Design and Construction Control of Warm Mix Epoxy Asphalt Mixture with Low Epoxy Content for Service Area Pavements

Abstract

Highway service area pavements are exposed to severe conditions such as heavy traffic, oil infiltration, and temperature fluctuations, which lead to issues like rutting and cracking in conventional asphalt mixtures. Although warm mix epoxy asphalt mixtures have high strength and corrosion resistance, their high epoxy content and stringent construction requirements limit their engineering applications. To address these challenges, a design and construction method for warm mix epoxy asphalt mixtures with low epoxy content (≤20%) was proposed. The mineral aggregate gradation was optimized using the CAVF volumetric method, and the impact of different epoxy asphalt-aggregate ratios was analyzed through various performance tests, including Marshall stability, high-temperature stability, low-temperature bending, and oil resistance tests. The construction available time was determined using viscosity tests, and process parameters were optimized based on infrared thermography and real-time compaction monitoring. The results show that a 5.4% epoxy asphalt-aggregate ratio yields the best overall performance, with significantly better dynamic stability, tensile strain, and oil resistance compared to SBS-modified asphalt mixtures. The recommended construction parameters, including temperature control and compaction process, ensure optimal performance and durability. The proposed methods provide essential technical support for the effective application of warm mix epoxy asphalt in service area pavements.

1. Introduction

The rapid expansion of China’s highway network has significantly improved transportation infrastructure, but it has also brought about new challenges, particularly in the functional demands of service areas [1,2]. Service areas, which are essential for vehicle parking, refueling, and loading/unloading, face an increasingly complex and extreme environment. These areas are subjected to long-term, high-frequency heavy traffic, oil contamination, fuel corrosion, and temperature fluctuations, which place considerable stress on the pavement materials used [3,4]. As a result, pavement performance in service areas is severely impacted, leading to issues such as rutting, oil contamination cracking, and fatigue damage [5,6]. These issues not only shorten the service life of the pavements but also undermine their quality and functionality. The service life of pavements in highway service areas is reportedly 40–60% shorter than that of pavements in regular highway sections, according to industry statistics [7,8]. The frequent maintenance and repair of service area pavements not only increase operational costs but also disrupt the normal flow of traffic, thus affecting the overall functionality of the service areas.

Given these challenges, there is a pressing need to develop new types of pavement materials that offer high strength, corrosion resistance, and ease of construction [9]. The goal is to enhance the durability and functional adaptability of service area pavements, ensuring that they can withstand the combined stresses of heavy traffic, oil contamination, and environmental fluctuations [10]. Among the various types of pavement materials, epoxy asphalt mixtures have attracted significant attention due to their outstanding chemical resistance and high-temperature stability [11,12]. However, conventional epoxy asphalt mixtures face significant limitations, including high epoxy resin content (typically ≥30%), stringent curing temperature requirements (≥120 °C), and short construction available times [13]. These characteristics lead to high material costs, substantial energy consumption, and difficulty in process control, making traditional epoxy asphalt unsuitable for large-scale use in service area pavements [14].

In light of the issues associated with traditional epoxy asphalt mixtures, research has begun to explore ways to reduce the epoxy content while maintaining or even improving the material’s performance [15,16]. One promising solution lies in combining low epoxy content systems with warm mix asphalt (WMA) technology. WMA technology has gained popularity due to its significant advantages in terms of energy conservation, emission reduction, and improved construction working conditions [17]. By lowering the mixing and compaction temperatures of asphalt mixtures by 20~40 °C, WMA technology not only reduces energy consumption but also extends the construction available time, making the paving process more efficient and environmentally friendly [18]. These advantages make WMA technology particularly attractive for service area pavements, where construction parameters are crucial for achieving optimal material performance.

In recent years, the concept of warm mix epoxy asphalt has emerged as a potential solution to combine the benefits of both low epoxy content and WMA technology [13,14,19]. This novel approach aims to reduce the epoxy content in the mixture to ≤20%, which helps lower material costs and energy consumption while still maintaining the high performance required for service area pavements [11,20]. However, achieving this balance of material performance, cost-effectiveness, and environmental friendliness presents a significant challenge, as it requires careful optimization of both the mixture composition and the construction process. Research on epoxy asphalt mixtures has mainly focused on modifying the resin system, controlling curing agents, and reinforcing the material with fibers to enhance its mechanical properties and durability. For example, Chen et al. [21] investigated the use of nano-silica-modified epoxy resin to improve the compressive strength of the mixture by 15–20%. However, even with these improvements, the epoxy content remained above 25%, which still poses challenges in terms of cost and process control. Similarly, the Zhang team [22,23] developed a low-temperature curing epoxy asphalt that reduced the construction temperature to below 100 °C. While this development holds promise, the long-term aging resistance of such mixtures remains uncertain, and further validation is needed to ensure their suitability for long-term use in service area pavements.

The construction process of epoxy asphalt mixtures involves several critical factors, including curing reaction kinetics, temperature control, and compaction parameters [24,25,26,27]. The curing process of epoxy asphalt is highly sensitive to temperature and time, and improper control of these factors can lead to deviations from the desired structural properties [13,28]. In particular, the coupling between the curing reaction and the paving process is a major challenge that can affect the long-term performance of the pavement. For instance, if the curing reaction is not adequately controlled during construction, the mixture may not achieve the desired strength or durability, leading to premature failures such as cracking or rutting [17,29].

Given the special service environment of service area pavements, existing studies mainly focus on improving single performance (e.g., rutting resistance modifiers, oil-repellent coatings), with insufficient attention to the synergistic optimization of the material’s comprehensive performance (mechanical properties, corrosion resistance, and constructability) [30,31,32]. Furthermore, during the construction process of epoxy asphalt mixtures, there is a strong coupling between the curing reaction kinetics and the paving process parameters [33]. Without fine process control, the actual formed structure may deviate from the design goals, directly affecting the pavement’s long-term performance [34,35,36]. Therefore, there is an urgent need to establish a comprehensive technical system that integrates “material design—performance verification—process control” to promote the engineering application of low epoxy content warm mix epoxy asphalt mixtures in service areas [37,38,39,40,41].

