Reclaimed Asphalt Pavement Research Articles

Experimental Investigation and Statistical Analysis of Recycled Asphalt Pavement Mixtures Incorporating Nanomaterials

Reclaimed Asphalt Pavement (RAP) materials are used as substitutes for new materials in asphalt pavement construction, leveraging the engineering and commercial benefits of the aged binders and aggregate matrixes in RAP. These asphalt mixtures impart significant variations in volumetric properties and asphalt mixture characteristics. The current study investigates the Marshall properties, moisture susceptibility, and rutting behavior of 24 recycled asphalt mixtures developed with nanosilica and nanoclay. RAP material percent, nanomaterial content, binder grade, and extra binder were considered the factors influencing asphalt mixture performance. The above factors were analyzed using the Response Surface Methodology (RSM) to predict the Marshall and volumetric properties. Also, this investigation covers the moisture susceptibility and rut characteristics of recycled nanomaterial-modified Hot Mix Asphalt (HMA) and Warm Mix Asphalt (WMA) mixes developed with Viscosity Grade 30 (VG-30) and Polymer-Modified Bitumen-40 (PMB-40). The chemical additive Zycotherm was used to develop WMA mixes. The test results indicate that adding RAP material at higher percentages and modifying the binder with nanomaterials affected moisture susceptibility with reduced moisture damage. Recycled nanosilica-modified HMA mixes developed with PMB-40 at higher RAP percentages reported higher tensile strength ratio (TSR) values in contrast with VG-30 mixes, indicating their greater susceptibility toward moisture-induced damage. The rutting potential of all of the recycled asphalt mixture combinations was enhanced by densely packed aggregate structures optimized with nanomaterials, total binder content, and RAP materials developed using the Marshall method. Overall, the nanosilica-modified recycled asphalt mixes developed with PMB40 at higher RAP percentages showed better performance in terms of strength and durability.

Recycling of RAP (Reclaimed Asphalt Pavement) as aggregate for structural concrete: experimental study on physical and mechanical properties

The replacement of natural aggregate in concrete with artificial and/or recycled one has recently gained attention as meaningful strategy to reduce the environmental impact of structural concrete and promote circular economy principles. This study investigated the possibility to use Reclaimed Asphalt Pavement (RAP), in the “as received conditions”, as a partial or complete substitution of natural aggregate for structural concrete. RAP aggregate was firstly characterized in terms of grain size distribution, density, assessment of fines, chloride content, moisture content and water absorption. Subsequently, a total of twenty-four concrete mixes were designed, considering two cement types, two w/c ratios and several aggregate substitution percentages. For each mix, properties at the fresh and hardened state were investigated, such as workability, density and total open porosity, compressive strength, dynamic modulus of elasticity, and electrical resistivity. Results showed that RAP has a good potential to be used in reinforced concrete, provided that different water absorption and moisture content are considered in the mix design. RAP concrete was characterized by a lower density and increased total open porosity; however, an accurate tailoring of the concrete recipe could compensate the strength loss for several applications. Other properties, such as electrical resistivity and the relationship between dynamic modulus of elasticity and compressive strength did not result significantly altered by the presence of RAP.

Research on the micro-mechanism and factors of hot recycled asphalt binder cracking

The poor low-temperature performance of recycled asphalt is the primary cause of the recycled asphalt disease. However, further strengthening is required to analyze the molecular-level cracking of recycled asphalt. This article presented a comprehensive investigation of the cracking phenomenon of recycled asphalt at both the macro and micro levels. This was achieved through the utilization of experimental tests and molecular dynamics simulations. In this study, pressure-aged asphalt was subjected to 40 h of aging. The aged asphalt was divided into three groups: 30% RAP (reclaimed asphalt pavement) recycled asphalt, 50% RAP recycled asphalt, and 70% RAP recycled asphalt. The three groups were then analyzed in terms of three major indexes. Linear amplitude scanning (linear amplitude sweep, LAS) was conducted on the three groups. The mechanical response law of asphalt was obtained, and it was found that the coefficient of strength of recycled asphalt increased with the increase of RAP dosage. At the same time, the fracture energy density decreased subsequently. Furthermore, the fracture energy density of aged asphalt was 7% lower than base asphalt's. Subsequently, this study examined the four fractions and C=O and S=O functional groups of five types of asphalt and constructed the molecular structures and models of stable base asphalt and aged asphalt. Subsequently, this article constructed a fusion model of new and old asphalt, analyzed the fusion process of new and old asphalt, and studied the factors affecting the blending degree of new and old asphalt. Finally, the factors affecting the cracking performance of rejuvenated asphalt, including the blending degree, the content of the four fractions, and the addition of the rejuvenator, were investigated. The impact of each influencing factor on the cracking performance of recycled asphalt was analyzed from multiple perspectives. In conclusion, the findings of this study provide a valuable guide for interpreting the tensile properties of reclaimed asphalt and the influence of rejuvenators.

