In-place Recycling Research Articles

Performance Restoration of Aged SBS-Modified Asphalt in RAP via Dry-Process SBS modifier Diffusion at the Interface of Aggregate-Aged Asphalt-Virgin Asphalt

To address and recycle more RAP (Reclaimed Asphalt Pavement), dry-process SBS(Styrene-Butadiene-Styrene) modifier was applied in hot in-place recycling. Molecular simulation and laboratory experiments were conducted to investigate the diffusion behavior of dry-process SBS modifier on RAP surfaces and its effects on the properties of recycled asphalt and recycled asphalt mixtures (RAM). Molecular simulation results revealed that after the diffusion of dry-process SBS on RAP surfaces, the fractured and aged SBS molecules in the aged asphalt were replenished, leading to a 59.49% enhancement in the interface energy between aged asphalt and aggregates. Dry-process SBS exhibited bidirectional diffusion between RAP and virgin asphalt, with a preference for the intact SARA components of Virgin asphalt. The diffusion coefficient of the dry-process recycling system was significantly higher compared to conventional wet-process recycling, achieving a higher degree of fusion (DOB) between virgin and aged asphalt. Laboratory experiments demonstrated that dry-process recycling was more effective in restoring the performance of aged SBS-modified asphalt compared to wet-process recycling. At high RAP contents (80%, 100%), the asphalt mixtures from wet-process recycling failed to meet specifications for high-temperature permanent deformation resistance, low-temperature cracking resistance, and resistance to water damage. However, using 10% dry-process SBS modifier by mass of aged asphalt in RAP enabled 100% RAP content asphalt mixtures to meet specifications.

Effects of Ground Coal Bottom Ash and Calcium Chloride on the Compaction Properties of Cement Stabilized Cold In-Place Recycling (CIPR) Pavement Base Course

Effects of Ground Coal Bottom Ash and Calcium Chloride on the Compaction Properties of Cement Stabilized Cold In-Place Recycling (CIPR) Pavement Base Course

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.

The use of asphalt waste dust for stabilization of sustainable pavement recycling

The asphalt waste dust as a sustainable material for stabilizing pavement recycling, which is cold in-place recycling (consists of reclaimed asphalt pavement, crushed rock aggregate base, and ordinary Portland cement), was presented in this study. To understand the effect of asphalt waste dust on stabilizing pavement recycling, 10–30 wt% asphalt waste dust was added and compared with the behavior of pure old-asphalt pavement material and old-asphalt pavement material mixed with 3.5 % OPC. The compaction test, unconfined compressive strength test (UCS), indirect tensile strength tests (IDT), and scanning electron microscope analysis were conducted under various mixing conditions. The addition of asphalt waste dust up to 20 wt% achieved desirable results of UCS and IDT involved with microstructural development, which were beyond the standard requirements of the base course from the Department of Rural Roads and the Department of Highway. A maximum of 20 wt% asphalt waste dust can be utilized for practical use with pavement recycling in the base course.

A Comprehensive Review of Life Cycle Cost Assessment of Recycled Materials in Asphalt Pavements Rehabilitation

This paper provides a comprehensive review of the use of life cycle cost assessment (LCCA) and life cycle assessment (LCA) methods for evaluating the sustainability and costs of using recycled materials in asphalt pavement rehabilitation projects. The review begins with an introduction to pavement rehabilitation strategies and the importance of choosing techniques based on thorough engineering and economic analyses. It then explores the different types of recycled materials that can be utilized, including reclaimed asphalt pavement, recycled concrete aggregate, and recycled asphalt shingles, discussing the key characteristics and properties of these materials based on previous laboratory studies. The review also examines the various rehabilitation methods that employ recycled content, such as cold in-place recycling, hot in-place recycling, and full-depth reclamation, providing a detailed breakdown of the construction, maintenance, and rehabilitation costs considered in LCCA and analyzing the environmental benefits of recycled material usage through a review of LCA techniques and criteria like carbon footprint reduction, impacts on air and water quality, and considerations of technological factors. Software tools for conducting LCA are compared and challenges to advancing the adoption of recycled materials are reviewed along with directions for future research efforts. The unique contribution of this work is its holistic assessment of LCCA and LCA methodologies to inform the sustainable and cost-effective deployment of recycled materials in asphalt pavement rehabilitation, a topic of growing importance for transportation infrastructure management. In summary, this current work provides a valuable review of how LCCA and LCA methodologies can assess the sustainability and costs of employing recycled content in asphalt pavement rehabilitation projects.

