**Experimental Investigation into the Structural and Functional Performance of Graphene Nano-Platelet (GNP)-Doped Asphalt**

#### **Murryam Hafeez 1,\*, Naveed Ahmad 1, Mumtaz Ahmed Kamal 1, Javaria Rafi 1, Muhammad Faizan ul Haq 1, Jamal 2, Syed Bilal Ahmed Zaidi <sup>1</sup> and Muhammad Ali Nasir <sup>3</sup>**


Received: 28 December 2018; Accepted: 13 February 2019; Published: 17 February 2019

**Abstract:** With the increase in the demand for bitumen, it has become essential for pavement engineers to ensure that construction of sustainable pavements occurs. For a complete analysis of the pavement, both its structural and functional performances are considered. In this study, a novel material (i.e., Graphene Nano-Platelets (GNPs)) has been used to enhance both of the types of pavements' performances. Two percentages of GNPs (i.e., 2% and 4% by the weight of the binder) were used for the modification of asphalt binder in order to achieve the desired Performance Grade. GNPs were homogeneously dispersed in the asphalt binder, which was validated by Scanning Electron Microscope (SEM) images and a Hot Storage Stability Test. To analyze the structural performance of the GNPs-doped asphalt, its rheology, resistance to permanent deformation, resistance to moisture damage, and bitumen-aggregate adhesive bond strength were studied. For the analysis of the functional performance, the skid resistance and polishing effect were studied using a British Pendulum Skid Resistance Tester. The results showed that GNPs improved not only the rutting resistance of the pavement but also its durability. The high surface area of GNPs increases the pavement's bonding strength and makes the asphalt binder stiffer. GNPs also provide nano-texture to the pavement, which enhances its skid resistance. Thus, we can recommend GNPs as an all-around modifier that could improve not only the structural performance but also the functional performance of asphalt pavements.

**Keywords:** Graphene nano-platelets (GNPs); asphalt; Scanning Electron Microscope (SEM); structural performance; functional performance

#### **1. Introduction**

Bitumen is considered to be an essential component of roadways and its demand is increasing with each passing day. According to the Asphalt Institute, 87 million tons of bitumen are produced per year around the globe [1]. A major chunk of this production, approximately 85% of the bitumen, is used in the paving industry. With the increase in traffic loads, the need for progress in pavement technology is also increasing. The early failure of pavement calls for its reconstruction, which results in an increased demand for bitumen. In order to conserve the resources, it is necessary to ensure the construction of sustainable pavements.

In Pakistan, two commonly observed highway failures are rutting and moisture damage. The poor mix properties of asphalt and the high temperature greatly contribute to these failures. The temperature cannot be controlled; however, the properties of asphalt can be improved for better temperature resistance. For over 50 years, researchers have been using various asphalt modifiers to achieve the desired material properties. Recently, the use of nanoparticles for asphalt modification has been brought into the hot spot because of their unique properties. Researchers have used various kinds of nanoparticles for enhancing the properties of asphalt, such as Nanosilica, Carbon Nanotubes (CNTs), Carbon Black Nanoparticles (CBNPs), and Graphite Nanoparticles (xGNPs). Graphene Nano-Platelets (GNPs) have commendable mechanical and thermal properties and as a result, their applications can be found in a broad range of fields [2–5]. A high specific surface area (SSA) and shape ratio (diameter/thickness) are responsible for imparting these properties on GNPs [6]. The strong carbon-carbon bond not only contributes to their exceptional strength but also provides chemical and structural stability [7]. The modification of asphalt with GNPs results in enhanced adhesive forces that increase the moisture resistance of asphalt. Asphalt modified by Graphene Nano-Platelets (GNPs) has been found to have improved mechanical and compaction properties when compared to the conventional asphalt [8]. The two challenges that researchers have to face while working with nanoparticles are their high cost and difficulty in homogeneously dispersing them in an asphalt binder. While working with GNPs, researchers overcame both of these challenges, as GNPs are low in cost and it is easier to achieve their homogeneous dispersion in an asphalt binder [8,9].

Researchers use a wet mixing technique to disperse nanoparticles in the binder. Nanoparticles are first dispersed in the solvent using a mechanical stirrer and then are mixed with the binder using a high shear mixer. The commonly observed issue with the wet mixing technique is that, if the solvent does not evaporate completely from the binder, it compromises the properties of the binder [1]. The usage of the solvent and the high shear mixer adds to the extra cost, as well as increases the processing time. On the other hand, it is comparatively easy to disperse GNPs in the asphalt binder, as no solvent or shear mixer is required. This makes the industrial application of GNPs favorable.

In order to evaluate a pavement, a structural and functional analysis of it is carried out. The structural performance is related to the pavement's strength and capacity to carry loads and traffic flow during its service life. The functional performance relates to the roughness of the pavement's surface. The skid resistance is an important parameter of the functional performance when it comes to the safety of the pavement. It is influenced by the micro-texture and the macro-texture of the pavement. The micro-texture affects the skid resistance in the pavement's early life, whereas the macro-texture influences the skid resistance over the service life of the pavement. To study the micro-texture and macro-texture of asphalt, Scanning Electron Microscopy and a British Pendulum Skid Resistance Tester are used. The structural and functional performances are inter-dependent. In a structurally sound pavement, the macro-texture remains intact for a longer period of time, providing skid resistance [10]. Normally, the gradation of aggregates is altered to get the desired skid resistance. In this study, we have worked with GNPs to improve the structural and functional performances of pavement, using a single modifier at the same time.

As the introduction of GNPs into the world of pavements happened quite recently, their impact on the rutting resistance, moisture susceptibility, and skid resistance of the asphalt needs further exploration. This paper not only aims to study the structural performance of asphalt modified by GNPs but to also explore its functional performance.

#### **2. Materials and Methods**

#### *2.1. Materials*

#### 2.1.1. Graphene Nano-Platelets

Graphene Nano-Platelets are basically sheets of graphene piled up together. A single graphene sheet is a monolayer of carbon atoms. These carbon atoms are tightly packed in a hexagonal arrangement. The stacking, rolling, and wrapping of graphene results in the formation of graphite, CNTs, and fullerenes, respectively. Thus, graphene can be labeled as a building block in the formation of the allotropes of carbon [11].

For this study, GNPs were procured from Advanced Chemical Suppliers (ACS) Materials, Pasadena, CA, USA. These GNPs were prepared using the interlayer cleavage method. Table 1 shows the properties of the procured GNPs. The shape ratio (diameter/thickness) and specific surface area are the two main properties of GNPs that are responsible for imparting the structural strength to GNPs-doped asphalt.


**Table 1.** The properties of Graphene Nano-Platelets (GNPs).

#### 2.1.2. Bitumen

For this study, 60/70 Penetration Grade bitumen was used. It was procured from the Attock Oil Refinery Limited, Pakistan.

#### 2.1.3. Aggregates

The source of the aggregates used for this research was Margalla Quarry, Punjab, Pakistan. It is a local quarry of limestone. Table 2 shows the qualitative properties of the aggregates obtained from the Margalla Hills [12].


**Table 2.** The properties of the aggregates procured from Margalla Quarry.

\* National Highway Authority.

The National Highway Authority Class B (for wearing course), which is specified as a finer gradation, was used for the preparation of the asphalt mixtures. Figure 1 represents the midpoint gradation curve for the National Highway Authority (NHA) Class B.

**Figure 1.** The aggregate gradation curve.

#### *2.2. Preparation of GNP-Modified Asphalt*

In Pakistan, the asphalt binder selection is currently based upon penetration grading, whereas pavement engineers around the globe are adopting performance grading (PG). The concept of performance grading is based around the idea that the properties of the asphalt binder should be in accordance with the requirements put forth by the environmental conditions of the area in which it is used. According to the temperature zoning of Pakistan, in 70% of areas the required performance grade of a binder is 70-10 [13,14]. Unfortunately, PG 70-10 binder is not produced in any of the oil refineries in Pakistan [13]. The PG of the binder used in this study was 58-22, which is much softer than the required PG 70-10. The high PG values are more of a concern, as the temperature hardly falls below 0 ◦C in most of the areas of Pakistan. The aim of the study was to achieve a PG of 70 using GNPs. For this research, two percentages of GNPs were used (2% and 4%) by weight (23% and 46% by volume) of the binder. This choice was made on the basis of the performance grade, as PG 64 was achieved when 2% of GNPs were added to the binder and PG 70 was achieved upon the addition of 4% of GNPs. For performance grading, a Dynamic Shear Rheometer was used. The frequency was set as 10 rad/s and 25 mm geometry was used. The initial temperature was set at 58 ◦C. The highest temperature, where the value of G\*/sinδ(kPa) was reduced to 1.0 kPa or less, was termed the high-temperature performance grade (PG) of the binder. The variation in the performance grade of the binder after its modification with GNPs is presented in Figure 2. The PG of the base binder was 58 ◦C and it failed at 62.7 ◦C. Upon the addition of 2% of GNPs, the PG increased by one level, i.e., 64 ◦C. The failure temperature also increased from 62.7 ◦C to 65.7 ◦C. A more pronounced change was observed when 4% of GNPs were added to the asphalt binder. PG 70 was achieved after modifying the binder with 4% of GNPs and the failure temperature was 71.5 ◦C. The trend shows that, with the addition of GNPs to the asphalt binder, its performance grade increases.