Study on use of recycled waste materials on the performance of asphalt binder and mixes: A comprehensive review

All aspects of daily living frequently involve the use of plastics. Due to the lack of effective waste treatment methods, massive global environmental problems will be caused by the accumulation of waste plastics. The use of waste materials in road building is of tremendous interest since it has the potential to reduce hazards and pollution while conserving natural resources. High-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), ethyl vinyl acetate (EVA), and polyvinyl chloride are the most appropriate and relevant of waste plastics. There are other types of waste materials used in the asphalt such as reclaimed asphalt pavement (RAP), crumb rubber (CR), steel slag, glass powder etc. These all-waste materials have been briefly discussed. In this review article several concerns have been raised about the physical and chemical qualities, of asphalt pavements that have been mixed with waste materials. Using waste materials in asphalt pavements can raise several concerns regarding the physical and chemical qualities of the resulting mixtures. These concerns typically revolve around factors such as durability, performance, environmental impact, and overall sustainability. This research article provides a thorough analysis of the wet and dry procedures used to mix different types of waste plastics. Overall, including waste plastics into asphalt mix improved rutting, fatigue, and moisture resistance. Additionally, recycling waste plastics in this manner helps in reducing environmental pollution and conserving natural resources. Overall, the inclusion of waste plastics in asphalt mixtures can contribute to the development of more sustainable and durable pavements with improved performance in terms of rutting, fatigue, and moisture resistance. There are still certain issues like compatibility and low temperature performances that need to be addressed. To overcome the aforementioned issues, many strategies are used, including the use of nanomaterials, chemical additives, and polymer additives. Additional research should be conducted to investigate the ageing effect, pavement performance monitoring, polymer stability, and the economic and environmental considerations associated with the usage of plastics in asphalt pavements.

Evaluation of the interface shear strength of concrete containing treated and untreated reclaimed asphalt pavement aggregates

Concrete production significantly contributes to carbon emissions and depletes natural resources, promoting increased interest in substituting traditional materials with recycled alternatives. This shift necessitates a thorough examination of recycled materials’ performance in concrete applications. This study aims to provide a better understanding of the shear transfer behavior of concrete containing reclaimed asphalt pavement (RAP) aggregates. To mitigate the potential negative impact of RAP aggregates, mechanical treatment was applied. The effects of RAP inclusion and mechanical treatment on the shear transfer strength of reinforced concrete were investigated. A total of twenty-four push-off specimens were tested to investigate the effects of aggregate replacement and the clamping reinforcement ratio on specimen behavior. Digital image correlation (DIC) was employed to monitor the strains and displacements, allowing for detailed tracking of the interface slip, crack width development, and reinforcement strain. The findings revealed that replacing natural aggregates with RAP aggregates resulted in lower compressive strengths and lower ultimate push-off strengths of the resulting concrete at equivalent effective water-to-cement (w/c) ratios. Specimens with higher reinforcement ratios retained residual strengths up to 77 % of the ultimate strength, even at relatively large slip and crack widths. The clamping reinforcement ratio was identified as the key factor influencing the ultimate and residual strengths for both types of concrete. The experimental results were compared to the strengths calculated by the ACI, PCI, AASHTO, and CSA design equations to evaluate their applicability when different aggregate types are used.

Embracing Sustainability in Rigid Pavement Construction: Unveiling Geopolymer Concrete’s Potential with Incorporated Reclaimed Asphalt Pavement Aggregates

Embracing Sustainability in Rigid Pavement Construction: Unveiling Geopolymer Concrete’s Potential with Incorporated Reclaimed Asphalt Pavement Aggregates

