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Article

The Effect of Air Void on the Laboratory Properties of Polyurethane Mixtures

by
Yunhao Zhou
1,
Shijie Ma
2,
Chenghua Gan
1,
Wenjian Wang
1,
Peihan Yu
1,
Xiangzhuo Zheng
1,
Peiyu Zhang
2,
Bokai Liu
3 and
Haisheng Zhao
2,*
1
Department of Road and Railway Engineering, School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, China
2
Key Laboratory of Highway Maintain Technology Ministry of Communication (Jinan), Jinan 250102, China
3
Shandong Provincial Communications Planning and Design Institute Group Co., Ltd., Jinan 250101, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 228; https://doi.org/10.3390/coatings15020228
Submission received: 10 January 2025 / Revised: 5 February 2025 / Accepted: 11 February 2025 / Published: 14 February 2025
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Abstract

:
Polyurethane (PU) materials, with their excellent mechanical properties, durability, and fatigue resistance, hold promise for addressing the challenges of aging, environmental pollution, and segregation during the storage of modified asphalt mixtures, thereby extending the lifespan of pavements and enhancing the level of service. Although studies have been conducted on the road performance of PU mixtures that compared them with asphalt mixtures, there is relatively less research on how the air void of PU mixtures affects their performance. This study systematically investigates the dynamic characteristics and road performance of dense-graded PU mixtures at three air void ranges—1%–3%, 3%–5%, and 5%–7%—and verifies the effectiveness through statistical methods. The research results show that air voids have a significant impact on road performance. Compared to low air voids, high air voids can increase high-temperature performance by 12%–33%. However, higher air voids also lead to a significant decrease in resistance to water damage, with a reduction of about 9%–24%. When the air void is in the range of 3%–5%, the mixture has better dynamic stability. Therefore, when designing PU mixtures, a reasonable air void should be selected based on engineering conditions to achieve the optimal pavement structure combination and save investment. This study provides a scientific basis for the design and application of PU mixtures and lays the foundation for further understanding of their performance mechanisms.

1. Introduction

In modern pavement engineering, asphalt mixtures and polyurethane (PU) mixtures have garnered widespread attention due to their unique properties. Asphalt mixtures, as traditional pavement materials, have been extensively studied and applied for their performance. In contrast, PU mixtures, as a new type of material, have shown significant potential to enhance service levels in pavement engineering due to their superior mechanical properties, durability, and fatigue resistance [1,2,3]. Volumetric indicators, as key factors affecting the performance of both types of mixtures, have become a hot topic of research for their impact on the dynamic characteristics and road performance of the mixtures. In the field of road engineering, optimizing the performance of asphalt mixtures is crucial for achieving road durability and functionality. Volumetric indicators, as core parameters in the design of asphalt mixtures, significantly influence the mechanical behavior, durability, and environmental adaptability of the mixtures [4,5,6].
In recent years, numerous scholars have conducted in-depth research on the volumetric indicators of asphalt mixtures and their impact on road performance, providing a solid theoretical foundation and technical support for this study. In the Al-Khateeb model [7], asphalt mixtures are composed of three parallel phases: asphalt binder, aggregate, and air void. From this perspective, the properties of asphalt mixtures must be influenced by these three phases, and many researchers have studied the impact of air void on asphalt binder. Ling et al. [8] quantitatively analyzed the 3D pore structure parameters of porous asphalt mixtures (PAM), such as air void, equivalent diameter, pore coordination number, pore throat size, and tortuosity, revealing the significant impact of air void and aggregate size on the complexity and spatial anisotropy of PAM pore structure. Seitllari et al. [9] assessed the impact of air void on the crack resistance of asphalt mixtures at room temperature, indicating that mixtures with lower air void may have better crack resistance. Ling et al. [10] established an air void prediction model to reveal the relationship between the road performance of asphalt mixtures and air void, optimizing design and enhancing the overall performance of pavement structures. Sun et al. [11] studied the impact of air void on the noise reduction performance of asphalt mixtures through the establishment of a sound absorption model, showing that an increase in air void corresponds to an increase in noise reduction performance. Islam et al. [12] demonstrated how an increase in binder content and air void affects the increase in the dynamic modulus of asphalt mixtures. The research conclusions of Solatifar et al. and Su et al. [13,14] indicate that the dynamic modulus of laboratory-tested asphalt mixtures is influenced by binder viscosity, binder content, air void, and gradation.
In the field of PU mixture research, Sun et al. [15] explored the road performance and mechanical properties of polyurethane concrete with crushed stones, noting that PU mixtures have good temperature stability and durability. Wang et al. [16] prepared PERS mixtures using two-component polyurethane materials and studied their mechanical and acoustic properties. Their research provided performance data for PERS mixtures with an air void of 35% and explored the performance of PERS mixtures with a polyurethane mass fraction of 15%, providing important references for the application of polyurethane in road engineering. Cong et al. [17] evaluated the anti-clogging performance of PPM through clogging tests and permeability measurements, deeply discussing the strength characteristics and failure mechanisms of porous polyurethane macadam (PPM) mixtures, providing a scientific basis for the engineering application of PPM. Lu et al. [18] studied the main mechanical properties of polyurethane combined with permeable mixtures (PUPMs) suitable for permeable pavements. Zhao et al. [19] concluded through the analysis of the anti-cracking factor (E*sin(δ)) that at lower temperatures, the anti-cracking factor of PU mixtures is much smaller than that of SBS-modified asphalt mixtures, indicating a smaller viscous component in PU mixtures. As the temperature rises, the loss modulus of PU mixtures changes little, showing a lower sensitivity of their viscous properties to temperature changes and further emphasizing the superiority of PU mixtures in high-temperature environments.
This study aims to systematically investigate the impact of different volumetric indicators on the dynamic characteristics and road performance of dense-graded polyurethane mixtures. Through dynamic modulus tests, conventional splitting tests, freeze–thaw splitting tests, MIST splitting tests, and Hamburg wheel-tracking tests, the performance of PU mixtures with air void in the ranges of 1%–3%, 3%–5%, and 5%–7% was evaluated. The research results reveal the role of volumetric indicators in the design of polyurethane mixtures, providing a reference for the gradation design of polyurethane mixtures. It offers a basis for laboratory testing to design polyurethane mixtures with optimal performance. It also provides theoretical guidance for the rational selection of air voids based on engineering conditions to achieve the best pavement structure. Moreover, it lays the foundation for a deeper understanding of the intrinsic performance mechanisms of polyurethane mixtures.

