Environmentally Tuning Asphalt Pavements Using Microencapsulated Phase Change Materials

Abstract

Environmental conditions are considered an important factor influencing asphalt pavement performance. The addition of modifiers, both to the asphalt binder and the asphalt mixture, has attracted considerable attention in potentially alleviating environmentally induced pavement performance issues. Although many solutions have been developed, and some deployed, many asphalt pavements continue to fail prematurely because of environmental loading. The research reported here investigates the inclusion of microencapsulated phase change material (μPCM) in asphalt binders and mixtures to help reduce environmental damage to asphalt pavements. The μPCM particles are formulated to absorb and release thermal energy as the particles liquify and solidify, depending on pavement temperature. As a result, μPCM can provide asphalt pavements with thermal energy storage capacities to reduce the impacts of drastic ambient temperature scenarios and minimize the appearance of critical temperatures within the pavement structure. By modifying asphalt pavement materials with μPCM, it may be possible to “tune” the pavement to the environment. Through rheology, differential scanning calorimetry, thermal cycling, and dynamic modulus testing, this work attempts to capture the μPCM effect and link the behavior between μPCM modified asphalt binders and mixtures. This study identifies a novel approach to determine when the μPCM effect occurs using rheological measurements. Additionally, the thermal and mechanical performances of μPCM modified asphalt mixtures are evaluated. An asphalt mixture design method is demonstrated to systematically incorporate a substantial portion of μPCM particles in a reference mixture. The findings extend the thermomechanical understanding of μPCM modified asphalt binders and mixtures.

Keywords

microencapsulated phase change material asphalt binder rheology asphalt mixture design thermal performance mechanical performance

In recent years, the asphalt pavement industry has recognized the need to design and construct roadways that satisfy the requirements of macro and global trends, such as the connectivity, automation, digitalization, virtualization, and electrification of road transportation services, as well as conservation of raw materials, climate change, and demographic shifts (1 –3). As a result, the advancement of pavement engineering will require the utmost care and use of state-of-the-art technology. The importance of adequate design provisions to mitigate the effect of environmental conditions on pavement performance cannot be overemphasized ( 4 ). The viscoelastic properties of asphalt binder determine the thermomechanical behavior of asphalt pavements. These binders have a complex chemical composition that exhibits both viscous and elastic properties, depending on temperature and loading time. The exposure of asphalt pavements to normal temperature fluctuations can result in pavement failures. For instance, pavement rutting typically occurs during high temperatures, when the binder stiffness is reduced, while thermal cracking is related to pavement contraction at low temperatures ( 5 ).

The ability of phase change material (PCM) to maintain specific temperatures for extended periods of time can help tune the service temperature of asphalt pavements. The modification of asphalt pavements with PCM can provide desired engineering properties, such as increased shear modulus and reduced plastic flow at high and intermediate temperatures, and increased resistance to thermal cracking at low temperatures (6 –9). However, the effects of different types and mass fractions of microencapsulated PCM (μPCM) on the properties and performance of asphalt materials have rarely been investigated. The advent of μPCM to modify asphalt pavements could be a significant breakthrough for asphalt pavement performance. However, before the large-scale practical application of this technology, it is necessary to address numerous research and development stages ( 10 ). An in-depth understanding of the effects of μPCM on the thermal and mechanical performance of asphalt binders and mixtures is still required. Lack of such understanding is likely to limit the application of μPCM to modify asphalt pavement materials, despite the potential advantages of their use.

By modifying asphalt binders with μPCM, it should be possible to “tune” the resulting asphalt pavement to the environment, thereby mitigating or eliminating pavement damage caused by the exposure of asphalt pavement surfaces to temperature fluctuations. It is hypothesized that comprehensive thermomechanical testing and modeling techniques can characterize and optimize μPCM modified asphalt binders and mixtures. However, research to date has not yet determined rational tests and parameters that can be used to explain the performance of μPCM modified asphalt binders and mixtures, develop standard specifications, and link binder and mixture behavior. As a step toward this goal, this study’s overall research objective is to identify testing techniques that characterize the thermomechanical performance of μPCM modified asphalt binders and mixtures within the linear viscoelastic range. This paper introduces a new approach to identify the effect of μPCM in asphalt binders using rheological measurements. Additionally, the findings of this study corroborate experimental and numerical investigations showing that a reduction in pavement surface temperatures between 2°C and 9°C can be obtained with μPCM modified asphalt materials, as compared with non-μPCM modified asphalt materials (6, 11). Finally, the re-design of an asphalt mixture with μPCM is demonstrated, and the mechanical performance implications of incorporating a significant portion of μPCM in the mixture are discussed.

It is important to point out that μPCM modified asphalt materials are not yet fully or partially implemented in practice. In recent years, μPCM has attracted considerable scholarly attention as an asphalt pavement modifier. However, the present understanding of μPCM modified asphalt materials is limited compared with well-established asphalt material modifiers, such as polymer, rubber, hydrated lime, and rejuvenators. This paper aims to gain knowledge about the potential implementation of this methodology. Several aspects that have been lightly discussed or ignored in previous research about μPCM modified asphalt materials are reported in this study. Such analysis will further the optimization of μPCM modified asphalt materials and help toward the prospective implementation of this technology.

Background

PCM has been extensively investigated as a thermoregulator for various engineering applications, including electronics, spacecraft, solar energy, textiles, buildings, and construction materials, to name a few ( 12 ). Accordingly, the research studies about PCM available in the literature are quantitatively vast. In relation to time, thermal energy storage (TES) started to receive increased attention in the 1970s ( 13 ). Thus, most of the information on PCM has been reported over the past 40 years ( 14 ). In the literature, the term “PCM” is widely used to refer to a substance that absorbs or releases thermal energy when it undergoes a phase change from solid to liquid, liquid to gas, or vice versa ( 15 ). The primary purpose of PCM is the storage of thermal energy in a latent form ( 16 ). It is well known that latent heat storage can be achieved through changes in the state of matter ( 13 ). If PCM changes from solid to liquid (liquification) or from liquid to vapor (vaporization, evaporation, boiling), a given system’s latent energy increases. Conversely, if the phase change is from vapor to liquid (condensation) or from liquid to solid (solidification), a given system’s latent energy decreases. If no phase change is occurring, there is no change in latent energy ( 17 ). A proper understanding of the fundamental concepts and principles that underlie the incorporation of PCM into any system is required for its optimization ( 18 ).

In general, the incorporation of PCM into asphalt pavement materials has been primarily motivated by the successful application of PCM in building and construction materials, such as glass and concrete (19, 20). The challenges currently faced by asphalt pavement scholars to understand this topic overlap with the obstacles experienced by scholars in other engineering applications ( 10 ). Typically, the five main requirements for PCM implementation have been: (i) stable phase transition at solid-liquid interface, (ii) marginal PCM segregation with continuous cycling, (iii) compatibility of PCM with surrounding materials, (iv) minimal changes in volume during phase transition, and (v) mass production of filled containers (or capsules) ( 13 ). Several approaches could be used to select and optimize PCM for asphalt pavement purposes properly. A fundamental understanding of PCM’s latent heat storage capacity is essential to promote PCM modified asphalt pavement materials. The technical specifications of PCMs for asphalt pavement purposes were grouped by Si et al. ( 21 ) into seven categories: (i) adequate thermal conductivity, (ii) enough heat storage capacity, (iii) suitable phase transition temperature, (iv) entirely reversible phase change, (v) harmless chemical properties, (vi) high-temperature performance and durability (i.e., above 135°C), and (vii) simple incorporation process (i.e., mixing and compaction).

