Rheological and Thermo-Oxidative Aging Properties of Asphalt Modified with a Combination of Sasobit and Linear Low-Density Polyethylene

Abstract

Research of different asphalt modifiers has been necessary for the attempt to construct durable roads with higher standards. Fischer Tropsch-paraffin wax (Sasobit) has recently attracted considerable attention over polymer modification due to its capacity to lower the energy requirements for asphaltic mix construction. In this study, Sasobit was used to recover the performance as well as the workability of 3 wt% linear low-density polyethylene (LLDPE) modified asphalt. A base asphalt binder with a penetration grade of 50/70 was blended with 3 wt% LLDPE and 3 wt% Sasobit separately and then combined with different Sasobit dosages (1–3 wt%). The performance of modified asphalt binders was evaluated using conventional, rheological, and thermal tests. As a result, it was found that loading Sasobit (1–3 wt%) into LLDPE-asphalt mixture steadily decreased the penetration and ductility at 25 °C from 25 to 12 dmm and 31 to 18 cm, respectively, and softening point increased by 20% indicating improved high-temperature performance. The binder workability and mix temperature were improved since the addition of Sasobit reduced the LLDPE-asphalt viscosity from 0.292 to 0.189 Pa.s (22% less). Sasobit improved the thermo-oxidative aging resistance of the binder by showing less weight variation (less than 0.001%) after the Rolling Thin-Film Oven Test (RTFOT) and high ductility retention (65%). Thermogravimetry (TG) and kinetics analysis results indicated that Sasobit-LLDPE delayed the initial and maximum decomposition temperature by 11 °C and hence increased the thermal stability of modified binders. Thus, the proposed binders are a suitable solution for asphalt pavement construction in regions that encounter high-temperature changes.

1. Introduction

Asphalt is a petroleum-derived material commonly used as a binder for aggregates in the pavement because of its impermeability and adhesion properties among others [1]. Traffic growth, tire pressure, larger and heavier trucks, and severe pavement environmental conditions have resulted in the demand for asphalt binders with special characteristics. These characteristics led to polymer modification being considered a general method for the improvement of asphalt binder properties [2].

Many polymers, among thermoplastics and elastomers, have been used for the modification of asphalt binders [3]. Elastomers such as styrene-butadiene-styrene (SBS), styrene isoprene styrene (SIS), and styrene-ethylene/butylene styrene (SEBS) are known for improving the elastic response of asphalt binders, and thermoplastics such as ethylene vinyl acetate (EVA), polyethylene (PE), ethylene butyl acrylate (EBA), and polypropylene (PP) improve asphalt binder rutting resistance [2,4]. Other asphalt binders’ properties, such as better moisture resistance, higher low-temperature cracking resistance, higher stiffness at high temperatures, and longer fatigue life are improved by these polymers [4,5,6].

Polyethylene (high-density—HDPE, low-density—LDPE, and linear low-density—LLDPE) has been used to decrease the asphalt pavements’ high-temperature deformation under heavy loads [7]. According to the research conducted by Hinislioğlu et al. [8], when using powdered HDPE as an asphalt binder modifier, the asphalt concrete mixtures containing 3 wt% HDPE had the highest Marshall quotient and, consequently, higher resistance to permanent deformation than unmodified asphalt concrete mixtures. Other studies carried out by Attaelmanan [9] concluded that HDPE addition into asphalt improves moisture susceptibility, increases rutting resistance, and reduces the low-temperature (−10 °C) cracking potential of the asphalt concrete mixtures. Bala et al. [3] investigated the use of LLDPE as a modifier for an 80/100 penetration grade asphalt binder. They showed that it leads to higher temperature susceptibility enhancement, increases stiffness, increases the higher resistance to thermo-oxidative aging, improves high temperatures rutting defects resistance, and exhibits better resistance against fatigue. In this study, LLDPE was chosen as the polymer modifier since previous studies showed that LLDPE was confirmed as the right modifier for enhancing high-temperature performance and has better compatibility with asphalt due to lower density and crystalline structure [10,11,12].

