Innovative Dual-Function Heated Pavement System Using Hollow Steel Pipe for Sustainable De-Icing

Abstract

Winter road safety is threatened by black ice, while traditional de-icing methods, such as chemical spreading and electrically heated pavement systems, raise concerns about environmental impact and economic costs. This study proposed a hydronic heated pavement system utilizing geothermal energy (HHPS-G)-integrated concrete pavement that ensures environmental sustainability and structural stability. The design utilizes hollow steel pipes as both reinforcement and heat exchange conduits, thereby eliminating the need for separate high-density polyethylene (HDPE) pipes. To enhance upward heat transfer, bottom-ash concrete was introduced as an alternative to conventional insulation, providing thermal insulation and structural strength. A validated numerical model was developed to compare the de-icing and snow-melting performance of different pipe types. The results show that hollow steel pipes reduced the time to reach 0 °C on the concrete pavement surface by 30.86% and improved heat flux by 10.19% compared to HDPE. The depth of pipe installation significantly influenced performance: positioning the pipes near the surface achieved the fastest heating (up to 70.11% faster), while mid-depth placement, recommended for structural integrity, still provided substantial thermal benefits. Variations in insulation thermal conductivity below 1 W/m·K had little effect, whereas replacing the base layer with bottom-ash concrete provided both insulation and strength without the need for separate insulation layers.

1. Introduction

Black ice and thin ice on road surfaces during winter are closely associated with traffic accidents. Even in the absence of snowfall or rainfall, icy patches may form on shaded road sections, underpasses, tunnel entrances and exits, or areas near rivers and lakes [1,2]. When such surface icing is detected in advance and appropriate de-icing or snow removal operations are conducted, winter traffic accidents can be reduced by up to 40% [3,4]. Accordingly, in regions with heavy snowfall or high humidity, conditions that result in elevated dew points, comprehensive Snow and Ice Removal Systems (SIRSs) are often implemented. A widely used traditional SIRS involves spreading calcium chloride on road surfaces [5,6]. However, the application of calcium chloride can lead to the corrosion of underground and bridge structures. In particular, it may cause oxidative hardening of pavement layers and corrosion of internal reinforcement bars, ultimately compromising the durability of the road system [7,8]. In addition, it can infiltrate the ground, leading to soil and groundwater contamination [9,10,11]. Although various control strategies have been explored to reduce the amount of calcium chloride used, such as optimized management and application techniques, these measures do not provide a fundamental solution [12].

As an alternative, electrically heated pavement systems (EHPSs) utilizing electric heating wires have increasingly been adopted in recent years [13,14]. These systems directly generate heat, offering excellent de-icing and snow removal performance. To further enhance this performance, studies have investigated ways to increase the electrical conductivity of concrete pavements through optimized mix designs [15,16,17], some of which have also considered economic efficiency and the use of recyclable materials [18]. However, EHPSs are known to entail high maintenance costs due to fuel consumption and electricity use. Moreover, the large amount of energy required for operation contributes significantly to greenhouse gas emissions over the road’s life cycle [19].

In contrast, hydronic heated pavement systems utilizing geothermal energy (HHPS-G) have been shown to be more environmentally sustainable and significantly reduce energy consumption and CO₂ emissions compared to traditional systems, such as calcium chloride spreading or EHPSs [19]. An HHPS-G involves the installation of closed-loop heat exchange pipes beneath the pavement, through which a heat-transfer fluid circulates. By circulating fluid warmer than the ambient winter temperatures, the system raises the pavement temperature, thereby preventing or removing snow and ice accumulation [20,21,22]. To supply a fluid warmer than the ambient air temperatures, a heat pump is typically employed to inject thermal energy. When coupled with renewable and low-carbon heat sources, the overall energy demand for the system can be substantially reduced. Common approaches include using deep boreholes to exploit relatively stable subsurface temperatures throughout the year [23,24] or, where conditions allow, utilizing naturally warm shallow groundwater [25]. Most notably, an HHPS-G not only enables winter heating of pavements but also provides summer cooling, helping to mitigate urban heat island effects and preventing thermally induced cracking and stress in asphalt or concrete pavements [26,27]. The heat absorbed during summer cooling can be stored using thermal storage technologies, such as bedrock, insulated tanks, or phase change materials, and reused for winter heating [28,29,30].

One of the most well-known real-world applications of the HHPS-G is the system installed by the Oregon Highway Department in Klamath Falls, Oregon, USA. This system has been in operation for nearly 50 years, demonstrating long-term stability and performance [31]. In Switzerland, another HHPS-G was implemented on a bridge deck, where solar heat accumulated in the deck during summer is stored in bedrock and reused for winter heating. This system also exhibited excellent de-icing and snow removal capabilities [32,33]. Additionally, various mock-up-scale experiments have been conducted to evaluate HHPS-G performance under different design conditions, leading to parametric analyses focused on factors such as pipe burial depth, pavement material types, and pipe spacing [34,35,36]. From an economic perspective, the HHPS-G has been reported to be more cost-effective than other SIRSs [37]. Its economic performance can be further improved by using pre-heating cycles or staged heating only when necessary rather than continuous operation [38].

Despite their proven durability, environmental benefits, and cost-efficiency, HHPS-Gs have not yet been widely adopted. A major reason is the lack of practical applicability. Most existing studies focus on performance verification or parametric evaluation, with limited attention to implementation challenges, such as pipe installation methods, constructability-optimized burial depths, or maintenance-friendly configurations [39,40]. Another critical limitation is the unresolved issue of structural integrity. Since heat exchange pipes must be embedded within the pavement or base layers, the cross-sectional area of the structural body is reduced, which can weaken the overall load-bearing capacity and strength. Furthermore, placing pipes too close to the surface in pursuit of higher heat transfer efficiency can result in increased localized stress, making the pavement more susceptible to cracking or failure. Although insulation layers are often added between the pipes and the base to enhance upward heat transfer, comprehensive analyses that address both thermal efficiency and structural stability are still lacking. Addressing these challenges is essential for the broader implementation of HHPS-Gs.

