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Application of crumb rubber in asphalt has demonstrated many performance advantages for the pavement industry while bringing a solution for the disposal of the scrap tire. However, the introduction of rubber to asphalt binder has challenges associated with rubber segregation and workability. To address the latter issues, continuous agitation and high mixing temperature is commonly used which increases energy consumption, disintegration of crumb rubber, and emission of greenhouse gases. This paper introduces a newly developed surface activated rubber (SAR) prepared via a hybrid method of microwave treatment and bio-modification to alleviate the associated problems. The properties and performance of SAR were compared with those of conventional crumb rubber modified (CRM) and microwave-activated crumb rubber modified (M-CRM) asphalt. Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and computational analysis using density functional theory (DFT) indicated successful grafting of biomolecules on the rubber particles. SAR showed 86% reduction in segregation index compared to the other CRM scenarios. Such enhancement was also reflected in a significant improvement in workability of SAR modified binder compared to the other CRM scenarios. Mechanical tests showed high fracture energy (167 J/m2) for SAR modified asphalt, which was approximately three times higher than that of conventional CRM asphalt (57 J/m2).

Produced with fine crumb rubber (CR) particles and at demanding conditions, Terminal Blend (TB) asphalt binder is an alternative approach to incorporating CR from scrap tires into asphalt binders. It offers the advantages of convenient transportation, storage, and usage at asphalt mixture plants. In the study as presented in this paper, Sasobit® and Polyethylene (PE) wax were used as warm-mix additives to further modify the properties of TB asphalt binders. These additives are anticipated to reduce the production temperatures of TB asphalt mixtures, hence reducing fuel consumption and greenhouse gas (GHG) emissions in mixture production and reducing asphalt fumes in construction. It is also anticipated that the engineering performance of TB asphalt binders modified with such additives are not adversely affected. To gain a comprehensive understanding of TB asphalt binders modified with the two additives, a variety of laboratory tests were conducted, including storage stability, high temperature performance, low temperature performance, fatigue, and other basic rheological properties. The costs of producing TB asphalt mixtures with or without the additives were compared, using the conventional hot-mix asphalt mixtures as a reference. Energy consumptions and GHG emissions generated in mixture production were also evaluated. Actual field construction was performed using TB asphalt mixtures with Sasobit® and using conventional asphalt mixtures. Both aerosol and volatile organic compounds (VOCs) samples were collected at zones close to the loose mixtures and were analyzed. The two wax-based additives were found to enhance the storage stability and workability of the TB binders. The other engineering properties related to the use of wax-based additives are mixed. The selection of right type of additive depends on local climate conditions. While the costs of using the TB binders (with or without additives) are higher than the costs of using the base (neat) asphalt binder. Energy consumptions and GHG emissions in mixture production are reduced when the additives are used. Asphalt fumes generated from the base asphalt binder and the TB binder with Sasobit® are comparable. In general, the incorporation of wax-based additives into TB asphalt binder offers a cleaner way of using recycled crumb rubber.

The focus of this paper is to evaluate the energy consumption and environmental impact of rubberized asphalt pavement during the life cycle. Reducing the environmental influences of the highway infrastructure has received the increasing attentions recently. Application of crumb rubber in asphalt pavements overcomes the used tires disposal problem and achieves materials savings and environmental benefits, due to the reduction of raw material and extension of pavement service life. In addition, rubberized asphalt production is a highly energy consuming process due to a higher mixing temperature. However, the warm mixing technology can lower the mixing temperature of rubberized asphalt mixture, leading to approximately 20–25% of fuel savings. Moreover, the consumed energy of rubberized asphalt during maintenance phase is lower than that of conventional asphalt. In addition, the greenhouse gas (GHG) emissions from the production and construction of rubberized asphalt mixtures are similar to those from hot mix asphalt, but their emissions of CO and CH4 are much lower. Meanwhile, the noise reduction is an environmentally friendly benefit. The leachate from asphalts used wet and dry-process crumb rubber modifiers (CRM) does not pose a measurable threat to the environment. Rubberized asphalt technology as a green technology in term of lowering GHG emissions, saving energy and raw material and reducing noise level of pavement should be widely used to achieve environmental sustainability.

