Structural behavior of the geothermo-electrical asphalt pavement: A critical review concerning climate change

Abstract

Particularly for asphalt pavement, where the temperature is a crucial driver in selecting construction materials, premature infrastructure failure and higher maintenance costs might be highly expected with the recently witnessed dramatic changes in climate. Numerous studies highlighted how the recent climate change might result in hazards to transportation infrastructure and affect all types of transportation modes. On the flip side, flexible pavement also contributes to global warming; various studies referred to the significant emissions percentages released by asphalt pavement upon subjection to solar radiation. With that in mind, several studies showed that the environmentally-friendly geothermal systems that mainly depend on heat exchanging with the soil have positive influences on reducing energy consumption, melting the ice on roadways in cold climates, or reducing the ambient temperature and the induced latent heat from the pavement in hot climates. However, very limited studies explored the influence of those geothermal systems on the structural behavior of the pavement concerning the associated distresses with extreme climate changes. In this paper, a critical review concerning climate change has been performed to investigate the structural performance and the associated distress of both conventional and geothermal asphalt pavement. This review underlines several advantageous physical and mechanical characteristics of geothermal pavement, which may recommend this system as a worthwhile alternative to conventional asphalt pavement. The paper also identified future research needs to overcome the shortcomings associated with the structural performance of the geothermo-electrical asphalt pavement.

Structural behavior of geothermal pavement; Asphalt pavement distresses; Temperature effect of pavement performance; Energy harvesting from Asphalt pavement.

1. Introduction

Safety, serviceability, and longevity are three key requirements that should be satisfied in the roadway structural design (Huang, 1993, Yoder and Witczak, 1975 [1, 2]). With that in mind, For the selection of paving materials, the present highway design standards depend on climate data spanning the years 1964 through 1995 (Underwood et al., 2017 [3]). Highway construction materials are, therefore, frequently specified with the premise of a stationary climate. Unfortunately, these climate-stationary premises led to the improper selection of 35 percent of the paving materials used in 799 distinct locations, as Underwood et al., 2017 [3] showed. This has led to various pavement failures and distresses, according to a recent evaluation study undertaken by Transportation for America, 2019 [4] to analyze the quality of the United states' roads between 2009 and 2017. This study found that the number of roads in poor condition rose from 14 to 20 percent; also, approximately $231,4 billion per year are needed to maintain and repair the nation’s roads within the next six years.

Climate change, as defined by the United Nations in 2022 [5], refers to long-term changes in temperature and weather patterns. Those alterations are either natural, as a result of oscillations in the solar cycle, or artificial, as a result of human actions such as the combustion of fossil fuels, oil, and gas. It is essential to mention that the globally averaged ocean and land surface temperature was 1.16 °C greater than the 20th century average of 12.7 °C, as conveyed in March 2020. This significant rise was described as the second-warmest record in the last 141 years (Climate Change Report, 2020 [6]). Given the flexible pavement temperature sensitivity, various failures and distresses could be projected.

The Intergovernmental Panel on Climate Change (IPCC, 2014 [7]) highlighted in the fourth assessment report that climate change represents a substantial threat not only to human life and nature but also to the built environment. In particular, the transportation infrastructures that are constantly exposed to the natural environment, such as the physical roadways network, railways, hydraulic structures, and bridges. The Fifth Assessment Report (IPCC, 2018 [8]) has proposed two scenarios concerning the continuous rising of greenhouse gases (GHGs): RCP4.5 (Thomson et al., 2011 [9]) and RCP8.5 (Riahi et al., 2011 [10]). RCP4.5 encompasses mitigation, while RCP8.5 describes a scenario in which all operations continue with current emission rates. Underwood et al., 2017 [3] revealed that in case of failure in upgrading the design standards concerning climate change and global warming threats to pavement infrastructure, the following practice for selecting the construction material might be anticipated to add a tremendous increase in the pavement cost. Only in the United States, a cost increase of around $13.6, $19.0, and $21.8 billion are respectively expected under the RCP4.5 scenario by 2010, 2040, and 2070. While an increase of $14.5, $26.3, and $35.8 billion are expected under the RCP8.5 scenario by the same years.

Gudipudi et al., 2017 [11] has ensembled nineteen different climate models and two individual models for RCP8.5 and RCP4.5 scenarios to study the climate change impact on pavement performance. Regardless of the variation in impact magnitude due to the different climate predictions, all the project models established in this study indicated that pavements would be subjected to greater distresses and early failure of pavements. In addition, the study also showed that fatigue cracking might increase by a percentage ranging from 2% to 9%. Also, AC rutting might significantly increase with a percentage between 9 and 40% upon future temperature increases.

In the same line, Humphrey, 2008 [12] investigated the possible influence of climate change on U.S. transportation systems and how the change in climate might result in hazards to transportation infrastructure. The study showed that those hazards could affect all types of transportation modes. For instance, the increased inland and coastal flooding events might affect the water transportation modes during the cold weather, resulting in operational challenges. On the other hand, the expected increase in temperature due to the heatwaves on summer days could substantially cause a decrease in pavement life from between 16 to 4 years, also causing an increasing percentage of 100% in the maintenance cost (Gudipudi et al., 2017 [11]). With that in mind, The National Cooperative Highway Research Program (NCHRP, 2008 [12]) expected that the standard design life for pavements (10–20 years) might help to alleviate these influences. However, the report enlightened that the possible influences of temperature and moisture changes on pavement behavior are not currently quantified. Nevertheless, the changes in moisture levels of subgrade soil, either by precipitation or the groundwater table changes, could affect the settlement and bearing capacity of the structural layer.

Asphalt pavement, on the flip side, contributes to global warming; several studies proved that asphalt induces up to 300% more emissions when exposed to solar radiation (i.e., Newburger, 2020 [13]). Since asphalt pavement constitutes a sizable portion of the built environment, this might seriously threaten air quality, particularly in the warm, sunny summer months. For instance, in the United States, ninety-four percent of paved roads are asphalt pavements (Qiao et al., 2020 [14]). In conclusion, adverse changes in the climate may hasten the deterioration of pavement and vice versa; the pavement harms the environment and makes a significant contribution to the amplification of global warming.

With that in mind, several studies showed that the environmentally-friendly geothermal systems that mainly depend on heat exchanging with the subgrade, such as the Ground Source Heat Pumps (GSHP), and the geothermal piles have a positive influence on reducing the consumption of energy in cooling and heating. Also, few others referred to the feasibility of using such systems either for ice melting purposes in cold climates or reducing the ambient temperature and the induced latent heat from the pavement in hot climates. Nevertheless, very limited studies explored the effect of those geothermal systems on the structural behavior of the pavement concerning the associated distresses with extreme climate changes.

Despite the outstanding merits of geothermal systems, the high implementation costs involved with their installation remain a substantial obstacle to the broader use of those systems (Motamedi et al., 2020 [15], and Al-Atroush et al., 2022 [16]). Keeping this in mind, relatively few recent research have delved into the possibility of harvesting renewable thermoelectrical energy from asphalt pavement using thermoelectric generators (TEG). Through field experiments, Zhu et al., 2019 [17] have confirmed that there is a sufficient temperature gradient between the pavement structures and the subgrades to apply the Seebeck effect and generate electricity using TEGs. Despite that, according to the results of this study, the amount of energy harvested by this system was insufficient. The investigation also found that the use of thermoelectric technology in roadways is in its preliminary stages of development and research, and more advanced cooling/heating sustainable systems might be needed to overcome this concern. Nevertheless, the expense of integrating geothermal with the roads may be worthwhile, upon the success in harvesting enough electricity from asphalt pavement using the TEGs.

This paper discusses the main factors affecting asphalt pavement structural performance. The different types of associated distress with asphalt pavement are identified in light of climate change. With that in mind, the study also highlights the key advances of integrating shallow geothermal systems into the pavement structure and how it could contribute to eliminating the adverse effects associated with extreme climate changes on asphalt pavement. It also intends to review the significant advances recently made in research related to investing the thermal differences between pavement layers and utilizing thermoelectrical generators (TEG) to harvest energy from the geothermal pavement. This would be necessary to identify future research needs to overcome the shortcomings associated with the structural performance of the geothermo-electrical asphalt pavement.

2. Climate effects on the flexible pavement

Flexible pavement structure often consists of four layers: subgrade, subbase course, base course, and the asphalt layer. Those four structural layers primarily resist the actions caused by the traffic loads and environmental conditions. Durability, smoothness, and safety are the three basic requirements for pavement structure. Durability refers to the long service life; pavement structure should have a satisfying strength and resistance against deformation to meet the durability requirement. Park and Kim, 2019 [18] defined pavement lifetime as either the period until maintenance is necessary to repair any construction problems or the period until a new overlay or rehabilitation is required.

