A Fundamental Study on an SAP Mixed Asphalt Mixture for Reducing the Urban Heat Island Effect

Abstract

As the average temperature in summer rises and heat waves occur more frequently, the urban heat island (UHI) phenomenon is becoming a social problem. Asphalt road pavement stores heat during the day, raising the surface temperature, and releases the stored heat at night, thereby aggravating the UHI phenomenon. Government authorities often spray water to lower the temperature of road pavement for the safety and convenience of citizens. However, the effect is immediate and does not last long. Therefore, in order to reduce the urban heat island phenomenon by spraying water, the recovery time of the surface temperature must be delayed. In this study, Super Absorbent Polymer (SAP), a highly absorbent polymer that absorbs 100 to 500 times its weight in water, was applied to asphalt road pavement. SAP is commonly used in diapers, feminine hygiene products, soil moisturizers, and concrete, and its scope is gradually expanding. The purpose of this study is to reduce the urban heat island phenomenon by mixing the SAP into asphalt and to increase the latent heat flux by evaporating the water absorbed by the SAP, thereby delaying the recovery time of the surface temperature of the road pavement. In this study, the performance of asphalt mixtures mixed with the SAP and the thermal characteristics according to the mixing amount were analyzed. In this study, the physical properties and temperature reduction performance of the asphalt mixture according to the SAP type and content were studied. The results of indoor and outdoor experiments on asphalt mixtures using the SAP showed that they satisfied the mechanical performance criteria as asphalt pavement materials and that the temperature recovery delay effect was improved.

1. Introduction

Due to industrial development and urban modernization, artificial structures in cities have increased, and temperatures in cities have also been steadily rising. According to the National Institute of Meteorological Sciences’ report on climate change in the Korean Peninsula, the average annual temperature over the past 30 years has increased by 1.4 °C compared to the early 20th century (1912–1941). Additionally, summers increased by 1.2 days and tropical nights increased by 0.9 days every 10 years. Figure 1 shows the temperature changes in the Korean Peninsula over the past 100 years [1].

Various problems due to rising temperatures have begun to appear. Microclimate changes caused by artificial structures in cities affect people’s health due to abnormal climates such as the urban heat island phenomenon. Furthermore, excessive heat exposure of the body can weaken the body’s temperature control function, causing fatigue and heatstroke. According to meteorological observations in 2018, the frequency and intensity of heat waves are increasing, and the damage from heat waves is also increasing [2,3,4].

Due to climate change, many problems are being experienced throughout society. The increase in annual average temperature has a direct or indirect effect on the health of people in cities. Due to the urban heat island phenomenon, many issues, such as the loss of lives of the elderly and the weak, the mental and physical decline of many people, and crop damage, are occurring as shown in

Table 1. Current status of heat wave damage by sector.

Content	Damage Type		2015	2016	2017	2018	2019	Unit
Health	A person with a fever	Thermal disease monitoring system	1056	2125	1574	4526	1841	Person
		National health insurance DB	21,398	30,602	27,032	44,094	-	Person
	A death from a heat illness	Thermal disease monitoring system	11	17	11	48	11	Person
		Statistics on causes of death by the national statistical office	39	67	35	145	-	Person
Animal husbandry	Chicken death		-	2573	5772	6426	5391	Heads
	Duck death		-	80	196	265	139	Heads
	Pig death		-	5	38	71	40	Heads

. Urban heat island phenomena are known to occur due to various factors. The urban heat island phenomenon is greatly influenced by the geographical characteristics, climate conditions, and seasonal changes in a specific location in the city [5].

Asphalt road pavement emissivity and atmospheric radiation are the most important factors in surface and atmospheric temperatures in urban canyons, resulting in heat island degradation at night, reduced evaporation cooling, increased surface roughness, and higher density. In addition, warmer urban environments amplify ozone concentrations at the surface, reduce air quality, and increase artificial energy consumption such as the use of cooling facilities due to tropical nights, while heat waves increase heat-related mortality, posing a great threat to human health [6,7,8].