In this study, an integrated HHPS-G concrete pavement was proposed to address both structural and practical challenges. Instead of embedding separate heat exchange pipes, the conventional steel reinforcement was replaced with hollow steel bars that serve a dual function as structural reinforcement and heat exchangers. This approach enables seamless integration of HHPS-G technology without compromising the structural inertia of the pavement. Moreover, the use of steel, which has higher thermal conductivity than conventional high-density polyethylene (HDPE) pipes, is expected to enhance de-icing and snow removal efficiency. Additionally, a thermally insulating layer composed of bottom-ash-blended concrete was introduced to improve upward heat transfer while maintaining structural strength. The results of field tests, laboratory experiments, and numerical simulations were analyzed to evaluate the thermal performance of the proposed HHPS-G-integrated pavement. Rather than focusing on the absolute values of thermal performance metrics, this study was designed to reveal practical implications by examining how various design parameters influence overall heating efficiency and thermal behavior. Based on these analyses, the feasibility and practical applicability of the system were comprehensively assessed.

2. Design of HHPS-G-Integrated Concrete Pavement

2.1. Structural Configuration

The structural configuration of the HHPS-G-integrated concrete pavement is illustrated in Figure 1a. In this system, conventional steel reinforcement is replaced with hollow steel pipes that preserve structural performance while simultaneously functioning as heat exchange conduits. This design eliminates the need for separate HDPE pipes, thereby minimizing potential reductions in structural integrity. Furthermore, as shown in Figure 1b, the system is designed for easy assembly and disassembly, enhancing constructability and maintainability. Figure 1 is intended as a conceptual illustration to convey the overall layout and functional concept of the HHPS-G system. Specific design parameters, such as pipe spacing, burial depth, and concrete cover thickness, are not provided in the schematic, as these are determined based on structural performance requirements and construction constraints.

The hollow steel pipe used as a reinforcement substitute was selected as STG800, which possesses mechanical properties comparable to those of conventional deformed steel bars, such as SD350. STG800 denotes a high-strength steel tube for ground reinforcement (STG) with a nominal yield strength of 800 MPa in accordance with KS D 3872; in this study it is used as a hollow, deformed bar [41]. A comparison of the mechanical properties of the SD350 rebar and the STG800 hollow steel pipe is summarized in

Table 1. Comparison of mechanical properties between conventional rebar and hollow steel pipe.

Type	Yield Strength	Ultimate Strength	Elastic Modulus	Elongation
Deformed rebar (SD350)	280–690 MPa	420–790 MPa	≥200,000 MPa	10–18%
Hollow steel pipe (STG800)	≥800 MPa	≥860 MPa	≥200,000 MPa	≥10%

. The values reported for STG800 summarize test results for a standardized material manufactured in Republic of Korea, in compliance with the relevant Korean Industrial Standards (KS) D 3872 [41]. The mechanical properties for the conventional rebar are taken from the nominal strengths specified in ASTM A615/A615M-22 [42]. Because STG800 is a standardized commodity supplied by multiple mills, a specific manufacturer is not identified here. Based on this comparison, the hollow steel pipe used in this study is considered a viable replacement for traditional steel reinforcement.

To evaluate the bond performance between the hollow steel pipes and concrete, a series of pullout tests was conducted, as reported in a previous study [43]. As shown in Figure 2, a 3000 mm long deformed rebar and a hollow steel pipe were embedded into concrete specimens, and pullout tests were performed using a custom-designed jig to measure the pullout force, which reflects the bond strength. The results demonstrate that STG800 hollow steel pipes with a diameter of 31.8 mm or more exhibited bond strength comparable to that of conventional deformed rebars. These findings preliminarily validate the feasibility of using hollow steel pipes as dual-purpose components (i.e., reinforcement and heat exchangers) in the proposed HHPS-G-integrated pavement system. However, it should be noted that the bond test results alone cannot fully represent the global structural performance of a reinforced concrete pavement. Although the material-level mechanical properties (e.g., bond strength, elastic modulus, and tensile strength) of the hollow steel pipes were verified, system-level behaviors, such as flexural capacity, crack control, and fatigue resistance under cyclic thermal and traffic loads, remain to be comprehensively evaluated. Further research involving full-scale slab tests is required to confirm the structural reliability of the system under real-world operating conditions.

2.2. Formation of Insulating Layer Using Bottom-Ash-Based Concrete

To improve the heat exchange efficiency of heated pavement systems (HPSs), it is essential to promote upward-directed heat transfer. Reducing heat loss toward the base layer can significantly enhance the de-icing and snow-melting performance of HPSs. Accordingly, the presence of an insulating layer between the pavement and the base plays a critical role in directing heat toward the surface. However, thermal insulation materials commonly used in buildings are not suitable for road applications, as they lack the mechanical strength required to withstand traffic loads.

In this study, an alternative insulating layer was considered by replacing fine aggregates in concrete with bottom ash to create a thermally insulating concrete layer beneath the HHPS-G-integrated pavement [44,45]. Aggarwal et al. [46] prepared five concrete mixtures by replacing fine aggregates with bottom ash at replacement rates ranging from 0% to 50%, while maintaining consistent amounts of coarse aggregate, cement, and water. Their results showed that increasing the bottom-ash replacement ratio led to a reduction in compressive strength. Following the same five mix designs proposed by Aggarwal et al. [46], concrete specimens were prepared in this study, and their thermal conductivity was measured. The measurements were conducted using a QTM-500 device (Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan), which utilizes the transient hot wire method. The replacement ratios and the corresponding thermal conductivity values are summarized in

Table 2. Thermal conductivity of concrete according to bottom-ash replacement ratio.