This paper systematically investigated the roles of aging and bio-oil regeneration on the self-healing evolution behavior of pure asphalt and terminal blend (TB) rubberized asphalt within wide temperature range, which was expected to promote the applications of self-healing asphalt pavement, meanwhile to accelerate the sustainable disposal of local bio-oil and rubber wastes. The self-healing, flow and elastic recovery (ER) abilities of asphalts within wide temperature range and their relationship were traced through the dynamic shear rheometer based fatigue-healing-fatigue, frequency sweep and creep recovery tests. Results revealed that different effects of temperature on ER and flow abilities of aged or rejuvenated asphalts resulted in the complexity of self-healing evolution behavior. The proposed ER-flow self-healing theory clarified that the ER ability of aged asphalts led the self-healing process in viscoelastic region, and the flow ability dominated as temperature rose into viscous flow region; however, the flow ability of regenerated asphalts dominated the self-healing at all measured temperatures. The aging growth percentage and regeneration recovery percentage (%RR) based on four self-healing characteristic temperatures were proposed to quantify the influences of aging and regeneration on self-healing evolution behavior of asphalts. The climate aging imparted severer impact on the self-healing evolution behavior of pure asphalt than long-term aging, whereas caused less impact on that of TB asphalt. The aging impact on the self-healing evolution behavior of TB asphalt was smaller than that of pure asphalt. Inversely, the %RR of rejuvenated asphalts increased as the growth of bio-oil content, and reached the maximum when adding 4%wt. waste cooking oil residues. The bio-oil elicited the greater regeneration improvement of self-healing performance than petroleum-based rejuvenator. Overall, the bio-oil regeneration recovery of the self-healing evolution performance of aged asphalts was limited.

This study investigates the potential use of crumb rubber and Reclaimed Asphalt Pavement (RAP) in Warm Mix Asphalt (WMA). In fact, the twofold purpose of this research is to improve the performance of asphalt pavement and mitigate the environmental impacts caused by waste tires and aged asphalt pavement. In the present paper, seven different asphalt mixtures including 30% and 60% RAP (by the weight of natural aggregate) were modified by 0, 10%, and 20% crumb rubber powder. To this end, natural aggregate was replaced by the different percentages of RAP and crumb rubber in the asphalt mixtures. Also, high percentages of organic additive, Sasobit, including 4% and 5.5% were employed to the mixtures including 30% and 60% RAP, respectively. The findings of this present investigation resulted from Marshall Stability, moisture susceptibility, fatigue cracking, and permanent deformation tests indicate that the high amount of Sasobit can not only reduce the mixing and compacting temperatures, but also provide a sophisticated asphalt mixture modified by crumb rubber, and simultaneously it can be considered as a practical method against global warming and stockpiling waste materials such as RAP and end-of-life tires. Overly, the rubberized asphalt mixtures including high RAP and high Sasobit additive provided a better behavior against moisture susceptibility, fatigue cracking and permanent deformation in comparison with the control mixture, CW.

The most common primary damage form of rubber asphalt pavements is fatigue cracking. At present, the indirect tension, direct tension, bending, and unconfined compression tests are the main considered fatigue parameters to design the structure of rubber asphalt pavement. However, the fatigue test results analyzed by various test methods have a quite significant divergence, which represents a lack of accuracy to access the durability of rubber asphalt mixture objectively. Thus, in the structural resistance design of rubber asphalt pavement, there exists a problem of artificial randomness of fatigue parameters. Hence, the primary objective of this study is to improve the procedure for determining the structural resistance of rubber asphalt pavement. To this end, the direct tensile, unconfined compressive, and indirect tensile strength tests of rubber asphalt mixture were performed under various loading rates to reveal the velocity-dependent of the strength of rubber asphalt mixture. The model of direct tensile strength, unconfined compressive strength, and indirect tensile strength increasing with loading rate are SD = 7.772v0.205, SC = 1.748v0.204, and ST = 1.077v0.207, respectively. The stress ratio related to both the traditional S–N fatigue equation and the loading rate was defined as the standard stress ratio and the velocity-dependent stress ratio, respectively. The fatigue test of rubber asphalt mixture was conducted. The results from those experiments were analyzed. A fatigue equation (Nf = tv−5.129 R2 = 0.853) related to the loading rate was established. The results indicated that the viscoelastic characteristics of rubber asphalt mixture are not taken into consideration in the traditional S–N fatigue equation, which contributed to the non-uniqueness and uncertainty of fatigue test results. The viscoelastic characteristics of rubber asphalt mixture are considered by the fatigue equation related to the velocity-dependent stress ratio. Thus, a new method (Kv = Nf0.0958) considering the influence of the loading rate for calculating the strength structure coefficient of the rubber asphalt mixture was proposed. The effectiveness and completeness of the mechanical behavior of pavement structure characterized by material parameters were improved. Thus, the utilization rate of waste rubber for pavement construction can be significantly increased and the life cycle cost of asphalt pavement is reduced.