2. Materials and Methods

2.1. Materials and Gradation of Polyurethane Mixtures

This paper utilizes a dense-graded polyurethane mixture, AC−13. Basalt from Jinan Xing’an Stone Co., Ltd. (Jinan, China) was selected as the aggregate and filler, and all technical standards meet the “Technical Specifications for Construction of Highway Asphalt Pavements” (JTG F40−2004). The particle pass rate of the selected gradation is shown in Figure 1, and the polyurethane dosage is 4.9%. The polyurethane mixture with AC−13 aggregate gradation is labeled as PUM−13.
The polyurethane was provided by Yantai Wanhua Chemical Group Co., Ltd. (Yantai, China), and the relevant technical specifications are shown in Table 1.
Taking polyurethane mixtures with the same gradation but with three different air void ranges (1%–3%, 3%–5%, 5%–7%) as the research subjects, performance tests were conducted on the mixtures with different air voids under controlled curing time and environmental conditions to derive performance parameters under various conditions. The polyurethane, provided by Yantai Wanhua Chemical Group Co., Ltd., in China, is of the wet-curing type, which cures under humid conditions, and the relevant technical specifications of the polyurethane are shown in Table 1.

2.2. Specimen Preparation

According to AASHTO: TP−62 (2009), the mixed aggregate is compacted using a Superpave Gyratory Compactor (SGC). By selecting different numbers of compaction cycles, the air void of the resulting specimens is controlled. Ultimately, 50 cycles, 100 cycles, and 160 cycles were chosen to correspond to air void ranges of 1%–3%, 3%–5%, and 5%–7%, respectively, with a diameter of 150 mm for all. The height of the dynamic modulus specimens is 170 mm, which is then cored and cut to 150 mm. The conventional splitting test, MIST water sensitivity test, and freeze–thaw splitting test specimens are all 118 mm. The Hamburg high-temperature wheel-tracking test specimens have a height of 170 mm, and then each specimen is cut into a pair of 60 mm thick standard specimens.
The production process is as follows: (a) Dry the graded aggregate in an oven for 4 h and cool to room temperature. (b) At room temperature, add the aggregate and polyurethane in proportion to the mixer and blend for 90 s, then add the filler and blend for another 90 s. (c) Store the mixed polyurethane mixture at room temperature for 2 h to allow initial hardening for easy molding. (d) Compact the polyurethane mixture in the Superpave Gyratory Compactor for 50, 100, and 160 cycles according to the required air void. (e) Cure the molded specimens in an environment of 35 °C and 70% humidity for 7 days. (f) According to the “Highway Engineering Asphalt and Mixture Testing Procedures” (JTG E20−2011), measure the specific air void of the specimens by the surface dry method and select the specimens that meet the requirements to complete the experimental preparation.
Experimental plan: (a) For each air void range (1%–3%, 3%–5%, 5%–7%), select three specimens for the dynamic modulus test to compare the modulus, phase angle, and crack resistance factor. (b) For the conventional splitting, freeze–thaw splitting, and MIST splitting tests, select four specimens from each of the three air void ranges for each test to compare the ultimate strength and resistance to water damage. (c) For the high-temperature rutting test, select four specimens from each air void range to compare the rutting depth.

2.3. Splitting Test of Polyurethane Mixture

According to the “Highway Engineering Asphalt and Asphalt Mixture Testing Procedures” (JTG E20−2011), select polyurethane mixture specimens, with 4 specimens chosen from each of the 3 air void ranges for the mixture splitting test. The test temperature is room temperature, and the loading rate is 50 mm/min.
The volumetric indicators of the selected specimens are shown in Table 2, with γb (asphalt and water have different relative densities) being 1.032 for all. γf represents the bulk relative density of the asphalt mixture, and γt represents the theoretical maximum relative density of the asphalt mixture.

