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Asphalt has an intrinsic self-healing character. However, the rate of damage formation is much higher than the rate of damage healing. To promote the self-healing capability of asphalt materials, micro-encapsulation technique has been regarded as the potential method. To illustrate the benefits of microencapsulation on the pavement engineering, the fatigue behavior of microcapsule-induced self-healing asphalt concrete was investigated. Firstly, the melamine urea formaldehyde (MUF) microcapsules containing rejuvenator were fabricated via an in-situ polymerization method. The surface morphology, the diameter and the shell thickness of the microcapsule were observed by scanning electron microscopy. The thermogravimetric analysis was used to ensure the high temperature stability of the microcapsule. The infrared spectroscopy analysis technology was used to investigate the components of the microcapsule material. Then, a four-point bending fatigue-healing-fatigue test was designed to study the effect of the microcapsule on the fatigue life of asphalt mixture specimen. From the test results, it can be concluded that the addition of 3 wt% microcapsules can double the fatigue life of the AC-10 asphalt mixture. [ABSTRACT FROM AUTHOR]

Microcapsules are considered as a potential method to enhance self-healing capability in asphalt materials. This study aims to investigate the optimization of self-healing microcapsule synthesis technology. According to prepolymer synthesis test results, the melamine-urea-formaldehyde (MUF) terpolymer, which contains 20 wt% urea, is selected as shell material. Fluorescence Microscope (FM) is employed to observe the rejuvenator emulsion and prepared microcapsules. The result reveals that Sodium Dodecyl Sulfate (SDS) has the best emulsion performance and can be determined as emulsifier of core material. By means of FM and microcapsule size calculation, sixteen sets of microcapsules prepared under different conditions are investigated to determine the optimization of reaction condition. The optimal MUF microcapsule is synthesized under core/shell ratio as 2:1, end point reaction temperature as 65 °C, emulsifying speed as 1500 r/min, emulsifier content as 0.7% and end point pH as 3. Scanning Electronic Microscope (SEM) result shows that the microcapsules are spherical and intact without damage. Fourier Transform Infrared Spectroscopy (FTIR) result indicates that with the increase of core/shell ratio, shell material in microcapsules becomes less. Thermo-gravimetric analysis (TGA) result shows that the mass loss of microcapsules is not distinct after maintained at 180 °C for 1 h, indicating that the optimal microcapsules can meet the temperature requirement of asphalt mixing and compaction. Finally, the self-healing process of microcapsules in asphalt binder is observed to prove the healing efficiency. [ABSTRACT FROM AUTHOR]

Asphalt binders have the self-healing ability to repair fatigue cracking automatically, and therefore the fatigue life can be extended. The extended fatigue life depends on the coupling effect between the chemical kinetics of asphalt molecules at cracking surfaces and the rate of the crack growth under fatigue loading. Obviously, the chemical characteristics of asphalt determine its inherent self-healing ability. However, uncertainty still remains in the effect of the chemical composition and structure of asphalt on the inherent self-healing ability. In this paper, four different Pen grade asphalt binders were characterized by Thin Layer Chromatography (TLC), Gel Permeation Chromatography (GPC), Fourier Transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR), and the self-healing abilities of asphalt binders were investigated by a fatigue-rest-fatigue test with DSR. It was found that asphalt with a higher small molecule content/large molecular content ratio combined with higher aromatics content has a greater self-healing ability. In addition to the chemical composition of asphalt, its molecule structure also plays an important role in determining its self-healing ability. A higher I 1460 / I 1376 , S / Ar ratio and H ar value, indicating a lower branched-chain and higher long and thin molecule content in asphalt, could promote self-healing. Based on Grey relational analysis, the microstructure of asphalt has a greater impact on the self-healing ability of asphalt binder compared with other factors. [ABSTRACT FROM AUTHOR]

