Laboratory evaluation of high-friction thin overlays for pavement preservation

Abstract

Traditional asphalt concrete (AC) and stone matrix asphalt (SMA), which are used as thin asphalt overlays, are common maintenance strategies to enhancing ride quality, skid resistance, and durability. Recently, several studies have used a novel asphalt mixture known as a high-friction thin overlay (HFTO) to improve surface characteristics. However, it remains uncertain whether the laboratory properties of HFTO differ significantly from those of conventional mixtures. This study aims to evaluate the laboratory properties of HFTO mixtures and compare them with those of AC and SMA. Those mixtures with nominal maximum size of 9.5 mm were produced in the laboratory, and performance tests were conducted, including wheel tracking test, low temperature flexural creep test, moisture susceptibility test, Cantabro Abrasion Test, Marshall Test, sand patch test, British pendulum test, and indoor tire-rolling-down test. The results showed that the HFTO exhibited a lower tire/pavement noise than the AC and SMA. Additionally, HFTO had superior high-temperature stability, larger macro texture, and higher skid resistance in comparison to those of AC, but lower than those of SMA. Consequently, HFTO mixtures may be considered a suitable replacement for traditional AC mixtures in regions where skid resistance and noise reduction are concerns.

Graphical abstract

Keywords

road engineering / pavement maintenance / high friction thin overlay / performance / skid resistance / tire/pavement noise

1 Introduction

Road surfaces are susceptible to various defects, such as rutting, cracking, potholes, and raveling, owing to the combined effects of vehicle loading and environmental factors during service. These flaws result in a shorter lifespan and a decreased driving experience. To extend service life, restore pavement performance, and improve driving experience in the early years of service, it is necessary to implement preventive maintenance measures on a life cycle basis. Applying thin asphalt overlays is an effective and economical maintenance method for pavements without structural distress. Thin asphalt overlays can enhance pavement performance and restore surface functions such as moisture susceptibility, durability, roughness, and skid resistance. Therefore, thin-asphalt overlays have attracted widespread attention in several countries.

Thin asphalt overlays are more prone to shoving, raveling, and fatigue cracking than traditional wearing courses, owing to their reduced thickness. Efforts have been made to improve the performance of thin asphalt overlays. For example, one study found that using large grain sizes in thin asphalt overlays can improve the rutting resistance compared with those made with small particle sizes [1]. Another study showed that a novel ultrathin-wearing course based on the course aggregate void-filling method exhibited better high-temperature stability and skid resistance compared to the common open-graded friction course OGFC-7 and Novachip-B asphalt mixtures [2]. Incorporating polyolefins and styrene-butadiene-styrene-modified (SBS-modified) asphalt can also improve the high-temperature performance of the ultrathin friction course [3]. Moreover, waste polymers such as crumb rubber (CR) and low-density polyethylene in thin asphalt overlays have been proven to enhance their permanent deformation resistance [4]. Šernas et al. [5] reported that using high-quality dolomite in thin asphalt overlays could improve the adhesion of the asphalt-aggregate system and reduce water damage. Furthermore, utilizing a multichain polyolefin modifier can enhance the high-temperature stability, low-temperature crack resistance, and moisture stability of ultrathin wear courses [6]. Fine dense-graded mixtures of thin asphalt overlays demonstrate good resistance to surface raveling, abrasion, and reflective cracking [7]. The addition of steel fibers and steel slag to mixtures can prolong the lifespan of thin asphalt overlays [8].

In addition to pavement distress and durability, some researchers have explored the skid resistance and tire/pavement noise of wearing courses. Thin asphalt overlays with coarse aggregates exhibit better skid resistance than those with finer aggregates [9]. Furthermore, the use of an emery with a particle size of 2–3 mm and a polyurethane binder in an ultrathin-wearing course has been shown to provide good skid resistance and wear resistance [10]. Vaitkus et al. [11] found that using a cubic shape, smaller-sized aggregates, special gradation, and optimal binder content can reduce the noise of the wearing course. It has also been noted that aggregate gradation significantly impacts tire/pavement noise, whereas the type of binder and thickness of the overlays have little effect [12,13]. However, the use of CR/SBS modified binder with 15% CR (60 mesh) and 2% SBS modifier (linear) can reduce the intermediate- and high-frequency noises of the ultrathin wearing course [14]. In summary, the quality and gradation of aggregates and modified asphalt can improve the performance of thin asphalt overlays to a certain extent. Nonetheless, further investigations are necessary to comprehensively consider the balance between performance and surface characteristics in the design of mixtures for thin asphalt overlays.

