A Rubberized-Aerogel Composite Binder Modifier for Durable and Sustainable Asphalt Pavements

Abstract

The United States produces approximately 500 million tons of asphalt mixtures annually, while generating vast amounts of waste materials that could be repurposed for sustainable infrastructure. Each year, 1.4 billion gallons of lubricating oils are available for reuse and recycling. Additionally, 280 million tires are discarded, contributing to significant environmental challenges. Given the critical role of the roadway network in economic growth, mobility, and infrastructure sustainability, there is a pressing need for innovative material solutions that integrate recycled materials without compromising performance. This study introduces a Rubberized-Aerogel Composite (RaC), a novel asphalt binder modifier combining crumb rubber, recycled oil, and a silica-based aerogel to enhance the sustainability and durability of asphalt pavements. The research methodology involved blending the RaC with the PG70-10 asphalt binder at a 5:1 ratio and conducting comprehensive laboratory tests on binders and mixtures, including rheology, thermal conductivity (TC), specific heat capacity (Cp), the Hamburg Wheel-Tracking Test (HWTT), and indirect tensile strength (IDT). Pavement performance was simulated using AASHTOWare Pavement ME under hot and cold climates with thin and thick pavement structures. Results showed that RaC-modified binders reduced thermal conductivity by up to 30% and increased specific heat capacity by 15%, improving thermal stability. RaC mixtures exhibited a 50% reduction in rut depth in the HWTT and lower thermal expansion/contraction coefficients. Pavement ME simulations predicted up to 40% less permanent deformation and 60% reduced thermal cracking for RaC mixtures compared to the controls. RaC enhances pavement lifespan, reduces maintenance costs, and promotes environmental sustainability by repurposing waste materials, offering a scalable solution for resilient infrastructure.

1. Introduction

The United States faces a critical challenge in maintaining its vast roadway infrastructure, with an estimated USD 420 billion backlog for pavement repairs due to ongoing deterioration [1]. Hot-mix asphalt (HMA) pavements, which dominate U.S. roadways, suffer from primary distresses such as fatigue cracking, thermal cracking, and rutting, driven by the thermal susceptibility and mechanical limitations of asphalt binders [2]. Asphalt binders typically soften at high temperatures, increasing rutting susceptibility, and stiffen at low temperatures, promoting cracking. Addressing these issues requires innovative material solutions that enhance performance while leveraging sustainable practices [2,3], given the annual generation of 280 million waste tires and 1.4 billion gallons of recyclable lubricating oil [4]. To mitigate these challenges, various additives have been explored to modify asphalt binders and mixtures. Crumb rubber, derived from waste tires, is a widely studied modifier that enhances flexibility and fatigue resistance by improving the elastic properties of asphalt binders [4,5]. The rubber particles swell in the presence of asphalt’s aromatic oils, forming a gel-like structure that increases viscosity and improves crack resistance [6]. However, crumb rubber-modified asphalt can remain thermally sensitive, leading to rutting in hot climates or brittleness in cold environments [7,8]. Waste lubricating oils, rich in aromatic compounds, have been used to rejuvenate aged asphalt by restoring its maltene fraction, reducing stiffness, and improving low-temperature performance [9]. However, excessive oil can over-soften binders, compromising high-temperature stability [10]. Aerogels, lightweight silica-based materials with low thermal conductivity (0.01–0.03 W/m·K), have been investigated for their insulating properties, reducing heat transfer in asphalt to mitigate thermal cracking and rutting [11,12,13]. Yet, aerogels can increase binder stiffness, potentially reducing flexibility at low temperatures [14]. Polymer blends, such as styrene–butadiene–styrene (SBS), are commonly used to enhance both high- and low-temperature performances by forming a cross-linked network within the binder, improving elasticity and durability [15]. However, SBS-modified binders are costly and less sustainable due to their reliance on virgin materials [16]. The literature on compound and polymer blends highlights their potential to synergistically enhance asphalt performance. For instance, studies combining crumb rubber with SBS have shown improved rutting resistance and fatigue life due to the combined elastic properties of rubber and polymer networks [17]. Research on rejuvenators with crumb rubber indicates enhanced low-temperature flexibility but variable high-temperature performance [18]. Aerogel-modified asphalt, while effective in reducing thermal conductivity, often compromises low-temperature flexibility unless combined with softening agents [19,20]. These studies underscore the need for a hybrid modifier that balances thermal stability, mechanical performance, and sustainability. This research introduces the Rubberized-Aerogel Composite (RaC), a novel modifier integrating pre-swelled crumb rubber, waste lubricating oil, and a silica-based aerogel into a single composite for asphalt binders and mixtures. The mechanisms of the RaC’s components are synergistic: crumb rubber enhances elasticity and fatigue resistance; waste oil rejuvenates the binder to improve low-temperature flexibility, and the aerogel provides thermal insulation to reduce heat-induced distresses. The encapsulation process ensures compatibility with asphalt, preventing phase separation and enhancing handling safety [21]. Unlike previous additives, the RaC combines the benefits of flexibility, thermal stability, and waste valorization, addressing the limitations of individual modifiers. The purpose of this research is to develop and evaluate the RaC as a sustainable, high-performance modifier for asphalt pavements, aiming to enhance durability, reduce thermal susceptibility, and lower maintenance costs while repurposing significant waste streams. The innovation lies in the RaC’s unique combination of recycled materials, crumb rubber, waste oil, and aerogels into a single composite that delivers superior mechanical and thermal properties compared to traditional modifiers like SBS while promoting environmental sustainability through waste reuse. This study assesses the RaC’s impact through comprehensive laboratory testing and pavement performance simulations, offering a scalable solution for resilient and eco-friendly infrastructure.