Measured Property	Bottom-Ash Replacement Ratio for Fine Aggregate (%)
	0%	20%	30%	40%	50%
Thermal conductivity (W/m·K)	3.21	3.30	2.73	2.41	2.12

.

The results indicate that thermal conductivity decreased as the bottom-ash replacement ratio increased. In particular, the specimen with a 50% replacement ratio exhibited a thermal conductivity approximately 34% lower than that of the control specimen without bottom ash. These findings suggest that substituting fine aggregate with bottom ash can effectively improve the thermal insulation performance of the HHPS-G. However, previous studies have shown that a thermal conductivity of 2.12 W/m·K is still relatively high for use as an effective insulation layer [47].

To further enhance insulation performance, additional concrete specimens were produced by replacing 100% of the fine aggregate with bottom ash and varying the porosity of the mix. The thermal conductivity and compressive strength of these specimens were then measured, as illustrated in Figure 3. The mix proportions for the three porous concrete specimens are provided in

Table 3. Mix design of porous concrete specimens with 100% bottom-ash replacement.

Mix Design	Specimen #1	Specimen #2	Specimen #3
w/c	0.3	0.3	0.3
Unit Water Content (kg)	127	113	109
Unit Cement Content (kg)	423	375	364
Unit Coarse Aggregate Content (kg)	440	503	517
Admixture (kg)	3.18	2.83	2.73
Measured Porosity (%)	15.4	19.4	27.1

, and the corresponding thermal and mechanical test results are summarized in

Table 4. Measured thermal and mechanical properties of porous bottom-ash concrete.

Specimen	Thermal Conductivity (W/m·K)				Compressive Strength (MPa)
	1st Test	2nd Test	3rd Test	Average
#1	0.721	0.718	0.719	0.72	17.5
#2	0.592	0.607	0.597	0.60	11.1
#3	0.372	0.353	0.351	0.36	9.6

.

The results show that even with identical bottom-ash content, increasing porosity led to a decrease in thermal conductivity, thereby enhancing thermal insulation. However, this also resulted in a reduction in compressive strength. These findings highlight the importance of optimizing the mix design to achieve a proper balance between thermal insulation performance and mechanical stability.

3. Experimental Validation of Heat Flux in HHPS-G Concrete Pavement

In this study, the potential de-icing and snow-melting performance of the HHPS-G-integrated concrete pavement was evaluated based on previously conducted indirect heat exchange experiments using STG800 hollow steel pipes. Lee et al. (2023) [48] installed a 123 m long STG800 hollow steel pipe in a spiral configuration with 400 mm spacing within a 50 m² concrete slab. A working fluid was circulated through the pipe to facilitate heat exchange [48]. A schematic illustration of the pipe installation is shown in Figure 4.

The working fluid had a flow rate of 7.4 L/min, with the inlet temperature maintained at 25 °C. The system was operated for 8 h per day, followed by a 16 h resting period, over a total of 7 days. During the experiment, the ambient air temperature averaged 11 °C, and the initial temperature of the concrete slab was 16.6 °C. Based on the measured data, the heat flux was calculated using Equation (1):

$$Q={\displaystyle \frac{\dot{m}·C·∆T}{A_{pipe}}}$$

(1)

where Q is the heat flux (W/m²), $\dot{m}·$ is the mass flow rate of the working fluid (kg/s), C is the specific heat capacity of the working fluid (J/kg·K), ∆T is the temperature difference between the inlet and outlet fluid (K), and A_pipe is the heat exchange surface area of the pipe.

The average temperature difference during operation was 4.54 °C, resulting in a calculated Q of 190.45 W/m². After 7 days of heating, the surface temperature of the concrete slab rose to 18.8 °C, indicating an overall temperature increase of approximately 2.2 °C due to continuous heating. According to previous studies on HPSs using HDPE pipes, typical Q values range from 100 to 180 W/m² [33,36,49,50]. In comparison, the HHPS-G proposed in this study, employing hollow steel pipes as both structural reinforcement and heat exchangers, demonstrated enhanced thermal performance for de-icing and snow-melting applications.

However, it is important to note that the referenced experiment was conducted in an indoor environment and did not account for external cold air exposure. Moreover, the effects of various influencing parameters were not explored. To address these limitations, a numerical model was developed and validated using experimental data. This model was subsequently employed to conduct a comprehensive performance analysis of the HHPS-G-integrated concrete pavement under diverse operating conditions.

4. Numerical Analysis for Heat Transfer Performance in HHPS-G-Integrated Pavements

4.1. Development and Validation of Numerical Model

A numerical model was developed using the commercial 3D finite element software, COMSOL Multiphysics (Ver. 6.3), to simulate the heat transfer mechanisms within the HHPS-G-integrated pavement. The model incorporates coupled computational fluid dynamics (CFD) and heat transfer analysis to represent heat transfer between the major components of the test structure reported by Lee et al. (2023) [48], including the pipe walls, concrete slab, and surrounding soil. The heat conduction transfer process within the solid was governed by the first law of thermodynamics, as described in Equation (2) [51,52]:

$${\mathsf{\rho }}_s{C}_p{\displaystyle \frac{\partial T}{\partial t}}=\nabla ·(k_s\nabla \mathrm{T})$$

(2)

where ρ_s is the solid density (kg/m³), C_p is the heat capacity at constant pressure (J/kg∙K), T is the absolute temperature (K), t is time (s), and k_s is the thermal conductivity of the solid (W/m∙K).