Although the great improvements in rheological and mechanical properties that polymers such as SBS, HDPE, LLDPE, and EVA provide to asphalt binders, the usage of these additives to produce asphalt concrete mixtures (hot mix asphalt—HMA) still requires high temperatures during mixing-compaction stages (generally from 140 to 190 °C) to compensate the high bitumen-polymer viscosity [13,14,15]. These high mixing-compaction temperatures not only increase energy requirements but are also a major source of pollution, mainly because of the release of harmful gases such as CO₂, SO₂, volatile organic compounds, CO, and NO_x [14,16,17]. This situation also provides a hazardous working environment and negatively impacts the health of workers involved in laying and compacting asphalt pavements [18,19,20].

Several methods and techniques have been developed to reduce the consumption of energy and, as a result, lower emissions during asphalt paving [18]. Warm mix asphalt (WMA) is one of the technologies where we can, at least, have some control over fuel consumption and emissions without having to compromise the performance of the road [19,20]. The use of Sasobit as a WMA is becoming increasingly widespread in the pavement construction industry due to its greater workability and numerous advantages, including energy savings, lower pollutants emissions (up to 30%), and lower mixing-compaction temperatures (about 20–40 °C reduction) [13]. Sasobit can generally improve the mechanical properties of asphalt mixtures [14] and was, thus, chosen as an LLDPE-asphalt additive.

During stirring, Sasobit can form a homogenous solution with asphalt binder, causing a considerable reduction in asphalt viscosity, resulting in enhanced binder capability to flow [21,22]. After crystallization, Sasobit generates a lattice structure in the asphalt, which is the basis of the Sasobit-modified asphalt binder’s structural stability and enhancement in asphalt stiffness and deformation resistance [23,24,25]. The recommended Sasobit loads in asphalt vary from 0.8 wt% to 3 wt% (by weight of the asphalt binder) [14]. It has been indicated that Sasobit-modified asphalt showed higher stiffness and intermediate testing temperatures compared with unmodified binders [17]. Studies conducted by Solouki et al. [26] showed that the addition of Sasobit improves asphalt binders’ high-temperature properties, hence increasing the softening point and decreasing the penetration value. The main objective of this research is to study the effect of Sasobit as a WMA additive in asphalt mixtures. Even though many researchers have used PE-based polymers and Sasobit separately to improve asphalt proprieties, very few studies have reported the combined effect of PE-based polymer and Sasobit on asphalt binder proprieties. The use of Sasobit was considered due to its ability to reduce the high viscous trend of LLDPE-asphalt binders and improve these binders’ workability during the mix and compaction stages.

2. Materials and Methods

2.1. Materials

The neat asphalt binder in this study was obtained from Puma Energy Mozambique (Maputo, Mozambique). This 50/70 penetration grade asphalt binder was selected as the base asphalt because it is the most common asphalt binder used on Mozambican roads. The properties of the asphalt base binder are shown in

Table 1. Base asphalt binder physical properties.

Properties	Method	Result	Min.	Max.
Penetration (25 °C), 0.1 mm	ASTM D5	54	50	70
R&B softening point, °C	ASTM D36	47	46	54
Ductility (25 °C, 5 cm/min), cm	ASTM D113	>100	100	-
Rotational viscosity at (60 °C), Pa s	ASTM D 4402	166	>145
Rotational viscosity at (135 °C), Pa s		0.248	-	<3

. LLDPE, in powder form, was used as a modifier to improve neat binder proprieties and it was supplied by Sasol (Sandton, South Africa) as well as the Sasobit, used as a WMA additive.

Table 2. LLDPE properties.

Properties	Values
Average particle diameter, mm	0.8
Density, g/cm3	0.915–0.95
Deflection temperature (at 0.46 Mpa), °C	35–42
Melting point, °C	115–135
Melt flow index, g/10 min	2–5

shows the properties of LLDPE. The physical properties of Sasobit can be seen in

Table 3. Sasobit properties.

Properties	Values
Form	Pellets
Composition	Aliphatic polymethylene hydrocarbon
Color	White
Melting point, °C	85–115
Softening point, °C	98 °C
Specific gravity (25 °C)	0.9
Viscosity (135 °C), cP	12

.