In fact, the majority of the current pavement design approaches were developed based on durability requirements (Sun, 2016-a [19]). With that in mind, the durability and the structural performance of flexible pavement are affected by many factors, which could be classified into extrinsic and intrinsic factors. The extrinsic are the external factors affecting the pavement, such as mechanical, vehicle-traffic, traffic average volume, traffic speed, rate of heavy vehicle traffic (Jianhong et al., 2021, Lingyun et al., 2020 [20,21]), and thermo-mechanical factors, in addition to the environmental factors such as groundwater, climate, and temperature cycles, freeze-thaw cycles. Secondly, the intrinsic factors related to the natural characteristics of the materials employed to assemble the pavement include fatigue, plastic deformation, cracking, or the pavement configuration, such as the thickness of the road base. This section mainly focuses on environmental factors that influence asphalt pavement performance.

2.1. Temperature effect on asphalt mixture

According to Sun, 2016-b [22], the asphalt mixture can be considered a temperature-sensitive material since temperature changes significantly impact its mechanical and structural performance. The asphalt mixture consists of an aggregate and a binder, and its performance is susceptible to change based on temperature variables. Upon the temperature increases, the stiffness of the binder is decreased. Because of this, many pavements distresses, including reflection and fatigue cracking, are directly and indirectly aligned to the asphalt mixture’s temperature. Dai et al., 2018 [23], Menapace et al., 2017 [24], and Sun et al., 2021 [25] used atomic force microscopy (AFM) to analyze the influence of heat aging on asphalt’s components and microstructure. As illustrated in Figure 1, Sun et al. 2021 [25] demonstrated that asphalt pavement would have distinct microstructures depending on the thermal aging durations (see Figure 1[a–c]), and there were considerable variances in the nanoscale size of the typical microstructure.

Figure 1.

Atomic force microscopy (AFM) analysis of an asphalt mixture under different thermal aging conditions (After Sun et al. 2021 [25]).

At high temperatures, the binder may soften to liquid form, as the binder’s viscosity and the adhesion among aggregates will dramatically decrease (Sun, 2016-c [26]). Consequently, the asphalt mixture stiffness decreases, and an enormous permanent accumulated deformation can be expected under repeating loading. This substantial change in the viscosity of the asphalt mixture is usually the main reason behind the induced rutting and compressibility of the asphalt structure in high-temperature environments. On the other hand, the binder behaves as a fragile solid at low temperatures, while at medium temperatures, it exhibits more viscoelastic behavior (Al-Atroush et al., 2022 [27]). Although the asphalt mixture’s strength will rise, in this case, but the thermal stress will rise too, which usually surpasses the material strength causing thermal cracking.

2.2. Groundwater effect on asphalt mixture

Moisture has a considerable impact on flexible pavement; it is one of the significant causes of weakening the natural aggregate materials and reducing the subgrade shear strength. Usually, water or moisture accesses the asphalt pavement structure through cracks and holes on the surface or through capillary action from the groundwater table. Particle lubrication, later particle displacement, and interlock loss are the most expected deteriorating consequences of water entry into the structure of asphalt pavement.

Significant climate changes have been recently experienced in different geographical zones. There have been several significant flooding disasters witnessed in the last years; Irma (Florida), Harvey (Texas), Sandy (New England), and Hurricanes Katrina (Louisiana) are just a few examples of the damaging event that occurred only in the Unites States' coastlines. Those events have resulted in billions of dollars of damage to infrastructure and devastating social and economic losses. When this happens, the pavement’s supporting components become increasingly susceptible to water intrusion over time. With excessive water infiltration into the moisture-sensitive aggregate layers, those layers are expected to lose their resilient modulus and strength (Haider et al., 2019 [28]), as shown in Figure 2. As a result, there will be more noticeable deflections. In addition to lowering the soil layers' shear strength and resilient modulus, water ingress into the structure of the pavement also causes stripping in asphalt pavements, fines migration into drainable layers, swelling in expansive soils, heave/frost, and thawing in cold territories (Huang et al., 1993 [1], and Al-Atroush and Sebaey, 2021 [29]). These premature failures of pavement for sure add unanticipated maintenance and rehabilitation costs for highway authorities.

Figure 2.

Asphalt resilient modulus (MR) reduction due to moisture increases upon different cracking levels. Wet-freeze (WF); wet-no freeze (WNF); Dry-freeze (DF); Dry-no freeze (DNF) (After Haider et al. 2019 [28]).

The early studies (Lottman, 1978 [30], Stuart, 1986 [31], and Parker and Benefield, 1991 [32]) revealed that pavement raveling and holes are the first signs of asphalt stripping when frequently heavy vehicle traffic and surface water soaking are combined. This is because of poor drainage or a lack of adhesion between the asphalt and aggregates, which leads to water accumulating on the pavement. With that in mind, those studies also reported that moisture could significantly deteriorate the bottom of the asphalt layer. An experimental study was carried out by Brown et al. 1990 [33]; aggregate particles were coated with bitumen and soaked in the water to investigate the process of pavement stripping. It was concluded that invasive water could permeate the interface of the bitumen-mineral aggregate through spontaneous emulsification and eventually substitute it because of water’s substantial surface tension. Fromm, 1974 [34], Plancher et al. 1977 [35], and Plancher et al. 1979 [36] obtained consistent conclusions.

Kandhal, 1994 [37] argued that the asphalt layer damage associated with the water is a bottom-up process. Deterioration of the asphalt layer bottom gradually grows, causing damage throughout the asphalt layer, according to the moisture distribution under the pavement’s surface. The capillary action and evaporation pressure raise subsurface water through the soil and primary pore. Capillary water can achieve the asphalt layer’s bottom when the groundwater level is high, or evaporation is intense, causing water erosion. Also, the asphalt layer’s air voids allow water from the pavement cracks to settle in the asphalt layer’s bottom for a prolonged period. So, the asphalt stripping typically starts from the bottom of the asphalt layer and progresses upward.

3. Failure mechanisms of flexible pavement and distresses classification

Surface deformation, cracking, disintegration, and surface defects usually characterize the failure of flexible pavements. However, those characteristics are often a result of several complex factors that affect the pavement structure. With that in mind, the relationship between the cause and effect of pavement failure is always complex. Therefore, its necessary to comprehensively and phenomenologically assess the failure mode and mechanism (Park and Kim, 2019 [18]).

In general, flexible pavement failure can be described in two manners; functional and conditional failures. The situation of not meeting an intended function is known as a functional failure. Conditional failure, on the other hand, is degraded to a specific condition in predetermined circumstances and within a predetermined time frame. In fact, due to the diversity of the affecting factors on the flexible pavement, as described in Figure 3, several failure mechanisms can be expected. However, due to traditional traffic and mechanical factors, shear (Figure 4a), tensile (Figure 4b), and bearing capacity (Figure 4c) failures are the three most common failure mechanisms associated with flexible pavement structures.

Figure 3.

Most common drivers for flexible pavement failures.

Figure 4.

Common three failure mechanisms of flexible pavement subjected to traditional traffic and mechanical factors. [a] Shear failure. [b] Tensile failure. [c] Bearing Capacity failure.

Fundamental to note that those three failure mechanisms mainly occur due to traditional traffic and mechanical factors. However, relatively limited studies discussed flexible pavement failure due to environmental influences such as groundwater, climate, temperature cycles, and freeze-thaw cycles, especially with the recently experienced variables like climate change and global warming. However, distresses of asphalt pavement can be generally categorized into four main types; cracks, deformations, surface damage, and other faults (Sun, 2016-c [26], and Sanghyun et al., 2015 [38]). Each one of those damages can appear in several forms on the pavement. This section reviews the most common types of cracks, deformations, and surface damage of asphalt pavement.

3.1. Cracks of asphalt pavement

Based on the causes of the cracks and their directions, pavement cracks could be categorized, as shown in Figure 5, into longitudinal, transverse, alligator, and block cracks (Sun, 2016-c [26]). Table 1 summarizes the detailed description and the leading causes of each pavement crack type.

Figure 5.

Common categories of flexible pavement cracks. [a] Longitudinal crack, [b] Transverse crack, [c] Alligator crack, [d] Block crack.

Table 1.

Types of flexible pavement cracks.

