Design of High-Modulus Asphalt Concrete for the Middle Layer of Asphalt Pavement

Abstract

This article investigates the application of high-modulus asphalt mixtures (HMM-13) in the intermediate layer of pavement, addressing rutting issues in asphalt pavements subjected to heavy traffic and high temperatures. The study utilized a 1% dosage of high-modulus modifier, and initially, the mix design of HMM-13 was determined using the gyratory compaction method. Subsequently, this study evaluated the road performance of HMM-13 through tests, including the −10 °C beam bending test, rutting tests at 60 and 70 °C, the freeze–thaw splitting test, and the single-axis compression dynamic modulus test. To ensure the effectiveness of the mixture’s on-site application, this study validated the raw material specifications at the construction site and adjusted the mix design accordingly. Water stability tests were also conducted. Finally, a survey of the mixing plant at the construction site was carried out, establishing the relationship between each bin’s flow rate and speed ratio. The suitable speed for the production of HMM-13 was calculated. The research results indicate that the optimal asphalt-to-aggregate ratio for HMM-13 is 4.2% (with a comprehensive asphalt-to-aggregate ratio of 5.2%), and the freeze–thaw splitting strength ratio can reach 84.2%. The dynamic stability is 11,217 cycles/mm at 60 °C and 6167 cycles/mm at 70 °C. The stiffness modulus at −10 °C is 5438 MPa, with a failure strain of 2049 με. At 10 Hz and 15 °C, the dynamic modulus is 15,488 MPa, and at 45 °C, it is 3872 MPa. All these indicators meet the requirements for construction technology and pavement performance.

1. Introduction

High-modulus asphalt mixtures are commonly used in road surfaces designed for heavy traffic and in high-temperature regions. Primarily applied in the intermediate layer of road surfaces, they possess superior mechanical properties, resistance to rutting, and fatigue resistance, [1,2]. Currently, common high-modulus asphalt mixtures are achieved through the following three methods:

(1) The use of hard asphalt or natural asphalt [3,4,5,6]. Initially, in the 1960s, France applied hard asphalt, with a penetration grade of 10–25, to road construction. Over the subsequent two decades, high-modulus asphalt mixtures with characteristics of a higher asphalt-to-aggregate ratio and a lower void ratio demonstrated excellent resistance to rutting, leading many countries to embark on research regarding high-modulus asphalt mixtures [7,8]. During this process, France explored adjustments to the asphalt-to-aggregate ratio and gradation to improve the low-temperature and fatigue performance issues caused by the insufficient flexibility of hard asphalt. In 2005 and 2007, the United Kingdom and France, respectively, issued design specifications for high-modulus asphalt concrete [9,10,11,12]. Portuguese scholar J. Tramp. Engrg et al. [13] proposed parameters for permanent deformation of high-modulus asphalt mixtures based on rutting performance, serving as a foundation for predicting rutting.

(2) The addition of reclaimed asphalt pavement (RAP) [14,15,16,17,18]. In 2007, the French LCPC institute first utilized RAP to produce high-modulus asphalt concrete, achieving favorable results with an RAP content exceeding 50%. Over the next five years, scholars in many European countries explored the road performance of high-modulus asphalt mixtures with an RAP content ranging from 15% to 50%. The results showed that mechanical properties, fatigue resistance, and the high-temperature stability of the mixtures improved with an increase in RAP content [14,19,20,21]. However, due to the variability of RAP materials, the low-temperature performance and water stability of the mixtures may decline. Jian Zhou’s research has validated the feasibility of achieving high-modulus asphalt mixture design through the substantial addition of reclaimed asphalt pavement (RAP) [14]. He believes that the penetration grade of the binder in RAP is similar to that of high-modulus asphalt mixtures. By optimizing the gradation and employing hard asphalt, he proposed a design method for high-modulus asphalt mixtures with a 100% RAP content.

