^4,15. New additives have recently been used to reduce the electrical resistivity of asphalt concrete, with promising outcomes in conductivity, pavement resistance, and cost-effectiveness^10,16–19.

Although considerable effort has been made to reuse waste rubber from end-of-life tires in asphalt pavements²⁰, less effort has been made to reuse other significant amounts of waste from these tires. RMF and pyrolytic recycled carbon black (CB) are among these materials^21–33. The use of these materials has positive effects on the rheological and mechanical properties of asphalt binders, especially in low and high temperature behavior. As a result, previous studies in this domain have recommended the use of these materials in asphalt mixtures^{10,12,14,19,34–39}.

It should be noted that limited research has been done on conductive asphalt pavements, so it has not yet led to a full-scale implementation. Failure energy, electrical resistivity, and temperature rise for different conductive asphalt mixture samples from previous studies have been mentioned in Tables 1 and 2. In addition, in Table 2, the magnetic field applied for each sample has been presented for the method that applied the electrical current to the pavement with an inductor carried by a snowplowing vehicle for deicing and self-healing programs. The results of these studies show that increasing the amount of graphite and graphene in electrically conductive asphalt mixtures has reduced their compressive strength, as well as lowered the electrical resistivity of the specimens⁴⁰. On the other hand, the use of carbon fiber in these asphalt mixtures has reduced the electrical resistivity and improved the crack resistance of the mixture⁴⁰. It should be noted that the required energy has not been calculated in these studies^18,41.

Table 1.

Failure energy and electrical resistivity for different samples.

Sample ID	Binder grade, content	Aggregation (%)			Air void (%)	Conductive material (additive)	Additive content (%)	Failure energy (Joule/m2)	Electrical resistivity (Ωm)	Reference
A1	PG 76 − 22, 7.6%	40.4 (0–6 mm)	30 (6–12 mm)	22 (12–19 mm)	7	Graphite	0	3034	N/A	41
A2						Graphite	28	1290	5.49
A3						Graphite	30	1651	2.29
A4						Graphite	40	1621	2.77
A5						Graphite Carbon fiber	28 1	N/A	1.66
A6						Graphite Carbon fiber	30 1		1.21
A7						Graphite Carbon fiber	40 1		1.21
AC-13	PG 64 − 22, 4.8%	11.7 (0–6 mm)	69.4 (6–9.5 mm)	18.9 (9.5–19 mm)	5.0	Graphite	2	N/A	1.0E + 12	31
							6		1.0E + 11
							10		1.0E + 08
							14		1.0E + 04
							18		1.0E + 03
							22		1.0E + 02
							26		1.0E + 02
					5.8	Steel fiber	0.1		1.0E + 10
							0.2		1.0E + 09
							0.4		1.0E + 06
							0.6		1.0E + 04
							0.8		1.0E + 03
							1.0		1.0E + 02
							1.2		1.0E + 02

N/A = not applicable.

Table 2.

Magnetic field applied and temperature rise for different samples.

Sample ID	Binder grade, content	Aggregation (%)			Air void (%)	Conductive material (additive)	Additive content (%)	Magnetic field (MT)	Temperature rise (°C)		Reference
									Surface layer	Conductive layer
A8	B50-70, 8%	60 (0–6 mm)	15 (6–12 mm)	10 (12–19 mm)	2	Metal fiber	5	4	0.31	0.38	18
A9								5.3	0.85	0.73
A10								7	3.14	1.53
A11		55 (0–6 mm)	14 (6–12 mm)	9 (12–19 mm)			11	4	2.11	2.10
A12								5.3	1.99	2.39
A13									8.64	6.78

Objective

The main goal of this system is to convert the asphalt mixture into a semi-conductor composite with a specific electrical resistivity to generate heat from an electrical current. Employing conductive additives in the mixture is a method previously applied for electrically conductive concrete pavement^{4,11,15,42–46}. A minimum amount of conductive material must be available to form a network of conductors (i.e., conductive components) in physical touch to serve as the electrically conductive path and establish the electric flow between the two electrodes through the conductive layer. This value was calculated and applied to a conductive concrete pavement containing 1% carbon fiber by composite volume^43,44. The presence of RMF used in previous studies varies between 1.5 and 11% of the specimen weight^18,37. Most of these studies focused on the self-healing of asphalt mixtures, with microwave and magnetic field heating methods used for this purpose. In order to investigate the effect of RMF on the performance of the system, a wide range of its percentages was tested in this study.

The design and demonstration of a full-scale implementation process for the operational electrically conductive asphalt pavement (ECAP) with various conditions and contents of RMF and two types of asphalt binders, namely virgin PG 58 − 22 and CBMB, were the targets of the current study. Four asphalt pavement samples were constructed at the full-scale pavement testing site of Urmia University in northwest Iran. These pavements were constructed to evaluate the heating efficiency and energy consumption of the ECAP samples under different conditions and investigate the capacity of RMF and CBMB to heat the pavement by applying an alternating electric current (AC).