No.	Crack Type	Description	Main Causes	Reference
1	Longitudinal cracking	⁃This type of crack substantially parallels the centerline of a highway or laydown direction. As shown in Figure 5a, longitudinal cracks are usually adjacent to about 3–5 cm from the edge of the pavement. ⁃Longitudinal cracks often take two patterns, straight and arc shapes. ⁃Most longitudinal cracks are caused by cracking at the location of the assembly lap under load.	⁃Inappropriate construction lap and inadequate paving lane joint. ⁃Subgrade settlement or differential settlement •Inappropriate widening of the Pavement. •Poor compaction of the cut or fill. ⁃Overweight loads and fatigue failure of asphalt.	Sun, 2016-c [26] Miller and Bellinger, 2014 [39]
2	Transverse cracking	⁃This type of crack is substantially vertical to the highway centerline, as shown in Figure 5b. ⁃Transverse cracks are usually classified as premature damage. ⁃They could be categorized into three types: •Low: crack width < 6 mm for unsealed crack or sealed crack with indeterminable crack width. •Moderate: crack width between 6 mm and 19 mm with adjacent random cracking of low level of severity. •High: crack width larger than 19 mm with the adjacent presence of random cracking of a high severity level.	⁃Low temperatures or temperature change. ⁃Undesirable construction lap, ⁃Reflective cracking. ⁃The pavement with the semirigid base course.	Sun, 2016-c [26] Miller and Bellinger, 2014 [39]
3	Alligator cracks	⁃A series of staggered interconnected cracks occur in localized areas and are usually companioned by subsidence. ⁃In the early stage, this type of crack shows intertwined patterns and, after that, develops into sharp-angled, many-sided pieces characteristically with an alligator pattern. ⁃As shown in Figure 5c, the fragments are usually fewer than 2.0 m on the longest side. ⁃Alligator cracks usually initiate at the asphalt surface bottom due to the high impact of the wheel load that causes tensile stress concentration at the bottom. Later, the cracks propagate to the surface, and by repeating the traffic loads, the cracks get connected and form many-sided pieces with the alligator pattern.	⁃Insufficient pavement structural tensile strength. ⁃Fatigue failure due to the repeated traffic loads ⁃Fatigue failure due to flexible/brittle base. ⁃The insufficient thickness of the pavement layer.	Sun, 2016-c [26] ASTM D6433-99, 1999 [40] (Zaltuom and Yulipriyono, 2011) [41]
4	Block cracking	⁃This type of crack could be described as a crossing crack able to splits the pavement surface into rectangular blocks. ⁃As shown in Figure 5d, the block size may range from 3 × 3 cm to 3 m × 3 m. ⁃Block cracks are usually an indication of the significant hardiness of the asphalt. ⁃Most probably, block cracking occurs over the large nontraffic pavements area.	•Temperature change •Reflective cracks of joints and in the underlying base •asphalt aging	(Zaltuom and Yulipriyono, 2011) [41] Sun, 2016-c [26]

3.2. Deformation of asphalt pavement

One of the main highway failure phenomena is asphalt pavement deformation. There are different patterns of pavement deformation, such as; rutting, subsidence, and shoving. Figure 6 demonstrates the various asphalt pavement deformation types, and Table 2 summarizes the detailed description and the leading causes of each pavement deformation type.

Figure 6.

Common types of flexible pavement deformations. [a] Rutting, [b] Shoving, [c] Depression/Settlement (Subsidence).

Table 2.

Deformation types of Asphalt Pavement.

No.	Deformation Type	Description	Main Causes	Reference
1	Rutting	⁃The rut is a longitudinal surface deformation that occurs along the wheel path under the traffic loads. Those rutting lengthy, thin grooves result from permanent pavement deformation. ⁃Shear deformation, re-compaction of the asphalt layers, and consolidated or lateral movement of the pavement materials are causing the various levels of rutting deformation along the wheel path. Due to the channelized traffic loads, the rut depth always upheaves locally due to the induced shear deformation flow, as shown in Figure 6a ⁃Significant rutting is one of the primary sources of pavement structural failure.	⁃There are two major causes of the rutting that could be classified into; •External causes: the repeated channelized traffic loads. •Internal causes: asphalt mixture, poor resistance to plastic deformation, and high temperature. ⁃The insufficient thickness of the pavement layer. •Low quality of the post-construction compaction. •Instability of base layers.	Sun, 2016-c [26]
2	Shoving and Corrugation	⁃Shoving is the longitudinal displacement that occurs in a localized pavement surface area and could be described as a crawl. In comparison, Corrugation is usually observed as ripples caused by the plastic movement of the pavement surface, as shown in Figure 6b. ⁃In the presence of high temperatures and horizontal loads such as breaking forces, a severe shoving deformation may occur due to the poor stability of the asphalt mixture. ⁃Those types of deformation are usually expected at locations subjected to frequent braking and accelerating of vehicles, such as ramps, intersections, entrances, and exit zones.	•High-temperature. •Horizontal force. •The poor bond between layers and insufficient stability of the mixture. •Lack of containment at the edges. •The insufficient thickness of the pavement layer.	Sun, 2016-c [26]
3	Depression/Settlement (Subsidence)	⁃Depressions or subsidence can be described as a slightly lower elevation localized pavement surface area compared with the surrounding pavement, as shown in Figure 6c. ⁃Settlement of the subgrade (Foundation) layer is the main cause of the subsidence. ⁃The roughness caused by the settlement can create hydroplaning if the depression is deep enough or filled with water.	•Settlements. •Failures in the underlying pavement layers, such as base course or road base. •Isolated consolidation. •Volume change of the subgrade.	Park and Kim, 2019 [18] Sun, 2016-c [26]

3.3. Surface damage of asphalt pavement

Surface damage is the third category of asphalt pavement distresses, including raveling, potholes, polishing, bleeding, and pavement patches. Figure 7 demonstrates the various asphalt pavement surface damages. Table 3 summarizes the detailed description and the leading causes of each pavement Surface damage.

Figure 7.

Common types of flexible pavement surface defects. [a] Bleeding [b] Polishing, [c] Patches, [d] Raveling, [e] Potholes [f] edge cracks.

Table 3.

Types of asphalt pavement surface damages.

No.	Failure Type	Description	Cause	Reference
1	Bleeding	⁃Bleeding is defined as an excess bituminous binder that occurs at the pavement’s surface. As demonstrated in Figure 7a, bleeding makes the asphalt surface smoother, like a mirror, which reduces the friction resistance and causes vehicle slippage. ⁃When the free asphalt in a pavement is heated to a point where it expands beyond the capacity of the air void in the asphalt mixture, a phenomenon known as asphalt bleeding occurs, and the asphalt overflows onto the pavement surface. ⁃Pavement bleeding can reduce the skid resistance and texture depth of the pavement, which reduces driving safety.	⁃There are two significant causes of bleeding that could be classified into; •External causes: the high temperature. •Internal causes: an improper asphalt mix design or construction, ⁃Mainly the asphalt’s high content, low viscosity, and minimal air voids in the asphalt mixture design. ⁃High saturation degree of the asphalt mixture. ⁃Lack of enough space available to accommodate volume changes of bitumen in the mixture in case the temperature increases.	Sun, 2016-c [26]
2	Polishing	⁃Asphalt pavement surface polishing can be described as the smoothness of the exposed aggregate particles to the road surface subjected to the friction of vehicle tires, as presented in Figure 7b. ⁃Pavement polishing can reduce the pavement’s structured texture and skid resistance, which reduces driving safety. ⁃Repeated traffic polishes aggregates. It’s present when the aggregate above the asphalt is either very small or lacks angular, rough aggregate particles to provide effective skid resistance. ⁃Controlling the quality of the aggregate material could be effective in eliminating pavement polishing.	⁃The low hardness of the utilized aggregate particles. ⁃The poor wear resistance of the aggregate particles. •Traffic movement and the related friction between vehicle tires and pavements.	Sha 2001 [42] Sun, 2016-c [26]
3	Patches	⁃Most probably, this type of damage occurs upon replacement of the pavement material in a localized surface area for repair purposes. ⁃As shown in Figure 7c, the patched area often not looks like the original pavement section.	⁃Inappropriate maintenance works, such as; •Asphalt hole filling without performing the essential cleaning works. •Asphalt hole filling without performing the essential proper leveling and compaction.	ASTM D6433-99, 1999 [40]
4	Raveling	⁃Loss of the asphalt binder and dislodging aggregate particles are the main characteristics of the raveling, as demonstrated in Figure 7d. ⁃The significant hardening of the asphalt binder is one of the significant raveling causes, in addition to the poor quality of the asphalt mix. ⁃This distress implies that either the asphalt binder has solidified significantly or that a low-quality mixture is evident.	•The insufficient adhesion between the aggregate particles and the asphalt binder. •Poor asphalt mix design. •Weathering due to environmental factors.
5	Potholes	⁃A pothole is any pit formed as a result of the loss of pavement materials., as exhibited in Figure 7e. ⁃The loose abjection of cracking fragments, failure, and abjection after subsidence is the leading cause of pavement potholes. Also, the poor adhesion between pavement structure layers.	•Poor drainage quality of asphalt layer. •Inadequate strength in one or more pavement layers.	ASTM D6433-99, 1999 [40]
6	Edge cracks	Edge cracking is a fracture in the side of the pavement that runs parallel to the edge of the pavement and expands longitudinally and transversely, branching towards the shoulders. It occurs at a distance of 0.3–0.5 m from the edge, as shown in Figure 7f.	•Inadequate width of the pavement. •Insufficient supports for the edges. •Traffic loads on the shoulders. •Weak adhesion and coating loss. •Inappropriate shoulder material that lacks plasticity.	Sha 2001 [42] Sun, 2016-c [26]

4. Geothermal systems: merits, mechanisms, applications, and challenges

With ever-increasing energy demand and the increased CO₂ emissions generated by burning fossil fuels, the transition to renewable and more clean energy resources has become an urgent need (Gu et al., 2021 [43]). Almost about 50% of the energy global consumption is encountered for heating and cooling purposes (REN21, 2019 [44]). With that in mind, geothermal energy has emerged as one of the most promising forms of renewable energy. The shallow geothermal systems are considered one of the more sustainable solutions that significantly contributed recently to reducing fossil fuel consumption (Narsilio and Aye, 2018 [45], Kim et al., 2016 [46]). This environmentally friendly solution is mainly employing the soil as a heat sink or heat source, which is efficiently contributed to providing the thermal energy needed for space cooling or heating requirements (Johnston et al., 2011 [47], Yoon et al., 2015 [48], Brandl, 2006 [49]). According to Narsilio et al., 2014 [50], for each 1 kW of electric power required to run the system, those geothermal systems can generate 4–5 kW of thermal energy. Compared with traditional electrical resistance heating elements, geothermal systems consume about 75% less energy, which could be considered effective energy-saving systems (Duffield and Sass, 2003 [51]).