(3) The addition of high-modulus modifiers to improve asphalt mixture modulus and reduce deformation under vehicle loads. France pioneered the specification for the use of high-modulus modifiers in 1992 [22]. Commonly used high-modulus modifiers in the market are primarily composed of PE polymers or other high molecular weight polymers [23,24,25]. Sometimes, for the purpose of replacing cementitious materials or improving the mechanical properties of asphalt, high-energy modifiers are also used in combination with fibers, such as in the case of the composite modification of basalt fibers and high-modulus modifiers. Most modifiers effectively enhance the modulus, high-temperature stability, water stability, and fatigue resistance of the mixture without compromising low-temperature performance [26,27,28,29,30]. Simultaneously, research has also explored the feasibility of combining high-modulus modifiers with other modifiers. Yan et al. [12]. conducted a comparative study on four types of hard asphalt binders, high-modulus modifiers, and polyester fibers to investigate the differences each made in improving the performance of high-modulus asphalt concrete. The conclusion drawn was that PRM (polymer-modified asphalt) and SBS (styrene-butadiene-styrene) composite modified asphalt mixtures exhibited superior resistance to rutting, as well as improved overall road performance. In order to address the issue of poor low-temperature crack resistance in asphalt mixtures caused by high-modulus modifiers, related studies have taken measures such as adding fibers (such as lignocellulosic fibers, basalt fibers, and polyester fibers), rubber powder, SBR (styrene-butadiene rubber), or diatomaceous earth to improve the low-temperature crack resistance of high-modulus asphalt mixtures.

Currently, many scholars have confirmed the significant socio-economic benefits of high-modulus asphalt concrete [31]. This paper presents the design of the intermediate layer of HMM-13 on the expressway in Jiangsu Province, China. By applying high-modulus modifiers to asphalt concrete, the preparation of high-modulus asphalt mixtures was achieved, and major road performance tests were conducted for validation. The gradation design of HMM-13 was determined based on raw material specifications, and the optimal asphalt-to-aggregate ratio (comprehensive asphalt-to-aggregate ratio) was determined through the gyratory compaction method. Subsequently, the low-temperature performance, high-temperature performance, water stability, and dynamic modulus of HMM-13 were evaluated. To ensure smooth on-site application, the relationship between the rotational speed ratio and the discharge flow rate of each material bin in the mixing plant was established, and adjustments were made to the cold material flow rate. Finally, the raw material specifications at the construction site were verified, and the gradation design was adjusted. This provides a reference for the widespread application of high-modulus asphalt concrete in the intermediate layer of road surfaces.

2. Materials and Methods

2.1. Asphalt Binder

The asphalt used in this article was SBS-modified asphalt sourced from LiaoNing Province, China. The basic properties of the asphalt, including relative density and penetration at 25 °C, ductility at 5 °C, and softening point, were tested. The test results are presented in

Table 1. Summary of asphalt test results.

Test Items	Unit	Specified Value	Actual Measurement Results
Relative Density (25 °C)	-	Actual Measurement	1.034
Needle Penetration (25 °C)	(0.1 mm)	40–70	56
Ductility (5 °C)	(cm)	≥25	31
Softening Point	(°C)	≥70	92.0

.

2.2. Aggregates

2.2.1. The Aggregate Used for the Indoor Mix Design

In order to ensure the effectiveness of high-modulus asphalt concrete, to explore its performance characteristics, and to provide guidance for practical engineering applications, this article initially conducted an indoor mix design. The so-called indoor mix design refers to the asphalt mixture mix proportion determined through scientific calculation and experimentation based on design requirements and construction conditions. It is the fundamental method of mixture composition design. As it is carried out in the laboratory, it is called an indoor mix design. The aggregate used for the indoor mix design was sourced from Heilongjiang Province, China. For convenience in packaging and transportation, the manufacturer defined the specifications of the aggregate based on the maximum nominal particle size when the aggregate left the factory. For the HMM-13 used in this article, coarse aggregate with particle size specifications ranging from 9.5–13.2 mm, 4.75–9.5 mm, 2.36–4.75 mm, and fine aggregate with a particle size range of 0–2.36 mm were selected.

The density tests were conducted on the aggregates, which include testing the apparent relative density, the gross volume relative density, and the water absorption rate. The apparent relative density and gross volume relative density of the coarse aggregate were determined using the basket method. Due to the smaller particle size of the fine aggregate, the basket method was not applicable, so the slump cone method was employed to determine the apparent relative density and gross volume relative density of the fine aggregate; at the same time, the water absorption rate was also tested. The test results are presented in Figure 1. Due to the importance of the apparent relative density in the mix design and the regulations regarding the related engineering design requirements, the minimum limit value of the apparent relative density is marked in the figure.