The objective of this study is to evaluate the laboratory properties of a high-friction thin overlay (HFTO) and compare its advantages and disadvantages with those of traditional thin asphalt overlays. The aggregate structures of asphalt concrete (AC) and stone matrix asphalt (SMA) were selected as the reference gradations. Three polymer-modified binders satisfying the PG76 specification were utilized to produce nine mixtures by combining the three aforementioned gradations. To characterize the typical performance of thin asphalt overlay mixtures, conventional testing procedures, such as wheel-tracking tests, low-temperature flexural creep tests, and moisture susceptibility tests, were employed. Additionally, the potential risks of raveling and aging were evaluated through the Cantabro Abrasion Test, which involved applying various intervals of moisture conditioning, and the Marshall Test after a long-term aging simulation. To obtain the macrotexture and skid resistance properties of the thin asphalt overlays, sand patch, and British pendulum tests were performed, respectively. Moreover, the tire/pavement noise characteristics of thin asphalt overlays were examined using an indoor tire-rolling-down test.

2 Materials and test methods

2.1 Raw materials

2.1.1 Asphalt binder

In this study, three types of polymer-modified asphalt (PMA) were used: PMA1 modified with linear SBS, PMA2 modified with 60 mesh CR, and PMA3 modified with a combination of CR and SBS. All the three modified binders satisfied PG76, and their properties are listed in Tab.1.

The distribution of its modifier significantly influenced the performance of PMA. To assess the homogeneity of the PMAs, fluorescence microscopy images were obtained (Fig.1). It can be seen from Fig.1(a) that the matrix asphalt presents a uniform color phase, indicating its homogeneous. In contrast, Fig.1(b) shows the clustering and interface of the CR-modified binder. However, the SBS-modified binder shown in Fig.1(c) demonstrates a desirable homogenous distribution with a uniform dispersion of SBS modifiers in the continuous phase of the matrix asphalt. Moreover, the continuous network structures formed by CR and SBS are visibly distinct, as shown in Fig.1(d).

2.1.2 Aggregates

Fine and coarse aggregates were crushed from diorite rocks sourced from a local quarry. The basic properties of these aggregates are listed in Tab.2 and Tab.3.

2.1.3 Calcareous filler

The conventional calcareous filler used in this study was manufactured from limestone and its fundamental properties are listed in Tab.4.

2.1.4 Additive

An anti rutting additive at a dosage of 0.3% (by weight of the total asphalt mixtures) was used to enhance the high-temperature performance of the tested mixtures. Their properties are presented in Tab.5.

2.2 Mixtures design

2.2.1 Gradation

The compacted thickness of hot-mix asphalt typically exceeds 2.5–3 times the nominal maximum size of the aggregates used to account for the compaction effect. Because classical thin asphalt overlays have a depth range of 20–30 mm, the nominal maximum size of the aggregates should be less than 9.5 mm. In this study, three gradations with a nominal maximum size of 9.5 mm (namely 9.5 mm AC, 9.5 mm SMA, and 9.5 mm HFTO) were designed to generate thin asphalt overlays mixtures. AC and SMA are commonly employed as wearing courses in asphalt paving projects, whereas HFTO is a novel dense-gradation mixture featuring approximately 5% voids, which differs from regularly used open-graded Novachip mixtures containing approximately 13% voids. Additionally, AC comprised more than 40% of particles smaller than 2.36 mm, SMA contained 26% fine aggregates, and HFTO fell between the two. The SMA mixtures consisted of a higher filler usage than the other two mixes. The HFTO mixtures had a filler dosage almost equal to that of AC. The three gradations are shown in Fig.2.

2.2.2 Optimum asphalt content

Each mixture, with varying gradations and binders, was investigated using the Marshall Test method in accordance with ASTM D6926 [15], to determine the optimum asphalt content. The evaluation results are listed in Tab.6.