Factors influencing the wetting performance of asphalt emulsion on RAP surface

The wetting performance of asphalt emulsion on reclaimed asphalt pavement (RAP) strongly affects the physical bonding ability of asphalt emulsion-RAP interface. For a more comprehensive understanding on the wetting performance of asphalt emulsion on RAP, many factors affecting the wetting performance of asphalt emulsion on RAP are studied, including the asphalt and emulsifier dosage in asphalt emulsion, RAP asphalt aged degree, and some additives (such as wetting agent, hydrated lime and silane coupling agent). The contact angle of asphalt emulsion on RAP asphalt surface and the other wetting parameters calculated by the surface wetting theory were used to comprehensively analyze the wetting performance. The results show that all of the tested factors have influence on the wetting performance of asphalt emulsions. The grey relational analysis indicates that the wetting agent, hydrated lime and silane coupling agent have significant effects on the wetting performance of asphalt emulsion on RAP. A suitable dosage of wetting agent, hydrated lime modified with silane coupling agent (HL-SCA) can decrease the contact angle and improve the wetting speed of asphalt emulsion on RAP. Meanwhile, the addition of both wetting agent and hydrated lime modified with silane coupling agent can greatly decrease the volume of air voids and improve the indirect tensile strength of cold recycled mixture with asphalt emulsion. Therefore, the wetting agent and the HL-SCA are recommended to improve the wetting performance of asphalt emulsion on RAP and the mechanical performance of cold recycled mixture with asphalt emulsion.

The influence of residual asphalt material in the mechanical behaviour of soil-RAP mixtures for use in paving

ABSTRACT The evaluation of Reclaimed Asphalt Pavement (RAP)’s suitability for pavement layers, optimises resource use, reduces environmental impact, and supports cost-effective practices. In this sense this study aimed to characterise soil-RAP mixtures to ensure they meet the quality standards required for pavement applications. The study examined two types of soil mixtures: one soil-RAP aggregates and the other soil-virgin aggregates (VAM) to understand the impact of residual asphalt binder on aggregate particles. Various mass contents (30%, 50%, 70%, and 90%) were tested alongside conventional soil-gravel mixtures with equivalent proportions and gradation of RAP and VAM. Additionally, the study investigated the effect of adding 2% cement on the performance of both recycled soil-RAP mixtures and conventional soil-gravel mixtures. The analysis includes granulometry, Atterberg Limits, MCT (Miniature, Compacted, Tropical) classification, soil chemical characterisation and microscopic images, compaction in Modified Proctor Energy, CBR, Resilient Modulus tests and stress–strain analysis. The results have revealed that the residual asphalt binder, surrounding RAP aggregates, has reduced the soil mixtures’ strength for different RAP contents. However, the study demonstrated that the addition of cement rendered the soil-RAP mixture feasible, enabling the application of the studied mixtures of RAP to constitute subbase and base layers.

Fracture properties of asphalt mixtures containing high content of reclaimed asphalt pavement (RAP) and eco-friendly PET additive at low temperature

Low-temperature cracking is one of the main types of distress that affects asphalt pavement performance over its service life. At the same time, the accumulation of plastics, especially polyethylene terephthalate (PET), and the potentially harmful effects of these plastics requires special attention in order to find ways to reduce them. Previous studies have paid less attention to the impact of PET on low-temperature cracking in asphalt mixtures and the simultaneous interaction between PET and reclaimed asphalt pavement materials (RAP) on fracture behavior of asphalt mixture. In this study, a value-added substance called PET Additive (PA), which is extracted from chemical processing of PET was blended with asphalt binder at different dosages (1–3 %) to use in the mixture of hot mix asphalt (HMA). Reclaimed Asphalt Pavement (RAP) was also added to the HMA mixtures at rates of 25 and 50 % by aggregate mass and the resulting asphalt mixtures were tested to characterize the load carrying ability and cracking resistance indexes of the modified HMA mixtures. Circular shaped test specimens with different mixture types and different geometries were manufactured. Results showed that the asphalt mixture containing PA has better performance than the control mixture; among all mixture types, a mixture containing 2 % PA and 25 % RAP had the better fracture resistance at low temperatures. Finally, the results demonstrate that the fracture behavior in Edge Notch Disc Bend (ENDB) and Semi-Circular Bend (SCB) specimens had similar trends.