The basket method and the slump cone method are based on the Test Methods of Aggregates for Highway Engineering (JTG E42–2005) and the T 0304–2005 and T 0330–2005 methods [32]. The basket method is suitable for aggregates with a particle size above 2.36 mm, while the slump cone method is applicable for determining aggregates with a particle size below 2.36 mm.

The basket method involves measuring the water mass (m_w), apparent dry mass (m_f), and apparent dry mass in air (m_a) of clean aggregate. Using the following formulas, the apparent relative density (γ_a) and gross volume relative density (γ_b) are then calculated.

$${\gamma }_a=\frac{m_a}{m_a-m_w}$$

(1)

$${\gamma }_b=\frac{m_a}{m_f-m_w}$$

(2)

The water absorption rate (w_x) is calculated according to the following formula:

$$w_x=\frac{m_f-m_a}{m_a}\times 100\%$$

(3)

The slump cone method involves measuring the oven-dry mass (m₀) of clean aggregate, the total mass (m₁) of water and the container, the mass (m₂) of the saturated surface-dry specimen, and the mass (m₃) of the saturated surface-dry specimen. Using the following formulas, the apparent relative density (γ_a) and the gross volume relative density (γ_b) are then calculated.

$${\gamma }_a=\frac{m_0}{m_0+m_1-m_2}$$

(4)

$${\gamma }_b=\frac{m_3}{m_3+m_1-m_2}$$

(5)

The water absorption rate (w_x) is calculated according to the following formula:

$$w_x=\frac{m_3-m_0}{m_0}\times 100\%$$

(6)

It can be observed that the density of the aggregate specimens meets the specifications and engineering requirements. Specimens with larger particle sizes have higher densities, and the density difference among the aggregates regarding different specifications is not significant. In contrast, the water absorption rate shows an opposite trend, suggesting that this may be due to a slightly higher distribution of internal pores in fine aggregates compared to coarse aggregates. Specifically, for the 0–2.36 mm aggregate specimens, the smaller particle size results in more surface contact with water, leading to a larger increase in water absorption rate.

2.2.2. The Aggregates Used for On-Site Mix Design

Usually, considering the changes in the source of aggregates at the construction site and the different proportions of each material bin in the mixing plant compared to those of the indoor mix design, it is necessary to screen the mineral materials in each hot material bin, design the gradation based on the screening results, and the indoor mix design is adjusted, resulting in the on-site mix design. In this case, due to construction scheduling, the limestone used for the on-site mix design differs from the aggregates used for the indoor mix design. The specifications include: 9.5–13.2 mm, 4.75–9.5 mm, 2.36–4.75 mm, and 0–2.36 mm. To ensure the effectiveness of high-modulus asphalt concrete, density tests were conducted on the aggregates taken from the hot material bin of the mixing plant, and the density tests of the aggregates include evaluating the apparent relative density, the gross volume relative density, and the water absorption rate. The results are shown in Figure 2.

The experiments revealed that the apparent relative density and gross volume relative density of the aggregate specimens used at the construction site were slightly smaller than those of the indoor test specimens, but the decrease was not significant. This difference can be attributed to variations in the origin of the aggregates, highlighting the necessity of validating aggregate specimen characteristics at the construction site. Although the water absorption rate of the aggregates used in the on-site tests showed a more noticeable decrease compared to that of those in the indoor tests, it generally complies with the requirements for the production and use of high-modulus asphalt mixtures. Therefore, the aggregates at the construction site are suitable for use.

2.3. Mineral Powder

The mineral powder used for the indoor mix design is sourced from Heilongjiang Province, China, and it is limestone with a particle size of <0.6 mm. Its apparent relative density is 2.691 g/cm³. The mineral powder used for the on-site mix design is sourced from Anhui, China, with an apparent relative density of 2.700 g/cm³.

2.4. Composite High-Modulus Modifier

The composite high-modulus modifier used in this article is sourced from Jiangsu Province, China. Its main function is to improve the high-temperature performance of asphalt mixtures. However, relevant literature and past application experiences indicate that the addition of high-modulus modifiers may lead to a slight decrease in the low-temperature performance of asphalt mixtures [3,4,5,9,10]. Considering various factors, the dosage of the high-modulus modifier was determined to be 1% of the mass of the asphalt mixture. This high-modulus modifier contains saturated fraction, aromatic fraction, asphaltene, and resin. Its characteristics are shown in

Table 2. Characteristics of high-modulus modifiers.

