Cold Recycled Mixtures Research Articles

Quantitative assessment and mechanism analysis of modification approaches for cold recycled mixtures with asphalt emulsion

Cold recycled mixtures with asphalt emulsion (CRMEs) are environmentally friendly and cost-effective materials widely used in asphalt pavement maintenance and rehabilitation (M&R). However, CRMEs exhibit lower mechanical performance and durability than hot-mix asphalt. This study aims to evaluate the effects of different modification approaches on the mechanical properties and durability of CRMEs. To this end, four optimal additives, including the rejuvenation agent (RA), basalt fibres (BFs), styrene–butadiene rubber (SBR) latex, and Buton rock asphalt (BRA), were added to CRMEs during the mixing procedure. Comprehensive laboratory tests including immersion splitting and freeze–thaw splitting tests, high-temperature rutting tests, low-temperature semi-circle bending tests, and indirect tensile fatigue tests were conducted to evaluate the mechanical properties of the modified CRMEs. In addition, the radar chart method was performed to quantitatively assess the effects of the multiple modification approaches. To investigate the influence mechanisms, microscopic features of the modified CRMEs, such as the compactness of the mortar and bonding sufficiency of the interface, were detected by scanning electron microscopy. The results showed that the addition of RA, SBR latex, and BRA can improve the comprehensive performance of the modified CRMEs. Among these additives, RA had the most significant effect, followed by BRA. These findings aid in selecting modifiers to improve the performance and durability of CRMEs in practical engineering applications.

Effect of polymer modified bitumen emulsion production method on the durability of recycled asphalt mixture in the presence of deicing agents

This study investigated the effects of polymer modified bitumen emulsion production methods on the microstructural properties of modified binders and the mechanical properties of cold recycled mixtures with a more emphasis on durability. Modified bitumen emulsions containing styrene butadiene rubber (SBR) produced using three methods, including soap pre-batching, post-blending and co-milling. Surface free energy (SFE) and atomic force microscopy (AFM) tests have been used to inquire into the fundamental and microstructure properties of samples. Indirect tensile strength and dynamic modulus tests were used to investigate the mechanical properties of cold recycled mixtures. Mechanical properties of mixtures were investigated in dry state, after saturation and placement in the freeze-thaw (F-T) cycles with and without deicing agents including calcium magnesium acetate (CMA) and calcium chloride (CaCl2). Accordingly, the soap pre-batching method had the most suitable performance in terms of dynamic modulus at high temperatures and superior durability under F-T cycles. The samples made by soap pre-batching method showed the lowest surface roughness, maximum adhesion and the highest cohesive energy and surface free energy based on the results of AFM and SFE tests, which were effective in improving the mixture performance. Overall, the post-blending and co-milling methods can be rated as the second and third ranks in terms of both mechanical and nano-mechanical properties, respectively. Finally, in comparison with CaCl2, CMA salt significantly reduced the destructive effects of F-T cycles on the mechanical properties of recycled mixture.

CALCULATION OF GRADING OF COLD RECYCLED MIXTURES USING NEW AGGREGATES ACCORDING TO THE DESIGNED THICKNESS OF A REHABILITATED LAYER

Technologies of cold recycling of asphalt concrete using various types of binders have been widely implemented in road building industry in Ukraine. Technologies of cold recycling allow to eliminate some types of deformations or destructions and also to enhance bearing capacity of flexible road pavements. If required, process of cold recycling assumes application of new aggregates to achieve standard grading of a recycled mixture. As the standard requirement does not state any restrictions on relative content of new aggregates, these can be applied to achieve the designed thickness of a rehabilitated layer. This article presents a method for calculation of grading of cold recycled mixtures according to the designed thickness of a rehabilitated layer assuming the amount of reclaimed asphalt pavement (RAP) equals to the amount of asphalt concrete in a milled layer. The proposed method is based on calculation of grading of a mixture of new aggregates that should be rather individual for each recycled mixture. The input data for calculation should be obtained from results of a field testing of “old” road pavement construction and also from laboratory investigation of “old” asphalt concrete cores. The article gives also some examples of calculation which are performed on base of mean values of standard grading of recycled mixtures. Nevertheless, the proposed method of calculation could be applied on the base of any values selected within the standard limits. The proposed method assures precise modeling of composition and properties of a recycled mixture and could be implemented when planning projects on rehabilitation of flexible road pavements either by cold in-plant or cold in-place recycling.