Urban areas have seen temperatures rise due to the increase in the usage of air-conditioning electricity and the lower quality of air, causing thermal damage to residents. Specifically, the heat island phenomenon worsens the living environment in the city and increases the incidence and mortality rate from heat-related diseases. Preventing diseases under extreme high-temperature conditions is becoming a major public health issue. As urban populations continue to grow and more severe heat waves are likely to occur in the future, measures to mitigate the urban heat island effect are essential [9,10,11].

Additionally, the problem of the urban heat island phenomenon is also occurring due to urbanization, with the main reason being the decrease in natural vegetation due to the increase in impermeable areas caused by urbanization and the increase in artificial surfaces that absorb heat. During midday, the temperature of an asphalt road is about 20 °C higher than that of grass, and the decrease in vegetation along with the increase in waterproof surfaces caused by urbanization seriously affect the UHI [12,13]. Likewise, artificial means of transportation in the urban cities have an adverse effect. In defining the heat island phenomenon, asphalt road pavement characteristics are classified according to the porosity or amount of asphalt added, and how much they contribute to the heat island phenomenon. Many previous studies have mentioned factors such as albedo, water permeability, and water conservation on the asphalt surface layer because these factors greatly affect the surface temperature [14,15,16].

Therefore, it is important to alleviate the urban heat island phenomenon caused by asphalt roads. Currently, in order to lower the high asphalt temperature, local governments are water spraying roads in a manner suitable for each region. According to the spraying method, a sprinkler truck is used, and a clean road system that automatically sprinkles water at certain intervals is operated. The effect of reducing the temperature of the road surface appears significant when spraying, but the duration is not long, so the problem is that the temperature reduction effect continues only when additional spraying is carried out or increased.

Recently, various studies have been conducted using an SAP (Super Absorbent Polymer) with water absorption properties as a means to effectively alleviate this urban heat island phenomenon. An SAP is a water-absorbing polymer with a network structure of polymer chains, commonly referred to as hydrophilic gels. The SAP is not soluble in water, but it absorbs it and then swells up. The SAP, in particular, can maintain absorbed water under some pressure unlike other absorbent materials such as tissue and polyurethane foam. Furthermore, the SAP has a three-dimensional network structure like a water-soluble polymer and can absorb water about 100 to 500 times its own weight without dissolving in water. The SAP types used in this study are widely used in everyday life. They are mainly used for disposable diapers, disposable items for women, and steaming packs, and recently an expanding scope means they are being used as soil repair agents and civil engineering and construction water-repellent materials [17,18].

The application of an asphalt mixture with an SAP can delay the road pavement’s temperature recovery time after water is sprayed to lower the road pavement’s temperature during a heat wave. Water cooling is performed on road pavement in each region every year during heat waves, and spray trucks or clean road systems are used. The water-cooling system is an effective way to lower the surface temperature of road pavement. However, after the spraying is completed, due to evaporation and drain effects, the water quickly disappears from the surface of the road pavement and the pavement tends to return to the temperature it was before spraying. Therefore, the usage of an SAP on asphalt pavement helps delay recovery time by absorbing the water longer [19,20,21,22].

In this study, an SAP was applied to asphalt mixtures to delay the temperature recovery time of road pavement, and various performance evaluations were performed. Two types of SAPs were applied to asphalt mixtures at various ratios to evaluate strength and durability. In addition, temperature change characteristics under various conditions were evaluated indoors and outdoors to evaluate the thermal characteristics of asphalt specimens applied with the SAP.

2. Materials and Methodology

2.1. SAP (Super Absorbent Polymer)

SAPs produced from LG Chem’s products (Seoul, Republic of Korea) were used in this study. Two types of SAPs known as SAP-500 and SAP-3500, which were named according to particle size, were utilized in this study, and the appearance and characteristics of the two SAPs are shown in Figure 2 and

Table 2. SAP properties.

Property	SAP-500	SAP-3500
Retained capacity (g/g)	Min. 31	Min. 25
Volume density (g/mL)	0.6 ± 0.05	0.52 ± 0.05
Absorption under pressure (mL/g)	Min. 26	Min. 22
Particle Size Distribution (%)
850 μm remaining	Max. 2	Max. 1
850∼500 μm	Max. 25	Max. 45
500∼150 μm	50–75
150 μm passing	Max. 7	Min. 40

.