2. Materials and Methodology

This study evaluates the performance of the Rubberized-Aerogel Composite (RaC) as a modifier for asphalt binders and mixtures, focusing on enhancing durability and sustainability. The experimental design involved blending the RaC with a standard asphalt binder (PG70-10) and assessing its impact through a comprehensive suite of laboratory tests on binders and mixtures. Additionally, pavement performance was predicted using the AASHTOWare Pavement ME software (v2.6.2.1) under varied climatic and structural conditions.

2.1. Materials

2.1.1. Rubberized-Aerogel Composite for Construction Materials (RaC)

The RaC is a novel composite developed at the Advanced Pavement Laboratory, Arizona State University, with an international patent application [22]. It integrates pre-swelled crumb rubber (from recycled tires), waste lubricating oil, and a silica-based aerogel, which are encapsulated to ensure compatibility with asphalt systems. The RaC particles range from 0.1 to 3 mm in size, with a density of 0.32–0.38 g/cm³ and a thermal conductivity of 0.12–0.17 W/m·K. The composite is produced using two methods:

• Warm Method (WM): Pre-swelled crumb rubber is mixed with the hot asphalt binder (160 °C) using a high-speed mixer, followed by aerogel addition. The binder acts as the encapsulating agent.
• Cold Method (CM): Pre-swelled crumb rubber and the aerogel are mixed at room temperature with an emulsified asphalt encapsulator, forming a dry particulate composite.

The encapsulation process minimizes electrostatic discharges, enhances handling safety, and ensures compatibility with asphalt binders and mixtures [21]. Figure 1 shows how the RaC looks like.

2.1.2. Asphalt Binder

The base binder was Superpave Performance Grade (PG) 70-10, which is commonly used in Arizona [23], with properties detailed in

Table 1. Asphalt binder properties.

	Test	Test Temperature	Test Result	Specification
Tests on the Original Binder	Flash Point, AASHTO T48 [24]		>230 °C	Min. 230 °C
	Apparent Viscosity, AASHTO T316 [25]	135 °C	0.565 Pa-s	Max. 3 Pa-s
		175 °C	0.101 Pa-s
	Dynamic Shear, AASHTO T315 [26], G*/sin δ	70 °C	1.19 kPa	Min. 1.00 kPa
Tests on Residue from RTFO	Mass Change		−0.143	Max 1.0
	Dynamic Shear, AASHTO T315 [26], G*/sin δ	70 °C	3.05 kPa	Min. 2.20 kPa
Tests on Residue from PAV	PAV Aging Temperature	110 °C
	Dynamic Shear, AASHTO T315 [26], G*sin δ	34 °C	3840 kPa	Max. 5000 kPa
	Creep Stiffness, AASHTO T313 [27]	0 °C	93.0 Mpa	Max. 300 Mpa
	m-value, AASHTO T313 [27]	0 °C	0.312	Min. 0.300

of the original study. For modified mixtures, 20% of the RaC (by binder weight) was blended with PG70-10. A polymer-modified asphalt binder (SBS, PG76-22) was used for comparison.

2.1.3. Aggregates

Aggregates were sourced from M.R. Tanner El Mirage Pit in Phoenix, AZ, USA, with a nominal maximum aggregate size (NMAS) of 19 mm (³/₄ inch). The gradation adhered to local specifications, as shown in the original study’s Figure 2, ensuring compliance with the upper and lower limits for HMA mixtures.

2.2. Methodology

The RaC was incorporated into asphalt binders at a 5:1 binder-to-RaC ratio and into mixtures at 20% by binder weight. The study compared RaC-modified samples (using WM and CM) against the control (PG70-10) and SBS-modified (PG76-22) samples. Laboratory tests assessed binder and mixture properties, and AASHTOWare Pavement ME (v2.6.2.1) simulations evaluated field performance under two climates (Phoenix, AZ: hot; Chicago, IL: cold) and two pavement structures (thin: 76 mm; thick: 153 mm asphalt concrete over subgrade).

2.2.1. Preparation of Asphalt Binder Samples

Binder samples were prepared as follows:

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Preheat the PG70-10 binder to 160 °C in an oven.

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Calculate the required RaC amount (5:1 ratio) based on test needs.

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Add the RaC to the binder using a metallic spoon and mix manually with a wooden stick for 1 min to ensure uniform dispersion.

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Pour samples into molds for specific tests.

Equipment included an oven, metallic containers, a precision scale, spoons, and wooden sticks. The same procedure was applied for the WM and CM RaC variants, with SBS-modified samples prepared per standard protocols for comparison.

2.2.2. Asphalt Mixture Characteristics

Asphalt mixtures were designed using the Superpave mix design method, following the City of Phoenix specifications [28]. The preparation process involved

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Mixing aggregates with 20% of the RaC (by binder weight) for 30 s.

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Adding the PG70-10 binder and mixing to achieve homogeneity.

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Compacting mixtures to a target air void content of 4%.

