Cold In-place Recycling Mixtures Research Articles

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.

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.

Evaluation of Curing Effects on Bitumen Emulsion-Based Cold In-Place Recycling Mixture Considering Field-Water Evaporation and Heat-Transfer Conditions

The strength growth of a bitumen emulsion-based cold in-place recycling asphalt mixture (BE-CIR) is time-dependent and time-consuming due to the addition of water. There is a great difference between the curing conditions of specification in the laboratory and the in situ conditions, which often leads to a great discrepancy between the results of lab specimens and the field cores. The main objective of this paper is to evaluate the curing effect on laboratory BE-CIR considering field-water evaporation and heat-transfer conditions. Four different curing methods were designed by using different combinations of waterproof layers, heat insulation layers, and variable temperature modes. The variations in temperature indexes, moisture content, air void, and indirect tensile strength (ITS) with curing time were tested, and the mutual influence of these indicators was analyzed. Furthermore, the results of the laboratory samples were compared with the field cores. Testing results show that the performance of the BE-CIR mixture is significantly different from that with no treatment, which is manifested as higher moisture content and lower air void and ITS under the same curing time. The internal temperature of the mixture is the main factor affecting the variation of moisture content, especially on the first curing day. The air void of the mixture has a strong linear relationship with the moisture content. Moisture content and ITS under different curing methods showed similar trends and could be divided into two stages. Taking the field cores as a benchmark, it can be concluded that the field-water evaporation condition should be considered in the setting of indoor curing methods, while the heat transfer could not.

Key Performance Analysis of Emulsified Asphalt Cold Recycling Mixtures of the Middle Layer of Pavement Structure

In the maintenance engineering of asphalt pavement, it is often encountered that both the surface and middle layers are damaged and need to be maintained. The cold in-place recycling technology can be used to simultaneously treat multi-layer diseases and reduce the waste of pavement materials. The cold in-place recycling mixture is rarely used for high layer of pavement structure in high-grade highway. In the supporting practical engineering, the emulsified asphalt cold in-place recycling mixtures were paved as the middle layer of pavement structure by the laying of an overlay. In order to comprehensively evaluate the material performances, coring samples were drilled after cold recycling pavement opening to traffic, and different performance tests were carried out based on the coring samples. The newly paved SMA mixtures were set as the control group. The high temperature stability of cold recycling mixture was analyzed by dynamic creep test and MMLS3 accelerated loading test. Then, the cracking resistance of cold recycling mixture was studied by semi-circular bending test. Finally, the effect of curing time on splitting strength of cold recycling mixture was measured, and the moisture susceptibility was analyzed by dry-wet splitting test and freeze-thaw splitting test. The test results showed that the high temperature stability of cold recycling mixture was worse than SMA mixture. For the cold recycling mixture, the deformation value at the early stage and deformation rate at the stable stage were larger than SMA mixture in the accelerated loading process, and shear failure at high temperature occurred earlier. The cracking resistance of cold recycling mixture was worse than SMA mixture because of the aging effect of the old asphalt and adverse influence of the added cement binder. The effect of curing time on splitting strength of cold recycling mixture was significant, and two stable periods of early strength were, respectively, reached after curing 3 days and 7 days. The indexes of moisture susceptibility, including dry-wet splitting strength ratio and freeze-thaw splitting strength ratio, were obviously lower than that of SMA mixture, and the test values not up to the standard requirement existed. For the emulsified asphalt cold in-place recycling mixture, the improvement of material performances should be focused on, especially the moisture susceptibility. In the research, the emulsified asphalt cold in-place recycling mixtures were acceptably used as the middle layer of maintenance pavement structure. The reliable discussions were summarized based on coring samples collected from real-life road sections. The case can provide guidance and reference for similar engineering applications.

Performance evaluation of cold in-place recycling materials through a simple semi-circular bending test

Cold in-place recycling (CIR) is a method of recycling existing Hot Mix Asphalt (HMA) pavements. In this study, a new performance-based test method, called Fracture Index Value for Energy (FIVE), is developed to capture the fracture energy of the CIR materials. Furthermore, various stabilisation agents were used including engineered emulsion (EE), high float asphalt emulsion (HFMS-2s), commodity asphalt emulsion (CSS-1) and foamed asphalt. The FIVE test was first validated using disk-shaped compact tension (DCT) test and then was implemented on the study mixtures at their optimum stabilisation agent contents. The ranking of the mixtures and reproducibility of the FIVE test was proved to be true through inter-lab comparisons of the test results between two independent testing laboratories. The statistical testing results postulate that the FIVE test is a simple test method that is capable of characterising the cracking performance of the CIR mixtures.

