Cold Recycled Mixtures Research Articles

Assessment of the Rheological and Mechanical Properties of Emulsion–Cement Paste

The main objective of this study was to evaluate the rheological and mechanical properties of emulsion–cement paste (ECP), which is generally formed during the mixing process of cold recycled mixtures. Forty-five combinations of ECP specimens were prepared at varying amounts of emulsion, cement, and water. Specimens were mixed using a low-shear mixer, then allowed to cure in an oven at 60°C for 72 h. The testing program for ECPs included multiple stress creep recovery, bending beam rheometer, linear amplitude sweep, penetration, and isothermal calorimetry tests. Statistical analyses were then conducted to assess the significance of the impact of ECP constituents on the low-, intermediate-, and high-temperature properties. Results showed that varying the proportions of emulsion, cement, and water significantly influenced the properties of ECPs. Higher cement and water contents improved the properties of ECP at high temperatures, but reduced their resistance to intermediate- and low-temperature cracking. Conversely, increasing the emulsion content improved the resistance of ECPs to fatigue and low-temperature cracking, while increasing their rutting susceptibility at high temperatures. The outcomes of this study provide more knowledge about the interactions among emulsion, cement, and water, which can be beneficial to the designers of asphalt mixtures that involve the use of emulsion, cement, and water, such as cold recycled mixtures.

Early Strength of Cold Recycled Emulsified Asphalt Mixtures

In order to study the influence of different factors on the early strength of the cold recycled emulsified asphalt mixtures, 16 different types of cold recycled mixtures were designed and produced, which were combined with different cement content, water content, and cement-water ratio. The indirect tensile strength (IDT), compressive strength (UCS), tensile strength at 15°C (TS), and indirect tensile strength ratio (ITSR) tests were used to test the early strength of the cold recycled mixture. The variance analysis method was utilized to analyze the influence of the single factor on the early strength of the cold recycled mixture. The results show that appropriate cement-water ratio content can effectively improve the early strength of cold recycled mixtures. According to the Spearman correlation coefficient and comparison of each evaluation indicator, IDT and ITSR are significantly affected by various factors, and the results are relatively stable and well-differentiated, which is recommended as indicators of early strength evaluation. Cement-water ratio is recommended as the main control factor to control the early strength of the cold recycled emulsified asphalt mixtures. The optimal cement-water ratio ranges from 0.6 to 1.0, while the cement content is controlled from 1.2% to 1.5% and the water content from 3.8% to 4.3%. This study is of great significance for promoting the design of emulsified asphalt cold regenerative mixture with cement-water ratio and improving and accelerating its early strength development.

A stress–temperature superposition approach to study the nonlinear resilient behavior of cold recycled mixtures (CRM) with active filler addition

The asphalt paving sector is currently embracing and enhancing cold mixture technologies to reduce carbon emissions and decarbonize its operations. Cold recycled mixtures (CRM) have proven to be a promising alternative in road construction and rehabilitation, becoming a primary research focus. This study evaluates the effects on compactability and volumetric properties of adding 1% Portland cement or 1% hydrated lime to CRM made with 100% reclaimed asphalt pavement and bitumen emulsion. Dynamic triaxial tests with different confinement pressures (ranging from 20 to 200 kPa) and temperatures (5, 15, 25, and 35 °C) were used to determine the influence of these active filler additions on the resilient modulus (Mr) of the mixtures. All the mixtures exhibited increased Mr values at lower temperatures and reduced stress–dependency. Cement addition had the most favorable effect on compactability, reducing the necessary compaction energy, while mixtures with hydrated lime had more substantial increases in Mr and reduced stress dependencies. Finally, the results were analyzed by plotting master curves using a proposed novel approach called Stress–Temperature Superposition Principle (STSP), which allows for simpler and more straightforward analyses.

Study on Road Performance of Polyurethane Cold-Recycled Mixture.