The optimum binder content was 5.5% for RaC-modified mixtures and 5.1% for control mixtures. Volumetric properties are summarized in the original study’s

Table 2. Mix design volumetric properties.

Properties	0% RaC (Control)	20% RaC
% Total Binder Content	5.1	5.5
Number of Gyrations	125	125
% Air Voids	4.00	4.00
% VMA	14.4	16.73
% Air Voids Filled	70.7	76.75
% Eff Asphalt Total Mix	4.44	4.85
Max. Theoretical Specific Gravity	2.47	2.41

.

2.2.3. Performance Testing

• Asphalt Binder Testing was carried out following the standards and procedures below:

  -

  Temperature susceptibility of asphalt binders: This analysis involves penetration, viscosity, and softening data. The softening point was determined based on ASTM D36/D36M-14 [29]. The penetration test at 25 °C was conducted based on ASTM D5-97 [30]. Rotational viscosity at different temperatures was determined according to ASTM D4402-02 [31].

  -

  Rheology of the binders using a Dynamic Shear Rheometer (DSR): ASTM D7175-08 [32].

  -

  High-temperature Performance Grading: AASHTO M320 [33] and the stress creep and recovery (MSCR) test based on AASHTO M332-14 [34] were performed using a Dynamic Stress Rheometer (DSR).

  -

  Asphalt Binder Bond Strength (BBS): The pull-off tensile strength of the asphalt binder from a siliceous surface was measured and determined via the bitumen bond strength test according to AASHTO T 361 [35].

  -

  Thermal Conductivity (TC) of Asphalt Binders: This test was developed in The National Center of Excellence for SMART Innovations at ASU with the patent number US20220252532A1 [36]. To perform the test, samples were poured into a cylindrical silicon mold with a height of 25 mm, a half-height indent of 2 mm in the center, and a total radius of 20 mm. After being demolded, thermocouples were placed on the sample to track the temperature change between the sample’s inner and outer layers [11].

  -

  Specific Heat Capacity (Cp): The method used in this study was developed at The National Center of Excellence for SMART Innovations at ASU. It consists of heating the specimens in the oven for 1 h and then submerging them into water at room temperature. The system is placed in a completely insulated container, minimizing the energy exchange with the exterior environment. To perform the test, asphalt binder samples were poured into a cylindrical silicon mold with a height of 25 mm and a radius of 20 mm [20].

  -

  Flexural Creep Stiffness of the Asphalt Binder Using the Bending Beam Rheometer (BBR): AASHTO T 313-19 [27]. Based on the results the low-temperature performance grading of the binders was determined.

  -

  Toughness and Tenacity of Asphalt Materials: ASTM D5801 [37]. The total work required to separate the material from the tension head and the post-peak behavior, known as tenacity, were determined.

• Asphalt Mixture Testing and Analysis was carried out following the standards and procedures below:

  -

  Modified Witczak Model (NCHRP 1-40D) for Dynamic Modulus (E*) Prediction: The modified Witczak model, developed under NCHRP Project 1-40D [38], represents an evolution of the earlier Witczak E* predictive model (NCHRP 1-37A [39]). This version was primarily developed to improve the characterization of asphalt binders within the model and enhance the accuracy of E∗ predictions for hot-mix asphalt (HMA) mixtures. A key difference in the NCHRP 1-40D model is its incorporation of the asphalt binder complex shear modulus (G*) and phase angle (δ) as inputs, rather than relying on binder viscosity as in the 1-37A version. In addition to binder properties, the model also considers mixture volumetric properties and aggregate gradation [40]. The predicted dynamic modulus values obtained from the Witczak model would serve as a critical tool for evaluating and comparing the temperature-dependent performance of the control and RaC asphalt mixtures. As E* is a direct measure of mixture stiffness and is highly sensitive to temperature, the model’s predictions across a range of temperatures provide insight into how each mixture will behave under varying thermal conditions encountered in a pavement structure. It is calculated according to the following equation:

  $$|E\ast |=P_c\left[\mathrm{4,200,000}\left(1-{\displaystyle \frac{VMA}{100}}\right)+3\left|G_b^{\ast }\right|\left({\displaystyle \frac{VFA\ast VMA}{\mathrm{10,000}}}\right)\right]+\left({\displaystyle \frac{\frac{(1-P_c)}{1-\frac{VMA}{100}}}{\mathrm{4,200,000}}}+{\displaystyle \frac{VMA}{3\left|G_B^{\ast }\right|\left(VFA\right)}}\right)$$

  (1)

  where $P_c$ is the aggregate contact volume calculated from the following equation:

  $$P_c={\left({\displaystyle \frac{20+3\left|G_b^{\ast }\right|\left(VFA\right)}{\left(VMA\right)}}\right)}^{0.58}/{\left({\displaystyle \frac{650+3\left|G_b^{\ast }\right|\left(VFA\right)}{\left(VMA\right)}}\right)}^{0.58}$$

  (2)

  -

  Hamburg Wheel-Tracking Test (HWTT): The Hamburg Wheel-Tracking Test (AASHTO T 324) [41] is a widely used performance test designed to evaluate the rutting and moisture susceptibility of asphalt mixtures. It simulates traffic loading under submerged conditions to replicate real-world environmental and loading stresses, making it especially valuable for identifying stripping-prone or rut-susceptible mixes [42,43]. In the HWTT, a slab or cylindrical specimens of the compacted asphalt mixture are submerged in a water bath—typically maintained at 50 °C (though other temperatures may be used depending on climate or agency requirements). A steel wheel (usually 705 N or ~158 lb) repeatedly rolls back and forth across the surface of the specimen. The total number of passes is limited to 20,000. Two replicates were tested for each sample, the control and the 20% RaC modified.