2.2. Preparation of the Modified Asphalt Binder

The preparation of the modified binders followed the procedures describe by [10,11,12] with slight modifications. Base asphalt binder was heated up until it became soft and 700 g (average weight) of the binder was poured into iron containers in an oven. A total of 6 samples for testing were prepared. The asphalt binder was modified by 3 wt% of LLDPE and Sasobit separately and hereafter in the same mixture. For the ternary compositions, the LLDPE content was set at 3 wt%, and Sasobit was added in amounts to make up 1, 2.5, and 3 wt%. For preparation, each poured sample was placed in a high shear mixer, IKA^® EUROSTAR 20, at 165 °C and 5000 rpm for 15 min, and 3 wt% of LLDPE/Sasobit was added gradually at a rate of 5 g/min. The shearing time was nearly 45 min. For the ternary compositions, after the addition of LLDPE, the shearing was done in 15 min and, on top of that, Sasobit was added to the binder and the mixture was stirred at a rate of 5000 rpm, for 30 min, at 165 °C. The samples were then cooled at room temperature for further testing. LLDPE-modified asphalt binder was designated as AL, and Sasobit-modified asphalt was designated as AS. The ternary binders were designated based on ALS-X format, in which A corresponds to asphalt, L stands for LLDPE, S is Sasobit, and X corresponds to additive value (1, 2.5, or 3 wt%). The experimental flowchart is shown in Figure 1 and

Table 4. Modified asphalt binders’ weight and code.

Sample	Sample Code	Weight (g)
		Asphalt	LLDPE	Sasobit
Unmodified asphalt	Asphalt	768.0	0.00	0.0
Asphalt + 3% LLDPE	AL3	698.4	21.0	0.0
Asphalt + 3% Sasobit	AS3	624.3	0.0	18.7
Asphalt + 3% LLDPE + 1% Sasobit	ALS-1	635.7	19.1	6.4
Asphalt + 3% LLDPE + 2.5% Sasobit	ALS-2.5	617.6	18.5	9.3
Asphalt + 3% LLDPE + 3% Sasobit	ALS-3	749.7	22.5	22.5

summarizes the binders’ names and the corresponding weight of each used additive.

2.3. Test Methods

2.3.1. Conventional Physical Tests

To evaluate the effect of Sasobit on LLDPE-modified asphalt binder standardized tests were used to characterize the rheological properties of asphalt binders including softening point, penetration (25 °C) ductility (25 °C), and dynamic viscosity (165 °C) were performed. The penetration value is an empirical test that provides an overview of asphalt binder consistency, and it was carried out following the ASTM D5 standard [27]. The softening point (the temperature at which an asphalt binder began to soften) was tested according to ASTM D36 [28]. A ductility test was performed by using the ductility testing machine according to ASTM D113 [29]. Furthermore, viscosity at 165 °C tests was carried out using a Brookfield viscometer (Model DV-I Prime) according to ASTM D4402 [30]. To assess how Sasobit and LLDPE affect the thermal sensitivity of modified asphalt the Penetration Index (PI) of asphalt was determined according to the van der Poel equation (Equation (1)) [31].

$$\mathrm{PI}=\frac{500\log {\mathrm{P}}_{\mathrm{e}\mathrm{n}}+20{\mathrm{T}}_{\mathrm{s}}(°\mathrm{C})-1952}{120-50\log {\mathrm{P}}_{\mathrm{e}\mathrm{n}}+20{\mathrm{T}}_{\mathrm{s}}(°\mathrm{C})}$$

(1)

2.3.2. Aging Process

Short-term laboratory aging of the unmodified and modified asphalt binders was carried out by using a Rolling Thin Film Oven (RTFO). The samples were kept in the RTFO for 85 min at 163 °C. The aged samples were tested for various properties such as weight change (%ΔW), ductility retention (DR), and viscosity aging index (VAI) calculated through Equations (2)–(4).

$$\%\Delta \mathrm{W}=\frac{{\mathrm{w}}_2-{\mathrm{w}}_1}{{\mathrm{w}}_1-{\mathrm{w}}_{\mathrm{o}}}\times 100\%$$

(2)

where %ΔW is the weight percentage change; ${\mathrm{w}}_{\mathrm{o}}$ is the weight of the test bottle; ${\mathrm{w}}_1$ is the weight of the bottle and sample before the RTFO; ${\mathrm{w}}_2$ is the weight in g of the bottle and sample after the RTFOT.

$$\mathrm{VAI}(\%)=\left(\frac{{\mathrm{V}}_{\mathrm{after}}-{\mathrm{V}}_{\mathrm{before}}}{{\mathrm{V}}_{\mathrm{before}}}\right)\times 100\%$$

(3)

where V_after and V_before represent the viscosity of the asphalt binders at 165 °C after and before the RTFO test, respectively.

$$\mathrm{DR}(\%)=\frac{{\mathrm{D}}_{\mathrm{after}}}{{\mathrm{D}}_{\mathrm{before}}}\times 100\%$$

(4)

where D_after and D_before represent the ductility at 25 °C of the asphalt binders after and before the RTFO test, respectively.