2.2. Asphalt Mixture

The asphalt mixture for wearing course (WC-2) was used in this study, and the maximum aggregate size was 13 mm according to the Korean standard. WC-2 is generally applied to the asphalt pavement surface layer, and the WC-2 gradation criteria and the combined aggregate gradation used for this study are shown in Figure 3. The asphalt binder used was asphalt of the PG-64-22 grade, and the optimum asphalt content was 5.5%. The SAP content was applied at 0%, 3%, 5%, and 7% based on the weight of the asphalt binder, and the SAP and asphalt binder were mixed using a high-shear mixer at a temperature of 150 °C.

2.3. Performance Evaluation of SAP Asphalt Mixture

Since the asphalt mixture mixed with the SAP was applied to serve the purpose of a reduction in urban heat island phenomenon in an environment exposed to moisture due to the water-absorbing characteristic of the SAP, the physical properties of the SAP asphalt mixture exposed to moisture were evaluated to verify the strength and durability of the mixture according to the type and content of SAP. The physical and mechanical properties of the SAP asphalt mixtures were analyzed through Marshall stability, indirect tensile strength, dynamic stability, and moisture resistance tests. In addition, Marshall stability and indirect tensile strength tests were conducted after exposing the specimens for 30 and 90 days, respectively, to evaluate the effectiveness of the SAP asphalt mixtures under outdoor environmental conditions.

2.4. Evaluation of Thermal Characteristics

2.4.1. An Indoor Experiment

Spraying tests were conducted to evaluate the temperature-reducing effect of the SAP asphalt mixtures. The most important goal in spraying is to maximize the cooling effect on the pavement relative to the total amount of water sprayed onto the pavement. When spraying water on road pavements, the temperature reduction effect is short-term because the water flows onto the surface and flows into the drain or evaporates. Therefore, an SAP can be mixed into asphalt mixtures to absorb water quickly so that the amount of water used in spraying can be utilized most efficiently and the temperature reduction effect can be maintained for a longer period of time. In this study, laboratory experiments were conducted to understand the thermal behavior of SAP asphalt mixtures that absorb water and delay the temperature recovery time of the asphalt surface, thereby reducing the surface temperature.

To measure the indoor thermal properties of asphalt mixtures containing an SAP, asphalt mixture specimens measuring 30 × 30 × 5 cm were prepared using different types and contents of SAPs. Two types of SAPs (SAP-500 and SAP-3500) were utilized. The contents of the SAPs were applied as 0%, 3%, 5%, and 7% of the asphalt mixture weight. The experimental process and setup are shown in Figure 4. The specimen surface was heated to approximately 70 °C, which is the highest temperature of the asphalt pavement surface in summer, using a 250 W heat lamp. The temperature change after water spraying was measured at 1 min intervals using a temperature logger (Tokyo Sokki Kenkyujo, TDS-540, Tokyo, Japan). An automatic sprayer (Kyeyang Industry, HY-76, Chungju, Republic of Korea) was used to spray 400 mL of water for 1 min.

2.4.2. Outdoor Experiment

Since the indoor experiments did not have many external environmental variables applied, outdoor experiments were conducted under conditions where external factors such as wind and humidity were directly applied. This experiment was conducted to simulate how thermal behavior would occur when SAP asphalt was applied to an actual road. As shown in Figure 5, asphalt specimens measuring 90 × 90 × 3 cm³ were manufactured and utilized. Based on the performance evaluation results, the 7% SAP was judged to be excessive and have a negative effect on the strength and durability of the asphalt mixture, so it was excluded. Therefore, outdoor experiments were conducted on specimens with 0%, 3%, and 5% of two types of SAPs (SAP-500 and SAP-3500). The experimental method was to spray 400 mL of water for 1 min on the surface of the specimen exposed to the outdoor environment using an automatic sprayer (Kyeyang Industry, HY-76, Chungju, Republic of Korea) to allow the asphalt pavement to absorb the water, and then the change in surface temperature over time was measured and analyzed using a thermal imaging camera (Bosch, GTC-400, Stuttgart, Germany).