This study presents an innovative design scheme for a warm mix epoxy asphalt mixture with a low epoxy content (≤20%). The mineral material gradation is designed using the volumetric design method, and the optimal epoxy asphalt content is determined by integrating the volumetric indices of the mixture with its road performance. A simulation of the typical oil-contaminated environment in the service area is conducted to evaluate the anti-erosion properties of the epoxy asphalt mixture. Leveraging the test road in the service area, application research on warm mix epoxy asphalt mixture is performed. Through the construction of the test road, appropriate construction process parameters are identified, providing technical support for the upgrading and replacement of pavement materials in the service area.

2. Physical Properties of Experimental Raw Materials

2.1. Asphalt Binder Materials

The selected warm mix epoxy asphalt is sourced from Jurong City, Jiangsu Province, China. This warm mix epoxy asphalt comprises two components: Component A (resin) and Component B (a curing agent mixed with asphalt in a ratio of 1:10.5). When the ratio of Component A to Component B is adjusted to 1:11.5, the epoxy resin content reaches 16%, classifying it as a low epoxy content (≤20%) epoxy asphalt binder. The primary technical indicators of Components A and B are detailed in

Table 1. Technical indicator test results of Component A.

Project	Unit	Requirement	Result	Method
Appearance	-	Light yellow	Qualified	Visualization
Viscosity (120 °C)	$\mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s}$	20 ± 10	21.8	GB/T 22314-2008 [42]
Water content	%	≤0.15	0.08
Chroma	Gardner	≤4	2.2
Density (20 °C)	$\mathrm{g}/\mathrm{m}\mathrm{L}$	1.15 ± 0.15	1.14
Flash point (open cup)	$\mathrm{℃}$	≥200	285

and

Table 2. Technical indicator test results of Component B.

Project	Unit	Requirement	Result	Method
Appearance	-	Brownish black	Qualified	Visualization
Acid value	$\mathrm{m}\mathrm{g}\mathrm{K}\mathrm{O}\mathrm{H}/\mathrm{g}$	≤150	90	GB/T 22314-2008 [42]
Flash point (open cup)	$\mathrm{℃}$	≥220	316
Water content	%	≤0.5	0.2
Viscosity (120 °C)	$\mathrm{m}\mathrm{P}\mathrm{a}·\mathrm{s}$	>320	2012
Density	$\mathrm{g}/\mathrm{m}\mathrm{L}$	1.00 ± 0.15	1.01

, respectively.

For comparative purposes, ordinary SBS-modified asphalt (type I-D) was also selected for the experiments, with its technical indicators outlined in

Table 3. Technical indicator test results of SBS-modified asphalt.

Project	Unit	Requirement	Result	Method
Penetration (25 °C, 100 g, 5 s)	0.1 mm	40~60	52	JTG E40-2007 [43]
Penetration index (PI)	-	≥0	0.35
Ductility (5 °C, 5 cm/min)	cm	≥25	28
Softening point TR&B	°C	≥75	>90
Flash point	°C	≥230	342
Solubility	%	≥99	99.8
Storage stability (163 °C, 48 h, Softening point Difference)	°C	≤2	0.8
Elastic recovery (25 °C)	%	≥95	96

.

2.2. Aggregates

The selected aggregates were sourced from Furong Quarry, located in Heyuan City, Guangdong Province, China. It consists of crushed stone in sizes of 10~15 mm, 5~10 mm, and 3~5 mm, as well as 0~3 mm machine-made sand, all derived from the same parent rock. The key performance indicators of these aggregates are detailed in

Table 4. Technical indicator test results of coarse aggregates.

Project	Unit	Requirement	Result	Method
Crushing value	%	≤18	8.6	JTG E42-2005 [44]
LA abrasion loss	%	≤22	9.2
Apparent relative density	-	≥2.6	2.922
Water content	%	≤1	0.2
Adhesion level	-	≥5	5
Particle content (<0.075 mm, washing)	%	≤0.8	0.3
Soft rock content	%	≤1	0.2
Polished stone value	BPN	≥42	45

and

Table 5. Technical indicator test results of fine aggregates.

Project	Unit	Requirement	Result	Method
Apparent relative density	$\mathrm{t}/{\mathrm{m}}^3$	≥2.5	2.918	JTG E42-2005 [44]
Ruggedness (>0.3 mm)	%	≤12	3.2
Sand equivalent	%	≥65	71
Methylene blue value	$\mathrm{g}/\mathrm{k}\mathrm{g}$	≤2.5	1.2
Angularity (flow time)	s	≥30	48

.

Technical indicators for aggregates, including particle size, distribution, shape, texture, and durability, are essential for evaluating the suitability of the material in asphalt mixes. Key performance tests, such as water absorption, abrasion resistance, and freeze–thaw durability, help ensure that the aggregates will perform well over time, contributing to the long-term durability of the asphalt mixture.

2.3. Mineral Powder

The selected mineral powder is made from finely ground limestone. It serves as an essential component in the warm mix epoxy asphalt mixture, providing important characteristics that contribute to the overall performance and durability of the pavement. The key performance indicators of the mineral powder are detailed in

Table 6. Technical indicator test results of mineral powder.

Project	Unit	Requirement	Result	Method
Appearance	-	No Agglomeration	Qualified	Visualization
Apparent relative density	$\mathrm{t}/{\mathrm{m}}^3$	≥2.5	2.717	JTG E42-2005 [44]
Water content	%	≤1	0.6
Particle size: <0.6 mm	%	100	100
<0.15 mm	%	90~100	96.6
<0.075 mm	%	75~100	86.7
Hydrophilicity coefficient	-	≤1	0.64
Methylene blue value	$\mathrm{g}/\mathrm{k}\mathrm{g}$	≤8	2.6

.

3. Design and Property Analysis of Warm Mix Epoxy Asphalt Mixture

3.1. Mineral Aggregate Gradation Design

The Coarse Aggregate Void Filling (CAVF) method is a well established volumetric approach widely utilized in the design of the core skeleton structure of asphalt mixtures. Its core principle is to form a stable skeleton structure through coarse aggregates and precisely fill the skeleton gaps with fine aggregates, asphalt, and mineral powder, thereby balancing the strength and density of the mixture. The design process involves three primary steps: First, the dry-rodded void ratio of coarse aggregates is measured to determine the optimal skeleton gradation. Subsequently, the volumetric proportions of fine aggregates and asphalt mortar are calculated to ensure complete filling of the skeletal voids. Finally, key performance indicators including void ratio and stability are validated through Marshall testing or gyratory compaction experiments.