Materials

The materials used in this system, are divided into two main categories: those utilized to produce an electrically conductive asphalt mixture, and those used to equip the site and prepare it for paving this mixture.

Electrically conductive asphalt mixture

Raw material

Four different mixture designs were used for the field samples. First, for the determination of the optimum binder content, the aggregates and the asphalt binder from which the samples had to be made were transported to the laboratory, where 18 specimens (three of each 4%, 4.5%, 5%, 5.5%, 6% and 6.5% binder content) were prepared, and the mixture design was drawn. The optimum binder content was calculated as 4.9% which was used for both asphalt mixtures containing virgin PG 58 − 22 and CBMB. The mix design of four field samples is shown in )Table 3).

Table 3.

Mix design of the field samples.

Sample	Component	Type	Content (kg/m3)
All samples	Coarse aggregate	Mineral 0–6 mm	1380
	Intermediate aggregate	Mineral 6–12 mm	529
	Fine aggregate	Mineral 12–19 mm	253
	Filler	Fly ash < 0.2 mm	138
3%, 6%, and 9%	Binder	PG 58 − 22	106
6% M		12% CBMB	106
3%	RMF	Recycled from end-of-life tires	69
6%			138
6% M			138
9%			207

3% = Asphalt mixture containing 3% of RMF per total weight of sample with virgin binder.

6% = Asphalt mixture containing 6% of RMF per total weight of sample with virgin binder.

6% M = Asphalt mixture containing 6% of RMF per total weight of sample with CBMB.

9% = Asphalt mixture containing 9% of RMF per total weight of sample and geo-membrane layer under the conductive asphalt layer.

Asphalt mixtures were prepared by an asphalt plant near the full-scale pavement testing site (located 5 km away). RMFs were added to the hot mix asphalt in the last step and blended for 5 more minutes. Before adding, they were heated to 100 °C to avoid decreasing the mixing temperature and to ensure better dispersion. During the mixing, clustering of RMFs was evident, and more clustering occurred by more blending.

Recycled metal fiber (RMF)

RMF is a harmonic steel with a high carbon content that can reach 0.80% and is bronze/copper-coated. The low electrical resistivity which is around 1.59 × 10^− 7 ohm-meters, considerable carbon contents, high melting point (between 1369 °C and 1704 °C) and low price are the key advantages of RMF that make it suitable for such a study. One gram of RMF was picked randomly, and its length was measured and reported in (Fig. 1). According to the data and reports, the diameter, length, and density of the fibers are respectively 0.381–0.457 mm, 0.5–30 mm and 300 kg/m³ on average. In addition, more than 90% of the RMFs are shorter than 14 mm. In total, 125 kg of RMF was prepared from a tire recycling company in Tabriz, Iran.

Fig. 1.

Frequency of different length groups in 1 gram of randomly collected RMF.

Carbon black modified binder (CBMB)

Carbon Black (CB) is a nanomaterial derived from the pyrolysis of waste tires. The recycled CB used in this study was obtained from a tire recycling company located in Khorramabad, Iran. The mixing procedure for the CBMB was previously investigated (see Table 4). To achieve the optimum electrical conductivity in the modified binder, the optimum content of CB was reported to be 12% by weight^17,19. Accordingly, a 12% CBMB was produced using PG 58 − 22 asphalt binder and a high-shear mixer as shown in (Fig. 2). The mixing temperature, mixing speed and mixing time were selected respectively as 160 °C, 67 Hz, and 30 min. According to the required contents of CBMB for a ECAP field sample, 25 kg of CBMB has produced at the pavement laboratory of Urmia University under mentioned circumstances.

Table 4.

CBMB production specifications.

Researchers	Base binder	CB content (%)	Mixing time (minutes)	Mixing speed (Hz)	Mixing temperature (°C)
19	PG 58 − 22	5–15	30–60	66.7	150–170
17		6–18	60	33.3	150

Fig. 2.

CBMB production using a high-shear mixer.

Site equipment

Electrodes

The shape, size, configuration, and type of electrodes chosen for all samples were kept constant. These embedded electrodes are responsible for applying electricity to the ECAP since they have high electrical conductivity. Galvanized steel, which also possesses a comparable level of corrosion resistance, was used to make the entire system of embedded electrodes inside the ECAP. The size and shape of the electrodes were derived from earlier research in this domain⁴. The electrodes utilized were 60 cm long, 2.54 cm (1-inch) in diameter, galvanized steel pipes. The tubular profile of the electrode is more resistant to the heavy loads the pavement will withstand. To avoid any physical contact between the electrodes and the base layer, and to prevent any displacement during the paving process, they were fixed to the base layer with 1 cm plastic spacers, as shown in (Fig. 3a).

Fig. 3.

ECAP base installation: (a) 1-inch galvanized steel electrodes; (b) Fished wires inside the polyethylene pipes; and, (c) Geo-membrane layer.