2.4. Freeze–Thaw Splitting Test of Polyurethane Mixture

According to the “Highway Engineering Asphalt and Asphalt Mixture Testing Procedures” (JTG E20−2011), select polyurethane mixture specimens and subject them to freeze–thaw conditions by storing them in a constant temperature freezer at −16 °C for 16 h, followed by keeping them in a constant temperature water bath at 60 °C for 24 h. The splitting loading rate is 50 mm/min to obtain the maximum load in the test.
Since the specimen diameter is 150 mm, the splitting coefficient is uniformly taken as 0.00425, and the volumetric indicators of the specimens are detailed in Table 3.

2.5. MIST Water Sensitivity Test

Water sensitivity is one of the important characteristic properties of polyurethane mixtures [20,21], and the air void index is directly related to the mixture’s resistance to water damage [22]. The MIST (Modified Indiana Stone Test) water sensitivity test machine uses a hydraulic pump and piston to inflate and deflate the airbag, creating positive and negative water pressure in the sealed chamber to simulate the impact of dynamic loads on the mixture under water-accumulated conditions [23,24,25]. The test temperature in this experiment is 35 °C, with a brushing count of 8000 times. After brushing, the mixture is stored in a water tank at room temperature for 24 h before undergoing a splitting test, during which the splitting strength is recorded.
The volumetric indicators of the specimens taken are shown in Table 4.

2.6. Hamburg Wheel Tracking Test at High Temperature

According to the “Standard Test Method for Hamburg Wheel—Track Testing of Compacted Asphalt Concrete” (AASHTO T324−04), the Hamburg high-temperature wheel-tracking test is conducted. Dense-graded polyurethane mixtures have better high-temperature performance compared to asphalt mixtures [26,27]. To make the effect of air void on high-temperature performance more intuitive [27,28,29,30,31,32], the final selected temperature is 60 °C, the wheel pressure is set at 0.7 MPa, and the maximum number of cycles is set at 20,000.
A total of 6 pairs, or 12 specimens, were used for the test, with 2 pairs, or 4 specimens, for each air void range. Detailed volumetric indicators can be found in Table 5.

2.7. Dynamic Modulus Test

According to the AASHTO: TP−79 (2010) standard, the Asphalt Mixture Performance Tester (AMPT) is used to conduct dynamic modulus tests on polyurethane with three different air void levels. The test temperatures are divided into five levels: 5 °C, 15 °C, 25 °C, 35 °C, 45 °C, and 55 °C, with loading frequencies set at 25 Hz, 20 Hz, 10 Hz, 5 Hz, 2 Hz, 1 Hz, 0.5 Hz, 0.2 Hz, and 0.1 Hz. To prevent specimen damage and keep the mixture within the viscoelastic range [33], the amplitude of the loading wave is set between 75 and 125 με, the loading waveform is sinusoidal, and the control mode is load-controlled uniaxial compression.
The volumetric indicators of the dynamic modulus specimens are shown in Table 6.

3. Results

3.1. Results of Splitting, Freeze–Thaw, and MIST Test

The results of the conventional splitting, freeze–thaw splitting, and dynamic water flushing splitting of the polyurethane mixture PUM−13 are plotted as scatter diagrams, with air void as the horizontal coordinate and splitting strength as the vertical coordinate. The results indicate that air voids have a significant impact on the ultimate strength of the mixture and that different air void levels significantly affect the resistance to water damage. The group with the lowest air voids performed significantly better than the other two groups under strength testing conditions. The obtained results are shown in Figure 2.

3.2. Results of Hamburg High-Temperature Wheel-Tracking Test

The Hamburg high-temperature wheel-tracking test data for the polyurethane mixture are shown in Figure 3, where the vertical coordinate represents the rut depth, and the horizontal coordinate represents the air void. The results show that there is a significant relationship between air voids and high-temperature performance, with a notable improvement in high-temperature performance when the air voids are larger.

3.3. Modulus at Different Temperatures and Frequencies

The dynamic modulus of the polyurethane mixture at 5 °C for various frequencies is shown in Figure 4a, at 15 °C in Figure 4b, at 25 °C in Figure 4c, at 35 °C in Figure 4d, at 45 °C in Figure 4e, and at 55 °C in Figure 4f. In these figures, the vertical coordinate represents the dynamic modulus, and the horizontal coordinate represents the frequency. The results show that specimens with different air voids follow the same pattern in terms of the influence of temperature and frequency under dynamic modulus testing conditions, but their sensitivity varies.

3.4. Phase Angle at Different Temperatures and Frequencies

The phase angles of the polyurethane mixture at various frequencies at 5 °C are shown in Figure 5a, at 15 °C in Figure 5b, at 25 °C in Figure 5c, at 35 °C in Figure 5d, at 45 °C in Figure 5e, and 55 °C in Figure 5f. In these figures, the vertical coordinate represents the phase angle, and the horizontal coordinate represents the frequency. The results indicate that the phase angle of specimens with different air voids follows the same pattern in terms of the influence of temperature, but their sensitivity varies. Additionally, the influence of frequency on the phase angle differs among specimens with different air voids.

3.5. Crack Resistance Factor at Different Temperatures and Frequencies

This paper uniformly refers to E*×sin(δ) as the crack resistance factor. The crack resistance factor (fatigue parameter) is crucial for understanding and predicting the durability and reliability of materials in practical applications and is an effective parameter for characterizing the fatigue crack resistance of asphalt mixtures [34]. The crack resistance factor of polyurethane mixtures at various frequencies at 5 °C is shown in Figure 6a, at 15 °C in Figure 6b, at 25 °C in Figure 6c, at 35 °C in Figure 6d, at 45 °C in Figure 6e, and at 55 °C in Figure 6f. In these figures, the vertical coordinate represents the crack resistance factor, and the horizontal coordinate represents the frequency.