Given that the diameter of the heat exchange pipe was small relative to the entire model domain, the pipe was modeled as a 1D line element using the Pipe Flow Module in COMSOL Multiphysics. This module utilizes built-in formulations for Darcy friction, supporting both laminar and turbulent flow conditions, as well as Newtonian and non-Newtonian fluids, across a range of pipe geometries and roughness values. The governing equations for energy and mass conservation in the pipe domain are presented in Equation (3) [47,52,53]:

$$\mathsf{\rho }\mathrm{A}{C}_p{\displaystyle \frac{\partial T}{\partial t}}+\mathsf{\rho }\mathrm{A}{C}_{p}u\nabla T=\nabla Ak\nabla T+f_D{\displaystyle \frac{\rho A}{2d_h}}{\left|u\right|}^3+Q_{wall}$$

(3)

where ρ is the fluid density (kg/m³), A is the pipe cross-section area (m²), k is the thermal conductivity of the working fluid (W/m∙K), f_D is the Darcy friction factor, d_h is the hydraulic diameter (m), and Q_wall is the heat exchange term with the surrounding media (W/m).

The pipe-wall heat-transfer term, Q_wall, is further defined in Equation (4) [47]:

$$Q_{wall}={\left(hZ\right)}_{eff}(T_{ext}-T)$$

(4)

where ${\left(hZ\right)}_{eff}$ is the effective value of the heat-transfer coefficient times the wall perimeter (W/m∙K), and T_ext is the external absolute temperature (K).

For a circular pipe cross-section, the effective value of the heat-transfer coefficient times the wall perimeter, ${\left(hZ\right)}_{eff}$, is automatically calculated by Equation (5) in COMSOL Multiphysics, considering the fluid velocity, the material properties of the pipe, and the flow regime of the working fluid [52]:

$${\left(hZ\right)}_{eff}={\displaystyle \frac{2\pi }{\frac{1}{r_0{h}_{int}}+\frac{1}{r_N{h}_{ext}}+{\displaystyle \sum _{n=1}^N}\left(\frac{ln\left(\frac{r_n}{r_{n-1}}\right)}{k_{w,n}}\right)}}$$

(5)

where $r_n$ is the outer radius of the nth pipe wall (m), $k_{w,n}$ is the thermal conductivity of the nth pipe wall (W/mK), and $h_{int}$ and $h_{ext}$ are the film heat-transfer coefficient son the inside and outside of the pipe, respectively (W/m²K).

The convective heat transfer between ambient air and materials was modeled using the Heat Flux Interface in COMSOL Multiphysics, assuming natural convection above a horizontal surface. Surface heat exchange was modeled using a convective boundary only; no separate long-wave radiation boundary was imposed. This choice preserves consistency with the indoor validation test (at approximately 11 °C). As all parametric cases share identical boundary conditions, omitting radiation affects the absolute values but not the comparative trends emphasized in this study. The convective heat flux is expressed in Equation (6):

$$q_c=h_c(T_{amb}-T)$$

(6)

where q_c is the convective heat flux (W/m²), h_c is the heat-transfer coefficient of concrete (W/m²K), and T_amb is the ambient air temperature (K).

The model geometry was constructed to replicate the actual test structure used in the field experiment. The geometry and mesh configuration are shown in Figure 5. The material properties used in the model were obtained from laboratory tests reported in a previous study, as summarized in

Table 5. Thermal properties of materials used in numerical model.

Materials	Density (kg/m3)	Specific Heat Capacity (J/kg·K)	Thermal Conductivity (W/m·K)
Concrete	2300	880	2.0
Ground	2400	2300	1.3
Fluid	990	4186	0.6

[47].

The numerical model was validated using the time-dependent heating test results reported by Lee et al. (2023) [48]. The simulation conditions were configured to replicate the field test setup, including time-varying inlet fluid temperature profiles and ambient air temperature applied to the exposed pavement and ground surfaces. Initial temperature distributions within each domain were assigned based on the measurements recorded at the start of the test. The outlet fluid temperatures predicted by the model were then compared with the experimental observations. The numerical model employed tetrahedral elements, which are suitable for three-dimensional analysis, with the smallest elements positioned around the heat exchange pipes, where the temperature gradients were most pronounced (refer to Figure 5b). To determine an element size that does not affect the numerical results, a series of mesh independence tests was conducted. In this test, the outlet temperature at a certain time was compared while the side length of the minimum element was gradually reduced, and the results are presented in Figure 6. The outlet temperature was found to converge to 10.2 °C when the side length of the minimum element was smaller than 0.205 m. Considering computational effort, the side length of the minimum element was determined to be 205 mm, resulting in a total of approximately 80,000 tetrahedral elements in the mesh. In addition, the average mesh quality was 0.68, and the minimum element quality was 0.02, exceeding the commonly accepted lower limit of 0.01, thereby ensuring numerical stability and accuracy [52].

The simulation results exhibit strong agreement with the measured data, with an average error of 1.50% and a root mean square error (RMSE) of 0.43 °C, as shown in Figure 7. These findings confirm that the developed numerical model can reliably reproduce the thermal behavior of HHPS-Gs under actual operating conditions. Therefore, the model is considered highly suitable for conducting parametric studies aimed at optimizing the design and performance of HHPS-G-integrated pavements.

4.2. Parametric Analysis

A parametric analysis was conducted using the validated numerical model to comprehensively evaluate the de-icing and snow-melting performance of the HHPS-G-integrated concrete pavement. For this purpose, a rectangular domain with dimensions was modeled. The vertical structure of the model consisted of a 250 mm thick concrete pavement layer, a 200 mm thick base layer, a 250 mm thick subbase layer, and a 3.0 m thick subgrade layer. In addition, the model assumed the installation of a conventional building insulation material of 70 mm between the concrete pavement and the base layer [54,55,56].

