Reducing the impact of the sunlight in urban areas using asphalt mixtures with phase change materials: a review in Scopus in the last three years

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Journal of Physics: Conference Series

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Reducing the impact of the sunlight in urban areas using asphalt mixtures with phase change materials: a review in Scopus in the last three years

To cite this article: Salmon Landi Jr. et al 2022 J. Phys.: Conf. Ser. 2407 012022

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Fifth International Conference on Applications of Optics and Photonics (AOP2022) Journal of Physics: Conference Series 2407 (2022) 012022

IOP Publishing doi:10.1088/1742-6596/2407/1/012022

Reducing the impact of the sunlight in urban areas using asphalt mixtures with phase change materials: a review in Scopus in the last three years

Salmon Landi Jr.^1,a, Iran Rocha Segundo^2,3,b, Natália Homem^4,c, Jorge Sousa³, Elisabete Freitas³, Manuel F. M. Costa⁵, and Joaquim Carneiro^2,d

1 Federal Institute Goiano, Rio Verde – GO, Brazil;

2 Centre of Physics of Minho and Porto Universities (CF-UM-UP), University of Minho, Azurém Campus, Guimarães, Portugal;

3 ISISE, Department of Civil Engineering, University of Minho, Guimarães, Portugal;

4 Digital Transformation CoLab (DTx), Building 1, Campus of Azurém, University of Minho, Guimarães, Portugal;

5 Centre of Physics of Minho and Porto Universities (CF-UM-UP), Gualtar Campus, University of Minho, Braga, Portugal

a[email protected], ^b[email protected],

c[email protected], ^d[email protected],

Abstract. Phase change materials (PCMs) have been incorporated into asphalt concrete pavements because they can regulate the temperature by absorbing and releasing heat during physical state changes. This effect reduces temperature gradients of pavements and, consequently, increases its service life. This work presents a systematic review of recent articles published in peer-reviewed journals (available in the Scopus database) involving asphalt mixtures with PCMs and focusing on mechanical characterization. It is observed that most of the selected papers investigated the benefits of polyethylene glycol as a PCM. The most common strategy to avoid leakage during the phase transition involved using a porous material that acts as a carrier matrix for the PCMs. Generally, asphalt pavements with PCMs are systems with favourable thermal transferability, thus demonstrating higher heat absorption and dissipation rates. Finally, the asphalt mixtures containing PCMs showed lower mechanical performance than the control mixtures. However, they still satisfy the required criteria. In any case, it is expected that with the incorporation of PCMs into asphalt pavements, the social and environmental effects (Urban Heat Island) of sunlight in urban areas can be mitigated by the thermoregulation phenomena.

Keywords:Phase change material; Asphalt mixture; Thermal properties; Permanent deformation

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2 1. Introduction

Urban heat islands (UHI) can be identified by the occurrence of higher temperatures in urban areas compared to surroundings (typically rural zones) and have been considered one of the most significant problems in the present century. As a result of urbanization's development, the natural ground is replaced by paved roads, sidewalks, and parking lots, among others [1]. The land surface alteration promotes lower solar albedo (percentage of solar energy that reaches a given surface and is reflected), higher heat store capability, and little water evaporation [2]. Thermal properties, namely thermal conductivity, specific heat capacity, albedo, density, and thermal emissivity, are the main factors that influence the thermal behavior of pavements. By making appropriate changes to these factors, the pavement and the air around it can be made cooler [3].

A large part of the studies on cool pavement strategies focuses primarily on reflectance since this characteristic is a determining factor in the maximum surface temperature of road pavements [4]. Increasing the albedo makes it possible to reduce the temperatures of the pavement's lower layers. The amount of heat available at the top of the pavement is smaller, and so is the heat within the subsequent layers. Yet another factor that greatly conditions reflectance is the effect of weathering and accumulation of dirt on the surface [5].

To prevent pavements from reaching too high temperatures, an approach is the use of phase change materials (PCM) [6]. These materials are characterized by absorbing significant amounts of energy when they change state, known as latent heat. The absorbed heat is then sent back into the surrounding environment when the material returns to its initial physical state [7]. Thermal conductivity is one of the main parameters related to the temperature conduction and distribution in asphalt materials. It is worth noting that if the thermal conductivity is too low, it will delay the thermal response of storing and releasing latent heat, and PCMs cannot absorb or release heat in the required temperature range, which is not interesting [10].