Characteristic	Technical Requirement
Appearance	Uniform Black Particles
Particle Size	≤8.0 mm
Density	1.0–1.2 g/cm3
Softening Point	110–140 °C
Melting Point	110–150 °C
Melt Mass-Flow Rate (140 °C, 2.16 kg)	≥50 g/10 min
Ash Content	≤5%

.

2.5. Experimental Methods

2.5.1. The Rotational Compaction Method (SGC)

Following the Superpave asphalt mixture design philosophy, the gyratory compaction method can effectively simulate the compaction state of the pavement under the pressure of tires. Using a gyratory compactor machine (SGC), cylindrical specimens of the high-modulus asphalt mixture are formed, and the asphalt content is selected based on volumetric design requirements.

Table 3. Requirements for rotary compaction equipment.

Test Parameters	Parameter Values
Rotating Compaction Angle	Internal Angle (1.16° ± 0.02°)
Number of Rotations	80 times
Rotation Rate	30 ± 0.5 rev/min
Vertical Pressure	0.6 ± 0.018 MPa
Test Piece Diameter	150 ± 0.1 mm

shows the parameters for the equipment requirements of the gyratory compaction method.

The mixing temperature is 160 °C, and the compaction temperature is approximately 150 °C. During compaction, the mass “m” of a specimen should be chosen to achieve a specimen height of 170 mm. Prior to compaction, it is necessary to preheat the mold and the compaction pedestal. Circular paper sheets should be placed on the compaction pedestal and on the top of the asphalt mixture to prevent the asphalt mixture from adhering to the mold and the compaction pedestal. Subsequently, the mold containing the high-modulus asphalt mixture is placed into the gyratory compactor, and after confirming the parameters, compaction is carried out. The freshly compacted hot specimen should be demolded after cooling to room temperature.

2.5.2. Freeze–Thaw Splitting Test

This experiment evaluates the water stability of asphalt mixtures by subjecting them to specified freeze–thaw cycles and measuring the strength ratio of the specimens before and after being subjected to water damage-induced cracking. It is important to note that the specimens for this test are standard Marshall specimens that have been compacted 50 times on both sides. These specimens are cylindrical, with a diameter of 101.6 ± 0.25 mm and a height of 63.5 ± 1.3 mm.

The test steps employed follow the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20–2011) [33]. Before the experiment, the specimens are divided into two groups, and the height of each specimen is measured. The control group is stored at room temperature for backup, while the experimental group is immersed in water. After being stored under a vacuum of 97.3–98.7 kPa (730–740 mmHg) for 15 min, the valve is then opened to restore atmospheric pressure, and the specimens are left in water for 0.5 h. Afterward, the specimens are removed, placed in sealed bags with 10 mL of water, and then the sealed bags are placed in a refrigerator at −18 ± 2 °C for no less than 16 h. Subsequently, the specimens are immediately removed and placed in a thermostatic water bath at 60 ± 0.5 °C for 24 h. Finally, both the control group and experimental group specimens are placed in a thermostatic water bath at 25 ± 0.5 °C for no less than 2 h to complete the preparation of the test specimens. Figure 3 shows the fixture for freeze-thaw splitting.

This experiment is conducted using a Marshall stability tester. The figure above shows the fixture used for the asphalt mixture splitting test, which is placed on the Marshall stability tester. The specimen should be positioned to ensure that the curved pressure strip of the fixture passes through the center of the specimen. The splitting test is conducted at a loading rate of 50 mm/min to obtain the maximum load.

The splitting strength and freeze–thaw splitting strength ratio are calculated using the following formulas:

$$R_T=0.006287P_T/h$$

(7)

In the following formulas:

R_T represents the splitting tensile strength of the specimen (MPa).

P_T represents the test load value of the specimen (MPa).

h represents the height of the specimen (mm).

$$TSR=\frac{\overline{R_{T2}}}{\overline{R_{T1}}}\times 100\%$$

(8)

TSR represents the freeze–thaw splitting test strength ratio (%).

$\overline{R_{T2}}$ denotes the average splitting tensile strength of the effective specimens after the freeze–thaw cycles (MPa).

$\overline{R_{T1}}$ denotes the average splitting tensile strength of the effective specimens, without undergoing freeze–thaw cycles (MPa).