2.3 Test methods

2.3.1 High-temperature performance test

The high-temperature performance of thin asphalt overlay mixtures was investigated using a wheel-tracking test per JTG E20-T0719 [16]. The specimens were pre-conditioned at a temperature of 60 °C for 5 h in a thermostatic chamber before undergoing testing. During the test, a wheel with a pressure of 0.7 MPa was applied to the slab surface, and the wheel velocity and track distance were 42 cycles per minute and 230 mm, respectively. The test lasted 60 min. To assess the high-temperature stability of thin asphalt overlays, the dynamic stability (DS) was derived using Eq. (1).

(1)\mathrm{D}\mathrm{S}=\frac{15\times 42}{d_{60}-d_{45}},

where d₄₅ represents the deformation of the asphalt slab surface after 45 min (mm); d₆₀ is the deformation of the asphalt slab surface after 60 min (mm).

2.3.2 Low-temperature performance test

The cracking resistance of thin asphalt overlays was evaluated via flexural creep testing in a low-temperature environment [17]. Prior to the testing, the specimens were preconditioned at a temperature of −10 °C for 5 h in a thermostatic chamber. A Universal Tester was used to perform tests at a predetermined loading rate of 50 mm/min. The load and deflection values at the mid-span position were recorded to calculate the ultimate flexure strain ({\epsilon }_{\mathrm{B}}) and flexure stiffness modulus (S_B), which are determined using Eqs. (2)–(4).

(2)R_{\mathrm{B}}=\frac{3\times L\times P_{\mathrm{B}}}{2\times b\times h^2},

(3){\epsilon }_{\mathrm{B}}=\frac{6\times h\times d}{L^2},

(4)S_{\mathrm{B}}=\frac{R_{\mathrm{B}}}{{\epsilon }_{\mathrm{B}}},

where R_B is the failure strength (MPa); {\epsilon }_{\mathrm{B}} is the maximum flexure strain as the beam damaged (μϵ); B and h are the width and height of the cross section, respectively (mm); L is the span of the beam (mm); d is the mid-span deflection as the beam damaged (mm); P_B is the failing load (N); S_B is the flexure stiffness modulus (MPa).

2.3.3 Moisture susceptibility test

1) Marshall immersion test

The moisture susceptibility of the thin asphalt overlay mixtures was assessed using the Marshall Immersion Test according to ASTM D6927 [18]. Prior to the testing, a subset of half specimens was submerged in a water bath at 60 °C for 30–40 min, while another subset was kept in the water bath for a period of 48 h. The ratio of the stability values obtained from the two subsets of specimens is referred to as the Marshall residual stability (MS₀), which was determined using Eq. (5).

(5){MS}_0=\frac{{MS}_2}{{MS}_1},

where MS₁ is the stability value of the subset of specimens immersed in a water bath for 30 min (kN), and MS₂ is the stability value of the subset of specimens immersed in a water bath for 48 h (kN).

2) Freeze–thaw splitting test

A freeze–thaw splitting test was also conducted to characterize the moisture susceptibility of the thin asphalt overlay mixtures, in accordance with ASTM D4867 [19]. The specimens were then randomly divided into two subsets. One subset was immersed in a water bath at a temperature of 25 °C for 2 h prior to the indirect tensile test. Another subset was saturated and moisture-conditioned in a water bath at room temperature for 15 min in a vacuum chamber. Subsequently, normal pressure was applied for an additional 30 min. The specimens were then tightly enclosed in a plastic bag containing roughly 10 mL of water and placed in an air bath freezer for 16 h at low temperature of −18 °C. After removal from the freezer, the specimens were immersed in a water bath at a temperature of 60 °C for 24 h, without the plastic bags. Finally, this subset of specimens was conditioned in a water bath at normal temperature of 25 °C for at least 2 h before performing the indirect tensile test. The tensile strength ratio (TSR) is calculated using the following equation.

(6)\mathrm{T}\mathrm{S}\mathrm{R}=\frac{R_{\mathrm{T}2}}{R_{\mathrm{T}1}}\times 100,

where R_T1 is the average tensile strength of the control group (MPa), and R_T2 is the average tensile strength of the freeze–thaw treated group (MPa).