The initial conditions were defined based on the average ambient temperature recorded on the coldest day in South Korea in 2023, which was set to −10 °C [57]. Following previous studies, the initial temperature of each pavement layer was assigned as follows: −10 °C for the concrete pavement, −4 °C for the base layer, 0 °C for the subbase layer, and 5 °C for the subgrade [58,59]. The thermal properties of each material used in the parametric analysis were maintained at the same values as those employed during model validation. Consequently, although the ambient air temperature and initial material temperature in the parametric simulations differed from the validation conditions, changes in material properties due to temperature variations, as well as adjustments to the boundary conditions, were not reflected in the parametric setup. This may potentially affect the reliability of parametric analysis. In particular, prior research has shown that the thermal conductivity of concrete may decrease when exposed to low-temperature environments. For instance, under cryogenic conditions (95 K), the thermal conductivity of concrete mortar can be reduced by nearly half [60].

However, it is important to note that −10 °C, the temperature considered in this study, does not fall within such extreme ranges, and the reduction in thermal conductivity is relatively minor at this level. Furthermore, arbitrarily modifying boundary conditions or material properties for low-temperature conditions, without additional experimental validation, could rather compromise the credibility of the simulation outcomes. Most importantly, the primary goal of this study is not to generalize absolute performance values but rather to investigate relative trends and comparative efficiency across different design configurations. Therefore, it was believed that conducting simulations by altering only the design variable of interest, while maintaining consistency with the validated model conditions, does not significantly compromise the validity of the conclusions.

The de-icing and snow-melting performance was evaluated under the assumption that the system provides sufficient thermal performance if the surface temperature of the concrete pavement reaches or exceeds 0 °C [35,61]. During the simulation, the inlet fluid temperature was maintained at 40 °C throughout a 24 h continuous heating period [62]. Accordingly, the de-icing and snow-melting performance was evaluated based on two key indicators: (1) the time required for the concrete surface temperature to increase from −10 °C to above 0 °C and (2) the heat flux delivered to the pavement during the 24 h heating cycle. For all simulation cases, the flow rate was set to 12 L/min, following previous research [35]. Under these conditions, the parametric analysis investigated the effects of pipe material, pipe geometry, pipe installation depth, and the presence or absence of an insulation layer on the thermal performance of the HHPS-G-integrated pavement.

Furthermore, it is assumed that the working fluid maintains its liquid phase throughout the heating period; hence, the simulation does not consider freezing risks under shutdown or emergency conditions. In practical applications, however, antifreeze additives should be incorporated at concentrations of 20–30%, ensuring a freezing point below −15 °C. These measures are crucial in preventing pipe rupture due to the thermal expansion of ice and in ensuring the long-term reliability of the system. In addition, a proper control strategy must be implemented to maintain minimum circulating temperatures during standby or idle periods, thereby minimizing the risk of freezing even in unforeseen operational interruptions.

4.2.1. Influence of Pipe Material and Geometry

HDPE heat exchange pipes are often installed in a spiral configuration and are generally considered to offer superior constructability [47]. In contrast, hollow steel pipes cannot be easily bent and must be installed in a linear configuration, as they are also intended to serve as structural reinforcement. Accordingly, as shown in Figure 8, a performance comparison was conducted between spiral and linear configurations, under identical conditions, with an equal pipe length of 53 m and a spacing of 400 mm.

The purpose of this comparison was to investigate the influence of pipe geometry on the de-icing and snow-melting efficiency of the pavement system. In addition, the enhancement in thermal performance was quantitatively assessed when conventional HDPE pipes were replaced with hollow steel pipes. This analysis aimed to clarify whether the higher thermal conductivity of steel contributes to improved surface temperature rise and heat-transfer rates. Figure 9 presents the simulation results of surface temperature variations for different combinations of pipe material and geometry, and

Table 6. Performance comparison of HHPS-G according to pipe material and configuration.

Case	Time to Reach 0 °C Surface Temperature (Hours)	Applied Heat Flux (W/m2)
HDPE pipe with linear layout	7.55	849.38
HDPE pipe with spiral layout	7.50	852.26
Hollow steel pipe with linear layout	5.22	935.90
Hollow steel pipe with spiral layout	5.19	940.28

summarizes the corresponding quantitative performance metrics.

The analysis indicates that the de-icing and snow-melting performance did not differ significantly between spiral and linear pipe configurations. Although the spiral layout exhibited slightly better performance in terms of heating rate and heat flux, the difference was negligible. This minor advantage is attributed to the fact that, in a spiral configuration, the inlet fluid circulates along the outermost region first, resulting in marginally enhanced heat transfer. In contrast, the choice of pipe material produced a notable impact. When hollow steel pipes were used instead of HDPE pipes, the time required for the concrete surface temperature to reach 0 °C was reduced by 30.86%, and the overall heat transfer, expressed as heat flux, increased by 10.19%. This improvement is primarily due to the significantly higher thermal conductivity of hollow steel pipes (33.6 W/m·K) compared to HDPE pipes (0.4 W/m·K).

An important observation is that the improvement in heating rate achieved by hollow steel pipes was greater than the corresponding increase in heat flux. A higher heat flux typically indicates a greater drop in outlet temperature, which would require an additional thermal energy input from the heat pump to restore the inlet temperature to 40 °C. In other words, excessive heat flux could increase electrical energy consumption. However, the simulation results demonstrate that the use of hollow steel pipes rapidly increased the concrete pavement temperature, leading to the quick stabilization of heat flux without excessive escalation. Consequently, replacing HDPE pipes with hollow steel pipes in an HHPS-G can provide faster and more effective de-icing and snow-melting performance without imposing significant additional energy demands.

