Effects of Physical Cooling on the Temperature and Performance of Newly Laid Porous Asphalt Mixtures

Abstract

Porous asphalt pavements need to be cured for 24 h~48 h before they can be opened to traffic. In an emergency, physical cooling methods, such as water sprinkling and air blowing, can be used to accelerate cooling, but the effects of the two methods on the mechanical properties and durability of porous asphalt mixtures are still unclear. In this research, firstly, the dropping and rising temperatures of the pavement surface during the water sprinkling process of newly laid porous asphalt mixtures in real projects were analyzed. The effects of the two conditions of water immersion and water sprinkling on the mechanical properties of porous asphalt mixtures were clarified, and water sprinkling technology for porous asphalt mixtures was proposed. Secondly, the effects of air blowing on the temperature reduction and strength loss of porous asphalt mixtures was analyzed, and the pavement surface temperature control standard that was suitable for air blowing was proposed. Finally, a seven-year observation was carried out on the water sprinkling cooling test section in the actual project. The research results show that water immersion or the sprinkling of water repeatedly during the curing period of porous asphalt pavements reduces the strength of the mixture. It is recommended to use a water amount of 0.3 kg/m² once and sprinkling four times before painting road markings and two times after painting road markings; this was the best water sprinkling cooling process for porous asphalt pavements. The use of air blowing can accelerate the temperature reduction of porous asphalt mixtures, but the mechanical properties of the mixtures are attenuated after air blowing. Air blowing can be carried out when the pavement surface temperature is lower than 70 °C. Compared with the road section without water sprinkling for cooling, the use of the determined process to cool the newly laid porous asphalt mixtures by water sprinkling does not have a significant adverse effect on their durability. There is also no significant difference in the performances of the two road surfaces within a seven-year service. In an emergency, physical cooling methods, such as water sprinkling or air blowing, can be used to accelerate the temperature reduction of the newly laid porous asphalt mixtures, so as to achieve the purpose of quickly opening to traffic.

1. Introduction

When water accumulates on the pavement surface during rainy days, the friction coefficient is greatly reduced due to the lubrication effects of the water between the tire and the pavement surface when the vehicle is driving, which makes traffic accidents more likely to occur. Especially on freeways, when vehicles travel at high speeds on the pavement surface with gathered water, splash and spray phenomena occur, thus affecting the driving vision. When the vehicle speed exceeds a critical speed, there is a risk of hydroplaning, which further increases the probability of traffic accidents [1]. Higher requirements for the safety, comfort, and other service functions of highways have been put forward by the public. The porosity of porous asphalt pavements is above 18%, and excellent drainage and noise reduction functions are found due to the porous characteristics [2,3]. In China, the cumulative application scale of the porous asphalt pavements of freeways was nearly 1000 km by 2023. In 2020, the Ministry of Transport of China issued JTG/T 3350-03-2020, named Technical Specifications for Design and Construction of Porous Asphalt Pavement [4], which further promoted the popularization and application of porous asphalt pavements in China. According to the specification JTG/T 3350-03-2020 [4], after the construction of the porous asphalt pavement is completed, traffic should be closed for more than 24 h before it is allowed to open to traffic. However, when it is necessary to pass in an emergency, it is recommended to wait until the pavement surface temperature drops below 50 °C. Long-term traffic control brings huge traffic pressure to the construction of porous asphalt pavements. Especially for maintenance projects, long-term traffic control may cause serious traffic congestion. It is very important to set a reasonable curing time for newly laid porous asphalt pavements.

Curing time is very important for the performance development of the asphalt, which in turn affects the mechanical performances and durability of the asphalt mixtures. When preparing SBS-modified asphalt, high-temperature development is an indispensable step. After the base asphalt and SBS modifier are completely sheared and mixed, they need to continue to react in the oven for no less than 90 min [5,6,7,8]. In order to further improve the comprehensive properties of SBS-modified asphalt, or for environmental protection and cost reduction, rubber powder, nano-clay, bio-oil, epoxy resin, polyurethane, and other materials are used as composite modifiers in the preparation of modified asphalt [9,10,11,12,13]. Due to the need for sufficient shearing, dissolving, and dilating between various modifiers and the asphalt to ensure the modification effect, the reaction time required for the preparation of composite modified asphalt is further extended [14]. In China, in order to ensure that porous asphalt pavements can adapt to high temperature and heavy load service conditions, it is stipulated that high-viscosity modified asphalt must be used as a binder. High-viscosity modified asphalt is modified by a variety of modifiers; this leads to porous asphalt pavements generally being opened to traffic 24 h after paving.

The porous asphalt mixture is obtained by mixing high-viscosity modified asphalt, coarse and fine aggregates, mineral powder, and additives in a certain proportion. During the mixing process, the high-viscosity modified asphalt and the coarse and fine aggregates need to be heated. Generally, the temperature of the freshly mixed porous asphalt mixture is between 170 °C and 185 °C [15]. When the porous asphalt mixture is paved on the road surface and rolled, the pavement surface temperature is between 70 °C and 80 °C. There is still a gap from the pavement surface temperature lower than 50 °C. The time required for the pavement surface temperature to drop to the specified level varies with the ambient temperature, and it generally takes 2 h to 6 h [16]. In the hot summer, it is even difficult for the pavement surface temperature to drop below 50 °C [17,18]. In this case, some physical cooling measures need to be taken to accelerate the temperature drop of the asphalt mixture, so as to achieve the purpose of opening the porous asphalt pavements to traffic earlier. Water sprinkling and air blowing are commonly used measures to accelerate the temperature drop of the asphalt mixture. However, during the curing stage of the asphalt mixture, the intrusion of moisture or high-speed airflow may have an adverse effect on the mixture performance.