The relevant literature in PCM modified asphalt materials research has focused on demonstrating this concept’s practical potential. A large and growing body of literature has investigated the morphological, thermal, and chemical performance of numerous PCM formulations for asphalt pavements. Scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis have been the most common tests used for such analyses (22 –24). In most cases, comparative test inputs are used across studies. These studies suggest that morphological, thermal, and chemical tests are appropriate to determine the feasibility of using a PCM formulation in asphalt pavement materials. However, previous studies have been primarily concerned with shape-stabilized PCM (22 –24), one of the earliest PCM formulations appraised for paving purposes. Shape-stabilized PCM is a composite material consisting of a dispersed PCM material (i.e., paraffin) supported by a carrier material, for instance high-density polyethylene. This type of PCM was found to be defective for asphalt material modification because of PCM leakage issues ( 23 ). Such findings led to the consideration of μPCM for the thermal adjustment of asphalt binders and mixtures. While a proper test method to evaluate the toughness and abrasion characteristics of PCM carriers and encapsulation techniques is missing, μPCM particles have demonstrated a more viable approach for tuning asphalt pavements to the environment than shape-stabilized PCM (6 –9). A reliable PCM encapsulation method is imperative for the progress of this technology. Previous research suggests that the direct interaction of PCM with asphalt increases penetration and decreases both softening point and complex shear modulus of the binder ( 25 ). Interaction of PCM with asphalt has been minimized by using μPCM particles under controlled conditions in the laboratory (6 –9).

One body of literature is concerned with developing numerical models to predict the heat transfer and temperature variations in PCM modified asphalt pavements (6, 26, 27). Similar volume-averaged energy governing equations have been applied to estimate the transient response of PCM modified asphalt pavement structures. Conversely, some published studies report the results of PCM modified asphalt pavement slabs exposed to laboratory and field environmental conditions (28, 29). A wide range of experimental setups has been employed to investigate the thermal response of PCM modified asphalt mixtures at low, intermediate, and high temperatures (28 –30). Such results have demonstrated that incorporation of PCM into asphalt materials could be beneficial over a broad temperature range; however, it should be tuned depending on the environment and targeted purpose (i.e., prevent extreme cold or hot temperatures). A few measuring indices have been proposed to analyze the effectiveness of PCM modification and changes in temperature rate ( 29 ). The experimental and numerical results show enough evidence to support the thermoregulation of asphalt pavements by using PCM. Still, some disagreement exists between the experimental and numerical results obtained across the studies available in the literature. Numerical studies tend to underestimate the effect of PCM in comparison with experimental results. Simulations typically yield a lower temperature difference between control and PCM modified asphalt pavements. Few models have been calibrated and validated using relatable experimental data.

The past decade has seen the rapid development of μPCM for diverse engineering applications ( 31 ), aiding the incorporation of this thermoregulating agent in asphalt materials. As a result, more recent attention has focused on conducting mechanical performance tests for μPCM modified asphalt binders and asphalt mixtures. The rheological behavior of μPCM modified asphalt binders has been studied using Superpave binder tests and non-standardized binder rheological protocols (7 –9). In contrast, the mechanical performance of μPCM modified asphalt mixtures has been examined mainly by conducting standard test protocols (i.e., indirect tensile test) ( 30 ), while the role of μPCM in the engineering behavior of compacted asphalt mixture specimens remains largely unexamined. Further studies are required to fully quantify the effect of μPCM on the performance of asphalt pavements. Several questions in relation to the mechanical characterization of μPCM modified asphalt materials remain to be addressed. For example, what is the thermomechanical response of μPCM modified asphalt materials under applied shear or strain loads when the core PCM is in solid or liquid phase? There is a lack of standard methods for testing the PCM to be used in asphalt materials and assessing the suitability of a given μPCM formulation to a specific asphalt pavement environment or target asphalt mixture distress. An investigation is needed to develop procedures that allow comparison and evaluation for μPCM modification in asphalt pavements. Given such information, this research study seeks to obtain data that will help to address these research gaps. Therefore, a series of asphalt binder and mixture testing procedures are demonstrated, the results of which could be instrumental in extending the knowledge of μPCM modified asphalt pavements.

Methods

Microencapsulated PCM

A commercially available μPCM with a solid-to-liquid phase transition of 43°C, referred to hereafter as μPCM-43, was used in the investigation. The μPCM-43 is described as microscopic bi-component particles consisting of a core PCM material and an outer shell, or capsule wall, and has a white to slightly off-white color appearance in dry powder form, with particle sizes typically ranging between 0.014 and 0.024 mm. The PCM core consists of paraffin, while the capsule wall is a polymer. The capsule composition percentages are approximately 85% PCM and 15% polymer shell by mass. The capsule specific gravity is approximately 0.900, and the latent heat of fusion is between 220 and 230 J/g. For this reason, μPCM-43 is used to regulate temperatures and heat storage in various applications, such as the textile industry, electronics, and building materials. Based on thermogravimetric analysis, the capsules are regarded as extremely stable, with less than 1% leakage when heated to 250°C, and can be exposed in the laboratory to multiple thermal cycles without damage.

Asphalt Binder Testing

For asphalt binder testing, about 20 g of binder was mixed with the corresponding amount of μPCM-43 at 160°C using a mechanical agitator for 5 min in a 120 cm³ tinned-metal container. The modified asphalt binders were reheated at 160°C in an oven after mixing and before pouring them into the dynamic shear modulus (DSR) silicone specimen molds. Although the μPCM exhibits good temperature stability, the mixing process took place just before specimen preparation. A PG 64-22 asphalt binder was used for this portion of the study.

The rheological properties of μPCM-43 modified asphalt binders were characterized using recently explored testing techniques. The DSR test protocol employed is based on the methods reported by Kakar et al. (7, 8). The thermal effect of μPCM-43 on the modified asphalt binder’s rheological response was determined by performing temperature sweep tests. The dynamic shear properties were measured using the parallel plate configuration of 2 mm thick specimens with 8 mm diameter. Temperature ramps from 60 to 20°C (and vice versa) were conducted while applying an oscillatory shear strain with constant strain amplitude (1.0%) and frequency (10 rad/s). These testing inputs were selected to keep the rheological measurements within the linear viscoelastic range and equipment torque limits. The control PG 64-22 binder was modified at six different levels: 0, 5%, 10%, 20%, 30%, and 40% by total binder mass. In addition, the rheological testing was conducted at five different temperature ramps between 60°C and 20°C, namely 3°C/h, 6°C/h, 9°C/h, 12°C/h, and 15°C/h. For each binder-μPCM combination, a single specimen was examined. After loading the specimen in the DSR, the material was initially conditioned for 20 min at 60°C. The rheological testing equipment was then configured to run a cooling and heating cycle for each temperature rate. The DSR Peltier system decreased the temperature from 60 to 20°C within a specific time duration and increased the temperature from 20 to 60°C within the same time span. The temperature ramps were conducted successively from slowest to fastest, without interruptions. The set of temperature ramps used is comparable to the temperature change rates experienced by in-service asphalt pavements ( 32 ). The viscoelastic properties were measured at 30 s intervals, meaning that every 30 s the DSR decreased a temperature step and reported the average results of the rheological measurements continuously taken within the time step.

Rheological measurements were complemented with differential scanning calorimeter (DSC) tests. DSC testing was performed to understand better the exothermic and endothermic processes of the μPCM modified asphalt binders. After pouring the asphalt binder specimens in the DSR silicone specimen molds, the tinned-metal containers holding the remaining material were left to cool down. Then, a microspatula was utilized to obtain a 10 ±5 mg sample from the remaining asphalt binder material. The extracted material was enclosed in a DSC aluminum sample pan with a lid and placed in a DSC chamber. The thermal experiments were conducted between 10°C and 70°C in a nitrogen atmosphere using a cooling/heating rate of 600°C/h (10°C/min). The test was initiated at 70°C, after stabilizing the temperature of the sample. The sample was cooled to 10°C and then heated back to 70°C to evaluate a complete thermal cycle.