Long-Term Performance of Pavement Structures with Cold In-Place Recycled Base Course

Properly designed and maintained asphalt pavements operate for ten to twenty-five years and have to be rehabilitated after that period. Cold in-place recycling has priority over all other rehabilitation methods since it is done without preheating and transportation of reclaimed asphalt pavement. Multiple researches on the performance of cold recycled mixtures have been done; however, it is unclear how the entire pavement structure (cold recycled asphalt pavement overlaid with asphalt mixture) performs depending on binding agents. The main objective of this research was to evaluate the performance of cold in-place recycled asphalt pavements considering binding agents (foamed bitumen in combination with cement or only cement) and figure out which binder leads to the best pavement performance. Three road sections rehabilitated in 2000, 2003, and 2005 were analysed. The performance of the entire pavement structure was evaluated in terms of the International Roughness Index, rut depth, and pavement surface distress in 2013 and 2017.

Effect of curing and compaction on volumetric and mechanical properties of cold-recycled mixture with asphalt emulsion under different cement contents

Cold-recycled mixture with asphalt emulsion (CMAE) has been widely used in pavement rehabilitation due to its environmental and economic advantages. Curing condition and compaction method can both affect the volumetric and mechanical properties of CMAE. The main objective of this paper is to propose a relative reasonable fabrication method of CMAE in consideration of curing and compaction history in the laboratory. The two fast curing conditions (single curing: 2 days curing at 60 °C and mixed curing: 3 days curing at 20 °C followed by 2 days curing 60 °C) and three impact compaction methods (single compaction-50 blows, single compaction-75 blows and double compaction) using Marshall compactor were used to evaluate the volumetric and mechanical properties of CMAE with different cement contents. The result analysis indicates that the single curing condition is beneficial to decreasing the voids content and increasing the indirect tensile strength (ITS), failure strain and critical strain energy density (CSED) of CMAE with low cement content. However, the mixed curing can decrease the voids content and greatly improve the ITS and CSED of CMAE with high cement content. This is because the single curing is better for the film formation of asphalt emulsion but more harmful to the cement hydration than the mixed curing. It is recommended to adopt single curing under relatively low cement content and mixed curing under relatively high cement content in the laboratory. Meanwhile, increasing the number of compaction blows and employing double compaction can greatly reduce the voids content and improve the ITS and CSED with different cement contents, which is beneficial to the mechanical properties of CMAE. Since the double compaction can better simulate the field construction, it is recommended to employ the double compaction to fabricate specimens in the laboratory.

Comparison of the Bearing Capacity of Pavement Structures with Unbound and Cold Central-Plant Recycled Base Courses Based on FWD Data

Bearing capacity changes over the year, depending on the water content in a pavement structure: the higher the water content, the lower the bearing capacity. As expected, the highest water content in a pavement structure is observed in the early spring as the ice lenses melt. Thus, spring is a critical period for pavement performance, because a decrease in bearing capacity results in faster pavement deterioration. The bearing capacity of pavement structures with an unbound base course and the negative effect of spring thawing on pavement performance have been analyzed by a considerable number of researchers. However, very little is known about the bearing capacity of pavement structures with a cold-recycled base course despite the significantly increasing usage of cold-recycled mixtures. This paper focuses on the bearing capacity of both unbound and cold central-plant recycled base courses at different seasons and their stability. A cold central-plant recycled (CCPR) base course was constructed from a mixture of 38.8% reclaimed asphalt pavement (RAP), 3.1% foamed bitumen and 2.3% cement. A virgin aggregate was added to achieve desirable aggregate gradation. The bearing capacity of the unbound and CCPR base layers, as well as the whole pavement structure, was evaluated by back-calculated E moduli from falling weight deflectometer (FWD) data. In addition to this, the residual pavement life was calculated using mechanistic-empirical pavement design principles. The results showed that the durability of pavement structures with a CCPR base course is more than seven times lower compared to that of pavement structures with an unbound base course, irrespective of season. Nevertheless, the bearing capacity (surface modulus E0) of the pavement structure with a CCPR base course gradually increases due to the curing processes of bituminous and hydraulic binders (in this study, within four years of operation, it increased by 28–47%, depending on the side of the road).