The attempt to apply PCM in asphalt mixtures is motivated by the success of these applications in other civil engineering materials, particularly in concretes and mortars. However, to incorporate PCM into any system, it is essential to properly understand the underlying concepts and principles to optimize the process [8]. Incorporating PCMs in asphalt mixtures presents challenges according to the different uses. Perhaps the most difficult challenge is preventing the PCMs from escaping when the pavements are subjected to high temperatures and mechanical loads exerted during manufacture [9].

This paper aims to review the work done over the last three years, focusing on the incorporation of PCM into asphalt mixtures as temperature regulators. It is essential to identify and analyze the main issues concerning the use of PCMs, namely, which ones can withstand the high mixing temperatures and which methods have been studied to avoid PCM leakage in the mixture. In addition, it is sought to know whether the latest studies analyze the impact of the incorporation of these materials concerning mechanical strength, as these materials are included in the composition and may influence these same mechanical characteristics.

2. Methods

The method applied in this study included identification of the papers that present both recent experimental results involving the application of PCMs in asphalt mixture and analyzes of their mechanical properties. The literature survey was carried out in the Scopus database using “phase change material” AND “asphalt” as keywords. Only articles published in the English language during the period 2019–2022 were selected. In fact, the works included in this literature review are those that mention experimental results of the permanent deformation of asphalt mixtures with PCMs evaluated using wheel tracking and/or dynamic creep tests. Moreover, literature reviews were not considered in this search. The obtained articles list was carefully scrutinized using the guiding questions below.

• Which type of PCM was used? And what are the corresponding main characteristics?

• What was the strategy used to prevent leakage of PCMs in asphalt mixture?

• What was the method of incorporating PCM into the asphalt mixture?

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• In comparative terms, what thermal improvement was achieved with the incorporation of PCM in the asphalt mixture?

• Were mechanic properties of the asphalt mixtures with PCMs, as permanent deformation, affected?

3. Results and Discussions

The search yielded 56 works considering the keywords “phase change material” AND “asphalt”.

However, according to the criteria presented in the previous section, the papers selected for the present study were only 9. Table 1 summarizes the information collected from these papers to help elaborate an answer to each question previously asked.

3.1. Raw material, obtaining and thermal performance of asphalt mixtures with PCMs

Most of the works presented in Table 1 investigated the benefits of polyethylene glycol (PEG) as PCM.

Other PCMs such as acids, formaldehyde, or NiTi alloys were also addressed. Among the advantages of PEG, the adequate phase change temperature range, large phase change enthalpy, non-toxicity, and good thermal and chemical stability are reported.

Regarding strategies to overcome the challenges due to material loss, that is, leakage of PCMs during the solid-liquid transition, porous materials that act as a carrier matrix for the PCMs are usually used.

For this purpose, SiO2 has been widely applied, as well as carbon-based materials and diatomite.

Polymerization and encapsulation also have been pointed out as options for obtaining core-shell systems in which the PCM is involved/protected by other materials, such as polyacrylamide.

About the method of incorporating PCMs into the asphalt mixture, in the majority of works addressed, these materials replaced the fine aggregates (dry mixing) as opposed to their addition to the asphalt binder (wet mixing).

Usually, asphaltic pavements with PCMs consist of systems with favorable thermal transferability and, therefore, demonstrate higher heat absorption and dissipation rates, which is beneficial for heat exchange between the pavement and the ambient environment. Three of the five works that evaluated the thermal conductivity reported an increase in this property due to the introduction of PCMs in asphalt mixtures. All the analyzed works mentioned positive characteristics when PCMs were added, such as peak temperature reduction compared to the control sample or even a reduction in the heating rates of asphalt mixtures.

3.2. Mechanical performance of asphalt mixtures with PCMs

It is well known that the incorporation of PCMs may affect the mechanical performance of pavements.

Thus, in the present work, special attention was given to papers that performed experimental tests such as wheel tracking and/or dynamic creep (Table 1). These are two of the most relevant tests for analyzing the permanent deformation of asphalt mixtures [11–13].

Except for the case where NiTi alloy is used [10], the asphalt mixtures containing PCMs have lower dynamic stability (permanent deformation resistance) when compared to control mixtures (without PCMs). This could be explained by the reduction of the stiffness of the asphalt mixtures when soft materials such as PCMs are incorporated [14,15]. However, they still satisfy the required criteria for this parameter.

