1. Introduction
According to the recent highway statistics published by the United States (U.S.) Department of Transportation (DOT) Federal Highway Administration (FHWA), there are around 2.9 million miles of paved public roads in the U.S., of which about 98% are paved with asphalt material [
1]. Moreover, in 2021, the estimated total amount of asphalt mixture produced in the U.S. was 432.4 million tons, a six percent increase from the previous year [
2]. Asphalt mixture is a blend of aggregates (stone, gravel, and sand) and asphalt binder (asphalt cement with or without modifiers), constituting around 95% and 5% of the total mixture weight, respectively. Asphalt binder is the binding agent between the aggregate particles and the main component responsible for pavement flexibility. Asphalt pavement is considered a non-renewable structure because good resources of natural aggregates and asphalt binders are limited. Meanwhile, the production of these materials consumes energy and releases emission. To meet the goals of sustainable development and low environmental impact, pavement engineers have developed and implemented strategies to reduce the massive demand for virgin aggregates and asphalt binders in pavement construction, maintenance, and rehabilitation [
3].
For the last few decades, the asphalt paving industry has consistently encouraged reusing materials such as waste tires and recycled asphalt shingle (RAS) in asphalt pavement applications. However, reclaimed asphalt pavement (RAP) has been of interest to engineers as a recycled material since the 1970s, when the price of crude oil climbed sharply after the oil embargo. Although the primary motivation in those days was to save the cost of producing hot mix asphalt (HMA) or later warm mix asphalt (WMA), the current interest in recycled materials aims to reduce costs, preserve natural resources, and minimize energy consumption and greenhouse gas emissions [
4,
5]. Currently, asphalt pavement is considered the most recycled material in the U.S. since more than 95% of RAP mixtures are reused in new pavements and the rest are used in other civil engineering applications [
2].
RAP is the material produced by removing and processing the old or rejected asphalt concrete layer of asphalt pavements during the resurfacing, rehabilitation, or reconstruction of roadways. Even after completing its service life on roadways, RAP is considered a high-value material whose asphalt binder and aggregates can be reused. In addition to its use in asphalt mixtures, there are also other applications of RAP, such as in pavement base or subbase course and as embankment or fill material [
6,
7]. RAP characteristics such as its aggregate gradation, dust content (percent passing through a 0.075 mm sieve), and asphalt content play a significant role in the performance of RAP mixtures. The physical characteristics vary based on the equipment used to crush and/or mill the old pavement [
8]. However, the asphalt pavement that produces RAP has undergone aging due to external and internal factors throughout its service life. External factors such as time, temperature fluctuations, moisture, oxygen exposure, and solar radiation interact with internal factors like thermal conductivity, heat capacity, activation energy, and reactive oxygen species. These factors collectively contribute to the aging process of bitumen. This aging results in the hardening of the asphalt binder, leading to pavement distresses such as cracking [
9].
RAP materials are used most efficiently in new HMA/WMA mixtures. It is ideal to produce new asphalt mixtures with 100% RAP materials. However, due to manufacturing limitations and the observed poor performance of HMA/WMA with high contents of RAP, the content of RAP materials in asphalt mixtures has been kept relatively low, around 21.9% in 2021 [
2]. RAP’s variability, the lack of mixing guidelines, and the mixture performance in cracking and fatigue resistance are some of the main reasons behind the low usage of RAP in asphalt mixtures. This indicates a knowledge gap and a research need in using higher RAP contents in asphalt mixtures.
A special asphalt modification method of adding epoxy resin and its curing agent, initially developed in the 1950s, has led to a unique asphalt binder known as epoxy asphalt. Unlike conventional asphalt binders, epoxy asphalt is thermosetting in that its stiffness and strength are much less affected by temperature [
10]. Owing to the high stability and strength of cured epoxy resin, epoxy asphalt has superior engineering performance [
11]. Due to its high cost, however, its application in pavement engineering has been limited to specific applications such as steel bridge deck paving. Recent advancements in research and construction practices have made it more economically viable to apply epoxy asphalt in road pavements. In addition to its superior performance, epoxy asphalt has a low initial viscosity and so can be mixed with aggregates during asphalt mixture production at a temperature (e.g., 120 °C) much lower than the common asphalt mixture production temperatures (135–180 °C). These unique features of epoxy asphalt have led to the conjecture that it may help increase the RAP content in asphalt mixtures.