Asphalt Mixture Testing

The thermal response of μPCM modified asphalt mixtures was also investigated. First, four asphalt mixtures gathered from three paving projects were modified with PCM microcapsules at three different levels: 0, 4%, and 8% by total mixture mass. Table 1 shows the characteristics of these mixtures. Mixture A was gathered from a paving project near Crawfordsville, IN, Mixture B from a pavement reconstruction section in Indianapolis, IN, and Mixtures C and D from a pavement overlay project near Fort Wayne, IN. Mixtures C and D have comparable aggregate blends, with the main difference being that Mixture C includes steel slag aggregate ( 33 ).

Table 1.

Asphalt Mixtures Used for Thermal Cycling Assessment

Mixture name	Mixture A	Mixture B	Mixture C	Mixture D
Mixture description	Surface mainline	Base mainline	Surface mainline	Surface shoulder
Mixture designation (mm)	9.5	19.0	9.5	9.5
Binder type	PG 70-22	PG 64-22	PG 76-22	PG 64-22
Aggregate material	9.5 mm dolomite	19.0 mm stone	9.5 mm limestone	9.5 mm limestone
		9.5 mm stone	9.5 mm limestone	9.5 mm limestone
		4.75 mm stone	4.75 mm limestone	4.75 mm limestone
		2.36 mm stone sand	9.5 mm steel slag	2.36 mm natural sand
	4.75 mm dolomite	12.5 mm RAP	2.36 mm manufactured sand	2.36 mm manufactured sand
	4.75 mm dolomite	9.5 mm RAP	2.36 mm manufactured sand	2.36 mm manufactured sand
	2.36 mm dolomite sand	Mineral filler	9.5 mm RAP	9.5 mm RAP
	9.5 mm RAP		Mineral filler	Mineral filler
	Mineral filler		Mineral filler	Mineral filler
Air voids content (%)	5.0	5.0	5.0	5.0
Voids in the mineral aggregate (%)	16.6	14.7	16.9	17.1
Voids filled with asphalt (%)	69.9	66.0	70.4	70.8
Asphalt binder content (%)	6.1	5.4	5.8	6.5
Effective binder content (%)	4.9	4.2	4.9	5.3
Virgin binder (%)	4.6	4.6	4.7	5.6
Binder replacement (%)	24.3	15.4	19.7	13.8
Dust-to-binder ratio	1.0	1.2	0.9	0.8
Number of design gyrations	50	50	50	50
SGC pill mass (g)	4,825	4,700	5,140	4,860

Note: RAP = Reclaimed Asphalt Pavement; SGC = Superpave Gyratory Compactor.

Compactor

The plant-produced paving materials were heated in the laboratory at 135°C for two hours and then manually mixed with μPCM-43 until all the particles exhibited a dark appearance (about 5 min), as shown in Figure 1a. The μPCM-43 modified asphalt material was again placed in the oven for approximately 20 min to get back to the 135°C established temperature for compaction. About 2,700 g of mixture was compacted using the Superpave Gyratory Compactor (SGC) to a specific height of 63.5 mm, or until a total of 50 SGC gyrations were reached. Little information is available in the literature in relation to the compaction and fabrication of μPCM modified asphalt mixture specimens. Thus, a 63.5 mm height was specified to ensure that 50 mm tall specimens could be fabricated for thermal cycling experiments. However, during compaction, it was observed that all the specimens were compacted until the number of gyrations criterion was achieved, except the control specimen for Mixture C. The specimens modified with μPCM at 0, 4%, and 8% by total mixture mass were compacted to an average height of 65.5 mm, 68.6 mm, and 76.2 mm, respectively. This increase in SGC pill height is primarily caused by the low specific gravity of the μPCM.

Figure 1.

(a) Mixing process of μPCM-43 and asphalt mixture and (b) specimens of Mixture B prepared for thermal cycling.

Following compaction, the SGC pills were cut using a saw to fabricate specimens 50 mm in height. Subsequently, three type-T thermocouples were placed on each specimen, one at the top surface (non-insulated circular face), one 25 mm from the top surface (mid-specimen), and one at the bottom surface. The mid-specimen temperature sensor was mounted at the center by drilling a hole to fit the thermocouple. Thermally conductive adhesive tape was applied to secure the thermocouples at the bottom and top surfaces. The bottom surface and outside diameter of each specimen were then insulated using a 50 mm insulating board (see Figure 1b). To minimize heat loss, the gap between the specimen and insulating board, plus the incisions made in the board to place the thermocouples, were filled with an insulating foam sealant. After insulating and instrumenting the specimens, they were placed in an environmental chamber and exposed to a thermal cycle. For the purpose of capturing the μPCM effect at about 43°C, the specimens were subjected to liquification and solidification transitions from 20 to 60°C and from 60 to 20°C, respectively.

Volumetric analysis was performed on the SGC pills before dimensioning the specimens to a 50 mm height. Despite the changes in SGC specimen mass and reduction in mixture design compaction temperature, from the 150°C allowed by the Indiana Department of Transportation (INDOT) specifications to the 135°C used for this study, the air void contents of the control mixture specimens fell within the permissible range. However, the air voids contents of the compacted μPCM-43 asphalt mixture specimens were outside the specification limits, between 3.6% and 6.4%. The air void contents of the μPCM-43 modified asphalt mixture specimens were significantly below the lower specification limit. Not surprisingly, as more μPCM-43 capsules were included, the air void content departed even more from specifications. These volumetric results strengthen the argument that, for the inclusion of μPCM-43 in asphalt mixtures, a portion of fine aggregate and mineral filler from the original mixture design should be replaced with μPCM-43 to satisfy the volumetric requirements (6, 30). Some μPCM-43 modified specimens showed an air void content even lower than 1%. To improve the feasibility of this technology, an appropriate mixture design procedure will need to be developed.

Table 2 illustrates how a reference mixture was re-designed to incorporate a significant portion of μPCM-43. The reference mixture had the same aggregate blend and volumetric properties as Mixture A (see Table 1), but it was prepared using a PG 64-22 asphalt binder. All the material passing the 0.600 mm sieve was substituted with μPCM-43. Considering that the capsules have a specific gravity of about 0.900, the replacement was estimated by volume and not by mass. Therefore, all the material passing the 0.600 mm sieve was assumed to have a specific gravity of 2.800.

Table 2.

Batching for μPCM-43 Re-Designed Asphalt Mixture

Component	Component mass (g)
Component	Reference mixture	μPCM-43 mixture
PG 64-22 asphalt binder	325.4	325.4
Retained sieve size (mm)
12.5	0.0	0.0
9.5	348.4	348.4
4.75	2533.9	2533.9
2.36	1796.3	1796.3
1.18	1036.6	1036.6
0.600	587.9	587.9
0.300	353.4*	0.0
0.150	179.1*	0.0
0.075	66.9*	0.0
Pan	172.1*	0.0
μPCM-43	0.0	248.0
Total mass	7400.0	6876.5
Proportion of μPCM-43 by total mixture mass (%)	0.0	3.6
Theoretical maximum specific gravity	2.544	2.341

Note: *Fine aggregate substituted with μPCM-43, including a portion of fine aggregate passing the 0.600 mm sieve (4.75 mm dolomite, 2.36 mm dolomite sand, and 9.5 mm RAP) and mineral filler.