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4 Table 1: Results of use of PCM in asphalt mixtures Ref.PCMIncorporation method of PCM in asphalt mixture

Asphalt mixture with PCMMain outcomes Type and size Melting point (°C) / enthalpy of fusion (J/g)

Strategy to prevent leakagePCM % incorporationThermal performanceMechanical performance [11]PEG 800 with polyacrylamide / 2-4 mm

17.1 / 72.2Copolymerization methodWet mixing 2.5-12.5% (by asphalt binder volume)

Greatly reduced the thermal conductivity (reduction up to 87.4%).

The asphalt mixtures containing PCMs show lower dynamic stability compared with the control mixture. However, they still satisfy the required criteria.

For best % PCM incorporation (7.5%) the temperature difference was up to 3.8 °C, and a delay was about 20 min compared to the control sample when the mixtures were cooled from 21 to 0 °C. [12]N-tetradecane with melamine formaldehyde resin and different graphene content

3.0-6.7 / 9.1- 100In situ polymerizationDry mixing 5% in mass proportionVolume-specific heat capacity of the asphalt mixture was enhanced by 43%.

The dynamic stability of all PCM-modified mixtures dropped (20-68%).

Microcapsules with 0.45% graphene content (best sample) provided an acceptable value for dynamic stability (2019 times/mm) and a 70% increase in bending deformation energy density. [13]PEG 2000 with hydrophobic fumed silica (HFS) and PEG 2000 with SiO2.

30.8 / 69.6 and 35.7 / 92.5 for PEG-HFS and PEG-SiO2, respectively.

PEG was filled or adsorbed in the spatial structure of SiO2 (carrier matrix).

Dry mixing Fine aggregate with the particle size range of 0.075-0.15 mm, 0.15-0.3 mm, 0.3-0.6 mm, and mineral filler were replaced by 25-75% of the corresponding particle size range granular PEG/SiO2 and powdered PEG/HFS in equal volume.

As the amount of PCM increases, the thermal conductivity of the asphalt mixture shows a downward trend (reduction up to 33.3%).

The presence of PCMs impaired the dynamic stability of the asphalt mixture.

When the PCM amount was 25% (best %), the impact on the mechanical still met the higher requirements. The temperature difference between the best sample and the control at a depth of 30 mm under tungsten iodine lamp irradiation for 3 h was about 4 °C.

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Table 1: continued Ref.PCMIncorporation method of PCM in asphalt mixture

Strategy to prevent leakagePCM % incorporation Thermal performanceMechanical performance [14]PEG 800 with polyacrylamide 17.1 / 72.2 and 16.5 / 68.9 for the 50th heating/ cooling cycling

Copolymerization using polyacrylamide

Wet mixing 2.5-12.5%(by asphaltbinder volume)

The thermal conductivityofthe different types of mixtures with PCMs decreases byupto 84.8% in relation to the control groups.

The dynamic stability of different mixturetypes decreases with the increase of PCM, and the smaller the maximum nominal particle size of the aggregate is, the higher the degree of reduction becomes.

All tested specimens meet the required dynamic stability. 7.5%, 7.5%, and 10.0% are recommended to apply in AC-10,AC-13,and AC-16, respectively [15]PEG with expanded graphite / 45-75 µm

42.7 / 122.9 Pores present in the expandedgraphite matrix werefilled with PEG.

Wet mixing 10-50%(by asphaltbinder volume)

The thermal conductivityofthe AC-13 asphalt mixtureincreased (7.91-56.80%) proportionally to the dosageofPCMs. Thermal diffusivities exhibited significant growth due tothe presence ofPCMs. Nonetheless, this benefitwas limited whenthe PCM dosagereached 30%.

The dynamicstabilityof the control sample was 1853 cycles/mm, while for samples with 10, 20, 30, 40, and 50 vol%, the values were1939, 1997, 2012, 2033, and 2052 cycles/mm, respectively.

PCMs increased the dynamic stability, thermal conductivity, and thermal diffusivity of asphalt mixtures.