To give full play to the advantages of polyurethane as a binder, such as mixing at room temperature, short curing time, and high curing strength, polyurethane was used as the binder of a waste asphalt mixture, and the pavement performance of PCRM (Polyurethane Cold-Recycled Mixture) was analyzed. Firstly, the adhesion performance of polyurethane binder with new and old aggregates was evaluated using the adhesion test. Then, the mix proportion was designed according to the material characteristics, and the reasonable molding process, maintenance conditions, design indexes, and the optimal binder ratio were proposed. Secondly, the high-temperature stability, low-temperature crack resistance, water stability, and compressive resilient modulus of the mixture were evaluated through laboratory tests. Finally, the pore structure and microscopic morphology of polyurethane cold-recycled mixture were analyzed by industrial CT (Computerized Tomography) scanning, and the failure mechanism of polyurethane cold-recycled mixture was revealed. The test results show that the adhesion between polyurethane and RAP (Reclaimed Asphalt Pavement) is good, and the splitting strength of the mixture increases greatly when the ratio of glue to stone reaches 9%. Polyurethane binder has low sensitivity to temperature and poor water stability. With the increase of RAP content, the high-temperature stability, low-temperature crack resistance, and compressive resilient modulus of PCRM showed a decreasing trend. When the RAP content was less than 40%, the freeze-thaw splitting strength ratio of the mixture was improved. After the incorporation of RAP, the interface was more complex and there were many micron-scale holes, cracks, and other defects; after high-temperature immersion, the polyurethane binder appeared to show a certain degree of peeling at the holes of the RAP surface. After freeze-thaw, the polyurethane binder on the surface of the mixture produced many cracks. The study of polyurethane cold-recycled mixture is of great significance to realize green construction.

Stiffness of cold-recycled mixtures under variable deformation conditions in the IT-CY test

Stiffness modulus belongs to the most important properties describing the cold-recycled mixtures (CRM) in terms of their usability in road pavement structures. Previous research proved that this property is strongly dependent on the scheme and conditions of the test (temperature and time of loading) and the time that has passed since the compaction of the specimen or pavement layer. It is a result of the influence of two different types of bonds – hydraulic bonds from cement and bituminous bonds from bituminous emulsion or foamed bitumen. Research presented in this paper showed that the target horizontal deformation values selected during the stiffness modulus test have a strong impact on the obtained results as well. In this paper the popular Indirect Tensile Stiffness Modulus (ITSM) test on cylindrical specimen (IT-CY scheme) was used to show the dependence of the stiffness modulus values on the selected target horizontal deformation level. Research was conducted on four different CRM mixtures and three reference materials. The research proved that even for a narrow deformation range the CRMs do not present linear viscoelastic behavior and display very high effort of material even for typical test conditions. In consequence, they are very prone to failure. Research also proved that CRM mixtures present different rheological behavior than cement concrete or asphalt concrete, and more attention should be given to establishing proper test conditions. Based on the research, it was determined that the recommended target horizontal deformation in IT-CY test of CRM should be reduced to 3 µm.

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.

Evaluating the evolution of fiber-reinforced emulsified asphalt cold-recycled mixture damage using digital image correlation

ABSTRACT The cold recycled mixture has insufficient resistance to cracking, and the process of fracture is incompletely understood. This study used digital image correlation (DIC) technology and a semi-circular bending test to investigate the anti-crack performances and fracture propagation of basalt fiber-reinforced cold recycled mixture (BFCRM). The load-displacement, full-field strain, full-field displacement, displacement rate and crack propagation map were obtained to evaluate the BFCRM’s anti-crack performances. The control cold recycled mixture without fibers was also prepared for comparison. Meanwhile, the fracture process zone (FPZ) during loading were compared and quantitatively assessed by digital image correlation analyses, and the fiber distribution and microscopic features at fracture surface was observed by scanning electron microscopy. The results show that the formation of microcracks of BFCRM takes more time than that of the control group without fiber. Based on DIC observations, the basalt fibers maintain strong anti-crack performance in cold-recycled mixtures and reduce fracture damage. The fracture surface morphology also revealed that basalt fiber exhibited adsorption, bridging, anchoring, stability, and strengthening effects at micro/nano scale, which further improved anti-crack performance e and hindered crack development of cold recycled asphalt mixtures.