4.2.2. Influence of Pipe Installation Depth

The effect of pipe installation depth on de-icing and snow-melting performance was next investigated. Localized heat transfer through embedded pipes can potentially induce thermal stress and surface cracking in concrete, particularly near the pavement surface. In particular, placing heat exchange pipes too close to the surface may increase the risk of spalling or cracking due to repeated thermal gradients. In this parametric study, however, to isolate and evaluate the thermal performance aspect, the heat exchange pipes were placed at three different depths below the pavement surface, 62.5 mm (top position), 125.0 mm (middle position), and 187.5 mm (bottom position), without considering constructability or structural safety. To evaluate the combined effects of placement depth and pipe material, six simulation cases were conducted using HDPE and hollow steel pipes, as described in Section 4.2.1. Since the previous analysis indicated negligible differences between spiral and linear configurations, only the linear arrangement was considered. Figure 10 shows modeling results with different pipe placement depths. In addition, Figure 11 presents the simulation results, while

Table 7. Performance comparison of HHPS-G according to pipe installation depth and material.

Case	Time to Reach 0 °C Surface Temperature (Hours)	Applied Heat Flux (W/m2)
HDPE pipe at bottom position	20.11	608.55
Hollow steel pipe at bottom position	15.89	653.26
HDPE pipe at middle position	7.50	852.26
Hollow steel pipe at middle position	5.22	935.90
HDPE pipe at top position	2.97	992.14
Hollow steel pipe at top position	1.56	1129.14

summarizes the quantitative comparison of performance metrics for HHPS-Gs based on pipe depth and material.

The analysis reveals that when the pipes were installed at the bottom position, the time required to achieve de-icing and snow-melting was excessively long, rendering this configuration impractical, even when hollow steel pipes were used. When comparing the middle position and top position, the heating rate improved by 60.4% for HDPE pipes and 70.11% for hollow steel pipes, while heat flux increased by 16.41% for HDPE pipes and 20.65% for hollow steel pipes. These findings indicate that positioning the pipes as close to the pavement surface as possible is recommended for achieving rapid and effective thermal performance.

However, this study aims to use hollow steel pipes as substitutes for reinforcement within concrete pavements. According to AASHTO [63] and several references [64,65], reinforcement in continuously reinforced concrete pavement (CRCP) should be positioned near the mid-depth. The CRCP design manual by Roesler et al. [66] further recommends placing reinforcement between the mid-depth and approximately 90 mm below the pavement surface. Furthermore, as previously noted, placing heat exchange pipes too close to the surface increases the risk of surface damage, such as thermal cracking and degradation of material strength, due to localized thermal stress and repeated thermal gradients. Considering practical design guidelines and structural requirements, it is, therefore, recommended that the hollow steel pipes in the proposed HHPS-G-integrated concrete pavement be installed at an intermediate depth.

Another key finding is that installing hollow steel pipes at the middle position significantly improved de-icing and snow-melting performance compared to bottom placement. Moreover, even when installed at mid-depth, hollow steel pipes provided performance improvements sufficient to compensate for the difference compared to HDPE pipes installed at the top position. Although a comprehensive structural analysis of thermally induced stress, cracking, and strength degradation was not conducted, the combination of a preliminary structural evaluation and the improvement in thermal performance suggests that the optimal installation depth for hollow steel pipes in the proposed system lies within the intermediate level of the pavement.

4.2.3. Influence of Insulation Layer and Corresponding Thermal Conductivity

To enhance the de-icing and snow-melting performance of pavement systems, it is desirable to install an insulation layer between the concrete pavement and the base layer to promote upward heat transfer. However, the use of conventional building insulation materials in pavement systems, where structural integrity under traffic loading is critical, can lead to severe structural issues. Therefore, based on the experimental results presented in Section 2.2, this study explored an alternative approach that ensures both structural support and thermal insulation by replacing the base layer with bottom-ash concrete, where fine aggregates are partially replaced with bottom ash. The bottom-ash concrete mixtures proposed in Section 2.2 exhibited varying porosity levels, resulting in thermal conductivities of 0.72 W/m·K, 0.60 W/m·K, and 0.36 W/m·K. A parametric analysis was then conducted to evaluate the impact of these thermal conductivities on de-icing and snow-melting performance while maintaining the hollow steel pipe at mid-depth within the concrete pavement. In the building-insulation case, the 70 mm insulation layer is retained, and the base remains 200 mm of the reference material. In contrast, in the bottom-ash concrete cases, the separate insulation is omitted, and the entire 200 mm base layer is replaced with bottom-ash concrete (with different thermal conductivities of 0.7196, 0.5986, and 0.3589 W/m·K). The results of this analysis are summarized in Figure 12 and

Table 8. Performance comparison of HHPS-G according to insulation material.

Case	Time to Reach 0 °C Surface Temperature (Hours)	Applied Heat Flux (W/m2)
Building insulation material	5.22	935.90
Bottom-ash concrete (0.7196 W/m·K)	4.61	1150.77
Bottom-ash concrete (0.5986 W/m·K)	4.61	1139.24
Bottom-ash concrete (0.3589 W/m·K)	4.61	1106.07

.