To facilitate this process, a set of equations is established, which accounts for the interactions between the different components of the mixture, as shown in Equations (1) and (2). These equations are derived from the principle that the volume of the mixture must be balanced in such a way that the void spaces are minimized while maintaining adequate filler and binder content for sufficient stability, workability, and resistance to deformation.

$$q_c+q_f+q_p=100\%$$

(1)

$${\displaystyle \frac{q_c}{100d_{f\left(s\right)c}}}(VCA-V_V)={\displaystyle \frac{q_f}{d_{tf}}}+{\displaystyle \frac{q_p}{d_{tp}}}+{\displaystyle \frac{q_a}{d_a}}$$

(2)

where $q_c$, $q_f$, $q_p$ are the mass ratios of coarse aggregate, fine aggregate, and mineral powder, $q_a$ is the asphalt-aggregate ratio, $VCA$ represents the voidage of coarse aggregate, $V_V$ is the target voidage of mixture design, $d_{tf}$, $d_{tp}$ are the apparent relative density of machine-made sand and mineral powder, $d_a$ is the relative density of asphalt, and $d_{f\left(s\right)c}$ is the compact relative density of coarse aggregate.

The final production proportion for the aggregate gradation, as determined through trial production, is shown in Figure 1.

3.2. Comprehensive Property Analysis of Epoxy Asphalt Mixture

(1)

Mashall indicator analysis

Marshall samples were prepared using asphalt-aggregate ratios of 4.8%, 5.1%, 5.4%, 5.7%, and 6.0%. The preheating temperature for Component A of epoxy asphalt is 85 °C, for Component B is 130 °C, and for the aggregates is 130 °C. The epoxy asphalt binder was prepared according to the proportional parameters in Section 2.1, and then it was added to the mineral material and stirred to prepare the Marshall specimen. The specimen formation should be completed within 30 min. For each oilstone ratio scheme, five Marschel specimens were formed. The specimens were cylindrical (with a diameter of 101.6 mm and a height of 63.5 ± 1.3 mm). After the epoxy asphalt mixture specimens were held at 120 °C for 30 min, the Marshall volume index test can be carried out, and this process takes approximately 30 min. Finally, the stability test in the uncured state is carried out, and this process will last for about 60 min.

Various key parameters of the warm mix epoxy asphalt mixture were tested and calculated, including the bulk specific gravity (G_mb), theoretical maximum specific gravity (G_mm), air void content (VV), voids in the mineral aggregate (VMA), voids filled with asphalt (VFA), uncured stage stability (F_N), and flow value (F_L). These parameters are essential for evaluating the performance and durability of the asphalt mixture under different conditions. The test results are summarized and presented in

Table 7. Mashall test results of warm mix epoxy asphalt mixture.

Asphalt-Aggregate Ratio (%)	Gmb	Gmm	VV (%)	VMA (%)	VFA (%)	FN (kN)	FL (0.1 mm)
4.8	2.559	2.683	4.6	14.5	68.0	10.1	28.8
5.1	2.560	2.671	4.1	14.7	71.9	10.8	29.1
5.4	2.561	2.659	3.7	14.9	75.3	11.7	30.2
5.7	2.563	2.648	3.2	15.1	78.8	11.2	31.6
6.0	2.576	2.637	2.3	14.9	84.6	10.6	33.3
Requirement	-	-	≤5	≥14	≥75	≥7.5	20~50

.

From the analysis of the data in Table 7, it can be analyzed that as the epoxy asphalt content increases, the gross volume relative density of the mixture continues to increase, while the theoretical maximum relative density decreases, and the corresponding voidage decreases. The Voids in Mineral Aggregate (VMA) of the compacted asphalt mixture is basically unaffected by the asphalt-aggregate ratio, remaining relatively stable. The Vapor Fraction of Asphalt (VFA) of the compacted asphalt mixture follows the opposite trend to the voidage, increasing as the asphalt-aggregate ratio increases. As the asphalt-aggregate ratio increases, the stability value of the uncured epoxy asphalt mixture first increases and then decreases, reaching a peak stability value at an asphalt-aggregate ratio of 5.4%. The reason for this is that when the asphalt-aggregate ratio is within a reasonable range, the asphalt can adequately coat the aggregates and form a complete bonding film, without excessive amounts that would cause a lubricating effect. At this point, the internal friction and asphalt bonding force of the aggregates are balanced. As the asphalt-aggregate ratio increases, the flow value of the mixture increases, and the asphalt content directly affects the deformation of the mixture at failure.

(2)

High-temperature stability analysis

Rutting plate samples of epoxy asphalt mixtures with different asphalt-aggregate ratios (300 mm × 300 mm × 50 mm) were formed. First, three rutting plate specimens were formed for each oil-to-stone ratio scheme. Then, the formed specimens were placed in a constant temperature oven for curing at 120 °C for 6 h. Finally, the loading test was carried out. A tire repeated loading test was then conducted at 70 °C to obtain the number of loading passes corresponding to a 1mm deformation of the samples. This is used to characterize the high-temperature stability performance of the asphalt pavement. The results are shown in Figure 2.

According to the results shown in Figure 2, the dynamic stability of the epoxy asphalt mixture at 70 °C exhibits significant non-linear characteristics with changes in the asphalt-aggregate ratio. When the asphalt-aggregate ratio increases from 4.8% to 5.4%, the dynamic stability increases from 8635 passes/mm to 14,319 passes/mm (a 65.8% increase). However, in the range of 5.4% to 6.0%, there is a 21.7% decline. This “increase-then-decrease” pattern reveals the dual effect mechanism of epoxy asphalt content on the high-temperature performance of the mixture. In the low asphalt-aggregate ratio stage (4.8–5.4%), the gradual increase in epoxy asphalt effectively improves the bonding condition at the binder–aggregate interface. At this point, the three-dimensional network structure formed by epoxy resin curing can fully encapsulate the aggregates, forming a continuous phase structure. As the thickness of the binder film increases from less than 5 μm to 8–10 μm (asphalt-aggregate ratio of 5.4%), the viscoelastic recovery ability of the mixture significantly enhances, while the interlocking effect between the aggregates is maintained. Once the asphalt-aggregate ratio exceeds 5.4%, the lubricating effect of the excess binder leads to a decrease in the effective stress at the contact points of the aggregates, resulting in a reduction in rutting resistance.