2.3.3. Thermal Analysis and Kinetics

The thermal stability of asphalt binders is one of the main important properties during mixing-compaction procedures. Furthermore, binders during the blending and construction processes are generally submitted to high temperatures. Thus, knowledge of binder thermal degradation behavior for paving purposes might be considered a key property for understanding pavement performance at service. Therefore, to comprehend the thermal behavior and stability properties of LLDPE and Sasobit-modified asphalt binders, thermogravimetric analysis (TGA) was proposed. The TGA was carried out with a TA instrument, model STA 6000 TGA/DSC. Each prepared binder (13 ± 2 mg) was placed in an aluminum holder and heated up at a heating rate of 10 °C min⁻¹, from room temperature to 600 °C, under an N₂ atmosphere with a flow rate of 100 mL min⁻¹. Kinetic parameters such as activation energy and pre-exponential factor were calculated using the Coats–Redfern model, shown by Equation (5) [32].

$$\ln \left[\frac{-\ln (1-\mathsf{\alpha })}{{\mathrm{T}}^2}\right]=\ln \frac{\mathrm{AR}}{{\mathsf{\beta }\mathrm{E}}_{\mathrm{a}}}\left[1-\frac{2\mathrm{RT}}{{\mathrm{E}}_{\mathrm{a}}}\right]-\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}$$

(5)

where, T denotes absolute temperature (K); β represents the linear heating rate (10 °C min⁻¹); A is the pre-exponential factor, E_a is the activation energy; R the gas constant (8.3145 J K⁻¹ mol⁻¹); and α is the sample decomposed fraction at a given time. In this method, the plot of $\ln \left[\frac{-\ln \left(1-\mathsf{\alpha }\right)}{{\mathrm{T}}^2}\right]$ versus $\frac{1000}{\mathrm{T}}$ gives a straight line and the slope is used to calculate E_a [33].

3. Results and Discussion

3.1. Conventional Tests

The results of conventional rheological properties, including penetration at 25 °C, softening point, and ductility at 25 °C are shown in Figure 2a–c, respectively. The penetration values and softening points of binders were used to investigate the high-temperature service performance of asphalt. Furthermore, a ductility test was used to study the modified binder service performance at low temperatures. These results show that the AL, AS and ALS-X binders had lower penetration and ductility and higher softening point compared with neat asphalt. The effect of adding Sasobit into the LLDPE-modified asphalt matrix resulted in a decrease in penetration and ductility and an increase in softening point with the increasing Sasobit content.

Figure 2a shows that the penetration value of neat asphalt binder decreases with the addition of 3.0 wt% of LLDPE and 3.0 wt% Sasobit from 54 dmm to 25 and 21 dmm, respectively. When 1.0, 2.5, and 3.0 wt% Sasobit is added to Sasobit-modified asphalt, the penetration values are, respectively, 33, 37, and 53% lower than that of AL3. Meanwhile, for modified asphalt with 3.0 wt% LLDPE and 3.0 wt% Sasobit, the softening temperature (Figure 2b) increases by 33% and 43%, respectively, compared to the neat asphalt. The increase in Sasobit load from 1.0 to 3.0 wt% increased the softening point of LLDPE-modified asphalt. This increase becomes less accentuated with the increase of the Sasobit content, practically stabilizing the softening for the Sasobit contents corresponding to 2.5 and 3%. This behavior, according to Din and Mir [18], is linked to the crystallization of Sasobit at temperatures below 100 °C, forming a very hard crystalline lattice, which reinforces the asphalt structure. Sasobit had a better effect than LLDPE on improving the high-temperature performance of neat asphalt as a result of lower penetration and higher softening points. Furthermore, it should also be noted that the addition of Sasobit into LLDPE-modified asphalt further increased the high-temperature performance of the modified binder.