Currently, the research on porous asphalt mixtures primarily focuses on the design methods of the mixture proportions, as well as studies on stability and durability performance. Li et al. [19] designed three gradations for porous asphalt mixtures and proposed that epoxy asphalt binder can significantly enhance the fatigue resistance of porous mixtures. Slebi-Acevedo et al. [20] emphasized that the use of polymer-modified binders and the inclusion of fibers and hydrated lime are crucial for the formulation of the mixtures. Ghafari Hashjin et al. [21] investigated the performance of porous asphalt mixtures prepared using different gradations of limestone and siliceous aggregates. Kusumawardani et al. [22] fabricated shape-controlled aggregates and applied them to porous asphalt mixtures to assess the impact of aggregate shape characteristics on the performances of porous asphalt mixtures. The performances of porous asphalt mixtures are significantly influenced by the composition of the materials. Wang et al. [23] discussed the bond failure and cohesive failure mechanisms of porous mixtures under different temperatures and humidities. Zhang et al. [24] proposed that common issues affecting the durability of porous asphalt mixtures are mainly related to humidity and temperature. The presence of moisture can easily lead to bond failure within the asphalt binder or between the binder and aggregates [25,26]. Temperature has a significant impact on the viscoelasticity of asphalt binders. So, it is necessary to take the adverse effects of moisture or high-speed airflow into consideration when reducing the pavement surface temperature by water sprinkling or air blowing methods.

The interaction between asphalt and aggregates is a crucial factor that affects the stability and crack resistance of porous asphalt pavement. After immersion in water, water-induced bulges occur on the asphalt binder surface; this reduces the surface energy of asphalt and the effective contact area between the asphalt and aggregates, thus resulting in the failure of adhesion or the loss of cohesion [27,28]. When the bonding between the asphalt and aggregates is lost, water damage, such as raveling and potholes, occurs, thus shortening the service life of the asphalt pavements [29,30,31]. Water migration in asphalt mixtures can also weaken their fatigue resistance [32]. In order to alleviate the adverse effects of water on the performances of the mixtures, many scholars have studied the influence of key materials, such as aggregates, fillers, and asphalt, on the water damage to the mixtures [33,34,35,36]. Measures have been proposed to improve the water stability of the mixtures by adding appropriate amounts of lime or cement and high-performance additives [37,38]. Air blowing also affects the development of asphalt performances. In the process of preparing asphalt materials from petroleum, air blowing contributes to asphalt with better properties without changing the asphalt grade. However, the asphalt temperature during air blowing is very critical, as temperatures exceeding 88 °C can cause asphalt oxidation and hardening [39,40].

Many scholars have studied the effect of moisture on water damage to porous asphalt mixtures, whereas most of the related research focuses on the effects of moisture on the water damage during the service period of porous asphalt pavements. The moisture effects on the pavement performances during construction are unknown. Water sprinkling or air blowing can be used as a physical cooling measure to accelerate the cooling of newly laid porous asphalt mixtures, allowing them to be open to traffic early. However, it is hard to find studies on the effects of water sprinkling or air blowing on the performances of newly laid porous asphalt mixtures. Therefore, this study analyzed the effects of these physical cooling measures on the temperature and performance of newly laid porous asphalt mixtures, providing technical measures for the rapid opening to traffic of porous asphalt pavements in emergency situations. Finally, based on actual projects, the effects of water sprinkling during the construction period on the durability of porous asphalt pavements were tracked and observed.

2. Experimental Design

2.1. Raw Materials and Asphalt Mixtures

2.1.1. High-Viscosity Modified Asphalt

High-viscosity modified asphalt was prepared by the composite modification of SBS-modified asphalt and the high-viscosity additive with their mass ratio of 92:8, and its key properties and corresponding test methods are shown in

Table 1. Key properties of high-viscosity modified asphalt and corresponding test methods [41].

Index	Unit	Test Results	Technical Requirement	Test Method
Penetration (25 ℃, 100 g, 5 s)	0.1 mm	52.7	≮40	JTG E20/T 0604
Softening point (TR&B)	°C	98.5	≮90	JTG E20/T 0606
Ductility (5 °C, 5 cm/min)	cm	54.8	≮30	JTG E20/T 0605
Dynamic viscosity (60 °C)	Pa·s	992,514	≮400,000	JTG E20/T 0620
Brookfield viscosity (170 °C)	Pa·s	1.465	≯3.0	JTG E20/T 0625
Solubility	%	99.9	≮99	JTG E20/T 0607
Elastic recovery (25 °C)	%	99.0	≮95	JTG E20/T 0662
Specific gravity (25 °C)	-	1.027	Actual test record	JTG E20/T 0603
Residue after TFOT				JTG E20/T 0609
Mass change	%	+0.37	±1.0
Penetration ratio (25 °C)	%	88.5	≮65	JTG E20/T 0604
Ductility (5 °C, 5 cm/min)	cm	32.3	≮20	JTG E20/T 0605

.