Erosion Potential of Stabilized Support Layers for Concrete Pavements and Overlays

The performance of concrete pavements and overlays is highly dependent on the uniformity, durability, and stability of the underlying support layers. Erosion of support layers can lead to pavement distresses and a reduction in pavement life. A review of existing erodibility performance tests that assessed stabilized support layers was first conducted to identify and evaluate their suitability for adaptation. The Hamburg wheel tracking device (HWTD) test was selected to assess the erosion potential of asphalt and cement stabilized support layers. Field testing with distress surveys, falling weight deflectometer, and coring was completed to obtain HWTD specimens and link laboratory results to pavement performance. The HWTD test was performed on cores obtained from in-service cement and asphalt stabilized support layers, a cold in-place recycling (CIR) mixture, and cement stabilized laboratory mixtures. As expected, an increase in cement content within cement stabilized mixtures decreases the likelihood of erosion with the HWTD. Additionally, conventional asphalt stabilized base layers were highly erosion resistant. Erosion resistant cement stabilized bases (including full-depth reclamation) should target an average HWTD erosion depth ≤2 to 4 mm (0.08 to 0.16 in.) after 10,000 load cycles based on the functional classification and expected traffic volume of the pavement section. Likewise, asphalt stabilized bases (including CIR and support layers for concrete overlays) should target an average HWTD erosion depth ≤12.5 mm (0.5 in.) after 7,500 load cycles with performance grade 64 binder.

Performance of High-Dose Reclaimed Asphalt Mixtures (RAPs) in Hot In-Place Recycling Based on Balanced Design.

This study endeavors to employ a balanced design methodology, aiming to equilibrate the resistance to rutting and cracking exhibited by hot in-place recycling asphalt mixtures containing a high dose of reclaimed asphalt pavement (RAP). The primary goal is to ascertain the optimal amount of new binder necessary for practical engineering applications, ensuring a balanced rutting and crack resistance performance of recycled asphalt mixtures. The investigation mainly employed wheel-tracking tests and semi-circular bending tests to assess the rutting and cracking performance of recycled asphalt mixtures with a different dose of RAP (in China, it is common to use RAP with 80% and 90% content as additives for preparing hot in-place recycling asphalt mixtures), and varying quantities of new binders (10%, 20%, and 30% of the binder content in the total RAP added). The results indicated that the addition of new binder reduced the resistance to rutting of the recycling asphalt mixtures but improved their resistance to cracking. Furthermore, for the recycling asphalt mixture with 80% RAP content aged for 5 days, the optimal new binder content is 1.52%, while the mixture with 90% RAP content requires 1.23% of new binder. After 10 days of aging, the optimal new binder content for the recycling asphalt mixture with 80% RAP content is 1.55%, while the mixture with 90% RAP content requires 1.28% of new binder.

Full-Scale Evaluation of Airfield Pavement Runway Reconstruction with Full-Depth Reclamation Technique

Full-depth reclamation (FDR) has been identified as a strategic in-place recycling technique that is readily available for effecting a rapid restoration of the structural capacity of abandoned airfield pavements in contingency environments. A full-scale pavement section was designed to investigate the performance of pavement sections containing an FDR layer under simulated loading conditions of a tactical heavy aircraft. The FDR materials were composed of various combinations of base material types (i.e., gravel and limestone) and different asphalt layer thicknesses (i.e., 51 mm and 102 mm). The FDR blends were stabilized with a combination of 3% asphalt emulsion and 2% Portland cement. The FDR pavement section consisted of six different test items to investigate the performance of various FDR material types, asphalt layer thicknesses (i.e., 51 mm and 76 mm), and unpaved landing zones. The performance of the existing pavement and the FDR pavement test sections was compared to assess the improvements resulting from the rehabilitation of airfield pavements using the FDR technique. The results indicated that FDR was effective in restoring the structural condition of a damaged asphalt pavement section to support additional aircraft operations. The quality of the FDR blend had a significant impact on pavement performance, whereas the impact of the asphalt layer thickness was insignificant. Finally, the unpaved FDR sections also demonstrated enough structural competency to support a low volume of aircraft traffic before further surface layers are added during pavement reconstruction in contingency environments.

