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• Proposed a discrete element method precise modeling based on X-ray CT sections. • Simulated the freeze–thaw cycles to cold recycled mixture with bitumen emulsion at micro-scale. • Explored the micro-damage characteristics of cold recycled mixture with bitumen emulsion under freeze–thaw cycles. Cold recycled mixture with bitumen emulsion (CRME) is an energy-saving and environmentally friendly material. However, the micro-scale damage characteristics under the coupling of water temperature are still unknown and need to be investigated. In this paper, based on the X-ray CT section, the two-dimensional discrete element method (DEM) was used to precise reconstruct the CRME under different freeze–thaw (F-T) cycles. Simultaneously, the fracture behavior, strength, and structural damage characteristics of CRME under F-T cycles were studied by DEM and digital image correlation technology. The results show that F-T cycle significantly damage the formation of strength force chain in CRME. After undergoing 20F-T cycles, the specimen exhibited tensile failure without any obvious crack penetration. The cracks required of the specimen after the F-T cycles to reach the peak load were less than the non-F-T. The stress–strain concentrations were reinforced, making the specimen more susceptible to unloading during the later stages of testing after F-T cycles. The repeated F-T cycles damaged the overall microstructure of the mixture, resulting in less noticeable tensile effects during the splitting test. Utilizing a precise DEM modeling method proved to be practical and effective in investigating the micro-damage characteristics of cold recycled mixture. [ABSTRACT FROM AUTHOR]

• The numerical model of cold recycled mixtures with bitumen emulsion (CRME) was established and the simulation splitting test was carried out by Particle Flow Code in 2 Dimensions (PFC2D). • The reliability and accuracy of the discrete element model (DEM) were verified by the simulation of CRME and the indirect tensile fatigue test in the laboratory. • The fatigue damage evolution process of CRME and the damage characteristics under dynamic load were described. 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. [ABSTRACT FROM AUTHOR]

• Four types of meso -structural indicators of CRME were revealed under repeated loading. • The factors between meso -structural and rutting depth were analyzed theoretically. • The accuracy of the predictive model was verified by K-CV algorithm optimization. The meso -structural characteristics of cold recycled mixes using asphalt emulsion (CRME) were investigated by wheel tracking tests and two-dimensional digital image processing technology in this study. The trends of contact points, long-axis states, fractional dimensionality, and the rule of aggregate distribution were analysed. Fitted equations of the CRME rutting depth and mesoscopic parameters were established in different loading periods. The number of aggregate contact points appears to increase as the loading time increasing. The long axis of the aggregate tends to be consistent. The long axis inclination and fractal dimension of the aggregate profile gradually decrease over time. The aggregate area ratio increased gradually and then the mixture became dense. The evolution of CRME could be divided into three stages under wheel tracking test: the stage of initial compaction, relative compaction, and destruction. The support vector machine prediction model of rutting depth and mesoscopic parameters was established by K-fold Cross Validation algorithm (K-CV) optimization. The proposed model showed high accuracy for the movement state of the asphalt mixture, which could provide scientific guide for pavement condition evaluation. [ABSTRACT FROM AUTHOR]

Cold recycled mixes using asphalt emulsion (CRME) is an economical and environmentally-friendly technology for asphalt pavement maintenance and rehabilitation. In order to determine the optimum range of cement contents, the complex interaction between cement and asphalt emulsion and the effects of cement on performance of CRME were investigated with different contents of cement. The microstructure and chemical composition of the fracture surface of CRME with different contents of cement were analyzed in this paper as well. Results show that the high-temperature stability and moisture susceptibility of CRME increased with the contents of cement increasing. The low-temperature crack resistance ability gradually increased when the content of cement is increased from 0% to 1.5%. However, it gradually decreased when the content of cement is increased from 1.5% to 4%. Cold recycled mixes had better low-temperature cracking resistance when the contents of cement were in the range from 1% to 2%. The results of microstructure and energy spectrum analysis show that the composite structure is formed by hydration products and asphalt emulsion. The study will be significant to better know the effects of cement and promote the development of CRME. [ABSTRACT FROM AUTHOR]

Research on asphalt fatigue and self-healing performance can more accurately model the damage and self-healing evolution principles in practical applications. In order to analyze the rationality of evaluation indexes based on rheology to evaluate the self-healing properties of TPU modified asphalt, a dynamic shear rheometer (DSR) was used in this study to perform time, temperature, and frequency sweep tests of TPU modified asphalt for analyzing the fatigue self-healing properties of TPU modified asphalt over a broader range of temperatures and loading frequencies. This method can improve the accuracy of the fatigue failure determination criteria of TPU modified asphalt and clarify the material performance parameters of TPU asphalt modifiers. The results showed that the increase in hard segment content (C h) and isocyanate radical index (r) can effectively alleviate the decrease in shear deformation resistance of modified asphalt caused by the increase in N (load repetition). N f50 (the number of loads when the normalized modulus drops to 0.5) is more suitable to evaluate the fatigue life of TPU modified asphalt compared with N p20 (the number of loads when the DER - N curve is offset by 20% deviation from the linear undamaged line) and N 2 (the number of load times corresponding to the intersection point of the tangent line of the DER – N fitted curve and the undamaged line). The degree of self-healing will also increase with the increase in C h and r. Besides, the higher the healing temperature and the healing time, the longer the fatigue life. The effects of the self-healing evaluation index (HI) on the self-healing index of TPU modified asphalt were as follows: healing temperature > healing time > soft segment > C h > r > degree of damage > degree of aging > deg r ee of self-healing. • The effects of different segments of polyurethanes on the fatigue self-healing ability of modified asphalt were analyzed. • A viscoelastic master curve for comprehensive analysis of wide temperature and wide frequency bands was established. • The versatility and accuracy of N f50 as an evaluation index for the fatigue life of TPU modified asphalt were determined. • Through comparative experiments, the appropriate synthesis parameters and dosage of TPU asphalt modifier were determined. • The effects of factors on the self-healing index HI were analyzed and sorted by gray correlation coefficient. [ABSTRACT FROM AUTHOR]

In this article, the structure characteristics of Liaoshu paving asphalts have been studied by the methods of element analysis, SARA and E-D-M. At same time, the relation between composition and structure and using performance of Liaoshu paving asphalt were established. The experimental results show that carbon and hydrogen contents were more than 95%, the hydrogen/carbon atom ratio was about 1.5 in the Liaoshu paving asphalts. The content of saturates is between 20 and 25% in the Liaoshu paving asphalt, aromatics 35 and 40%, resin 25 and 30%, and asphaltene 8 and 10%. There are 45 carbon atoms in the average asphalt molecule, which contain 16–17 aromatic carbon atoms. The number of total ring in the average asphalt molecule is 5–6, the aromatic ring is 3–4, and the naphthene ring is 1–2. [ABSTRACT FROM AUTHOR]