Prior to the mixing process, the asphalt binder was heated in an oven for 30 min at 158 ± 5 ◦C. A glass rod was used to mix the GNPs into the binder for 10 min at 158 ± 5 ◦C [8]. In order to ensure homogeneous mixing of the GNPs in the asphalt binder, Scanning Electron Microscope (SEM) (Vega3, TESCAN, Czech Republic) images were taken. The Scanning Electron Microscope has been shown in

Figure 3. Figure 4 shows the SEM image of the GNPs, in which the plate-like/flaky morphology of the GNPs can be seen. Figure 5 is the SEM image of the GNPs dispersed in the asphalt binder. In the image, homogeneously dispersed GNPs are visible in the binder.

**Figure 2.** The performance grades (PGs) of the asphalt binders.

**Figure 3.** The Scanning Electron Microscope (SEM) in the Mechanical Engineering Department (University of Engineering and Technology, Taxila).

**Figure 4.** A Scanning Electron Microscope (SEM) image of the Graphene Nano-Platelets (GNPs) (100 μm).

**Figure 5.** A Scanning Electron Microscope (SEM) image of the Graphene Nano-Platelets (GNPs) in the asphalt binder (500 μm).

According to the General Specifications of the National Highway Authority (NHA) of Pakistan, two aggregate gradations named class "A" and "B" are used for the construction of asphalt pavement wearing courses. Class A is coarser, while class B is finer [15]. For this study, the asphalt mixtures were prepared using the NHA class B aggregate gradation for the wearing course (19.5 mm Nominal Maximum Aggregate Size). To determine the Optimum Binder Content (OBC) of the GNP-modified asphalt, the Marshall Method of Mix Design was adopted. The asphalt mixes were prepared using a base binder and GNP-modified binders. Throughout the design, 5.5% of the air voids were maintained, which is the mean of the specified range of the in-place air voids (3% to 8%) according to the Asphalt Institute Manual Series-2 (MS-2). The OBCs of the three types of mixes are reported in Table 3.


**Table 3.** The Optimum Binder Content (OBC) of the asphalt mixtures.

#### *2.3. Storage Stability Test*

The storage stability is an integral parameter to consider while working with binders modified with nanoparticles. A Hot Storage Stability Test was carried out to check whether the suspension of the GNPs in the bitumen was stable for storage or not. This test was performed according to BS EN 13399 (2017). During this, 50 g of GNP-modified binder was poured into an aluminum tube. It was kept in the oven at 163 ◦C for 72 h. After the sample was taken out, it was cut into three sections. The top and bottom sections were heated and poured into rings for a softening point test. For a stable storage sample, the difference in the softening point of the top and bottom sections should be less than 2.2 ◦C. Although the softening point test is the most common method to determine the storage stability, it is not highly accurate [16]. For accuracy, the performance grades of all of the three sections were also determined by using a Dynamic Shear Rheometer (DSR) to ensure the homogeneous mixing of the GNPs in the binder.

The softening point results for the top and bottom sections of the tube are shown in Table 4.


**Table 4.** The softening point test results for storage stability.

According to the results, the difference between the softening point of the top and bottom sections of the 2% and 4% GNP-modified binder was less than 2.2 ◦C. This indicates that the GNP-modified asphalt binder is a stable storage blend [17].

The performance grades of the top and bottom sections of the samples for all of the blends were also determined using a DSR to ensure the homogeneous dispersion of the GNPs in the asphalt binder, as the rheological characterization of the binder is affected by its composition [16]. The performance grades of the binders did not vary between the sections.

#### *2.4. Study of the Structural Performance*

#### 2.4.1. Conventional Testing and Rheological Analysis of the Binder

#### • Conventional Binder Testing

The penetration, softening point, and ductility tests were performed in accordance with ASTM D5-13, ASTM D36-76, and ASTM D113-99, respectively.

• Study of the Rheology of the Binder Using a Dynamic Shear Rheometer

The rheological parameters of the binder, such as the Complex shear modulus (G\*), Phase angle (δ), and Superpave Rutting factor (G\*/sinδ), were determined using an Anton Paar Dynamic Shear Rheometer (DSR). Two tests were performed in accordance with AASHTO T 315 (High-Temperature Performance Grading and a Frequency Sweep Test).

The range of frequencies used for the frequency sweep test was 10 to 0.1 rad/s. The range of temperatures used was 20 ◦C to 70 ◦C. Both of the geometries (8 mm and 25 mm) were used. In this test, oscillatory shear stress is used at a constant strain level to determine the storage and loss modulus at the given range of frequencies and temperatures. The test data obtained were fitted to develop master curves using the sigmoidal function, as described by the mechanistic-empirical pavement design guide (MEPDG). Master curves were constructed at a reference temperature of 50 ◦C by giving a horizontal shift to the data obtained from each test temperature. A temperature of 50 ◦C was selected because the main focus of the study was to analyze the high-temperature performance of the binder. Before the Frequency Sweep test, a Strain Sweep test was performed at each testing temperature in strain-controlled mode. The complex shear modulus (G\*) was measured and the percent strain value at which the complex shear modulus was reduced to 95% of its initial value was noted as a threshold for the linear viscoelastic (LVE) region. Based on this, the strain limit for the base binder was kept at 10%, whereas it was 0.45% for the GNP-modified binder.

#### 2.4.2. Permanent Deformation Analysis

#### • Study of the Rut Resistance

The rut resistance of asphalt was studied at 40 ◦C and 55 ◦C using a Cooper Wheel Tracking Machine. The standard followed for this test was BS EN12697-22. The asphalt mix was prepared at 158 ± 5 ◦C and compacted to form slabs, with the help of a compacting machine, until the target of 5.5% air voids was achieved. Three slabs were prepared for each test condition. The dimensions of the slabs were 305 mm × 305 mm × 50 mm. The load applied by the wheel tracking machine was 700 N. The thickness of the wheel was 50.8 mm and its diameter was 203.2 mm. The speed of the machine was 26.5 rpm. Each slab was subjected to 10,000 loading cycles and the rut depth was measured. The wheel-tracking slope should be expressed in mm per 10<sup>3</sup> load cycles and calculated using Formula (1).

$$WTS\_{AIR} = (d\_{10000} - d\_{5000})/5 \tag{1}$$

where *WTSAIR* is the wheel-tracking slope (mm/10<sup>3</sup> load cycles) and *d*<sup>5000</sup> and *d*<sup>10000</sup> are the rut depths (mm) after 5000 load cycles and 10,000 load cycles, respectively.

• Determination of the Dynamic Modulus of Asphalt

The ratio of the peak-to-peak stress to the peak-to-peak strain is termed the Dynamic Modulus. A Dynamic Modulus test was performed using the Servo-Pneumatic Universal Testing Machine NU-14 (Cooper, Ripley, UK). This test is also termed as a viscoelastic test in which sinusoidal loading is applied. The standard followed for this test was in accordance with AASHTO TP 62-8. Cylindrical specimens of asphalt were prepared using a Superpave Gyratory Compactor (Controls, Milan, Italy), with a diameter of 152.4 mm and a height of 172.72 mm. Samples with a diameter of 101.6 mm and a height of 152.4 mm were extracted using a core cutter machine. Three samples were prepared against each test condition. The test was performed at 40 ◦C and 55 ◦C. The loading frequencies were 25, 10, 5, 1, 0.5, and 0.1 Hz. The loads applied at 40 ◦C and 55 ◦C were 195 kPa and 53 kPa, respectively. The Dynamic Modulus was recorded with the help of Linear Variable Differential Transformers (LVDTs).

#### 2.4.3. Durability Analysis

• Moisture Susceptibility Analysis (Rolling Bottle Test)

A Rolling Bottle test was carried out to study the moisture sensitivity of the asphalt. The test was performed in accordance with BS EN 12697-11:2005. The 6.3 mm to 10 mm of aggregates' fraction sieved was used as per the standard EN 12697-2. The weight of the aggregates was 170 g. The weight of the binder was 8 g. The aggregates were heated and uniformly coated with the binder. 150 g of aggregate coated with binder was placed in the bottle. 400 mL of distilled water was added to the bottle. Two samples were prepared against each binder. All of the bottles were placed in the rolling bottle machine. The speed of the machine was 60 revolutions per minute. The samples were taken out and studied for the percentage of bitumen coverage after 6, 24, 48, and 72 h.