(3)

Low-temperature crack resistance analysis

According to the test method T0715 of the “Standard Test Method of Bitumen and Bituminous Mixtures for Highway Engineering” (JTG E20-2011) in China, two rutting plate specimens were formed and placed in a constant temperature oven for curing and curing at 120 °C for 6 h. Then, a double-sided saw was used to cut the rutting board into six small beam samples. The length of the small beams was 250 mm, the width was 30 mm, and the height was 35mm. A beam bending test was conducted at −10 °C, with a loading rate of 50 mm/min, until the sample failed. The maximum flexural tensile strain at the bottom of the beam at the time of failure was calculated. The test results are shown in Figure 3.

The low-temperature bending test is a crucial method for assessing the low-temperature cracking resistance of asphalt mixtures. By measuring the flexural tensile strain of beam samples at varying epoxy asphalt contents, it was observed that as the asphalt-aggregate ratio increased from 4.8% to 6.0%, the flexural tensile strain initially increased significantly, followed by a trend toward stabilization, as shown in Figure 3. Specifically, when the asphalt-aggregate ratio increased from 4.8% to 5.7%, the flexural tensile strain rose substantially from 2364 με to 2922 με, marking an increase of 23.6%. However, as the asphalt-aggregate ratio further increased to 6.0%, the flexural tensile strain slightly decreased to 2917 με, entering a stable phase.

This phenomenon can be attributed to the synergistic effect of the epoxy asphalt binder within the mixture. In the lower asphalt-aggregate ratio range (4.8–5.7%), the increase in epoxy asphalt content effectively enhanced the bonding performance and stress transfer efficiency of the binder. This improvement strengthened the weak points in the interface transition zone between aggregates, thereby significantly enhancing the mixture’s ability to deform under low-temperature conditions. At this stage, the binder fully encapsulates the aggregates, forming a continuous phase, and the three-dimensional network crosslinking structure of the epoxy binder maintains good flexibility even at low temperatures.

When the asphalt-aggregate ratio reaches 5.7%, the internal voids in the mixture are completely filled, and the flexural tensile strain peaks at 2922 με. However, when the asphalt-aggregate ratio is further increased to 6.0%, the excess free asphalt begins to create a “lubricating effect” within the mixture’s skeleton structure. This lubrication partially weakens the interlocking effect between the aggregates, reducing the overall mechanical interactivity of the mixture. Additionally, the excessive binder content leads to an increase in the binder’s modulus, which may impair the mixture’s ability to coordinate deformation. Consequently, the strain value decreases slightly as the mixture becomes less able to absorb and distribute stress under low-temperature conditions.

In summary, the results indicate that an optimal asphalt-aggregate ratio exists within the range of 4.8% to 5.7%, where the epoxy asphalt binder improves the mixture’s low-temperature flexibility and cracking resistance. Beyond this optimal ratio, the excess binder can adversely affect the mixture’s structural integrity, leading to a reduction in the material’s overall deformation capacity.

(4)

Oil corrosion resistance analysis

For highway service area pavements, oil and diesel leaks often occur during truck stops, and the areas contaminated by oil stains are prone to asphalt pavement corrosion. To address this issue, following the T0709 test method from the “Standard Test Method of Bitumen and Bituminous Mixtures for Highway Engineering” (JTG E20-2011) in China, cylindrical samples with a diameter of 101.6 mm and a height of 63.5 mm were formed. Three Marshall specimens were formed for the epoxy asphalt mixture and SBS-modified asphalt mixture. The formed epoxy asphalt mixture specimens were placed in a constant temperature oven for curing at 120 °C for 6 h. These samples were then immersed in diesel for 48 h (with the liquid level raised 10 mm above the sample’s top), and the changes in appearance and mass loss of the samples before and after immersion were observed. Finally, the intact samples were subjected to loading (at a rate of 50 mm/min) to evaluate the strength of the samples after corrosion.

Based on the oil immersion test observations presented in Figure 4, after 48 h of diesel immersion, the SBS-modified asphalt mixture samples exhibited significant degradation. The asphalt quickly dissolved, exposing the aggregates and resulting in a loose, compromised structure, with a measured mass loss of 10%. In contrast, the epoxy asphalt mixture samples displayed minimal mass loss, less than 0.2%, and maintained their structural integrity, with the oil film fully enveloping the sample.

In the Marshall stability test, the epoxy asphalt samples exhibited exceptional stability. After 48 h of oil immersion, the epoxy samples maintained an ambient temperature stability of approximately 40 kN, while the oil-immersed samples still retained a stability of over 20 kN. This demonstrates the excellent mechanical properties of the epoxy asphalt mixture, both before and after oil immersion. The calculated residual stability was greater than 50%, further highlighting the strong oil resistance of the epoxy asphalt mixture.

These results indicate that epoxy asphalt mixtures exhibit superior oil resistance compared to SBS-modified asphalts, making them a more suitable choice for applications such as truck parking areas in service zones, where exposure to oil and fuel may be frequent.

(5)

Recommended epoxy asphalt mixture scheme

Finally, based on the Marshall volumetric parameters, pavement performance, and economic considerations, the optimal asphalt-aggregate ratio for the epoxy asphalt mixture was determined to be 5.4%. Subsequently, both SBS-modified asphalt and epoxy asphalt mixtures were prepared. The gradation of the SBS-modified asphalt mixture is consistent with that of the epoxy asphalt mixture, as shown in the gradation curve presented in Figure 1. The oil-to-stone ratio is determined to be 5.4%. The molding temperature of the SBS-modified asphalt mixture is approximately 165 °C, which differs from that of the warm mix epoxy asphalt mixture. The relevant tests were conducted to evaluate the effects of different asphalt materials on the mixture’s performance. The key technical indicators are presented in

Table 8. Property comparation of different asphalt mixtures.

Project	Unit	Epoxy Asphalt Mixture	SBS-Modified Asphalt Mixture	Requirement
Mashall stability	kN	30.6	14.34	≥7.5
Void content	%	3.7	3.8	≤5
Vapor Fraction of Asphalt	%	75.3	75.1	≥75
Immersion Mashall Residue stability ratio	%	95.1	91.8	≥90
Freeze–thaw tensile strength ratio	%	94	90.2	≥80
Flexural tensile strain	με	2863	2558.0	≥2000
Dynamic stability (70 °C)	pass/mm	14,319	5329	≥5000
Texture depth	mm	1.1	1.08	≥0.8

.