The performance of RAP mixtures has been evaluated by many researchers, with a focus on the impact of the RAP content. It is generally found that incorporating RAP materials into asphalt mixtures often improves their stiffness and resistance to rutting but also significantly reduces their resistance to fatigue cracking [
12,
13]. The impact of RAP content on mixture performance therefore should be investigated in terms of both rutting and fatigue resistance, especially at high RAP contents [
8]. The findings of some recent studies on RAP mixtures are summarized as follows.
In 2017, Porto et al. [
14] tried adding up to 70% RAP materials to new HMA with the use of a bio-based rejuvenation agent. They assumed that adding 4% pine-based rejuvenation agent, by the weight of the RAP binder, would restore the aged RAP binder by two grades and lead to good performance, especially at low temperatures. Three mixtures (0% RAP, 70% RAP, and 70% RAP + rejuvenating agent) were prepared in their laboratory and evaluated for their susceptibility to low-temperature cracking using a thermal stress restrained specimen test. The complex modulus was also measured at various temperatures and frequencies to characterize the asphalt mixtures. The results showed that the 70% RAP mixture without the rejuvenation agent had the highest stiffness modulus and did not perform well against fatigue at low temperatures. The 70% RAP mixture with the rejuvenation agent, however, performed similarly to or better than the virgin mixture.
Sabouri [
15] studied the effect of the RAP content on the mixture performance in terms of both fatigue and rutting resistance. Sabouri prepared and tested specimens of seven mixtures with RAP contents of 0%, 20%, and 40%. In order to observe the impact of the RAP content only, all the other variables, such as the aggregate gradation and asphalt binder type, were kept constant. The mixture performance was evaluated using a simplified viscoelastic continuum damage (S-VECD) model and a triaxial stress sweep (TSS) test. The long-term fatigue and rutting behaviors of the specimens were forecasted using FlexPave (Version 1.0.0) software. The results showed that the rutting resistance increased while the fatigue resistance deteriorated with an increase in the RAP content.
Yi et al. [
16] evaluated the influence of epoxy asphalt on RAP mixture performance. They assumed that adding epoxy asphalt to a 100% RAP mixture would produce a mixture with performance equal to or even better than a virgin epoxy asphalt mixture. The epoxy asphalt used in their study was produced by mixing virgin asphalt and epoxy resin components (including epoxy resin and a curing agent, which were both produced by a Japanese company). Two HMA mixtures were evaluated and compared: a reference mixture (made of virgin aggregate, epoxy resin components, and virgin asphalt) and an epoxy asphalt recycled mixture (made of RAP, epoxy resin components, and virgin asphalt). An optimal binder–aggregate ratio of 7.2% was determined for the epoxy asphalt recycled mixture using the Marshall method. As a result, the contents of the epoxy resin components and virgin asphalt were 1.71% and 0.18% (by weight of RAP), respectively. The performance of the mixture was evaluated using four tests: a wheel tracking test to measure the high-temperature rutting resistance, a bending test to measure the low-temperature (15 °C and −10 °C) performance, a freeze–thaw test to evaluate the moisture susceptibility, and a fatigue test to assess the fatigue performance. The results showed that using epoxy asphalt with 100% RAP produced a mixture with better rutting resistance. The low-temperature performance and moisture susceptibility were almost identical to those of the conventional mixture, while the fatigue resistance was poor.
Considering the above background and the industrial recycling intentions, it is necessary to explore more options that would increase the RAP content in asphalt mixtures. This study aims to investigate the use of epoxy asphalt in asphalt mixtures containing a high percentage of RAP materials. Different from the study by Yi et al. [
16], this study will investigate the potential of a domestic (i.e., United States) epoxy asphalt product, which is mixed at a much lower temperature and has a proven track record of superior performance in both dense- and open-graded asphalt mixtures in pavement projects around the world [
10]. In this study, the objectives are (1) evaluating and analyzing the performance of asphalt mixtures containing high contents of RAP and epoxy asphalt, where the comparison will be based on a serious of laboratory experiments on performance, and (2) developing recommendations on the design and evaluation of RAP mixtures with epoxy asphalt.