• Bitumen-Aggregate Adhesion Analysis

The Bitumen Bond Strength was studied using a Pneumatic Adhesion Tensile Test Instrument (PATTI). The standard followed for this test was ASTM D 4541. Plates of limestone were prepared by cutting out slabs. The dimensions of the slabs were 381 mm × 152.4 mm × 50.8 mm. The slabs were cleaned for 60 min in an ultrasonic cleaner at 60 ◦C and then heated in the oven at 150 ◦C for an hour. Prior to the start of the test, the binder, stubs, and slabs were heated in the oven for 30 min at 75 ◦C. The type of stubs used for the test was F-4, with a diameter of 12.7 mm. 0.04g of bitumen was placed on the stub and then the stub was placed on the slab. Three samples were prepared against each binder. The samples were tested after 12 h of conditioning at room temperature. The PATTI gives the value of burst pressure at which the stub gets detached from the substrate, which was converted to the Pull-Off Tensile Strength (POTS) by using the following Formula (2).

$$POTS = \left[ \left( BP \, \ast A \, \text{g} \right) - \mathbb{C} \right] / A \, ps \tag{2}$$

where:

*Ag* = Contact area of the gasket with the reaction plate = 2619.35 mm<sup>2</sup> *BP* = Burst pressure (MPa) *Aps* = Area of the pull stub = 126.64 mm2 *C* = Piston Constant = 129.73 g.

#### *2.5. Study of the Functional Performance*

#### 2.5.1. Study of the Surface Texture

In order to study the surface texture of aggregates coated with GNP-modified binder, images from a digital camera were captured. To observe it in detail, SEM images were used. The samples were prepared and sputtered before observation under the SEM.

#### 2.5.2. Study of the Skid Resistance and Polishing Effect

A British Pendulum Skid Resistance Tester was used to study the skid resistance of the asphalt. This test was performed in accordance with ASTM E303-93(2013). Before subjecting the asphalt slabs to the Wheel Tracking Machine, their surfaces were tested for their skid resistance. The British Pendulum Skid Resistance Tester gave us a British Pendulum Number (BPN) against each asphalt mix. The higher the BPN, the higher the skid resistance is. After the completion of the Wheel Tracking Test (10,000 loading cycles), the skid resistance was checked again on the path developed due to the continuous movement of the wheel. This was done in order to study the polishing of the asphalt surface due to the movement of the wheel on the asphalt.

#### **3. Results and Discussion**

#### *3.1. Conventional Testing Results*

The results of the conventional tests performed in the laboratory to study the physical properties of the binders are presented in Table 5.


**Table 5.** The physical properties of the base and modified binders.

It was observed that the addition of the GNPs to the binder reduced its penetration value. Reductions of 34% and 48% in the penetration values were recorded after the addition of 2% and 4% of GNPs, respectively. An inverse relationship between the GNP content and binders' penetration value can be seen. An increase in the GNP content led to a decrease in the penetration value, which validated the stiffening of the binder. In the same way, the results of the softening point also depict the stiffening of the binder. The softening point of the binder increased up to 19% when 4% of GNPs were added. The improvement in the physical properties is due to the increase in the bonding strength after the homogeneous dispersion of GNP layers in the binder, which restricts the flow of bitumen and makes it stiffer [18]. The extremely small size and high surface area of GNPs enable them to form a strong bond with bitumen, which leads to a reduction in the penetration value and an increase in the softening point [19]. A massive reduction in the ductility was also recorded in the case of GNP-modified binder, which can be attributed to increased stiffness [20]. The trend in the physical properties of the GNP-modified binder indicates an enhancement in its high-temperature performance.

#### *3.2. Rheology of the Binder*

#### Frequency Sweep Test

Rheological changes were observed in the asphalt binder upon the addition of GNPs. An increase in the complex shear modulus and a decrease in the phase angle values were recorded. Figures 6 and 7 show the Master Curves for the Complex Shear Modulus and Phase Angle, respectively. It is evident in Figure 6 that modification of the binder with GNPs has led to an increase in the complex modulus. The increment in the complex modulus was more pronounced when 4% of GNPs were added in comparison to 2%. Figure 7 shows a decrement in the phase angle, which is more prominent at a low frequency/high temperature. A maximum decrease in the phase angle was observed for the higher weight content of the GNPs. The value of the Superpave rutting factor also increased massively at a low frequency upon modification of the binder. This showed the same trend as the Complex Shear Modulus.

**Figure 6.** The Master Curve for the Complex Modulus at 50 ◦C. GNPs = Graphene Nano-Platelets.

**Figure 7.** The Master Curve for the Phase Angle at 50 ◦C. GNPs = Graphene Nano-Platelets.

The reason behind the increase in the complex modulus can be attributed to the high elasticity and the large surface area of the nanoparticles, which gives them a higher affinity to bond with the functional groups of the binder [21]. A cover is created, which helps in preventing the viscous nature of bitumen and causes a delay in the conversion of the elastic behavior to the viscosity at a high temperature [22]. The downward shift of the phase angle can also be attributed to the elasticity of the GNPs. The elastic behavior of the asphalt binder contributes to the rutting resistance. Increasing the GNP content intensifies the increase in the complex modulus and the decrease in the phase angle.

The increase in the Superpave rutting factor can be explained by the fact that the extremely small size of the GNPs leads to an increase in Van der Waals interaction energy between the GNPs and the asphalt binder [23]. This results in a more stable asphalt binder. The enhancement in the Superpave rutting factor due to the addition of the GNPs validates the improvement in the deformation resistance at a high temperature.

#### *3.3. Rutting Resistance*

The Wheel Tracking Test was performed at two temperatures (40 ◦C and 55 ◦C). Figures 8 and 9 show the relationship between the number of passes and the rut depth for all of the samples. The results depict an enhancement in the rut resistance of the asphalt following the addition of the GNPs. The increment in the rut resistance is more pronounced at 55 ◦C, where the rut depth decreased from 7.3 mm to 4.7 mm after the addition of 4% of GNPs. At 40 ◦C, the rut depth decreased from 2.8 mm to 2.59 mm when 2% of GNPs were added, and further decreased to 2.45 mm following the addition of 4% of GNPs. Table 6 shows the results of the Wheel Tracking Slope. It can be seen that, with the increase in the content of the GNPs, the values of the slope decrease at 55 ◦C and remain constant at 40 ◦C. A decrease in the slope means that there is an increase in resistance to a permanent deformation [24]. According to the results, the maximum resistance to a permanent deformation can be achieved at 55 ◦C by adding 4% of GNPs. Due to the high surface area of the GNPs, they are reactive and form a strong bond with bitumen, imparting strength [25]. The GNPs may also act as filler, increasing the structural asphalt and significantly reducing the temperature susceptibility of asphalt [26].

**Figure 8.** The Wheel Tracking Test results at 40 ◦C. GNPs = Graphene Nano-Platelets.

**Figure 9.** The Wheel Tracking Test results at 55 ◦C. GNPs = Graphene Nano-Platelets.


**Table 6.** The Wheel Tracking slope. GNPs = Graphene Nano-Platelets.

#### *3.4. Dynamic Modulus*

The Dynamic Modulus Test was performed using NU-14 at 40 ◦C and 55 ◦C. Figures 10 and 11 show the relationship between the frequency and the dynamic modulus for the samples at both of the temperatures. Dynamic Modulus is a direct measure of the rut resistance. It is evident from the results that the value of the Dynamic Modulus is higher for GNP-modified asphalt. Asphalt modified with 4% of GNPs gave the highest values of Dynamic Modulus, regardless of the temperature and loading conditions. The same trend has been observed in the Wheel Tracking Test too. In general, the values of the Dynamic Modulus at 55 ◦C are lower than the values at 40 ◦C, which indicates a greater rut susceptibility at a high temperature. It can also be seen that the load response lag phenomenon is quite obvious when the loading frequency increases. This indicates that the Dynamic Modulus or strength of the asphalt mixture increases with an increment in the loading frequency [27]. The well-dispersed GNPs can provide support to asphalt by increasing the interaction and adhesion strengths within it [28]. These effects may lead to an increase in the Dynamic Modulus and resistance to the rutting of GNP-doped asphalt.

**Figure 10.** The Dynamic Modulus at 40 ◦C. GNPs = Graphene Nano-Platelets.

**Figure 11.** The Dynamic Modulus at 55 ◦C. GNPs = Graphene Nano-Platelets.

#### *3.5. Moisture Susceptibility*

The percentage of bitumen coverage was noted after 6, 24, 48, and 72 h during the rolling bottle test. Figure 12 is the graphical representation of the relationship between the time of rolling and the percentage of bitumen coverage. Figure 13 shows the affinity between the bitumen and aggregate after 72 h of rolling. The rolling bottle test is a measure of the binder's moisture susceptibility. A higher percentage of bitumen coverage indicates a strong adhesive bond between the binder and the aggregate, as well as a high resistance of the binder to moisture damage. It is evident from the results that, after the completion of the test, the percentage of bitumen coverage for the base binder was 15%. It then increased to 60% when 2% of GNPs were added. For the binder modified with 4% of GNPs, it jumped to 70%. The extremely small size of the GNPs gives them a larger surface area compared to their parent material, which results in the increase of their affinity to form a strong bond. Thus, GNPs have the ability to absorb more free asphalt binder and maximize the quantity of the structural asphalt [26]. The results prove that the addition of GNPs leads to an increased adhesion between the binder and aggregate, making the asphalt highly resistant to moisture.