First, the aggregate was batched as is typically done for specimen fabrication. After batching, the aggregate was sieved to determine the mass of the material to be substituted. The approximate volume of the material passing the 0.600 mm sieve was then calculated by dividing its total mass by 2.800. The estimated volume was multiplied by 0.900 to obtain the mass required to produce a similar volume replacement of μPCM-43. The mass of the virgin PG 64-22 asphalt binder was kept the same for the μPCM-43 re-designed mixture. The retained material at the 0.600 mm and virgin binder were heated at 135°C for 2 h and then mixed in the laboratory. Following mixing, the asphalt mixture was short-term conditioned for mixture mechanical property testing at 135°C, in accordance with AASHTO R 30-02. After conditioning, the μPCM-43 was added and manually mixed to the laboratory-prepared mixture until all the μPCM-43 particles appeared to be coated with asphalt binder (about 5 min). The μPCM-43 modified asphalt mixture was again placed in the oven for approximately 20 min to re-gain the 135°C required temperature for compaction. The modification is reported by the masses of raw material needed to prepare SGC specimens that were 180 mm in height (see Figure 2a), from which specimens with air void contents of 5.0 ± 0.5% could be extracted for mechanical testing. The same batching and mixing procedures were performed to prepare the required amount of material to determine each mixture’s theoretical maximum specific gravity (G_mm), according to AASHTO T 209-19.

Figure 2.

(a) μPCM-43 mixture, SGC specimen 180 mm in height and (b) dynamic modulus testing setup of small μPCM-43 mixture specimen.

A wide variety of PCM dosages have been investigated in experimental and numerical studies. The proportion of μPCM-43 by total mixture mass evaluated in this study is in good agreement with recent investigations and comparable to those reported in the literature (7 –9, 11, 29, 30). Perhaps the most interesting aspect of this study is the variation in aggregate components to allow for μPCM incorporation. Few studies have attempted to entirely re-design an asphalt mixture using μPCM. Instead, most investigations have added the μPCM directly to an existing asphalt mixture design without performing variations to the aggregate blend or by just replacing the mineral filler ( 30 ). As indicated by the G_mm values of the reference and μPCM-43 mixtures, a significant G_mm reduction is observed when μPCM is substituted for fine aggregates. Simultaneously, a lower amount of material is required to fabricate a desirable SGC specimen, 7400.0 g and 6876.5 g for the reference and μPCM-43 mixtures, respectively. Although replacing fine aggregate and mineral filler with μPCM-43 seems straightforward, such a change will cause a difference in volumetric properties and, consequently, mechanical performance. To better understand the mechanical behavior of the μPCM-43 mixture, the dynamic modulus for the re-designed asphalt mixture was determined using two specimen geometries, namely 100 mm diameter by 150 mm tall (large specimen) and 38 mm diameter by 110 mm tall (small specimen). Mechanical testing of large specimens was performed according to AASHTO T 378-17, small specimen dynamic modulus testing was conducted according to AASHTO TP 132-19 (see Figure 2b). In addition, the thermal response of μPCM-43 mixture specimens was investigated at three different air void content levels under controlled environmental conditions. The thermal response specimens were 100 mm in diameter and 50 mm thick, and were instrumented with thermocouples and insulated, as shown in Figure 3.

Figure 3.

(a) Insulation and instrumentation of specimens and (b) specimens of reference mixture and μPCM-43 mixture in environmental chamber.

Experimental Results

Asphalt Binder Testing

Studies into the viscoelastic behavior of asphalt binders have received increased interest from various researchers since the early 1990s, following the Strategic Highway Research Program (SHRP) (34, 35). The SHRP research effort reported that over most of the range of interest in asphalt binder applications, the rate of change of complex shear modulus (G*) with respect to temperature ranges from 15 to 25% per degree Celsius ( 34 ). This means that asphalt binders should exhibit a relatively constant G* change rate when subjected to a continuous temperature gradient. Such a concept has not been thoroughly investigated over the years, in significant part because of equipment limitations. Many of the commercial rheological testing devices available in the early 1990s were designed primarily for use with polymers, foodstuffs such as cheese, and other materials, many of which exhibit low-temperature dependency in comparison to asphalt binder ( 34 ). In such a situation, most rheological testing devices provided just enough temperature control accuracy for maintaining suitable repeatability of test data. Consequently, as per specifications, asphalt binder tests are typically performed under steady-state temperature conditions. However, today, rheological testing devices can perform temperature fluctuations and ramps with a great deal of accuracy. Some rheological testing devices can guarantee a homogeneous convection temperature distribution and thus accurate and stable temperature control for various testing protocols.

Figure 4a confirms that an asphalt binder subjected to a constant temperature increase (from 20 to 60°C) will show a relatively constant G* change rate. This calculation will vary depending on the temperature ramp applied and the interval at which rheological measurements are taken. For this study, the measurements were taken at 30 s intervals. This time step is used to calculate the G* change rate, as shown in Equation 1.

G^{*} Change Rate = \frac{G_{i}^{*}}{G_{i - 1}^{*}}

(1)

where $G_{i}^{*}$ is the complex shear modulus at a given temperature and $G_{i - 1}^{*}$ is the complex shear modulus obtained at the previous time step for the predetermined temperature change rate during the solidification or liquification cycle phase.

Figure 4.

Asphalt binder experiments using dynamic shear rheometer (DSR) and differential scanning calorimeter (DSC): (a) control binder, (b) DSR testing, 3°C per hour, (c) DSR testing, 9°C per hour, (d) DSR testing, 15°C per hour, and (e) DSC testing.

Data from several studies have demonstrated that the temperature can be shifted or reduced by incorporating μPCM into asphalt binders (6 –9). Although previous studies have encouraged the use of DSR measurements, a method to fully understand the effect of μPCM in asphalt binders using rheological data was not identified. For example, Kakar et al. ( 8 ) modified control asphalt binders with μPCM having a phase transition temperature close to 6°C, at three different levels: 0%, 1%, and 3% by total binder mass. In their research study, the rheological measurements were taken by performing temperature ramps between 20°C and −10°C, using a cooling/heating rate of about 26°C/h. The constant strain amplitude and frequency applied to run the tests were 0.1% and 6.3 rad/s, respectively. The rheological measuring system had a diameter of 8 mm and specimen thickness of 2 mm. To interpret the results, the master curve, complex shear modulus versus temperature, and phase angle versus temperature plots were generated. After analyzing the data, Kakar et al. ( 8 ) suggested that during cooling ramps, no thermal effect resulting from μPCM incorporation was noticed on the rheological response of modified asphalt binders. They concluded that, perhaps at higher μPCM concentrations, the effect could be detectable. The work presented in this paper differs from previous research on the idea that a μPCM effect cannot be captured by performing rheological measurements. If the data is normalized utilizing the G* change rate, then the μPCM effect is noticeable during solidification and liquification transitions, at low and relatively high μPCM dosages, and temperature fluctuations comparable to those experienced by asphalt pavements in the field.

Figure 4 shows that the rate of change concept can help quantify the μPCM effect in asphalt materials. This rheological data analysis is in good agreement with the results obtained from DSC measurements, as can be inferred from Figure 4e. The evidence is obscure because DSC measurements were performed at a cooling/heating rate of 600°C/h (or 10°C/min), which is a typical testing parameter for this thermal procedure. This cooling/heating rate is inconceivable for the rheological analysis of asphalt binders. Although DSC measurements were performed at a significantly higher cooling/heating rate, the μPCM effect peaks observed are connected to the rheological results obtained at cooling and heating rates between 3°C/h and 15°C/h. When the μPCM-43 releases heat (solidification phase), a reduction in the G* change rate is observed, primarily from a thermal lag in the asphalt binder specimen. The specimen is no longer transitioning at a constant temperature rate because of the μPCM-43. In contrast, when the μPCM-43 absorbs heat (liquification phase) from the system, an increase is noticed in the G* change rate, meaning the DSR must apply a higher torque (or activation energy) to generate the predetermined strain amplitude of 1.0% in the asphalt binder specimen. In both phases, a countereffect is observed in the rheological measurement after the μPCM-43 effect is completed compared with DSC results. A possible explanation for this might be that the binder experiences an accelerated temperature transition. Because of the thermal lag, there might be a significant temperature gradient between the DSR plates and μPCM-43 modified binder, causing a sudden temperature shift in the specimen. Additionally, in DSR testing, the asphalt binder is susceptible to heat losses as opposed to DSC testing, where the specimen is enclosed in a thermal chamber. More research is necessary to understand the cause of this countereffect and its potential ramifications.