3. Results and Discussion

3.1. Strength Characteristics of the SAP Asphalt Mixtures

3.1.1. Marshall Stability Results

Figure 6 and Figure 7 present the results of Marshall stability tests of asphalt mixtures manufactured by SAP type and content. The Marshall stability of all specimens satisfied the Korean asphalt mixture design standard of 7500 N. In addition, the flow values of all specimens, except for the specimens with an SAP-3500 content of 7%, satisfied the design standard of 20~40 mm. In the case of the Marshall stability test, the specimen is immersed in a 60 °C water bath for 30 min before the test is conducted. Therefore, when the asphalt and SAP are mixed, the SAP that absorbs water may expand and the Marshall stability value may decrease. The Marshall stability of the SAP-500 mixture decreased rapidly at 7% because the SAP absorbed and expanded while being coated on the aggregate together with the asphalt binder, weakening the bonding strength between the binder and the aggregate. The SAP absorbs and expands water to fill the internal voids, but when it exceeds a certain amount, expansion stress acts inside the specimen, widening the gap between the aggregates and weakening the stability [19]. The Marshall stability results of the specimens exposed outdoors for 30 and 90 days showed that the stability decreased after 30 days and increased again after 90 days. The asphalt mixture was affected by repeated expansion and contraction due to the active reaction between SAP and moisture during the 30-day outdoor experiment, but after that, the reactivity decreased and the effect on the mixture decreased, so the Marshall stability increased as shown in the figures.

3.1.2. Indirect Tensile Strength Result

Figure 8 and Figure 9 show the results of the indirect tensile strength test. As a result of the test, all SAP asphalt mixtures satisfied the Korean standards of an indirect tensile strength of 0.8 N/mm² and toughness of 8000 N/mm. However, both the indirect tensile strength and toughness tend to decrease as the SAP content increases.

For the mixtures with the SAP, the SAP added in the asphalt mixture replaced the asphalt binder; hence, when a load is applied to the asphalt mixture, the indirect tensile strength and toughness decreased since the asphalt binder binds together with the aggregate and makes the mixture stronger. The higher the SAP content, the more the SAP replaced the asphalt binder, and therefore there was a decrease in indirect tensile strength and toughness.

Regardless of the SAP type, the lowest results were an indirect tensile strength of 1.34 N/mm² and a toughness of 11,094 N/mm for SAP-500 at 7%. The lowest results for SAP-3500 (7%) were 1.35 N/mm² and 11,178 N/mm. It was expected that a mixture of two SAPs with different particle sizes would have a greater effect on the mixture than SAP-500 with a large single expansion property, but the difference in volume in the mixture has a similar effect on the crack resistance and rigidity of the two asphalt mixtures. The test results of specimens after being exposed outdoors for 30 days and 90 days showed that the indirect tensile strength was either similar or decreased over time. However, in the case of toughness, the results at 30 days and 90 days showed that the toughness increased. The toughness seems to have increased due to the aging of the asphalt binder, which causes rigidity. Previous studies have shown that when aged binders are used, the indirect tensile strength is lower and the toughness is higher [23].

3.1.3. Dynamic Stability Test Result

The results of the dynamic stability tests of asphalt mixtures according to SAP type and content are shown in Figure 10. After mixing in the SAP, the dynamic stability met the requirement of more than 750 times/mm in all specimens except for the specimen with 7% content. The results showed that adding the SAP to the asphalt mixture improves dynamic stability and flow resistance.

The SAP results increased when the SAP content increased to 1091 times/mm at 3% and 1507 times/mm at 5%, and SAP-3500 showed the same trend, but the result was 429 times/mm for the 7% SAP content for both types, which did not meet the standard. For the 7% SAP mixture, it was determined that the excessive amount of SAP causes insufficient asphalt to bind in the aggregate, resulting in poor durability.