**Figure 12.** The Rolling Time vs the Percentage of Bitumen Coverage. GNPs = Graphene Nano-Platelets.

**Figure 13.** The bitumen/aggregate affinity after 72 h of rolling: (**a**) The base binder, (**b**) 2% Graphene Nano-Platelet (GNP)-modified binder and, (**c**) 4% GNP-modified binder.

#### *3.6. Bitumen Bond Strength*

The idea of the PATTI was first conceived by the paint industry. It determines the POTS of the binder while keeping a constant loading rate of 0.69 MPa/s. Figure 14 shows the testing assembly. It has been learned, from the results presented in Table 7, that an increase in additive content produces a higher Bitumen Bond Strength. When 2% of GNPs were added to the binder, the POTS increased significantly from 8.76 MPa to 11.95 MPa. With the addition of 4% of GNPs, it further increased to 13.66 MPa. The mode of failure for all of the samples was cohesive, i.e., bitumen-bitumen interface

breakage. The reason behind the increment in the tensile strength of the binder can be attributed to the increased surface texture of the GNP-modified asphalt binder [29]. This is because a rough binder surface is obtained when nanoparticles are added to it, due to more particle interlocking. The elasticity of the nanoparticles also plays a part in enhancing the tensile strength of the binder. Thus, it is evident that the GNPs improve the adhesive and cohesive bond strength of the asphalt binder.

**Figure 14.** The Pneumatic Adhesion Tensile Test Instrument (PATTI) Assembly.



#### *3.7. Surface Texture of the Binder-Coated Aggregates*

The images to study the surface texture of aggregates coated with GNP-modified asphalt binder were taken both from a digital camera and an SEM. Figure 15 shows the images of a piece of aggregate coated with base binder. Figure 16 shows the images of GNP-modified asphalt binder.

**Figure 15.** A piece of aggregate coated with base binder.

**Figure 16.** A piece of aggregate coated with Graphene Nano-Platelet (GNP)-modified binder.

It can be observed from the images that the surface of the aggregate coated with GNP-modified binder is rough in comparison to the one coated with the base binder. This is because nanoparticles add their nano-texture to the surface, which is also the reason that nanoparticles are used to make the pavement surface rough in order to increase its skid resistance [30].

#### *3.8. Skid Resistance and the Polishing Effect*

A British Pendulum Skid Resistance Tester was used to obtain the BPNs of the asphalt samples. Table 8 shows the results of the Skid Resistance test. It has been established from the results that the asphalt modified with GNPs has a higher skid resistance in comparison to non-modified asphalt. The reason behind this is the nano-texture provided by the GNPs.


**Table 8.** The British Pendulum Numbers (BPNs).

The reduction in the skid resistance due to the polishing of the asphalt surface (caused by the continuous movement of the wheel) is also lesser for GNP-doped asphalt. In the case of the non-modified asphalt, the reduction in the skid resistance was around 47%. On the other hand, the reduction in the skid resistance in the case of 2% and 4% GNP-modified asphalt was 27% and 28%, respectively. This is due to an increase in the stiffness of the GNP-modified binder which improves the structural performance of highways [18]. An improved structural performance ensures the intactness of the macrotexture, resulting in a high skid resistance [10].

Typically, BPN values are obtained in the field after the pavement has been subjected to traffic but, in this case, they were obtained in a laboratory to compare the skid resistance of GNP-doped asphalt with that of conventional asphalt. It is pertinent to mention here that the focus of this investigation was

to determine the effect of the surface texture/roughness provided by the GNPs on the skid resistance of the asphalt.

#### **4. Conclusions**

In this study, Graphene Nano-Platelets (GNPs) were added to asphalt to modify its properties. Various tests were carried out to study rheology, moisture susceptibility, temperature susceptibility, bond strength, and skid resistance of the GNP-doped asphalt in comparison to conventional asphalt. Based on the results of the performance testing, the following conclusions have been drawn:


free or available asphalt binder, imparting structural strength to the asphalt [30]. A reduction in the moisture susceptibility indicates high durability of the asphalt.

• The bitumen bond strength test carried out using a PATTI shows that GNPs contribute to improving the adhesive and cohesive bonding of asphalt. The addition of GNPs to binder leads to an increase in the Pull-Off Tensile Strength of around 60%. This can be attributed to the hydrogen bonds and Van der Waals forces in nano-hybrid material [31].The addition of GNPs improves an important safety parameter, the skid resistance, and also increases the asphalt resistance against polishing. The inclusion of the GNPs decreased the percentage reduction of the skid resistance due to polishing from 47% to 27%. GNPs impart nanotexture to the asphalt, which contributes to enhancing the skid resistance.

The current research on the use of nanomaterials in asphalt mixtures is being carried out in two phases. The first phase involves the laboratory characterization of selected materials. The second phase includes the field investigation through the formation of test tracks in the field exposed to the actual environmental or traffic conditions. This paper presents the findings of the first phase only. The materials shortlisted during the first phase will be subjected to field investigation in the second phase. The handling and transportation of a stiff binder in the field will also be studied. Enhancing the skid resistance of asphalt using GNPs is a new concept and needs further exploration. This will also be carried out in the second phase.

**Author Contributions:** Conceptualization, N.A.; Data curation, M.H., J.R., and M.F.u.H.; Formal analysis, M.H., N.A., M.A.K., J.R., M.F.u.H., and J.; Investigation, M.H.; Methodology, M.H. and M.A.K.; Supervision, N.A. and M.A.K.; Writing—original draft, M.H.; Writing—review and editing, N.A., M.A.K., J., S.B.A.Z., and M.A.N. All of the authors approved and studied the final paper.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to acknowledge the support of the Civil Engineering department at the University of Engineering and Technology, Taxila.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **E**ff**ect of Chemical Composition of Bio- and Petroleum-Based Modifiers on Asphalt Binder Rheology**

#### **Punit Singhvi 1,\*, Javier J. García Mainieri 1, Hasan Ozer 2, Brajendra K. Sharma <sup>3</sup> and Imad L. Al-Qadi <sup>1</sup>**


Received: 4 March 2020; Accepted: 3 May 2020; Published: 7 May 2020

**Featured Application: This study provides guidelines for asphalt binder modifier selection to produce low modulus binders (softer binders) to meet desired quality. The research focuses on reducing expected cracking susceptibility of modified asphalt binders after long-term aging. The study recommends the development of engineered modifiers for specific paving applications.**

**Abstract:** In recent years, increased use of recycled asphalt materials (RAP) has created a need for softer binders to compensate stiffer binder coming from RAP. Economic alternatives, like recycled oils and proprietary bio-based oils, can be potential modifiers that will reduce the dependence on petroleum-based alternatives. However, there is limited information on the long-term rheological performance of binders modified with proprietary modifiers. These modifiers are chemically complex and their interaction with binders further complicates the binder chemistry. Therefore, the objective of this study was to evaluate the impact of modifier chemistry on modified binders' long-term cracking potential. A base binder of Superpave Performance Grade (PG) 64-22 was used to develop PG 58-28 binder using six different modifiers. An unmodified PG 58-28 was included for a comparative analysis. A few modified binders rheologically outperformed the base binder and others performed similarly. The modifier derived from recycled engine oil showed the worst performance. Chemical analysis indicated that the best performing modified binders had significant amounts of nitrogen in the form of amines. On the other hand, poor performing modified binder had traces of sulfur. Additionally, modifiers with lower average molecular weights appeared to have a positive impact on the performance of aged binders.

**Keywords:** Asphalt modification; modifier chemistry; long-term aging; asphalt rheology; phase angle; delta Tc

#### **1. Introduction**

Asphalt concrete (AC) is one of the most commonly used pavement materials in the United States and worldwide. More than 400 million tons of AC are produced in the US annually, which requires 20 million tons of asphalt binder [1]. AC mixture is a heterogeneous composite of asphalt binder, mineral aggregates, and air voids. The performance of AC mixtures is greatly affected by aggregate characteristics, binder chemistry and rheology, mixture volumetrics, and aging. Aging of AC is a

continuous process; it is dominated by volatilization and oxidation of asphalt binder in the short- and long-term, respectively. Volatilization in binders refers to loss of lighter fractions when exposed to high temperatures and occurs during the production and construction stages. Oxidation in binders is caused by photo-oxidation and thermal oxidation during pavement's service-life [2]. Aging increases AC brittleness that may result in cracking. Hence, binders with superior aging resistance characteristics may delay AC cracking and increase pavement service life.