The rheological results are also consistent with the present evidence suggesting that the intensity of the latent heat peak is shifted or augmented under different cooling and heating rates because thermal gradients are built up in the PCM. A study on the phase change process and latent heat of PCM-impregnated lightweight aggregate concluded that DSC tests with higher cooling and heating rates intensify the verticality of the curve at the peak temperature ( 36 ). Likewise, the rheological results of this study demonstrate that as the cooling/heating rate increases, the G* change rate peak intensifies, as shown in Figure 4, b–d. It should be noted that, regardless of the cooling/heating rate in DSC testing, the amount of latent heat absorbed and released by the μPCM-43 is constant. This fundamental concept can be demonstrated by calculating the integral under the DSC peak above the baseline ( 36 ). The analogous DSR and DSC plots suggest that a similar analysis might be applicable for the rheological results. Although the rheological findings should be treated with a degree of caution, the results are interesting because they identify an approach for capturing the μPCM effect in asphalt binders. This analysis warrants further investigations to correlate the G* change rate to the latent heat capacity of the μPCM-43 modified asphalt binders and their overall mechanical performance. Table 3 reports the latent heat characteristics of μPCM-43 modified asphalt binders according to the DSC measurements.

Table 3.

Differential Scanning Calorimeter Test Results for μPCM-43 Modified Asphalt Binders

μPCM-43 content by binder mass (%)	Solidification phase		Liquification phase
μPCM-43 content by binder mass (%)	Temperature range (°C)	Latent heat (J/g)	Temperature range (°C)	Latent heat (J/g)
5	29.3–39.5	8.3	38.5–43.8	8.8
10	29.3–39.1	16.4	38.8–44.0	16.8
20	28.9–37.9	31.1	39.3–44.5	31.6
30	28.9–37.7	42.7	39.4–42.4	42.8
40	28.6–36.7	50.7	39.5–44.5	50.7

Thermal Cycling of Asphalt Mixtures

Figure 5 shows the thermal response of asphalt specimens with and without μPCM-43. The control asphalt mixture is a 9.5 mm plant-produced mixture gathered in situ through plate sampling on the road, namely Mixture A. The specimens were exposed to a temperature cycle between 20°C and 60°C to capture the effect of μPCM at about 43°C. When the μPCM particles liquify, the μPCM-43 product delays the modified asphalt specimens from reaching a threshold temperature of 45°C and creates a thermal lag in the specimens. As a result, the temperature of the μPCM modified asphalt specimens dwells at about 43°C for a longer time than if the μPCM was not present. Likewise, during solidification, the μPCM effect provides additional relaxation time to the asphalt specimen during the temperature transition from 60 to 20°C, at approximately 43°C. A closer examination reveals that a second thermal lag is experienced at about 30°C when the μPCM-43 is cooling. This observation is more evident for the specimen modified with 8% μPCM-43 by total mixture mass. Overall, the thermal responses of the μPCM-43 modified asphalt specimens mimic the rheological analysis found for the asphalt binders, as presented in the previous section. As the μPCM-43 solidifies, two sharp peaks are detectable for the G* change rate, at about 43°C and 30°C. Conversely, when the μPCM-43 is transitioning to a liquid, the DSR, DSC, and thermal cycling measurements agree that the μPCM effect occurs more uniformly between 35°C and 45°C. Similar observations were obtained for the other three asphalt mixtures tested, Mixtures B, C, and D.

Figure 5.

Mixture A: thermal response of asphalt specimens with and without μPCM-43 at different depths from top surface: (a) top, (b) mid, and (c) bottom.

Table 4 reports the differences in temperature obtained at different depths from the top surface for the specimens with μPCM-43 relative to the control specimens. The liquification (from 20 to 60°C) and solidification (from 60 to 20°C) μPCM-43 transitions demonstrate disparities in the absolute temperature differences observed. The temperature differences are consistently lower for all mixture types during PCM solidification. Thus, the μPCM-43 appears to be more beneficial when liquifying, meaning that the μPCM-43 does a better job delaying the appearance of temperatures higher than 43°C in comparison to hindering temperatures lower than 43°C. This outcome is contingent on the type of μPCM and ambient temperature profile, along with other factors. Still, at first glance, the μPCM-43 effect seems promising in delaying temperatures above 45°C. The effect of μPCM is more meaningful as the depth from the top surface increases, primarily because of the one-dimensional heat flow generated under laboratory conditions. The bottom layer is not exposed to the ambient temperature and is safeguarded by the insulating material. In the field, the bottom layers of pavements are exposed to heat flux from the underlying layers. Thus, this outcome is not necessarily constant under realistic pavement conditions. Another important observation of this study is that the thermal cycling tests included mixtures composed of a wide variety of raw materials. The A, B, C, and D mixtures were gathered as part of a research project focused on implementing asphalt mixture design changes in Indiana ( 33 ). As a result, the mixtures used in the experiments come from various areas of the state. Consequently, the thermal cycling results suggest the μPCM effect is applicable for a broad array of asphalt mixture materials.

Table 4.

Absolute Maximum Temperature Difference Between Control and μPCM-43 Modified Specimens

Control mixture	μPCM-43 content by total mixture mass (%)	Phase transition	Absolute maximum temperature difference (°C)
			Depth from top surface (mm)
			0 (top surface)	25 (mid-specimen)	50 (bottom surface)
Mixture A	4	Liquification	4.64	5.68	6.16
	4	Solidification	2.61	2.81	2.92
	8	Liquification	7.93	9.48	10.13
	8	Solidification	4.56	5.22	5.43
Mixture B	4	Liquification	4.70	5.27	6.25
	4	Solidification	2.70	3.04	3.32
	8	Liquification	6.10	8.46	9.30
	8	Solidification	3.53	4.66	4.99
Mixture C	4	Liquification	4.72	5.62	6.29
	4	Solidification	2.47	2.85	3.19
	8	Liquification	7.53	9.06	10.32
	8	Solidification	4.10	4.74	5.46
Mixture D	4	Liquification	3.74	5.65	6.34
	4	Solidification	1.91	2.89	3.15
	8	Liquification	7.23	8.83	9.64
	8	Solidification	3.75	4.39	4.64

Figure 6 shows the thermal responses of the reference mixture, without μPCM, and the μPCM-43 mixture with different air void contents. The results resemble the thermal behavior of the μPCM-43 modified asphalt mixtures previously discussed, Mixtures A–D (see Figure 5), but this μPCM-43 mixture has air void contents within the acceptable air void content range. The mixture was purposely re-designed to incorporate the μPCM without including an excessive amount of fine particles (combination of aggregate passing the 0.600 mm sieve, mineral filler, and μPCM). In the μPCM-43 mixture, all the material passing the 0.600 mm sieve is μPCM-43.

Figure 6.

Thermal response of μPCM-43 mixture specimens with various air void contents at different depths from top surface: (a) top, (b) mid, and (c) bottom.