Figure 11 shows the change in rutting over time in the wheel tracking test results. SAP-3500 showed less rutting than Non-SAP at all contents, while SAP-500 showed significant rutting at 3% and 5% contents. Another point to note in the experimental results is that although SAP-500 7% and SAP-3500 7% were similar in the dynamic stability test in Figure 10, the rutting changes over time in Figure 11 show that the two mixtures have very different tendencies. The dynamic stability of the two mixtures was the same at 429 times/mm, but the rutting depth at 60 min for SAP-500 7% was 10.8 mm and that for SAP-3500 7% was 8.3 mm, showing a large difference. This is because only the difference in a rutting depth of 45 to 60 min is considered when determining dynamic stability. This method was not an issue when evaluating similar existing asphalt mixtures, but it shows that evaluating only dynamic stability is problematic when evaluating new asphalt mixtures as in this study. This issue was reported in other previous studies that evaluated the correlation between dynamic stability and maximum rutting depth obtained from existing wheel tracking tests and showed a very low correlation (R2 = 0.28), which has many problems in practical application [24,25].

Looking at the maximum rutting depth and the rutting change in Figure 11, the maximum rutting depth is relatively large when SAP-500 is applied and the rutting change slope changes more rapidly compared to that of SAP-3500. Looking at the rutting change slope, it seems that the initial rutting increased quickly because of the smaller fineness of SAP-500 and its effect on the mixture is less than that of SAP-3500. The dynamic stability appears to have increased because mixing the SAP into the asphalt binder acts to limit the flowability of the binder. However, both types of SAPs show very low dynamic stability values when the SAP content is 7%, which is believed to be due to an excessive amount of SAP and is similar to that found in the Marshall stability and indirect tensile strength results. When the SAP content was excessively added, it caused the reduction in the adhesive force between the binder and the aggregate, thereby reducing the rigidity of the binder itself.

3.1.4. Tensile Strength Ratio (TSR) Results

The moisture resistance of the SAP asphalt mixtures was evaluated using the TSR test. The TSR test measures the indirect tensile strength of un-moisture-treated and moisture-treated specimens with air voids of 7 ± 0.5%, and calculates the ratio of the two values. The moisture treatment was applied by immersing the moisture vacuum-saturated specimen in water at a temperature of 60 °C for 24 h.

The TSR test results are shown in Figure 12, and it can be seen that as the content of the SAP increased, the TSR tended to decrease. Between the Non-SAP and SAP-500 3% there was no significant difference in TSR values where the results were 0.83 and 0.82 and both these values met the required standards of 0.8. Furthermore, SAP-500 5% and SAP-3500 3% had similar TSR values of 0.73 and 0.74, respectively, but SAP-3500 5% had a significantly lower TSR value of 0.64. The reason for the decrease in the TSR value is that the SAP available between the aggregate and the binder absorbs moisture and expands, and expansion stress occurs inside, thereby weakening the durability of the asphalt mixture. Since the SAP absorbs water, when it comes into contact with moisture, the intensity decreases. However, the TSR decreased with the increase in SAP content. Additionally, it was expected that SAP-3500, which has a small fineness and a larger volume increase during the water reaction, would have a greater effect on the asphalt mixture under moisture. But even in the other tests, the results showed that SAP-500, which has a large fineness, had a better performance in the asphalt mixture. Since SAP-3500 is finer than SAP-500, during mixing SAP-3500 acted as a filler and combined with the asphalt binder, and hence the amount of SAP-3500 was large in the area of the micro-zone, which has a large effect on the binder binding ability, thereby affecting the asphalt mixture.

3.1.5. Dynamic Immersion Results

The SAP has a volumetric capacity that increases when absorbing moisture, so a stripping resistance experiment was conducted to verify whether the adhesion strength between the aggregate and binder was affected by the change in volume when the moisture penetrated it while being coated on the aggregate. The dynamic water immersion test method is a test that measures the rate of peeling of the bond between the aggregate and asphalt in water, and the European EN-12697-11 test method [26] was applied. In addition, in order to examine the effect according to the outdoor exposure time, the specimens were left outdoors for 30 and 90 days, and then the test was conducted.