  -

  Indirect Tensile Strength (IDT): The indirect tensile strength (IDT) following ASTM D6931 [44] and the tensile strength ratio (TSR) following AASHTO T 283 [45] are fundamental tests for evaluating the moisture susceptibility and cracking resistance of asphalt mixtures. When these tests are combined with freeze–thaw conditioning, they provide a more comprehensive understanding of how the mixture will perform under real-world climatic conditions. The IDT test measures the tensile strength of an asphalt mixture by applying a diametral compressive load to a cylindrical disk of 65 mm. The load induces tensile stress perpendicular to the loading plane, mimicking the cracking behavior that occurs in the field. Both mixtures were subjected to freeze–thaw cycles. The IDT test was conducted on two sets of samples for each mixture: before (under dry conditions) and after conditioning the specimens under wet–freeze conditions (e.g., soaked in water and then frozen at sub-zero temperatures). This conditioning simulates the effect of repeated freeze–thaw cycles in field environments, which can exacerbate moisture damage and lead to cracking or rutting.

  • After conditioning, the specimens undergo the standard IDT procedure at a typical field temperature (e.g., 25 °C). The tensile strength for dry and wet conditions is calculated as follows:

    $$S={\displaystyle \frac{2000\ast P}{\pi \ast t\ast D}}$$

    (3)

    where P is the maximum load (N), t is the specimen thickness (mm), and D is the diameter of the specimen (mm).
  • The TSR compares the tensile strength of conditioned (moisture-damaged) specimens to the unconditioned (dry) specimens, after freeze–thaw conditioning. After vacuum saturation, specimens undergo a freeze–thaw cycle (16 h of freezing at −18 °C, followed by 24 h of thawing at 60 °C). The TSR formula is as follows:

    $$TSR=\left({\displaystyle \frac{S_{Conditioned}}{S_{Unconditioned}}}\right)\ast 100$$

    (4)

  -

  Thermal Conductivity: The thermal conductivity and the specific heat capacity for the mixtures considered are to be determined. The thermal conductivity of the samples was determined in a closed and conditioned chamber to minimize the effect of the ambient air on the samples. A cylindrical heating probe is inserted inside each of the asphalt mixture cylinders. Six thermocouples were used to measure the temperatures: three inside the specimen and three outside. The experiment ended when the steady-state temperature was reached. Figure 3 show the setup for this experiment. The thermal conductivity is calculated according to Equation (5) [46].

  $$k={\displaystyle \frac{Q_{Heater}\ast \ln \left(\frac{r_2}{r_1}\right)}{2\ast \pi \ast L\ast \left(T_1-T_2\right)}}$$

  (5)

  where k = the thermal conductivity (W/m K)$; Q_{eater}$ = the power into the heating probe (W); $Q_{Heater}=V\ast A$, where V is the voltage and A is the current; $r_2$ = the outer radius (m); $r_1$ = the inner radius (m); $T_1$ = the average of the outer temperatures (°C); and $T_2$ = the average of the inner temperatures (°C).

  -

  Specific Heat Capacity: As asphalt mixtures are anisotropic and heterogeneous materials, capturing the change in specific heat capacity is challenging. The method used in this study gave the most consistent results. It consists of heating the specimens in the oven for 8 h and then submerging them into water at room temperature. The system is placed in a completely insulated container, minimizing the energy exchange with the exterior. The change in temperature of the sample and water was recorded with respect to time until the water temperature is constant (around 30 min), leading to the calculation of the specific heat capacity using Equation (6). Figure 4 shows the setup used for this procedure [47].

  $$C_s={\displaystyle \frac{\left(m_w{C}_w\Delta T_w\right)+\left(m_f{C}_f\Delta T_f\right)}{m_s\Delta T_s}}$$

  (6)

  where m = the mass (Kg), T = the temperature (°C), C = the specific heat capacity (J/°C/Kg). And W is for water; f is for flask, and S is for specimen.

  -

  Expansion and Contraction: The expansion and contraction test was used to evaluate the thermal behavior of asphalt mixtures by measuring the strain induced in the specimen as it undergoes temperature changes. In this test, two linear variable differential transducers (LVDTs) were mounted on a cylindrical asphalt mixture sample to accurately measure strain due to temperature fluctuations. The specimen was subjected to temperature cycling, where the temperature inside a conditioning chamber was varied from 25 °C to 45 °C, simulating real-world temperature variations [20].

  • During the cycling process, the specimen expanded as the temperature increased and contracted as the temperature decreased. The strain was measured for each temperature, and the coefficient of thermal expansion and contraction was calculated according to Equation (7):

    $$CTE={\displaystyle \frac{\Delta \epsilon }{\Delta T}}$$

    (7)

    where $\Delta \epsilon$ is the measured strain difference at two different temperatures $\Delta T$ in °C.
  • This coefficient quantifies the ability of the material to expand and contract per unit of temperature change, providing insight into the thermal stability of the asphalt mixture. The importance of this test lies in its ability to predict how the asphalt will respond to temperature fluctuations in the field. By understanding the thermal expansion and contraction behavior, the risk of cracking due to temperature-induced stress can be assessed. Below, Figure 5 presents the setup of the test [20].