The results indicate that since the thermal conductivity of the insulation layers considered in this study was sufficiently low (below 1 W/m·K), variations in conductivity produced only minor differences in de-icing and snow-melting performance. Interestingly, when a conventional building insulation material with extremely low thermal conductivity was applied, a slight delay in surface temperature rise was observed due to heat accumulation near the pipe. However, this effect was negligible in practical terms. These findings suggest that even without installing a separate insulation layer, replacing the base layer with bottom-ash concrete offers an effective solution to ensure both the structural stability and thermal performance of the HHPS-G. Furthermore, based on the results presented in Section 2.2, which demonstrated that increasing porosity reduces thermal conductivity but also decreases compressive strength, the mixture exhibiting a thermal conductivity of 0.72 W/m·K (corresponding to Specimen #1 in Table 3) was identified as the optimal mix for HHPS-G-integrated pavements. Although this mix exhibited a reduced compressive strength of approximately 17.5 MPa due to increased porosity, this value still satisfies the typical strength range required for non-structural base concrete layers [67]. It is important to note that this bottom-ash concrete layer is not intended to serve as a primary structural component but rather as a hybrid functional layer that provides thermal insulation while contributing partial load-distributing capability. However, for heavy-duty applications, such as airfield pavements, industrial roads, or other high-load-bearing scenarios, alternative mixes with enhanced strength or composite layering strategies must be considered to ensure mechanical reliability and long-term durability.

5. Discussion

5.1. Environmental and Life-Cycle Considerations of HHPS-G-Integrated Concrete Pavement

Numerous prior studies have identified HHPS-Gs as environmentally favorable relative to other SIRSs. Calcium chloride-based SIRSs can cause significant soil and groundwater contamination when de-icing agents infiltrate the subsurface [9,10,11] and may promote oxidative hardening of the pavement, accelerating rebar corrosion and increasing the likelihood of structural distress [7,8]. These effects compound over time, driving up operation and maintenance needs (e.g., repairs, rehabilitation, resurfacing, etc.) and incurring additional economic costs and life-cycle carbon emissions associated with increased material production, transportation, and disposal. They also impose social costs through heightened traffic congestion and elevated safety risks. By contrast, EHPSs avoid the environmental and economic issues associated with calcium chloride and can deliver immediate de-icing and snow-melting through high-efficiency heat exchange. However, prior studies report that substantial fuel and electricity use leads to markedly higher maintenance costs and life-cycle greenhouse gas (GHG) emissions. Shen et al. [19] compared four systems (HHPS-G, HHPS-NG, EHPS, and calcium chloride-based SIRS) across life-cycle stages, including power generation, operation, material production, and antifreeze/wastewater treatment. They found that calcium chloride approaches exhibit higher life-cycle energy demand than HPSs owing to the production, transport, and disposal of de-icing chemicals, with this gap widening as snowfall duration increases. Accordingly, in very cold regions, the HPS tends to be more cost-effective. Meanwhile, the EHPS exhibited a sharp increase in GHG emissions during prolonged snowfall due to increased fuel and electricity consumption. In comparison, the HHPS-G could substantially reduce life-cycle energy demand and GHG emissions by leveraging geothermal heat sources, with further reductions possible when coupled with higher-performance heat pumps or thermal-energy storage systems. In short, improving the efficiency of an HHPS-G decreases energy demand and GHG emissions, enabling a more environmentally sustainable system.

In this context, the present study proposes enhancing HHPS-G performance by replacing conventional HDPE heat exchange pipes with hollow steel pipes. Under the same burial depth (mid-depth within a 250 mm pavement), the HDPE case reached 0 °C in 7.50 h, with an applied heat flux of 852.26 W/m², whereas the hollow steel case reached 0 °C in 5.19 h, with an applied heat flux of 940.28 W/m². Assuming an identical coefficient of performance (COP) of the heat pump and shutdown upon reaching 0 °C surface temperature, the cumulative heat input was 6.392 kWh/m² for the HDPE configuration and 4.880 kWh/m² for the hollow steel configuration. This indicates an estimated 23.7% reduction in energy demand and, by extension, a similar proportional decrease in GHG emissions. When including the holding phase above 0 °C, the relative savings may be larger, as the hollow steel configuration exhibits a steeper surface temperature rise and a faster convergence of heat flux. It should be noted that these values are approximations based on observed heat-flux convergence. Therefore, total energy use may vary with control strategy, ambient fluctuations, flow rate, and COP.

From a material-cost perspective, the HHPS-G-integrated pavement also offers an advantage because a separate HDPE pipe is no longer required. The hollow steel pipe simultaneously provides reinforcement and heat exchange. Typical procurement ranges indicate that hollow steel pipes are approximately 10–20% more expensive per meter than conventional rebars, whereas HDPE pipes cost roughly half the price of rebars per meter. Under these representative assumptions, replacing the “rebar + HDPE” combination with hollow steel pipe yields an estimated 20–27% reduction in pipe and reinforcement material costs per meter. These percentages reflect material costs only and exclude fittings, manifolds, corrosion protection, pumping accessories, and installation. Thus, a project-level estimate should include those items. End-of-life aspects further support the use of hollow steel pipes. Steel components can be readily recovered from construction and demolition waste through magnetic separation and remelted without loss of intrinsic properties, enabling high scrap recovery rates and closed-loop recycling. By contrast, although HDPE is recyclable, buried pipes often carry soil contamination and residual heat transfer fluid, and black pigmentation can complicate optical sorting. These factors tend to increase processing costs and raise the likelihood of down-cycling. Consequently, it can be expected that the proposed system replaces a polymer waste stream with a steel stream that already feeds established scrap-based production routes, potentially improving life-cycle recoverability and residual value.

Finally, the proposed HHPS-G-integrated pavement system advances beyond technical feasibility to align with the United Nations Sustainable Development Goals (SDGs). With respect to SDG 9 (Industry, Innovation and Infrastructure), the hollow steel, integrated design promotes infrastructure innovation while improving the operational reliability and maintenance efficiency of winter de-icing/snow-melting systems. In relation to SDG 11 (Sustainable Cities and Communities), it supports urban heat-island mitigation, enhances winter road safety, and reduces salt-induced corrosion. Consistent with SDG 12 (Responsible Consumption and Production), it promotes resource efficiency through the use of recyclable steel and the circular utilization of bottom ash while minimizing the use of HDPE pipes and the associated polymer waste.