Experimental study on the fatigue performance of emulsified asphalt cold recycled mixtures

With advantages such as energy saving, environmentally friendliness, green, low carbon emission, and high economic efficiency, emulsified asphalt cold recycled mixtures (CRM) have been used increasingly extensively, and their range of application has also extended from the base layer to the lower surface layer. Thus, studying their fatigue performance is particularly important. The inevitable secondary compaction in the construction process significantly affects the fatigue performance of emulsified asphalt CRM. However, relevant studies concerning the effect of secondary compaction on the fatigue performance of emulsified asphalt CRM are still lacking, and the conclusions of existing studies investigating the effect of emulsified asphalt CRM composition on its fatigue performance are inconsistent. In this study, the effect of emulsified asphalt content, cement type, cement content, and reclaimed asphalt pavement (RAP) content on fatigue performance was investigated through controlled strain four-point bending fatigue tests, and the effect of secondary compaction on the fatigue performance of CRM was comparatively studied by simulating the secondary compaction with indoor wheel rolling tests. The results showed that the fatigue performance of CRM increased significantly with the increase of emulsified asphalt content. With the increase of ordinary Portland cement content, the fatigue performance improved and then degraded. The fatigue performance peaked at the cement content of 1.5% and then decreased sharply. Mixing with high early-strength cement resulted in little improvement in fatigue performance but reduced it at the content of 3%. The fatigue performance increased with the increase of RAP content. Secondary compaction substantially improved the fatigue performance of CRM, and the fatigue performance was underestimated by the existing evaluation methods. This study could provide guidance and reference for the composition design of CRM and the objective evaluation of its fatigue performance.

The Influence of Recycled Materials on Cold Mix with Foamed Bitumen Properties.

The utilization of recycled materials is an important issue in the context of environmental protection. The large amounts of recycled material recovered from the demolition of asphalt road structures indicate the need to find new ways of utilizing them. In the case of road renovation projects, large amounts of recycled materials are, in most cases, recovered in the form of reclaimed asphalt pavement (RAP), reclaimed concrete (RC) and recycled aggregate (RA). To focus on the effects of the use of recovered materials (RAP, RC and RA), the same composition was used for all of the analyzed mixtures in terms of foamed bitumen (FB) and Portland cement (CEM) content. The scope of laboratory tests included the specification of the following parameters: the amount of air void content Vm, the determination of axial compression strength at +25 °C, indirect tensile strength (ITS) at +25 °C, water resistance, TSR, water and frost resistance, WRW+M stiffness modulus (IT-CY) at 13 °C, dynamic dynamicmodulus. The plan of the experiment assumed addition recycled material in quantities between 20% and 80% in increments of 20%. The obtained results indicate that both the type and quantity of recycled material significantly affect the properties of the cold-recycled mixture with foamed bitumen. Using reclaimed asphalt pavement and recycled cement concrete guarantees high levels of stiffness in the recycled mixture. Howeverin the case of recycled aggregate, the authors did not observe any visible changes in the dynamicdynamic modulus, irrespective of the loading conditions. It was also indicated that it is necessary to reduce the quantity of reclaimed asphalt pavement in the composition of the FB-RCM mix to maintain the required air void content.