4. Discussion

4.1. The Impact of Temperature and Loading Frequency on the Dynamic Modulus of Mixtures with Different Air Voids

From Figure 4, it can be concluded that for polyurethane mixtures of any air void, the dynamic modulus increases monotonically with the increase in loading frequency and decreases with the rise in test temperature [3,5,35]. This phenomenon indicates that when the temperature rises, the impact of the polyurethane binder on the dynamic modulus weakens. Polyurethane mixtures mainly exhibit elastic behavior at low temperatures and viscous behavior at high temperatures, a pattern similar to that of asphalt mixtures. When the loading frequency is within the range of 0–10 Hz, the dynamic modulus of polyurethane mixtures with various air voids increases rapidly with the increase in loading frequency. When the loading frequency exceeds 10 Hz, the dynamic modulus curves of polyurethane mixtures with various air voids tend to flatten. It can be seen that the dynamic modulus of dense-graded polyurethane mixtures with different air voids is more sensitive at low loading frequencies. Moreover, the polyurethane binder mainly exhibits elastic behavior at low temperatures and viscous behavior at high temperatures [35]. High temperatures weaken the impact of the polyurethane binder on the mixture, making the load-bearing role of the aggregate more pronounced and leading to an overall decrease in the dynamic modulus of the mixture.
When the loading temperature increases from 5 °C to 55 °C, the dynamic modulus of polyurethane mixtures with an air void less than 3% decreases by about 58% to 60%, those with an air void between 3% and 5% decrease by about 45% to 48%, and those with an air void greater than 5% decrease by about 50%. Therefore, the dynamic modulus of mixtures with an air void less than 3% is more sensitive to temperature changes, and it is least sensitive when the air void is between 3% and 5%. For asphalt mixtures, the dynamic modulus can decrease by 67% to 80% when the temperature rises from 21.1 °C to 37.8 °C [36], indicating that polyurethane binders are less sensitive to temperature than asphalt binders. At all temperatures, the maximum dynamic modulus is achieved when the air void is around 3%. The dynamic moduli of mixtures with air voids of 2.48% and 4.09% are similar at low and room temperatures, but at higher temperatures, the dynamic modulus of the mixture with 4.09% air void is significantly greater than that with 2.48%. When the air void is greater than 5% or less than 2.48%, the dynamic modulus significantly decreases at all temperatures. Therefore, when the air void is around 3%, dense-graded polyurethane mixtures achieve the best resistance to deformation. When the air void is greater than 5%, the dynamic modulus of the mixture decays rapidly.
Using the SPSS 26 software to analyze the coefficient of variation (COV) of the dynamic modulus for the different air void ranges, the dynamic modulus COV of mixtures with air void less than 3% is not significantly affected by temperature, with the COV ranging between 6% and 10%. For mixtures with air void between 3% and 5%, the dynamic modulus COV increases with the temperature rise, with the COV ranging between 12% and 19%. For mixtures with air void greater than 5%, the dynamic modulus COV also increases with the temperature rise, with the COV ranging between 8% and 13%. There are differences in the dynamic modulus coefficient of variation for mixtures with different air voids; among them, the COV for the group with an air void greater than 5% increases with the increase in frequency at the same temperature, while the other two groups show no clear trend of COV changing with frequency.

4.2. Phase Angles of Mixtures with Different Air Voids at Various Temperatures and Loading Frequencies

The phase angle is an indicator of the relative proportions of viscosity and elasticity in the binder. The larger the phase angle of the binder, the more it implies an increase in viscosity and a decrease in elasticity; conversely, the smaller the phase angle, the less viscous and more elastic it is [37]. Regardless of the air void of the mixture, when the loading frequency increases from 0 Hz to 10 Hz at the test temperature, the phase angle rapidly decreases, indicating that under these frequency conditions, the viscosity of the polyurethane mixture decreases while its elasticity increases. At this time, the performance of the aggregate framework and the polyurethane binder has a more significant impact on the mechanical properties of the mixture. When the loading frequency exceeds 10 Hz, the phase angle slowly decreases with the reduction in loading frequency, and the phase angle curve flattens out. This proves that under high loading frequencies, the phase angle of the polyurethane mixture is not sensitive to frequency changes, and the viscoelastic properties of the polyurethane binder do not change significantly.
Regardless of the air void, the phase angle of polyurethane mixtures increases monotonically from 5 °C to 55 °C. Some studies have indicated that for asphalt mixtures, when the temperature reaches 35 °C, the phase angle first increases and then significantly decreases with the reduction in loading frequency. As the temperature continues to rise to 50 °C, the phase angle continues to decrease with the increase in temperature and the reduction in loading frequency [38]. The correlation between the phase angle and temperature of polyurethane mixtures is different from that of asphalt mixtures.
When the loading temperature increases from 5 °C to 55 °C, the phase angle of polyurethane mixtures with an air void of less than 3% increases by about 110% to 118%, those with an air void between 3% and 5% increase by about 135% to 169%, and those with an air void greater than 5% increase by about 120% to 138%. The phase angle change for mixtures with an air void between 3% and 5% is greater than the other two air void ranges, and mixtures with an air void in this range have higher upper and lower limits of phase angle. Polyurethane mixtures are linear viscoelastic materials, and the larger the phase angle value, the greater the viscosity of the linear viscoelastic material. Therefore, a lower value indicates better elastic behavior of the linear viscoelastic material. From the results, it can be seen that polyurethane mixtures with an air void between 3% and 5% exhibit stronger viscous behavior at high temperatures and also have better elastic behavior at low temperatures.