2.5.3. Rutting Test

This test employs the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) and is used to determine the high-temperature rutting resistance of asphalt mixtures. This test is applicable for determining the high-temperature rutting resistance of asphalt mixtures. The specimens used in the test are plate-shaped and are formed by a rolling compactor, with dimensions of 300 mm in length, 300 mm in width, and 50 mm in height. The rutting test is conducted using a rutting test machine, as shown in the diagram below. This machine includes a solid rubber tire with a width of 50 mm and a rubber layer thickness of 15 mm. The test wheel travels a distance of 230 ± 10 mm, with a reciprocating rolling speed of 42 ± 1 cycle/min.

By adjusting the loading device, the contact pressure of the rubber tire on the specimen is set to 0.7 ± 0.05 MPa, and the applied total load is approximately 780 N. Additionally, the rutting test machine is equipped with a displacement sensor and a constant temperature chamber. Before starting the test, the specimen should be preheated in a constant temperature chamber at 60 °C for 5 to 12 h and maintained at 60 °C until the end of the test. Figure 4 shows the process of rutting test.

In this article, rutting tests were conducted at temperatures of 60 °C and 70 °C. During the test, the rubber wheel should be positioned at the center of the specimen, and its movement direction should align with the rolling or traffic direction of the specimen. The test machine is started, and the time–deformation curve is automatically recorded until the test duration reaches 1 h or the maximum deformation reaches 25 mm, indicating the end of the test.

The dynamic stability of the asphalt mixture in the rutting test is calculated using the following formula:

$$DS=\frac{(t_2-t_1)\times N}{d_2-d_1}\times C_1\times C_2$$

(9)

In the formula:

DS represents the dynamic stability of the asphalt mixture (cycles/mm).

d₁ is the deformation corresponding to time t₁ (mm).

d₂ is the deformation corresponding to time t₂ (mm).

C₁ is the machine type coefficient, which is 1.0 for the crank connecting rod-driven reciprocating wheel mode.

C₂ is the specimen coefficient, which is 1.0 for specimens with a width of 300 mm, prepared in the laboratory.

2.5.4. Bending Test

This test is conducted at a test temperature of −10 ± 0.5 °C, using a universal material testing machine with a loading rate of 50 mm/min. It is employed to evaluate the low-temperature tensile performance of asphalt mixtures, which follows the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). The specimens are obtained by cutting them from the material compacted by a rolling compactor. The specified specimen dimensions are a prism-shaped beam with a length of 250 ± 2.0 mm, a width of 30 ± 2.0 mm, and a height of 35 ± 2.0 mm, with a span of 200 ± 0.5 mm.

The universal testing machine (UTM) should be equipped with an upper platen and a beam-type support with a fixed steel rod with a radius of 10 mm. The center distance of the lower support is 200 mm. During the test, the upper platen is positioned centrally and in close contact with the specimen to avoid impact loads caused by any gap between the platen and the specimen. Additionally, it should be equipped with a mid-span displacement sensor and a data acquisition system. Figure 5 shows the process of low-temperature small beam bending test.

Before the test begins, it is necessary to measure the height (h) and width (b) of the specimen. If the difference between the test height and width at two supporting points exceeds 2 mm, the specimen is considered non-compliant. If the specimen is compliant, the average value is taken as the specimen height/width. The specimen is then placed in a freezer at −10 °C for at least 12 h before the test. Additionally, 45 min before the test, the environmental warming box of the universal material testing machine should be adjusted to −10 °C.

During the test, the specimen is symmetrically placed on the supports, with the specimen’s vertical direction aligned with the direction of specimen formation. The testing machine is operated to apply a concentrated load at the center of the span at a specified rate until the specimen fails. This process generates a load–deflection curve for the span.

2.5.5. Dynamic Modulus Test

According to Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011), this study conducted uniaxial compression dynamic modulus tests on asphalt mixtures within the range of viscoelasticity at temperatures of 15 °C and 45 °C. The testing apparatus used was a universal testing machine (UTM). Test specimens were cored from the gyratory compacted specimens, with a diameter of 100–104 mm and a height of 150 ± 2.5 mm, wrapped in polyethylene film, and stored at room temperature for one week. Figure 6 shows the dynamic modulus specimen.