Laboratory Investigation of Factors Affecting the Evolution of Curing in Cold In-Place Recycled Materials

Most cold in-place recycling (CIR) construction uses asphalt emulsion or foamed asphalt as stabilizing agent, which requires placing and compacting at optimum moisture or fluid content in the field. To ensure the CIR layer gains appreciable mechanical capacity to support traffic (including that of construction vehicles), the CIR layer must cure. Curing is typically expressed in relation to moisture content in the CIR layer; however, there is need to explore if a direct link exists between the amount of moisture in CIR and the mechanical properties of CIR. Presently, to ensure sufficient curing, construction specifications recommend time estimates with minimal consideration of how various factors such as material variations, climatic inputs, and construction process differences may affect the curing evolution. The objective of this study was to investigate and identify the critical factors that affect the curing evolution of CIR materials. Curing was evaluated in relation to moisture loss (using gravimetric measurements) and gain in mechanical properties (using indirect tensile strength and resilient modulus tests). The results show that there is further gain in mechanical properties after the CIR mixtures have reached an equilibrium moisture condition. Additionally, the following factors were identified to most significantly affect the curing evolution of CIR materials: stabilizer type and amount, active filler type and amount, initial moisture content, curing temperature (when active filler is present), and moisture reintroduction through external sources (e.g., rainfall). On the other hand, density and curing temperature (in the absence of active filler) did not seem to affect the rate of curing.

Evaluation of the shear and permanent deformation properties of cold in-place recycled mixtures with bitumen emulsion using triaxial tests

The cold in-place recycling (CIR) technique is a road rehabilitation method suitable for a circular economy. Triaxial tests have proven to be suitable for studying the mechanical behaviour of cold mixtures. In this study, CIR specimens were prepared using a gyratory compactor with different proportions of bitumen emulsion. The Mohr–Coulomb diagrams and shear parameters were obtained by conducting monotonic triaxial tests at different confining pressures. The binder content provided adequate cohesion; however, an excess of binder reduced internal friction. Subsequently, repeated load permanent deformation triaxial tests were performed. Accordingly, critical stress ratios between 20% and 30% were obtained, and the mix with a 2.50% binder exhibited the best response. Lastly, two permanent deformation prediction models were fitted to the measured results. For high deformations, the Paute model underestimated the deformations, whereas the Huurman model demonstrated the best fit.

Performance Characteristics of Cold In-Place Recycling Mixtures

AbstractCold in-place recycling (CIR) of asphalt pavements is a process that has successfully been used for many years. The use of CIR for rehabilitation offers many advantages over reconstruction....

Structural Contribution of Cold In-Place Recycling Base Layer

Cold in-place recycling (CIR) of asphalt pavements is a process that has successfully been used for many years. The use of CIR for rehabilitation offers many advantages over traditional overlays due to its excellent resistance to reflective cracking and its environmentally friendly impacts. Despite the good performance and positive sustainability aspects of CIR, the structural contribution of the CIR base layer has not been well defined. In this research, CIR mixtures were designed with different asphalt emulsions. The mixtures were then subjected to dynamic modulus, repeated load triaxial, and flexural beam fatigue testing over a range of temperature and loading conditions. The performance test data generated were then used to develop CIR rutting and fatigue performance models used in the mechanistic analysis of flexible pavements. The technique used to develop the performance models leveraged the fact that the rutting and fatigue models for individual CIR mixtures were all within the 95 percent confidence interval of each other. A mechanistic analysis was conducted using the 3D-Move Mechanistic Analysis model. With the laboratory-developed performance models, the structural layer coefficient for the CIR base layer were developed for use in the 1993 AASHTO Guide for the Design of Pavement Structures. This analysis led to the determination of an average structural coefficient of the CIR base layer of 0.25.

Compaction and volumetric analysis of cold in-place recycled asphalt mixtures prepared using gyratory, static, and impact procedures

Compaction is one of the most important factors to be considered while manufacturing laboratory samples of cold in-place recycled (CIR) mixtures.In this study, the effect of three laboratory compaction procedures (static, gyratory, and impact) on the volumetric performance of CIR mixtures was investigated. CIR specimens were manufactured with the same proportion of added water and bitumen emulsion, and using several levels of compaction, varying the number of gyrations and blows. The volumetric properties were evaluated by different procedures, both before and after curing. A comparison between the laboratory results of the different tests, the design target values, and actual in-field values was performed. It was observed that gyratory compaction proved to be the most versatile and one that best represented the real compaction. The bulk density procedure by dimensions was useful for laboratory design; however, it overestimated the sizes of the air void content by more than 10% compared to the field values.