Evaluation of water loss and stiffness increase in cold recycled mixes during curing

Implementing cold solutions is one of the primary efforts in the road sector to counteract CO2 emissions and climate change. Using cold recycled mixtures (CRM) permits the reuse of up to 100% of reclaimed asphalt pavement (RAP). This research examines how CRM develop their resilient response during the curing process. Using cyclic loading dynamic triaxial (DTx) testing, resilient moduli of CRM made with various proportions of bitumen emulsion were determined. Curing times from 0 days to 18 months were assessed. Binder content and water loss speed proved to be fundamental factors in the evolution of the resilient moduli of the CRM studied. CRM with a binder content of 2.50% exhibited the best short-term response, while those with a binder content of 3.00% showed greater stiffness values over more extended curing. Once water losses stopped, the CRM continued showing changes in stiffness, proving their evolutionary response.

Multiple Optimization of Cold Reclaimed Asphalt Pavement Mixture

Many asphalt pavements fail structurally after having performed well for some years. In China, when low volume of traffic highway is recycled, the asphalt surface course and the base course stabilized by inorganic stabilizers are sometimes recycled and mixed together to form a new base. Portland cement, lime-fly ash, lime or Portland cement-fly ash is added to improve the strength of the new base, but they are not used at the same time, so it is difficult for a road engineer to select an effective and economical stabilizer. To evaluate the effect and economical efficiency of different stabilizers comprehensively, this paper used Portland cement, lime-fly ash, lime and Portland cement-fly ash to form four kinds of cold recycled mixtures and every kind of cold recycled mixture has 3 mix proportions. The unconfined compression strength, flexural tensile strength, splitting strength, compression resilient modulus, shrinkage performance, water stability performance, fatigue failure and engineering cost of 12 cold recycled mixtures were compared. Finally, the Portland cement-fly ash stabilized mixture with proportion of 5:15:80 was recommended.

Investigation of the influence of bitumen emulsion and Reclaimed Asphalt Pavement content on the mechanical properties of Bitumen Stabilised Materials with high fine particle content

Cold recycling technology is gaining popularity as an economically and environmentally effective way of pavement rehabilitation. The main benefits of the use of cold recycling mixtures (CRMs) in road base courses are associated with the reduction of raw materials consumption by replacing them with recycled materials such as Reclaimed Asphalt Pavement (RAP), as well as the decrease in energy consumption during production and paving compared to Hot Mix Asphalt (HMA). Bitumen Stabilised Materials (BSMs), described as non-continuously bound CRMs, exhibit granular and viscoelastic materials properties due to the presence of local bituminous bonds and a reduced amount of cement. Their mechanical properties are still being investigated.In this paper, Bitumen Stabilised Materials with bitumen emulsion (BSM-E) were investigated. Mixtures characterised by high fine particle content and designed with different Reclaimed Asphalt Pavement (RAP) and bitumen emulsion contents were subjected to Indirect Tensile Strength (ITS) tests at 25°C. TSR water resistance parameter for the mixtures was calculated. Test results were analysed using two-way ANOVA statistical tests.The laboratory test results showed that BSM-E mixtures exhibited different trends of an increase in ITSdry, ITSwet and TSR values as a function of bitumen emulsion content, depending on the RAP amount used. The statistical significance of the effects of bitumen emulsion content, RAP content, and the interaction between these two factors on the tested ITS values was proven.

Evaluation of mixture grading and binder composition effects on the characteristics of cold-recycled mixtures with foamed bitumen

The investigation was carried out to evaluate the effects of the hydraulic binder's composition on the properties of cold recycled mixtures with foamed bitumen of different gradations. The evaluated hydraulic binders were composed of Portland cement, hydrated lime and cement by-pass dusts. The ratios of these three components simultaneously varied from 20 to 60% in relation to the mass of the resultant hydraulic binder. The mixtures were composed of reclaimed asphalt from a road base (coarse or fine RAP), natural aggregates (0/2 and 0/31.5 mm) in different ratios, hydraulic binder and 70/100 foamed paving grade bitumen. It was shown that the fine and coarse mixtures responded differently to the changes in the composition of the hydraulic binder. It was found that the optimum performance of the fine and coarse graded mixtures required different binder compositions.