The visual evaluation of the coating condition of the asphalt binder was conducted after the test, but all samples were almost the same at 60–70%. Therefore, the weight of the samples before and after the test was measured, and the amount of binder loss was analyzed as shown in Figure 13. After the asphalt loose mix was exposed outdoors for 30 and 90 days, the amount of binder loss in all SAP contents was not significantly different from that of the Non-SAP. The weight loss after 90 days for all samples was large when comparing the 1st and 30th days, which appears to be due to the reaction of the asphalt mixture and the environment conditions such as rain, sunshine, and wind.

3.2. Thermal Properties Results

3.2.1. Results of Indoor SAP Asphalt Thermal Characteristics

Table 3. Temperature recovery delay effect of SAP asphalt.

Content	1~2 Times Spraying	2~3 Times Spraying
SAP-500 3%	+5 min	+12 min
SAP-500 5%	+9 min	+24 min
SAP-500 7%	+16 min	+49 min
SAP-3500 3%	+6 min	+14 min
SAP-3500 5%	+10 min	+26 min
SAP-3500 7%	+33 min	+60 min

shows the surface temperature recovery delay time of SAP-500 and SAP-3500 asphalt compared to Non-SAP asphalt. It took about 20 min for the Non-SAP asphalt to recover to its original temperature after the first spray, and about 24 min after the second spray. The SAP content of 3% for both SAP types showed a similar delay, as shown in Table 2. However, as the content of the SAP increased, a significant delay time difference occurred. As per previous results, it was expected that the absorption performance of SAP-500 would be excellent, so the temperature recovery time delay effect would be better than that of SAP-3500 even when asphalt was mixed, but SAP-3500 was found to be more effective. It seems that SAP-3500, which has a faster initial absorption rate, absorbs a lot of moisture due to having an insufficient moisture supply, resulting in a relative temperature delay effect.

After the observations, it was confirmed that the higher the SAP content, the better the temperature recovery time delay effect. Generally, porous asphalt moisture is stored in a large air void and then evaporated, thereby delaying the recovery temperature of the pavement. The thermal behavior of porous asphalt with large porosity curves upward when it is sprayed [27,28,29]. In this way, it was determined that the SAP can be mixed with asphalt to exhibit a temperature recovery delay effect in a similar tendency to porous asphalt, thereby causing a cooling effect on the road pavement surface [29].

Figure 14 and Figure 15 show changes in the asphalt surface temperature based on the SAP type and content. The results obtained showed that the temperature recovery delay time increased as the SAP content increased. Furthermore, the thermal behavior of the asphalt mixture with an SAP content of 7% was similar to the thermal behavior of porous asphalt with a large porosity. For both SAP types, the SAP 7% specimens had much better temperature recovery delay times than the other SAP specimens. This was because the SAP 7% specimens had a large amount of SAP that was not fully covered by the asphalt binder, which made a large amount of SAP readily available for water absorption, resulting in a longer time delay. For the 3% and 5% SAP specimens, almost all the SAP was covered and well mixed during manufacturing.

3.2.2. Results of Outdoor SAP Asphalt Thermal Properties

Based on the results of the indoor experiment, an outdoor experiment was conducted to verify the performance of the SAP asphalt mixtures under external environmental conditions. Samples were prepared and evaluated according to the type and content of the SAP, and the average air temperature during the test was 26.3 °C. The specimens were sprayed for 1 min (400 mL) using an automatic sprayer (Kyeyang Industry, HY-76, Chungju, Republic of Korea), and then the temperature change in the specimens was photographed over time using a thermal imaging camera (BOSCH, GTC400C, Stuttgart, Germany). During this test only the Non-SAP and 5% SAP contents from both types were produced and experimented on because the 3% and 7% SAP contents were less efficient and durable. Figure 16 shows images of specimens before spraying, and the Figure 17 images were taken immediately after spraying. The Figure 18 images were taken 1 h after spraying and the Figure 19 images were taken 2 h after spraying, and analysis was conducted based on the temperature distribution bar. Before spraying, all specimens had a similar temperature distribution, and the maximum temperature was measured to be about 52.2 °C.