Fracture Parameters and Cracking Propagation of Cold Recycled Mixture Considering Material Heterogeneity Based on Extended Finite Element Method.

Cold recycled mixture (CRM) has been widely used around the world mainly because of its good ability to resist reflection cracking. In this study, mixed-mode cracking tests were carried out by the designed rotary test device to evaluate the cracking resistance of CRM. Through the finite element method, the heterogeneous model of CRM based on its meso-structure was established. The cracking process of CRM was simulated using the extended finite element method, and the influence of different notch lengths on its anti-cracking performance was studied. The results show that the mixed-mode fracture test method can effectively evaluate the cracking resistance of CRM by the proposed fracture parameters. The virtual tests under three of five kinds of mixed-cracking modes have good simulation to capture the cracking behavior of CRM. The effect of notch length on the initial crack angle and the crack propagation process of the CRM is mainly related to the distribution characteristics of its meso-structure. With the increase of the proportion of Mode II cracking, the crack development path gradually deviates, and the failure elements gradually increase. At any mixed-mode level, there is an obvious linear relationship between the peak load, fracture energy, and the notch length.

Compaction Characteristics of Cold Recycled Mixtures with Asphalt Emulsion and Their Influencing Factors

The objective of this study is to investigate the compaction characteristics of cold recycled mixtures with asphalt emulsion (CRME) using the Superpave gyratory compactor (SGC) method. Five characteristic parameters were proposed and calculated including the compaction energy index, the compaction energy index, three compaction energy indicators at different compaction stages. The influence of these parameters and material compositions were analyzed for the pavement performance. The difference between SGC and Marshall double-sided compaction/heavy compaction method was compared. The results show that the proposed parameters can better reflect the compaction characteristics of CRME, and the mixture effect with SGC of 50 gyrations was close to that with 75 blows using the Marshall compaction. The asphalt emulsion contents and compaction temperatures had a significant effect on compaction characteristics, but the effect of aggregate gradations was not significant. The appropriate asphalt emulsion and the new aggregate content can increase the capability of the CRME to resist the permanent deformation. The optimum mixing water content of CRME obtained by the SGC method was reduced by 18%, but the density increased by 3.5%, compared with the heavy compaction method. Finally, a new idea to determine the optimum emulsified asphalt content of CRME was provided through analyzing the compaction characteristic parameters.

Диференціація методів виготовлення зразків сумішей на основі гідравлічних в'яжучих

Introduction. To the particular group of road materials that are applied by close technologies belong hydraulically bound mixtures, roller-compacted concrete and cold recycled cement bound mixtures. As it is known, the composition and also the technology of application of road materials both affect the choice of method for their specimens manufacturing. For the Ukrainian road building industry, only one method for such specimens manufacturing was implemented, but in the world-wide practices there exist a developed differentiation of said methods which considers a lot of factors. The paper is aimed on the analysis and issues to be resolved in that direction.General classification, analysis and criteria for selecting the standard methods for manufacturing the specimens of mixtures based on hydraulic binders.Taking into consideration the methods of compaction applied in various systems of standardization, the classification of methods of manufacturing of specimens of the specified types of mixtures was offered. A comparative analysis of methods of specimens manufacturing is performed taking into account the requirements for specimens, features of the procedure of selection of the mixture composition, conditions of specimens manufacturing and also mixture grading. Particular attention was paid to the choice of criteria for the manufacture of specimens of cold recycled mixtures, which, according to world practice of road construction, shall be subjected to the rheological properties testing.Conclusions.Accounting for the world-wide experience in road building industry, established in Ukrainian practice methods of manufacturing specimens of mixtures based on hydraulic binders used by close technologies need to be improved in order to:–provide a selective approach to the procedure for selecting the mixture composition;–realize the possibility for making specimens on the construction area;–ensure performing of tests accounting for the extended requirements for said mixtures.Keywords: hydraulically bound mixture; roller-compacted concrete; cold recycled mixtures; methods of specimens manufacturing; compaction methods