The Early Performance Development of Hot In-Place Recycled Asphalt Mixture

With increasing societal attention being directed to resource and environment problems, the research focus on high reclaimed asphalt content mixtures has become pertinent. The degree of asphalt fusion in the thermal regeneration process of a high RAP content reclaimed asphalt mixture has a great influence on its performance. In order to explore the development process of hot in-place recycling mixture performance along with internal asphalt fusion, this study conducted research on a geothermal regeneration mixture with 80% RAP content. Dynamic shear rheology (DSR), infrared spectroscopy, and scanning electron microscopy were used to investigate the fusion of recycled mixture under different placement times (1 day, 4 days, and 7 days), and the road performance and fatigue life of the recycled mixture under different placement times were then studied. The results showed that the fusion degree of old asphalt and new asphalt in a recycled asphalt mixture reached 100%, and gradually increased with the extension of placement time. With the increase in placement time, the high temperature performance of the regenerated mixture gradually decreased, the water stability gradually increased, and the low-temperature performance and fatigue life significantly increased from 1 day to 7 days, by 19% and 32%, respectively.

Laboratory evaluation and field demonstration of cold in-place recycling asphalt mixture in Michigan low-volume road

Cold-in place recycling (CIR) is a promising technology for the rehabilitation of asphalt pavements. However, the CIR asphalt pavement constructed in the field often has high air voids due to the presence of moisture during construction, and the moisture susceptibility of the pavement is crucial in determining its service life. Therefore, the objective of this research was to assess the laboratory performance of CIR asphalt mixes under freeze-thaw conditions (The mix prior to being subjected to freezing and thawing is labeled as "dry condition", while the mix that was placed in a refrigerator at –18 °C for 16 h and then transferred to a distilled water bath at 60 °C for 24 h is referred to as "post-freeze-thaw") and forecast in-situ pavement performance through pavement mechanistic-empirical design (PMED). Moisture susceptibility was evaluated by preparing CIR asphalt mixtures under dry condition and freeze-thaw conditions. The Disk-Shaped Compact Tension (DCT) test was utilized to approximate the low-temperature characteristics of the CIR mixture, while the rutting resistance properties were assessed using the Hamburg wheel tracking device (HWDT). The dynamic modulus test was used to assess the elastic deformation characteristics. The asphalt was extracted from the loose material, and the viscoelasticity properties were analyzed using the Dynamic Shear Rheometer (DSR) test and Asphalt Binder Cracking Device (ABCD) test, respectively. Based on the M-E inputs, the projected pavement deterioration was determined through calculations. The outcomes indicated that the dynamic modulus of the asphalt mixture after the freeze-thaw cycle dropped by 11–42 % under varying temperatures and frequencies. The number of wheel passes for the dry condition CIR asphalt mixture was 8.4 % higher than that of the post-freeze-thaw CIR asphalt mixture. The fracture energy dropped by 6 % after freeze-thaw conditions. The emulsion asphalt used in the RAP (reclaimed asphalt pavement) made the binder softer, the fatigue and low-temperature cracking resistance of the material was enhanced, and decreased its resistance to rutting. the results of the M-E pavement design demonstrated that the freeze-thaw condition increased rutting, cracking, and IRI (International Roughness Index) distress. In conclusion, the application of CIR technology in the project enhanced the performance of the asphalt pavement with regard to its resistance to cracking and fatigue. As a result, CIR pavement may be suitable for use in low-traffic road.

Hot In-Place Recycled Asphalt Mixtures: RAP Analysis, Compaction Characteristics and Field Evaluation

The substantial accumulation of reclaimed asphalt pavement (RAP) poses a pressing issue in road construction. The hot in-place recycling (HIR) technique has garnered widespread attention due to its high recycling rates of RAP and minimal environmental hazards. This study focuses on the RAP analysis, compaction characteristics, and field evaluation of hot in-place recycled asphalt pavements (HIRAP). Firstly, a novel test method of RAP analysis was proposed to evaluate the suitability of RAP. Subsequently, compaction tests reveal the compaction characteristics of hot in-place recycled asphalt mixture (HIRAM). Finally, the field performance of HIRAP was assessed. The research findings indicate that the RAP analysis method can accurately characterize the status of RAP. Increasing the RAP temperature improves the compaction characteristics of HIRAM. The field tests show that using HIR technology improves the performance of the pavement, in particular with a compaction of 99.7%. This study will establish a theoretical foundation for further promoting the HIR technique.