Table 5 highlights the importance of air void content to the temperature profile and heat flow within the μPCM-43 mixture specimens. As the air void content increases, the μPCM-43 effect seems to decrease, as demonstrated by the absolute maximum temperature difference between reference mixture and μPCM-43 mixture specimens. The minimum temperature differences observed at the surface of specimens with 6% air void content are 3.06°C and 2.09°C for the PCM liquification and solidification transitions, respectively. This outcome agrees with the work reported in Refaa et al. ( 6 ), which through numerical analysis determined that replacing a similar portion of fine aggregate and mineral filler with μPCM could reduce the surface temperature by about 2.70°C.

Table 5.

Absolute Maximum Temperature Difference Between Reference Mixture and μPCM-43 Mixture Specimens

Air void content of μPCM-43 mixture (%)	Phase transition	Absolute maximum temperature difference (°C)
		Depth from top surface (mm)
		0 (top surface)	25 (mid-specimen)	50 (bottom surface)
4	Liquification	4.16	4.66	5.89
4	Solidification	2.49	2.73	3.28
5	Liquification	3.12	4.43	5.22
5	Solidification	2.28	2.81	3.29
6	Liquification	3.06	4.02	4.92
6	Solidification	2.09	2.51	2.90

It should be emphasized that all temperature differences are relative to the control reference mixture specimen, which has an air void content of 5%. However, the tendency is undeniable and corroborates previous research studying the effect of air void content on thermal properties of asphalt mixtures. Hassn et al. ( 37 ) determined that asphalt mixtures with high air void content have lower thermal conductivity and specific heat capacities than those with lower air void content. Consequently, asphalt mixtures with high air void content are more suitable to alleviate the urban heat island effect. In contrast, asphalt mixtures with low air void content are recommended for harvesting solar energy from the environment. Given the results obtained for the μPCM-43 mixture, it is plausible that slightly lower air void content might be more beneficial to the μPCM effect.

Link Between Rheological and Thermal Cycling Results

Figure 7 illustrates the link between the rheological testing for asphalt binders and thermal cycling results obtained for compacted asphalt mixtures. The absolute temperature difference plots delineate the gaps in temperature between the reference mixture and μPCM-43 mixture specimens with 5% air void content. The disparities in temperature caused by the presence of μPCM have been the subject of investigation in previous studies (21, 29). Based on the fundamental theory of latent heat, Ma et al. ( 29 ) proposed quantifying the accumulation of temperature difference between a μPCM asphalt mixture and a non-μPCM modified asphalt mixture in a time range as a measurement of the effectiveness of μPCM modification. The parameter’s theoretical background suggests estimating the accumulated temperature difference (or area below the curve) as soon as a discrepancy is observed. Thus, Ma et al. ( 29 ) failed to account for inherent temperature differences because of dissimilar material properties and proportions or thermocouple measurement errors. As can be seen in Figure 7, the differences in temperature below 1°C are ignored as these disparities are not necessarily because of the latent heat effect of the μPCM.

Figure 7.

Comparison between rheological measurements and thermal cycling experiments: (a) thermal cycling, mid, (b) liquidification, and (c) solidification.

To further assess the capability of the G* change rate, the rheological results reported in this section correspond to PG 64-22 asphalt binder specimens modified with μPCM at 80%, 100%, and 120% by total binder mass. These asphalt binder-to-μPCM ratios are in good agreement with the raw quantities of material included in the μPCM-43 mixture, 248.0 g for μPCM-43 and 325.4 g for PG 64-22 binder. The percentages also resemble typical dust-to-binder ratios. The G* change rate was determined by conducting DSR tests at 6°C/h. The number, shape, and intensity of the G* change rate peaks are analogous to the temperature difference curves observed between the reference mixture and μPCM-43 mixture specimens. As previously reported, the peak in the PCM liquification transition is more uniform, generating a higher temperature difference than the solidification transition. When the paraffin material inside the μPCM-43 is crystallizing, the latent heat effect is divided into two parts, as confirmed in Figure 7c. It should be highlighted that as more μPCM-43 particles are included in the DSR testing, the G* change rate slightly drifts from the baseline, or the G* change rate reported by the control PG 64-22 binder specimen. However, peaks caused by μPCM-43 are still observed.

Mechanical Performance of μPCM-43 Mixtures

As shown in Figure 8, a comparison of the dynamic modulus master curves reveals that the reference mixture has greater stiffness than the μPCM-43 mixture, especially at the intermediate test temperatures (i.e., loading frequencies between 0.1 and 100 Hz). Previous studies have not discussed the reduction in dynamic modulus from the presence of μPCM-43 particles (6 –9, 30, 38). Several investigations have implied that the decrease in temperature caused by the μPCM effect translates directly to a higher asphalt mixture or binder stiffness without acknowledging the mechanical impact of the μPCM-43 particles. For conventional asphalt mixtures, some rules of thumb have been validated and can be used for quantifying the improvement of performance in asphalt mixtures caused by temperature reduction ( 21 ), where a slight shift in asphalt pavement temperature could lead to a significant increase in stiffness or relaxation, and enhance pavement life-cycle performance. For example, a 5°C reduction in temperature can delay the risk of cracking for about three years ( 21 ). However, the results presented here suggest that these relationships may not apply to μPCM modified asphalt mixtures. The reduction in dynamic modulus may be associated with the stiffness, surface area properties, and interfacial adhesion of the μPCM capsules. These factors should be further investigated through experimental testing and thermomechanical modeling, along with the thermal relaxation benefits that μPCM might provide.

Figure 8.

Mechanical performance of reference mixture and μPCM-43 mixture: (a) small specimens and (b) large specimens.

The data suggest there are issues related to the repeatability and reproducibility of the test methods for the μPCM-43 mixture at high temperatures. This result is consistent with laboratory observations. The dynamic modulus testing was conducted at three temperatures: 4°C, 20°C, and 40°C. During temperature conditioning and testing, a waxy appearance was noticed in the μPCM-43 mixture specimens at 40°C. Presumably, some of the capsules did not survive the mixing and specimen fabrication process, leading to paraffin leakage. As demonstrated by the DSC measurements, at temperatures below 38°C the core material of the μPCM-43, paraffin, is solid. At temperatures above 38°C, it transforms into liquid. Research is currently underway to improve μPCM formulation for paving purposes to better quantify the toughness of the μPCM-43 particles and their resistance to abrasive forces during mixture production and compaction. Additionally, both small and large specimens will be used to conduct fatigue cracking and flow number tests ( 39 ). Overall, despite all the current limitations of the technology, the μPCM-43 mixture stiffness is not too dissimilar to the behavior of a conventional asphalt mixture, as illustrated by the master curve analysis.

Summary and Conclusions

This study attempts to understand the environmental tuning of asphalt pavement materials using μPCM. The relevance of μPCM modified asphalt binders and mixtures is supported by the current findings available in the literature. The work presented in this paper advances the environmental tuning of asphalt pavement materials using μPCM and is significant because it establishes a rheological measurement and mixture design method for μPCM incorporation in asphalt binders and mixture. By strategically modifying asphalt binders and mixtures, optimum dosages of μPCM in asphalt paving materials are feasible. The thermal and mechanical tuning of asphalt materials to their anticipated environment is expected to improve asphalt mixture performance and extend asphalt pavement service lives, leading to more sustainable paving technologies. The following conclusions can be drawn from the present study:

Rheological measurements using the DSR equipment can help to identify the latent heat effect of μPCM particles in asphalt binders. This study’s findings propose using the G* change rate parameter to determine the temperatures at which the μPCM effect occurs. This approach can also provide insights into the intensity of the impact of μPCM on asphalt mixtures.