Influence of the Compaction Method in the Volumetric Design of Cold Recycled Mixtures with Emulsion.

Compaction of cold asphalt mixtures is a subject that has not been thoroughly studied, and, for this reason, requires new efforts from researchers to have a better understanding. Unlike hot mixtures, cold mixtures and mainly recycled mixtures require specific considerations for compaction. There is a lack of consensus about the methodology to select the optimum premix water and emulsion contents. In the absence of specific regulations, the use of soil tests or hot mixtures procedures is common. For these reasons, this investigation’s main goal was to evaluate two compaction methods used to design cold recycled mixtures with emulsion: the modified Proctor procedure and the gyratory compaction. It was concluded that both methods could be useful to study compactability since consistent results were obtained by applying the maximum bulk density criterion. However, the higher bulk densities achieved, the smaller specimens used, and the suitability of the gyratory specimens to be later tested for mechanical properties make them preferable to the modified Proctor samples. A new approach has been proposed using iso-density lines on dual-axis premix water content-emulsion content graphs that facilitates the study of the influence on compactability of these two factors combined. These contributions can alleviate the laboratory works during the design of cold recycled asphalt mixtures and contribute to a more in-depth knowledge of the combined effect of premix water and emulsion contents on the compactability of cold recycled mixtures with emulsion.

Use of calcium oxide as active filler for bituminous stabilised materials

With the increasing usage of cold recycling techniques on maintenance of flexible pavements, increases also the need to better characterise the properties of cold recycled mixtures, usually composed of aggregates, reclaimed asphalt – RAP, active filler and bitumen. Understanding the mechanical behaviour of cold recycling mixtures is necessary to set the design process on a performance based approach. This study was designed to evaluate evolution of mechanical behaviour related with the use of quicklime (calcium oxide) as active filler in a BSM mixture; the quicklime was added on different contents up to 5%, considering that its chemical reaction with water can be very convenient to deal with environmental limitations on the construction of a BSM layer. Alongside, this study also compares in a qualitative way the changes in mechanical properties when lime or cement are used as active filler. Indirect tension tests and monotonic triaxial tests were run on BSM mixtures produced with either quicklime or cement added in different amounts. The results show that the use of quicklime may bring various advantages to cold recycling operations, with negligible performance variation even when added in higher amounts. Results also indicate that mixtures designed with cement are more susceptible to mechanical behaviour changes when compared to the ones designed with lime, which suggests that mixtures designed with cement need a more precise mix design and specially a more accurate mixing control in field operations.

Investigation of the Shear Behavior between Layers of Foamed Asphalt Cold-Recycled Pavement

AbstractFoamed asphalt cold-recycled mixtures have been increasingly used in China. However, the interlayer shear performance between the foamed asphalt cold-recycled layer and its top and bottom l...

Effect of RAP Source on Compactability and Behavior of Cold-Recycled Mixtures in the Small Strain Domain

AbstractCold recycled materials (CRMs) are products of modern recycling techniques that are used in maintenance and rehabilitation of pavement structures with significant economical and environment...

Study of the Stiffness of the Bitumen Emulsion Based Cold Recycling Mixes for Road Base Courses.

