Investigating mechanical performance and interface characteristics of cold recycled mixture: Promoting sustainable utilization of reclaimed asphalt pavement

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Abstract

The recycling and sustainable utilization of reclaimed asphalt pavement (RAP) are the key method to improve resource conservation and reduce energy consumption in roadway construction. The cold-recycled technology has been regarded as a promising recycling one because it can achieve mixing at room temperature and high utilization of RAP. Investigating the mechanical mechanism of cold-recycled asphalt mixture (CRAM) is the basis for large-scale application. Thus the splitting tests were carried out to study the influencing law of the material components on the strength performance of CRAM by varying RAP, cement and asphalt content. Then the effects of material component on the strength mechanism were further revealed by the interface characteristics analysis, including void and filler parameters, binder and interface transition zone (ITZ) thickness. The results show that the CRAM with more RAP has a higher splitting strength and the aged asphalt contributes to the reduction of void number around the aggregate interface, indicating a stronger bonding effect with binder. The asphalt mixture has lower void size, void number and porosity than cement mixture, and shows more uniform distribution of binder thickness and lower void distribution difference between the ITZ and the mortar phase, which is conducive to force uniformly and prevent stress concentration within mixture. The above outcomes confirm the effects of strength improvement from RAP and the void increment from cement, and the contents could be reasonably increased for RAP and strictly controlled for cement to ensure strength performance of CRAM while promoting saving energy and environmental protection.

Introduction

With the rapid development of road industry in the past decades all over the world, the early built asphalt pavements have been damaged seriously and various distresses have occurred such as rutting, cracking, moisture damage, raveling etc. In order to treat these pavement distresses during the road renovation, the upper asphalt layers are usually milled and asphalt overlays are paved. Then a large amount of waste milled mixtures are subsequently produced. Traditional treatment methods such as open-pit stacking and landfill may cause environmental issues and waste of non-renewable resources. Therefore, improving resource conservation and recycling of asphalt waste materials has become the research focus of pavement engineering (Jahanbakhsh et al., 2020).

Hot recycling and cold recycling techniques have been regarded as an effective way to pavement rehabilitation (Abdalfattah et al., 2021; Zhu et al., 2020). Compare to the hot recycling, the cold recycling technology can achieve room temperature mixing by adding asphalt emulsion or foamed asphalt and higher utilization of RAP up to eighty to ninety percent (Dołżycki and Jaskuła, 2019; Xiao et al., 2018). The total consumption energy of cold-recycled technology is 60% less than HMA. In addition, implementing cold-recycled mixture can reduce the emission of 52% and 54% in respect of GHG, nitrogen oxide and sulfur dioxide and reduce the aggregate consumption up to 90%, while the overall cost saving is 41%–47% when considering energy, construction and environmental costs (Offenbacker et al., 2020). The CRAM generally refers to the mixture containing asphalt emulsion, cement, new aggregate particle (NAP) and RAP. The complex material compositions lead to multi-interface forms, which affect the strength formation of CRAM. However, the effects of RAP and microscopic interface characteristic on the mechanical mechanism are not yet fully clear. This impedes the wide application of cold recycling technology.

The mechanical properties of CRAM are characterized by low early strength, continuous strength increment with curing time and high porosity (Chen et al., 2020a). Cement is generally added to increase early strength and improve overall long-term performances (Wang et al., 2010). It may be viewed as a modifier that improve the interface adhesive property between aggregate and asphalt mastic (Du, 2014, 2016). Asphalt emulsion interweaves and interpenetrates with hydration products to form a spatial network structure, reducing the pore diameter in the ITZ. Asphalt droplets could cover the cement particles and the covered asphalt membrane to prevent further hydration of cement (Du, 2014; Lin et al., 2017). The interaction mechanism between the asphalt emulsion and cement has been studied by many researchers (Brand and Roesler, 2017a, p.; Du et al., 2019; Ma et al., 2015). In addition, the relative contents of cement and asphalt emulsion also significantly affect the mechanical properties of CRAM (Li et al., 2019). As for the RAP, it contains more micro pores than the virgin aggregate and the interface bonding effects of RAP with binders are significantly different from the virgin aggregate due to the existence of aged asphalt (Imtiaz et al., 2020; Cardone et al., 2018; Abraham and Ransinchung, 2018a). Hence, various interactions exist between material components, causing difficulty in analyzing the mechanical mechanism of CRAM. A comprehensive research is needed to study the effects of each component and relative content on the strength performance.

