Reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) contain valuable components, making them more suitable for use in asphalt mixes [1]. The use of 20% RAP in asphalt mixes reduced energy consumption by 5.0 to 7.5% [2]. Hong and Prozzi[3] found that using 35% RAP in a 1-mile-long overlay section decreased cost by 25%. The use of 5% RAS in asphalt mixes resulted in cost savings ranging from $1.00 to $2.80 per ton [4]. Nevertheless, incorporating RAP and/or RAS into asphalt mixes alters the performance of the total binders—virgin asphalt binders (VABs) and aged binders in RAP and/or RAS—within mixes [5]. Using RAP and/or RAS in asphalt mixes enhanced the rutting resistance of extracted asphalt binders (EABs) due to the stiff binders in RAP and RAS [6, 7]. However, increasing the percentages of asphalt binder replacement (ABR) by RAP in asphalt mixes deteriorated the fatigue resistance of EABs [8–10].
The chemical and thermal properties of EABs have been found to be good indicators of fatigue resistance of EABs [8]. Thermal characteristics of asphalt binders are analyzed by thermogravimetric analysis (TGA) by monitoring changes in thermograph (TG) parameters and derivative of thermograph (DTG) shapes [8–11]. TG reflects the relationship between temperature and mass loss, and DTG explicates the relationship between temperature and decomposition rate [12–14]. TG parameters are the onset temperature (Tonset), the endset temperature (Tendset), and the percentage of residue (%R) or char [8–10]. Tonset for mass loss during thermal degradation is used to predict binders’ compositional changes and fatigue resistance [8, 15]. Tonset is defined in ISO 11358-1 [16] as the intersection point of the starting-mass baseline and the tangent to the TG curve at the point of the maximum gradient, known as the inflection point. In addition, DTG shapes, during thermal degradation, reflect asphalt binders’ compositions [8, 11]. The shape of the DTG curve during thermal degradation reflects the aging condition of asphalt binders [8]. Usually, the DTGs of asphalt binders show three regions: no mass loss happens in the first region, thermal degradation initiates in the second region, and the fastest molecules’ cracking occurs in the third region [17]. However, Deef-Allah and Abdelrahman [8] found that the second region disappeared for EABs from long-term aged field mixes containing RAP. Asphaltene has one peak in the DTG; however, maltene presents two peaks [11]. Thus, the disappearance of the second region in the DTG indicates a decrease in the maltene component of the EABs [8]. Chemical composition changes in asphalt binders are investigated by Fourier transform infrared (FTIR) spectroscopy [8, 18]. The aging components exchanged between RAP/RAS and VABs altered the FTIR aging indices [carbonyl index (ICO) and sulfoxide index (ISO)], aromatic index (ICC), and aliphatic index (ICH) [6, 7].
The RAP binder has a high fraction of asphaltene [19], and thus the fractions of EABs—maltenes and asphaltenes—change based on the interactions between the RAP binder and VAB. These interactions and related changes in asphalt fractions can be explored through TGA. Nciri et al. [20] found that using waste pig fat as a rejuvenator with RAP binders decreased the %R and Tonset. Previous studies [21, 22] examined the thermal stability of the asphalt binder fractions [saturates, aromatics, resins, and asphaltenes], and it was found that the thermal stability was the highest for asphaltene as the heaviest fraction with the highest molecular weight. Nevertheless, the thermal stability was the lowest for saturates as the lightest fraction with the lowest molecular weight. Thus, asphaltene has the highest Tonset and %R. The molecular chains of naphthene structures in saturates are easily broken at high temperatures. Thus, light volatiles and a small amount of coke—char or carbonaceous—are the decomposition components of saturates [21, 23]. The aromatics are composed of aromatic rings and associated side chains that are easily split from the aromatic rings. Therefore, the aromatics are easily decomposed to form coke [21], [23]. A larger number of aromatic rings exist in resins and asphaltene, which are not opened during the pyrolysis process. Additionally, asphaltenes are consisted of polynuclear aromatic compounds, contain heteroatoms attached to the oxygen-containing functional groups, that are considered the main source of coke formation [21, 23, 24].
Deef-Allah and Abdelrahman [8] correlated FTIR indices with Superpave fatigue cracking parameters (|G*|.sinδ) for EABs from field cores; direct relationships were noted between ICO or ICC and |G*|.sinδ. However, inverse relationships were noticed between |G*|.sinδ and ISO or ICH. A direct relationship was observed between Tonset and |G*|.sinδ; however, an inverse relationship was found between Tonset and number of load repetitions to failure (Nf) at 2.5% strain. Hence, direct relationships between ICO or ICC and Tonset have been recorded, and inverse relationships between Tonset and ISO or ICH have been observed. In addition, inverse relationships were noticed between ICO or ICC and Nf, and direct relationships were noted between Nf and ISO or ICH. Based on these relationships, EABs from field cores with the highest fatigue cracking resistance had the lowest |G*|.sinδ, Tonset, ICO, ICC, the highest Nf, ISO, and ICH.
The main objective of this study was to characterize the fatigue cracking resistance of EABs from field cores. This objective was achieved by relating the fatigue cracking resistance, through rheological testing, to the chemical and thermal properties of EABs. The rheological testing of EABs was conducted on the dynamic shear rheometer at intermediate temperatures. The chemical properties of EABs were evaluated using FTIR, and the thermal properties of EABs were investigated by TGA.