Economic and Environmental Cost Analysis of Cold In-Place Recycling

The purpose of this study is to evaluate and quantify the construction costs, energy costs, and environmental benefits of using cold in-place recycling (CIR) as a pavement rehabilitation technique. For this study, the costs of using CIR mixes prepared using either asphalt emulsion (CIR-E) or foamed asphalt (CIR-F) were compared with conventional 5.1-cm (2-in.) hot-mix asphalt (HMA) mill and overlays. Three different costs were considered for the production of each pavement rehabilitation technique: (1) construction, (2) energy, and (3) environmental costs. To facilitate the cost comparisons, an equivalent pavement structural analysis was conducted between each pavement rehabilitation technique using a standard design method. The results of the study showed that initial construction costs of using CIR are less than the conventional HMA mill and overlay. The energy and environmental costs were also found to be much lower for CIR mixtures compared with the HMA mill and overlay, with energy savings of up to 63%. Overall, when considering all costs, the use of CIR has lower costs compared with traditional HMA overlays. Cost savings of approximately $34,000–$39,000 were observed when using CIR with HMA overlays, with a total cost savings between 41% and 47% over traditional HMA mill and overlays. This study gives an overall procedure for assessing the economic and environmental costs of different pavement rehabilitation techniques and provides further evidence of the cost benefits associated with CIR to practitioners.

Assessment of two laboratory design methods for CIR mixtures with bitumen emulsion based on static and gyratory compaction

To expand the body of knowledge regarding cold in-place recycled mixtures, this study presents two different laboratory design methods. The main differences between the methods are the compaction procedures (static and gyratory) and the required strength tests (unconfined compressive strength and indirect tensile strength).Specimens were manufactured using both methods, with different contents of bitumen emulsion and added water. The effects of adding Portland cement and increasing the compaction energy were also investigated.The compliance with strength criteria was reviewed, and the optimal bitumen emulsion and water contents were identified. The requirements of the specification based on gyratory compaction proved to be excessively high. A reduction of the values is suggested, and further research is encouraged to allow new benchmarks to be set.

Assessment of Permanent Deformation Properties of In-Place Recycled Pavement Mixtures

Cold in-place recycling (CIR), cold central plant recycling (CCPR), and full depth reclamation (FDR) are techniques that can be used to rehabilitate deteriorated pavements or used in new pavement construction projects. Use of these techniques has been found to be both fiscally and environmentally beneficial. However, there is a lack of literature that examines the mechanistic behavior of these mixtures, particularly when it comes to asphalt mixture-related test methods. This paper presents a comprehensive analysis of data from repeated load permanent deformation (RLPD) testing, a laboratory test that describes a mixture’s rutting behavior, conducted on bituminously stabilized CIR, CCPR, and FDR mixtures. The mixtures were collected as a part of NCHRP 9-51, Material Properties of Cold In-Place Recycling and Full-Depth Reclamation Asphalt Concrete for Pavement Design. The permanent deformation behavior of specimens from 23 projects sampled between 12 and 24 months after construction is presented. The impact of stabilizing or recycling agent as well as the presence of chemical additives on the permanent deformation behavior was considered. Density, one of the most prevalent factors in field-based quality control and acceptance of cold recycled mixes was also examined with respect to its impact on permanent deformation behavior.

The influence of combination of binding agents on fatigue properties of deep cold in-place recycled mixtures in Indirect Tensile Fatigue Test (ITFT)

The publication presents fatigue properties of cold recycled mixtures for eight combinations of binding agents (cement and bituminous emulsion). Cold recycled mixtures were evaluated in Indirect Tensile Fatigue Test (ITFT) at the temperature of 20 °C in controlled stress mode. As a function of horizontal stress, fatigue life is strongly influenced by combination of the binding agents. When fatigue life is analyzed as a function of initial horizontal strain, the difference between different combinations decreases. In all cases, the influence of a specific binding agent is more visible for combinations with lower total amounts of the two agents.

A Balanced Mix Design Method for Selecting the Optimum Binder Content of Cold In-Place Recycling Asphalt Mixtures

The objective of this paper is to present a procedure for designing cold in-place recycling (CIR) mixtures through balancing rutting and cracking for these mixtures. Four CIR mixtures were prepared using two recycling agents (foamed and emulsified asphalts), and compacted at two gyration levels (30 and 70 gyrations). The CIR mixtures were prepared at a constant water content of 3% and a constant cement content of 1% while curing of the compacted samples was conducted by placing them in an oven for three days at 140°F (dry curing). The CoreLok device was used for measuring air voids in compacted samples. The rutting susceptibility of these mixtures was then evaluated using the asphalt pavement analyzer (APA) and dynamic complex modulus (|E*|) while resistance to cracking was assessed using the indirect tensile strength (ITS) test and fracture energy as determined using the semi-circular bend (SCB-FE) test. A demonstration of how these tests were utilized to select a performance balanced optimum binder content for each of the four CIR mixes was also presented. The developed balanced mix design approach was used successfully for designing four CIR mixtures and selecting the optimum binder content for each mix. The results also showed that using a higher compaction level leads to increasing both foamed and emulsified asphalt CIR mixtures’ ability to resist rutting. In terms of cracking, SCB-FE results showed that foamed asphalt mixtures were better at resisting cracking than emulsified asphalt CIR mixtures.