2.1.2. Other Raw Materials

The coarse aggregates used were basalt. The fine aggregates were limestone, and the finely ground limestone powders were used as fillers. The gradations are shown in

Table 2. Gradation of the coarse aggregates, fine aggregates, and fillers.

Aggregate Type	Mass Percentage (%) Passing Through Different Sieve Sizes (mm)
	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Mineral powder	100	100	100	100	100	100	100	99.9	98.8	94.2
Fine aggregates of 0~3 mm	100	100	100	100	86.4	57.1	38.0	25.8	19.6	15.7
Coarse aggregates of 5~10 mm	100	100	98.9	17.8	0.9	0.6	0.6	0.6	0.6	0.6
Coarse aggregates of 10~15 mm	100	83.9	19.8	0.4	0.4	0.4	0.4	0.4	0.4	0.4

. All of them meet the technique specification JTG/T 3350-03-2020 [4]. Polyester fiber was used with a dosage of 0.1% of the mass of the asphalt mixture.

2.1.3. Asphalt Mixtures

The porous asphalt mixture with a nominal maximum aggregate size of 13.2 mm, named PAC-13, was used. Its gradation composition is shown in

Table 3. Gradation composition of PAC-13.

Mixture Type	Mass Percentage (%) Passing Through Different Sieve Sizes (mm)
	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
PAC-13	100	93.2	65.8	20.4	11.5	9.0	7.5	6.5	6.0	5.5

, and the asphalt/aggregate ratio is 4.9%. The diagram of the formed specimen is shown in Figure 1.

2.2. Research Design

Two physical cooling measures, including water sprinkling and air blowing, were adopted to analyze their influences on the temperature and performance of the newly laid PAC-13 mixture, and a suitable cooling process for the newly laid porous asphalt mixture was proposed. In 2016, a porous asphalt pavement section with water sprinkling cooling was paved in the mileage from K126 + 900 to K126 + 700 in the Xuzhou to Huai’an direction on the G2513 Huai’an-Xuzhou Freeway in Jiangsu Province, China, and the section was opened to traffic on the day of construction. Through a seven-year follow-up observation, the influence of water sprinkling cooling on the long-term performances of the porous asphalt pavements was evaluated. The specific research ideas are shown in Figure 2.

2.3. Test Methods

2.3.1. Evaluation Indexes of Asphalt Mixture Performances

It is essential to select reasonable mixture performance indicators to accurately evaluate the effects of physical cooling on the performance of porous asphalt mixtures. Splitting tensile strength is one of the most important performance evaluation indicators of asphalt mixtures. In the curing stage of cold recycled asphalt mixtures, splitting strength is used as a key performance indicator to determine the time before opening to traffic [42,43,44]. Raveling is one of the most typical forms of diseases in porous asphalt pavements, and it is also the most important factor affecting the service life of porous asphalt pavements [15,45,46]. Therefore, the splitting tensile strength and raveling loss were employed as key performance indicators for porous asphalt mixtures. Splitting strength was tested according to T0716-2011 in the Chinese standard JTG E20-2011 [41], and the calculation formula is shown in Equation (1). The raveling loss was tested according to T0733-2011 in the Chinese standard JTG E20-2011 [41,47], and the calculation formula is shown in Equation (2). The splitting test and raveling loss measurement are shown in Figure 3 and Figure 4, respectively.

$$R_T=0.006287\times P_T/h$$

(1)

where $R_T$ denotes splitting tensile strength, and the unit is MPa; $P_T$ denotes the maximum tested load, and the unit is N; h denotes specimen height, and the unit is mm.

$$\Delta S={\displaystyle \frac{m_0-m_1}{m_0}}\times 100$$

(2)

where $\Delta S$ denotes the raveling loss of the asphalt mixture, and the unit is %; $m_0$ denotes the mass of the specimen before the test, and the unit is g; $m_1$ denotes the residual mass of the tested specimen after the test, and the unit is g.

2.3.2. Long-Term Performance Indexes of Porous Asphalt Pavement

For the highways in operation, according to the Chinese standard JTG 5210-2018 [48], the pavement technical condition is comprehensively assessed on a 100-point scale based on indicators such as the pavement surface condition index (PCI), pavement riding quality index (RQI), pavement rutting depth index (RDI), pavement skid resistance index (SRI), pavement structure strength index (PSSI), etc. The PSSI represents the overall pavement structure strength condition, which is mainly affected by the base layer. For the observed pavement section with the water sprinkling cooling method in this research, the PSSI data over the years after its opening to traffic showed 100 points with no decline trend. At the same time, no obvious damage occurred on the pavement surface. Therefore, PSSI and PCI were not analyzed in this research. The calculations for RQI, RDI, and SRI are shown in Equation (3), Equation (4), and Equation (5), respectively. In order to analyze the pavement performances more intuitively, various basic indicators, namely the international roughness index (IRI), rutting depth (RD), and sideway force coefficient (SFC), were used to observe the long-term performances of the pavement section with the water sprinkling cooling method.

$$RQI={\displaystyle \frac{100}{1+{\mathrm{a}}_0{\mathrm{e}}^{{\mathrm{a}}_{1}IRI}}}$$

(3)

where $IRI$ denotes the international roughness index and the unit is m/km; ${\mathrm{a}}_0$ and ${\mathrm{a}}_1$ denote model constants; the values of 0.026 for ${\mathrm{a}}_0$ and 0.65 for ${\mathrm{a}}_1$ are adopted for freeways.