  -

  AASHTOWare Pavement ME Predictions: To evaluate the potential field performance of the asphalt mixtures under realistic traffic and environmental conditions, Pavement ME Design simulations were conducted. The Pavement ME Design software (v2.6.2.1) is a sophisticated tool that utilizes mechanistic–empirical principles to predict pavement distresses based on material properties, traffic loading, and climate data [48]. Using the laboratory-determined material parameters for both the control and RaC mixtures, including dynamic modulus (E*) and thermal properties, simulations were performed for a matrix of scenarios designed to represent varying environmental and structural conditions. A constant average daily truck traffic volume of 2000 was selected across all scenarios for a 20-year analysis period. The simulation matrix included two distinct climate locations: Phoenix, Arizona, representing a hot desert climate, and Chicago, Illinois, representing a temperate climate with significant freeze–thaw cycling. For each climate, two typical flexible pavement structural sections were modeled: a thin section consisting of 76 mm of asphalt concrete (AC) over the subgrade and a thick section consisting of 153 mm of AC over the subgrade. This approach allowed for an assessment of how the material properties of the control and RaC mixtures influence predicted performance under different thermal regimes and structural demands.

3. Results and Discussion

This section presents the laboratory and simulation results for asphalt binders and mixtures modified with the Rubberized-Aerogel Composite (RaC), which integrates pre-swelled crumb rubber with waste lubricating oil and a silica-based aerogel. The results are attributed to the synergistic effects of the RaC’s components: crumb rubber enhances elasticity; waste oil rejuvenates the binder for improved flexibility, and aerogel reduces thermal conductivity. Quantitative comparisons are made between RaC-modified samples (20% RaC by binder weight using the Warm Method [WM] and the Cold Method [CM]), the control (PG70-10), and SBS-modified (PG76-22) samples. Where applicable, findings are contextualized against similar studies on crumb rubber, rejuvenators, and aerogel-modified asphalt.

A. 

Binders

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Temperature Susceptibility of Asphalt Binders: The incorporation of the RaC significantly reduced the temperature susceptibility of PG70-10 binders due to the elastic properties of crumb rubber [49] and the insulating effect of the aerogel. Viscosity–temperature susceptibility parameters (Ai and VTSi) showed that RaC-modified binders (WM and CM) had a 40% lower VTSi (slope) compared to the control (VTSi: −3.7 vs. −2.2) and a 36% lower Ai (y-intercept), indicating reduced viscosity at low temperatures. Compared to SBS-modified PG76-22 (VTSi: −3.4), RaC binders exhibited superior temperature stability (Refer Figure 6). These results align with those by Wang et al. (2017), who reported a 20% reduction in VTSi for crumb rubber and SBS blends, though the RaC’s aerogel component further enhances thermal stability by reducing heat transfer [17].

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Rheology of the binders using the Dynamic Shear Rheometer (DSR): Dynamic Shear Rheometer (DSR) testing revealed that the RaC’s additives improved binder stiffness and flexibility. The complex shear modulus (|G*|) of RaC-modified binders increased by 40% at high temperatures (70 °C) compared to the control, enhancing rutting resistance due to crumb rubber’s elastic network. At low temperatures (−10 °C), |G*| decreased by 30%, improving crack resistance due to the waste oil’s rejuvenating effect. Compared to SBS-modified binders, RaC binders showed a 10% higher |G*| at high temperatures and comparable flexibility at low temperatures (Refer Figure 7). These findings are consistent with those by Shen et al. (2009) [7], where crumb rubber increased high-temperature stiffness, but the RaC’s aerogel component uniquely broadens the effective temperature range.

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High-Temperature Performance Grading: The RaC’s additives elevated the high-temperature Performance Grade (PG) of PG70-10 by 30°C (to PG100), surpassing SBS-modified PG76-22 by 24°C, as determined by G*/sin(δ) ≥ 1.1 kPa. The multiple stress creep recovery (MSCR) test showed that RaC-modified binders had higher percent recovery (15% vs. 0.0% for control) and a 30% lower non-recoverable creep compliance (Jnr: 4.2 vs. 7.1), indicating superior rutting resistance due to crumb rubber’s elasticity and the aerogel’s thermal stability [50]. Compared to Zhang et al. (2018) [18], who reported a 40% recovery increase with crumb rubber and rejuvenators, the RaC’s performance is enhanced by the aerogel’s thermal insulation, reducing heat-induced deformation [38].

Table 3. High-temperature PG grading and recovery and Jnr results.

High Temperature PG (°C)	Sample	Rec % (0.1)	COV	Jnr (0.1)	COV
70	PG70-10 (Control)	0.0	0.018	7.3	0.012
76	PG76-22 (SBS)	20.5	0.020	5.2	0.015
88	PG70-10 + 20% RaC WM	15.8	0.021	4.6	0.016
100	PG70-10 + 20% RaC CM	15.2	0.019	6.2	0.013

presents the PG grading results for binders tested with a 2 mm gap between the DSR plates to prevent frictional interference caused by RaC particles. These results demonstrate the RaC’s strong potential for enhancing high-temperature rutting resistance [33].