Optimization of Hot Mix Asphalt Using Diesel Engine Waste Oil and Reclaimed Asphalt Pavement to Improve Stability and Durability

Optimization of Hot Mix Asphalt Using Diesel Engine Waste Oil and Reclaimed Asphalt Pavement to Improve Stability and Durability

Sustainability of Asphalt Mixtures Containing 50% RAP and Recycling Agents

The substitution of virgin asphalt binder with reclaimed asphalt pavement (RAP) has environmental and economic merits, however, cracking susceptibility arises due to the aged asphalt binder within RAP. The objectives of this study are to (1) enhance the cracking resistance of asphalt mixtures containing 50% RAP utilizing recycling agents (RAs) derived from six petroleum-based and bio-based materials, (2) conduct an environmental impact assessment (represented by global warming potential “GWP”) for high-RAP mixtures including RAs, and (3) estimate the cost effectiveness of including high-RAP content in asphalt mixtures. Based on the RAP asphalt binder performance grade (PG), base asphalt binder PG, and RAP content, the RA contents were determined to achieve a target asphalt binder of PG 76-22. A control mixture was benchmarked for comparison, specified for high-traffic volume roads, and contained PG 76-22 polymer-modified asphalt binder. The engineering performance of studied asphalt mixtures was evaluated using the Hamburg wheel-tracking (HWT), semi-circular bend, Illinois flexibility index, Ideal cracking tolerance, and thermal stress-restrained specimen tensile strength tests. It was found that petroleum-derived aromatic oil, soy-based oil, and tall oil fatty acid-based RAs demonstrated a successful restoration of aged RAP asphalt binder without compromising the permanent deformation resistance. The 50% RAP mixtures emitted less GWP by 41% and 42.9% using petroleum- and bio-oil RAs, respectively, and achieved a 31% cost reduction compared to the control mixtures.

Influence of field aging on viscoelastoplastic performance of rubberized asphalt mixtures incorporating reclaimed asphalt pavement in arid urban climate

This study investigates the effects of reclaimed asphalt pavement (RAP) and crumb rubber-modified asphalt (CRMA) on the viscoelastoplastic properties of asphalt mixtures under a range of aging conditions. Six asphalt mixtures, with varying RAP contents (0 %, 30 %, and 50 %) and CRMA modifications, were evaluated through dynamic modulus (|E*|) and flow number (FN) analyses. The specimens were subjected to authentic field-aging conditions for periods of 0, 3, 6, and 9 months. The results reveal that increasing RAP content enhances volumetric properties, Marshall stability, and rutting resistance. CRMA significantly improves fatigue performance and further bolsters rutting resistance, although some detrimental effects are observed at lower temperatures. Aging has a pronounced impact on the mixtures’ performance, particularly on |E*|, Prony series coefficients, viscoelastic strain (εve), and viscoplastic strain (εvp). Newly proposed aging indices, the dynamic modulus aging index (DMAI) and viscoplastic aging index (VPAI), effectively capture the changes in viscoelastoplastic behavior under different aging durations. These findings underscore the potential of RAP and CRMA to enhance pavement durability, prolong service life, and contribute to the development of sustainable and resilient infrastructure. The research offers valuable insights into the performance of asphalt mixtures incorporating RAP and CRMA under real-world conditions.

Micro-Structure Analysis and Mechanical Behaviour of Hot Mix Asphalt Modified with Reclaimed Asphalt Pavement Using Palm Kernel Shell Ash as Mineral Filler

In pavement construction and production of hot mix asphalt HMA, the use of industrial and agricultural waste has gained much relevance because of its economic and environmental benefits. This research examined the effects of palm kernel shell ash PKSA on the physical and volumetric properties of HMA modified with reclaimed asphalt pavement RAP. All preliminary test conducted on the modified asphalt mixture in accordance with relevant standards showed adequacy for use in production of HMA. Marshall method of mix design was adopted for the HMA production. The bitumen content was varied from 4.5 to 6.5% (at intervals of 0.5%). The palm kernel shell ash was varied from 25% to 75% (at interval of 25%). A maximum stability of 7.1kN was recorded at 5.5% bitumen content which is a little increment in strength but good significance in material (virgin bitumen) when compared to the maximum stability of 6.9kN at 6% bitumen obtained from the control mix. The microstructural analysis of the hot mix asphalt done on the palm kernel shell ash PKSA showed a rough surface texture needed in flexible pavement construction and when comparison was done between the control specimen and the modified specimen, it shows an improvement in the interlocking arrangement of aggregates resulting in a denser mixture for the modified hot mix asphalt. In conclusion this study confirms that a blend of a 50% RAP and 50% virgin aggregates with 50% palm kernel shell ash PKSA as mineral filler at 5% bitumen content can improved strength performance of HMA, hence, the effect of PKSA as a mineral filler in HMA containing RAP is significant.