$$RDI=\left\{\begin{array}{l}100-{\mathrm{a}}_{2}RD\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}(RD\le R{D}_{\mathrm{a}})\\ 90-{\mathrm{a}}_3(RD-R{D}_{\mathrm{a}})\hspace{1em}\hspace{1em}\hspace{1em}(R{D}_{\mathrm{a}}<RD\le R{D}_{\mathrm{b}})\\ 0 \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} \hspace{1em}\hspace{1em}\hspace{1em}(RD>R{D}_{\mathrm{b}})\end{array}\right.$$

(4)

where $RD$ denotes rutting depth, and the unit is mm; ${\mathrm{a}}_2$ and ${\mathrm{a}}_3$ are model constants, and the values of 1.0 for ${\mathrm{a}}_2$ and 3.0 for ${\mathrm{a}}_3$ are adopted; $R{D}_{\mathrm{a}}$ and $R{D}_{\mathrm{b}}$ denote $RD$ parameters with $R{D}_{\mathrm{a}}$ of 10.0 and $R{D}_{\mathrm{b}}$ of 40.0, respectively.

$$SRI={\displaystyle \frac{100-SR{I}_{\min }}{1+{\mathrm{a}}_4{\mathrm{e}}^{{\mathrm{a}}_{5}SFC}}}+SR{I}_{\min }$$

(5)

where $SFC$ denotes the sideway force coefficient; $SR{I}_{\min }$ denotes calibration constant, and 35.0 is usually adopted; ${\mathrm{a}}_4$ and ${\mathrm{a}}_5$ denote model constants, and the values of 28.6 for ${\mathrm{a}}_4$ and −0.105 for ${\mathrm{a}}_5$ are adopted.

3. Effect of Water Sprinkling Cooling on the Temperature and Performance of Porous Asphalt Mixtures

3.1. Effect of Water Sprinkling on Mixture Temperature

3.1.1. Basic Information of the Test Section

The G2513 Huai’an-Xuzhou freeway is a two-way four-lane freeway with a design speed of 120 km/h. It was completed and opened to traffic in 2003. In 2016, in order to improve the bearing capacity of the overall pavement structure and enhance the service function of the freeway, a 4 cm thick porous asphalt mixture PAC-13 overlay was paved on some sections of the road. Due to the closure of traffic during construction, there was immense traffic pressure faced during the construction process. In order to accelerate temperature reduction and shorten the curing time, water sprinkling cooling was carried out when the surface temperature of the newly paved mixture dropped to 60 °C. The pavement section with water sprinkling cooling is located in the mileage from K126 + 900 to K126 + 700 in the Xuzhou-Huai’an direction, with a length of 200 m.

During the construction of the porous asphalt mixture, the construction temperature at each stage was tested using a handheld infrared temperature tester, as shown in

Table 4. The construction temperature at each stage for the porous pavement section with water sprinkling cooling.

Test Index	Construction Procedure
	Paving	Initial Compaction	Secondary Compaction	Final Compaction	The End of Compaction
Temperature (°C)	Above 155	Above 150	Below 90	Above 75	70~75

. It can be seen in Table 4 that the surface temperature of the porous asphalt pavement after rolling was between 70 °C and 75 °C, which is much higher than the temperature range for opening to traffic required by the specification [4].

3.1.2. Effects of Water Sprinkling on Surface Temperature of Mixtures

The water sprinkling is divided into two stages: sprinkling water four times when the surface temperature of the newly laid mixture drops to 60 °C and sprinkling water twelve times after the road marking is applied. The water sprinkling amount at the site was 0.3 kg/m². The water sprinkling equipment employed was an engineering watering truck, whose sprinkling pipes can be controlled in multiple directions, such as single or double sides, as well as upper and lower sides. It can also provide sufficient pressure to ensure mist spraying when driving at low speeds, as shown in Figure 5. During the entire water sprinkling process, the surface temperature of the porous asphalt pavement was tested, as shown in Figure 6.

As shown in Figure 6, during the entire water sprinkling process, the surface temperature of the newly paved porous asphalt pavement shows an overall downward trend, but three instances of temperature rebounds exist. The first occurs during the gap between the second and third water sprinkling. This is consistent with previous research showing that the temperature transfer rate inside the porous asphalt mixture is greater than its surface [49]. The internal temperature of the mixture is very high, and the heat is transferred to the pavement surface, thus causing the pavement surface temperature to rebound from 40 °C to 50 °C. The second appears during the road marking construction stage. The road marking time is from 14:00 to 16:00 in the afternoon, taking a total of about 2 h. The combined effects of the internal temperature transfer of the mixture and solar radiation cause the pavement surface temperature to rebound from 40 °C to 50 °C. The third occurs after water is sprinkled sixteen times. The pavement surface temperature gradually rises to 40 °C and then slowly falls back to 37 °C. The pavement surface temperature on the adjacent section without water sprinkling cooling remains at about 46 °C. The pavement surface temperature is reduced by about 9 °C by water sprinkling cooling, indicating that water sprinkling can effectively reduce the pavement surface temperature of porous asphalt pavements.