The experimental results confirm that μPCM modified asphalt mixture specimens can experience temperature differences between 1.9°C and 10.3°C lower, as compared with non-μPCM modified asphalt mixture specimens subjected to the same ambient temperatures. In practice, this suggests that μPCM can delay the appearance of undesirable temperatures in asphalt pavements. However, this outcome depends on the amount and characteristics of μPCM, asphalt mixture materials, ambient temperature, phase change transition (solidification or liquification), depth from the pavement surface, and density of the compacted asphalt mixture.

DSR and DSC results verify that asphalt binder viscoelastic flow is a thermally activated process. The rate at which asphalt binder stiffness changes is conditional on the temperature fluctuations experienced by the material.

Although the thermal performance of asphalt pavements can be tuned by including μPCM, the variations in mechanical performance arising from the addition of μPCM should be assessed. As demonstrated in this research, proper mixture design procedures can facilitate the incorporation of μPCM capsules in asphalt pavements.

A countereffect in the viscoelastic flow of μPCM modified asphalt binders was found based on the G* change rate analysis. Therefore, a natural progression of this work is to determine the causes and possible implications of this finding.

This study has demonstrated an interrelation between the volumetric, mechanical, and thermal properties of μPCM modified asphalt mixtures. However, this investigation is unable to encompass the entire set of tradeoffs among these properties from the μPCM incorporation. Further research is needed to provide greater insight into the effects of μPCM on asphalt mixtures’ volumetric, mechanical, and thermal properties, such as binder absorption of μPCM particles and particle-to-particle interlock and specific heat capacity of μPCM modified asphalt mixtures.

Future work is needed to evaluate the survivability of μPCM during the production, placement, and compaction of asphalt materials. In addition, the long-term performance of the μPCM capsules under repetitive vehicle loading and temperature fluctuations must be assessed.

Finally, this research study’s overall significance should not be limited to the optimum characterization and design of μPCM asphalt materials. The widespread implementation of μPCM modified asphalt pavements can potentially benefit not only the pavement performance but also society. As demonstrated by this study, the environmental tuning of asphalt materials using μPCM could mitigate the appearance of intense surface and inner temperatures on asphalt pavements, which could help alleviate the urban heat island effect. Consequently, this research’s findings could have positive impacts in various areas, such as transportation safety and electricity demand. To that end, further studies are required to fully understand the implications of actively controlling the temperature of asphalt pavements. More research is needed to perform field trials that demonstrate the modification of asphalt materials with μPCM. Such a process could help validate and address μPCM modified asphalt pavements’ benefits, design, and challenges. The additional costs related to the use of μPCM particles will be hard to justify without reliable field performance histories. Although this study is limited to laboratory methods, the findings of this paper are expected to contribute to the prospective field implementation of μPCM modified asphalt materials.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: M. A. Montoya, D. Betancourt, R. Rahbar-Rastegar, J. Youngblood, C. Martinez, J. E. Haddock; data collection: M. A. Montoya, D. Betancourt; analysis and interpretation of results: M. A. Montoya, R. Rahbar-Rastegar, J. E. Haddock; draft manuscript preparation: M. A. Montoya, J. E. Haddock. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research work was supported by the Indiana Department of Transportation (INDOT) and Joint Transportation Research Program (JTRP), Award Number: SPR-4335. The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the sponsor. These contents do not constitute a standard, specification, or regulation.

ORCID iDs

Miguel A. Montoya

Daniela Betancourt

Reyhaneh Rahbar-Rastegar

Jeffrey Youngblood

Carlos Martinez

John E. Haddock

References

Mallick

R. B.

Radzicki

M. J.

Daniel

J. S.

Jacobs

J. M.

Use of System Dynamics to Understand Long-Term Impact of Climate Change on Pavement Performance and Maintenance Cost. Transportation Research Record: Journal of the Transportation Research Board, 2014. 2455: 1–9. https://doi.org/10.3141/2455-01.

Asphalt Institute Foundation. STAR Symposium Report2017. https://bookstore.asphaltinstitute.org/STAR17report. Accessed March 16, 2020.

Kodippily

Yeaman

Henning

Tighe

Effects of Extreme Climatic Conditions on Pavement Response. Road Materials and Pavement Design, Vol. 21, No. 5, 2020, 1413–1425. https://doi.org/10.1080/14680629.2018.1552620.

Papagiannakis

A. T.

Pavement Design and Materials. John Wiley, Hoboken, NJ, 2008.

Behnood

McDaniel

Shah

Olek

Analysis of the Multiple Stress Creep Recovery Asphalt Binder Test and Specifications for Use in Indiana. Joint Transportation Research Program Publication No. FHWA/IN/JTRP-2016/07. Purdue University, West Lafayette, IN, 2016. https://doi.org/10.5703/1288284316330.

Refaa

Kakar

M. R.

Stamatiou

Worlitschek

Partl

M. N.

Bueno

Numerical Study on the Effect of Phase Change Materials on Heat Transfer in Asphalt Concrete. International Journal of Thermal Sciences, Vol. 133, 2018, pp. 140–150. https://doi.org/10.1016/j.ijthermalsci.2018.07.014.

Kakar

M. R.

Refaa

Worlitschek

Stamatiou

Partl

M. N.

Bueno

Effects of Aging on Asphalt Binders Modified With Microencapsulated Phase Change Material. Composites Part B: Engineering, Vol. 173, 2019, p. 107007. https://doi.org/10.1016/j.compositesb.2019.107007.

Kakar

M. R.

Refaa

Worlitschek

Stamatiou

Partl

M. N.

Bueno

Thermal and Rheological Characterization of Bitumen Modified With Microencapsulated Phase Change Materials. Construction and Building Materials, Vol. 215, 2019, pp. 171–179. https://doi.org/10.1016/j.conbuildmat.2019.04.171.

Wei

Wang

Effects of Microencapsulated Phase Change Materials on the Performance of Asphalt Binders. Renewable Energy, Vol. 132, 2019, pp. 931–940. https://doi.org/10.1016/j.renene.2018.08.062.

10.

Sharma

Tyagi

V. V.

Chen

C. R.

Buddhi

Review on Thermal Energy Storage with Phase Change Materials and Applications. Renewable and Sustainable Energy Reviews, Vol. 13, 2009, pp. 318–345. https://doi.org/10.1016/j.rser.2007.10.005.

11.

Jin

Lin

Liu

Xiao

Zheng

Qian

Liu

Wen

Preparation and Thermal Properties of Mineral-Supported Polyethylene Glycol as Form-Stable Composite Phase Change Materials (CPCMs) Used in Asphalt Pavements. Scientific Reports, Vol. 7, No. 1, 2017, pp. 1–10. https://doi.org/10.1038/s41598-017-17224-1.

12.

Agyenim

Hewitt

Eames

Smyth

A Review of Materials, Heat Transfer and Phase Change Problem Formulation for Latent Heat Thermal Energy Storage Systems (LHTESS). Renewable and Sustainable Energy Reviews, Vol. 14, No. 2, 2010, pp. 615–628. https://doi.org/10.1016/j.rser.2009.10.015.

13.

Lane

G. A.

Solar Heat Storage: Latent Heat Materials. CRC Press, Boca Raton, FL, 2018.

14.

Zalba

Marín

J. M.

Cabeza

L. F.

Mehling

Review on Thermal Energy Storage with Phase Change: Materials, Heat Transfer Analysis and Applications. Applied Thermal Engineering, Vol. 23, No. 3, 2003, pp. 251–283. https://doi.org/10.1016/S1359-4311(02)00192-8.

15.

Dutil

Rousse

D. R.

Ben Salah

Lassue

Zalewski

A Review on Phase-Change Materials: Mathematical Modeling and Simulations. Renewable and Sustainable Energy Reviews, Vol. 15, No. 1, 2011, pp. 112–130. https://doi.org/10.1016/j.rser.2010.06.011.

16.