Marshall samples and rutting plates were prepared at the optimal asphalt-aggregate ratio, followed by volumetric and pavement performance testing. The results, shown in Table 8, indicate that for mixtures made with different asphalt materials, the volumetric indicators (such as VV, VFA, texture depth, etc.) were relatively consistent and all met the required technical specifications.

The Marshall stability, residual stability, and freeze–thaw splitting test results revealed that the warm mix epoxy asphalt mixture exhibited a Marshall stability 2.13 times higher than that of the SBS-modified asphalt mixture, and its high-temperature deformation resistance was 2.69 times greater. Furthermore, the immersion Marshall residual stability ratio and freeze–thaw tensile strength ratio were approximately 3% higher than those of the SBS-modified asphalt mixture, indicating superior resistance to water-induced damage. The low-temperature cracking resistance, as measured by the flexural tensile strain, was 12% higher for the epoxy asphalt mixture compared to the SBS-modified asphalt mixture.

Given the high temperature, heavy rainfall, and intense UV conditions typical of southern China, combined with the frequent braking and accelerating on service area ramps, the pavement performance requirements are demanding. Therefore, warm mix epoxy asphalt offers significant performance advantages over SBS-modified asphalt, particularly in terms of durability and resistance to environmental stresses.

3.3. Viscosity Variety Analysis of Epoxy Asphalt

Viscosity tests of epoxy asphalt were conducted using the Brookfield viscometer method outlined in the U.S. SHRP Strategic Plan, at varying temperatures and durations. After preparing the epoxy asphalt samples, they should be kept warm immediately, which takes about 5 to 8 min. Then, the viscosity test can be carried out. The test temperatures were set at 120 °C, 100 °C, 90 °C, and 80 °C, with each test lasting for 180 min. The test results are presented in Figure 5, a construction viscosity of 3 Pa·s is recommended.

The viscosity/temperature/time test results for epoxy asphalt demonstrate that at 120 °C, the warm mix epoxy asphalt has a retention time of approximately 110 min. At 100 °C, the retention time is around 100 min. As the temperature decreases, the viscosity of the epoxy asphalt increases significantly, making low-temperature construction less favorable for proper compaction and shaping of the asphalt mixture.

As the temperature of the epoxy asphalt rises, the viscosity difference gradually diminishes, stabilizing over time. However, excessively high temperatures can accelerate the curing rate of the epoxy binder, leading to performance aging, which may complicate construction and potentially degrade the quality of the epoxy asphalt pavement. Therefore, further increasing the temperature beyond this point is not recommended.

The effective construction window for warm mix epoxy asphalt is determined by both temperature and reaction time. Considering the entire process, including production, transportation, paving, and compaction, it is advised to maintain the mixing and construction temperature within the range of 100 °C to 120 °C, with an operational time not exceeding 100 min. This ensures optimal workability and prevents adverse effects on the mixture’s performance.

4. Construction Quality Control of Warm Mix Epoxy Asphalt Mixture

4.1. Project Profile

The construction of warm mix epoxy asphalt pavement was carried out at the truck parking area and A1 ramp in Zone A of the Lantang Service Area along the Zihui Highway in Guangdong Province, China, covering a total area of 23,564 m². The pavement structure design for this service area includes multiple layers aimed at ensuring optimal durability and performance under high traffic and environmental stress.

The structure consists of the following layers: a 4 cm thick warm mix epoxy asphalt concrete GAC-13 upper course, treated with 0.45 kg/m² of epoxy asphalt bonding agent to enhance layer adhesion; a 6 cm thick SBS-modified asphalt GAC-20 middle course; followed by an 8 cm thick conventional 70# asphalt GAC-25 lower course. Beneath these asphalt layers, the base course consists of a 38 cm thick hydraulically bound mixture, with a 20 cm thick hydraulically bound mixture sub-base course beneath it, providing the necessary structural support for the pavement.

This multi-layered pavement structure was designed to meet the demands of the service area’s operational environment, where high traffic volumes and frequent braking and acceleration are common. The selection of warm mix epoxy asphalt for the upper course was driven by its superior performance characteristics, including enhanced durability, resistance to cracking, and better workability during construction. The construction site layout and process are illustrated in Figure 6, which provides a visual representation of the pavement layers and the construction activities involved.

The design and construction of this pavement structure aim to provide long-term durability, stability, and performance under varying traffic loads and environmental conditions, making it particularly suitable for high-traffic areas such as truck parking zones and service ramps.

4.2. Preparation of Construction Machinery

Before the construction of the warm mix epoxy asphalt pavement, it is essential to modify the mixing plant to accommodate the specific requirements of epoxy asphalt. This modification primarily involves adding delivery pipelines for Components A and B of the epoxy asphalt, along with the necessary connecting devices. Component A, which is required in smaller quantities, is stored in a mobile storage tank, as illustrated in Figure 7a. Component B, needed in larger quantities, is stored in the contractor’s asphalt tank. Both components are heated using thermal oil to ensure they reach the appropriate processing temperatures before being delivered through the pipelines from the respective storage tanks.

To precisely control the metering of the two components, a pumping system, as shown in Figure 7b, is installed. This system comprises a motor and a flowmeter, which enable the accurate regulation of the flow rates for both components. By setting the correct ratio for Components A and B, the computer-controlled system ensures the precise mixing of the required quantities. Any excess asphalt is recirculated to maintain consistency and prevent wastage.

The compaction process is a critical phase in ensuring the quality of the pavement. The selected compaction sequence, designed to optimize compaction efficiency and achieve the required density and performance characteristics, is detailed in

Table 9. Compaction sequence for warm mix epoxy asphalt mixture.

Compaction Stage	Number of Passes
Initial rolling	1 Pass, static, 2 steel-wheel (2 drum) rollers
Repeated rolling	2~3 Passes, oscillatory, 2 steel-wheel (2 drum) rollers 2 Pass, static, 1 pneumatic-tire roller
Finish rolling	1~2 Passes, static, 2 steel-wheel (2 drum) rollers

. This carefully managed sequence ensures that the warm mix epoxy asphalt pavement attains superior durability, resistance to cracking, and long-term performance under high traffic and environmental stresses.

By addressing the specific requirements of mixing and compaction, the construction process can achieve the desired quality and longevity for the warm; mix epoxy asphalt pavement.