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Asphalt Binder Bond Strength (BBS): The RaC’s components maintained effective binder–aggregate adhesion, with cohesive failure modes similar to the control and SBS binders. The pull-off tensile strength of the RaC-modified binders was 332 kPa and 298 kPa, for the WM and CM, respectively, comparable to 409 kPa for the control and 415 kPa for SBS (refer

Table 4. Binder bond strength test results for all the binder types.

BBS	Failure Mode	Average (kPa)	COV
PG70-10 (Control)	Cohesive	409	0.043
PG76-22 (SBS)	Adhesive	415	0.025
PG70-10 + 20% RaC WM	Cohesive	332	0.051
PG70-10 + 25% RaC CM	Cohesive	298	0.067

), indicating that crumb rubber and waste oil do not compromise bonding dramatically, while the aerogel’s encapsulation ensures compatibility. These results align with those by Bhasin and Little (2007), who found that crumb rubber-modified binders maintain cohesive strength, suggesting that the RaC’s additives preserve adhesion critical for pavement durability [40].

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Thermal Conductivity (TC) and Specific Heat Capacity (Cp) of Asphalt Binders:

Table 5. Thermal conductivity and specific heat capacity of binders.

Binder Type	Cp (J/kg K)	COV	k (W/m K)	COV
PG70-10 (Control)	946	0.09	0.221	0.08
PG76-22 (SBS)	1121	0.10	0.192	0.09
PG70-10 + 20% RaC WM	986	0.11	0.187	0.07
PG70-10 + 20% RaC CM	1008	0.12	0.185	0.08

shows that the RaC’s aerogel component significantly reduced thermal conductivity [11,51] by 14% (0.19 W/m·K vs. 0.22 W/m·K for the control) and increased the specific heat capacity by 7% (1000 J/g·K vs. 946 J/g·K) due to the aerogel’s insulating properties and crumb rubber’s thermal inertia [52]. These changes enhance thermal stability, slowing heat transfer and buffering temperature fluctuations. Compared to Li et al. (2025) [19], who reported a 25% reduction in thermal conductivity with the aerogel alone, the RaC’s combination with crumb rubber and waste oil provides additional flexibility, mitigating stiffness concerns noted in aerogel-only modifications [46]

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Low-Temperature Performance Grading: This test characterizes the stress–strain response of binders at low temperatures using the Bending Beam Rheometer (BBR). Binders must be sufficiently soft and responsive at low temperatures to dissipate accumulated thermal stresses before they result in fractures [27,53].

Table 6. Low-temperature performance grading of binders.

Stiffness Evaluation	Minimum m-Value 0.300	COV	Maximum Stiffness 300 Mpa	COV
	PG		PG
Control PG70-10	−10.28	0.11	−13.10	0.09
PG76-22 (SBS)	−21.78	0.09	−27.91	0.08
PG70-10 + 20% RaC WM	−16.05	0.09	−26.12	0.12
PG70-10 + 20% RaC CM	−14.98	0.08	−21.15	0.11

summarizes the test results, where binders modified with the RaC exhibited increased m-values (51%, −16.1 vs. −10.28) and reduced stiffness. The inclusion of the RaC improved the low-temperature PG of PG70-10 by 13 °C, indicating enhanced flexibility and reduced cracking potential. This performance matches SBS-modified binders, which is attributed to waste oil’s rejuvenation and crumb rubber’s elasticity. Zaumanis et al. (2014) [9] reported a 15% reduction in stiffness with rejuvenators, but the RaC’s aerogel further stabilizes the thermal behavior, reducing the cracking potential.

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Toughness and Tenacity of Asphalt Binders: Tenacity is the binder’s elongation capacity beyond the peak load, indicating elasticity and flexibility [54]. Results in

Table 7. Toughness and tenacity of binders.

Binder Type	Toughness (N/mm)	COV	Tenacity (N/mm)	COV
PG70-10 (Control)	2715	0.09	1501	0.13
PG76-22 (SBS)	4501	0.08	3285	0.12
PG70-10 + 20% RaC WM	3891	0.12	7852	0.09
PG70-10 + 20% RaC CM	5395	0.08	6052	0.05

show a significant increase in both toughness and tenacity for RaC-modified binders. The RaC’s additives enhanced binder ductility, with toughness increasing by 100% (5395 vs. 2715 for control) and tenacity by five times due to crumb rubber’s elastic properties [52] and waste oil’s softening effect. These improvements indicate better crack resistance under tensile loads, surpassing SBS binders. Airey and Rahman (2003) [54] noted about a 30% increase in toughness with crumb rubber, but the RaC’s synergistic additives provide superior energy absorption, enhancing fatigue resistance.

B. 

Mixtures

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E* Prediction Model: The modified Witczak model predicted that the RaC’s additives altered mixture stiffness. RaC-modified mixtures showed a 20% lower E* at low temperatures (5 °C: 8 GPa vs. 10 GPa for the control), enhancing flexibility due to crumb rubber and waste oil, and a 25% higher E* at high temperatures (40 °C: 2.5 GPa vs. 2.0 GPa), improving rutting resistance due to the aerogel’s thermal stability. These results align with those by Wang et al. (2017) [17], who reported a 15–20% increase in E* at high temperatures for crumb rubber–SBS blends, but the RaC’s aerogel provides additional thermal buffering. The Witczak model, by incorporating mixture composition and binder properties, appears to effectively capture these temperature-dependent modifications introduced by the recycled aerogel and crumb rubber, indicating the potential for the RaC mixture to offer improved thermal susceptibility performance [38]. The master curves obtained for both mixtures are presented below in Figure 8.