2.3.4 Raveling resistance test

The Cantabro Abrasion Test was used to estimate the raveling resistance of the thin asphalt overlay mixtures in accordance with AASHTO TP108 [20]. All specimens were placed in a water bath at a temperature of 60 °C for varying durations of 0, 1, 2, 3, and 5 d. Three specimens were removed at each interval and dried at room temperature for 24 h to determine their average original weights. Subsequently, each specimen was placed in a Los Angeles Abrasion Testing Machine and rotated at a speed 30–33 r/min for a total of 300 revolutions. The residual mass of the specimens in each time group was recorded, and the average coefficient of mass change for each group was calculated to show the raveling resistance characteristics according to Eq. (7).

(7)\mathrm{R}\mathrm{R}\mathrm{C}=\frac{m_0}{m_1}\times 100,

where RRC is the average raveling resistance coefficient of the mass change for each interval (%), m₀ is the dry weight of the three specimens prior to the Cantabro Abrasion Test (g), and m₁ represents the residual weight of the three specimens after the Cantabro Abrasion Test (g).

A higher RRC value indicates a better resistance to raveling.

2.3.5 Aging resistance test

To assess the anti-aging performance of thin asphalt overlays, a Marshall Test was conducted on specimens that underwent long-term aging in accordance with AASHTO PP2 [21]. Cylindrical specimens were placed in an oven at a temperature of 85 °C for 5 d to simulate long-term aging during service. The aging resistance of the thin asphalt overlay mixtures was determined using Eq. (8).

(8)\mathrm{A}\mathrm{R}\mathrm{I}=\frac{{MS}_3}{{MS}_1}\times 100,

where ARI is the aging resistance index (%), MS₁ is the Marshall stability value of the subset without long-term aging (kN), and MS₃ is the Marshall stability value of the subset with long-term aging (kN).

2.3.6 Macrotexture and skid resistance test

The skid resistance of thin asphalt overlays is an important safety parameter. In this study, traditional sand patch and British pendulum tests were performed on slabs. The mean texture depth (MTD) and British pendulum number (BPN) were obtained to quantify the macrotexture and skid resistance characteristics of the thin asphalt overlays, respectively, as shown in Fig.3.

2.3.7 Tire/pavement noise test

Traditional tire/pavement noise is generally measured in the field using methods such as statistical pass-by, close-proximity, controlled pass-by, and coast-by methods [22]. Currently, there is no internationally recognized test method for evaluating tire/pavement noise during the design stage of asphalt mixtures. Chang’an University developed an indoor tire rolling-down test to simulate tire/pavement noise caused by tire vibrations and aerodynamic effects [23–25]. This test was conducted to measure the tire/pavement noise between a car tire and a thin asphalt overlay. A schematic of the indoor tire rolling-down test system is shown in Fig.4, and the entire test process is shown in Fig.5. The A-weighted sound pressure level was calculated to describe the noise characteristics of the various thin asphalt overlays.

2.3.8 Specimens’ preparation

In this study, an asphalt roller compactor was used to produce asphalt slabs with a total thickness of 6 cm for a wheel-tracking test. The lower layer, consisting of 4 cm AC-13 asphalt mixtures, was compacted prior to the application of the 2 cm thin asphalt overlays, which were overlaid to simulate real conditions. Information regarding the asphalt slabs is presented in Tab.7.

The preparation of the specimens for the other tests was carried out as follows.

1) The slabs were prepared using the same method as that used for the wheel-tracking test and then sawed into small beam specimens for the low-temperature flexure creep test.

2) Cylindrical specimens for the Marshall Immersion test were compacted using a Marshall compactor, with 75 blows on both sides for the AC and HFTO mixtures and 50 blows on both sides for the SMA mixtures, as per the procedure described in the Chinese Specification T0729 [26].

3) The cylindrical specimens for the freeze–thaw splitting test were also compacted using the Marshall compactor, with 50 blows on both sides for all mixtures, following the same method as the Marshall Immersion Test.

4) The aging resistance test was conducted by initially placing loose mixed asphalt mixtures of thin asphalt overlays in an oven at 135 °C for 4 h, to simulate short-term aging during construction. Subsequently, the specimens were manufactured using the same method used for the Marshall immersion tests.