3.2. Effect of Water Sprinkling on Mechanical Properties of Mixtures

3.2.1. Effects of Water Sprinkling Times

Considering that after water was sprinkled six times in the actual project, as shown in Figure 6 in Section 3.1.2, the continued water sprinkling had little impact on the surface temperature of porous asphalt pavements, water sprinkling was applied only six times during the laboratory simulation; that is, no further water sprinkling treatment was performed. The experimental temperature in the laboratory was maintained at approximately 15~20 °C through air conditioning; the water temperature was controlled at around 15 °C.

The Marshall specimens were pretreated according to the following test conditions. (a) Eight groups of Marshall specimens were fabricated, with three specimens in each group, and the surface temperature of the specimens was tested using a handheld infrared temperature tester. (b) When the surface temperature of the specimens dropped to approximately 50 °C, water was sprinkled for the first time using a watering can, with the water flowing out from the bottom of the test specimen as the control condition for stopping the sprinkling water (the same is the case below), which was considered to be water sprinkling once (sprinkling water at about 20 mL/time in the laboratory). (c) The temperature on the surface of the specimen was monitored using the temperature tester. After the temperature became stable, the monitoring was stopped; then, water was sprinkled for the second time using the watering can, which was considered to be water sprinkling for the second time. (d) This process was repeated until water was sprinkled six times, and the water sprinkling was terminated. (e) Splitting tests were conducted on four groups of Marshall specimens, including those without water sprinkling, with water sprinkling once, with water sprinkling three times, and with water sprinkling six times, and the corresponding splitting tensile strengths were recorded. Another four groups of Marshall specimens, which had no sprinkled water, water sprinkled once, water sprinkled three times, and water sprinkled six times, were subjected to Cantabro tests and the raveling losses were recorded. The water sprinkling and the surface temperature monitoring process of the specimen are shown in Figure 7. The splitting tensile strength and raveling loss of the specimens were tested under different watering times. The test results are shown in Figure 8 and Figure 9.

As shown in Figure 8 and Figure 9, with the increase in water sprinkling times, the splitting tensile strength of the porous asphalt mixture decreases and the raveling loss increases. This demonstrates that water sprinkling in the early stage of the curing of porous asphalt mixture has a certain adverse effect on the mixture performance. Water causes deterioration of asphalt–aggregate adhesion and reduces the fracture resistance of the mixture [50]. It can be seen from Figure 8 that the splitting tensile strength of the porous asphalt mixture tends to be stable after water is sprinkled six times, with a small change in amplitude. The splitting tensile strength and raveling loss of the porous asphalt mixture are approximately 82.5% and 175.9% of those without water sprinkling, respectively. Although the performance declines, it still meets the requirements of JTG/T 3350-03-2020 that the splitting tensile strength should exceed 0.7 MPa and the raveling loss should be less than 15%, as shown in Figure 8 and Figure 9, respectively.

3.2.2. Effects of Water Sprinkling and Water Immersion

During the water sprinkling process in the laboratory, the specimen endures sprinkled water from a water mist state to an immersion state with increasing water amounts. Therefore, in order to analyze the effect of water sprinkling on the mechanical properties, water immersion was also included to represent the large water sprinkling amount. The Marshall specimens were pretreated according to the following test conditions. (a) Four groups of Marshall specimens were fabricated, with three specimens in each group, numbered No. 1, No. 2, and No. 3, and the temperature on the specimen surfaces was tested using a handheld infrared temperature tester. (b) When the surface temperature of the specimens dropped to approximately 50 °C, two groups of Marshall specimens were placed in a constant-temperature drying oven at 25 °C. After being placed in the drying oven for 6 h, they were immediately taken out and demolded and marked as Dry-Group 1 and Dry-Group 2; the other two groups of Marshall specimens were placed in a constant-temperature water bath at 25 °C. After being placed in the water bath for 1.5 h, they were immediately taken out and demolded and marked as Immersed-Group 1 and Immersed-Group 2. (c) Splitting tests were performed on the Dry-Group 1 and Submerged-Group 1 Marshall specimens and the splitting tensile strength was recorded. Cantabro tests were performed on the Dry-Group 2 and Submerged-Group 2 Marshall specimens, and the raveling loss was recorded. In this manner, the splitting tensile strength and raveling loss of the specimens under different pretreatment conditions were tested. The test results are shown in Figure 10 and Figure 11.

From Figure 10 and Figure 11, it can be that seen during the curing stage of the porous asphalt mixture, compared with the situation without water cooling, the strength of the porous mixture shows a certain degree of attenuation after immersion treatment, with an average decrease of 35.1% in the splitting tensile strength and an average increase of 41.4% in the raveling loss. However, the average splitting tensile strength of the porous asphalt mixture decreases by 17.5% after water is sprinkled six times, while the raveling loss increases by 75.9%. Compared with the situation without watering, the decrease in splitting tensile strength after water sprinkling is relatively small; yet, the increase in the raveling loss is significantly larger. The reasons for this phenomenon lie in the fact that the raveling loss test primarily focuses on the detachment of aggregates on the surface of asphalt mixtures under specific impact conditions. Although the Cantabro test can reflect the durability of the material when subjected to external impacts, the effect of aggregate strength has a significant impact on the test results, making it less accurate and comprehensive in assessing the adhesion performance between asphalt and aggregates. In contrast, the splitting tensile strength test not only considers the adhesion force between asphalt and aggregates but also encompasses the skeleton effect among aggregates and the overall strength of the asphalt mixture, thus making its results more representative and practical. During the early strength formation process of the porous asphalt mixture, a long period of immersion in water results in a loss of mixture strength, indicating that the porous asphalt mixture should not be exposed to water for a long time or in large quantities in the early stage after paving [51]. Therefore, the adoption of the water sprinkling cooling process is a more preferable option.