4.3. Analysis of Anti-Cracking Factors for Mixtures with Different Air Voids at Various Temperatures and Loading Frequencies

Fatigue is one of the most significant damages to asphalt pavements, and the formation and propagation of cracks are closely related to the energy dissipation caused by external loads [39,40]. The Strategic Highway Research Program (SHRP) mentions that the crack resistance factor (E*×sin(δ)) is an effective parameter for characterizing the resistance of asphalt mixtures to fatigue cracking [41]. Mixtures with higher crack resistance factor values are also generally more capable of resisting cracking distress. This finding is crucial for understanding and predicting the durability and reliability of materials in practical applications.
At all temperatures, the crack resistance factor for air voids of 2.92% and 4.09% both show a clear decreasing trend as the frequency increases. The crack resistance factors for these two air voids are significantly affected by temperature, with both achieving the maximum crack resistance factor at room temperature and the minimum at 55 °C. Therefore, the fatigue crack resistance of these air voids is influenced by the loading frequency, with higher frequencies leading to poorer resistance to fatigue cracking. Both low and high temperatures affect the fatigue crack resistance performance, with low temperatures having a less significant impact and high temperatures having a marked effect. In the two groups of specimens with air void greater than 5%, the loading frequency hardly affects the change in the crack resistance factor. The factor influencing the crack resistance factor is only temperature, with little difference in the crack resistance factor between low and room temperatures and a decrease of about 15% at high temperatures, slightly lower than the 18% decrease in the 4.09% air void group. This indicates that the fatigue crack resistance of dense-graded polyurethane mixtures with air void greater than 5% is not strongly correlated with loading frequency and is mainly affected by high temperatures, with the decrease in fatigue crack resistance being less than that of the 4.09% group. The crack resistance factor for the 2.28% air void group, like the group with air void greater than 5%, is not significantly affected by loading frequency, with a decrease of about 16% in the crack resistance factor at high temperatures. The crack resistance factor for the 2.48% air void group varies unstably with changes in loading frequency, and the change in crack resistance factor between high and low temperatures is the greatest, reaching 31%. Overall, it can be concluded that the fatigue crack resistance of polyurethane mixtures with an air void of 5%–7% is not easily affected by loading frequency, and polyurethane mixtures with an air void of 3%–4% exhibit the best fatigue crack resistance at high temperatures.
These experimental results highlight the importance of air voids in the dynamic characteristics of polyurethane mixtures and provide a basis for laboratory testing to design polyurethane mixtures with optimal performance. The experiments demonstrate the linear elasticity of polyurethane mixtures and identify the air void ranges that offer better dynamic characteristics in dense-graded mixtures. These findings also lay a foundation for further research to reveal the intrinsic performance mechanisms of polyurethane mixtures under laboratory conditions.

4.4. Strength Analysis of Polyurethane Mixtures with Different Air Voids

For ease of analysis, the conventional splitting results are compared together with MIST splitting and freeze–thaw splitting data, as shown in Figure 7, with the three air void ranges separated by blue lines.
Under conventional splitting conditions, the splitting strengths of the mixtures in the three air void ranges are not significantly different. Taking the strength of the mixture with air void between 1% and 3% as the baseline, the strength of the mixture with air void between 3% and 5% decreases by about 5%, and the strength of the mixture with air void between 5% and 7% decreases by about 12%. Under freeze–thaw conditions, the strength of the range 1%–3% does not decrease; instead, it increases by about 12%. The freeze–thaw splitting strength of the mixture with air void between 3% and 5% significantly decreases relative to conventional splitting, with a reduction of about 42%. The freeze–thaw splitting strength of the mixture with air void between 5% and 7% shows the largest decrease relative to conventional splitting, with a reduction of up to 57%. The experiments indicate that the higher the air void of the polyurethane mixture, the more significant the decrease in freeze–thaw splitting strength. When the air void is less than 3%, polyurethane mixtures may experience a certain increase in strength due to freeze–thaw cycles. Some reports have pointed out that under freeze–thaw conditions, an increase in air void in asphalt mixtures leads to a decrease in relative strength [42,43]. Compared to base asphalt mixtures, SBS-modified asphalt mixtures and thermosetting asphalt mixtures show a significant reduction in air void growth before and after freeze–thaw cycles, with thermosetting asphalt mixtures having the lowest air void growth, especially those with smaller air voids [42,44]. It is inferred that dense polyurethane mixtures may have freeze–thaw characteristics similar to those of thermosetting asphalt mixtures with small air voids and may experience a slight increase in strength under certain influencing factors.
In the MIST dynamic water spray splitting test, the strength of all three groups of mixtures decreased compared to conventional splitting. The decrease in strength for the three groups of air voids from highest to lowest was 24%, 39%, and 48%, respectively. It can be quite directly concluded that under dynamic water spray conditions, the air void of polyurethane mixtures is inversely proportional to the splitting strength; the lower the air void, the stronger the resistance to water damage.
In general, polyurethane mixtures possess better strength and water damage resistance at lower air voids. When the air void is between 1% and 3%, the polyurethane mixtures perform better in strength under all three experimental conditions compared to the other two groups.
These experimental results highlight the significance of air voids in determining the ultimate strength and water damage resistance characteristics of polyurethane mixtures and provide a basis for laboratory testing to design polyurethane mixtures with optimal performance. The air void ranges provided by this study can help pavement designers and construction professionals enhance the durability and overall performance of pavements. This not only increases the service life of the pavement but also reduces maintenance costs and environmental impact, making polyurethane mixtures a viable and advantageous option for modern pavement engineering. Additionally, these findings lay a foundation for further research to reveal the intrinsic performance mechanisms of polyurethane mixtures under laboratory conditions.