5) The slabs for the macrotexture and skid-resistance tests were prepared using the same method as that for the wheel-tracking test. The specimen surfaces of the three mixtures are shown in Fig.6.

6) The mixtures for the tire/pavement noise test were prepared in accordance with the designed proportions and optimum asphalt contents, as outlined in Tab.6. Subsequently, the slabs were compacted in the laboratory using a mini-road roller.

3 Result and discussion

In this study, the results were statistically analyzed at a significance level of 5% (corresponding to 0.05 probability of a Type I error) using a two-way independent analysis of variance with respect to the effects of gradation or binder type.

3.1 Dynamic stability

The DS values of the thin asphalt overlay mixtures, which varied with gradation and binder, are shown in Fig.7. All DS values were above 2400 passes/mm, which is the threshold for modified asphalt mixtures used in the hot regions of China. The HFTO mixtures exhibited significantly greater DS values than the AC mixtures but were slightly lower than those of the SMA mixtures. In addition, the binders significantly affected the DS values of the thin asphalt overlay mixtures, as shown in Fig.7. The mixtures prepared with PMA2 exhibited higher DS values than those of the other two binders, indicating better high-temperature stability when applied to hot regions, which is consistent with the results reported by Geng et al. [6]. For example, the HFTO mixtures produced using PMA2 were approximately 44% and 10% higher than those produced using PMA1 and PMA2, respectively. It was established that HFTO mixtures demonstrated excellent high-temperature stability compared to traditional AC mixtures.

3.2 Low-temperature cracking resistance

3.2.1 Flexure strain

The results shown in Fig.8 indicate that the flexural strain values changed with the mixture type. As seen from Fig.8, the flexure strain values of all mixtures were greater than 2500 × 10⁻⁶, satisfying with the Chinese engineering threshold for modified asphalt mixtures. The HFTO mixtures had lower flexural strain values than the other mixtures in terms of gradation, indicating poor resistance to low-temperature cracking. The HFTO mixtures demonstrated a reduction of approximately 11% compared to the AC mixtures, regardless of the type of binder used. Additionally, the HFTO mixtures prepared with the CR-modified binder exhibited the lowest flexural strain values among all mixtures. However, utilizing the SBS-or CR/SBS-modified binder can improve the resistance of HFTO to cracking. This may be related to the crosslinking structure of SBS, which restrains crack propagation [27]. Thus, it can be concluded that the HFTO mixtures displayed a slightly lower cracking resistance than the other mixtures made with either AC or SMA.

3.2.2 Flexure stiffness modulus

As shown in Fig.9, with respect to the flexural stiffness modulus of the various mixtures, the HFTO mixtures had slightly higher values, exhibiting a poorer stress relaxation capacity in a low-temperature environment compared to the AC and SMA mixtures. However, the incorporation of SBS as a modifier in the binder significantly decreased the flexural stiffness modulus values of the HFTO mixtures, resulting in reductions of 13% and 8% compared to those produced with the CR and CR/SBS binders, respectively. The mixtures made with the CR/SBS-modified binder also improved the stress relaxation capacity by decreasing the flexural stiffness modulus compared to the CR-modified binder. This may be due to the crosslinked structure of SBS in the binder, which provides mixtures with better deformability at low temperatures. Therefore, it can be concluded that the stress relaxation capability of HFTO at low temperatures is also inferior to that of AC or SMA.

3.3 Moisture susceptibility

3.3.1 Residual Marshall stability ratio

The results shown in Fig.10 indicate that there was minimal variation in the residual Marshall stability ratio among the various thin asphalt overlays. It was observed that the MS₀ values for all the mixtures were greater than 85%, which meets the criterion of China for modified asphalt mixtures. However, the HFTO mixtures exhibited a slight reduction in MS₀ values (approximately 1%) compared to the AC mixtures, suggesting a slight decrease in moisture stability. By contrast, the MS₀ values of the SMA mixtures were approximately 3% higher than those of the HFTO mixtures. Moreover, the HFTO mixtures prepared with the CR-modified binder exhibited the lowest MS₀ values compared with the other mixtures, a finding consistent with that of Walaa et al. [28]. However, the use of SBS-modified binders in HFTO mixtures has the potential to partially close the gap in conventional AC or SMA mixtures. In conclusion, the HFTO mixtures displayed slightly inferior moisture stability compared to conventional thin overlay mixtures in terms of the binder.