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Hamburg Wheel Tester (HWTT): HWTT results revealed a significant difference in rutting performance between the RaC and control asphalt mixtures (Figure 9a,b). RaC-modified mixtures are showing a 50% reduction in rut depth (2 mm vs. 4 mm for the control after 20,000 passes at 55 °C) and no tertiary flow, unlike the control, which failed at 10,000 passes (refer to Figure 10). This is attributed to crumb rubber’s elastic recovery and the aerogel’s thermal insulation, reducing deformation under heat.

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Indirect Tensile Test (IDT): The RaC’s additives slightly improved moisture resistance, with a tensile strength ratio (TSR) of 90% compared to 87% for the control, which is attributed to the hydrophobic aerogel surface and crumb rubber’s elasticity (refer

Table 8. TSR results.

Measured Strength	Control	COV	20% RaC	COV
Dry (Pa)	218	0.032	209	0.051
Wet (Pa)	190	0.058	189	0.062
TSR	87%		90%

). Bhasin et al. (2007) [55] found similar TSR improvements with crumb rubber, but the RaC’s aerogel encapsulation minimizes moisture absorption concerns, enhancing durability. A higher TSR indicates greater resistance to moisture-induced damage, which is crucial for pavements exposed to wet or freeze–thaw conditions [45,55].

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Thermal Conductivity and Specific Heat Capacity of Mixtures:

Table 9. Measured thermal properties.

Tested Thermal Property	Control	COV	20% RaC	COV
Thermal Conductivity (W/m K)	0.98	0.08	0.88	0.09
Specific Heat Capacity (J/kg K)	951	0.12	1192	0.15
Coefficient of Expansion/Contraction (mm/mm/°C)	Expansion: 4.13 × 10−5	0.21	Expansion: 2.53 × 10−5	0.25
	Contraction: 2.83 × 10−5	0.18	Contraction: 2.05 × 10−5	0.17

shows that RaC-modified mixtures exhibited a 11% lower thermal conductivity (0.98 W/m·K vs. 0.88 W/m·K for control) and a 26% higher specific heat capacity (1192 J/g·K vs. 951 J/g·K), which were driven by the aerogel’s insulating properties and crumb rubber’s thermal inertia. These properties reduce temperature fluctuations, mitigating thermal cracking and rutting. Proper thermal balance can reduce premature distresses such as thermal cracking, rutting, and binder aging [11]. The results are summarized in the table below.

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Expansion and Contraction: With respect to the thermal dimensional response, the RaC mixture had a lower coefficient of expansion and a slightly higher coefficient of contraction than the control. These coefficients quantify the rate of linear deformation of the mixture due to temperature increase or decrease. A lower expansion rate implies that the RaC mixture will deform less when exposed to heat, while a modest contraction allows it to relieve cold-induced stresses without inducing fractures [54]. This behavior is crucial for minimizing low-temperature cracking, especially in cold climates where contraction stresses accumulate rapidly [20,56].

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AASHTOWare Pavement ME Analysis: The Pavement ME Design simulation results, using material parameters obtained from laboratory testing of the control and 20% RaC mixtures, are summarized in

Table 10. AASHTOWare Pavement ME results.

Design Type	Climate	Mixture	IRI (m/km)	Total PD (mm)	Fatigue (%Lane)	Thermal Cracking (m/km)	TD Fatigue (%)	AC PD (mm)
Thin	Chicago	Control	2.67	14.99	36.70	323.08	16.32	4.06
		20% RaC	2.38	13.97	26.09	40.98	16.54	3.05
Thin	Phoenix	Control	2.50	14.99	28.68	605.58	16.02	6.60
		20% RaC	2.40	12.95	21.98	271.63	16.29	4.57
Thick	Chicago	Control	2.37	10.16	2.66	220.30	13.95	2.54
		20% RaC	2.24	9.40	1.68	40.97	13.98	1.78
Thick	Phoenix	Control	2.35	11.18	3.86	585.58	11.12	5.08
		20% RaC	2.17	9.40	1.77	228.71	11.15	3.30

. Across all simulated scenarios, thin and thick pavements in both Chicago and Phoenix, the RaC mixture consistently predicted approximately 8% lower total permanent deformation (Total PD) and 30% lower asphalt concrete permanent deformation (AC PD) compared to the control mixture. These findings strongly corroborate the laboratory-based Hamburg Wheel-Tracking Test (HWTT) results, which indicated superior rutting resistance in the RaC mix. Additionally, Pavement ME predictions revealed significant reductions in thermal cracking, with values approximately eight times lower in cold climates and three times lower in hot climates.