In general, these results show that LLDPE and Sasobit can improve the asphalt binder resistance against high-temperature effects and make it more resistant to pavement failures such as permanent deformations. The observed improvements can be explained, accordingly to Bala et al. [3], by the good diffusion of asphalt maltene oil fraction into the LLDPE polymeric phase, causing higher interactions and swelling between the LLDPE and polar molecules of the binder (asphaltenes). For Sasobit, the improvements observed at higher temperature performance (lower penetration and higher softening point) of binders might be explained by the large chain length of the hydrocarbons in Sasobit (around 40–115 carbon atoms) [6,18]. It is contemplated that the large hydrocarbon chain length increases the elastic and plastic limits, hence decreasing the penetration value and increasing the softening point. According to Bala et al. [3], for paving applications, a higher asphalt softening point temperature value and a lower penetration value describe the higher stability of the binder under high service temperatures.

Figure 2c shows that Sasobit-modified asphalt exhibits more ductility at 25 °C reduction effect compared to LLDPE, and the increase of Sasobit loads in LLDPE-modified binder from 1.0 to 3.0 wt% leads to a reduction of ductility at 25 °C. Such a result refers to a better low-temperature performance of LLDPE than that of Sasobit. On the other hand, in comparison with neat asphalt, the warm-mix asphalt prepared shows lower performance at low temperatures due to the decreased ductility, which would lead to moisture damage and low-temperature cracking. Therefore, this could bring adverse effects on the performance of asphalt pavements incorporated with Sasobit in low-temperature regions. So, the additive dosage should be limited to such a proportion up to which the performance of modified asphalt, at low temperatures, should not be adversely affected.

Asphalt binder materials are generally very sensitive to variations in temperature. These make them display diverse properties under varying service temperature conditions. The effects of Sasobit and LLDPE on binder penetration indices are given in Figure 1d. As LLDPE and Sasobit are added, PI value increase by 104 and 147%, respectively, meaning that modified binders are less susceptible to temperature variations. The increase in PI value and consequently lower temperature susceptibility is more pronounced when Sasobit is used as a stand-alone modifier. As Sasobit content increases from 1 wt% to 3 wt% in the triple composite, the PI value increases further. These results show that the addition of Sasobit into the neat asphalt and LLDPE-asphalt binders significantly enhanced the susceptibility of the binders toward service temperatures. These higher PI values indicate less susceptibility to temperature variations and more rubbery elastic behavior [3].

According to Lesueur [31], for paving applications, the asphalt binder has to have PI values ranging from −2 (high-temperature susceptible asphalt) to +2 (low-temperature susceptible asphalt). This means that the binders containing Sasobit are less susceptible to temperature variations in in-service temperatures. On the other hand, these higher PI values of the asphalt binders render them less brittle, and under high strains, they might exhibit higher elastic properties. Thus, these warm-mix binders are likely to be more resistant to temperature cracking and permanent deformation [3].

3.2. Viscosity

The viscosity of asphalt is a significant parameter for asphalt binder performance since it provides an idea about the capability of asphalt to coat aggregates in asphalt mixtures. The results of dynamic viscosity testing are shown in Figure 3. The incorporation of LLDPE leads to higher viscosity values of the asphalt binder. However, the increase of viscosity in Sasobit-modified asphalt binders is not appreciable. Sasobit loading into LLDPE-modified asphalt from 1.0 wt% to 3.0 wt% leads to a decrease in viscosity at 165 °C by 14, 10, and 20%, respectively.

The observed reduction in binders’ viscosity can be attributed to the longer carbon chain of Sasobit than LLDPE, which keeps it in solution and hence induces a reduction in viscosity. This effect can also be explained by the melting point of Sasobit, which ranges from 85 to 115 °C. Thus, when the temperature is higher than the melting point, Sasobit appears in asphalt as a liquid and hence decreases the viscosity of LLDPE-modified asphalt [18]. The ASL-X binders have better workability during service temperatures and conditions in comparison with AL binders, meaning that these binders can be mixed and compacted at relatively lower temperatures which might result in less energy consumption, operation time, as well as in the reduction of environmental pollutants emissions.

3.3. Aging Properties

The thermo-oxidative aging for both unmodified and modified binders was simulated using RTFO, the aging index for viscosity, and weight variation. The ductility was estimated to evaluate the binder’s resistance against aging defects. The values of weight change (%ΔW), ductility retention (DR), and viscosity aging index (VAI) are shown in Figure 4, Figure 5 and Figure 6, respectively.