The primary mechanism of the shallow geothermal system is heat exchange with shallow soil layers at depths ranging from 1 m to 200 m. Those systems are recognized as ground heat exchangers (GHEs), or Ground Source Heat Pumps (GSHP). They were originally developed in 1940 by Robert Webber (Akrouch 2014 [52]). In those systems, the subgrade act as a heat exchanger, as shown in Figure 8a. Due to the fact that the soil temperature at the ground’s surface is extremely dependent on the air’s ambient temperature, various studies (Motamedi et al., 2021 [53]; Gashti et al., 2014 [54]; and Muhammad et al., 2016 [55]) revealed that the optimal embedded depth for achieving a steady and reliable subsurface temperature for use in the heat transfer process should be at least 4 m Figure 8b illustrates the temperature-depth relationship for London clay at Guildford (Sani et al., 2019 [56]). As demonstrated, after around 5 m deep, the temperature of the subsurface soil tends to remain consistent regardless of the surface ambient temperature over different sessions. In addition, the geothermal heat pump transmits thermal energy from or to the soil by means of vapor-compression of a carrier fluid contained within vertical or horizontal loops of pipes buried in the soil.

Figure 8.

[a] Heat exchange mechanism of the shallow geothermal systems. [b] An example of the variations of the ground temperature with depth (After Sani et al., 2019 [56]).

The heat exchanger could generally be categorized into two types, the closed-loop and open-loop (Suryatriyastuti et al., 2012 [57]; Singh et al., 2019 [58]). This classification is based on the thermal energy extraction or exchanging method from or to the ground. As explained in Figure 9 a, the open-loop system collects water from an aquifer or waterway. It circulates the fluid through the heat pump system and then returns to the original, which is usually located close to the distance to avoid any interaction. Maintenance expense is the main disadvantage of open-loop solutions, as it is relatively expensive because of the risk of contamination and blockage (Suryatriyastuti et al., 2012 [57]). In most cases, the blockage may occur because of the minerals' cumulation, e.g., iron-manganese hydroxides (FeH₄MnO₄). Those minerals are induced by precipitation or temperature changes (Brandl, 2006 [49]). On the other side, the closed-loop approach circulates a persistent volume of fluid through an enclosed pipe circuit network that can be installed vertically, horizontally, or slantingly. Vertical closed-loop heat exchangers, such as conventional geothermal boreholes or energy piles, are favored due to the high seasonal temperature variations and restricted land space with other closed-loop forms (See Figure 9b) (Akrouch 2014 [52]; Suryatriyastuti et al., 2012 [57]; Zagorscak and Thomas, 2016 [59]; Kovačević et al., 2013 [60]).

Figure 9.

Heat exchanging systems. [a] Open-loop systems. [b] Closed-loop Systems.

Ground heat exchangers (GHEs) could also be classified into vertical and horizontal systems. The horizontal geostructures incorporate carrier fluid pipes for the exchange heat process (İnallı and Esen, 2004 [61]). However, with little additional cost, vertical GHEs could be used for the dual purpose of thermal provision as well as structural stability (Hepbasli et al., 2003 [62]). That’s why geothermal systems have recently become more useable and feasible with the different structural elements (See Figure 10), as reported in several case studies, such as foundation piles (Brandl, 2006 [49]; Bourne-Webb et al., 2016 [63]; Mustaffa et al., 2022 [64], Makasis et al., 2018a [65]), diaphragm walls (Barla et al., 2020 [66]; Makasis et al., 2020 [67]; Shafagh et al., 2020 [68]), and tunnel linings (Adam and Markiewicz, 2009 [69]; Bidarmaghz and Narsilio, 2018 [70]).

Figure 10.

Geothermal pipes mounted along the reinforcing cage. [a] Geothermal pile. [b] Geothermal diaphragm wall. [c] Geothermal slab. [d] Geothermal tunnel segmental lining (After Laloui and Loria 2019 [73]).

As explained before, GSHP systems have several benefits, but their high installation costs prevent them from being extensively used. Moreover, the implementation and design of GHEs extremely depend on the geometry of the underground structures and site layout. In addition to the extra stresses that could be imposed on the structural element due to the transferred thermal loads, which must be carefully considered during the design phase (Abdelaziz et al., 2011 [71]; Al-Qadami et al., 2022 [72]). Due to these limitations, it can be more difficult to design geothermal systems that can fulfill the required thermal loads. Nevertheless, to compensate for the GSHP installation cost, few recent studies (e.g., Zhu et al., 2019 [17]) in the field of electrical engineering have examined the renewable thermoelectrical energy harvested by asphalt pavement thermoelectric generators (TEG) (See Figure 11a) and investing the Seeback effect explained in Figure 11b to produce electricity through the thermal gradient. It was also concluded that future studies might work on improving the efficiency of those energy harvesting systems by promoting the thermal convection of the TEGs, proposing various configurations for the system, and improving the thermoelectrical properties of pavement using the composite material approaches.

Figure 11.

[a] Geothermoelectrical asphalt pavement (After Hasebe et al., 2006 [74]. [b] Schematic diagram explaining the Seebeck effect and generating energy through the temperature difference (After Zhu et al., 2019 [17]).

5. Geothermal pavement components and performance

Similar to the other geostructures, the geothermal pavement basically comprises the heat exchanger horizontal system. As shown in Figure 12, The geothermal pavement consists of three essential components: the primary unit, which comprises heat exchanging loops placed in the pavement layer, and the secondary unit, which consists of geothermal pipes (GEP) that transmit heat energy to or from the subgrade layer. Lastly, the heat pump system circulates carrier fluid continuously (Ho and Dickson, 2017 [75]).

Figure 12.

Schematic illustration of a geothermal pavement system (After Ho and Dickson, 2017 [75]).

The heat pump is hydro-mechanical equipment that circulates hot carrier fluid (HCF) via pipes to elevate the temperature of heat retrieved from the soil to a degree suitable for room cooling and vice versa. In reverse order, its functioning mechanism is similar to the one of a refrigerator. The heat pump is comprised of four parts: a compressor, a condenser, an evaporator, and an expansion valve. During heating, HCF is pumped through geothermal pipes (GEP). The HCF takes heat from the earth and feeds it to the evaporator, which comprises refrigerant (heat transfer fluid). As a result, the refrigerant absorbs the thermal energy from the HCF, thus raising the refrigerant’s temperature. The high-temperature refrigerant passes into the compressor, which compresses and transforms it into a high-pressure, high-temperature fluid. At the condenser, the vapor (refrigerant) transfers its heat to the heating system or secondary unit, which then heats the structure. The cooled refrigerant is delivered through the expansion valve, lowering its pressure and temperature. After the expansion valve, it goes through the evaporator to restart the cycle. Similarly, for cooling purposes, the whole process is reversed (Akrouch 2014 [52]).

Geothermal pipes (GEP) exchange heat with the soil or rock they are embedded in. The temperature difference between the pipe and the soil causes heat to flow. However, the temperature difference between the outlet and inlet HCF guarantees efficient heat flow. This depends on several factors like geological soil properties, groundwater flow, soil type, moisture content, initial soil temperature, and geographical location. Soil heat transfer is a sophisticated process. It happens through the processes of conduction, convection, and radiation. Other important heat transfer mechanisms include vaporization, thawing, freezing, and condensation (Brandl, 2006 [49]; Brandl et al., 2006 [76]).

On the other hand, the primary unit is a system of embedded pipes in the asphalt layer that forms a closed loop. It delivers the extracted or removed heat from the pavement to be transferred to the underlying subgrade. Those closed loops could also be employed in bridge decks, roads, and airport runways for de-icing purposes (Kovačević et al., 2013 [60]). Those pipes are commonly known as energy loops, heat transfer pipes, or absorber pipes.