4.5. Analysis of Hamburg Wheel-Tracking Test for Polyurethane Mixtures with Different Air Voids

The Hamburg wheel-tracking test (HWTT) is a laboratory testing method used to evaluate the resistance of asphalt mixtures to rutting under high-temperature conditions, as well as the adhesion of asphalt binder to aggregates and their resistance to water damage. Typically, modified asphalt mixtures often develop rutting depths of about 20 mm after 5000 cycles of loading at high temperatures [45]. The high-temperature performance of polyurethane mixtures is generally far superior to that of modified asphalt mixtures [46,47], and the experimental results also support this view. The maximum rutting depth of polyurethane mixtures with three different air voids did not exceed 5 mm under a loading frequency of 60 °C and 20,000 cycles.
Taking the rutting depth of the mixture with an air void of 3%–5% as the baseline, the rutting depth of the mixture with an air void of 1%–3% increased by 21% compared to the baseline, while the rutting depth of the mixture with an air void of 5%–7% decreased by 12% compared to the baseline. Therefore, among the three air voids tested at a loading frequency of 60 °C, the polyurethane mixture with an air void of 5%–7% exhibited the best high-temperature performance. Overall, the high-temperature performance of the three air voids is not significantly different, and even the worst-performing group, with an air void of 1%–3%, is far superior to that of dense-graded asphalt mixtures.
The excessively high temperature and number of loading cycles resulted in a large coefficient of variation (COV) and scattered data for the 5%–7% air void group in this experiment, with a COV of approximately 86%.

4.6. Significance Analysis of the Influence of Air Void

A loading frequency of 10 Hz is commonly used to simulate traffic loads on roads because it is close to the frequency of actual vehicle passages [48]. Testing the dynamic modulus at 10 Hz can more accurately simulate and predict the performance of materials under actual traffic loads. Therefore, 10 Hz at 15 °C and 55 °C was chosen for the ANCOVA (Analysis of Covariance) significance analysis.
Under controlled testing temperatures and loading frequencies, the statistical analysis aims to determine whether air void has an impact on the road performance of PU mixtures. A 95% significance level is used to determine whether the samples are statistically different. If the calculated p-value (i.e., the significance level) is less than 0.05, the variable has a statistically significant impact on performance; otherwise, the compared groups can be considered statistically equivalent [49]. From the significance values, it can be seen that after excluding the effects of temperature and frequency, the dynamic modulus, phase angle, strength, crack resistance, and rutting resistance of PU mixtures are significantly affected by air void, with p-values all less than 0.05 (p = 0.000). Therefore, air void has a significant predictive effect on the performance of PU mixtures.
Figure 8 compares the performance data of mixtures with different air voids at 10 Hz, 15 °C, and 55 °C. The results of the ANCOVA analysis are presented in Table 7.