The asphalt and cement are seen as composite binder to form cohesive strength in CRAM, then the adhesion status between the aggregate and binder and ITZ characteristic, including physical parameters (ITZ thickness, void size and number, binder thickness, etc.) and mechanical properties (visco-elastic parameter, elastic modulus, etc.) also have a significant impact on the strength performance of CRAM (Abraham and Ransinchung, 2018b; Imtiaz et al., 2020; Abd et al., 2018; Hu and Qian, 2018; Ma et al., 2015). This is because the cracks generally initiate and develop at weak positions when the failure occurs, especially for micro-voids within the ITZ and asphalt mastic (Abraham and Ransinchung, 2018b; Liu et al., 2020, Van Breugel et al., 2004). Microstructural characterization is considered as an effective way to explore the ITZ and corresponding microstructure parameters, further helping reveal strength mechanism of CRAM. Various testing approaches have been used, including Scanning Electron Microscopy (SEM), Nano-indentation, Atomic Force Microscopy (AFM) and so on (Cai et al., 2020) (Huang et al., 2020; Yao et al., 2017). SEM has been widely applied to explore the morphology and distribution of each-phase microstructure along the ITZ (Brand and Roesler, 2017b; Liu et al., 2020). Combined with image processing analysis such as backscattered electron (BSE), microstructure characteristic parameters and ITZ thickness could be quantitatively analyzed (Shadmani et al., 2018).

However, the complex component compositions of CRAM are adverse to the analysis of the effects of a specified component on the microstructure characteristic, the separate analysis of two-phase compound mixtures including RAP/virgin aggregate and asphalt/cement may be an effective method to remove interferences from other components (Chen et al., 2020b).

Section snippets

Research scope and objective

This paper is aimed to improve the RAP utilization and promote resource conservation and recycling in roadway construction, and the most important issue is that the effects of material components on the strength mechanism of CRAM should be confirmed. This is studied by mechanical tests and microstructure analysis in the paper. For the mechanical test, the splitting tests with varying RAP, asphalt emulsion and cement contents were carried out, which could reflect the interaction degree and

Mix design of the CRAM

The new aggregate particle (NAP), RAP, mineral powder (MP), cement, asphalt emulsion and water consisted of CRAM. The gradation of CRAM was medium-gradation with the nominal maximum aggregate particle of 19 mm (Shadmani et al., 2018; Technical Specifications for Highway Asphalt Pavement Recycling, 2019). The mixing ratio was RAP: NAP: MP = 86.8: 10: 3.2, and NAP was applied in the particle range of 19–26.5 mm. The NAP and minder powder were limestone. The gradation curve of CRAM is shown in

Effect of cement and aggregate content

The constant emulsified asphalt content was 4.5%, and the variations of cement contents were 1%, 1.5%, 2% and 2.5%, respectively. Splitting strength test results are shown in Fig. 7. For 100R mixture (shown in Fig. 7 (a)), the indirect tensile strength increases almost linearly with respect to cement amount. This indicates that cement plays a positive role in the early strength. However, the 100R mixture has a decreasing trending of strength increment amplitude when increasing the cement

Conclusion

This paper studies the effects of material composition and relative content of CRAM on the strength performance. The interface microstructure analysis are performed to investigate the contribution of different material component to the strength mechanism of CRAM. The main conclusions are as follows:

  • The interfaces involving RAP have denser micro morphology and less micro-voids than that involving NAP, indicating a deeper bonding effect between the aged asphalt and binder.

  • The asphalt mixture has

CRediT authorship contribution statement

Yingcheng Luan: Conceptualization, Writing – original draft. Tao Ma: Visualization, Investigation. Siqi Wang: Project administration, Writing – review & editing. Yuan Ma: Supervision, Writing – original draft. Guangji Xu: Methodology, Formal analysis. Meng Wu: Operating experiment, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The study is financially supported by the National Natural Science Foundation of China (No. 51878164 and No. 51922030), the National Key Research and Development Project (No. 2020YFB1600102 and No. 2020YFA0714302), Southeast University “Zhongying Young Scholars” Project, Jiangsu Highway Engineering Maintenance Technology Co., Ltd.

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