The weight change percentage shows that the unmodified asphalt binder sample gains mass during the test, while the AL3 binder shows a loss in the mass of the sample. On the other hand, the AS3 binder, like the neat asphalt sample, gained mass during the RTFOT, however in a lower extension. Thus, Sasobit reduced the oxidation of asphalt binders during service temperature conditions. All ASL-X binders showed mass gain after the test; however, the extension of oxidation is lower compared to that which was observed during the testing of the neat asphalt. This fact shows that Sasobit can reduce the loss of volatile compounds of asphalt and prevent the oxidation of the binders. The sample ALS-3 did not show mass variation which implies that it is less affected by thermo-oxidative agents. These results suggest that the addition of Sasobit in the asphalt polymer mixture enhances its aging resistance, and the observed mass changes are within the acceptable range of ±0.5%, according to the European standard EN 12607-1 [34].

The asphalt binders aging resistance can also be evaluated by the viscosity aging index (VAI). Figure 5 shows that the VAI values of AL and AS samples were significantly lower than that of the unmodified asphalt binder. Furthermore, with the addition of 1.0 and 3.0 wt% of Sasobit in ALS-X samples, the VAI values were significantly higher while the AL samples showed lower improvements compared to unmodified asphalt binder. These results suggest a poor Sasobit effect in LLDPE-modified asphalt binder VAI but still a greater index than neat asphalt. According to Zhang et al. [34], lower VAI values lead to greater aging resistance. Therefore, all modified binders have more aging resistance than neat asphalt. This might be explained by considering that during binder aging, naphthene aromatics of asphalt are partly converted into polar aromatics, and the polar aromatics are also converted into asphaltenes, causing an increase in asphaltene content [3].

Figure 6 shows the aging effects on the retention of ductility at 25 °C of the unmodified and modified binder. It was observed that the ductility of both LLDPE-modified and Sasobit-modified binders reduces after thermo-oxidative aging, which leads to a decrease in the retention of ductility ratio. Sasobit has a positive effect in increasing the retention of ductility at 25 °C, as shown in Figure 6. Furthermore, higher Sasobit content in the triple composite led to higher retention of ductility after the RTFO test.

According to Galooyak et al. [35], a lower retained ductility value reflects a more aged binder. Thus, the ALS-1 binder is the most aged sample since it presented the lowest ductility retention value. On the other hand, ALS-2.5 and ALS-3 had higher ductility retention values showing that the aging susceptibility of these binders decreased after modification, and the thermo-oxidative aging resistance of the binders was enhanced.

3.4. Thermal Properties

3.4.1. TGA Analysis

TGA and the corresponding derivative thermogravimetric (DTG) diagrams of unmodified asphalt and Sasobit-LLDPE modified asphalt were examined, as shown in Figure 7a,b, respectively. TGA thermograms presented a single-stage mass loss step for both unmodified and modified asphalt. Generally, under an inert (N₂) atmosphere, the pyrolysis of asphalt binders occurs in a single stage and these findings correspond to phenomena reported in previous studies conducted by [18,36,37,38]. The unmodified asphalt binder is stable against heating below 370 °C. However, when the temperature is raised above 370 °C, starts the evaporation of low molecular weight compounds, such as aromatics, oils, and saturates during the thermal condensation process and the breakage of the side chains of the compounds [39,40]. In contrast, the modified binders exhibited higher resistance against heat compared to the unmodified ones. When asphalt is modified with 3.0 wt% of LLDPE and 3.0 wt% of Sasobit, the initial decomposition temperature is delayed by 11 and 10 °C, respectively. Figure 7a and

Table 5. TGA and DTG data of unmodified and modified binders at a heating rate of 10 °C min⁻¹.

Binders	Temperatures, °C				Residue at 600 °C, %
	Tonset	T0.5	Tmax	Toffset
Neat asphalt	372.94	442.45	447.63	505.56	15.38
AL3	383.54	453.10	461.10	508.07	14.46
AS3	382.22	445.75	455.86	510.11	11.97
ALS-2.5	376.69	446.99	453.59	506.00	15.21
ALS-3	380.38	447.65	456.41	510.77	13.91

show that the initial decomposition temperature is reduced when 2.5 wt% of Sasobit is added to LLDPE-modified asphalt, despite that the temperature is raised when 3.0 wt% of Sasobit is added. The mass loss of all binders is mainly concentrated in the range of 400–600 °C. When the temperature is raised to 400 °C, the binders gradually crack into chain-segment molecules. As the temperature increases, the chain-segment molecules continue to break down into smaller molecules, and, finally, form a coke residue [39].