5. Conclusions

This study conducted dynamic modulus tests, conventional splitting tests, freeze–thaw splitting tests, MIST splitting tests, and Hamburg wheel-tracking tests on dense-graded polyurethane mixtures with three different air voids. The influence of different air voids, temperatures, and loading frequencies on the dynamic modulus, phase angle, and crack resistance factor of polyurethane mixtures was compared and discussed. The strength of the three air void mixtures under conventional, freeze–thaw, and dynamic water spray conditions was analyzed, and finally, the rutting resistance of the three air voids at high temperatures was compared. Based on the above discussions, the following conclusions can be drawn:
(1) Under normal conditions, the splitting strength of the three types of air voids of the mixture is not significantly different. Under freeze–thaw and MIST test conditions, polyurethane mixtures with an air void of 3% to 5% still maintain high strength, showing good resistance to water damage. Additionally, the larger the air void of the polyurethane mixture, the more significant the reduction in splitting strength under freeze–thaw and MIST conditions.
(2) Under water bath high-temperature loading conditions, polyurethane mixtures with an air void of 5% to 7% have the best resistance to rutting. Overall, the high-temperature performance of the three types of air voids is not significantly different, and the group with the worst high-temperature performance, 1% to 3%, is still much higher than the corresponding asphalt mixtures.
(3) The dynamic modulus of PU mixtures increases with the increase in loading frequency but decreases with the rise in test temperature; conversely, the phase angle exhibits the opposite trend. This behavior is similar to that of asphalt mixtures.
(4) The dynamic modulus of dense-graded polyurethane mixtures with various air voids is more sensitive at low loading frequencies. When the air void is around 3%, the mixture has the best resistance to deformation, and when the air void is less than 3%, the dynamic modulus of the mixture is more sensitive to temperature changes. The dynamic modulus of dense-graded polyurethane mixtures with various air voids is more sensitive at low loading frequencies.
(5) The phase angle of polyurethane mixtures is more sensitive at loading frequencies between 0 Hz and 10 Hz. When the loading frequency exceeds 10 Hz, the phase angle gradually decreases with the decrease in loading frequency. According to the phase angle data, polyurethane mixtures with an air void of 3% to 5% exhibit stronger viscous behavior at high temperatures and better elastic behavior at low temperatures.
(6) The fatigue cracking resistance of polyurethane mixtures with an air void of 5% to 7% is not significantly affected by the loading frequency, while polyurethane mixtures with an air void of 3% to 4% have the best fatigue cracking resistance at high temperatures.
Overall, these experimental results highlight the significance of air voids in determining the performance characteristics of polyurethane mixtures and provide a basis for laboratory testing to design polyurethane mixtures with optimal performance. By carefully selecting the air void range based on the specific requirements of a project, pavement designers and construction professionals can enhance the durability, deformation resistance, and overall performance of pavements. This not only improves the service life of the pavement but also reduces maintenance costs and environmental impact, making polyurethane mixtures a viable and advantageous option for modern pavement engineering. Additionally, these results lay a foundation for further research to reveal the intrinsic performance mechanisms of polyurethane mixtures under laboratory conditions.
The impact of air void on the road performance of dense-graded polyurethane mixtures, which involves research on the material’s resistance to deformation, water damage, high-temperature performance, and fatigue crack resistance, necessitates further study on the performance of mixtures with various air voids under low-temperature conditions. Issues such as the large coefficient of variation in the Hamburg test data and the lack of mechanistic explanations for the unexpected strength of mixtures with small air voids under freeze–thaw conditions remain to be addressed in future research.

Author Contributions

Conceptualization, Y.Z.; methodology, H.Z. and P.Z.; software, S.M.; validation, Y.Z., S.M. and P.Z.; formal analysis, Y.Z.; investigation, Y.Z. and W.W.; resources, W.W., S.M., B.L. and C.G.; data curation, Y.Z., P.Y., X.Z., W.W. and S.M.; writing—original draft preparation, Y.Z.; writing—review and editing, H.Z.; visualization, Y.Z., X.Z., P.Y. and B.L.; supervision, H.Z.; project administration, H.Z.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author, Haisheng Z, upon reasonable request. Data available on request due to restrictions.

Acknowledgments

We thank Fan Wei and Xiaomin Fan for their assistance with experiments and valuable discussion.