4.3. Mixing and Transportation of Epoxy Asphalt Mixture

The calibration of the intermittent mixing plant was conducted initially to ensure its capability to capture and print data for each batch. This feature allows for real-time monitoring of raw material usage, temperature, and other critical parameters, with a particular emphasis on strictly controlling the quantities of raw materials. To prevent asphalt aging and ensure balanced construction performance, stringent control over the heating temperature of both raw materials and the mixed material is imperative.

The mixing parameters were established as follows: the mixing time for aggregates and mineral powder was set to 6 s, and the mixing time after the addition of asphalt was set to 38 s. To guarantee thorough mixing, each batch processed 4.5 tons of mixture.

During the production process, the heating temperature of the asphalt in the tank was maintained at 120 °C to ensure the fluidity of the dual-component epoxy asphalt. The heating temperature of the aggregates was adjusted to regulate the discharge temperature of the mixture. For the warm mix epoxy asphalt, the discharge temperature needs to be controlled between 100 °C and 120 °C. A higher temperature accelerates the curing reaction of the epoxy resin, while a lower temperature impedes compaction. Hence, the batch mixing plant adjusts the flame size of the heating drum based on capacity, raw material moisture content, and gradation composition to maintain the desired temperature range.

During transportation, the chemical reaction between the epoxy resins generates additional heat, which may further increase the temperature. Consequently, the temperature of the hot material during the production process is maintained between 101.3 °C and 105.2 °C. The temperature of the hot material is measured using a portable infrared imaging device, with the results illustrated in Figure 8.

By meticulously controlling the mixing and heating parameters, the production process can ensure the high-quality performance of the warm mix epoxy asphalt mixture. This rigorous approach to temperature management and material handling is essential for achieving the desired durability and longevity of the constructed pavement.

The transport vehicles follow a “front, rear, middle, front supplement, rear supplement” loading sequence to ensure the even distribution of the discharge temperature of the mixture, as illustrated in Figure 9a. Based on the test results, the surface temperature of the transported material was measured at 108.7 °C using a FLIR infrared detection device from a distance of 3 to 4 m, as shown in Figure 9b. This careful loading sequence plays a crucial role in maintaining temperature consistency across the entire load.

Moreover, the mixing plant effectively utilizes the temporary storage and insulation capabilities of the finished product storage bin to minimize temperature loss during the waiting period. This setup ensures that the transport vehicles can be loaded in a single operation, thereby reducing thermal degradation of the mixture prior to transportation.

To further enhance operational efficiency and reduce the discharge cycle time of the epoxy mixture, the loading capacity of each transport vehicle is carefully controlled. Each vehicle is loaded with approximately 40 tons of mixture, and the loading process takes around 12 min. This systematic approach improves both production and transportation efficiency, while simultaneously ensuring that the mixture’s temperature remains within the optimal range for subsequent construction operations.

By optimizing the loading and transport procedures, this method ensures that the warm mix epoxy asphalt maintains its required temperature for proper handling and compaction, thus contributing to the overall quality and durability of the pavement.

4.4. Distributing Construction of Epoxy Asphalt Bonding Treatment

The application of the epoxy asphalt waterproof bonding material is carried out using a specialized epoxy asphalt distributor vehicle, which was modified from a synchronous stone chip sealing vehicle. This modification involves the use of two separate containers to store the A and B components of the epoxy bonding material. These components are mixed through specially designed pipes, and the resulting mixture is sprayed onto the surface, as shown in Figure 10a.

The epoxy asphalt distributor vehicle is employed to spray the epoxy bonding oil, with the actual spraying rate ranging from 0.4 to 0.5 kg/m², which falls within the design specifications of 0.2 to 0.5 kg/m². This ensures that the application meets the required bonding material coverage for effective waterproofing. However, during the construction process, issues such as the curing of epoxy bonding oil and the subsequent clogging of the spray nozzles are common. To address this, workers must be present on-site to promptly clean the spray nozzles to ensure consistent application.

Additionally, it is crucial to promptly cover the applied bonding oil with an appropriate layer to reduce the potential for contamination at the interface, particularly in the event of cross-construction activities. The on-site application of the epoxy bonding oil is shown in Figure 10, demonstrating the careful handling required to maintain the quality and performance of the waterproof bonding layer. By maintaining a strict operational protocol and addressing common challenges such as nozzle clogging, the application process can be optimized to ensure the effectiveness and durability of the epoxy asphalt bonding material.

4.5. Stability Control of Raw Material Mixing Production

According to data from the batching plant, the trial section construction commenced at 12:40 p.m. and concluded at 5:30 p.m., spanning a total duration of 4 h and 50 min. During this period, a total of 769.802 tons of material was produced, resulting in an average production rate of approximately 159.379 tons per hour. A detailed analysis was conducted on the fluctuations in the material proportions added during each batch.

Statistical analysis was performed on the raw material proportion data for the 178 batches of asphalt mixture produced in the trial section. The data were analyzed using mathematical statistical methods, and the results are summarized in

Table 10. Fluctuation statistics of raw material mixing proportion.

Indicator	Design (%)	Item	Average (%)	Max (%)	Min (%)
0~3.5 mm	26	Actual Value	25.99	26.56	24.84
		RE	−0.04	2.11	−4.67
3.5~6 mm	6	Actual Value	6.00	6.13	5.70
		RE	0.00	2.12	−5.26
6~11 mm	32	Actual Value	32.03	33.18	31.64
		RE	0.09	3.56	−1.14
11~16 mm	32	Actual Value	31.98	32.87	30.62
		RE	−0.06	2.65	−4.51
Mineral powder	4	Actual Value	4.00	4.17	3.89
		RE	0.00	4.08	−2.83
Asphalt-aggregate ratio	5.4	Actual Value	5.39	5.63	5.20
		RE	−0.19	4.09	−3.85

Note: RE represents relative error.

. After accounting for potential measurement fluctuations caused by material overflow or waiting times during the process, it was found that the proportions of all raw materials were highly consistent with the design values. This indicates that the overall composition of the asphalt mixture remained stable throughout the production process.

These findings suggest that the batching process was effectively controlled, ensuring that the material proportions were accurately adhered to and that the final mixture composition met the specified requirements. Such consistency is crucial for maintaining the desired quality and performance characteristics of the pavement.