Comparative analysis of cold in-place recycling for roadway maintenance and rehabilitation from the perspectives of technical-cost-environmental nexus

Cold-in-place recycling (CIR) is a sustainable road pavement maintenance technology for its on-site operation and complete reuse of reclaimed asphalt pavement. However, its sustainable performance has not been thoroughly investigated. This study aims to systematically assess and compare CIR against three alternative maintenance technologies from technical, cost and environmental perspectives, specially taking into account different numbers of repaired layers. The technical improvement in full-depth loading resistance is evaluated using a newly developed multi-sequenced repeated load (MSRL) test. Environmental impacts are assessed through life cycle assessment (LCA), while cost analysis is conducted using life cycle cost analysis (LCCA), with consistent system boundary applied. Additionally, a nexus analysis is performed using a modified Boston Consulting Group (BCG) matrix to comprehensively examine the sustainable performance of different treatments and further explore the synergies and trade-offs among various assessment aspects. The multi-dimensional sustainable assessment results reveal the influence factors in each perspective. Final BCG matrix shows that CIR can achieve the comparable technical performance to the conventional maintenance treatment, while reducing GHG emissions by 60.8% and cost by 45.8%. The methodology and findings of this study are expected to provide helpful insights for decision-makers in reducing negative impacts and promoting integrated management for sustainability.

Characterization of the Cracking Resistance Gradient of Bitumen Emulsion-Based Cold In-Place Recycling Mixtures over Curing by Semi-Circular Bending Test

To better reveal the performance development of bitumen emulsion-based cold in-place recycling (BE-CIR) mixture over curing, a semi-sealed laboratory curing method was proposed in this research to simulate the in situ moisture evaporation process and cracking resistance of the BE-CIR specimen at different depths during a curing time of 28 days, which was also investigated by the semi-circular bending (SCB) test. The influencing factors of cement content (1.5% to 2.5%), initial moisture content (3.5% to 4.5%), curing temperature (25 °C to 45 °C) and relative humidity were investigated, and the significance of different factors affecting the performance development was also analyzed. The results indicate significant variations in cracking performance parameters at different depths, with the top part exhibiting notably higher tensile strength and fracture energy compared to the bottom part, and a gradient index (GI) is proposed to describe the difference. Cement content affected early tensile strength and fracture energy, while the initial moisture content affected the development rate of the performance. The influence of curing temperature was extensive, and as the temperature increased beyond 40 °C, the strength of the effect decreased. High humidity during the early stage of curing inhibited the strength formation and development of fracture energy. The performance development of the BE-CIR mixture is more significantly influenced by the moisture migration process, which is governed by curing temperature and relative humidity, as opposed to the cement content and initial moisture content.

Compaction Characteristics of a Foam Asphalt Hot In-Place Recycling Asphalt Mixture

This study investigates the application of foam asphalt (FA) to enhance the compaction effectiveness of a hot in-place recycling asphalt mixture (HIR-AM) during the HIR process of old road surfaces. Initially, the process parameters for FA preparation were determined through expansion-rate and half-life tests. Subsequently, the study focused on evaluating the impact of FA on the compaction quality of HIR-AM. Performance assessments were conducted through rutting tests, low-temperature bending tests, Hamburg wheel tracking tests, dynamic modulus analyses, and various other experiments to evaluate the road performance of HIR-FAM. Finally, the research findings were validated through practical engineering applications, and the construction process for HIR-FAM was summarized. The research results reveal that the optimal foaming temperature for SBS asphalt is 170 °C, with an ideal water content of 1.7%. Under the same compaction temperature, HIR-FAM demonstrated a significant reduction in void content, ranging from 3.8% to 21.2% compared to HIR-AM. Moreover, a higher proportion of FA usage resulted in a more substantial decrease in void content. Compared to HIR-AM, HIR-FAM exhibited notable improvements, including an 11.6% increase in dynamic stability, a 13.4% enhancement in bending strength, a 13.3% increase in maximum bending strain, an 8.1% improvement in residual stability, and an 8.5% boost in freeze–thaw splitting strength. Furthermore, HIR-FAM demonstrated superior water-thermal stability and resistance to low-frequency loads. Paving a test road verified that the adoption of foam asphalt in thermal recycling led to a compaction density increase of over 0.79% compared to traditional in situ thermal recycling sections, with improved compaction uniformity.