4. Performance Tests
After specimen fabrication, the performance of each mixture was evaluated in the laboratory in terms of its air void content, Marshall stability and flow, indirect tensile strength, moisture resistance, fatigue resistance, and fracture resistance, which are good indicators of the mixture performance in field pavements, such as high-temperature stability, low-temperature cracking resistance, fatigue cracking resistance, and moisture stability. By default, three replicate specimens were measured in each test. The test methods are described as follows.
4.1. Air Void Content
The volumetric properties of a compacted asphalt mixture are essential parameters that affect the long-term performance of the mixture in the field [
35]. For example, the air void content of a dense asphalt mixture is recommended to be within the range of 3–8% during pavement service life to avoid early distress [
36]. In this study, the air void content (V
a) of each specimen was calculated from the measurements of the theoretical maximum specific gravity (G
mm) and the bulk specific gravity (G
mb) of each specimen, following Equation (1).
where V
a = air void content in a compacted specimen (%); G
mb = bulk specific gravity of the compacted mixture; G
mm = theoretical maximum specific gravity of the mixture. The G
mm and G
mb measurements were conducted per the AASHTO T 275 [
37] and AASHTO T 209 [
38] procedures, respectively. One variation in the measurement of G
mb is that parafilm M sealing film was used instead of paraffin to seal the specimens.
The calculations of G
mm and G
mb are shown in Equations (2) and (3), respectively.
where A = the mass of the loose sample in the air (g); D = the mass of the container in water (g); E = the mass of the container and loose sample in water (g).
where A1 = weight of the dry specimen in air (g); D1 = weight of the dry specimen plus parafilm film (g); E1 = weight of the dry specimen plus parafilm film in water (g); F1 = specific gravity of the parafilm film.
4.2. Marshall Stability and Flow
The load and deformation resistance of all the mixtures were evaluated using the Marshall stability and flow test according to AASHTO T 245 [
33]. In this test, a Marshall specimen was loaded inside a Marshall breaking head in the diametrical direction at a loading rate of 51 mm/min to determine the maximum load supported and the corresponding flow value. The ratio of Marshall stability and flow value, defined as the Marshall quotient, can be used to assess the stiffness of the specimen. Before the test, the specimens were immersed in a water bath at 60 °C for 90 min instead of the 35 min specified in AASHTO T 245 [
33]. This change in the immersion duration was made to ensure that the inner part of the specimens reached the desired temperature (60 °C) before testing. Such an alternation was applied to all the specimens tested to be sure that the test results were comparable. For each mixture, three replicate specimens were tested.
4.3. Indirect Tensile Strength
The tensile strength of each mixture under relatively uniform tensile stress was measured using the indirect tensile strength (ITS) test following the procedure in ASTM D6931 [
39]. In this test, a vertical compressive load is applied on the diametral plane of a Marshall specimen at a loading rate of 51 mm/min. The maximum load is recorded to calculate the ITS of the specimen using Equation (4). Like the Marshall stability and flow test, specimens were conditioned in a water bath at 60 °C for 90 min before testing.
where S
t = ITS (kPa); P = maximum load (N); t = specimen height immediately before test (mm); D = specimen diameter (mm).
4.4. Moisture Resistance
Asphalt mixtures can be sensitive to moisture damage if the internal bonding between the binder and aggregate deteriorates in the presence of water. Moisture damage is considered one of the dominant causes of asphalt mixture distress. The moisture resistance of an asphalt mixture is often evaluated according to the tensile strength ratio (TSR) of the mixture, which is the ratio of the indirect tensile strength of an asphalt mixture after moisture conditioning to the indirect tensile strength of an asphalt mixture without moisture conditioning, as shown in Equation (5).
where TSR = tensile strength ratio (%); ITS
1 = average ITS of unconditioned specimens (kPa); ITS
2 = average ITS of specimens after moisture conditioning (kPa). Typically, a minimum TSR of 80% is required for the mixture to be considered resistant to moisture damage, as recommended by many transportation agencies [
40].
In this study, the procedure in the AASHTO T 283 specification was followed with minor modifications to measure the TSR [
41]. For each mixture, three specimens were tested for their ITS at 60 °C following the procedure in
Section 4.3. Three additional specimens went through a moisture conditioning process and were then measured for their ITS at 60 °C. The moisture conditioning procedure is as follows.
First, the specimens were subjected to partial vacuum saturation to fill air voids with water and reach a level of saturation between 70% and 80%. Afterward, the specimens were frozen in a freezer at −18 °C for at least 16 h. After that, the specimens were immersed in a 60 °C water bath for 24 ± 1 h. Finally, the moisture-conditioned specimens were measured for their ITS at 60 °C. This process is illustrated in
Figure 1.
