1. Literature Review
Asphalt concrete (AC) has two primary components, namely, aggregates and asphalt binder. Aggregates on average range from 94 to 96% of the mixture which implies that alternative aggregates can help to create sustainable mixtures for road construction. Nowadays, numerous recycled and waste materials, such as reclaimed asphalt pavement, RAP, crumb rubber, plastics, and waste glass are being used and/or evaluated as a suitable replacement of virgin aggregates in the construction of pavement layers such as asphalt and base layers [
1,
2,
3,
4]. Recyclability and waste utilization not only decrease the harmful effects of waste disposal, but also reduce the depletion of natural resources, resulting in cost savings and economic benefits [
5]. In addition, the use of such materials might lead to further enhancement of the performance of asphalt paving materials, thus representing a value-added use for solid waste. For these reasons, many research projects have been dedicated to searching for and assessing the utilization of alternative recycled materials in bituminous mixtures [
3,
4,
6].
When exploring recycling based AC, besides material type, aggregate gradation, aging conditions, frequency of loading and temperature, additional factors need to be investigated. In the case of using waste glass (WG) materials, the effect on asphalt mixture properties (including dynamic modulus |E*| and phase angle Φ) and impact on performance need to be assessed. The resistance of AC to permanent deformation and fatigue cracking are two major performance measures that are typically considered along with durability [
7]. Among the various alternatives, permanent deformation potential of AC can be evaluated using compressive uniaxial tests such as flow number (FN) [
8], whereas fatigue cracking potential can be evaluated using indirect tensile testing [
7,
9].
Over the last decade, researchers opted to use crushed waste glass (CWG) in construction applications in order to divert WG from landfills and to reduce the environmental effect in the construction sector. CWG has been studied for use as an aggregate in asphalt subbase and unbound base layers, as well as concrete [
10]. Ali and Arulrajah [
11] and Chen et al. [
12] examined the use of waste glass in the base and sub-base courses of pavements indicating that sufficient shear strength, bearing capacity, and crushing resistance are achieved. Similarly, Saberian et al. indicated that crushed glass had beneficial effects on unconfined compression strength, California bearing ratio, and resilient modulus of the base and subbase layers when combined with crumb rubber [
13]. This was attributed to the larger angularity of glass particles having a higher internal angle of friction that resulted in stronger interlocking between aggregate particles.
With regards to the asphalt layer, good performance was reported in asphalt pavements with glass contents of 10% to 15% in surface course mixtures [
14]. In this context, Bachand et al. [
15] conducted a complex modulus test to compare the stiffness of HMA with 10% glass and different binder contents to that of a traditional HMA. It was concluded that small variation between all HMA were observed. Furthermore, a maximum aggregate size of 4.75 mm was recommended to control durability impact [
16]. Arabani assessed the stiffness modulus of specimens with varying glass (0% to 20%) and hydrated lime levels at 5 °C, 25 °C, and 40 °C [
17]. The results showed that the stiffness modulus of the specimens with glass and hydrated lime had an increasing value due to the higher cohesiveness between aggregates and binder produced by the hydrated lime’s anti-stripping properties. However, the stiffness modulus dropped as the glass content approached a predetermined threshold of 15%, defined as the ideal content. Likewise, Shafabakhsh and Sajed found an optimal glass content of 15% above which the stiffness modulus decreased [
18]. The decrease in stiffness modulus was attributed to an abundance of glass particles, which can slip on together because of their smooth texture or break into smaller particles under heavy loads at the surface layer of the pavement structure. Moreover, Arabani and Kambooza [
19] found that the dynamic modulus of traditional HMA was reduced by 70% compared to 50 % for mixes containing WG materials when the testing temperature was raised to 40 °C. This indicated that HMA containing glass had a lower susceptibility to temperature changes that might be the result of the heat transfer ability of glass in comparison with the stone aggregates. On the other hand, Airey et al. [
20] used the indirect tensile strength method to examine specimens that contained 50% glass at 20 °C and found that the stiffness modulus is barely affected when glass is used in place of conventional aggregate. Similar conclusions were reached by other researchers, indicating as well that large proportions of recycled glass in asphalt mixtures may result in undesirable Marshall testing properties, such as a reduction in the mixture’s strength, density, voids filled with binder, and air void content [
21].
Past investigations on replacing filler with waste materials explored either partial or full replacements [
22,
23]. The aggregate portion which passes the No. 200 sieve (75 μm) is termed as filler, which influences the mechanical behavior and durability of asphalt mixtures. Mineral fillers were generally treated as being suspended in the asphalt binder without particle to particle contact and thus without contributing to binder stiffness [
24,
25]. Additionally, mineral fillers are part of the aggregate skeleton as they provide contact and/or friction between particles [
26]. Thus, fillers play an important role in the performance of asphalt pavement mixtures. The filler present in the asphalt mix interacts with asphalt binder to form asphalt mastic. The filler activity in the mastic is due to the physical hardening and chemical interaction [
27]. Based on this activity, fillers can be generally classified into two categories known as active fillers and passive (inert) fillers. The fillers which exhibit chemical activity in the mastic due to their alkaline nature and the acidic nature of the binder are termed as active fillers [
27]. This chemical reaction is reported to improve the anti-aging potential, adhesion, and high temperature resistance in asphalt mastic and mixes [
28]. The fillers such as hydrated lime, cement, steel slag, and so on fall under the category of active fillers. On the other hand, the inert or passive fillers exhibit little to no chemical activity in asphalt mastic but are usually responsible for causing stiffness or physical hardening in the asphalt mastic owing to their physical characteristics. Overall, the performance of asphalt mixes against distress such as permanent deformation, load and non-load dependent cracking, aging, and moisture sensitivity is largely dependent on the physical and chemical characteristics of fillers [
29,
30]. As a result, the type and quantity of fillers used in flexible pavements are critical to their cost-effectiveness and long-term performance. Cement, stone dust, and lime are the most common traditional fillers assessed in HMA [
31,
32]. Sebaaly et al. indicated that hydrated lime has the ability to improve the resistance of HMA mixtures to moisture damage, reduce oxidative aging, and enhance the resistance to fatigue and rutting, which led to observed improvements in the field performance of lime-treated HMA pavements [
32]. Likewise, Choudhary [
29] investigated the effects of different types of filler on the performance of asphalt concrete and found that mixes containing red mud and limestone dust worked well in terms of cracking and rutting deformation, while compositions containing carbide lime performed better in terms of moisture resistance. Moreover, Qassim et al. [
33] and Murana et al. [
34] investigated the performance of HMA containing Metakaolin as a partial or full replacement of fillers, and found that the addition of different proportions of Metakaolin has a significant influence on the mechanical behavior of the HMA. The Marshall’s stability, flow and, density grow as the Metakaolin content grows until 50%, whereas the indirect tensile strength increases continuously as the Metakaolin content increases up to 100% at 25, 40, and 60 °C.
Previous literature revealed a focus on one side of waste glass utilization as a replacement of fines aggregates in AC, generally limited to percentages between 10% to 15% [
16,
17,
18], but a replacement of mineral fillers solely by waste materials showed improvement to the properties and performance of HMA [
31,
32,
33]. Therefore, this study aims to investigate the utilization of recycled laminated waste glass as a replacement of mineral fillers in Superpave AC mixes from a performance perspective. In the process, the main characteristics of dynamic modulus, (i.e., stiffness), phase angle and the permanent deformation potential of asphalt concrete containing different glass powder percentages (0, 25, and 50%) are evaluated. Additionally, the flow number (FN), dynamic modulus (DM), and simple performance indicators results were compared and used to characterize the mechanical behavior of asphalt mixtures.