3.2.3. Water Sprinkling Cooling Process for Porous Asphalt Mixtures

During the trial section of porous asphalt pavements with water sprinkling cooling, the water amount for each sprinkling was calculated based on the total water consumption. The net weight of the water stored in the watering tank was about 12 t. The area of the trial section was about 2350 m², and it was divided into sixteen sections to complete the sprinkling. The calculations show that the water amount sprinkled per square meter each time was about 0.32 kg. Therefore, according to the research results, it is recommended to use a water sprinkling amount of 0.3 kg/m² once, and to sprinkle water four times before road marking and two times after road marking as the best water sprinkling cooling process for porous asphalt pavements. By observing the water infiltration phenomenon during the water sprinkling process, it is believed that the amount of water sprinkled at a time should not be too small, so as to avoid excessive evaporation of water and uneven temperature reduction of the porous asphalt pavements when the pavement surface temperature is high. The amount of water sprinkled at a time should not be too large to avoid water overflow and waste of water resources. At the same time, excessive water sprinkling also exacerbates the impact of water damage on the durability of porous asphalt pavements.

3.3. Long-Term Performance Observation of Porous Asphalt Pavement Trial Section with Water Sprinkling Cooling in the Actual Project

The actual engineering section for water sprinkling cooling is in the mileage from K126 + 900 to K126 + 700 in the Xuzhou to Huai’an direction, with a length of 200 m. The section from K128 + 000 to K127 + 000 in the Xuzhou to Huai’an direction was selected as the control section without water sprinkling, with a length of 1000 m. The test results of the pavement technical condition indicators of the two sections over a continuous period of seven years from 2017 to 2023 are shown in

Table 5. The test results of the pavement technical condition indexes over the seven years in the trial section with water sprinkling cooling.

Pavement Quality Index	Year
	2017	2018	2019	2020	2021	2022	2023
International Roughness Index (m/km)	0.98	1.01	1.23	1.24	1.37	1.22	1.12
Rutting Depth (mm)	3.7	4.4	3.5	3.3	2.8	3.5	2.8
Sideway Force Coefficient	45	58	49	48	46	49	50

and

Table 6. Test results of the pavement technical condition indexes over the seven years in the trial section without water sprinkling cooling.

Pavement Quality Index	Year
	2017	2018	2019	2020	2021	2022	2023
International Roughness Index (m/km)	1.06	1.07	1.06	1.18	1.20	1.24	1.21
Rutting Depth (mm)	3.3	3.9	3.3	3.6	2.8	3.5	2.6
Sideway Force Coefficient	47	54	50	47	45	48	47

, respectively.

According to Table 5 and Figure 6, there is little difference in the pavement performance indicators between the water sprinkling cooling trial section and the control section without water sprinkling over the seven years of service. The IRI for both sections is within 1.4 m/km, and the rutting depth is within 5 mm, maintaining an “excellent” level throughout the seven-year period. The SFCs for both sections increase initially and then decrease after their opening to traffic. After seven years, the average SFC for the trial section with water sprinkling cooling is 50, achieving an “excellent” level, while the average SFC for the control section without water sprinkling is 47, achieving a “good” level. However, the difference between them is not significant. Based on seven years of continuous observation, it is concluded that the use of the specified water sprinkling process for cooling does not have a significant adverse impact on the long-term performance of the porous asphalt pavements. In an emergency, water sprinkling can be used to accelerate the cooling of porous asphalt pavements, thereby achieving the purpose of quickly opening to traffic.

4. Effects of Air Blowing Cooling on the Temperature and Properties of Porous Asphalt Mixtures

4.1. Effect of Air Blowing on the Mixture Temperature

4.1.1. Air Blowing Speed Test for Different Settings for the Air Blowing Device

A floor-standing industrial fan was used as the air blowing equipment to analyze the temperature drop of the porous asphalt mixture under varying air blowing durations. Different fan settings resulted in different wind speeds, thus leading to variations in the cooling effects. In this study, an anemometer was placed 30 cm in front of the fan blades in the horizontal direction. After turning on the fan, the wind speed displayed on the anemometer was recorded, as shown in

Table 7. Wind speeds of the industrial fan in different gears.

Fan Gear	Wind Speed in Different Test Numbers (m/s)				Average
	No.1	No.2	No.3	No.4
First gear	2.9	3.1	3.1	2.9	3.0
Second gear	4.2	4.4	4.5	4.4	4.4
Third gear	5.7	5.8	6.3	6.0	6.0

. The wind speed measurement process is illustrated in Figure 12. According to Table 7, the wind speed is about 3.0 m/s when the fan is set to the first gear, 4.4 m/s in the second gear, and 6.0 m/s in the third gear.