During the test, first, the diameter of the specimen was measured, and the specimen was placed in a constant temperature chamber set to the specified test temperature for 4–5 h. Subsequently, displacement sensors were positioned on three nails fixed along the circumference of the specimen, allowing for the measurement of the compressive deformation at the center of the specimen. The upper loading plate was adjusted to make slight contact with the specimen, and a 5% contact load was applied continuously for 10 s as a pre-load. Sinusoidal axial stress test loads were then applied to the specimen, repeating the loading test from high frequency to low frequency at frequencies of 25, 20, 10, 5, 1, 0.5, and 0.1 Hz under the set temperature. The test collected the load and deformation curves of the last five waveforms, recording and calculating the applied load, axial recoverable deformation, dynamic modulus, and phase angle of the test.

3. Results and Discussion

3.1. Indoor Mix Design

3.1.1. Preliminary Gradation

In the initial stage of the indoor mix design, we conducted the sieving of aggregates to prepare for the subsequent gradation adjustment for the HMM-13. Figure 7 shows the results of the aggregate sieving. As we referred to the French asphalt mixture design philosophy, namely the gyratory compaction method, we also followed the French standards for sieving the aggregates. From the results, it can be observed that the particle sizes of all specifications of aggregates generally meet the factory specifications. There are some cases of oversize particles in certain size ranges (such as aggregates with a specification of 2.36–4.75 mm) and some cases of undersize particles (such as aggregates with a specification of 4.75–9.5 mm), but this does not affect the subsequent gradation design or the performance of HMM-13.

Using the reference limits of gradation control points for high-modulus asphalt mixture design, we have preliminarily selected coarse, intermediate, and fine gradations. Based on past engineering experience, we chose an asphalt-to-aggregate ratio of 4.2% (comprehensive asphalt-to-aggregate ratio of 5.2%, percentage of the sum of asphalt and high-modulus agent mass to aggregate mass) to prepare gyratory compacted specimens and to determine the volumetric properties of the mixture. Figure 8 provides a detailed table of the proportions of various specifications of aggregates in the preliminary gradation.

The synthetic gradation of the three selected gradations passing through the square sieve (mm) is shown in Figure 9. It can be seen that the synthetic passing rates of the three preliminary gradations all meet the control point limits for high-modulus asphalt mixture design. However, Gradation 1, which has coarser aggregate particle sizes, has a design curve too close to the control point limits. Considering the potential errors in indoor experiments, along with the construction processes, it is likely to result in the volume indicators of the rotating compacted specimens and the void ratio of the high-modulus asphalt concrete surface layer not meeting the requirements. Therefore, Gradation 1 is not considered. However, the final gradation selection also needs to consider the volume indicators of the rotating compacted specimens, as shown in Figure 10.

The theoretical maximum relative density for HMM-13 is calculated according to the following formula:

$${\gamma }_t=\frac{100}{\frac{P_{si}}{{\gamma }_{se}}+\frac{P_{ba}}{{\gamma }_{ba}}+\frac{P_h}{{\gamma }_h}}$$

(10)

In the formula:

γ_t: theoretical maximum relative density of the high-modulus asphalt mixture (dimensionless).

P_ba: modified asphalt content (%).

P_si: aggregate content (%).

P_h: high-modulus agent content (g).

γ_se: effective relative density of aggregate (dimensionless).

γ_ba: relative density of asphalt (dimensionless).

γ_h: relative density of high-modulus agent (dimensionless).

It can be observed that there is a small difference in the relative density between the gross volume and the calculated theoretical maximum relative density for the three gradations. In practical applications, this difference can be negligible. However, based on past practical experience, the design void ratio is controlled within 1.5%–2.5%, and the mix workability is used as the criteria for asphalt mixture gradation design. Only the volume indicators for Gradation 2 meet the requirements; thus, Gradation 2 is selected as the design gradation.

3.1.2. Determination of Asphalt Content

Weighing the aggregate for Gradation 2 proportionally, three asphalt content ratios, namely 3.9%, 4.2%, and 4.5% (corresponding to overall asphalt contents of 4.9%, 5.2%, and 5.5%), were used for the gyratory compaction test. The void ratios were measured, and the corresponding theoretical maximum relative densities for each group were determined. The summarized test results are presented in the Figure 11, where VV is the void rate of the specimen.

Based on the compaction results for the three asphalt content ratios, the density differences among the specimens for the three ratios are relatively small. However, the void ratio of the specimen with a 5.2% overall asphalt content meets the technical standard of 1.5%–2.5%. Considering its practical application in engineering, the overall asphalt content of 5.2% is selected for the next step of testing.