Microstructure characterization of cold in-place recycled asphalt mixtures by X-ray computed tomography

This study investigated the microstructure features of cold in-place recycled (CIR) asphalt mixtures by the X-ray computed tomography (CT) and digital image processing (DIP) methods. A variety of CIR asphalt mixtures were prepared considering four types of aggregate gradation and three type of compaction method. The air void distribution characteristics of CIR specimens were analyzed based on the concept of air void gradation. A two-parameter Weibull function was used to fit the air void gradation, of which the statistical parameters are sensitive to both the aggregate gradation and compaction method. The concept of thickness spectrum was then developed to characterize the thickness distribution of cement asphalt (CA) mortar. Lognormal function was observed of good fitness of thickness spectrum. Finally, the uniformity index (UI) was built to evaluate the homogeneity of CIR mixtures and found the UIs of CIR mixture decreases gradually with the gradation from fine to coarse, and among the three compaction methods, the Superpave gyratory compaction is the best in terms of homogeneity. The findings of this study provide a solid foundation in exploring the internal structure and guiding the mix design of CIR mixtures.

Application of laboratory asphalt cracking tests to cold in-place recycled mixtures

Cold in-place recycling (CIR) is an asphalt pavement rehabilitation technology which is rapidly gaining popularity. This technique consists in milling the top 75–100 mm of an existing asphalt pavement, mixing the crushed material with a recycling agent, and reusing it to repave the roadway. Hence, there is a need to identify laboratory tests for developing CIR mixtures with improved resistance to cracking. This study investigates the applicability of semi-circular bend (SCB) and disk-shaped compact tension (DCT) tests to CIR mixtures. Tests were performed on five similar laboratory and field prepared mixtures. Since the CIR mixtures were relatively softer and more brittle than typical hot mix asphalt, minor modifications to the testing parameters and data analysis approach were found necessary. Fracture energies computed from SCB and DCT data resulted in being statistically similar and detected considerable differences among the five mixtures. Overall, both fracture tests showed good repeatability and potential applications in mix design engineering and optimisation of CIR mixtures, quality control and field validation usages.

Three-dimensional fracture simulation of cold in-place recycling mixture using cohesive zone model

Cold in-place recycling (CIR) shows a good performance in the resistance to cracks in the field and a newly developed Arcan configuration has been used to investigate the Mixed-Mode cracking behaviour of CIR mixture. In this research, a three-dimensional (3D) heterogeneous fracture modeling method was presented to simulate complex cracking development in CIR mixture. The X-ray computed tomography (CT) technique and digital image processing (DIP) method were employed to create 3D heterogeneous numerical model of CIR mixture. In the modeling, a cohesive zone model (CZM) was utilized to estimate the crack resistance of the interface based on laboratory fracture test. The coalescence of micro-cracks and the inception and propagation of macro-cracks in CIR mixture were carefully studied under fracture Mode I and Mode II, respectively. The fracture process zone was also investigated. Three types of displacements, including the Load-Line Displacement (LLD), Crack Mouth Opening Displacement (CMOD), and Crack Tip Opening Displacement (CTOD) were utilized to plot load-displacement curves and the corresponding fracture energies were calculated. The analyzing results of virtual numerical test are proved to be quite consistent with the actual experimental test. The validity of numerical model is verified.

Rational Mix-Design Procedure for Cold In-Place Recycling Asphalt Mixtures and Performance Prediction

AbstractA new volumetric mix-design procedure utilizing the Superpave gyratory compactor (SGC) was developed for cold in-place recycling (CIR) asphalt mixtures with assistance from the Federal Highway Administration (FHWA). It was developed for partial-depth CIR using asphalt emulsions as the recycling additive. This procedure was calibrated using materials from five geographically varied locations in North America: Connecticut, Kansas, Ontario, Arizona, and New Mexico. It required that specimens be prepared at densities similar to those found in the field. The performance of CIR mixtures prepared in accordance with the new mix-design procedure was evaluated in the laboratory with mechanistic-empirical pavement design guide (MEPDG) models as well as in the field. Creep compliance and strength of the mixtures were determined at 0, −10, and −20°C using the Superpave indirect tensile tester (IDT) with satisfactory results. A field test section also had been established with CIR mixtures in Arizona using this...