The most common materials for heat transfer pipes are high-density polyethylene and polypropylene (HDPE/HDPP), polyvinyl chloride, and polybutylene (Fisher et al., 2006 [77]; Kakaç and Yener, 2008 [78]; Cullin et al., 2015 [79]; Zeng et al., 2002 [80]). The HDPE diameter is ranged between 20 and 44 mm, as reported by several studies (Mimouni and Laloui, 2015 [81]; Lamarche and Beauchamp, 2007 [82]; Bidarmaghz, 2015 [83]). In addition, a nominal pipe diameter to pipe wall thickness ratio was recommended to be eleven. This ensures the pipe is durable enough to withstand a nominal pressure of 160 psi (Signorelli et al., 2007 [84]). Furthermore, several studies commended the use of copper material for the pipe network, as it achieves the most outstanding thermal efficiency of the system (e.g., Bobes-Jesus et al., 2013 [85]). Table 4 compares the thermal conductivity of the most common pipe materials.

Table 4.

Thermal conductivity of the most common pipe materials.

No.	Pipe Materials	Thermal Conductivity (W/m∗K)	Reference
1	High-Density Polyethylene (HDPE)	0.48	Mimouni and Laloui, 2015 [81]; Lamarche and Beauchamp 2007 [82]; Bidarmaghz 2015 [83] Bobes-Jesus et al., 2013 [85]
2	Silver	429
3	Copper	398
4	Aluminum nitride	310
5	Aluminum	247
6	Silicon carbide	270

The pipes could be mounted in different forms and configurations within the asphalt pavement, as demonstrated in Figure 13. U-shape (Figure 13a), W-shape (Figure 13a), triple U-shape, helical shape, spiral shape (Figure 13c), direct and indirect double-pipe are examples of the most common configurations reported in several studies (Gao et al., 2008-a [86]; Park et al., 2013 [87]; Hamada et al., 2007 [88]; Wang et al., 2013 [89]; You et al., 2016 [90]; Zarrella et al., 2013 [91]).

Figure 13.

Typical layout of heat transfer pipes in the geothermal pavement: [a] U-shape pipe, [b] W-shape. [c] spiral or helical shape (After Loveridge et al., 2020 [92]).

Hamada et al., 2007 [88] highlighted that selecting the most proper configuration is vital for maintaining system performance effectiveness. In the same line, an extensive comparison has been conducted for different pipe arrangements by Noorollahi et al., 2018 [93]. While Ho and Dickson, 2017 [75] compared surface heat production, considering different pipe arrangements, to investigate the feasibility of geothermal systems for ice-melting purposes. This comparison was based on the ratio (S/D) between the spacing (S) and the pipe diameter (D). As shown in Figure 14, the adapted S/D values ranged from 6.0 to 10. The S/D ratio was also utilized to obtain the pavement surface heat production at different fluid temperatures (ranging from 20 °C to 80 °C). It can be seen that the produced heat at the pavement surface decreased with the S/D increases.

Figure 14.

Heat production versus pipe spacing diameter (S/D) ratios (Ho and Dickson, 2017 [75]).

Motamedi et al., 2020 [15] carried out a comprehensive in-situ test on a 20 m by 10 m pavements portion that was modified with a geothermal heating and cooling system. Thermal response testing, often known as TRT, was carried out in the city of Adelaide in the Australian state in order to investigate the influence zone of the geothermal pipes. It was found that 0.50 m was the effective radius of influence around the geothermal pipe; also, the heat exchange rate achieved was around 25 W/m for each 1 m length of the pipe.

6. Thermo-mechanical behavior of the geothermal pavement

The structural behavior of flexible pavement, such as the viscoelastic hot-mix asphalt (HMA), is significantly influenced by the temperature (Solaimanian and Kennedy 1993 [94]; Hermansson 2004 [95]; Yavuzturk et al. 2005 [96]). Variations in pavement temperature could result in freeze-thaw cycles that often reduce the pavement load-carrying capacity and long-term stability (Dempsey and Thompson 1970 [97]; Mrawira and Luca 2002 [98]). On the other hand, the pavement itself significantly contributes to the Urban Heat Island (UHI) and substantially influences the ambient atmosphere (i.e., Gui et al., 2007 [99]; Mallick et al., 2009 [100]). Therefore, it is fundamentally vital to study the thermo-mechanical behavior of the asphalt pavement to highlight the change in this behavior in the case of the geothermal system integration with the pavement. This section discusses the heat transfer mechanism of both traditional asphalt pavement and geothermal pavement.

6.1. Heat transfer mechanism of the traditional asphalt pavement

In general, as defined by Cengel 2002 [101], Due to a temperature difference, heat is a form of energy that can be transferred from one system to another. Therefore, the driving factor for heat transmission is temperature differential, and the greater the temperature gradient, the greater the heat transfer rate. In the presence of a temperature gradient, heat could be transported through three different modes; convection, conduction, and radiation. Through those three means, the heat is transferred from the high-temperature medium to a lower-temperature one. Typically, asphalt materials have a significant absorption and storage capacity for solar energy (Al-Jabri, et al., 2005 [102]; Grimmond and Oke, 1999 [103]; Fortuniak, 2008 [104]; and Qin and Hiller, 2014 [105]). As shown in Figure 15a, Xu et al., 2021 [106] expressed that the induced solar energy by pavements could be characterized into the following three components (Eq. (1)):

E_H = H_S + H_L + H_C (1)

Where EH: is the pavement-induced solar energy (W/m²); H_S: is the sensible released heat (W/m²); H_L: is the latent released heat (W/m²); H_C: is the conductive heat through pavement structure (W/m²).

Figure 15.

[a] The temperature balance on the top of the pavement (After Wang et al., 2016 [107]). [b] Schematic Cross-section of surface heat energy equilibrium with the depth (After Banks 2008 [108]).

The pavement thermal equilibrium could be determined using the budget of induced solar radiation, also known as the net radiation, as represented in Eq. (2). As Wang et al., 2016 [107] expressed, the algebraic difference between the radiation collected by pavement and the released terrestrial radiation can represent the net radiation (R_n). Also, the radiation reflected from the pavement (R_r) can be expressed as in Eq. (3):

R_n = R_i – R_r + R_a − R_s (2)

where R_n: is the pavement collected net radiation (W/m²); R_r: is the reflected radiation; R_i: is the incident radiation (W/m²); (W/m²); R_s: is the pavement surface radiation; R_a: is the atmospheric radiation (W/m²).

R_r = α × R_i (3)

where α: the albedo of the pavement surface.

Any surface’s incident light fraction is represented by the reflection coefficient (α), often known as the albedo (Bobes-Jesus et al., 2013 [85]; Golden and Kaloush, 2006 [109]; and Sen and Roesler, 2016 [110]). It incorporates the diffuse and specular components of the solar spectrum while taking into consideration the hemispherical reflection of solar energy. The factors affecting the energy balance at the pavement surface and the heat transport processes that occur within the pavement layers are illustrated in Figure 15b. To sum up, the principal Eq. (4) could express the energy absorbed by pavement structures (Xu et al., 2021 [106]).

R_n = R_i – R_r + R_a – R_s = H_S + H_L + H_C (4)

6.2. Heat transfer mechanism of the geothermal asphalt pavement

As Yun and Santamarina 2008 [111] described, the heat transfer mechanisms throughout binder-coated aggregate particles in pavement materials are complex. Figure 16a shows the radiation transfer between particles, phase change, vaporization, convection in the pores, condensation, and freeze-thaw processes are expected through this internal heat transport mechanism.

Figure 16.

Heat transport mechanisms through different types of pavement. [a] Traditional pavement heat transfer among binder-coated aggregate particles (After Yun and Santamarina 2008 [111]). [b] Heat transfer mechanism of the geothermal pipes (After Brandl 2006 [49]).

In most cases, heat transfer is a three-dimensional heat conduction problem (Bopshetty et al. 1992 [112]; Sokolov and Reshef 1992 [113]; Chiasson et al. 2000 [114]; Esen et al. 2007 [115]; Demir et al. 2009 [116]). However, Brandl 2006 [49] concluded that a satisfactory prediction model could be obtained by lessening the complexity of the heat transfer procedure to an equivalent conduction model. This was explained by the fact that pore sizes are typically insignificant in comparison to the volume of the pavement body under investigation. Pavements also have a large surface area. As a result, the prediction model could be established as a one-dimensional conduction transient model in conjunction with a surface energy balance technique to forecast pavement temperature variations under specified meteorological factors by ignoring edge effects. On the other side, considering the geothermal system, Brandl 2006 [49] highlighted that the heat transfer processes behaving upon a pavement slab could be expressed, as shown in Figure 16b. In contrast, Turner 1987 [117] argued that by considering the fluid flow within the pipe, the heat transfer in the pavement would not strictly be a one-dimensional model anymore.

The heat transfer mechanism in geothermal pavement mainly involves conduction and convection. As shown in Figure 17, conduction is the dominant mechanism between the ground and the pipe wall. At the same time, convection is associated with the carrier fluid. As highlighted by Motamedi et al., 2020 [15], the heat exchange from the complete heat exchanger (Geothermal system) to its surrounds can be expressed as shown in Eq. (5);

Q = m C ρ (T_inlet − T_outlet) (5)

where m: flow rate (in L/s), C: heat capacity of water (4186 J/kg/°C), ρ: density (kg/m³), T_Inlet and T_Outlet: fluid temperatures of the inlet and outlet of the circuit (measured by sensors in °C).

Figure 17.