4.1.2. Effects of Air Blowing on Surface Temperature of Mixtures

The cooling effects of air blowing at different wind speeds were analyzed with the specimens without air blowing as a comparison. During the blowing process, the Marshall specimens were arranged in a row and placed 30 cm away from the fan blades in the horizontal direction. The fabricated Marshall specimens were subjected to air blowing at three different speeds, corresponding to the first, second, and third gears, respectively. The blowing started when the surface temperature of the specimens reached approximately 120 °C after their fabrication. The surface temperature of the specimens was tested every five minutes, and blowing was stopped when the temperature dropped below 50 °C. The air blowing process of the specimen is shown in Figure 13. The surface temperature of the porous asphalt mixture Marshall specimens at different blowing times is shown in Figure 14.

As shown in Figure 14, the use of air blowing can accelerate the temperature reduction of porous asphalt mixtures and significantly shorten the time required for the surface temperature of the mixture to drop to 50 °C. With an ambient temperature of about 15 °C, the surface temperature of the porous asphalt mixture specimen drops below 50 °C after being placed for 35 min with no air blowing. With an air blowing speed of 3.0 m/s, the surface temperature of the porous asphalt mixture specimen drops below 50 °C after being placed for about 25 min. With an air blowing speed of 4.4 m/s, the surface temperature of the porous asphalt mixture specimen drops below 50 °C after being placed for 18 min. With an air blowing speed of 6.0 m/s, the surface temperature of the porous asphalt mixture specimen drops below 50 °C after being placed for 15 min.

4.2. Effects of Air Blowing on Mechanical Properties of Mixtures

4.2.1. Effects of Air Blowing Speeds

The Marshall specimens of the porous asphalt mixture, cooled by air blowing at different blowing speeds, were used to carry out splitting tests, and the test results of the splitting tensile strength are shown in Figure 15. Additionally, Cantabro tests under 300 and 600 rotations were conducted. The test results of the raveling loss are shown in Figure 16. The standard number of rotations for the Cantabro test is 300. In order to simulate the anti-raveling performance under harsh conditions, Cantabro tests at 600 rotations were also carried out.

As shown in Figure 15 and Figure 16, the strength of the porous asphalt mixture is damaged to a certain extent after being exposed to air blowing, resulting in significant reductions in its anti-raveling performance and splitting tensile strength. Taking the raveling loss as an example without air blowing, the raveling loss of the porous asphalt mixture is 5.4% after 300 rotations and 10.9% after 600 rotations. When the air blowing speed is 3.0 m/s, the raveling loss of the porous asphalt mixture increases to 7.4% after 300 rotations and 11.1% after 600 rotations. Moreover, the greater the air blowing speed, the more pronounced the changes in the splitting tensile strength and raveling loss of the porous asphalt mixture after air blowing. This is due to the fact that when the wind blows across the surface of the porous asphalt mixture, its strong airflow may disturb the particles in the mixture. This disturbance is not limited to the surface particles but may also penetrate deep into the interior of the mixture, affecting the interaction between the particles. This disturbance destroys the skeleton structure of the mixture, resulting in a decrease in mechanical properties [52,53].

4.2.2. Effects of Air Blowing at Different Pavement Surface Temperatures

When blowing air on the surface of the porous asphalt mixture at high temperatures, the performance loss of the mixture is relatively significant. After the specimens were fabricated in the laboratory, they were subjected to air blowing for cooling at three different wind speeds when the surface temperatures were approximately 120 °C and 70 °C, respectively. The surface temperature of 70 °C simulates the surface temperature of the mixture after final compaction in onsite construction. The surface temperature and raveling loss of the porous asphalt mixture after air blowing were tested, and the results are shown in Figure 17 and

Table 8. Raveling loss of porous asphalt mixture in different temperature conditions for air blowing.

Mechanical Performance Index		Raveling Loss in Different Temperature Conditions for Air Blowing (%)
		Unblown	Blown at About 120 °C	Blown at About 70 °C
Raveling loss (%)	300 rotations	5.4	12.6	6.0
	600 rotations	10.9	22.4	11.3
Splitting tensile strength (MPa)		1.14	0.74	1.03

.

As shown in Figure 17, when the air blowing was carried out at the surface temperature of about 120 °C, the surface temperature of the porous asphalt mixture specimen dropped below 50 °C after being placed for 15 min. When the air blowing was carried out at the surface temperature of about 70 °C, that is, when the air blowing began after natural cooling at the ambient temperature for 25 min, the surface temperature of the porous asphalt mixture specimen fell below 50 °C after being placed for nearly 35 min. As shown in Table 7, there is a significant difference in the influence on the mechanical properties of the porous asphalt mixture when air blowing is initiated at the specimen temperatures of 120 °C and 70 °C, respectively. In the former case, the raveling loss is 12.6% after 300 rotations and 22.4% after 600 rotations, and the splitting tensile strength is 0.74 MPa, showing a significant change compared with those without air blowing. In the latter case, the raveling loss is 6.0% after 300 rotations and 11.3% after 600 rotations, and the splitting tensile strength is 1.03 MPa, which is only slightly different from the case without blowing. Asphalt is actually a temperature-dependent material, and temperature has a great influence on the viscoelasticity of asphalt binder [24]. The porous asphalt pavement has a large number of interconnecting pores [24,54], and it is easier for the air flow to blow into the specimen to achieve the purpose of cooling. Air blowing at high temperatures may also affect the strength formation due to air flow in the interconnecting pores. This indicates that if air cooling is used, the temperature of the porous asphalt mixture must be strictly controlled, and air blowing can be carried out when the pavement surface temperature is at least below 70 °C.