Moisture damage mechanism of asphalt mixtures containing reclaimed asphalt pavement binder: A novel molecular dynamics study

Reclaimed asphalt pavement (RAP) stands at the forefront of sustainable practices in road construction and rehabilitation. However, concerns regarding the moisture susceptibility of aged RAP binder have arisen due to its potential hazard to pavement durability. So far, the underlying mechanisms of how moisture infiltrates into the mixture and results in binder stripping remain controversial and scarcely investigated. To address this challenge, a novel molecular dynamics (MD) simulation method has been developed to explore the nanoscale stripping mechanisms, considering the effects of asphalt oxidative aging and rejuvenation. The results indicate that hydrogen bonding interactions between water and the aggregate surface reduce the contact distance compared to the distance between asphalt and aggregate. Under pore water pressure, moisture diffuses into the interface, occupying the van der Waals adhesion sites of asphalt molecules and consequently causing detachment. Asphalt oxidative aging, on one hand, increases the oxygen concentration and hydrophilicity, thereby increasing moisture susceptibility from the energy perspective. On the other hand, it increases the intermolecular friction between aged asphalt molecules, leading to reduced flexibility and age hardening, consequently triggering asphalt premature detachment and diminishing its coating quality on the aggregate surface. Rejuvenators can restore the moisture sensitivity of severely aged asphalt, with a mechanism involving two aspects: firstly, reducing the agglomeration and proportion of polar components in asphalt, thereby weakening its hydrophilicity; secondly, lubricating aged asphalt molecules to restore their flexibility and surface coating properties. Findings from this study contribute to revealing the moisture-induced damage mechanism when recycling RAP binder.

Mechanical Analysis through Non-Destructive Testing of Recycled Porous Friction Course Asphalt Mixture

This study assessed the mechanical performance of porous asphalt mixtures, specifically the porous friction course (PFC), incorporating 10% Reclaimed Asphalt Pavement (RAP) and rubberized asphalt. Three different methods were investigated to evaluate the stiffness of the mixtures: the resilience modulus (RM) test at a single temperature and loading frequency, the complex modulus |E*| test from compressive loading conducted at various temperatures and frequencies, and the impact resonance (IR) tests performed at three temperatures with five impacts applied to the mixture. The results demonstrated that the RAP-containing mixture exhibited a higher resilience modulus at all tested temperatures, indicating greater stiffness compared to the mixture without RAP. Additionally, the IR and |E*| tests revealed similar behavior between the two evaluated mixtures. These findings suggest that both quasi-static and vibrational tests are suitable for characterizing the stiffness of porous asphalt mixtures due to the similarity in the viscoelastic parameters of the two investigated mixtures. This study provides important insights into the practical and scientific application of recycled and modified materials in porous asphalt mixtures.

Analysis of Reclaimed Asphalt Pavement Heating with Microwave Radiation

Analysis of Reclaimed Asphalt Pavement Heating with Microwave Radiation

Quantifying the blending efficiency of warm mix asphalt-synchronous rejuvenated SBS-modified asphalt through a dynamic shear rheometer (DSR) testing approach

The degree of blending (DOB), which is controlled by the diffusion between virgin and aged/rejuvenated asphalt binder, has a major impact on the performance of the asphalt mixture incorporated with reclaimed asphalt pavement (RAP). Despite numerous studies investigating the DOB through experimental methods, the blending efficiency of warm mix asphalt (WMA) and synchronous rejuvenated SBS-modified asphalt (SBSMA) is still unexplored. In this study, a dynamic shear rheometer (DSR) testing approach was developed to quantify the blending efficiency of WMA modified SBSMA and synchronous rejuvenated SBSMA. The novelty of this approach lies in the innovative use of a DSR asphalt double-layer structure to determine the DOB in combination with a developed theoretical calculation framework. The individual and synergistic effects of WMA additive and rejuvenator type on the DOB of WMA modified SBSMA-Synchronous rejuvenated SBSMA were investigated, while the influence of blending temperature, blending time, and dosage of aged/rejuvenated SBSMA on the blending efficiency were systemically evaluated. Results indicated that the SBS repairing agent in the synchronous rejuvenator can dramatically increase the DOB, while the presence of WMA additive can further increase the DOB by improving the fluidity of the virgin SBSMA. Furthermore, increasing the blending temperature/blending time and decreasing the dosage of aged/rejuvenated SBSMA were also conducive to enhancing the DOB. In addition, a blending chart of the WMA modified SBSMA-Synchronous rejuvenated SBSMA was developed to facilitate the accurate prediction of the DOB at any given blending temperature and blending time.