Wires

Because of the high temperature and roughness of the paving procedure, fire-proof coated 6 mm aluminum wires were responsible for transmitting the electrical current from the transformer to the electrodes. The longest wire used for the farthest electrode from the transformer was 8 m long and can withstand up to 50 amps of electrical current. Wires were threaded through a polyethylene pipe between the transformer and electrodes for protection, as shown in (Fig. 3b).

Geo-membrane layer

It is an insulated sheet made from 98% polyethylene resin and 2% CB. These sheets are typically used in agricultural reservoirs to prevent water loss and to waterproof the reservoir. Due to their thermal and electrical isolation properties, one of the samples (Fig. 3c) was used to isolate the electrically conductive asphalt layer from the base layer for three reasons:

• To Prevent water penetration from the base layer during the wet season due to high underground water levels;

• To retain the generated heat inside the layer and enhance the thermal efficiency of ECAP; and,

• To ensure electrical isolation of the ECAP and base layer, eliminating electricity loss.

Methodology

Methodology includes three main parts: first, the design overview; second, the field operation steps; and third, the testing program.

Design overview

The design of the system became further complicated by the fact that the asphalt material behaves as an insulator of both electrical current and heat. To overcome this challenge, the binder was prepared in two varieties. The first is a virgin PG 58 − 22 binder, and the second is CBMB, which was employed as a semi-conductive binder, coupled with RMFs from waste tires.

There are two electron transition mechanisms between electrodes through the ECAP: first, when RMFs are in physical contact, and second, when RMFs are at a close proximity. A high electrical potential difference is required for electron transition at close proximity and without physical contact (electron tunneling phenomena⁴⁷. High voltage can damage the fiber-shaped path of highly conductive material. Particles that are not physically connected or not close enough to the connection path, will not accompany the electric path and will only act as thermal conductors, transferring heat from the bottom of the layer, where the electrodes and electric path are placed, to the surface – allowing the surface layer temperature to rise faster.

Four samples with a width of 70 cm, a length of 400 cm, and a thickness of 7.5 cm were considered for construction at a full-scale pavement testing site. Six embedded electrodes with distances of 50, 75, and 100 cm were implanted within the samples. To prevent electrode movement during the paving operation and to avoid physical contact between the electrodes and the ground, the electrodes were secured to the base concrete layer using plastic spacers. For one of the samples (i.e., the 9% RMF sample), a geo-membrane layer was used to evaluate the effects of electrical and thermal isolation of the conductive layer from the ground. Two polyethylene pipes were fixed to the concrete base layer to protect the wires. One wire was used for each electrode to make the electrical experiments more flexible. A 0-220-Volt transformer was also prepared to provide a wide range of voltage to the system. Figure 4 shows an overview of the implemented ECAP system.

Fig. 4.

An overview of implemented ECAP system.

Field operation steps

Electrodes and polyethylene pipes fixation

A 5 cm thick concrete layer was built in a 5 m × 6 m area to create a surface for the electrodes and polyethylene pipes to be attached. 8 mm plastic drywall anchors were used in the drilled holes. The electrodes and polyethylene pipes were then fixed to the concrete base layer.

Wiring and connections

Wires were threaded inside the polyethylene pipes for added protection. In order to provide an efficient connection area and the proper strength for connecting the wires to the electrodes, a screwed 2.54 cm (1-inch) clamp was used. To prepare the same test condition between samples, ring-type cable lugs were used for the wire-transformer connection.

Paving

On the 22nd of May 2022, the surface was first cleaned with a blower, and then the surface of the electrodes was polished. A wet sponge was used to clean the surface of the electrodes to remove any possible oil or dust. Electrically conductive asphalt mixtures were prepared individually at the asphalt plant. RMFs were added to the mixture during the final step of production, while the hot aggregates were mixed with the binder and blended adequately with the asphalt mixture. Although the promising effect of Carboxymethyl cellulose (CMC) on the dispersion of fiber-shaped materials at specific percentages has been mentioned in a few studies^48,49, other researchers argue that CMC negatively impacts electrical conductivity and power density⁵⁰. Therefore, in this research, it was decided not to use CMC as metal fiber dispersant. As a result, poor distribution of the RMFs and clustering was expected and was evident in the asphalt plant while mixing procedure and on-site before paving as shown in (Fig. 5). The ECAP samples were compacted in two steps (every 3.5 cm thickness) using a suitable compactor. Every sample weighed 520 kg weight, hence, according to standard asphalt density, compaction continued until the sample reached the standard volume (7.5 cm average thickness). Figure 6 shows the final ECAP samples constructed on-site.

Fig. 5.

Poor distribution of the RMFs and clustering in the ECAP field operation.

Fig. 6.

Final ECAP samples.