5. Comparison of the Mechanical Properties for Porous Asphalt Mixtures with Water Sprinkling Cooling and Air Blowing Cooling

In order to analyze the effects of the cooling methods, including water sprinkling and air blowing, on the mechanical properties of the porous asphalt mixtures, and to give suggestions for cooling in an emergency in actual projects, the splitting tensile strength and raveling loss after air blowing in different treatment conditions were compared with those after water was sprinkled six times, as shown in

Table 9. The comparison of mechanical properties between air blowing in different treatment conditions and water sprinkling six times.

Mechanical Performance Index	Air Blowing			Water Sprinkling
	Unblown	Blow at About 70 °C	Attenuation Rate (%)	Dry	Water Sprinkling Six Times	Attenuation Rate (%)
Splitting tensile strength (MPa)	1.14	1.03	9.6	1.14	0.94	17.5
Raveling loss (%)	5.4	6	11.11	5.8	10.2	75.9

.

As shown in Table 9, the splitting tensile strength of the porous asphalt mixtures subjected to water sprinkling six times is lower than that of the mixtures treated with the air blowing process at 70 °C. Additionally, the raveling loss of the porous asphalt mixtures after water is sprinkled six times is higher than that of the air blowing process at 70 °C. A comparative analysis reveals that the difference in the reduction degree of the splitting tensile strength between the two pretreatment methods is not significant, but the raveling loss is greater after water is sprinkled six times. This aligns with previous research showing that water is identified as a critical factor affecting the bonding stability at the asphalt–aggregate interface [55]. Considering the adverse effects of the cooling methods on the mechanical properties, the air blowing cooling process seems a superior option.

Considering the significant difference between the floor-standing industrial fans used for laboratory-simulated air blowing and the high-power air compressor employed on the construction site, a survey of the wind speeds of the commonly used air compressors in the market was investigated in this research, as detailed in

Table 10. Wind speeds of air compressors at different power levels.

Power (kW)	Pressure (MPa)	Air Displacement (m3/min)	Wind Speeds (m/s)
			Outlet Diameter 5 cm	Outlet Diameter 10 cm
7.5	0.8	1.2	10.2	2.5
11	0.8	1.65	14.0	3.5
15	0.8	2.5	21.2	5.3
22	0.8	3.8	32.3	8.1
30	0.8	5.3	45.0	11.2
37	0.8	6.8	57.7	14.4

. According to Table 10, the wind speeds of the air compressor are significantly higher than those of the industrial fans used for laboratory simulation. Considering the adverse effects of wind speeds on the mechanical properties of porous asphalt mixture, it is evident that the use of air compressors for air blowing cooling on the construction site will cause more serious damage to porous asphalt mixtures. To sum up, water sprinkling six times can be used to accelerate the temperature drop of porous asphalt pavements.

6. Conclusions

In this research, water sprinkling and air blowing were used to accelerate the cooling of porous asphalt pavements. Their impacts on the temperature and mechanical properties were analyzed. Finally, a seven-year period of observation was conducted in the trial section using the water sprinkling cooling method, with the section without water sprinkling as a control section. Based on the test results and analysis, the following conclusions were obtained.

(1) Water immersion or the frequent sprinkling of water during the curing period of porous asphalt pavements can reduce the mixture strength. The use of a water amount of 0.3 kg/m² once is proposed, with water sprinkling four times before marking and water sprinkling two times after marking; this is regarded as the optimal water sprinkling cooling process for porous asphalt pavements. The splitting tensile strength and raveling loss of the porous asphalt mixture are about 82.5% and 175.9% of those without water sprinkling, respectively. Although the performance exhibits a certain decline, it still meets the requirements of the relevant specifications.

(2) The use of air blowing can accelerate the temperature reduction of the porous asphalt mixture, but the raveling loss of the mixture increases significantly and the splitting tensile strength decreases significantly after air blowing at high pavement surface temperatures, adversely affecting the mechanical properties. The impact on the mechanical properties of the porous asphalt mixture will be minimal if air blowing is conducted when the pavement surface temperature is below 70 °C.

(3) Sprinkling water or blowing air can be used to accelerate the cooling of porous asphalt pavements, thereby achieving the goal of opening to traffic early, but it will have an adverse effect on the mechanical properties of porous asphalt mixtures.

(4) After a seven-year period of tracking observation, it is believed that the adoption of the specified process for water sprinkling to reduce temperature has no significant adverse effects on the long-term performance of porous asphalt pavements. In an emergency, water sprinkling can be used to accelerate the temperature drop of porous asphalt pavements, thereby achieving the purpose of quickly opening to traffic.

In this research, the cooling effects achieved by water sprinkling and air blowing technologies were clarified for a newly laid porous asphalt mixture, and the effects on the mechanical properties of the mixtures were revealed. The technology control parameters of the two methods were proposed with minimal comprehensive impact. This research provides a quick and effective technical solution for the construction of porous asphalt pavements with limited curing time. Through the long-term observation of actual engineering, it was determined that the water sprinkling process had little effect on the durability of porous asphalt pavement, and it can be used as the optimal method for early opening to traffic. Due to the difference in the work efficiency of the air blowing equipment in the laboratory and in onsite projects, the proposed control parameters based on the laboratory test may not be suitable for the project site, and they need to be further verified in combination with the actual project.