Baetens

Jelle

B. P.

Gustavsen

Phase Change Materials for Building Applications: A State-of-the-Art Review. Energy and Buildings, Vol. 42, No. 9, 2010, pp. 1361–1368. https://doi.org/10.1016/j.enbuild.2010.03.026.

17.

Incropera

F. P.

Fundamentals of Heat and Mass Transfer, 6th ed. John Wiley, Hoboken, NJ, 2007.

18.

Oró

de Gracia

Castell

Farid

M. M.

Cabeza

L. F.

Review on Phase Change Materials (PCMs) for Cold Thermal Energy Storage Applications. Applied Energy, Vol. 99, 2012, pp. 513–533. https://doi.org/10.1016/j.apenergy.2012.03.058.

19.

Ismail

Salinas

C. T.

Henriquez

J. R.

Comparison Between PCM Filled Glass Windows and Absorbing Gas Filled Windows. Energy and Buildings, Vol. 40, No. 5, 2008, pp. 710–719. https://doi.org/10.1016/j.enbuild.2007.05.005.

20.

Cui

Liao

Memon

Dong

Tang

Thermophysical and Mechanical Properties of Hardened Cement Paste with Microencapsulated Phase Change Materials for Energy Storage. Materials, Vol. 7, No. 12, 2014, pp. 8070–8087. https://doi.org/10.3390/ma7128070.

21.

Zhou

X. Y.

Ren

J. P.

Chang

Y. J.

The Mechanism of Different Thermoregulation Types of Composite Shape-Stabilized Phase Change Materials Used in Asphalt Pavement. Construction and Building Materials, Vol. 98, 2015, pp. 547–558. https://doi.org/10.1016/j.conbuildmat.2015.08.038.

22.

Wang

Peng

Preparation and Properties of Composite Shape-Stabilized Phase Change Material for Asphalt Mixture. Applied Mechanics and Materials, Vol. 71–78, 2011, pp. 118–121. https://doi.org/10.4028/www.scientific.net/AMM.71-78.118.

23.

L. H.

J. R.

Zhu

H. Z.

Analysis on Application Prospect of Shape-Stabilized Phase Change Materials in Asphalt Pavement. Applied Mechanics and Materials, Vol. 357–360, 2013, pp. 1277–1281. https://doi.org/10.4028/www.scientific.net/AMM.357-360.1277.

24.

Leng

Chen

Zheng

Theoretical and Experimental Studies on Preparation of OMMT-Based Composite Phase Change Materials Used in Asphalt Pavement. Key Engineering Materials, Vol. 599, 2014, pp. 355–360. https://doi.org/10.4028/www.scientific.net/KEM.599.355.

25.

Kakar

M. R.

Refaa

Bueno

Worlitschek

Stamatiou

Partl

M. N.

Investigating Bitumen’s Direct Interaction with Tetradecane as Potential Phase Change Material for Low Temperature Applications. Road Materials and Pavement Design, Vol. 21, No. 8, 2020, pp. 2356–2363. https://doi.org/10.1080/14680629.2019.1601127.

26.

Athukorallage

Dissanayaka

Senadheera

James

Performance Analysis of Incorporating Phase Change Materials in Asphalt Concrete Pavements. Construction and Building Materials, Vol. 164, 2018, pp. 419–432. https://doi.org/10.1016/j.conbuildmat.2017.12.226.

27.

Ren

Zhou

Tian

Temperature Responses of Asphalt Pavement Structure Constructed with Phase Change Material by Applying Finite Element Method. Construction and Building Materials, Vol. 244, 2020, p. 118088. https://doi.org/10.1016/j.conbuildmat.2020.118088.

28.

Jin

Xiao

Zheng

Liu

Qian

Xie

Wei

Zhang

Liu

Preparation and Thermal Properties of Encapsulated Ceramsite-Supported Phase Change Materials Used in Asphalt Pavements. Construction and Building Materials, Vol. 190, 2018, pp. 235–245. https://doi.org/10.1016/j.conbuildmat.2018.09.119.

29.

Chen

S.-S.

Wei

Liu

F.-W.

Zhou

X.-Y.

Analysis of Thermoregulation Indices on Microencapsulated Phase Change Materials for Asphalt Pavement. Construction and Building Materials, Vol. 208, 2019, pp. 402–412. https://doi.org/10.1016/j.conbuildmat.2019.03.014.

30.

Bueno

Kakar

M. R.

Refaa

Worlitschek

Stamatiou

Partl

M. N.

Modification of Asphalt Mixtures for Cold Regions Using Microencapsulated Phase Change Materials. Scientific Reports, Vol. 9, No. 1, 2019, pp. 1–10. https://doi.org/10.1038/s41598-019-56808-x.

31.

Jamekhorshid

Sadrameli

S. M.

Farid

A Review of Microencapsulation Methods of Phase Change Materials (PCMs) as a Thermal Energy Storage (TES) Medium. Renewable and Sustainable Energy Reviews, Vol. 31, 2014, pp. 531–542. https://doi.org/10.1016/j.rser.2013.12.033.

32.

Yavuzturk

Ksaibati

Chiasson

A. D.

Assessment of Temperature Fluctuations in Asphalt Pavements due to Thermal Environmental Conditions Using a Two-Dimensional, Transient Finite-Difference Approach. Journal of Materials in Civil Engineering, Vol. 17, No. 4, 2005, pp. 465–475. https://doi.org/10.1061/(asce)0899-1561(2005)17:4(465).

33.

Haddock

J. E.

Rahbar-Rastegar

Pouranian

M. R.

Montoya

Patel

Implementing the Superpave 5 Asphalt Mixture Design Method in Indiana. Joint Transportation Research Program Publication No. FHWA/IN/JTRP-2020/12. Purdue University, West Lafayette, IN, 2020. https://doi.org/10.5703/1288284317127.

34.

Anderson

D. A.

Christensen

D. W.

Bahia

H. U.

Dongre

Sharma

M. G.

Antle

C. E.

Button

National Research Council, Report No. SHRP-A-369: Binder Characterization and Evaluation, Volume 3: Physical Characterization. Strategic Highway Research Program, Washington, D.C., 1994.

35.

Yusoff

N. I. M.

Shaw

M. T.

Airey

G. D.

Modelling the Linear Viscoelastic Rheological Properties of Bituminous Binders. Construction and Building Materials, Vol. 25, No. 5, 2011, pp. 2171–2189. https://doi.org/10.1016/j.conbuildmat.2010.11.086.

36.

Kheradmand

Castro-Gomes

Azenha

Silva

P. D.

De Aguiar

J. L. B.

Zoorob

S. E.

Assessing the Feasibility of Impregnating Phase Change Materials in Lightweight Aggregate for Development of Thermal Energy Storage Systems. Construction and Building Materials, Vol. 89, 2015, pp. 48–59. https://doi.org/10.1016/j.conbuildmat.2015.04.031.

37.

Hassn

Aboufoul

Dawson

Garcia

Effect of Air Voids Content on Thermal Properties of Asphalt Mixtures. Construction and Building Materials, Vol. 115, 2016, pp. 327–335. https://doi.org/10.1016/j.conbuildmat.2016.03.106.

38.

Tian

Y.-X.

Liu

F.-W.

Zhou

X.-Y.

Thermoregulation Effect Analysis of Microencapsulated Phase Change Thermoregulation Agent for Asphalt Pavement. Construction and Building Materials, Vol. 221, 2019, pp. 139–150. https://doi.org/10.1016/j.conbuildmat.2019.05.184.

39.

Montoya

Environmentally Tuning Asphalt Pavements Using Phase Change Materials: Benefits, Design, and Challenges. Doctoral dissertation. Purdue University, West Lafayette, IN, 2021.