Conflicts of Interest

Author Bokai Liu was employed by the company Shandong Provincial Communications Planning and Design Institute Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Coatings 15 00228 g001
Figure 1. Gradation chart of the selected mixture.
Figure 1. Gradation chart of the selected mixture.
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Figure 2. Three different splitting test results: (a) conventional splitting; (b) freeze–thaw splitting; (c) dynamic water spray splitting.
Figure 2. Three different splitting test results: (a) conventional splitting; (b) freeze–thaw splitting; (c) dynamic water spray splitting.
Coatings 15 00228 g002
Coatings 15 00228 g003
Figure 3. Results of the Hamburg high-temperature wheel-tracking test for polyurethane mixture.
Figure 3. Results of the Hamburg high-temperature wheel-tracking test for polyurethane mixture.
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Coatings 15 00228 g004aCoatings 15 00228 g004b
Figure 4. Dynamic modulus at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
Figure 4. Dynamic modulus at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
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Coatings 15 00228 g005aCoatings 15 00228 g005b
Figure 5. Phase angle at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
Figure 5. Phase angle at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
Coatings 15 00228 g005aCoatings 15 00228 g005b
Coatings 15 00228 g006aCoatings 15 00228 g006b
Figure 6. Crack resistance factor at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
Figure 6. Crack resistance factor at various frequencies: (a) at 5 °C; (b) at 15 °C; (c) at 25 °C; (d) at 35 °C; (e) at 45 °C; (f) at 55 °C.
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Figure 7. Splitting strength under conventional, dynamic water spray, and freeze–thaw conditions.
Figure 7. Splitting strength under conventional, dynamic water spray, and freeze–thaw conditions.
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Figure 8. Significance analysis results: (a) dynamic modulus at 15 °C and 10 Hz frequency; (b) dynamic modulus at 55 °C and 10 Hz frequency; (c) phase angle at 15 °C and 10 Hz frequency; (d) phase angle at 55 °C and 10 Hz frequency; (e) crack resistance factor at 15 °C and 10 Hz frequency; (f) crack resistance factor at 15 °C and 10 Hz frequency.
Figure 8. Significance analysis results: (a) dynamic modulus at 15 °C and 10 Hz frequency; (b) dynamic modulus at 55 °C and 10 Hz frequency; (c) phase angle at 15 °C and 10 Hz frequency; (d) phase angle at 55 °C and 10 Hz frequency; (e) crack resistance factor at 15 °C and 10 Hz frequency; (f) crack resistance factor at 15 °C and 10 Hz frequency.
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Table 1. Technical specifications of polyurethane performance.
Table 1. Technical specifications of polyurethane performance.
IndexTraditional Cure SpeedSlow Cure Speed
Viscosity (25 °C) (MPa·s)17071691
Dry time (30 °C, 90%RH) (min)7083
Tensile strength (MPa)24.529.4
Breaking elongation (%)212516
Table 2. Volumetric indicators of conventional splitting test specimens.
Table 2. Volumetric indicators of conventional splitting test specimens.
Specimen Identification (Air Void)γfγtAir Void (%)
Range 1 (1%–3%)2.5592.6071.84
2.5442.6072.42
2.5562.6071.96
2.5482.6072.26
Range 2 (3%–5%)2.4972.6074.22
2.5252.6073.15
2.5142.6073.57
2.5172.6073.45
Range 3 (5%–7%)2.4532.6075.92
2.4642.6075.47
2.4482.6076.08
2.4452.6076.21
Table 3. Volumetric indicators of freeze–thaw splitting test specimens.
Table 3. Volumetric indicators of freeze–thaw splitting test specimens.
Specimen Identification (Air Void)γfγtAir Void (%)
Range 1 (1%–3%)2.5642.6071.65
2.5482.6072.27
2.5362.6072.72
2.5462.6072.34
Range 2 (3%–5%)2.492.6074.46
2.4792.6074.92
2.4852.6074.67
2.4972.6074.21
Range 3 (5%–7%)2.4582.6075.73
2.4352.6076.59
2.452.6076.01
2.4422.6076.3
Table 4. Volumetric indicators of MIST water sensitivity test specimens.
Table 4. Volumetric indicators of MIST water sensitivity test specimens.
Specimen Identification (Air Void)γfγtAir Void (%)
Range 1 (1%–3%)2.5582.6071.87
2.5572.6071.92
2.5372.6072.69
2.5452.6072.37
Range 2 (3%–5%)2.4782.6074.95
2.4872.6074.6
2.5142.6073.57
2.5222.6073.26
Range 3 (5%–7%)2.4322.6076.69
2.4432.6076.28
2.442.6076.42
2.472.6075.27
Table 5. Volumetric indicators of Hamburg high-temperature wheel-tracking test specimens.
Table 5. Volumetric indicators of Hamburg high-temperature wheel-tracking test specimens.
Specimen Identification (Air Void)γfγtAir Void (%)
Range 1 (1%–3%)2.5512.6072.14
2.5512.6072.14
2.5572.6071.92
2.5572.6071.92
Range 2 (3%–5%)2.5112.6073.68
2.5112.6073.68
2.52.6074.12
2.52.6074.12
Range 3 (5%–7%)2.4432.6076.29
2.4432.6076.29
2.462.6075.64
2.462.6075.64
Table 6. Volumetric indicators of dynamic modulus specimens.
Table 6. Volumetric indicators of dynamic modulus specimens.
Specimen Identification (Air Void)γfγtAir Void (%)
Range 1 (1%–3%)2.5312.6072.92
2.5422.6072.48
2.5482.6072.28
Range 2 (3%–5%)2.52.6074.09
2.4812.6074.82
2.5172.6073.47
Range 3 (5%–7%)2.4452.6076.21
2.4542.6075.86
2.4292.6076.82
Table 7. Significance of air void on the performance of polyurethane mixtures.
Table 7. Significance of air void on the performance of polyurethane mixtures.
VariablesEffect on Dynamic ModulusEffect on Phase Angle
air void (%)Yes (p = 0.000)Yes (p = 0.000)
VariablesEffect on Fatigue ParameterEffect on Splitting Strength
air void (%)Yes (p = 0.000)Yes (p = 0.000)
VariablesEffect on Rutting Depth
air void (%)Yes (p = 0.000)
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MDPI and ACS Style

Zhou, Y.; Ma, S.; Gan, C.; Wang, W.; Yu, P.; Zheng, X.; Zhang, P.; Liu, B.; Zhao, H. The Effect of Air Void on the Laboratory Properties of Polyurethane Mixtures. Coatings 2025, 15, 228. https://doi.org/10.3390/coatings15020228

AMA Style

Zhou Y, Ma S, Gan C, Wang W, Yu P, Zheng X, Zhang P, Liu B, Zhao H. The Effect of Air Void on the Laboratory Properties of Polyurethane Mixtures. Coatings. 2025; 15(2):228. https://doi.org/10.3390/coatings15020228

Chicago/Turabian Style

Zhou, Yunhao, Shijie Ma, Chenghua Gan, Wenjian Wang, Peihan Yu, Xiangzhuo Zheng, Peiyu Zhang, Bokai Liu, and Haisheng Zhao. 2025. "The Effect of Air Void on the Laboratory Properties of Polyurethane Mixtures" Coatings 15, no. 2: 228. https://doi.org/10.3390/coatings15020228

APA Style

Zhou, Y., Ma, S., Gan, C., Wang, W., Yu, P., Zheng, X., Zhang, P., Liu, B., & Zhao, H. (2025). The Effect of Air Void on the Laboratory Properties of Polyurethane Mixtures. Coatings, 15(2), 228. https://doi.org/10.3390/coatings15020228

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