Coring

After completing the field test, twelve cores measuring 7.62 cm (3-inch) in diameter were extracted from four ECAP samples – three cores per sample – and transferred to the laboratory, as shown in (Figs. 6 and 7). In all samples, cores were taken between grids e - f (as shown in Fig. 4), a segment with a 100 cm electrode spacing, in order to minimize system damage and preserve the setup for further experiments.

Fig. 7.

Core samples for laboratory testing.

Testing program

Testing site weather information

Summers at the site location are warm, while winters are extremely cold, snowy, and partly cloudy. During the year, the air temperature typically ranges from − 6 °C to 32 °C, with temperatures rarely falling below − 12 °C or rising above 34 °C. As it can be seen in Fig. 8, the snowy period of the year occurs for 3.6 months, from November 18 to March 6, with a sliding 31-day snowfall of at least 2.54 cm (1-inch). January has the most snow at the operation location, with an average snowfall of 7 cm (2.76-inches)⁵¹.

Fig. 8.

Average monthly snowfall at the site location⁵¹.

Field tests were conducted to measure the conductivity of ECAP a few days after the paving operation was completed. The air temperature was 32 °C, while the pavement surface temperature was measured at 37 °C. Surface temperature was measured using an infrared thermometer. It should be noted that, due to the high temperatures of the air and pavement surface, the operational deicing performance under snowy and icy conditions was re-evaluated during the following winter.

Field testing

Six rows of electrodes were installed (labeled a-f). As previously shown in Fig. 4, the distances between grids a - b, b - c, c - d, d - e, and e - f are 50, 50, 75, 75 and 100 cm, respectively. Due to the independent wiring of the electrodes, tests could be conducted either between two individual electrodes or across the entire sample. Any test between two electrodes indicates the electrical resistivity of the mixture. The tests were performed across varying electrode spacings, using an AC voltage range of 0–220 V. Current between the positive and negative poles was recorded at 30-second intervals, as illustrated in (Fig. 9). According to Ohm’s law, the electrical resistivity of each sample at different electrode distances was calculated using Eq. 1, where U (volts) is the applied voltage, I (amperes) represents the electric current and R (ohms) denotes the electrical resistance of the material. Because Ohm’s law is valid only under constant temperature and stable physical conditions, calculations were limited to the initial measurement phase, prior to any significant temperature rise or alteration in the sample’s physical properties.

1

Fig. 9.

Electrical test of ECAP samples.

Testing of cores

In the laboratory, after freezing the cores to -20 °C, the same transformer was used for core testing. Two 5 cm × 5 cm aluminum foils were used as electrodes and placed on opposite sides of each cored sample. A digital thermal probe was drilled 2 cm inside the surface of the core, and the voltage was applied gently until a stable current was established. Every 30 s, the current and the corresponding temperature were recorded. The experiments were continued until the surface temperature reached to 0 °C. This was due to the fact that high temperatures can damage the conductive path inside the sample and also accelerate binder aging.

Skid resistance test

One of the most important indices regarding driving safety is the pavement skid resistance index. The British pendulum number (BPN) is an index that shows the friction between the tire and the pavement. In this study, a BM-III British pendulum tester was employed to obtain the BPN values for four ECAP samples and one reference sample. The BPN was measured three times for each ECAP sample under both dry and wet conditions between grids c and d as shown in (Fig. 10). The average BPN for each location was calculated as the BPN value under wet and dry conditions. All experiments were conducted in the morning at a consistent ambient temperature of 20 °C.

Fig. 10.

Skid resistance experiment with British pendulum tester on ECAP samples.

Safety considerations

An earth wire was connected from the transformer to the grounding system of the nearest building to discharge any probable excess electricity on the body of the transformer. In laboratory experiments, the same action was taken into consideration.

Results and discussion

Field experiments

Figures 11a,c show the electrical resistivity of each sample at electrode distances of 50, 75, and 100 cm, respectively. For 50 and 75 cm electrode distances, the results are the average performance of two successive similar segments, while only one result was obtained for the 100 cm electrode distance, as previously shown in (Fig. 4). Electrical experiments between electrodes at a distance of 50 cm reveal that 6% M sample, i.e., the sample containing 6% of RMF and CBMB, performed best, followed by the 9% (the sample containing 9% of RMF and geo-membrane layer under the ECAP layer) and the 6% (the sample containing only 6% of RMF) samples. The sample with the worst performance was 3%, which is the sample mixed only with 3% of RMF (Fig. 11a). The reason for the poor performance of 3% sample is that the RMF content in this sample is not sufficient to make a channel of RMFs in physical contact in order to establish a path through the mixture. These results show that the percolation threshold zone is between 3% and 6% of RMF content, where the material becomes semi-conductive^46,51. For electrode distances of 75 cm, the same performance order was obtained (Fig. 11b). Due to the considerable distance between the electrodes of grids e - f (100 cm), only the 6% M sample functioned, as shown in Fig. 11c).

Fig. 11.

ECAP measured electrical current in different applied voltages at the same time period of 5 min and several electrodes’ distances of: (a) 50 cm; (b) 75 cm; and, (c) 100 cm.