Asphalt binder (AB) is an important component for the construction of AC pavements. It is currently produced from the fractional distillation of crude petroleum at refineries. In recent years, the increasing use of harder and aged recycled asphalt materials in pavement applications has significantly increased the need for softer ABs [1]. Logistical limitations and the cost of refineries to produce softer ABs instead of products of higher financial value, result in a shortage of soft straight-run or unmodified AB in the market [3].

To overcome the current demand for softer ABs, traditional petroleum-based "softeners", like AB flux and aromatic oils are blended with straight-run AB [4]. Blending "softeners" with readily available products, like recycled oils and bio-based oils, provides an opportunity to manufacture required AB economically and reduce the dependence on petroleum-based products.

A variety of proprietary products are available to modify AB to achieve softer grades. They are used with limited knowledge about their long-term performance. Poor durability and/or extended cracking issues have been identified when re-refined engine oil bottoms (ReOB) and waste engine oils (WEO) have been blended in AB [5–9]. On the other hand, use of certain bio-based oils in ABs increases the oxidation potential which makes pavements vulnerable to cracking over the long-term [10]. This limits or restricts the use of these products. However, there are some specialized bio-based softeners that have shown enhanced long-term performance of ABs and reduced cracking potential [11]. Therefore, the potential of bio-based modifiers to produce softer PGs is investigated in this study. A ReOB-based modifier was included in this study for comparison purposes.

Superpave Performance Grading (PG) is a rheology-based system, currently used to trade and specify AB in the United States. The PG system has limitations in identifying long-term performance of modified asphalt binder (MAB). The use of bio-based modifiers can significantly impact AB chemistry and rheology without changing its Superpave grading. The current challenges for the market-entry of MAB are: (i) their complex chemistry, (ii) their uncertain long-term rheological performance, and (iii) the lack of a robust grading system that can discriminate them to ensure long-term performance.

Low-temperature performance of AB is critical to prevent adverse effects of thermal cracking. Use of recycled asphalt pavements (RAP)—obtained from milling of old asphalt layers—and recycled asphalt shingles (RAS)—typically obtained from either tear-off shingles or manufactured waste shingles—has increased in AC pavement and requires adequately-performing binders to avoid premature cracking [12–18]. The ΔTc parameter has been used to assess the low-temperature induced cracking performance of AB [19,20]. Low-temperature ductility of AB has been related to pavement cracking performance [21]. Glover et al. [22] developed a rheological parameter based on dynamic shear rheometer (DSR) frequency sweep tests, that strongly correlates to low-temperature ductility; a simplification of this parameter is known as the Glover–Rowe (GR) parameter [23]. Literature provides ΔTc and GR parameters as good low-temperature cracking indices [19]. Therefore, frequency sweeps and bending beam rheometer (BBR) tests were considered in this study to obtain them and evaluate asphalt binder's expected cracking performance.

Limitations of using Superpave's |G\*|sinδ to assess intermediate-temperature cracking susceptibility of asphalt binders are well reported [3,24]. Black space diagrams may provide insights to the rheological properties that drive cracking susceptibility at intermediate temperature.

Asphalt cracking is further aggravated with aging. Field aging depends on geographical location and environmental factors and varies along the pavement depth. The current pressure aging vessel (PAV) aging for 20 h at 90, 100, or 110 ◦C and a pressure of 2.1 MPa is not sufficient to represent realistic long-term aging of binders [22,25]. Meanwhile, researchers are investigating 40-h PAV as an alternate solution [26]. Aging up to 60-h PAV has also been used to evaluate AB long-term performance [27]. Laboratory aging conditions of 20-h, 40-h, and 60-h PAV were investigated in this study.

Chemical composition of AB plays an important role in durability of asphalt pavements [28]. Modifiers' chemical composition can affect AB compatibility and susceptibility to oxidation, changing its rheological properties and long-term performance [3,27,29,30]. Chemical characterization of ABs and MABs was conducted, in many studies, using elemental analysis, gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FTIR), and thin-layer chromatography flame ionization detection (TLC-FID). Carbonyl and sulfoxide indices from FTIR have been widely used to track oxidation and evaluate the impacts of long-term aging on binder characteristics [31–35]. Molecular weight tends to increase with aging in binders and has been reported by several authors [30,33,36–38]. Asphalt binders are composed of maltenes and asphaltenes and their interaction with aging drives the mechanical properties of the binder [28,30,38–41].

There is a need for understanding the impact of various modifiers on ABs and their resulting long-term field performance. Simple laboratory protocols with reasonable and consistent predictive capabilities of field performance are essential to optimize modifier selection and dosage. Therefore, the objective of this study was to evaluate modifiers' chemical characteristics and their impact on MAB's long-term cracking potential. The fundamental relationship between chemical composition of modifiers and its effect on MAB's rheological properties was investigated.

#### **2. Materials and Methods**

#### *2.1. Materials*

Various sources of bio-based products were selected to produce "softer" MABs with equivalent or better long-term performance as the binder obtained from crude oil sources. Softer binders are defined as binders with lower modulus and lower PG. Such binders are commonly used to control cracking in colder climates or neutralize RAP's relatively stiff binder. PG 58-28 was the target binder grade to be produced using commonly available PG 64-22 as base binder, referred to as S1. The base binder was selected and sampled from a refinery terminal in Illinois, USA. Five bio-based AB modifiers and one ReOB modifier available in the US industry were procured for modification. The names of the modifiers used in this study are kept confidential and are designated as shown in Table 1 from hereon. Table 1 lists the modifier type as provided by the suppliers. MABs were labelled with base AB's designation and modifier's designation followed by dosage (% by weight) of modifier in the blend, e.g., S1-A-3.5 represents MAB obtained from the base binder S1 blended with 3.5% (% by weight) of modifier "A". A softer unmodified binder, PG 58-28 (labelled as S5), was also included in the study as a benchmark for modified binders.


**Table 1.** Modifiers used in this study and their classification.

<sup>1</sup> as reported by suppliers (from US); <sup>2</sup> not provided by the suppliers.

#### *2.2. Asphalt Binder Modification*

Modifier blending methodology, binder heating cycles, temperatures applied during splitting process, and storage are discussed in this section. A high shear mixer (Cafarmo BDC1850) with a Heidolph PR31 ringed propeller (33-mm diameter fan) was used for blending. The blending was performed at a steady temperature of 130 ± 10 ◦C. The temperatures were maintained using a Glas-Col LLC heating mantle capable of handling 1-L capacity aluminum can. The methodology is illustrated in Figure 1.

**Figure 1.** Modifier blending approach (**a**) labelled cans of base binder divided in five sets; (**b**) 3.8 L cans split into 6–1.0 L cans; (**c**) shear mixer for blending and modifier addition; (**d**) modified binder split into 4–240 mL cans; and (**e**) 3–240 mL cans from different sets combined to produce materials for all aging conditions.

The following were the steps for modifying binders used for chemical and rheological characterization:


Aging conditions used in the study were: Unaged (UA), Rolling Thin-Film Oven (RTFO) in accordance with American Association of State Highway and Transportation Officials (AASHTO) specification T240-13, 20-h Pressure Aging Vessel (PAV) including vacuum degassing and in accordance with AASHTO specification T28-12, 2PAV and 3PAV. 2PAV and 3PAV conditions were obtained by running continuous 40-h and 60-h PAV cycles, respectively. Once samples reached their required aging conditions, they were stored in small 30 mL cans until tested to avoid multiple heating cycles. To avoid changes in chemical and rheological properties, cans once heated for testing were not re-used. Same treatment was given to the unmodified base binder S1 and unmodified reference binder S5.

#### *2.3. Modifier Chemistry Experimental Program*

#### 2.3.1. Elemental Analysis

Elemental analyses of modifiers were conducted in an Exeter Analytical (Chelmsford, MA, USA) CE-440 elemental analyzer. The proportions of carbon, hydrogen, nitrogen, and sulfur (CHNS) elements in the material's composition were expressed in percent. The proportion of oxygen (O) was obtained by subtracting CHNS percentage from 100.

#### 2.3.2. Chemical Functional Groups

Thermo Nicolet Nexus 670 FTIR spectrometer was used to detect the chemical functional groups present in modifiers in the range of wavenumbers 600–4000 cm<sup>−</sup>1. Data was collected at a resolution of 4 cm−<sup>1</sup> with number of scans set to 128. Three replicates were tested for each modifier. The method was based on attenuated total reflection (ATR).