Use of recycled rubber particles in asphalt pavements has shown to be advantageous to extend pavement service life. The challenge with the use of rubber particles which limits its wide-spread application is the segregation of rubber from asphalt matrix demanding continuous agitation; this in turn increases the cost of applying rubberized asphalt pavement. This study compares the efficacy of chemisorption and physisorption of bio-derived molecules to rubber particles and their effect on rubberized asphalt properties. The chemisorption is studied via grafting biomolecules onto the rubber surface by heat-treatment to create surface activated rubber (SAR). The physisorption is studied via bio-oil’s absorption into rubber particles to produce pre-swelled crumb rubber (PsCR). The grafting process was accomplished by microwave-assisted bio-chemical treatment while the rubber particles pre-swelling was carried out by direct mixing rubber with bio-oil. Both SAR and PsCR are then added to asphalt to create rubberized asphalt for further analysis. The efficacy of each rubberized asphalt was studied via thermo-mechanical characterization. The study of storage stability showed SAR was more effective to reduce segregation index, reducing segregation of rubber and asphalt by 41%. However, the study of elastic recovery showed higher recover for PsCR. The elastic recovery of PsCR was 156 percent higher than rubberized asphalt containing non-swelled particles. PsCR also presented improved anti-cracking performance with 39% creep stiffness decrease and 17% m-value increase. The fluorescence microscopy showed that bio-oil was successfully absorbed by rubber particles significantly improving the dispersion of rubber in asphalt. Both grafting and pre-swelling of rubber particles using biomolecules improved properties of rubberized asphalt. The study results promote the use of recycled rubber to enhance the sustainability of pavement construction while promoting resource conservation and environmental protection.

This study examines the merits of surface activation of rubber using various bio-oils to improve rubber-asphalt interaction. To do so a hybrid method combining microwave irradiation and bio-chemical treatment was used to graft biomolecules onto the exterior surface of the rubber. Five surface activated rubbers were prepared using waste vegetable oil, wood pellet, miscanthus, corn stover, and castor oil. The effectiveness of each oil was examined by measuring the chemisorption of the bio-oil and elastic recovery of bitumen containing rubber particles treated with each bio-oil. Our quantum-based density functional theory calculations showed presence of both physical and chemical interactions between polar aromatic components of bio-oils and rubber. Among studied bio-oils, wood-based bio-oil found to have the highest content of polar aromatics such as phenolic resins leading to its enhanced interaction with rubber. This was evidenced in percent recovery, which was nearly doubled (from 13% to 24%) when wood-based bio-oil molecules were grafted onto the surface of rubber. Overall, wood-based bio-oil was shown to adsorb well to the rubber surface and reduce its tendency to separate from bitumen by 82%. The study results showed how composition of bio-oil affects its efficacy to activate rubber surface. It also proved the technical merits of using surface activated rubber to reduce segregation between rubber and bitumen which commonly occurs in rubberized asphalt. Therefore, the outcome of this study promotes recycling of waste tire to promote sustainability in pavement construction.

Crumb rubber modified asphalt mixture (CRMA) has gained rapid-growing interest as a sustainable paving material, because it allows value-added recycling of waste tire into durable and low-noise asphalt pavements. However, the poor workability of CRMA leads to higher construction temperature, which results in more energy consumption and hazardous emission. Surfactant, a typical warm mix asphalt additive, has been proven to be effective in alleviating the workability concern of CRMA without obviously deteriorating its mechanical properties. Nevertheless, performance of CRMA with surfactant may be influenced by the mixing sequence of different components (raw asphalt, crumb rubber, surfactant and aggregates), which unfortunately has not been well investigated yet. This study aims to address this issue by characterizing the engineering performance of warm CRMA prepared by six different mixing sequences. Properties including Marshall stability and flow value, workability, rheological property, rutting resistance, moisture sensitivity and fatigue resistance of prepared mixtures were measured and compared. Analytic hierarchy process (AHP) was employed to determine the optimal mixing sequence of warm CRMA considering the overall engineering performance. Test results showed that the effect of warm CRMA’s mixing procedure on its engineering performance is noticeable. Earlier incorporation of surfactant additive had limited negative influence on mechanical properties of warm CRMA but allows for more energy saving during the production of rubberized asphalt binder. The AHP analysis results indicated that among the six mixing sequences, the optimal option is to make rubber absorb surfactant first, then incorporating the rubber-surfactant compound to raw asphalt and finally blending the modified binder to aggregates.

In recent years, transportation agencies and the general public alike are demanding increased considerations of sustainability in transport infrastructure. Warm mix asphalt (WMA) is developed for reducing energy consumptions and emissions in asphalt paving industry. In addition, the use of rubberized asphalt concrete (RAC) has proven to be economically and environmentally sound and effective in improving the performance of pavements around the world. The combination of WMA and RAC, namely WarmRAC, is a novel and promising paving technology that can realize pavement sustainability from principles to practices. This study summarizes the best practices and recent research findings on warm mix rubberized asphalt concrete, including mix design, construction techniques, performance evaluation, feasibility of recycling, and environmental and economic benefits. Although most research findings to date about WarmRAC are positive, it still has a long way for WarmRAC to be fully adopted worldwide. Therefore, life cycle assessment including environmental and economic impacts, and long-term performance of WarmRAC need further research with involvement of transportation agencies, industry and academia.