Evaluation of the Mechanical Performance of Warm Bio-Recycled Asphalt Mixtures

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Evaluation of the Mechanical Performance of Warm Bio-Recycled Asphalt Mixtures


1. Introduction

Roads are the most crucial infrastructure in every nation, and they must be constructed to provide an adequately strong and durable surface for the smooth movement of traffic during their design life to ensure the safety of their users. Regarding this, the asphalt mixtures used to build road pavements are designed to be able to endure climatic fluctuations and traffic loads. Aggregates and bitumen are the main components of an asphalt mixture. Bitumen is a viscoelastic component that provides tensile resistance and cohesion. Aggregates are mineral particles that provide the mixture with resistance to compression due to friction between them. Aggregates and binders need to work together to ensure the combination performs as intended.

Sustainability is currently a growing requirement and a fundamental objective included in international policies and strategies. Current concern about environmental issues has motivated the asphalt paving industry to look for more efficient utilization and management of energy and resources, which has led to the development of mixtures processed at lower temperatures and containing an increasing quantity of reusable aggregates, such as the use of reclaimed asphalt (RA) [1].
Reclaimed asphalt (RA) is produced by grinding or completely removing the original pavement when it has reached the end of its useful life. Since reusing RA lowers CO2 emissions and does not deplete nonrenewable aggregate and asphalt binder supplies, it has positive environmental and energy-saving effects. The authors of [2] showed that utilizing 30% RA in hot-mix asphalt (HMA) reduces CO2 emissions by 20% and energy requirements by 16%. Therefore, the higher the percentage of RA used in the mix is, the greater the environmental benefits that can be obtained are. Regarding mechanical performance, several works noticed that the inclusion of low percentages of RA (less than 15%) might not affect the behavior of new asphalt mixes [3,4]. Therefore, several European Union projects have mainly focused on using a high percentage of more than 50% RAP in the mix design [5]. On the other hand, it must be highlighted that there are some negative effects of using high percentages of RA and the aged binder that it contains. Oxidation occurring during the service life of pavement alters the binder chemical composition by decreasing the level of maltenes and increasing the asphaltenes-to-maltenes ratio, leading to a stiffening effect in the binder [6], which may adversely affect the mechanical performance of the asphalt mixture [7].
On the other hand, petroleum-based binders are the main binding materials used in the paving sector. As a result of high consumption worldwide, the scarcity of this raw material is an important concern in this industry nowadays. In addition, the heating of bitumen at high temperatures for HMA production results in high greenhouse gas emissions [8]. For these reasons, several researchers have focused their work on studying alternative sources and technologies for binders in asphalt mixtures, such as using biobinders and bio-oils derived from different biomass resources, such as cooking oil residues [9], soy oil [10], wood residues [11], or the usage of swine manure [12,13], for paving applications. Research has shown [14] that biobinders have great potential not only to reduce bitumen demand but also to exhibit acceptable behavior, with responses similar to that of petroleum-based asphalt binders; therefore, biobinders can be used as the binders in mixtures for the surface layers of road pavements if they have been previously characterized. Similarly, other researchers have found that the inclusion of the biomodifier improved the fatigue characteristic by decreasing the loss modulus of asphalt binder samples [15] and showed that that the addition of modified bamboo fiber as a biomodifier increased the stability and the tensile strength of the asphalt mixture [16]. Utilizing bio-asphalt serves to positively impact the environment by preventing air and water contamination by repurposing urban waste and animal refuse. Additionally, biomaterials can be used to decrease the viscosity of asphalt binders, subsequently resulting in reduced mixing temperatures, energy consumption, and greenhouse gas emissions [17]. When compared to petroleum asphalt, biobinders offer a more advantageous solution in terms of environmental benefits. However, the authors of [18,19] have indicated that despite the abundance of sources for obtaining bio-bitumen and its good environmental impact, there are still several restrictions that limit bio-bitumen’s widespread use in the pavement industry for several reasons. Each biobinder shows different properties depending on its source and processing. This means that there is still a lack of comprehensive and sufficient standards for the use of biobinders in asphalt mixtures, and their long-term performance has not been adequately proven and reported yet. The current tendency for biobinders in the road industry is focusing on studying them as a partial replacement for bitumen.
The recent quest to create solutions to promote sustainability and minimize costs in asphalt paving has prompted researchers to investigate novel ways to use more recycled materials and biobinders in mixtures. While the RA contains an aged binder, the concern regarding increasing RA rates in new mixtures is that it leads to the production of stiff overlay asphalt mixtures. Biobinders can be used as soft binders in new asphalt mixtures with high RA rates to try to compensate such an issue. In fact, laboratory performance tests and full-scale experiments [20] on HMA containing 50% RA and biobinders (i.e., bio-recycled asphalt mixtures) have shown that the bio-recycled asphalt mixtures outperformed the traditional one. Similar results were obtained by other authors [21], who presented the full-scale RA validation of asphalt mixtures with the incorporation of biomaterials as a reclaiming agent. Consequently, in terms of durability and property evolution, the bio-asphalt mixtures outperformed the control mixture.

Within this framework, and to go a step further towards more sustainable asphalt mixtures, the main purpose of this investigation is to use biobinders as total bitumen replacement in new asphalt mixtures with high RA content manufactured at decreased temperatures. The combination of these three points leads to the production and evaluation of warm bio-recycled asphalt mixtures. The laboratory campaign was focused on characterizing both the binders and the warm bio-recycled asphalt mixtures. Bearing capacity, cohesion, resistance to moisture damage, and resistance to permanent deformations were studied as the main distresses affecting such asphalt mixtures.

4. Conclusions

This study analyzed the performance of asphalt mixtures produced using biobinders as the only virgin binders, high reclaimed asphalt rates, and reduced manufacturing temperatures, i.e., warm bio-recycled asphalt mixtures. Based on the results, the following conclusions can be drawn:

  • At the binder level, the rheological characterization of the binders showed that the combination of the stiff RA binder and the soft biobinder might be suitable to achieve a compromise for the rutting and fatigue resistance of the warm bio-recycled asphalt mixtures.

  • At the mixture level, the warm bio-recycled asphalt mixtures exhibited adequate mechanical properties, including enhanced bearing capacity and resistance to permanent deformations, shedding light on the possible issues regarding these distresses and the use of soft biobinders. The improvement in cohesion and water sensitivity reduced the concern about moisture damage in these types of asphalt mixtures.

  • The soft consistency of the biobinder combined with the high stiffness of the RA provided favorable results and allowed the positive reduction in manufacturing temperatures. This temperature reduction implies energy consumption savings and decreased greenhouse gas emissions, which must be specifically measured in future research.

These findings reveal that warm bio-recycled asphalt mixtures might be viable solutions for flexible road pavements. The innovative approach of manufacturing warm bio-recycled asphalt mixtures may represent a pivotal advancement in sustainable pavement engineering. The utilization of biobinders and the strategic integration of reused materials, coupled with reduced manufacturing temperatures, not only aligns with sustainable goals but also demonstrates promising performance characteristics. Future research will be focused on incorporating further low-temperature manufacturing technologies for bio-recycled asphalt mixtures, studying their aging and evaluating their reduced environmental impact.


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