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6 Table 1: continued Ref.PCMIncorporati on method of PCM in asphalt mixture

Strategy to prevent leakagePCM % incorporationThermal performanceMechanical performance [16]PEG 4000 with SiO2 / 0.6-1.18 mm

57.4 / 166.8 The sol-gel method was used to synthesize a composite with PEG serving as the core and SiO2 as the shell.

Dry mixing 0-3.0% (by mass) PCMs particles were replaced by aggregates of the same size.

The indoor heating test showed that the maximum peak temperature drop in the lab was 3.0 °C when comparing the 3.0% sample to the reference (without PCM)

Dynamic stability decreased about 7500 times/mm for the reference sample at 5000 times/mm with a dosage equal to 3% PCM.

Dynamic stability has decreased with the percentage of PCMs; however, the modified mixtures generally had a satisfactory mechanical performance. Besides that, the indoor heating test showed that the PCMs positively influence reducing slab temperature. [17]Stearic acid (SA) with diatomite (DI) or expanded perlite (EP) / Maximum 0.15 mm.

Both about 67 / 143.7 (SA- DI) and 105.5 (SA-EP)

Preparation of mineral-supported SA.

Dry mixing 50% of the same size fine aggregate was replaced with 0.3–0.6 mm and 0.15–0.3 mm of PCMs

After 530 min of irradiation with artificial light, the temperature of the upper surface of the sample with the PCM was 2.3 °C lower than the corresponding value considering the conventional asphalt mixtures.

Asphalt mixtures with PCMs show much higher values of rutting depth than conventional asphalt mixtures at high temperatures.

The thermal storage and release rate of SA-DI was much lower than that of SA-EP. The addition of PCMs reduced the rutting resistance of pavement. [18]Stearic acid and palmitic acid with diatomite

52.9 / 106.7 The diatomite (DI) was selected as the load matrix.

Dry mixing The fine aggregate of 0.075 mm and filler were replaced with (SA + PA)/DI PCM for the corresponding particle size.

Both the albedo and specific heat capacity of specimens with PCM are larger than those with PCM.

The conventional asphalt mixture can achieve the maximum round-trip times of 20,000 times, and the final rutting depth was 9.53 mm. With the PMC, these values were about 18,000 times and 20 mm, respectively.

The thermal performance results reveal that the presence of PCM can reduce the peak temperature of the upper and bottom surface by up to about 8 and 6 °C, respectively, during a summer day.

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Table 1: continued Ref.PCMIncorporation method of PCM in asphalt mixture

Strategy to prevent leakagePCM % incorporationThermal performanceMechanical performance [10]NiTi alloy / diameter 1.0 mm and length 2.0 mm

54 / 26.9- Dry mixing Substituting the equal volume (3- 12 wt%) of NiTi particles for 0/3 mm level of fine aggregate.

The thermal conductivity of asphaltic mixtures shows an increasing linear trend with the increase of the substitution rate of NiTi particles. The latent heat thermoregulation index is the highest at a 6 wt% substitution rate of NiTi particles.

Dynamic stability of asphalt mixtures gradually increased with the substitution rate of NiTi particles: 1186.4 time/mm (reference sample) to 3795.2 time/mm (12wt% of NiTi).

Outdoor tests show that the 3 wt% (12wt%) sample has 0.3 °C (3.5 °C) daily maximum temperature difference from the ordinary asphalt mixture specimen.

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8 4. Conclusions

This research aimed to review PCM’s incorporation into asphalt mixtures as temperature regulators in the last three years. The main conclusions were:

• The most used PCM in asphalt mixtures is Polyethylene glycol (PEG);

• The leakage is a concern for the researchers to maintain the thermal regulation and mechanical properties of the asphalt mixtures. The main techniques used to produce the PCM to prevent leakage are the development of porous materials and copolymerization and encapsulation;

• The most used material as shell of PCM capsules is polyacrylamide;

• The PCMs are incorporated into asphalt mixtures by two techniques: bulk incorporation (called dry mixing) and asphalt binder modification (called wet mixing);

• On the one hand, the incorporation of the PCM into asphalt mixtures presents improvements in thermal properties; on the other hand, there is a worsening in mechanical properties, namely permanent deformation.

Acknowledges

This work was partially financed by FCT - Fundação para a Ciência e a Tecnologia - under the projects of the Strategic Funding UIDB/04650/2020, MicroCoolPav project EXPL/EQU- EQU/1110/2021, and NanoAir project PTDC/FISMAC/6606/2020.

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