Statistical analysis was performed on the field data, and the results are presented in Table 5 for non-zero electrical current values. A two-way analysis of variance (ANOVA) revealed that electrode distance had a significant effect on current magnitude (p-value = 0.003), with 50 cm producing consistently higher currents than 100 cm. Sample type showed a marginally significant effect (p-value = 0.07), with the 6% M and 9% samples demonstrating superior performance compared to other samples. Variability between replicates was generally low, as indicated by coefficients of variation (CV) below 20% for most conditions. Similar to CV, the standard deviation (SD) between replicates was also low, indicating good repeatability.

Table 5.

Statistical analysis for non-zero electrical current values for the ECAP field samples.

Distance (cm)	Time (s)	Electrical current (A)
		Sample 6% M				Sample 6%				Sample 9%
		Mean	SD	Mean ± SD	CV (%)	Mean	SD	Mean ± SD	CV (%)	Mean	SD	Mean ± SD	CV (%)
50	150	2.0	0.4	[1.6, 2.4]	21.0	-	-	-	-	-	-	-	-
	180	15.4	2.4	[13.0, 17.8]	15.6	-	-	-	-	24.4	2.3	[22.1, 26.7]	9.3
	210	24.5	1.0	[23.5, 25.5]	4.0	3.5	0.6	[2.9, 4.1]	16.3	-	-	-	-
	240	-	-	-	-	23.1	0.6	[22.5, 23.7]	2.5	-	-	-	-
75	90	5.8	0.4	[5.4, 6.2]	7.2	-	-	-	-	-	-	-	-
	120	23.5	1.0	[22.5, 24.5]	4.2	-	-	-	-	-	-	-	-
	180	-	-	-	-	-	-	-	-	22.1	4.5	[17.6, 26.6]	20.5
	240	-	-	-	-	24.6	1.0	[23.6, 25.6]	4.0	-	-	-	-
100	240	24.5	0.9	[23.7, 25.4]	3.5	-	-	-	-	-	-	-	-

During testing, some segments of field samples failed after only a few seconds of activity, due to the fact that the electrical current was disconnected. A sound from samples accompanied this issue during the field electrical experiment. When the electrical current meets the sporadic fibers’ section, the RMFs path between the electrodes is disrupted. This is schematically shown in Fig. 12. Because of the low area of the electrical path in that region, high applied voltage can damage the electrical path between electrodes in a specific place, and a localized failure mechanism occurs. In other words, system capacity depends on the smallest electrical path area through the ECAP. Each failure causes irreparable damage to the system. This means that after a specific number of failures, system will not activate anymore. As a result, applied voltage should be carefully monitored to avoid such failures.

Fig. 12.

Schematic mechanism of electric path disruption (portion failure mechanism).

As shown in this figure, there are three types of fiber diffusion in the conductive asphalt mixture. The first category includes physically connected fibers that make a conductive network in which the electrical resistivity of RMFs generates heat from the electrical current that flows through the electrical path. The second group of RMFs is in close proximity. This group of RMFs requires a higher voltage to establish an electrical current. The last part fibers are independent of the electrical path and only contribute to the transfer of heat generated by the network to other parts of the conductive asphalt mixture. Since the asphalt mixture is an isolated composite, it cuts off the conductive network of conductive fibers between two electrodes at some sections. Therefore, it is not possible to establish an electrical current between two electrodes with a specific applied voltage through the RMFs in the conductive network. Finally, a higher applied voltage is required for electron hopping at short distances between the fibers (electron tunneling phenomena as shown in Fig. 13)⁴⁷. Increasing applied voltage, combined with the low electrical resistivity of fibers, results in network failure. The electrical network of the ECAP is irreparably damaged when a portion of the conductive network is burned due to high voltage application.

Fig. 13.

Electrical conductive network inside the ECAP: (a) Physically connected fibers; and, (b) Electron tunneling phenomena^47,52.

To elaborate further, two 6% M core samples were cut and processed with image processing tools after the laboratory experiments. Results demonstrate clustering and different dispersion rates in two sections. Figure 14a–d show different RMF contents at the constant area of 6% M core samples. According to the image analysis, sample 6% M-1 contains at least 14% of RMFs on its section (clustering), while sample 6% M-2 consists of only 2.6% of RMFs (poor dispersion), which could potentially fail.

Fig. 14.

Image processing of RMF dispersion in different sections of field samples: (a) 6% M-1 sample core; (b) 6% M-1 image processing; (c) 6% M-2 sample core; and, (d) 6% M-2 image processing.