The benefits of the use of cold recycling mixtures (CRMs) in pavement rehabilitation are associated with both the reduction of natural resource consumption by replacing them with recycled materials and the reduction of energy consumption during their production and paving. The evolution of the stiffness of CRMs in road construction and the fatigue life of pavements with CRM base layers are still being investigated. In this paper, CRMs with 1% cement content, called bitumen-stabilized materials with bitumen emulsion (BSM-Es), were examined. Mixtures that were differentiated in terms of Reclaimed Asphalt Pavement (RAP) content, as well as the amount and type of bitumen emulsions, were subjected to indirect tensile stiffness modulus (ITSM) tests at 5 °C, 13 °C, and 20 °C. The thermal sensitivities of the BSM-E mixtures were analyzed. BSM-E mixture stiffness modulus levels at various temperatures were determined using a statistical approach. On the basis of the results obtained, a discussion on the mechanistic-empirical design of flexible pavements with BSM-E base layers is presented. The potential benefits of using BSM-E materials in road construction in certain aspects of pavement life are indicated.

Assessment of the Impact of Hydraulic Binder on the Properties of the Cold Recycled Mixture with Foamed Bitumen and Bitumen Emulsion: Field Tests

The paper presents the results of tests of the impact of hydraulic and bituminous binders on the properties of the cold-recycled mixture (CRM). The composition of the cold-recycled mixture includes two types of different binders, i.e., bituminous binder in the form of foamed bitumen and bitumen emulsion, as well as Portland cement (CEM I 32.5R) and hydraulic binder. The hydraulic binder was produced by mixing three base ingredients in the following ratio: 40% CEM I 32.5R; 20% Ca(OH)2 and 40% CBD (cement bypass dust). The cold-recycled mixtures were produced under industrial conditions on a test section. The prepared CRM with bitumen emulsion (MCE) and foamed bitumen (MCAS) was collected from the test section and compacted under laboratory conditions. The impact of the type and kind of the binder was assessed in terms of physical properties, mechanical properties and deformation modulus (bearing capacity of subbase) of the recycled base course after 1, 7 and 28 days. It was found that the use of hydraulic binder in the recycled base course, regardless of the type of bituminous binder, reduced cohesion without reducing the remaining parameters.

Investigation on the designing method of asphalt emulsion cold recycled mixture based on one-time compaction

The asphalt emulsion cold recycled mixture (CRME) technology is one of the main utilization method when RAP is used in the asphalt pavement construction. However, there is not a unified material design method for this technology. In this paper, according to the Marshall test method, the test samples were prepared by two molding methods: one-time compaction and two-time compaction. The effects of compaction work and moisture content on the performance of CRME were studied. The test results showed that after the one-time compaction of CRME, as the compaction work increased, the air void of the mixture gradually decreased, and indirect tensile strength (ITS) gradually increased. When the first compaction times exceeded 125 times, the density of CRME could meet the requirement, only limited changes could be observed when the compaction work was continuously increased. However, the increase in compaction work may cause lead to the occurrence of internal cracks and reduce the strength of the mixture. The moisture content of CRME was basically the same at 60 °C for 120 h and 70 °C for 48 h, indicating that the moisture content parameters of CRME met the principle of time-temperature equivalence. CRME was cured for 8 h at 60 °C, which had met the requirements of the technical specification with a moisture content of less than 2%. When the samples were cured for more than 24 h, there was no remarkable difference in ITS and air void of the mixtures formed by one-time compaction and two-time compaction. Therefore, there was no need to perform a second compaction for the mixture. On this basis, a material design method for CRME with one-time compaction was proposed.

Influence factors of strength and performance of foamed asphalt cold recycled mixture

This paper investigates the factors that influence the strength and moisture damage resistance of foamed asphalt cold recycled mixtures. The relevant factors include gradation, asphalt penetration, mixing temperature, particle size of virgin aggregates, and the curing time and moisture content. Indirect tensile strength (ITS) tests under dry and wet conditions were conducted to evaluate the mixture properties. The test results indicated that minute variation in foaming parameters (i.e. expansion ratio and half-life) for asphalt binders with different penetration grades, had no significant effect on the strength and performance of foamed asphalt cold recycled mixtures. The results also revealed that the mixture with medium-grained gradation exhibited higher ITS than mixtures with coarse- and fine-grained gradation. As the temperature of the added virgin aggregates increased, the ITS and moisture damage resistance of the mixture increased as well, but the difference was quite small. It was also observed that the mixture demonstrated better ITS and moisture damage resistance when using filler in material design. The relationships between curing time, moisture content, and strength of the mixture during curing were also investigated, and the variations of strength formation was summarised. The significance of the studied factors was finally analysed using a one-way analysis of variance based on the test results.