Comparative performance evaluation of basalt fiber–modified hot in-place (HIP) recycling asphalt mixtures: site mixture versus lab mixture

The performance improvement of hot in-place recycling asphalt mixtures has been a hot topic recently due to the widespread application of HIP recycling technology. Based on the maintenance project of the provincial pavement G233 Baoying section, basalt fibers were introduced into HIP recycling mixtures. The effect of basalt fiber on the comprehensive performance of recycled mixtures was investigated using high temperature stability tests, cracking resistance tests, water stability tests, and dynamic modulus tests. Moreover, the performance of site mixtures was comparatively investigated with that of lab-made mixtures to further explore the site mixing effect on the mixture performance. The results showed that the recycled mixtures without basalt fiber presented unqualified cracking resistance even though proper mixture design was performed. The addition of basalt fibers could greatly enhance the rutting resistance, low-temperature cracking resistance, and stripping resistance of HIP recycled mixtures by 105.2%, 102.3%, and 46.9%, respectively. Moreover, the mixing method also had a significant impact on the properties of mixtures. The recycled mixtures produced by the site re-mixing method showed inferior performance compared to that of mixtures produced by the lab mixing method. Specifically, the dynamic stability, low temperature failure strain, and stripping point values reduced by 44.1%, 16.2%, and 11.7%, respectively, indicating that the site re-mixing process was not as effective as the lab mixing process due to the weaker blending and mixing procedures of the site equipment. The results could be beneficial for the utilization of basalt fiber in HIP recycling technology.

Drying behavior modeling of bitumen emulsion-based cold in-place recycling pavement considering heat-moisture coupling effects

Bitumen Emulsion-based Cold In-place Recycling (BE-CIR) pavement has been widely applied for its energy-saving and environmental friendliness. BE-CIR is manufactured at ambient temperature and requires a specific drying duration to develop sufficient strength before overlay placement. However, it is challenging to characterize the drying behavior of BE-CIR pavement due to the complex field curing environments and strength formation process. Hence, this research aims to develop a drying behavior model of BE-CIR pavement considering the heat-moisture coupling effects, which can help optimize the timing for overlay placement under different curing environments. Numerical models of mixture and pavement were established based on the heat and mass transfer theories. Actual temperature and humidity data obtained from laboratory tests and field monitoring were used for model calibration and validation. Time-variable coefficient and depth correction coefficient developed based on functional relationships of physical parameters were proposed to calibrate the model. The finite element calculation results indicated that the calibrated model could effectively simulate the drying behavior of BE-CIR pavement under natural curing environments. Two typical working conditions based on natural pavement curing environments were simulated. The calculation results suggested that the curing time required to reach moisture equilibrium was 120 h at medium temperature and high humidity and 50 h at low temperature and medium humidity, both higher than the measured 30 h at high temperature and low humidity.

Molecular study on diffusion behavior and performance recovery of aged asphalt binder containing functional rejuvenators

In hot in-place recycling (HIR) technology, there are many shortcomings and challenges such as low mixing temperature, short mixing time and less virgin asphalt binder, demanding multifunctional rejuvenators with higher diffusion rates. Therefore, there is an urgent need to develop a rejuvenator that balances diffusion rate and performance recovery. In this study, molecular dynamics (MD) simulation was employed to establish the basic structure of oil fractions, including chain alkanes, cycloalkanes and aromatic hydrocarbons. To improve the diffusion rate of rejuvenators, functional groups in penetrants and surfactants, such as hydroxyl, carboxyl, amide, ether, and ester groups, were incorporated into the basic structure for artificial design and enhancement. The diffusion rates of the primary and enhanced structures were investigated, and the internal factors for diffusion were analyzed. Furthermore, two rejuvenation methods for rejuvenators were proposed based on the changes in non-bond interaction energy in different asphalt binder systems. Moreover, the performance recovery of different rejuvenators were investigated. The results show that the diffusion rate of chain alkanes in the basic structure is the fastest, while that of aromatic hydrocarbons is the slowest. When ether and ester groups are used as enhanced structures, the diffusion rates of chain alkanes increase by 34.7% and 61.1%, respectively. Regarding performance recovery, chain alkanes exhibit the best performance recovery effect, which is further enhanced when ether and ester groups are employed as enhanced structures.