In order to distinguish the wetting healing stage and the molecular diffusion healing stage during the self-healing process of asphalt binders, the fluorescence microscope (FM) together with the image processing technique was employed. The healing index was put forward based on the variation of crack area. On the basis of the established capillary diffusion theory, the Two-Stage Model was built to fit the healing curves characterizing the healing index variation with healing time. It is found that the established method is quite suitable to identify the wetting and the diffusion stages. The wetting healing occurs firstly, during which the asphalt molecules spontaneously wet the both faces of crack. Although the strength recovery is not distinct at this stage, SBS modified asphalt shows greater wetting healing strength than the other neat asphalt binders. Once the wetting process is nearly finished, the molecular diffusion healing ratio accelerates significantly, and the descending order of the molecular diffusion rate is: SBS > PEN 100 > PEN 70 > PEN 50 > PEN 20. The test results also show that the disappearance of visible crack does not indicate that the strength has been fully recovered. Here, the tensile strength test is developed to identify the strength recovery rate after the crack is fully closed, which implies that the strength regeneration is a long process. [ABSTRACT FROM AUTHOR]

Understanding the self-healing mechanism and efficiently evaluating healing potential of asphalt materials is a crucial topic, which is especially necessary for a precise prediction of fatigue life. Most existing laboratory test methods to evaluate healing are empirical or phenomenological in nature, leading to an underestimation of fatigue life significantly. Self-healing processes are inherently of hierarchical, multiscale character. Therefore, researches of healing mechanisms may effectively require discrete atomistic treatments. In this paper, molecular dynamics (MD) simulation was used as a tool to examine the hypothesis of healing mechanism and evaluate the self-healing capability of neat and SBS modified asphalt binders. Densities and glass transition temperatures of neat and SBS modified asphalt binders were figured out based on the MD simulation results. An artificial crack was created in MD model to simulate the diffusion of molecules across a crack interface. Three indices calculated from MD simulation results were put forward to evaluate the self-healing capability of asphalt binders, including diffusion coefficient, activation energy, and pre-exponential factor. Finally, the values of these three indices obtained from MD results were compared with the values derived from fatigue-rest-fatigue test, to assess the reliability of above indices as self-healing performance indicators. [ABSTRACT FROM AUTHOR]

Self-healing, as a very valuable characteristic, should be considered when selecting and designing constitutions of asphalt materials. It is necessary to develop a fundamental and universal healing evaluation indication based on the healing mechanism. However, most existing healing evaluation indication lacks physical meaning and hardly can predict the time-dependent healing process under different conditions. In this paper, the formulation of activation energy as a self-healing indication is developed based on the Arrhenius law. The procedure to get the activation energy of different asphalt mastic is put forward. Nine samples of different asphalt mastic are prepared. Then, the fatigue–rest–fatigue tests are employed to get the activation energy of nine asphalt mastic. Finally, the potential of the activation energy as the healing evaluation index for asphalt mastic is studied, and the effect of type of asphalt and filler, asphalt-filler ratio, damage degree on the value of activation energy is discussed. It is found that the healing activation energy can obviously distinguish the healing ability of different asphalt mastic. Healing activation energy represents the minimum energy required for the time-dependent strength gain and reflects the time-dependent strength gain rate, also can rank the time-dependent healing capacity of different asphalt mastic. [ABSTRACT FROM AUTHOR]

Asphalt aging and fatigue cracking are common forms of distresses in flexible pavements. Microencapsulated rejuvenator has been generating significant interest in the feasibility and potentiality applications of asphalt materials self-healing over the past decade and is likely to remain so for the foreseeable future. Despite multiple attempts, the prerequisite is crack propagation through the microcapsules and resulting in the broken shell, which is a passive release. Strategies toward self-healing of uncracked aged asphalt by microcapsules remain a significant challenge. Herein, numerous successful applications of controlled release microcapsules in other fields, including biomedical engineering, food industry, agriculture, smart coating, textile industry, and cosmetics, are classified and referenced. Special attention is given to the mechanisms of release and their triggers, followed by recent advances of microencapsulation technologies with detailed experimental demonstrations. In particular, the challenges and promising solutions of novel controlled release systems for asphalt materials are proposed in detail for the first time. This review aims to present future directions for research in this field, which provides new ideas for the smart maintenance of asphalt pavements. [ABSTRACT FROM AUTHOR]