Laboratory Performance Evaluation of Asphalt-Treated Cold Recycled Mixtures Using Existing Practical Tests

Agencies currently utilize several proxy tests, related to level of compaction (density) and moisture content, to assess the quality of a recycled pavement during construction. These proxy tests are not completely able to indicate sufficient quality or stability of recycled pavements under trafficking or surfacing, especially at early ages. The lack of sound and rapid tests for use in process control and acceptance during construction limits the use of the recycling techniques by agencies, therefore they do not fully benefit from the advantages that pavement recycling offers. This study evaluates the viability of currently available simple and practical stiffness tests (i.e., soil stiffness gauge and light weight deflectometer tests), deformation resistance tests (i.e., Marshall hammer test), and penetration resistance tests (i.e., dynamic cone penetration test) to characterize the properties of asphalt-treated cold recycled mixtures. The scope of the work included laboratory testing of 18 different cold recycled mixtures, prepared through different processes combining different binding agents and filler types, at different curing times via the selected tests. The collected data were analyzed to evaluate these tests in relation to their repeatability characteristics and versatility to capture the performance of recycled materials with respect to changes in curing time, type of stabilizing/recycling agent, and presence of active filler. The data obtained showed the ability of these tests to capture the effect of curing. Moreover, these tests were found to be sensitive to changes in the recycling agent type, recycling agent content, and presence of active filler.

Simulation and evaluation of fatigue damage of cold recycled mixtures with bitumen emulsion

In order to explore the fatigue damage mechanism of CRME under dynamic load, DEM of CRME was established based on PFC2D. The micromechanical response characteristics of CRME were obtained by simulation splitting test, and verified by indoor indirect tensile fatigue test. It shows that the results of simulation test are basically consistent with those of indoor test. The numerical value and quantity of tensile force chain in DEM increase obviously with the increase of the stress ratio, the fatigue life decays rapidly, and the horizontal displacement at different width positions in the specimen increases significantly. The tensile force chain is generated, accumulated and concentrated in the mortar and the aggregate-mortar interface. Eventually, the micro structure and bonding force of CRME are damaged. Tensile force chain is the main factor of fatigue damage. The improvement of the bonding performance of bitumen emulsion cement mortar (BECM) and RAP-mortar interface is suggested to improve the fatigue performance of CRME.

Study on Low-Temperature Index and Improvement Measures of Emulsified Asphalt Cold Recycled Mixture.

With the promotion of cold recycled mixture (CRM) in cold areas and the improvement of its application layer, the enhancement of the low-temperature performance of mixtures is particularly important. The applicability of the current low-temperature bending test method to CRM is controversial. Firstly, the low-temperature crack resistance of CRM with different gradations and emulsified asphalt contents was studied by the indirect tensile (IDT) test and the semi-circular bending (SCB) test. Thereafter, the low-temperature performance evaluation index suitable for CRM was put forward. Then, the low-temperature performance of CRM with different gradations, fiber types, and contents was evaluated by using the above low-temperature evaluation index. The results show that the low-temperature performance of CRM with different gradations and emulsified asphalt contents can be distinguished by fracture work (W) and fracture energy (Gf). Not only do the test results have little variability (about 12% and 15%, respectively), but also the correlation coefficient with the new asphalt film thickness is the highest (0.8595 and 0.8939, respectively). Compared with coarse gradation (AC-25) and fine gradation (AC-13), medium-gradation (AC-20) CRM has higher low-temperature performance, and polyester fiber can significantly improve the low-temperature performance of CRM. Compared with non-fiber, the W and Gf of CRM of polyester fiber (0.3% content) can be increased by at least 42% and 30%, respectively.