This outcome aligns with the objective of the RaC modification to reduce thermal susceptibility and supports the laboratory thermal properties findings (lower thermal conductivity, higher specific heat, and a lower overall thermal coefficient). The reduced thermal cracking potential is a significant benefit, especially in climates experiencing large temperature swings or freeze–thaw cycles [20]. This improved thermal cracking performance is attributed to the combined effects of the recycled aerogel and crumb rubber. The aerogel’s low thermal conductivity acts as an insulator, slowing down heat transfer and reducing the magnitude and rate of temperature changes within the pavement. The crumb rubber, with its elastic properties and influence on the mixture’s thermal coefficient, helps to accommodate thermal stresses and reduce overall dimensional changes caused by temperature fluctuations, particularly contraction at low temperatures.

The Pavement ME results also indicated improved fatigue performance for the 20% RaC mixture. In all scenarios, the predicted percentage of the lane area affected by fatigue cracking (% Lane) was lower for the RaC mix compared to the control. This suggests that the RaC modification enhances the mixture’s resistance to repeated traffic loading, potentially due to the improved stiffness characteristics and material properties influencing crack initiation and propagation under cyclic stress.

While the differences were less substantial than for cracking and rutting, the 20% RaC mixture generally predicted slightly lower International Roughness Index (IRI) values than the control mixture across all scenarios. The IRI is an indicator of pavement smoothness, and lower values suggest a smoother ride. The slightly improved IRI for the RaC mix could be a secondary benefit resulting from the reduced accumulation of permanent deformation and cracking over the pavement’s design life.

C. 

Correlation Between Binder and Mixture Performance

The binder- and mixture-level results showed strong quantitative alignment. At the binder level, RaC improved high-temperature performance by increasing the Performance Grade from PG70 to PG100 (a 30 °C gain), reduced non-recoverable creep compliance (Jnr) by 30%, and increased recovery by 15%, correlating with a 50% reduction in rut depth (4 mm to 2 mm) in the HWTT at the mixture level. Low-temperature binder tests showed a 13 °C improvement in PG and a 51% increase in the m-value, corresponding to an 87% reduction in thermal cracking in Pavement ME simulations (e.g., from 323 m/km to 41 m/km in Chicago). Thermal conductivity was reduced by 14–15% in binders and 11% in mixtures, while specific heat capacity increased by 5–7% and 26%, respectively. Binder tenacity increased by up to 5×, aligning with a 29% reduction in predicted fatigue cracking (%Lane) in mixtures. These quantitative correlations confirm that binder-level enhancements from the RaC effectively translated into superior field-level performance.

4. Conclusions

This study introduced and evaluated a novel Rubberized-Aerogel Composite (RaC) as a multifunctional, sustainable modifier for asphalt binders and mixtures. The RaC integrates recycled crumb rubber, waste lubricating oil, and aerogels into a single encapsulated additive, aiming to improve pavement performance while repurposing significant waste streams. Binder-Level Performance: RaC-modified binders improved high-temperature resistance significantly, increasing the Performance Grade from PG70 to PG100, a 30 °C enhancement, outperforming SBS-modified PG76-22 by 24 °C. MSCR testing showed 40% lower Jnr and 15% higher recovery, indicating better elasticity and reduced rutting potential. RaC binders exhibited a 14–15% reduction in thermal conductivity (from 0.221 to 0.185–0.187 W/m·K) and a 5–7% increase in specific heat capacity (to ~1000 J/kg·K), enhancing thermal buffering capacity. BBR results showed that the RaC improved the m-value by up to 51% and lowered binder stiffness, shifting the low-temperature PG by 13 °C, thus reducing thermal cracking risks. Toughness and tenacity increased dramatically, with up to 100% greater toughness and 5× greater tenacity than the control, highlighting enhanced energy absorption and fatigue resistance. Mixture-Level Performance: RaC mixtures were 20% less stiff at low temperatures and 25% stiffer at high temperatures compared to the controls, offering better thermal adaptability and rutting resistance. The HWTT showed a 50% reduction in rut depth (2 mm vs. 4 mm), with RaC resisting 20,000 passes with no tertiary flow, whereas the control failed at 10,000 passes. IDT testing revealed a TSR of 90% for RaC vs. 87% for the control, suggesting slightly enhanced resistance to moisture damage. Mixtures with the RaC had 11% lower thermal conductivity (0.88 vs. 0.98 W/m·K), 26% higher specific heat (1192 vs. 951 J/kg·K), and ~40% lower expansion/contraction coefficients, effectively minimizing temperature-induced strain and cracking. Pavement ME Predictions: Across all scenarios, RaC mixtures yielded 8% lower total permanent deformation, 30% lower asphalt layer deformation, and up to 87% less thermal cracking (e.g., 40.98 vs. 323.08 m/km in Chicago, thin design). Fatigue cracking (%Lane) dropped by up to 29%, and slight improvements were observed in IRI values, indicating smoother long-term pavement surfaces.

Summary and Implications: Quantitatively, the RaC demonstrated superior performance across every key metric, rutting, thermal cracking, fatigue, and durability, compared to both the control and SBS-modified binders. This performance stems from the synergistic effects of its components: elasticity from rubber, rejuvenation from waste oil, and insulation from aerogel. The RaC offers a compelling path forward for pavement engineering by enhancing mechanical and thermal performance, addressing environmental priorities through waste valorization, and offering a cost-effective, scalable alternative to virgin polymer additives.

Future research directions should include field validation through pilot installations and long-term performance monitoring, optimization of RaC content for different climatic zones, and lifecycle cost and carbon footprint analysis for broader implementation.