5.2. Limitations of Study and Directions for Future Research

This study proposed an innovative HHPS-G-integrated pavement system by replacing conventional reinforcement in concrete with hollow steel pipes, enabling dual functionality as both structural reinforcement and heat exchangers. Through comparative performance evaluations based on numerical simulations, the proposed system demonstrates potential advantages in both structural stability and heat transfer efficiency compared to conventional HDPE-based HHPS-Gs. However, as a foundational investigation of this novel approach, several limitations must be acknowledged, and further studies are needed to ensure long-term reliability and field applicability.

First, the numerical model was developed and validated based on a heating experiment conducted under ambient conditions at approximately 11 °C. However, the actual de-icing and snow-melting performance must be evaluated under low-temperature and snow-covered environments (e.g., −10 °C or below). Since the current model has not been calibrated for such conditions, its absolute output values should be interpreted with caution. Future work should focus on model validation under extreme winter conditions and field-scale testing to provide reliable data for design. Additionally, the system’s applicability across diverse climate zones and ground conditions should be assessed to enhance generalizability and practical feasibility.

Second, although the thermal efficiency of the proposed system was extensively analyzed, its long-term structural behavior under field conditions remains unverified. The current study evaluated only the bond strength between steel pipes and concrete using pullout tests and provided preliminary comparisons of mechanical properties with conventional reinforcement. However, real-world pavements are subject to repeated thermal cycling and vehicular loads, which can induce micro-cracking or interfacial delamination at the pipe–concrete interface. Consequently, integrated structural validation, such as full-scale fatigue testing or advanced finite element analysis, is required to confirm the mechanical durability of the system under dynamic loading. Moreover, while various pipe placement depths were explored, the shallowest condition (62.5 mm) may pose risks, such as surface cracking or spalling, under freeze–thaw cycles or heavy traffic. Although this condition was included solely for sensitivity analysis, not practical application, future design guidelines should prioritize intermediate burial depths that optimize both structural and thermal performance.

Finally, the operation of embedded heat exchange systems can generate localized thermal stresses in the pavement, especially under cyclic thermal gradients. These stresses may lead to material degradation, micro-cracking, or interface weakening over extended periods. Therefore, it is essential to conduct long-term structural evaluations that account for the coupled effects of thermal and mechanical stresses in real-world conditions. To address this, the development of a comprehensive thermo-hydro-mechanical (THM) numerical model is strongly recommended. Such a model would enable comprehensive simulations of coupled temperature and stress behavior in multi-layer pavement systems incorporating hollow steel pipes. By enabling robust parametric and sensitivity analyses, this model can support the construction of a performance-based database for long-term durability assessment and design optimization. Furthermore, to mitigate localized stress concentrations and ensure structural stability, additional reinforcement strategies, such as the inclusion of supplemental top steel reinforcement or the application of fiber-reinforced concrete around the hollow pipe zones, should be considered and investigated in future studies.

Given these considerations, future research should develop a comprehensive validation framework that incorporates laboratory fatigue testing under cyclic thermal–mechanical loads, full-scale winter field trials, and the creation of calibrated THM-based design tools. These efforts will collectively enhance the mechanical and functional reliability of HHPS-Gs, providing actionable guidance for real-world implementation across various field conditions.

6. Conclusions

This study was undertaken to propose an HHPS-G-integrated concrete pavement capable of simultaneously ensuring environmental sustainability and structural stability. To achieve this goal, a comprehensive approach was adopted, encompassing system design, experimental evaluation of system components, and performance assessment through validated numerical analysis, with the aim of confirming the practical applicability of the proposed solution. The major findings are summarized as follows:

(1)

A novel HHPS-G was proposed in which hollow steel pipes serve dual functions as structural reinforcement and heat exchange conduits, eliminating the need for separate HDPE pipes and thereby preserving structural integrity. This design improves heat transfer efficiency due to the higher thermal conductivity of steel compared to HDPE while maintaining the reinforcement performance required by structural design standards.

(2)

To enhance upward heat transfer and reduce heat loss to the sublayers, the concept of incorporating an insulation layer was examined. Based on experimental testing, bottom-ash concrete was identified as an alternative to conventional building insulation materials, ensuring both mechanical strength and thermal insulation. Among the tested mixtures, the mix with a thermal conductivity of 0.72 W/m·K provided the most balanced performance, making it the optimal choice for HHPS-Gs.

(3)

The applicability of the proposed system was initially verified through analysis of experimental results from a previous study, in which a concrete slab embedded with hollow steel pipes was tested under heating conditions. Based on these results, a 3D numerical model was developed using COMSOL Multiphysics. Comparison with experimental data showed excellent agreement, with an average error of 1.50% and an RMSE of 0.43 °C, confirming the reliability of the model for conducting parametric simulations.

(4)

The parametric study reveals that replacing HDPE pipes with hollow steel pipes significantly enhanced thermal performance, reducing the time to reach 0 °C by 30.86% and increasing heat flux by 10.19%. Pipe depth strongly influenced performance: the top position offered the fastest heating (up to 70.11% faster for steel pipes), but structural requirements necessitate placement near the mid-depth zone, where hollow steel pipes still achieved comparable performance to HDPE pipes positioned at the top.

(5)

For insulation materials, variations in thermal conductivity below 1 W/m·K had minimal impact on overall performance. In fact, using extremely low-conductivity building insulation materials caused slight heat accumulation near the pipe, marginally delaying surface temperature rise. Conversely, replacing the base layer with bottom-ash concrete effectively provided both thermal insulation and structural stability, eliminating the need for separate insulation layers.