3.3.2 Tensile strength ratio

As shown in Fig.11, all mixtures had TSR values above 80%, which is in line with the Chinese standard for modified asphalt mixtures in practical applications. However, the TSRs of the HFTO mixtures were slightly lower than those of the AC mixtures. Conversely, the SMA mixtures displayed the highest TSR values, indicating excellent resistance to the combined effects of moisture and temperature. This difference in the TSR values may be attributed to variations in the voids and skeleton structure of the mixtures. The SMA mixtures have fewer voids, whereas the HFTO mixtures have more voids. Additionally, the HFTO mixtures prepared with the SBS-modified binder presented an improved TSR of 5% compared with those prepared with the CR-modified binder, likely owing to the interplay of the asphalt film thickness, asphalt viscosity, and mixture porosity. A high asphalt viscosity generally leads to better cohesion and a thicker film, but may also result in insufficient compaction and larger voids in the mixtures. In summary, the moisture stability of the HFTO mixtures was found to be comparable to that of the AC mixtures.

3.4 Raveling resistance coefficient

As shown in Fig.12, the raveling resistance coefficients of the mixtures composed of different gradations and binders declined with prolonged soaking times. This decrease was more pronounced during the initial two days and then plateaued as the soaking time increased. Additionally, the HFTO mixtures consistently had the lowest coefficients of raveling resistance, regardless of the soaking time. Conversely, the SMA mixtures exhibited higher coefficients of raveling resistance than those of the other two gradation mixtures when the same binder was used. This difference can be attributed to the slightly larger voids in the HFTO mixture. Moreover, the mixtures created with the CR-modified binder showed lower coefficients of raveling resistance than those of the other two modified binders at each corresponding soaking interval. Interestingly, Fig.12 indicates that the HFTO mixtures prepared with the SBS-modified binder exhibited enhanced resistance to water attack, with higher coefficients of resistance. Thus, it could be deduced that the HFTO mixtures had better raveling resistance characteristics than traditional AC mixtures.

3.5 Aging resistance index

Fig.13 illustrates the effect of long-term aging on the Marshall stability of various thin asphalt overlays. The results indicated that all the aging resistance index values for the mixtures were above 80%. No significant variation was observed between the AC and SMA. However, HFTO had slightly lower values, approximately 2% less than those of AC and SMA, regardless of the binder used. In addition, the mixtures prepared with the CR-modified binder slightly outperformed those prepared with the other binders. The HFTO mixtures produced with the SBS-modified binder exhibited the lowest aging resistance index among all the samples. Therefore, it can be concluded that the long-term aging resistance of HFTO is slightly weaker than those of the AC and SMA mixtures.

3.6 Mean texture depth and British pendulum number

3.6.1 Mean texture depth

Fig.14 presents the variations in MTD among various thin asphalt overlay mixtures. All of the mixtures displayed MTD values higher than 0.55 mm, which is Chinese standard for regions with an average annual rainfall greater than 1000 mm. The results showed that the HFTO mixtures exhibited significantly higher MTD values, approximately 21% greater than those of traditional AC mixtures. There were minimal differences in the MTD among the HFTO mixtures with different binders. However, the mixtures made with the SBS-modified binder displayed a slightly higher MTD than those produced with the CR or CR/SBS-modified binder. These findings suggest that the HFTO mixtures have excellent skid resistance at high speeds when used as thin asphalt overlays. Consequently, it can be concluded that the use of HFTO mixtures can enhance the safety of road surfaces under rainy conditions, particularly in regions with high annual rainfall.

To evaluate the effect of the mixtures on the MTD, a statistical analysis was performed, and the results are presented in Tab.8. The results indicate significant differences in the MTD among different mixtures, suggesting that the type of mixture has a notable effect on the MTD. Consequently, the HFTO mixtures exhibited a significant difference in MTD when compared with the other two conventional thin asphalt overlay mixtures, demonstrating their potential as suitable alternatives in areas with high rainfall.