Heat flow mechanism in geothermal pavement (After Saifuddin et al. 2019 [118]).

Wu et al., 2011 [119] studied the temperature distribution of asphalt geothermal pavements (see Figure 18) by comparing the thermal performance of small-scale asphalt slabs without and with embedded copper pipes. The carrier fluid utilized in these experimental tests was water. In addition, the temperature variation through the pavement depth has been monitored using embedded thermal sensors. The asphalt slabs were heated in a lab irritation simulation test, and the slabs' heat transfer and the absorbing heat capability of the geothermal asphalt were investigated. It was found that compared with the controlled slabs, a noticeable temperature reduction of 12.4 °C in the geothermal slab was detected when water started to circulate. This was indicated by the temperatures difference change between the outlet and inlet water (Delta T), as shown in Figure 19a.

Figure 18.

Schematic diaphragm showing the heat transfer mechanism of the embedded geothermal pipes into the pavement layer (After Wu et al., 2011 [119]).

Figure 19.

[a] Variations in pavement surface temperature upon water circulation. [b] Effects of the water circulation on the test slabs' depth’s vertical temperature distribution (After Wu et al., 2011 [119]).

Figure 19b reveals the vertical temperature distribution along with the test slabs' depth. The temperature distributions of the slabs with the circulation of water differ significantly from the controlled slabs with no water circulation (highlighted in red), notably in areas above the pipe. It was also concluded that the circulated water could extract the heat stored in the asphalt slabs. The effect of flow rate on the temperature distribution was almost similar. Also, raising the flow rate has only resulted in minor changes in temperature distributions in the vertical direction.

Nooralhuda et al., 2020 [120] performed a numerical study to simulate the thermal behavior of a field case study of a geothermal pavement (Dakessian et al., 2016 [121]). After calibrating the numerical model, a comprehensive parametric study was carried out to explore the effect of different influencing factors on the thermal behavior of geothermal pavements. The utilized metric for parameter sensitivity was the change in surface temperature induced by changing the parameter value, as shown in the numerical results below (Figure 20). The parametric study revealed that pipe spacing (Figure 20a) and pavement conductivity were the highly dominant parameters affecting the efficiency of the geothermal pavement. However, the depth of the pipes was mildly sensitive to pipe diameter (Figure 20b and c); also, the flow rate showed a minor sensitivity. Figure 21 summarizes the various parameters' effects on the heat transfer mechanism of the geothermal pavement.

Figure 20.

Results of a numerical parametric study (After Nooralhuda et al., 2020 [120]). [a] Effect of spacing on the cross-sectional temperature distribution. [b] Depth effect [c] Diameter effect.

Figure 21.

Effect of various parameters of the heat transfer mechanism of the geothermal pavement (After Nooralhuda et al., 2020 [120]).

7. Structural behavior of the geothermal pavement

The structural behavior of asphalt pavement generally defines the relationship between the characteristics of pavement materials and the physical features of pavement structure, aiming to estimate the mechanical response of the pavement structure under the effects of the different external traffic loads, environmental factors, and time (Sun 2016-d [122]). With that in mind, geothermal pavement structural design should be given special attention due to the combined influence of temperature and traffic-induced stresses. Unlike ordinary asphalt pavement, where the temperature gradient is primarily vertical, geothermal pavement can show a strong temperature gradient in all directions, as explained in the previous section (See Figure 18). On the other side, geothermal systems might positively contribute to the pavement, as the asphalt pavements could be cooled down using the geothermal systems. This may increase the rutting resistance and reduce the effect of heat-island.

Unfortunately, very limited studies investigated the change in the structural performance of asphalt pavement upon integrating the geothermal systems. In this section, the most direct indicators of the pavement structural behavior and service performance are identified and utilized to investigate the change in the asphalt pavement response with the presence of the geothermal system. The pavement performance indicators could be conveyed as the function of the pavement’s mechanical and physical properties. Those indicators could be classified into two categories; first, the structural behavior indicators such as; shear strength, modulus of resilience, pavement condition index (PCI), riding, deflection (L), and quality index (RQI). Second, the service performance indicators include rutting, roughness, skid resistance, loss of aggregate/pumping, the reflective character of the pavement surface, the injection of materials from underneath layers, and traffic noise.

7.1. Interaction between embedded pipes and the asphalt pavement

The most governmental difference between traditional asphalt pavement and geothermal one is the integration of the geothermal pipes inside the pavement structures (see Figure 12). First of all, the interface zone between the pipe and the asphalt material should be investigated to highlight the main changes in the pavement’s structural response.

Dehdezi 2012 [123] investigated the interfacial zone (IZ) between the asphalt pavement and the embedded pipes using X-Ray Computed Tomography (XRCT). Asphalt slabs of 300 × 300 × 100 mm with an embedded pipe in the center were prepared using different pipe materials. Copper and polyethylene pipes were the primary materials utilized in this experiment. Cylinders (300 mm in height and 100 mm in diameter) were cored out from those slabs to be X-rayed. Figure 22 shows the XRCT images taken across asphalt specimens with embedded copper (Figure 22a) and polyethylene (Figure 22b) pipes.

Figure 22.

[a] XRCT images of asphalt specimen with integrated copper pipe. [b] XRCT images of asphalt specimen with embedded polyethylene pipe (After Dehdezi, 2012 [123]).

It can be clearly observed from Figure 22a that the asphalt specimen with the embedded copper pipe has better bonding between the pipe and the pavement’s body. The air voids' presence can indicate this. The air voids around the pipe could limit the heat exchange effectiveness between the asphalt pavement and the pipes (Dehdezi, 2012 [123]). In contrast, the hot asphalt and compaction allowed polyethylene pipes in the asphalt mixture to soften and deform. Therefore, a clear interfacial zone (IZ) was detected between the polyethylene pipe and the asphalt pavement. Furthermore, crack growth has been seen in the XRCT images, which could have been caused by pipe recovery after the mix compaction.

Along the same line, Sheeba and Rohini 2014 [124] performed numerous laboratory tests combined with finite element analyses to investigate the impact of geothermal pipes on pavement structure performance concerning the structural aspects of asphalt heating and cooling. It was concluded that a pipe’s presence in asphalt might result in a complex propagation of strains and stresses with peak stresses all over the pipe. This may reduce the lifetime as compared to the traditional pavements without pipes. In the coming subsections, the effect of geothermal pipes on different mechanical properties of the asphalt structure will be critically discussed.

7.2. Pipe effect on the asphalt pavement tensile strength

Zhou et al., 2021 [125] performed a structural analysis of geothermal road pavement to study the effect of geothermal pipes on the structural responses of the road pavement. Three loading conditions were applied for this investigation: static vehicle load, temperature load, and coupled vehicle static and temperature loads. A pavement prototype with dimensions of 600 × 400 × 130 mm was adopted in this analysis. Also, a copper pipe (8 mm diameter and 1 mm thickness) was integrated into the center of the prototype. A comparison between conventional pavement (without geothermal pipe) and geothermal pavement was carried out by assessing the change in the pavement’s stress and strain relationship and the bottom tensile stress concerning the cracking of the asphalt pavement. Figures 23 and 24 compare the bottom maximum tensile stress of the geothermal and traditional asphalt pavement under three different loading cases.

Figure 23.

Maximum tensile stress at the bottom of the geothermal and conventional pavement slabs (after Zhou et al., 2021 [125]).

Figure 24.

Structural response of conventional and geothermal asphalt pavement slabs. [a] Under Vehicle load [b] Under thermal load. [c] Under Coupling mechanical-thermal loads (after Zhou et al., 2021 [125]).

Figure 23 indicates that the maximum tensile stress at the bottom of geothermal pavement was nearly ten times the traditional one under static vehicle load (Figure 24a). In contrast, it was about one-third of the traditional asphalt pavement under temperature load and coupling static load (Figure 24b and c). The reduction in thermal stress, which was a vital aspect of the mechanical reaction, was explained by the fact that the heat from the pavement was absorbed by the moving water inside the heat pipe. Also, as shown in Figure 24, the maximum bottom tensile stress was considerably subsided due to inserting the geothermal heat pipe. Thus, Zhou et al., 2021 [125] concluded that the heat pipe could reduce asphalt pavement disease and cracks.

7.3. Pipe effect on the asphalt pavement compressive strength

Fourteen laboratory compression tests combined with finite element analyses have been carried out by Van Bijsterveld et al., 2001 [126] to compare the response of asphalt structures with and without geothermal polyethylene pipes under the compression stresses. The experiment setup is presented in Figure 25 a; displacement transducers (LVDTs) were utilized to detect the specimens' horizontal deformations. The compression strength of the specimen was obtained by dividing the maximum achieved compression force over the loaded surface area.

Figure 25.

[a] Compression test set-up. [b] The flattening of the pipe. [c] The specimen failure (after Van Bijsterveld et al., 2001 [126]).