3.1.3. The Verification of the Road Performance Test for Asphalt Mixtures

Freeze–thaw splitting tests were conducted under the optimal asphalt content ratio. Due to the effects of water in the voids of the specimen on the asphalt bonding characteristics during the freeze–thaw cycles, and considering that the splitting test evaluates the tensile stress applied to the internal bonding strength of the specimen, this test can effectively assess the water stability performance of high-modulus asphalt concrete. The test results are shown in Figure 12. CVV represents the void ratio of the control group (without freeze–thaw treatment), EVV represents the void ratio of the test group (after freeze–thaw treatment), CP represents the splitting strength of the control group, and EP represents the splitting strength of the test group.

The test results indicate that the freeze–thaw treatment resulted in only about a 2% change in the void ratio for the high-modulus asphalt mixture specimens. The tensile strength ratio (TSR) was 84.2%, suggesting that the dynamic effects of water in the specimen voids, which continually change during freeze–thaw cycles, may be relatively small. Therefore, the void ratio of the specimen does not change significantly. However, water disrupts some of the asphalt bonding films on the aggregate surface, weakening the adhesion properties of the asphalt and leading to a decrease in splitting strength. Despite this, the TSR still meets the technical construction requirements.

Furthermore, rutting tests were conducted at temperatures of 60 and 70 °C. Numerous studies indicate that dynamic stability can intuitively reflect the ability of asphalt mixtures to resist high-temperature deformation, and rutting tests have become the test of choice for assessing high-temperature stability. The rutting test specimen’s gross volume relative density is 2.460, the calculated theoretical maximum relative density is 2.524, and the void ratio is 2.5%. The test results are shown in Figure 13.

The lower coefficient of variation indicates that there is a small degree of variation among different rutting test data, and the data have high reliability. From the test results, it can be observed that as the temperature increases by 10 °C, the dynamic stability of the high-modulus asphalt mixture decreases by approximately 45%. This result is closely related to the temperature sensitivity of the asphalt materials. However, the dynamic stability test results at both temperatures are significantly higher than the technical construction requirements, demonstrating the effectiveness of the high-modulus modifier.

In cold weather, the bottom tensile area of asphalt pavement is prone to develop low-temperature cracks. Therefore, a −10 °C beam bending test was conducted to evaluate the low-temperature crack resistance of the asphalt mixture; the test results are as

Table 4. Bending test results of asphalt mixture.

Maximum Load (kN)	Mid-Span Deflection (mm)	Bending Tensile Strength (MPa)	Stiffness Modulus (MPa)	Destructive Strain (με)	Requirement of Strain (με)
1.369	0.390	11.12	5438	2049	≥1800

shows.

Generally, it is assumed that the failure strain and bending stiffness modulus can intuitively reflect the low-temperature performance of asphalt mixtures. The greater the tensile strain at the bottom of the beam during failure, and the smaller the bending stiffness modulus, the better the low-temperature crack resistance of the mixture. Some studies suggest that high-modulus asphalt concrete primarily improves high-temperature performance and may sacrifice some low-temperature performance. However, the low-temperature crack resistance of the mixture in this article meets with the design and usage requirements.

Dynamic modulus specimens can reflect the dynamic response characteristics of asphalt mixtures under traffic loads. Due to the hysteresis of the strain response during load application, it is necessary to conduct dynamic modulus tests to verify the viscoelastic properties of high-modulus asphalt mixtures. The test results are shown in Figure 14. The results indicate that the high-modulus asphalt mixture used in this study exhibits good axial mechanical performance at a loading frequency of 10 Hz.

3.2. On-Site Mix Design

3.2.1. Verification of Raw Material Specifications

When conducting an on-site mix design, to validate the variability of incoming raw material specifications and to ensure the accuracy of the synthetic gradation in an on-site mix design, the technical service team performed a recheck of the raw material sieve analysis on materials that entered the site after the completion of the indoor mix design. Simultaneously, a comparison was made with the materials used in the indoor mix design. The sieve analysis results for the hot material bin are shown in Figure 15. The comparative results indicate that the single gradation of each raw material is not significantly different from the gradation used in the indoor mix design. The different values from the sieve analysis are shown in Figure 16, where the y-axis represents the difference between the sieving results of materials used in on-site mix design and the sieving results of materials used in indoor mix design.