Table 5 shows the initial decomposition temperature (T_onset), 50% weight loss temperature (T_0.5), maximum decomposition temperature (T_max), final decomposition temperature (T_offset), and the residue at 600 °C. Such as the T_onset of unmodified, the peak values for T_0.5 and T_max are shifted to higher temperatures due to the higher thermal stability of the modifiers. The temperature at which 50% of the initial sample material of the modified binders is decomposed AL3, AS3, ALS-2.5, and ALS-3 is, respectively, 11, 3, 4, and 5 °C higher than the neat binder. The T_max of neat asphalt is 13, 8, 6, and 9 °C lower compared to AL3, AS3, ALS-2.5, and ALS-3 binders, respectively. So, the inclusion of LLDPE and Sasobit improved the binders’ thermal stability.

In contrast, Table 5 shows that the yield of charred residue at 600 °C for unmodified binder (15%) is higher than all modified binders mainly due to the higher offset temperature. According to Hu et al. [39], the narrow mass loss temperature range leads to a higher residue which is related to higher temperatures of pyrolysis process finalization. Despite that, Sasobit and LLDPE had a good effect on the thermal properties of unmodified asphalt binders, which indicates that the Sasobit-LLDPE modified binders would not become thermally degraded at actual storage and mixing temperatures (below 200 °C) as well as during the construction stage.

3.4.2. Kinetic Analysis

TGA is an excellent tool for studying thermal decomposition kinetics. Figure 8 shows the kinetic curves of thermal degradation of asphalt-LLDPE-Sasobit ternary composites fitted using the Coats–Redfern model as described by Equation (5). The activation energy (E_a) values were directly obtained from the slope of the linear regression line of $\ln \left[\frac{-\ln \left(1-\mathsf{\alpha }\right)}{{\mathrm{T}}^2}\right]$ versus $\frac{1000}{\mathrm{T}}$ at a constant heating rate of 10 °C min⁻¹.

There was a significant change in the activation energy values between the composites and the neat asphalt binder. Both LLDPE and Sasobit, separately and combined, increased the activation energy of asphalt as shown in Figure 9. This implies that the thermal stability of pure asphalt increases with the inclusion of Sasobit and LLDPE. For ternary blends, it can be inferred that the composite containing 3.0 wt% of Sasobit exhibits higher activation energy and the increase of Sasobit content steadily increases the thermal stability of ternary composite binders.

The thermal stability of the asphalt was substantially increased by the incorporation of both modifiers due to the good solvation of Sasobit and LLDPE in the asphalt maltenes fraction. The formed composites appear to be in a continuous phase and swollen by the miscible components of the asphalt binder and form an interconnected three-dimensional structure, that leads to better compatibility between the components [41].

4. Conclusions

In the present work, laboratory tests were used to investigate the effects of using Sasobit on the rheological, aging, and thermal properties of asphalt and LLDPE-modified asphalt. Based on these research outcomes, it can be concluded that:

• The addition of Sasobit (1–3 wt%) into LLDPE-modified asphalt reduced penetration and ductility and increased softening point and PI values. Therefore, asphalt binders containing Sasobit and LLDPE result in higher PI, which could increase the resistance of the asphalt mixtures to thermal cracking and rutting;
• The use of Sasobit as a warm-mix additive steadily reduced the viscosity values of LLDPE-modified asphalt. This fact indicates that proposed binders require less temperature mixing-compacting leading to lower energy consumption and reduced pollutants released during service;
• The short-term aging test was conducted to ensure that the blending processes were effective and that the modified binders would be stable during the mixing-compacting stage. These tests showed that LLDPE-Sasobit improved the asphalt binder’s ability to resist thermo-oxidative aging;
• The modified asphalt binders exhibited significantly higher decomposition temperature and activation energy compared to neat asphalt. Thus, the addition of these additives restricts the decomposition to some extent and improves the thermal stability of asphalt.

The key factors determining the performance of binder for paving, namely rheology, thermal behavior, and aging revealed that the asphalt binder consistency has been improved. By the same token, the short-term aging showed that Sasobit-LLDPE greatly improved the high in-service properties of asphalt, and these binders are more resistant to permanent deformation and rutting. Nevertheless, these results also revealed that extensive reduction in penetration brings adverse effects on the performance of asphalt pavements incorporated with Sasobit-LLDPE in low-temperature regions due to a high tendency to undergo thermal cracking.