After 1 min of spraying, the surface temperature of the Non-SAP specimen was measured to be approximately 3 °C higher than the other specimens, whereas the surface temperature of the SAP specimen was measured to be lower over the entire pavement surface. This means that in the case of Non-SAP asphalt, the air void was small, so the amount of water exceeding the surface air void after spraying flowed to the side or was collected, and also evaporation begun at the same time as spraying causing the Non-SAP specimen to quickly recover to its original temperature. However, in the case of the SAP asphalt, the temperature recovery was delayed because the SAP absorbed water flowing inside the pavement quickly and it slowly evaporated. When comparing between SAP-500 and SAP-3500, the SAP-500 surface temperature distribution tended to be slightly lower. This is because SAP-3500 is finer than SAP-500, which made it more widely distributed than SAP-500 and hence it had a better temperature distribution even though the same amount of water was sprayed for 1 min for both specimens.

After 1 h of spraying, the temperature distribution was lower in the SAP asphalt than in the Non-SAP asphalt. There were areas where the SAP-500 asphalt differed from the Non-SAP asphalt by a range from at least 3 °C to a maximum of 9 °C, and SAP-3500 differed from the Non-SAP asphalt by a range from at least 2 °C to a maximum of 10 °C. Compared to the Non-SAP asphalt, the temperature distribution gap of the SAP asphalt was severe in the marked area, which is considered to contain a large amount of moisture as it was a SAP-intensive area at the time of mixing.

After 2 h of spraying, the temperature distribution was higher in the Non-SAP asphalt than other specimens, and there was a difference of about 3 to 5 °C in the SAP-500 asphalt and 2 to 5 °C in the SAP-3500 asphalt. In the indoor experiment, the original temperature was restored in about 40 min, but the outdoor experiment maintained a low temperature for a long time. Unlike the indoor experiment conducted at the highest temperature, the maximum temperature of the asphalt surface outdoors was about 52 °C. Additionally, the influence of wind, humidity, and sprinkling amount was believed to be the reason the cooling effect was maintained for a longer time.

4. Conclusions

This study was conducted to evaluate the reduction effect of the urban heat island phenomenon of asphalt pavement applied with an SAP. The durability and temperature reduction performance of asphalt mixtures applied with various types and contents of SAPs were studied, and the conclusions are as follows:

• Based pm the results of the performance of the asphalt mixture mixed with an SAP, when the SAP content was less than 7% the mixture design criteria of the mixture were satisfied. In the case of indirect tensile strength, the Marshall stability and crack resistance of the Non-SAP asphalt mixture had better results than the asphalt mixtures with SAP. However, the dynamic stability results showed that the rutting resistance increased by more than three times for asphalt mixtures with the SAP compared to the Non-SAP asphalt mixture.
• As a result of the indoor experiment on the thermal characteristics of the SAP asphalt, it was confirmed that the effect of delaying the temperature recovery was improving as the content of the SAP increased. Compared to the Non-SAP asphalt, the surface temperature immediately after spraying decreased by 10 °C, and the temperature recovery was delayed by 5 to 30 min depending on the SAP content.
• In the case of a high content of the SAP (7% or higher), the surface temperature lowered after spraying was maintained or tended to rise after lowering. This thermal behavior is generally the same as the temperature change behavior of the porous asphalt surface, which has the effect of improving the temperature recovery delay effect similar to that of the SAP asphalt mixture with a high content of 7% or more.
• The SAP asphalt indoor and outdoor experiments showed that the delay effect became better as the SAP content increased when a sufficient amount of spray was applied. The recovery temperature was delayed for more than 1 h after the surface temperature decreased by about 10 °C, and there was a clear difference in the Non-SAP.
• Although an SAP of 7% or higher exhibits thermal behavior characteristics similar to porous asphalt, the most important thing is to establish an SAP optimal content and required porosity that can exhibit temperature recovery delay effects while ensuring the porosity of asphalt. There is a space for absorbing the expansion stress of the SAP. It is concluded that further research on porosity and thermal behavior is needed by applying the SAP to porous asphalt.