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,
3,
4,
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.
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:
In comparison to other nanomaterials, Graphene Nano-Platelets are easy to disperse. Unlike GNPs, other nanoparticles (i.e., CBNPs and CNTs) either require high shear mixing or solvent-based dispersion. The usage of smaller percentages of GNPs produces more pronounced results than with other nanoparticles [
31]. GNPs save the cost of solvent and high shear mixing, making it possible for pavement engineers to use GNPs for asphalt modification on a larger scale.
As per the results of conventional testing, GNPs have the potential to reduce the penetration value of asphalt binder by up to 48% and increase its softening point by up to 19%. The enhancement in these properties can be attributed to the small size and large surface area of GNPs, which aids in stronger bonding. The lower softening point values of the locally produced non-modified binders make them susceptible to rutting during the summers. Upon the addition of 4% of GNPs, the softening point increases to 57 °C, which is desirable due to the high temperature in Pakistan.
According to the result of the storage stability test, GNP-modified asphalt binder is stable and can be stored for longer periods without the settling down of the nanoparticles. This makes it suitable for use in the pavement industry.
Upon the addition of 4% of GNPs, PG 70 is achieved, which caters to the environmental conditions of Pakistan [
13].
The study of the rheology of the binder shows a significant increase in the stiffness properties of the GNP-modified binder. A maximum increase in the complex shear modulus and a decrease in the phase angle is recorded when 4% of GNPs are added to the binder. The Superpave rutting factor also increases for the GNP-doped asphalt binder, suggesting a better performance at a high temperature. The elastic nature of the GNPs contributes to this improvement.
The optimum binder content (OBC) increases with an increase in the content of the GNPs. This trend shows that the initial cost of GNP-modified asphalt would be slightly more than conventional asphalt. But, by observing the modified asphalt performance in the results, we can conclude that doping asphalt with GNPs would reduce the life cycle cost of the pavements. This is due to the fact that pavements in Pakistan fail prematurely. A highway section designed for 20 years sometimes fails within the first two to three years of service because of excessive deformations during summers. GNP-doped asphalt pavements would require far less frequent maintenance or reconstruction cycles, thus positively affecting the life cycle costs.
The addition of GNPs significantly reduces the temperature susceptibility and increases the resistance to permanent deformation. GNP-modified asphalt shows around a 35% reduction in the rut depth at a high temperature. At 55 °C, the wheel-tracking slope (WTS) decreased from 0.58 to 0.35 when 4% of GNPs were added to the asphalt binder. This reduction in the WTS is also suggestive of high resistance to permanent deformation.
GNPs act as supporting material in asphalt and give it strength, similar to a steel reinforcement in concrete. This leads to an increase in the dynamic modulus of the asphalt. The dynamic modulus of GNP-modified asphalt is around 21% higher than that of non-modified asphalt, which represents a greater resistance to permanent deformation.
GNPs significantly reduce the moisture susceptibility of asphalt. The addition of 2% and 4% of GNPs to the binder increases the percentage of bitumen coverage from 15% to 60% and 70%, respectively. This could be due to the high surface area of the GNPs, which lets them absorb the 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.