4.5. Fatigue Resistance
The literature review shows that asphalt mixtures with a high content of RAP often have poor resistance to fatigue damage. In this study, the fatigue performance of a few mixtures was evaluated and compared through sinusoidal loading applied on cylindrical specimens, utilizing a testing machine (Instron E10000) owned by a collaborator in California, as shown in
Figure 2.
During the test, the two ends of a cylindrical specimen (51 mm in diameter and 102 mm in height) were first glued to the testing plates of the loading machine with structural adhesive. A sinusoidal load of a maximum value of 0.5 kN or 1 kN (in tension) and a minimum value of −0.1 kN (in compression) was then applied to the specimen at a frequency of 10 Hz. The peak load of 0.5 kN or 1 kN was determined based on the direct tensile strength measured on replicate specimens in the same testing machine at a loading rate of 0.51 mm/min and a temperature of 22 °C. For a specimen of a RAP mixture with 1% epoxy asphalt, the measured direct tensile strength is about 1.7 kN, so it was decided to use 0.5 kN (about 30% of the direct tensile strength) as the peak tensile load in the fatigue test and increase it in 0.5 kN increments in further fatigue tests. This load was used for all the specimens in the fatigue test so that their results could be compared. Furthermore, all the specimens were tested at the same temperature of 22 °C, which simulates the temperature environment at which fatigue damage in asphalt mixtures is most likely to occur.
4.6. Fracture Resistance
Another type of distress in asphalt pavement is cracking, which can significantly reduce the pavement service life. Several factors contribute to cracking, including traffic load, temperature variations, aging, and moisture damage. As a result, considering the fracture resistance of asphalt mixtures under various conditions is essential to mitigate cracking distress. The semi-circular bend (SCB) test is commonly used to measure the cracking resistance of asphalt mixtures under tensile loading [
42]. This test simulates the tensile stresses that occur at the bottom of the asphalt layer due to traffic loading and temperature fluctuations and determines the stiffness and strength of the asphalt mixture. It can be used to determine the effects of different factors on the cracking resistance of asphalt mixtures, including the type and amount of asphalt binder, aggregate gradation, and compaction methods.
In this study, the SCB test was performed in accordance with AASHTO TP 124 to assess the fracture resistance of the asphalt mixtures [
43]. For this test, a specimen compacted using the Superpave gyratory compactor, which had a diameter of 150 mm and a height of 160 mm, was first cut into two circular slices, each with a thickness of 50 ± 1 mm, from the specimen’s center region. After that, each slice was cut along its diameter into two equal semi-circular pieces. Then, a straight vertical notch was cut along the symmetrical axis of each semi-circular specimen. The notch was 15 ± 1 mm in depth and 1.5 ± 0.1 mm in width. During the test, a vertical load was applied on a steel strip placed on top of the specimen at a loading rate of 50 mm/min, as shown in
Figure 3. Specimen samples before and after fracture are shown in
Figure 4.
The vertical load and deflection during the test were recorded and used to calculate the following parameters:
Fracture energy (
), as shown in Equation (6) [
43], represents a mixture’s resistance to crack initiation and propagation.
Tensile strength (
), as shown in Equation (8) [
44]
The flexibility index (
), as shown in Equation (9) [
43], was used to evaluate the specimens’ flexibility and the rate of crack propagation.
The crack resistance index (
CRI), as shown in Equation (10) [
45], which provide a better measure of cracking resistance than fracture energy for brittle fractures with large loads and small displacements.
where
= fracture energy (J/m
2);
= work of fracture (J), which is the area under the load versus the average load line displacement curve, as shown in Equation (7);
= ligament area (mm
2), which is the product of the ligament length (60 mm) and the thickness of the specimen (50 mm).
where
= applied load (kN);
= displacement at the 0.1 kN cut-off load.
where
= tensile strength (MPa);
= maximum applied load (N);
= specimen diameter (150 mm);
= specimen thickness (50 mm).
where
= flexibility index;
= fracture energy (J/m
2);
= absolute value of post-peak load slope (
) at the inflection point of the unloading portion of the load versus the average load line displacement curve (kN/mm).
where
= crack resistance index;
= fracture energy (J/m
2);
= maximum applied load (kN).