Quantifying the Workability of Cold In-Place Recycling in the Laboratory

Abstract The Dongre Workability Test (DWT) is a method for determining an asphalt pavement’s relative workability based on stress–strain relationships during compaction in a Superpave gyratory compactor. Although the DWT has seen favorable results with warm-mix and hot-mix asphalt, there has been limited attempts to apply it to cold-mix asphalt (CMA), and those limited attempts did not provide conclusive results. This research re-examined the curves developed during the DWT data collection in order to better quantify the workability of CMA, specifically, cold in-place recycling (CIR). Phase one consisted of defining ten possible metrics from the stress–strain curve and applying them to samples with three different curing conditions. From these preliminary results, the five metrics that showed the greatest differences between curing conditions with small relative coefficients of variation were chosen for continued evaluation. In phase two, these five metrics were applied to three different asphalt emulsions at four different curing conditions (temperature-minutes: 10C-30, 10C-120, 60C-30, and 60C-120). Two metrics based on different areas under the stress–strain curve yielded the most logical and consistent results, with higher values indicating a more workable mixture. Samples cured for 30 minutes at either 10°C or 60°C showed the most noticeable distinction between workability of each asphalt emulsion. Moving forward, it is recommended these metrics be applied to additional asphalt emulsions, reclaimed asphalt pavement sources and gradations, and curing conditions to determine if the DWT method can be modified for application to CIR mixtures.

Micropore structure characterization of the bitumen emulsion-based cold in-place recycling mixture considering water gradient migration condition through multiple XCT scanning

Bitumen Emulsion-based Cold In-place Recycling (BE-CIR) has been widely used all around the world due to its superior environmental benefits. Unlike the hot mix asphalt (HMA), BE-CIR mixture presents a unique moisture migration behavior after compaction. The migration of water can alter the micro-pore structure of the BE-CIR mixture and then change its mechanical performance. Therefore, it is essential to characterize the variation of internal structure, especially the air voids, in depth direction during the curing process of BE-CIR mixture. A one-way evaporation method was developed to simulate the field moisture migration of BE-CIR mixture specimens in laboratory. To track the internal structure change with time, multiple X-ray Computed Tomography (XCT) scanning examinations were performed on the lab-prepared specimens with three different curing periods (0 h, 60 h, and 153 h). The effects of curing temperature and initial moisture content on the micropore structure development were also investigated. The morphology variations of air voids including the content, number, volume distribution and void gradation in the depth direction with curing time were further characterized. The results indicate that there is a 0.5% difference in air void content between the top and bottom of the BE-CIR mixture. The BE-CIR mixture's micropore structure evolved over the course of curing with two key characteristics: a fast increase in the number of small voids and a sustained development of large voids. The development of more tiny pores and the gradient properties of these pores, which predominated during the curing time of 60 to 153 h, were a reflection of the internal migration of free water. The initial moisture content mainly affects the magnitude of the large void variation during the curing time of 0 to 60 h, while the curing temperature affects the proportion of pore increase between the two curing periods. The outcomes can provide a better understanding on the dynamic volumetric characteristics of the BE-CIR pavement over the curing process.

Determination of Virgin Asphalt Mixture Content in Hot In-Place Recycling Based on Field Rutting Depth Variability

Hot in-place recycling (HIR) technology, 100% recycling reclaimed asphalt pavement (RAP), has been widely used as a sustainable maintenance treatment. However, the inhomogeneity of RAP makes it impossible to securely assume that the produced mixture properties will match the laboratory design. For HIR projects, a virgin asphalt mixture (VAM) is added to modify the gradation of the existing rutted pavement. However, once the quantities of VAM added to areas of rutting are varied along the driving direction, the desired construction quality cannot be maintained. In this paper, a method to estimate the quantities of VAM needed for areas with rutting along the driving direction is proposed. To verify the feasibility of the proposed method and explore the effects of varying VAM content caused by rutting depth (RD) distribution variation on construction quality, performance tests of all samples, including two gradation types of virgin aggregates and five VAM contents, were conducted in the laboratory. Test results indicate that the pavement performance of the recycled mixture with continuously graded virgin aggregates is more stable due to the similar gradation of RAP, while the performance of the recycled mixture with gap-graded virgin aggregates possesses substantial fluctuation. Specifically, when the required VAM content is lower than 15%, namely, all monitored RDs less than 10 mm, gap-graded virgin aggregates are more highly recommended from a high-temperature stability perspective. When the required VAM content ranges from 10% to 15%, namely, all monitored RDs ranging from 5.9 to 10 mm, the continuously graded virgin aggregates should be considered first with respect to low-temperature performance. To improve moisture susceptibility, the continuously graded virgin aggregates are more appropriate.