#### 2.3.3. Molecular Weight Analysis

The molecular weight analysis was conducted using GPC. The system consists of a Waters 2695 separation module connected to two Styragel HR1 SEC columns (7.8 mm × 300 mm) in series followed by a Waters 2414 RI detector and a computer with Empower Pro and data acquisition software. Samples of 3% *w*/*w* were prepared in tetrahydrofuran (THF), a carrier solvent with a flow rate of 1.0 mL/min and an injection volume of 20 μL; they were filtered using a 0.45 μm millipore polytetrafluoroethylene (PTFE) syringe filter to remove suspended particulates. To detect analytes, a constant flow of fresh eluent was supplied to the column via a pump.

The resulting chromatographic data was processed for number-average molecular weight (*Mn*), weight-average molecular weight (*Mw*), and polydispersity index (PDI) using Equations (1)–(3), respectively. The molecular weights were calculated based on the component molecular weights (*Mi*) determined from the retention time calibration curve and signal intensities (*Ni*).

$$M\_{\rm II} = \frac{\sum M\_i N\_i}{\sum N\_i} \tag{1}$$

$$M\_{\rm w} = \frac{\sum M\_i^2 N\_i}{\sum M\_i N\_i} \tag{2}$$

$$PDI = \frac{M\_w}{M\_n} \tag{3}$$

The retention time calibration curve was developed by fitting log-scale molecular weights to their retention time for standard material with known molecular weights using a 3-degree polynomial. The fitted curve was then used to measure the molecular weights of unknown modifiers using the chromatographic data. The distributions with shorter retention times correspond to larger molecular size whereas longer retention times represent smaller sizes. The molecular weights are reported in Daltons.

#### 2.3.4. Binder Fractionation

Two percent (weight by volume) solutions of the modifiers were prepared in dichloromethane and filtered through a 0.45 μm millipore PTFE syringe filter to remove insoluble suspended particles from the solution. The suspended particles are referred to as residue from hereon. The sample solution (1 μL) was spotted on chromrods coated with a thin film of silica gel, using a microsyringe. The separation of bitumen into four generic fractions: saturates, aromatics, resins, and asphaltenes (SARA) was performed in a three-stage development process using n-heptane, toluene, and THF. The chromrods were dried for 10 min and humidified in NaNO2 for 10 min between each development. The chromrods

were scanned with an Iatroscan MK-5 analyzer (Iatron Laboratories Inc., Tokyo, Japan) with a flame ionization detection (FID), which provided chromatograms with peaks for SARA composition.

One of the used modifiers (G) was insoluble in dichloromethane. However, the modifier dissolved in water, acetone and methanol and hence the sample was prepared in methanol for performing the test.

#### *2.4. Binder Rheology Experimental Program*

Rheological characterization of binders included determining Superpave performance grade, ΔTc, and frequency sweep test parameters using a dynamic shear rheometer (DSR). Two replicates were tested for each rheological test and an average value was reported. AASHTO-intra-laboratory precision limits were obeyed. Specifically, in the case of frequency sweeps, the coefficient of variation was limited to 7% on any complex modulus measurement. All DSR measurements were performed in a Kinexus KNX2712 equipment with an active hood for temperature control while the BBR measurements were performed on a Cannon instrument.

The modifier dosage was selected to achieve similar PG for all modified binders in this study. Superpave system of grading is the current method of selecting binders in the US. Even though the binders tested for this study (except the base binder and one of the modified binders) would be assigned the same grading by this system, their chemistry could affect their rheological characteristics and susceptibility to oxidation; hence, it may affect their long-term performance. Given that the same base binder (S1) was used, the differences reported in the study's experimental program were caused by the modifiers.

#### 2.4.1. Superpave Performance Grading (PG)

All the binders used in the study were tested for Superpave PG. The tests were performed in accordance with AASHTO specifications: T315-19 (DSR) and T313-19 (BBR). Continuous PGs (true grades) were also determined.

#### 2.4.2. ΔTc Parameter

BBR measurements for PAV-aged samples were obtained for PG. Additionally, BBR measurements were recorded for all samples at 2PAV-aged and 3PAV-aged conditions in accordance with AASHTO T313-19 specification.

Δ*Tc* was computed using the following Equation (4):

$$
\Delta T\_{\mathcal{E}} = \mathcal{P} \mathcal{G}\_{\mathcal{S}diffuses} - \mathcal{P} \mathcal{G}\_{m-value} \tag{4}
$$

where *PGSti*ff*ness* and *PGm-value* are the temperatures at which samples pass PG criteria for stiffness and slope of stiffness curve (relaxation), respectively. *PGm-value* at all aging conditions and *PGSti*ff*ness* value at PAV condition (except for K-modified binder) were interpolated as stated in [20]. *PGSti*ff*ness* value at 2PAV and 3PAV conditions, and K-modified binder at all aging conditions, were extrapolated using the same equation.

#### 2.4.3. Frequency Sweep Test

Frequency sweeps were performed after short conditioning (10 cycles of 0.1% strain at 15 ◦C and a frequency of 0.5–0.61 rad/s). Complex shear modulus (G\*) and phase angle (δ) were obtained at all frequencies for all samples at UA, RTFO, PAV, 2PAV, and 3PAV. Data was measured in isotherms of 15 ◦C, 25 ◦C and 35 ◦C. An additional 5 ◦C isotherm was included for UA and RFTO-aged samples to ensure crossover (when phase angle δ is 45 degrees) data were measured. Eighteen data points were collected per isotherm in the frequency range from 1.00 rad/s to 62.83 rad/s at constant shear strain of 1.6% for UA, 1.2% for RTFO and PAV-aged samples, and 1.0% for 2PAV and 3PAV samples.

These strains were selected to ensure measurements within linear viscoelastic range (LVER) of the samples. Harmonic distortion between strain excitations and stress responses was lower than 1% for all measurements. In addition, measured torque was in the operational range of equipment for all sweeps performed. Additionally, a built-in sequence to verify whether measurements for each sample were taken in the LVER was incorporated before the isotherms.

Data from different isotherms were then manually shifted to match G\* at 15 ◦C (reference temperature) to create master curves. Polynomial fitting was performed to obtain the presented black space diagrams.

#### Glover–Rowe Parameter (GR)

*GR* parameter was computed using Equation (5):

$$GR = \frac{G^\* \left(\cos \delta\right)^2}{\sin \delta} \tag{5}$$

*G*\* and δ are the complex shear modulus and phase angle at 15 ◦C and 0.005 rad/s. *G*\* and δ at 15 ◦C and 0.005 rad/s were obtained from measured data. In some cases, the data were extrapolated using polynomial-fits from master curves and black space diagrams.

#### **3. Results**

This section presents the results from chemical compositional testing of the modifiers and rheological testing of the modified and base binders.

#### *3.1. Chemical Characterization of Modifiers*

#### 3.1.1. Elemental Analysis

The results from elemental analysis are presented in Table 2. Modifiers A, D, and E show similar elemental composition of carbon, hydrogen, nitrogen and oxygen with additional sulfur in E (0.33%). Modifier K has slightly higher carbon (79.7%) and hydrogen (12.7%) content but is lower in oxygen (5.7%) compared to A, D, and E. Modifier K also has higher sulfur (0.98%) content than E. Modifier C has relatively higher nitrogen (3.5%) and oxygen (14.9%) and is low on Carbon (70.1%) in comparison to A, D, E, and K. Modifier G possesses very different composition compared to all other modifiers. Its elemental oxygen (33.1%) and nitrogen (9.0%) contents are the highest while the carbon (47.9%) is the lowest compared to other modifiers.


**Table 2.** Elemental analysis of modifiers.

#### 3.1.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 2a shows the full FTIR spectra for all modifiers, from which chemical functional groups present in the modifiers can be identified. The majority of the absorbance peaks were observed in wavenumbers ranging from 1000 to 1800 cm−<sup>1</sup> (Figure 2b) and 2700 to 3200 cm−<sup>1</sup> (Figure 2c).

**Figure 2.** FTIR spectra for modifiers from (**a**) 600–4000 cm<sup>−</sup>1; (**b**) 1000–1800 cm<sup>−</sup>1; and (**c**) 2700–3200 cm<sup>−</sup>1.

Following are the observations from the FTIR spectra shown in Figure 2:


#### 3.1.3. Gel Permeation Chromatography

The average molecular weights (Mn and Mw) for modifiers A, C, D, and E ranged from 3700 to 4700, and for G it is around 2500 with polydispersity index (PDI) in a range of 1.03–1.13 as shown in Table 3. Interestingly, modifier K has a high Mn of 8933 and significantly higher Mw of 48,784 which is the reason for high PDI of 5.46. The plot showing retention times (Figure 3) suggests that even though modifiers A, C, D, and E have similar range of molecular weights, Mn and Mw (Table 3), their molecular size distribution varies. Following are the observations from GPC analysis as per Figure 3:



**Table 3.** Number-average molecular weight, weight-average molecular weight, and polydispersity index of modifiers.

**Figure 3.** Signal intensity vs. retention time for molecular size determination of modifiers.

#### 3.1.4. Thin Layer Chromatography Flame Ionization Detection (TLC-FID)

Table 4 shows the SARA composition and the unfiltered residue for different modifiers. Following are the observations from SARA analysis:



**Table 4.** Percentage composition of saturates, aromatics, resins, asphaltenes and residue for modifiers.

Note: Results for modifier G cannot be determined using TLC-FID.