The finite element analyses indicated that the overall stiffness modulus was hardly affected by the presence of the pipe (assuming similar asphalt qualities). Consistently, it was observed from the laboratory compression tests that parts of the asphalt structure had been pushed out to the sides, and the pipe was flattened; also, it was noticed that the asphalt body above the pipe was intact. This was explained as the pipe was deformed, and the material next to the pipes grabbed the load and failed. Van Bijsterveld et al., 2001 [126] concluded that the presence of a pipe in the asphalt results in principal stresses approximately fifty percent higher than the mean vertical stress, which may reduce the lifetime compared to pavements without pipes (Figure 25b and c).

Fundamental to note that the compression tests presented above simulated the mechanical response of the geothermal pipes without considering the effect of thermal loading or the significant role of the fluid inside the pipes. With that in mind, Zhou et al., 2021 [125] equivalent stress FEM analyses were carried out to compare the stress distribution of the conventional and geothermal asphalt pavement under the following three loading cases; static vehicle load, temperature load, and coupled vehicle static and temperature loads. The obtained equivalent stress results are presented in Figures 26 and 27.

Figure 26.

Equivalent stress of the geothermal and traditional pavement (after Zhou et al., 2021 [125]).

Figure 27.

Equivalent stress analyses of conventional and geothermal asphalt pavement slabs. [a] Under Vehicle load [b] Under thermal load. [c] Under Coupling mechanical-thermal loads (after Zhou et al., 2021 [125]).

It can be seen from Figure 26 that the embedded geothermal pipes have significantly enlarged the equivalent stresses under both the thermal and mechanical static load (Figure 27a and b). Under coupling loads, the corresponding stress of geothermal pavement was more than twice that of conventional pavement (Figure 27c). According to the findings, the temperature difference between the heat pipe and the surrounding asphalt mixture was significant, resulting in considerable stress concentration. However, Zhou et al., 2021 [125] concluded that in comparison to ordinary pavement, the overall stiffness of asphalt pavement had been enhanced by the embedment of geothermal pipe.

7.4. Resilience modulus of the geothermal asphalt pavement

In general, the asphalt complex modulus measures the dynamic mechanical properties of a material, considering the energy dissipated as heat during deformation and recovery. It is one of the principal properties of asphalt binder that mainly affects the pavement structure’s deformation. The temperature of the asphalt structure is a significant factor influencing the asphalt complex modulus (Chen et al., 2010 [127]), as demonstrated in Figure 28.

Figure 28.

The change in complex modulus of asphalt binder with temperature (after Chen et al., 2010 [127]).

It can be seen in Figure 28 that the complex modulus of asphalt binder is significantly affected by temperature increases. Chen et al., 2010 [127] found that When the temperature of asphalt concrete rises, the stability and structural strength of the concrete rapidly deteriorate. Higher temperatures reduce both the rutting resistance and stiffness modulus of the asphalt; thus, the asphalt exhibits more significant distress due to the volume expansion, softening, and aging of asphalt under steady high temperatures (Xingdong and Xiwu, 2007 [128], Qi-lai et al., 2005 [129]). Therefore, the rutting of asphalt pavement exposed to high temperatures is more critical among asphalt pavement problems. It is mainly noticed during the hottest days. Because of the asphalt’s high heat-absorbing capacity, the asphalt pavement temperature might reach 70 °C by solar irradiation during the summertime. With that in mind, integrating the geothermal systems with pavement might positively contribute to the asphalt pavement’s cooling down and maintaining its complex modulus within the accepted ranges, as proven by Shaopeng et al., 2009 [130].

7.5. Pipe effect on the pavement surface deformation

The asphalt pavement surface deformation is one of the primary indirect indicators that reflect the pavement’s overall strength and stiffness. The asphalt pavement surface deformation is substantially affected by temperature variations. Therefore, Zhou et al., 2021 [125] have considered it as one of the fundamental indexes to assess the properties of geothermal pavement and compared the response of the geothermal pavement with the conventional one under the effect of vertical load, thermal load, and coupled loads, as presented in Figure 29.

Figure 29.

Surface deformation of the geothermal and ordinary pavement (after Zhou et al., 2021 [125]).

The surface deformation generated by the temperature load was significantly more than the one caused by the vehicle load for conventional pavement, as can be shown in Figure 29. Conversely, for the geothermal pavement, the structural strength decreased due to the integration of thermal pipes; thus, its surface deformation has been dramatically increased only under the vehicle load.

The comparative finite element analyses (Figure 30) conducted by Zhou et al., 2021 [125] have also indicated that the surface deformation of geothermal pavement was roughly ten times the conventional one under the vertical static loading case (Figure 30a). In comparison, the surface deformation of geothermal pavement was only 14.3% greater than conventional asphalt pavement under coupling loads (Figure 30c). Fundamental to note that, Under the offset of the upward deformation generated by the temperature load (Figure 30b), geothermal pavement surface deformation was also greater than conventional pavement.

Figure 30.

Surface deformation results of conventional and geothermal asphalt pavement slabs. [a] Under Vehicle load [b] Under thermal load. [c] Under Coupling mechanical-thermal loads (after Zhou et al., 2021 [125]).

7.6. Temperature effect on the behavior of different subgrade soils

One of the main advantages of geothermal pavement is the heat transferring from the asphalt pavement layer to be exchanged with the subgrade layers (see Figure 12). This was effective in cooling down the asphalt layer, which somehow might be positively reflected in the structural performance of the asphalt, as discussed in the previous sections. With that in mind, it is also fundamental to inspect the effect of the transferred temperature on the subgrade soils and how this temperature affects the thermo-mechanical behavior of soils.

High-temperature exposure results in changes in soil properties. Those changes are expected to affect dynamic performance, such as permeability, Infiltration, and shear behavior. The temperature effect on the soil behavior depends on the soil composition, cohesionless or cohesive mixtures, the mineralogical composition, and the grain size of the soil particles (Zihms et al., 2013 [131]).

Table 5 summarizes the available data in the literature about the variation or change in different natural sandy soils' behavior due to the temperature change or, in other words, when exposed to drained thermal cyclic loading. In the same line, Di Donna 2014 [132] has developed and calibrated a new experimental setup consisting of four oedometric cells equipped with temperature control and LVDTs systems to measure natural clay’s volumetric change upon temperature change to be used in the framework of energy pile applications.

Table 5.

Summary of the temperature effect on the behavior of different soil types.

No.	Soil Types	Sandy soils	Clayey Soils	References
1	Thermo-mechanical behavior	⁃Sandy soils behave thermo-elastically with a volumetric thermal expansion coefficient of the order of 3 · 10−5 °C−1. ⁃The bonding between fines and sand grains is temperature-dependent. Therefore, when temperatures rise, the particle size in sandy soils is subsided due to the mobilization of fines. ⁃Dense sand samples were examined under undrained heating cycles, and volumetric expansion was observed with temperature increases, it was noted by the upward shift of the consolidation line. Fundamental to mention that the consolidation line has shifted up but with the same slope. ⁃Under the drained heating and shearing cycles, the state of the dense sand samples was moved to the right due to the effect of negative pore water pressure. However, there was no obvious effect on the critical state line.	⁃ Cays show a thermoelastic-thermoplastic behavior, depending on their OCR. In OC conditions, they behave thermo-elastically with a volumetric thermal expansion coefficient in the order of 3 · 10−5 °C−1, while in NC conditions, they undergo irreversible contractive deformation during heating, including between 0.3 and 1 % for an increase in temperature from 20 to 60 °C; ○ Under cyclic thermal loading, NC clays show a thermal accommodation phenomenon: most of the irreversible deformation is developed during the first cycle, while the following ones contribute less and less; ○ Thermal cycles on NC clays induce the phenomenon of thermal consolidation. Without changing the vertical applied effective stress, after the thermal cycles, the soil tends to be overconsolidated; ○ The typical thermal collapse of NC clays is thought to be related to the thermal-induced effect on the clay micro-structural assessment, but further analyses are required to clarify this aspect.	Di Donna 2014 [132] Zihms et al., 2013 [131]
2	Shear Strength	⁃A series of temperature-controlled, isotropically-consolidated, hollow cylinder triaxial compression tests were used to explore the shearing response of saturated dense sand upon the temperature change. Specimens were heated under drained settings before being sheared in undrained conditions. ⁃For the undrained condition, at the peak state, deviatoric stress was increased with the initial mean effective stress increases. However, the undrained shear strength was noted to decrease with the temperature increase. This was explained by the negative pore-water pressure induced at the peak state with the temperature increase. ⁃For drained conditions, shear strength was investigated in terms of the critical state, and it was found that there were no noticeable changes in the critical state line. Also, the friction angle was not impacted by the temperature change.	⁃In an experimental program utilizing modified triaxial apparatus, the temperature effect on the shear strength of soft Bangkok clay soil was investigated within a temperature range of 25–90 °C. ⁃The stiffness, undrained and drained shear strength of the soft clay was temperature dependent, as it was increased with the soil temperature increases or after being exposed to a temperature history. ⁃Thermal effects on clay shear strength, stiffness, and yielding behavior were explained by the thermally induced fabric changes and the thermally induced volume change.	Liu et al., 2018 [133] Abuel-Naga et al., 2007 [134]
3	Volume Change Behavior	⁃The relationship between the change in temperature and the sand thermal volumetric strain showed an expansion during the drained heating, which led to a rise in the void ratio, and the mean effective stress. However, it had little impact on the thermal volume change.	⁃The temperature-induced volume change of an isotropic consolidating clay specimen is stress history-dependent and stress level-independent. ⁃As a result of heating, the status of the clay soil changes from normally consolidated to over-consolidated, beyond a particular OCR value, the thermally generated contractive volumetric strain exhibits dilatant behavior. ⁃Temperature impacts on the clay interparticle interactions and viscous shear resistance of adsorbed water that affect clay particle resistance to fabric changes can be used to explain the thermally induced volume change behavior. The swelling line, the compression line, and the critical state line are temperature-independent.	ASTM D6433-99, 1999 [40] by Ng et al., 2016 [135]. Liu et al. 2018 [136].