Research on Low Temperature Performance of Emulsified Asphalt Cold Recycled Mixture and Improvement Measures Based on Fracture Energy.

At present, there are no specific indicators and requirements for the low-temperature crack resistance of emulsified asphalt cold recycled mixture (CRME) in the Chinese road mixture specifications. In order to expand the application of this technology in the asphalt surface layer in cold areas, this paper studied the influence of 10 influencing factors on the low-temperature anti-cracking performance of CRME through the semicircular bending test (SCB) with fracture energy as the evaluation index. The research results show that the fracture energy index of the SCB test can be used to evaluate the low temperature crack resistance of CRME. After 10 kinds of influencing factors were analyzed, it was found that the biggest factor affecting the low-temperature cracking resistance of the mixture was the recycling agent, which had an effect on the fracture energy index of over 60%. Followed by cement, fiber and compaction work, the degree of influence exceeded 30%. Finally, combined with engineering application experience, some specific measures to improve the low-temperature anti-cracking performance of CRME were proposed.

Performance of Hot and Cold Recycled Mixtures with High Reclaimed Asphalt Pavement Content

Florida Department of Transportation yearly maintenance and rehabilitation activities include milling and resurfacing of approximately 2,000 lane miles of roadway, with an average resurfacing depth of about 2.1 in. (55 mm). These activities result in the generation and accumulation of roughly 1.8 million tons of reclaimed asphalt pavement (RAP) each year. The use of elevated quantities of RAP in asphalt pavement provides an environmentally responsible solution to the accumulated RAP surplus in some urban areas, while at the same time offering an economical pavement maintenance and rehabilitation option to local agencies facing budget constraints. The objective of this project was to compare the performance of mixtures with 60% RAP content to be used primarily on low volume roads (average daily traffic <750 vehicles) and suggest adequate test methods within the context of a balanced mix design approach. To accomplish this objective, three types of mixtures were considered: hot, cold with emulsion, and cold with foamed binder. Performance testing included intermediate temperature cracking, rutting, moisture susceptibility, and durability. The hot recycled mixtures showed good moisture susceptibility and cracking resistance, especially when rejuvenators were incorporated. The cold recycled mixtures showed poor moisture susceptibility and durability characteristics, which were alleviated when hydrated lime or Portland cement was incorporated in the emulsified and foamed mixtures, respectively. Most hot recycled mixtures, and all the cold recycled mixtures, exhibited poor rutting behavior, likely because of the harsh conditions of the test. A life-cycle cost analysis demonstrated more favorable savings when cold recycled mixtures were used.

Cold asphalt mix with emulsion and 100% rap: Compaction energy and influence of emulsion and cement content

The present work aims at determining the conditions for the compaction of cold recycled mixtures with emulsion and 100% RAP. Additionally, the influence of the emulsion and cement on the final properties of the recycled mixtures has been investigated. To achieve these purposes, reclaimed asphalt material coming from the milling of a distressed pavement was mixed with different proportions of water, asphalt emulsion and cement. The mixtures were compacted by a gyratory compactor. As a result of the investigation, it is recommended to apply a compaction energy level of 100 gyrations for specimens 100 mm diameter and 60 mm height, and an energy of 200 gyrations for specimens 100 mm diameter and 100 mm height. As for the mechanical properties of the mixtures, the laboratory tests showed that the presence of cement contributes to improving the sensitivity to water attack, the resistance to permanent deformations, the stiffness modulus and the fatigue life of the mixtures.