4.6. Construction Available Time of Epoxy Asphalt Mixture

According to the viscosity/temperature–viscosity/time test results for the warm mix epoxy asphalt, the construction window for the epoxy asphalt mixture is approximately 90 min. To optimize transportation efficiency, the selected construction area at the service station was located approximately 7 km from the mixing plant, minimizing the transportation time for material trucks traveling from the plant to the construction site.

Based on the timing data collected for each material truck at both the front and rear sites on the day of construction, the time from the mixing of the epoxy asphalt mixture to its application on-site ranged from 34 to 62 min, with an average of 46 min, as shown in Figure 11. This time frame was consistent across the transportation process, indicating stable control over the production and delivery sequence.

The effective time management for both production and transportation ensured that the epoxy mixture was delivered within the optimal construction window, thus meeting the required 90 min construction available time. This stability in the production and transportation process contributed to the overall efficiency and quality of the construction, ensuring that the epoxy asphalt mixture was applied within the necessary time parameters for proper curing and compaction.

4.7. Construction Temperature Control of Epoxy Asphalt Mixture

An embedded thermometer was utilized to measure the internal temperature of each truckload of the mixture, as depicted in Figure 12. The out-of-plant temperature of the epoxy asphalt mixture ranged from 100 °C to 120 °C, while the on-site temperature ranged from 106 °C to 116 °C, indicating minimal temperature loss during transportation, which was nearly negligible. This preservation of temperature can be attributed to two primary factors: (1) the short transportation distance combined with effective insulation measures, and (2) the heat generated by the chemical reaction of the epoxy mixture compensating for any temperature loss.

The paving temperature was maintained between 103 °C and 112 °C, which meets the established construction requirements. However, significant temperature loss (approximately 5~10 °C) was observed during the unloading process. This temperature drop was largely due to the low ambient temperature (15.3 °C) and higher wind speeds.

To mitigate this temperature loss, it is recommended that insulation measures, such as covering the mixture with thick tarps, be maintained throughout the unloading process until completion. Additionally, a steel-wheel roller should be promptly arranged for breakdown rolling to ensure proper compaction. These measures are crucial to maintain the mixture’s temperature within the optimal range for effective application and to ensure the quality and durability of the final pavement.

4.8. Compaction Process Control of Epoxy Asphalt Mixture

Epoxy asphalt mixtures are highly sensitive to moisture, as the presence of water directly impacts the bonding performance of the epoxy resin. To mitigate issues related to wheel adhesion, both steel-wheel and pneumatic-tire rollers were manually coated with vegetable oil. In order to determine the optimal rolling timing for the epoxy asphalt mixture and ensure proper compaction of the surface, the PQI-380 nuclear density gauge was used to monitor changes in compaction under different rolling passes during the trial section.

Three measurement points were selected on-site for the testing. Measurement points 1 and 2 were evaluated when the mixture’s paving temperature was around 100 °C, considered the normal temperature condition, while measurement point 3 was evaluated when the mixture’s paving temperature was around 80 °C, considered the low-temperature condition. The test results, shown in Figure 13, reveal the following key observations.

(1)

Under normal temperature conditions, after five passes of steel-wheel rolling (including the initial compaction), the compaction reached a stable state. Subsequently, two additional passes with the pneumatic-tire roller were sufficient to achieve peak compaction.

(2)

Under low-temperature conditions, compaction progressed more slowly. Even with the same number of rolling passes, the final compaction achieved was significantly lower than that under normal temperature conditions. The difference in final compaction was approximately 2.3%.

The variations in temperature at different locations also contributed to the differences in the final compaction results. Based on these findings, it is recommended that for warm mix epoxy asphalt, the initial compaction temperature should be maintained at ≥100 °C, with at least six rolling passes. Furthermore, the final compaction mixture temperature should not drop below 70 °C to ensure that the compaction level reaches ≥98%.

In addition to temperature control, attention must also be paid to the rolling time. Based on the viscosity test results of the epoxy asphalt, the overall construction window for warm mix epoxy asphalt (from production to completed rolling) should be kept within 90 min. At lower temperatures, compaction becomes more difficult, and over time, the chemical reaction of the epoxy resin accelerates, causing the mixture to solidify. Once this occurs, the mixture should be discarded to prevent any detrimental impact on the quality of the pavement.

These findings underscore the importance of carefully managing temperature and rolling time to ensure optimal compaction and the long-term performance of the warm mix epoxy asphalt.

4.9. Determination of Curing Time for Epoxy Asphalt Mixture

The strength of epoxy asphalt mixtures requires a specific temperature and time for adequate development [33]. Therefore, immediately after paving and rolling, the mixture lacks sufficient strength to open the road to traffic and must undergo a curing process. Previous studies have indicated that the rate of strength gain in epoxy asphalt mixtures is influenced by material properties and curing temperature [33,45]. Elevated temperatures accelerate the curing reaction, thereby shortening the necessary curing time [46].

For the day’s construction, 10 sets of Marshall samples were prepared and cured at ambient temperatures near the trial section. Periodic testing of Marshall stability was conducted, and the results are presented in Figure 14. On the first day after the formation of the warm mix epoxy asphalt, the initial Marshall stability after 24 h of curing at ambient temperature was 8.8 kN, which is comparable to the stability of conventional modified asphalt mixtures. As the curing time increased, the strength of the epoxy asphalt samples continued to rise, reaching 16.6 kN on day 27. However, the stability had not yet reached the design strength required for opening the road to traffic (20 kN). This delay in strength development can primarily be attributed to the low ambient temperatures during the winter construction period, with daily average temperatures around 12 °C (ranging from 7 °C to 17 °C).

A linear regression analysis between Marshall stability and curing time revealed a strong linear correlation. The fitted equation is given by Equation (3).

y = 0.289x + 9.016

(3)

where y is the Marshall stability (kN), x is the curing time (d), and the correlation coefficient R² is 0.955. Using this equation, it is predicted that after 38 days of curing, the stability value of the warm mix epoxy asphalt mixture will reach the design standard of 20 kN.

For this warm mix epoxy asphalt pavement, it is recommended to ensure the full curing period, particularly during winter when low temperatures prevail. If it is not possible to extend the curing time, it is crucial to restrict heavy vehicle traffic to prevent early load-induced damage, which could lead to irreparable defects in the pavement. These findings emphasize the importance of appropriate curing time management to achieve the desired strength and durability of the pavement, ensuring its longevity and performance under traffic loads.