Evaluating dynamic modulus of cold in-place recycling mixture with foamed bitumen using field core samples

This study evaluates the dynamic modulus of Cold In-Place Recycling with Foamed Bitumen (FB-CIR) pavements, focusing on the influence of various recycled materials—reclaimed asphalt pavement (RAP), reclaimed inorganic binder stabilized aggregate (RAI), and their combination (RAP+RAI). Core samples from three representative FB-CIR projects in China were tested using the MTS-130 system across temperatures of −10°C, 5°C, 20°C, 35°C, and 50°C, and frequencies ranging from 25 Hz to 0.1 Hz. Statistical analysis and the Sigmoidal model were employed to analyze the viscoelastic behavior and establish the master curve for the FB-CIR dynamic modulus. The dynamic modulus of the three FB-CIR mixtures showed pronounced frequency-dependence, decreasing with reduced frequencies, and exhibited temperature sensitivity, with lower values at elevated temperatures. FB-RAI consistently demonstrated higher modulus values due to the hydration of previously unhydrated cement particles, while FB-RAP and FB-RAP+RAI displayed lower values, as RAP behaves similarly to unbound granular material. The construction of the master curve, based on the time-temperature superposition principle, enhanced our understanding of FB-CIR mixtures' mechanical behavior and enabled accurate extrapolation of dynamic modulus values across diverse conditions. Furthermore, FB-RAI's narrow phase angle variation indicates its superior mechanical stability and reduced sensitivity to external changes, making it ideal for areas with extreme climate fluctuations or heavy traffic. The study also identified three distinct phase angle variation patterns in FB-CIR mixtures, underscoring the importance of nuanced pavement design strategies that consider temperature, frequency, and material composition to optimize the performance and durability of FB-CIR pavements.

Enhancing waste asphalt durability through cold recycling and additive integration

The longevity of waste asphalt can be considerably improved through cold recycling techniques combined with various additives. This research investigates the cold regeneration of aged asphalt concrete using Reclaimed Asphalt Pavement (RAP), Portland cement, cationic bitumen emulsion, and additional aggregates. The primary goal is to evaluate the performance enhancements in terms of average density, compressive strength, water resistance, and swelling across different mix compositions. Three distinct mixtures were formulated and assessed. Mix No. 1, composed solely of RAP, showed the lowest average density and highest swelling, indicating poor performance due to the lack of binding agents. Mix No. 2, which incorporated RAP, Portland cement, and water, exhibited the highest density and compressive strength, highlighting the crucial role of Portland cement in improving structural integrity. Mix No. 3, a more complex mixture including RAP, aggregates, Portland cement, water, and bitumen emulsion, displayed balanced properties with enhanced moisture resistance and reduced swelling. The experimental findings emphasize the effectiveness of adding Portland cement and bitumen emulsion to improve the mechanical and durability characteristics of recycled asphalt mixtures. Specifically, Mix No. 2 and Mix No. 3 demonstrated significant performance improvements, making them suitable for road maintenance applications. This study advocates for the widespread use of cold recycling methods with additive integration to achieve sustainable and cost-effective pavement restoration solutions.

RAP chunks produced in cold milling operation of asphalt pavement: Evaluation, mechanism, and engineering investigation in China

Cold milling is a widely used method for rehabilitating asphalt pavement, generating reclaimed asphalt pavement (RAP) chunks. Within this process, aggregates within the asphalt pavement will be crushed, forming RAP agglomerates and aggregate breakdown. However, the mechanism of these phenomena has remained unclear, and a unified evaluation method has yet to be established. In this study, RAP agglomeration and aggregate fragmentation were characterized, five distinct methods were systematically assessed, and the mechanism of RAP agglomeration and breakdown was analyzed by discrete element method (DEM) simulation based on setting different particle contact parameters, then followed by a mechanical analysis, and demonstrated in engineering. The results revealed that both agglomeration and aggregate breakdown occur within RAP particles of various sizes, with the five methods showing similar trends in quantifying these effects. Through DEM simulations and mechanical analyses, the aggregate breakdown predominantly occurs at the cutter's motion trajectory of the cutter and during crack propagation, while agglomeration was mainly related to the sliding surface's area. The milling speed and depth positively impact RAP agglomeration, while negatively affecting aggregate breakdown, and milling drum speed exerts minimal influence on these phenomena. RAP agglomeration varies considerably in different engineering projects, and cold milling parameters should be determined based on the material composition of the asphalt pavement and design requirements to control agglomeration and breakdown rates of RAP.