3.6.2 British pendulum number

The differences in the BPN values among the different mixtures are depicted in Fig.15. The findings indicated that all BPN values exceeded the minimum requirement of 45 for safe driving in China. The HFTO mixtures achieved higher BPN values than the traditional AC mixtures but displayed slightly lower BPN values than the SMA mixtures. Minor variations were observed among mixtures with the same gradation. The BPN values for the mixtures produced with CR or CR/SBS were slightly higher than those made with SMA, which could be due to the increased roughness of the asphalt film caused by the presence of CR particles in the binder. Therefore, the HFTO mixtures exhibited an improved skid resistance when utilized as thin overlays. Consequently, it can be concluded that the use of HFTO mixtures can enhance the safety of road surfaces, particularly in areas where high skid resistance is required.

A statistical analysis was conducted to assess the impact of the mixture on the BPN, as presented in Tab.9. These findings indicate that there were significant differences in the BPN values between the different mixtures. Therefore, it can be concluded that the HFTO mixtures demonstrated a significant distinction in BPN when compared with the other two conventional thin asphalt overlay mixtures.

3.7 Sound pressure level

Fig.16 illustrates the sound pressure level variations of the thin overlay mixtures depending on their gradation and binder. The results demonstrated that the HFTO mixtures had lower sound pressure levels owing to their slightly larger voids and surface textures. Conversely, the SMA mixtures exhibited higher sound pressure levels than the AC mixtures, resulting in incremental tire/pavement noise. The mixtures incorporating SBS exhibited higher sound pressure levels than those containing CR or CR/SBS. The mixtures manufactured with CR had the lowest sound pressure levels owing to the superior damping characteristics of the CR-modified binder, which effectively converted the vibration energy into internal energy compared to the SBS-modified binder [29]. Therefore, it can be concluded that the HFTO mixtures exhibited excellent tire/pavement noise reduction properties in comparison with the traditional AC and SMA mixtures, indicating their viability as an alternative in areas where noise is a concern.

Statistical analysis was performed to evaluate the effects of gradation and binder on tire/pavement noise, as shown in Tab.10 and Tab.11. The findings revealed significant differences in tire/pavement noise among the various gradations. Tab.10 highlights the significant variation in the tire/pavement noise between the CR-modified binder and the SBS-modified binder or CR/SBS-modified binder at the 5% significance level. Therefore, the HFTO mixtures exhibited a considerable distinction in tire/pavement noise compared to the other two conventional thin asphalt overlay mixtures.

4 Conclusions

This study reported the outcomes of a laboratory investigation that examined the characteristics of a novel HFTO and compared them with the benefits and drawbacks of conventional asphalt overlays. The results of this study can be used as a guide for designing thin asphalt overlay mixtures in the paving industry. The following conclusions were drawn.

1) The DS values of the HFTO mixtures were lower than those of the SMA mixtures, but greater than those of the AC mixtures. However, the HFTO mixtures had slightly lower resistance to low-temperature cracking and moisture stability than the control mixtures. The SBS-modified binder was found to be effective in enhancing the low-temperature performance of HFTO mixtures.

2) The raveling resistance of the HFTO mixtures decreased as the soaking time increased and was inferior to those of the AC and SMA mixtures. However, the incorporation of the SBS-modified binder in the HFTO mixtures helped reduce the gap in the raveling resistance. No statistically significant variations in the aging resistance capacity were observed among the different mixtures evaluated.

3) The HFTO mixtures exhibited higher MTD values than the AC mixtures primarily because of their coarser gradation and slightly larger voids. The type of binder had little impact on the MTD values. Furthermore, the BPN values of the HFTO mixtures were lower than those of the SMA mixtures but higher than those of the AC mixtures.

4) The HFTO mixtures demonstrated a decline in the sound pressure level of the tire/pavement noise compared with the AC and SMA mixtures. Moreover, the use of the CR-modified binder further diminished tire/pavement noise by effectively absorbing vibrations. Consequently, HFTO mixtures displayed remarkable superiority over conventional AC mixtures in terms of high-temperature stability, surface texture, skid resistance, and noise reduction.

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