Numerical and Experimental Analysis of the Raise-Temperature Effect of Quicklime in Cold Recycled Mixtures

This study focuses on the development of a thermodynamic model to individuate the increment of temperature generated by the addition of quicklime as an active filler in cold recycled mixtures (CRMs). CRMs are increasingly popular mixtures primarily composed of reclaimed asphalt pavement (RAP) and produced at low temperatures. CRMs are composed of RAP aggregates, an asphalt stabilizing agent (emulsified or foamed asphalt), fillers and active fillers (traditionally cement or hydrated lime), and water. One of the main limitations of CRMs consists of their workability that requires high air temperature. For this reason, the construction season for the application of cold recycling technologies is usually limited to summertime, especially in colder areas. A solution to this limitation is the use of quicklime as an active filler in the mixture. As shown by previous research, calcium oxide causes an exothermic reaction with water generating heat and increasing the temperature (20°C–25°C) of the entire mixture. This paper presents a thermodynamic model to identify the temperature increase and its evolution during preparation and transportation to the site of CRMs. The scope is to individuate the correct procedure to identify the most suitable application rate of quicklime in the mix design in order to reach the desired CRM temperature and workability.

Characterization of Residual Moisture Distribution in Cold-Recycled Mixtures with Foamed Asphalt Using Hyperspectral Technology

Cold-recycling technology with foamed asphalt is used widely because it has the incomparable advantages of environmental protection and resource conservation. In this study, cold-recycled mixtures with foamed asphalt were prepared with different gradation types, mixing water contents, and curing times to study the distribution characteristics of residual moisture. An optimum foaming water content of 2.5% and foaming temperature of 165°C were determined for producing the foamed asphalt that was used for the cold-recycled asphalt mixtures. Hyperspectral imaging tests and spectral processing technology were used to obtain the image information and spectral information of residual moisture from 44 specimens; the moisture content of all specimens ranged from 0.25% to 4.70%. Pseudocolor-coding technology was utilized to realize visualization of moisture distribution. In addition, the residual moisture content prediction model was established based on the partial least-squares regression (PLSR) model. Correlations of the residual moisture distribution characteristics and moisture damage resistance evaluation indicators were developed. The residual moisture distribution characteristics and moisture damage resistance are affected significantly by aggregate gradations, mixed water content, and curing time. After 3 days of curing, the strength of the recycled mixture tends to be stable, and the moisture damage resistance can reach the specification requirements, and is suitable for open traffic. It was confirmed that there is a certain correlation between the internal residual moisture content value distribution range and the moisture damage resistance of the cold-recycled mixtures with foamed asphalt. This study could contribute to a better application of cold-recycled asphalt pavement for environmental protection and carbon stabilization.

Research on the Performance of Emulsified-Asphalt Recycled Mixture–Based Cement Dosing

In order to solve the problem of semirigid base damage in the initial period of use, this paper focused on emulsified-asphalt cold recycled mixtures with all old mineral and analyzed its strength formation mechanism. The optimum moisture content and amount of emulsified asphalt in the regenerated base were determined through tests, whereas the performance of the mixtures was verified according to the designed optimum ratio. The gray correlation method was used to analyze the performance-influencing factors and determine the rationality of the base designs. The research results can be used for pavement base recycling with a new solution for old roads, and the efficiency of pavement construction was greatly improved. It also reduced the heat and energy loss and contributed to the sustainable environmentally development of road construction.

Preliminary study of linear viscoelasticity limits of cold recycled mixtures determined in Simple Performance Tester (SPT)

The publication presents methodology developed for determination of linear viscoelasticity limits for cold recycled mixtures with cement and bituminous emulsion using Simple Performance Tester (SPT). Methodology was verified on reference materials (PCV and steel dummy specimens, cement concrete and asphalt concrete) to comply with elasticity and viscoelasticity theory. The developed methodology enabled determination of linear viscoelasticity limits for the tested cold recycled mixtures for base course. Tests were conducted for controlled strain ranging from 10 up to 110 µstrain. Linear viscoelasticity limits were determined based on stiffness moduli and phase angles. The combination of binding agents and test temperature had the greatest influence on the obtained values.