In winter 2023, after the first snow event, samples were evaluated for the deicing capability of the ECAP system. Snow began at 23:30 on January 4th and continued until 09:30 the following day, causing 8 cm of snow to accumulate on the pavements. The weather was − 1 °C during the experiment. The 3% and 6% samples failed after a few seconds of activation and were not able to melt the snow. However, due to the lower temperature in comparison to the summer season when electro-thermal experiments were conducted (around 35 °C colder), the 6% M and 9% samples began heating for 5 and 6 min, respectively, longer durations before failure. High electrical current due to the high conductivity of RMF led to a failure in one segment (grids e - d) of the 9% sample. For this reason, the system was turned off after 5 min to avoid further failure in the electrical network. After three cycles of 5-minute activation (with controlled voltage to prevent electrical network damage) followed by 10-minute deactivation periods – totally 45 min, the accumulated snow melted in the 6% M and 9% samples. Figure 15 illustrates the performance of the samples in the winter snow event.

Fig. 15.

Deicing capability of 6% M and 9% ECAP samples.

Evaluation of the cores in freezing conditions

As mentioned earlier, to better simulate the ECAP deicing conditions, 7.62 cm (3-inch) cored samples were frozen down to -20 °C, and tested using the same transformer. Because of the small size of the cores, the system was activated at very low voltage levels. The 6% M, 6%, and 9% cored samples started to pass electric current at 2, 4 and 3 Volts, respectively and then the applied voltage was kept constant. All 3% cores showed the same behavior as the field samples and failed after requiring a high voltage application because of low density of RMFs inside them. These results verify the field observations and show that the 3% field samples and core specimens did not meet the percolation threshold zone. Figure 16 shows the results of the electrical current and thermal efficiency of the cores. Note that the temperature rise in the 3% cores was due to the room temperature.

Fig. 16.

Electrical current and temperature rise of the ECAP cored samples: (a) Temperature per time (s); and, (b) Electrical current per time in constant applied voltage.

Descriptive statistics including mean, standard deviation, and confidence intervals were calculated for each sample, as shown in Tables 6 and 7. Error bars in Fig. 16 represent ± SD, demonstrating the variability across replicates. The ANOVA revealed statistically significant differences in temperature and current profiles across samples (p-value < 0.05). In addition, statistical significance was assessed using two-tailed t-tests (α = 0.05), confirming that differences between samples (e.g., 6% vs. 9% at 120 s, p-value < 0.01) were robust. Confidence intervals (95%) for temperature and current further validated the precision and repeatability of the measurements. These results demonstrate high reliability and statistical robustness in the thermal and electrical behavior of the samples.

Table 6.

Statistical analysis for temperature values for the ECAP core samples.

Time (s)	Temperature (°C)
	Sample 6% M			Sample 3%			Sample 6%			Sample 9%
	Mean	SD	95% CI	Mean	SD	95% CI	Mean	SD	95% CI	Mean	SD	95% CI
0	-20	0	0	-20	0	0	-20	0	0	-20	0	0
30	-16	1.2	1.4	-20	0.2	0.2	-18.5	1.2	1.4	-17	1.2	1.4
60	-10	1.2	1.4	-20	0.2	0.2	-14.5	1.2	1.4	-12	0.6	0.7
90	-5	1.3	1.5	-19.5	0.5	0.6	-10	1.2	1.4	-8	1.2	1.4
120	-1	1.4	1.6	-19.5	0.5	0.5	-5	0.8	0.9	-6.5	1.2	1.4
150	2	1.1	1.3	-19	0.6	0.7	-3	1.2	1.4	-3	0.6	0.7
180	3.5	0.7	0.8	-19	1.2	1.4	-1	1.4	1.6	1	0.9	1.0
210	4	1.1	1.3	-18.5	1.5	1.6	1	0.9	1.0	-2	1.2	1.4
240	3.5	1.1	1.2	-18	1.3	1.5	-1	1.4	1.6
270	1.5	1.1	1.3	-17.5	1.3	1.5
300	-3	1.1	1.2

Table 7.

Statistical analysis for electrical current values for the ECAP core samples.

Time (s)	Electrical Current (A)
	Sample 6% M			Sample 3%			Sample 6%			Sample 9%
	Mean	SD	95% CI	Mean	SD	95% CI	Mean	SD	95% CI	Mean	SD	95% CI
0	2.5	1.5	1.7	14.0	2.1	2.4	5.5	1.7	2.0	6	1.7	1.9
30	4.5	0.7	0.7				6.5	0.8	0.9	6	1.0	1.1
60	4.0	1.8	2.0				6.1	0.6	0.6	6	0.5	0.6
90	3.9	0.4	0.4				5.2	2.2	2.4	5.5	2.3	2.5
120	3.6	1.3	1.4				0	0.0	0.0	0	0.0	0.0

Electrical current intensity and temperature rise

The use of CBMB showed promising results in both electrical current and temperature rise, as illustrated in (Fig. 16). In the 6% M sample, the electrical current initially increased; however, as the system temperature rose, the operational deicing workability of the system decreased, leading to a gradual decline in electrical current. In the 6% and 9% samples, a greater electrical potential difference was required, resulting in higher electrical current consumption. The high current consumption caused excessive heat generation within the fibers, and the system began to fail after two minutes. It can be inferred that the capacity of the ECAP was 2 Volts for the 6% M cored sample. A higher applied voltage or extended use will lead to irreparable damage to the ECAP system. Regarding the temperature rise, the 6% M, 6% and 9% samples reached 0 °C from − 20 °C at 128, 198 and 176 s, respectively.