Modifier G possesses unique characteristics which are different from other modifiers. It is a water-soluble modifier and did not dissolve in the solvent used for other modifiers. Modifier G is also toluene insoluble which suggests there is no presence of asphaltic materials. Hence, the SARA approach seems inappropriate to characterize the chemical composition of modifier G. Furthermore, solubility of G in water can have moisture durability issues in AC designed from this modified binder which is not in the scope of this study. Therefore, AC properties to evaluate the effect of moisture should be investigated.

#### *3.2. Rheological Characterization of Modified Binders*

#### 3.2.1. Superpave Grading

Base binder, S1 (PG 64-22), was modified to PG 58-28 using modifiers provided in Table 5. All the modifiers were able to convert the base binder to the acceptable limits of PG 58-28 except modifier K (ReOB). Figure 4a shows the continuous PG for base binder, unmodified PG 58-28 (S5), and modified binders. The selection of modifier dosage was based on (i) achieving similar high temperature true-grades (0.6 ◦C standard deviation) and (ii) obtaining similar true-grade results to S5 s (±1.2 ◦C).

**Table 5.** Continuous and Superpave Performance Grade (PG) of base binder, unmodified binder and modified binders.


**Figure 4.** (**a**) Continuous PG of binders; for modifier K, effect of dosage on (**b**) High-temperature PG; (**c**) intermediate-temperature PG; and (**d**) Low-temperature PGStiffness and PGm-value.

For modifier K, none of the dosages from 6% to 12% were able to produce a PG 58-28 binder. It was observed that increase in the addition of modifier K had a softening effect on high and intermediate temperature grades (Figure 4b,c). In case of low temperature PG, the stiffness was reduced with increasing modifier dosage whereas m-value was not affected (as shown in Figure 4d). Moreover, the low temperature PG for K was controlled by the m-value which failed to meet required criteria. The 10% dosage for K was chosen to obtain the continuous PG closest to PG 58-28.

#### 3.2.2. Delta Tc (ΔTc)

ΔTc for PAV, 2PAV, and 3PAV conditions was computed for all binders (Figure 5). The modification to S1 increased the ΔTc values, at all aging conditions, for all modified binders except when modifier K (ReOB) was used. A higher, or less negative, ΔTc can be related to better resistance to low-temperature cracking. Departments of Transportation in Maryland, Kansas, Pennsylvania, New York, New Jersey, Delaware, and Vermont restrict ΔTc to be greater than −5 ◦C after 40-h PAV conditioning while some other states use a limit for ΔTc after 20-h PAV [20].

**Figure 5.** ΔTc values of different modified binders, base binder and unmodified binder for aging conditions (**a**) pressure aging vessel (PAV); (**b**) 2PAV; and (**c**) 3PAV.

On comparing unmodified binder S5 to G and C modified binders, the later ones showed similar ΔTc in PAV aging condition. However, with prolonged aging (2PAV and 3PAV), G-modified binder outperformed S5, followed by C-modified binder. MABs with modifiers A, D, E, and K had lower ΔTc than S5. It is widely accepted that the presence of ReOB increases low-temperature cracking susceptibility of AB [5–9], which is also observed in this study. K-modified binder has the lowest ΔTc at all aging conditions.

It is important to note that with aging, the effect on ΔTc values are predominantly driven by the m-value criterion. In all cases, ΔTc becomes more negative with aging, indicating more pronounced loss of relaxation (reduction of m-value) than stiffening of the material. Stiffnesses were not as greatly affected by aging.

The ΔTc parameter suggests that modified binders may have similar long-term cracking resistance with the unmodified S5. On the other hand, K-modified binder (ReOB) has the lowest ΔTc, which is well beyond the acceptable thresholds suggested in the literature [20].

#### 3.2.3. Frequency Sweep Test

In this section, complex modulus master curves, black space diagrams, and rheological parameter, GR, are presented for, RTFO, PAV, 2PAV, and 3PAV aging conditions.

#### Complex Shear Modulus Master Curves

Complex modulus master curves were determined at a reference temperature of 15 ◦C. Figure 6a–d show the progression of complex modulus at different aging conditions, for limited low reduced-frequency range. This range was selected to represent low-temperature (non-load associated) cracking conditions. Following are the observations from Figure 6.

**Figure 6.** Complex shear modulus curves at reference temperature of 15 ◦C for aging conditions (**a**) Rolling Thin-Film Oven (RTFO); (**b**) PAV; (**c**) 2PAV; and (**d**) 3PAV.


#### Black Space Diagram

Black space diagrams were plotted for RTFO, PAV, 2PAV, and 3PAV aging conditions. A black space diagram shows the variation of complex shear modulus (G\*) with phase angle (δ). A reference temperature of 15 ◦C was used to obtain these curves. A higher δ at a fixed temperature and G\* represents a material more prone to flow in a viscous manner and a lower δ represents a more elastic behavior [42]. Materials with higher δ are less likely to crack in a brittle way. In other words, a lower δ indicates that more energy would be stored, and faster accumulation of stress would be observed during repeated deformation [3]. A good correlation has been demonstrated between isothermal phase angle (especially at 50 ◦C) and test-pavement cracking severity [43]. Phase angle (at G\* = 8967 kPa) has

been found to be more repeatable measurement than G\* and capable of identifying phase-incompatible asphalt binders [3].

Figure 7a–d show complete black space diagrams for specific aging conditions, whereas the inset figure highlights the effect on δ for G\* ranging from 10 MPa to 11 MPa. The motivation for the selection of this G\* range was the intermediate-temperature PG criteria [3]. However, a different range of G\* selection does not affect the trends in δ and could represent other loading conditions in the field. Some observations from the black space diagrams are the following:


**Figure 7.** Black space diagrams at reference temperature of 15 ◦C for aging conditions (**a**) RTFO; (**b**) PAV; (**c**) 2PAV; and (**d**) 3PAV.

#### Glover–Rowe (GR) Parameter

GR values are indicated in the black space diagrams, shown in Figure 8, for all aging conditions. Modified binders containing A, D, and E show similar characteristics to unmodified binder (S5) at all aging conditions (Figure 8a). Differently, in case of modifier C, higher resistance to low-temperature cracking was observed.

**Figure 8.** Black space diagrams showing G\* vs phase angle (δ) at reference temperature of 15 ◦C and reduced frequency of 0.005 rad/s for all aging conditions for (**a**) modifiers A, C, D, and E compared to unmodified binder S5; and (**b**) modifiers G and K compared to unmodified binder S5.

A GR of 180 kPa is a criterion for damage onset for non-load-associated low-temperature cracking (shown in the red-dotted line in Figure 8), while 450 kPa is indicative for severe cracking (shown in the red-solid line in Figure 8) [23]. As per these thresholds, damage onset initiates around 2PAV while severe damage takes place at around 3PAV condition for S5 and binders containing modifiers A, D, and E.

In Figure 8b, for G-modified binder the evolution of GR parameter suggests higher resistance to aging compared to S5. The GR value for 3PAV of G is similar to that of 2PAV for S5. In contrast, GR for K-modified binder increases at a much faster rate than S5, suggesting potential early damage. The GR values of PAV and 2PAV for K are similar to 2PAV and 3PAV of S5, respectively. Figure 8a,b show that G-modified binder does not reach onset of severe cracking criterion after 3PAV while K-modified binder surpasses the criterion after 2PAV.

#### **4. Discussion**

#### *4.1. Summary of Modifier Chemistry*

The Elemental analysis showed that modifiers G and C had higher nitrogen content (9% and 3.5%, respectively), while E and K had some sulfur (0.33% and 0.98%, respectively) compared to other modifiers. Modifier G has significantly low carbon (47.9%) and high oxygen (33.1%) content.

FTIR spectra further validated the presence of nitrogen in modifiers C and G with peaks in the ranges of 1000–1250 cm−<sup>1</sup> and 3100–3500 cm−<sup>1</sup> which are characteristic of C-N stretching and N-H stretching (from secondary amines), respectively. A distinct peak at 2860 cm−<sup>1</sup> and a small peak at 1650 cm−<sup>1</sup> further validate the presence of nitrogen, as these peaks are representative of amine salt and amides, respectively. Modifiers A, C, D, and E show the presence of carbonyl functionality, which was observed from the carbonyl peaks at 1742 cm<sup>−</sup>1.