3.2.2. Construction Site Gradation Adjustment

To ensure the effective application of high-modulus asphalt concrete, along with the on-site aggregate specifications and the flow rates of various bins in the mixing plant, we conducted on-site mix design work. The results are shown in Figure 17 and Figure 18.

3.2.3. Determination of the Optimal Asphalt Content Ratio through On-Site Mix Design

Based on the on-site mix design results, rotational compaction tests were conducted using asphalt content ratios of 4.0%, 4.2%, and 4.4% (equivalent to overall asphalt contents of 5.0%, 5.2%, and 5.4%). The compaction cycle was set at 80 times, and the maximum theoretical relative density for each group was calculated. The test results are shown in Figure 19, where VV is the void rate of the specimen.

Based on the rotational compaction test results for mixtures with different asphalt content ratios, and following the principle of achieving a void ratio within the technical standard of 1.5%–2.5%, along with relevant engineering experience, the choice for the final asphalt content ratio is 4.2% (equivalent to an overall asphalt content of 5.2%).

3.2.4. The Water Stability Test of On-Site Mix Design

The freeze–thaw splitting test was conducted on the high-modulus asphalt mixture with a 4.2% asphalt content ratio. The validation results are shown in Figure 20.

From the test results, it is evident that the performance of the high-modulus asphalt mixture designed at the construction site is nearly identical to that for the indoor design, with some cases even showing an improvement in TSR. Therefore, the performance meets the research objectives; therefore, we proceeded with the final paving work.

3.2.5. The Cold Material Flow Rate Test in the Mixing Plant

The second test utilized the Marini MAC360 mixing plant, with the cold material feed rate controlled by rotational speed. During the construction process at the site, the material proportions in each bin were ensured by setting the rotational speed for individual bins. The screen sizes of the mixing plant were 1717 mm, 1111 mm, 66 mm, and 44 mm. For this flow rate test, three trials were conducted for each material at different rotational speed ratios. Each flow rate test employed a feeding duration of 5 min. The relationship between the flow rate and the rotational speed ratio is illustrated in Figure 21.

Based on the indoor mix design results and the flow rate relationship curve derived from the cold material flow rate test, the rotational speed ratios for each cold material bin at a normal production rate of 200 t/h were calculated. The results are shown in Figure 22.

4. Conclusions

This article explores the application effectiveness of a high-modulus modifier in asphalt mixtures. Firstly, an indoor mix design was conducted, and a preliminary evaluation of its road performance was performed. Subsequently, the mix proportion of HMM-13 was adjusted, and water stability was verified based on on-site construction conditions. Simultaneously, the rotation speed ratios of the material bins in the mixing plant were adjusted. The main conclusions are as follows:

• A laboratory indoor mix design was carried out. The optimal aggregate gradation was determined first. Then, through the rotational compaction method, the void ratio and density of the high-modulus asphalt mixture were tested. The optimum asphalt content ratio for the indoor mix design was determined to be 4.2%, with a 1% dosage of high-modulus modifier. Under these conditions, the void ratio of the high-modulus asphalt mixture specimen was 2.0%, the gross volume relative density was 2.473, and the calculated theoretical maximum relative density was 2.524.
• The performance verification was conducted on the high-modulus asphalt mixture using a 5.2% comprehensive asphalt content ratio, based on the indoor mix design results. The tests included −10 °C beam bending, 60 °C and 70 °C rutting, freeze–thaw splitting, and single-axis compression dynamic modulus tests. The results indicated that the mixture met the requirements for low-temperature performance, high-temperature performance, water stability, and dynamic modulus.
• An on-site mix design was conducted at the construction site. Sieve analysis revealed minor differences in the aggregate between the construction site and indoor design, leading to adjustments in gradation. The optimal asphalt content ratio was determined based on the on-site mix design, which is still 4.2%, with the modifier content remained at 1%. Finally, the water stability was verified, and was found to be in compliance with the requirements.
• The model of the mixing plant at the construction site was investigated, and an assessment of its cold material flow rate was completed. The relationship between the rotation speed of the cold material bins and the corresponding discharge weight of the aggregate was established and adjusted. The rotation speed ratios of the cold material bins were determined for a production rate of 200 t/h in the mixing plant. This effectively reduced the delay and overflow of materials in the mixing plant, thereby reducing the variability of the asphalt mixture aggregate gradation.