Energy consumption

In this study, energy consumption is expressed as power density, defined as the total electrical energy consumed by the ECAP per unit area. To calculate power density, average electrical current and the constant voltage applied to the samples were used. Each ECAP sample was measured through three cores, and the resulting standard deviations were used to generate error bars in the temperature and electrical current plots (Fig. 16). The data showed consistent trends with low variability, indicating reliable and repeatable energy performance across the tested conditions. These statistical measures reinforce the robustness of the energy consumption values reported.

The power densities of the 6% M, 6% and 9% samples were calculated as 1623, 4087 and 3092 W/m², respectively, based on core temperature rise and energy consumption results obtained in the laboratory. Although the power density of the designed ECAP is higher than that of electrically conductive cement concrete (ECON)⁴, ECAP heats the pavement much more rapidly. While the average power density of ECON is 265.1 W/m², its average net temperature rise was only 5 °C over a 2-hour period⁴. Table 8 presents a comparison of average energy consumption between ECON and ECAP systems. The results indicate that the total energy consumption of ECAP for deicing could be less than ECON, due to its significantly shorter activation period. However, it should be noted that the higher power density of the ECAP requires greater voltage application, control instrumentations and safety considerations in comparison to ECON which was implemented in Des Moines Airport⁴.

Table 8.

Comparison of ECON and ECAP average energy consumption.

Deicing pavement type	Additive/s	Average power density (W/m2)	Temperature rise capability	Reference
ECON	Carbon fiber (CF)	265.1	5 °C at 2 h	4
ECAP	6% RMF and 12% CB	1623	20 °C at 128 s	This study
	6% RMF	4087	20 °C at 198 s
	9% RMF	3092	20 °C at 176 s

Overall comparison of samples

Rapid temperature rise and establishment of electrical current under low voltage application were the key parameters used to select the best sample. Fast temperature rise enables efficient deicing, while low activation voltage to have safer electrical system and reduced electrical energy consumption. According to the electrical and thermal experimental comparisons of all ECAP samples, the sample containing 6% RMF with CBMB (sample 6% M) demonstrated the best overall performance.

Skid resistance of the samples

Based on the measured BPN indices for the ECAP and reference samples, the road surface BPN values were calculated using the average of three recorded data points under both wet and dry conditions, as presented in (Table 9). The reference sample and the ECAP sample containing 3% RMF reported the same BPN values under both dry and wet conditions. This index showed higher values for the 6% M, 6%, and 9% samples under dry conditions, with measured values of 62, 63, and 63, respectively. The same trend was observed under wet conditions, with measured BPN values of 52, 52, and 53 for the 9%, 6% M, and 6% samples, respectively. It can also be interpreted that the ECAP sample containing virgin binder recorded slightly higher BPN values than the sample with CBMB, despite having the same RMF content.

Table 9.

BPN values for the ECAP and reference samples in both dry and wet conditions (At 20 °C).

Test condition	Sample ID
	Reference	3%	6% M	6%	9%
Dry	60	60	62	63	63
Wet	51	51	52	53	52

Cost analysis

This section performs a cost analysis of the ECAP and calculates the construction cost per square meter of this pavement. The implementation location of this pavement affects the construction cost. These research estimates have been based exclusively on the costs of the asphalt pavement samples implemented at the full-scale pavement testing site at Urmia University. It should be noted that the sample containing 6% RMFs with CBMB and 50 cm electrode spacing has been selected as the research case for cost analysis because it has the best performance among the tested samples. Table 10 presents the cost of each unit of the components used in the conductive asphalt pavement. Because of the same costs for implementing the base and sub-surfacing in both conventional and conductive asphalt pavements, these components have not been taken into consideration. In addition, this table compares the usage rate and price per unit area of each component for the 7.5 cm layer of conventional and conductive asphalt pavements. The price of conventional asphalt mixture is 35 USD/ton. This price includes transporting the asphalt mixture up to 5 km from the asphalt plant. The results show that the cost of required materials and instruments for construction of the ECAP is 2.26 times that of conventional asphalt pavement. However, in previous studies, conductive asphalt concrete material has been estimated to be 5 times more than conventional asphalt⁵². This is because of the difference in the prices of carbon fiber and RMF. In addition, the cost of installation of this new pavement which includes electrodes fixation and wiring operations, has been estimated to be 1.67 times that of installing conventional pavement in this field study which is close to the value obtained in previous research (1.5 times)^47,53. The installation cost is estimated to be 5 USD/m², which is lower than values reported in previous studies due to differences in sample scale and labor wages across countries. In full figures, the total cost of implementation of electrically conductive asphalt pavement is 2.12 times that of conventional pavement (26.82 and 12.65 USD/m², respectively for conductive and conventional asphalt pavements).