The review performed indicates a significant difference between the thermo-mechanical behavior of clayey and sandy soils. Liu et al., 2018 [133] reported that Although there is considerable variation in the thermal volume change at the maximal temperature, dense sand specimens typically expand during drained heating, resulting in an increase in the void ratio (Figure 31b). However, the mean effective stress had no significant effect on the thermal volume change, as shown in Figure 31a.

Figure 31.

Stress-strain relationships for sand in temperature-controlled hollow cylinder triaxial tests under different temperature conditions (After Liu et al., 2018 [133]). [a] Axial strain (ε_a) change with deviatoric Stress (q). [b] Axial strain change with pore water pressure.

Conversely, clays show a thermoelastic-thermoplastic behavior, depending on their over-consolidation ratio (OCR), as highlighted by Abuel-Naga et al., 2007 [134] in Figure 32a. In OC conditions, they behave thermo-elastically, while in NC conditions, they undergo irreversible contractive deformation during heating. Under cyclic thermal loading, NC clays show a thermal accommodation phenomenon: most of the irreversible deformation is developed during the first cycle, while the following ones contribute less and less. Also, thermal cycles on NC clays induce the phenomenon of thermal consolidation (Figure 32b). Without changing the vertical applied effective stress, after the thermal cycles, the soil tends to be overconsolidated (Di Donna 2014 [132]).

Figure 32.

Results of drained triaxial compression test of soft Bangkok clay isotropic consolidated specimens at different OCR and temperatures (After Abuel-Naga et al., 2007 [134]). [a] Deviatoric stress (q) and strain relationship. [b] Volumetric strain (ε_v) relation with the axial strain (ε_a).

8. Discussion

As a material, asphalt is highly susceptible to temperature changes, which can significantly affect its mechanical characteristics and structural performance. Recent increases in average worldwide land surface temperature pose a significant hazard to the built environment, especially pavement. However, asphalt pavement is a major contributor to climate change because, as multiple studies have shown, it emits up to 300 percent more carbon dioxide when heated by the solar system. With that in mind, this review has shown that the environmentally-friendly geothermal systems could be an effective solution able to cool down the asphalt pavement in hot climates through the heat exchange with the subgrade and vice versa; it can warm the asphalt up and be utilized for the ice melting purposes in cold climates. As a result, asphalt pavement could be maintained at moderate temperatures, ensuring that it retains its viscoelastic structural behavior and maintains deformation within safe limits.

Several factors may affect the thermal and structural performance of the geothermal pavement. The thermal behavior of geothermal pavement is influenced by the spacing between the pipes, the embedded depth into the pavement layer, the flow rate of the carrier fluid, the diameter of geothermal pipes, and the asphalt conductivity. In comparison, several experimental and numerical studies emphasized that the structural performance of the geothermal pavement is influenced by the volume loss resulting from the pipe integration into the asphalt structure. In addition, the interaction between the geothermal pipes. The geothermal pavement was also criticized in different studies as the induced air voids between the pipe affect both the pavement’s thermal and structural performance. Copper and polyethylene pipes are the most common materials utilized with geothermal systems. It was evidenced through X-ray images that asphalt specimens with embedded copper pipes have better bonding between the pipe and the pavement’s materials, which was indicated by the presence of air voids; in contrast, the polyethylene pipes in the asphalt mixture were deformed due to pipe softening resulting from the hot asphalt and the compaction process.

In the same line, few studies explored the structural performance of geothermal pavement. It was concluded that integrating geothermal pipes has positively enhanced pavement tensile strength under the different loading cases and contributed to eliminating the asphalt pavement thermal cracking resulting from extreme climate events. On the other hand, integrating polyethylene pipes also negatively impacted the compressive strength of the asphalt structure, as reported in other studies. The presence of a polyethylene pipe in the asphalt resulted in principal stresses roughly fifty percent greater than the vertical mean stress, which may reduce the lifetime compared to conventional pavements without pipes. From the serviceability point of view, it was proven in different studies that integrating the geothermal systems with pavement might positively contribute to the asphalt pavement’s cooling down and thus maintain its complex modulus within the accepted ranges. With the fact that the complex modulus of asphalt binder is significantly affected by temperature increases, the geothermal pavement might help in raising the AC rutting resistance and stiffness modulus of the asphalt. However, the decrease of the geothermal pavement’s structural strength due to the integration of thermal pipes was reported as a fundamental reason for its surface deformation increases compared to the conventional pavement, despite the improvement of the pavement’s thermal behavior.

On the other hand, few field studies delved into the possibility of harvesting renewable thermoelectrical energy from asphalt pavement using thermoelectric generators (TEG). Field experiments have confirmed that there is a sufficient temperature gradient between the pavement structures and the subgrades to apply the Seebeck effect and generate electricity using TEGs. Wide implementation of so-called geothermo-electrical asphalt pavement could be invested in charging the electric vehicles and reducing the energy consumption of the highway utilizes, besides its significant positive contributions towards eliminating the climate change effects and the associated earlier failures of the infrastructures. Therefore, the expense of integrating geothermal with the roads may be worthwhile, upon the success in harvesting enough Pavement Analysis and Design, Prentice electricity from asphalt pavement using the TEGs.

9. Conclusions and future studies

A critical review was presented of the available literature related to the structural performance of asphalt pavement and the associated different types of failures and distresses due to traffic loads and environmental conditions. The review has fundamentally shed light on the recent rapid changes in the climate and its expected effects predominantly on the pavement. The study also highlighted a potential environmental-friendly solution to overcome the climate-oriented problems that result in earlier infrastructure failure and roadway maintenance cost increases.

Concerning climate change, integrating the geothermal systems with the pavement may remarkably contribute to cooling down the pavement temperature and reducing the induced emissions from the pavement. This, as discussed in this study, will help in mitigating the associated earlier failures of the pavement with extreme climate hazards. Nevertheless, several concerns still prevent adopting geothermal systems more efficiently. Therefore, this study tried to critically discuss and review the significant advances that have been recently made in research related to the thermal and structural performance of the geothermal pavement. Based on the findings from the reviewed studies, three fundamental research gaps have become apparent, and the following needs for future research have emerged:

• 1. Structural performance of the geothermal pavement: The air voids in the interfacial zone and the compression stress concentration around the pipes are the main reasons for the relatively greater surface deformation associated with the geothermal pavement. Using corrugated pipes instead of mild ones could increase the bond between the asphalt mixture and the pipe surface. They may also reduce the percentage of the air voids around the geothermal pipes, which will be positively reflected in the heat exchange mechanism and the structural performance as well. Besides, using customized asphalt mixes manufactured mainly for the geothermal pavement may give a chance to eliminate the induced air voids without affecting the stability of the asphalt mixture. In the same line, several additives, such as polymers and different ashes, could be utilized with the asphalt mixture to increase the bond between the mix and the pipe also to fill the air voids generated around the pipe’s cross-section without affecting the thermo-mechanical behavior of the pavement.

• 2. Thermal performance of the asphalt pavement: The relation between the asphalt pavement and climate change is sophisticated. The negative environmental rapid changes are causing faster pavement deterioration. The current roadway design standards assume a stationary climate and rely on outdated climate information. With the rapid climate changes experienced nowadays, updated design methodologies considering the environmental and thermal factors in roadway design become an urgent need for future research.

• 3. Energy harvesting from the geothermo-electrical pavement: Success in generating sufficient electric power through the asphalt and TEG system may justify the implementation cost of the geothermal with the pavements. Future research can also be extended experimentally to promote the thermal difference between the asphalt and subgrade layer utilizing the phase change materials (PCM). Also, different pipe materials with greater thermal conductivity may be examined in full-scale tests to assess their effectiveness for both heat exchange and ice melting purposes.

Declarations

Author contribution statement

All authors listed have significantly contributed to the development and the writing of this article.

Funding statement

Dr. Mohamed Ezzat Al-Atroush was supported by the Structures and Material (S&M) Research Lab of Prince Sultan University, Saudi Arabia [PSU-CE-SEED-74, 2021].

Data availability statement

Data included in article/supp. material/referenced in article.

Declaration of interest’s statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.