Molecular weight analysis showed that modifiers A, C, D, and E have average molecular weights in a similar range, modifier G has the lowest weight and K has the highest. In addition, modifier K possesses significantly high PDI indicating a wide variation of molecular species presence. Modifiers, however, have different molecular size distributions, with some modifiers having multiple peaks (A, C, D, and K) and hence, are composed of distinct molecules. On the other hand, others (E and G) had single peak which means they are composed of single molecular size.

Modifier G has a distinct chemical composition compared to other modifiers. It is significantly high on nitrogen and oxygen and relatively low on carbon compared to others. The presence of nitrogen was validated by the FTIR spectra. Peaks corresponding to amine salt and primary and secondary amines were observed. It was found that the molecular weight of G was the lowest single peak distribution. Additionally, modifier C showed relatively higher content of elemental nitrogen in the form of secondary amines which was verified by the FTIR spectrum.

SARA analysis of G was inconclusive and additional characterization with a different method is required to evaluate the chemical characteristics of modifier G. Moreover, its solubility in water requires additional investigation of the AC's susceptibility to moisture. Modifiers C and E are largely composed of resins and asphaltenes with limited or no saturates and aromatics. While A and D have some aromatics and asphaltenes along with a majority of resins. Modifier K has large amount of insoluble residue with high saturate content and limited resins with traces of aromatics.

Furthermore, modifier A shows similar chemical functional groups, molecular weight and molecular weight distribution to modifier D, which indicates that modifier A may belong to bio-oil category.

#### *4.2. Summary of Modified Asphalt Binder Rheology*

Modifiers' dosages were selected to meet PG 58-28 with true grades close to each other for all MABs to ensure reasonable comparison of rheological properties, except for binder modified with K (ReOB). It was observed that increasing the dosage of K increased high PG, decreased intermediate PG, and decreased low PG stiffness. However, there was no improvement in the relaxation properties of the modified binder with increasing amount of modifier K in the blend. Therefore, dosage for modifier K resulting in a continuous PG closest to PG 58-28 was selected for further investigation. The selected dosage varied from one modifier to the other. This might be one of the reasons for the observed differences in rheology. For instance, increasing the dosage of modifier C can result in a similar performance as that of AB modified with modifier G; but would result in a different Superpave continuous-PG. The focus of the study was to compare binders with similar Superpave characteristics. Dosage variation/optimization is not within the scope of this study, but appears to be a promising research path.

ΔTc was determined to evaluate the low temperature cracking susceptibility of modified binders for PAV, 2PAV, and 3PAV aging conditions. Relatively high ΔTc indicates better relaxation properties at low temperatures, which results in better resistance to cracking. Modification of S1 improved the ΔTc for all MABs except K. Significant improvements were observed when modifiers G and C were used, which even showed better relaxation properties than unmodified binder (S5). G-modified binder has the highest ΔTc for 2PAV and 3PAV conditions, followed by binder modified with C. Differently, K-modified binder has the lowest ΔTc values in all aging conditions. MABs containing A, D, and E have ΔTc values close to S5 only after 2PAV and 3PAV aging conditions.

Complex shear modulus master curves show that modulus consistently increased for all aged binders. K-modified binder showed distinctly stiff behavior at PAV that was also observed in 2PAV and 3PAV conditions. Other MABs stiffness trends shifted with aging conditions. At UA condition, G-modified binder was the stiffest and after 2PAV and 3PAV, it was the softest binder; which is desired. However, mechanisms of change in modulus for G-modified binder after aging need to be investigated. Modifier G, as discussed before, has distinctive characteristics and needs to be explored with additional testing. Aging after 2PAV and 3PAV, other modifiers have master curves closer to unmodified binder (S5).

The black space diagram was used to evaluate the impact of aging on phase angle (δ). The δ for selected range of G\* shows similar trends after PAV, 2PAV and 3PAV aging. The differences in δ of MABs become more distinct with aging but are always noticeable, which makes δ at a certain G\* a useful parameter to distinguish MABs. Note that this might not be the case when polymers are in the blend [44]. As mentioned earlier, a higher δ at a certain G\* indicates that the material is less prone to cracking in a brittle way at service conditions. Again, MAB containing G has the highest phase angles, followed by C, S5, E, D, and A, while K has the lowest. In addition, the evolution of GR parameter also suggests that G-modified binder is the most resistant to aging while K is the least.

Based on rheological testing, G-modified binder is least susceptible to cracking followed by C-modified binder whereas K-modified binder is the most susceptible. MABs modified with A, D and E show similar rheological characteristics to S5.

#### *4.3. Relationship between Modifier Chemistry and Binder Rheology*

Modifier's chemical make-up contributed significantly to the long-term rheological response of MABs. Nitrogen-based compounds are known for their antioxidant properties [45]. The presence of higher nitrogen content was validated with elemental analyses and FTIR spectra and its impact was observed in the change of frequency sweep measurements as aging progressed. Modifier C, containing 3.5% nitrogen, shows similar or better crack resisting properties than the unmodified product (S5) at PAV, 2PAV, and 3PAV conditions. Superior rheological properties of G-modified binder can be attributed to the presence of high nitrogen content (9.0% in modifier G), which is composed of nitrogen-based compounds like amines. This validates the impact of antioxidants on resisting binder aging, and hence, reducing cracking susceptibility.

On the other hand, sulfur presence in modifiers E (0.33%) and K (0.98%) is accompanied by lower expected performance based on the reported rheological parameters. Excessive sulfur content (>4%) in binders can cause increased oxidation due to the formation of additional sulfoxides causing embrittlement in binders [46].

Lower molecular weight of modifiers could be promoting phase compatibility. Rheological test results and Mw distinguish three groups: MABs containing A, D, and E have similar characteristics, MAB containing K has lower expected performance and highest Mw, and MABs containing C and G have higher expected performance and lower Mw.

The rheological parameters: ΔTc, GR, and phase angle from black space diagram have consistent trends among all modifiers and are able to distinguish MABs based on their expected cracking performance.

#### **5. Summary and Findings**

The focus of this paper is to evaluate the impact of modifiers' chemical properties on the rheological properties of respective modified binders. Binders blended with various types of modifiers, intended to soften (reduce) the grade of an unmodified binder, were tested at various aging conditions (unaged, RTFO, PAV, 2PAV, and 3PAV). Performance progression indicators were used to predict their long-term performance. Low-temperature cracking susceptibility was assessed using GR and ΔTc, and intermediate-temperature cracking susceptibility was assessed using black space diagrams. Chemical characteristics of modifiers were evaluated using elemental analysis, FTIR, GPC, and TLC-FID. The results show that modifier chemistry impacts modified binder performance. The presence of certain elements, chemical functional groups and molecular size can affect the rheological properties of the binder. Following are the findings of this study:


In conclusion, modifier chemistry was shown to have a relationship with rheological behavior of modified binders. The experimental program presented in this paper can be used to choose modifiers that may control cracking development and could also be used as guidance to engineer asphalt binder modifiers.

**Author Contributions:** All authors have read and agree to the published version of the manuscript. Conceptualization, B.K.S., H.O. and I.L.A.-Q.; Methodology, B.K.S., H.O. and I.L.A.-Q.; Validation, P.S. and J.J.G.M.; Formal analysis, P.S. and J.J.G.M.; Investigation, B.K.S., H.O. and I.L.A.-Q., J.J.G.M. and P.S.; Resources, I.L.A.-Q., H.O. and B.K.S.; Data curation, P.S. and J.J.G.M.; Writing—original draft preparation, P.S. and J.J.G.M.; Writing—Review and editing, I.L.A.-Q., B.K.S., H.O.; Visualization, P.S. and J.J.G.M.; Supervision, B.K.S., H.O. and I.L.A.-Q.; Project administration, B.K.S., H.O., and I.L.A.-Q.; Funding acquisition, B.K.S., H.O. and I.L.A.-Q.

**Funding:** This research was funded by the Illinois Department of Transportation, under the project number ICT R27-196 HS.

**Acknowledgments:** This publication is based on the results of "ICT-R27-196 HS: Rheology-Chemical Based Procedure to Evaluate Additives/Modifiers used in Asphalt Binders for Performance Enhancements (Phase 2)." ICT-R27-196 HS is currently conducted in cooperation with the Illinois Center for Transportation (ICT); Illinois Sustainability Technology Center (ISTC); the Illinois Department of Transportation (IDOT); and the U.S. Department of Transportation, Federal Highway Administration. Special thanks to ICT and ISTC students and research staff, Kirtika Kohli, Uthman Mohammed Ali, Greg Renshaw, and Marc Killion, for their input and support during this study. The contributions of the technical review committee are acknowledged; special thanks to Kelly Morse, Jim Trepanier, Ronald Price, Clay Snyder and Brian Hill. We also thank binder and modifiers suppliers for providing the requested quantities of the samples. The contents of this paper reflect the view of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of ICT, ISTC or IDOT. This paper does not constitute a standard, specification, or regulation.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


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