5. Evaluation of Construction Quality for Warm Mix Epoxy Asphalt Pavement

The test section results indicate that the average thickness of the warm mix epoxy asphalt surface layer is 40 mm, with a pass rate of 100%. The average compaction rate of the core samples is 99.9%, also with a pass rate of 100%. Additionally, the average permeability coefficient is 36.7 mL/min, with a pass rate of 100%, demonstrating excellent performance in terms of water resistance. The pavement texture depth ranges from 1.0 to 1.2 mm, which is indicative of good skid resistance, thus meeting the required specifications for the highway service area pavement.

Further evaluation of the construction quality reveals excellent overall uniformity in both the paving and compaction of the warm mix epoxy asphalt, as shown in Figure 15.

Six measurement points were selected on the road surface and the three-dimensional morphology of pavement texture was tested using a 3D laser scanner. Referring to the ASTM E2147 method, the MPD (Mean Profile Depth) index was calculated, and the calculation was carried out based on the micro-texture distribution density S_MI value proposed in the previous study [47]. The MPD values for the pavement ranged from 1.02 mm to 1.12 mm (with an average of 1.06 mm), while the texture depth of traditional AC-13 pavements is typically 1.0 mm to 1.1 mm. The calculated S_MI values varied from 1.52 to 1.74 (with an average of 1.61), compared to S_MI values of 1.4 to 1.7 for traditional AC-13 pavements. Despite the relatively high asphalt content in the epoxy asphalt AC-13 pavement of this project, its micro-texture remains relatively rich. This outcome was achieved through the optimization of mixture gradation design and the strict control of construction quality, thereby providing the road surface with effective anti-slip textures.

Core samples exhibit good density, and the aggregate skeleton is well interlocked [48]. During the construction process, careful control of the hopper frequency effectively minimized material segregation, while workers were promptly assigned to remove excess edge material, ensuring a continuous paving operation. However, due to the large area of the service area parking lot, local coarse segregation strips appeared along the edges of the single-lane paving. Additionally, the multi-lane overlap areas were constructed with cold joints, which are susceptible to water infiltration and cracking, representing potential weak points in the pavement structure.

In response to this issue, subsequent measures were implemented based on observed water infiltration conditions, including the application of emulsified asphalt or water-repellent agents to enhance the waterproofing performance of the surface. Furthermore, a drainage well area exists within the site, where the waterproofing measures around the well openings require further improvement to prevent water penetration, which could compromise the pavement structure.

6. Conclusions

The research primarily focuses on the design and application technology of warm mix epoxy asphalt mixtures with low epoxy content, specifically for service area applications. The study includes aggregate gradation design, performance analysis of mixtures with varying epoxy asphalt contents, the evaluation of oil resistance and corrosion resistance, and other raw material and mixture tests to determine the optimal design for warm mix epoxy asphalt mixtures. Additionally, construction process parameters were monitored and controlled to ensure that the construction quality meets the design specifications for service area pavements. The main conclusions are as follows:

(1)

The CAVF volumetric method for designing the gradation of warm mix epoxy asphalt mixtures effectively balances the mixture’s density and skid resistance. The high-temperature stability of the epoxy asphalt mixture exhibits significant non-linear characteristics with varying asphalt-aggregate ratios. When the asphalt-aggregate ratio is 5.4%, the dynamic stability of the GAC-13 mixture peaks at 70 °C. As the epoxy asphalt content increases, the low-temperature crack resistance of the mixture initially increases significantly and then stabilizes.

(2)

The structural strength and high-temperature stability of the warm mix epoxy asphalt mixtures are 2.13 times and 2.69 times greater, respectively, than those of the SBS-modified asphalt mixtures. The water stability and low-temperature crack resistance are also slightly superior to those of the SBS-modified asphalt mixtures. After oil immersion, the Marshall stability of the epoxy asphalt mixture samples reached 20 kN, with a residual stability of 50%. The oil resistance is significantly better than that of the SBS-modified asphalt mixtures, making it particularly suitable for service area parking lot pavements.

(3)

Viscosity tests of epoxy asphalt mixtures show that as the temperature decreases, the initial viscosity of the asphalt increases. Excessively high temperatures and prolonged reaction times can accelerate the curing reaction of the epoxy resin, leading to a rapid increase in asphalt viscosity. Therefore, the effective construction window for warm mix epoxy asphalt mixtures should be controlled within 100 min at temperatures between 100 °C and 120 °C.

(4)

Key construction processes were monitored, including infrared thermography, production data analysis from the mixing plant, transportation time statistics, and compaction measurements. Based on the results, it is recommended that the initial compaction temperature of the warm mix epoxy asphalt mixture be ≥100 °C, with at least six passes during the repeated compaction process. The final compaction temperature should be ≥70 °C to ensure a compaction rate of ≥98%.

(5)

Marshall stability tests of epoxy asphalt samples during ambient temperature curing showed that the initial stability is comparable to that of conventional modified asphalt mixtures. As curing time increases, the strength development rate is strongly correlated with temperature (R² = 0.955). After 38 days of curing, the strength of the epoxy asphalt pavement structure reaches the design standard of 20 kN, allowing the road to be opened to traffic.

In conclusion, the performance of the warm mix epoxy asphalt mixtures with low epoxy content is significantly superior to that of the SBS-modified asphalt mixtures, making it more suitable for the complex conditions of service area pavements. Additionally, the use of low epoxy content warm mix epoxy asphalt materials offers significant economic and ecological benefits.

The development and application of a warm mix epoxy asphalt mixture with low epoxy content present promising advancements for service area pavements. While this study demonstrates significant improvements in mechanical performance, high-temperature stability, and oil resistance compared to conventional SBS-modified asphalt mixtures, further research is needed to fully understand the long-term benefits and environmental impacts. As the construction temperature of the epoxy asphalt mixture in this project is approximately 50~60 °C lower than that of traditional HMA construction, fuel consumption during production and construction is significantly reduced, and environmental impact is minimized. However, given the limitations of the project’s testing conditions and the novelty of this pavement material, our team plans to continue long-term performance monitoring of this pavement in the future to further conduct a comprehensive life cycle assessment. These future efforts will provide a more detailed understanding of the economic and environmental sustainability of this innovative pavement material.