Table 10.

Comparison of required costs of construction per square meter of a 7.5 cm layer of conventional and conductive asphalt pavements.

Component	Unit	Price per unit (USD)	Usage rate for one square meter of 7.5 cm thickness	Price (USD)/m2
				Conductive asphalt pavement	Conventional asphalt pavement
Aggregates and mixing process in asphalt plant (except binder)	ton	35	0.18	6.3	6.3
Binder	kg	0.38	8.82	3.35	3.35
CB	kg	0.37	1.06	0.39	0
RMF	kg	0.37	10.80	3.96
Concrete layer	m3	28.40	0.05	1.42
Electrode (1-inch Galvanized pipe)	m	1.60	2	3.20
Wire (20 Conductor – 600 V)	m	3.20	1	3.20
Total cost of raw material per square meter				21.82	9.65
Field installation cost				5	3
Total cost per square meter				26.82	12.65

The maintenance costs for this pavement cannot be calculated because the pilot sample has not been implemented yet. These costs may include electrode oxidation, failure of a section of the ECAP, and electrical inspections. On the other hand, implementation of this pavement could significantly reduce the damage caused by deicing chemicals, heavy snow machines, and low temperature cracking. Simultaneously, ECAP can prevent blocked access to sensitive facilities, potential crises, and financial losses caused by flight cancelations, delays, as well as help save lives by reducing accidents caused by snowy conditions.

Conclusions and recommendations

The goal of this research was to technically evaluate the electro-thermal performance of various asphalt mixture designs containing RMF and CBMB to implement a full-scale electrically conductive asphalt pavement (ECAP) system. Four 70 cm × 400 cm ECAP samples with 3, 6, and 9% RMF content including PG 58 − 22 and CBMB in a 7.5 cm thickness were constructed. A geo-membrane layer was utilized to evaluate the ECAP performance with electrical and thermal isolation from the base layer in winter. The electrodes, prepared using 60 cm long and 2.54 cm (1-inch) diameter galvanized pipes, were embedded at distances of 50, 75, and 100 cm within the ECAP samples. Afterward, a 0-220-Volt transformer was used for both field and laboratory experiments. The earth wire of transformer was connected to the grounding system of the near building to discharge any excess electricity for safety considerations. Results, findings, and recommendations for future research and investigations are summarized in the following:

• Electrical conductivity comparison of samples containing the same content of RMF (i.e., 6% M and 6% field samples), and corresponding cored samples show the considerable effect of CBMB presence on reducing electrical resistivity of the ECAP. The best performance in temperature rise and power density in both field and cored samples was for ECAP including 6% RMF with CBMB (sample 6% M). Accordingly, CBMB is a promising material for reducing electrical resistivity in the ECAP.

• The high conductivity of RMF resulted in faster heating in cores and consequently low energy consumption. The lower electrical resistivity of RMF, coupled with poor dispersion and RMF cutting mechanism in sporadic cross sections of ECAP during operation, led to the failure of a few field samples and cores. In addition, field core samples’ image processing showed a considerable gap in RMF dispersion rates (2.6 and 14) despite a constant percentage of RMF (6%). As a result, RMF’s cutting mechanism and dispersion simulation are required for further studies.

• The ECAP samples with 6% M (6% RMF with CBMB) and 9% (9% RMF with neat PG 58 − 22 binder), were able to melt 8 cm of snow in 45 min during the first snowfall of the 2023 winter.

• Laboratory electro-thermal experiments on the ECAP field cores showed a power density of 1623 W/m². Higher power density requires more instrumentation and safety considerations compared to ECON implemented in Des Moines Airport with an average power density of 265.1 W/m². However, the total energy consumption of ECAP for deicing could be less than ECON, due to its significantly shorter activation period.

• Regarding skid resistance experiments, ECAP samples containing RMF showed better performance compared to the reference conventional asphalt pavement under both wet and dry conditions.

• The results indicate that the cost of required materials and instruments for construction of the conductive asphalt layer is approximately 2.26 times higher than that of conventional asphalt layer. In addition, the cost of implementing conductive pavement is estimated to be 1.67 times greater than that of installing conventional pavements. Overall, the total construction cost of Electrically Conductive Asphalt Pavement (ECAP) is 2.12 times that of traditional asphalt pavement.

• RMF and CB are both recycled materials from scrap tires. Using these materials in asphalt pavement will considerably mitigate environmental pollution from two aspects: first, by reducing chemical deicers in the environment; second, by reusing the waste material from scrap tires.

Author contributions

E.A. prepared experimental work and collected data. N.S. carried out supervision and methodology. E.A. and N.S. analyzed the data. E.A. prepared figures and tables, and wrote the manuscript text. N.S. edited and reviewed the manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.