Fabrication, characterization and assessment of the capsules containing rejuvenator for improving the self-healing performance of asphalt materials: A review

Abstract

The intrinsic self-healing efficiency of asphalt pavement is insufficient to timely repair the cracks generated due to the asphalt aging, traffic loading, and other unfavorable factors. Hence, the possible solutions to promote the self-healing process of asphalt have been extensively explored by scholars in recent years. This review introduces the asphalt self-healing phenomenon, mechanism and enhancement methods, elaborates the fabrication methods of the capsules encapsulating asphalt rejuvenator, and further analyzes the characteristics of the capsules by the surface morphology, internal structure, chemical composition, thermal stability and mechanical strength. Finally, the assessment of the capsules is made in view of their effects on the self-healing property and the road performance of asphalt materials. It is found that the capsules have great differences in the physical and chemical properties that are mainly affected by the materials used and the encapsulatioin methods. All the capsules have certain abilities to improve the self-healing performance of asphalt, but the improvement degree varies significantly with the test specimen, test method, healing condition and evaluation indicator. Meanwhile, the road performance of asphalt mixture with an appropriate content of capsules can still meet the requirements. This review provides support for the improvement of asphalt self-healing performance and the construction of smart pavements.

Introduction

Asphalt mixture mainly consisted of asphalt binder and mineral aggregates has been widely used in road construction, providing excellent riding comfort, obvious noise reduction and improved driving safety. However, asphalt pavement inevitably suffers the influence of adverse factors, such as traffic loading, thermal cycling and ultraviolet radiation, resulting in the aging of asphalt binder and the occurrence of micro-cracks (Saoula et al., 2013; Tschegg et al., 2011; Wang et al., 2016a; Wu et al., 2008). If no appropriate treatment is applied timely, the micro-cracks will expand and propagate to the visible macro-cracks, severely decreasing the riding comfort and service life of asphalt pavement. Conventional cracking treatments, including crack seal, fog seal, slurry seal, micro-surfacing, etc., are passive methods adopted after the occurrence of visible macro-cracks (Burningham and Stankevich, 2005), which require a long maintenance period, hinder the traffic flow, and consume lots of material resources. Moreover, the optimum maintenance time is usually difficult to determine.

In recent years, the self-healing material that can heal itself spontaneously when subjected to a certain damage has become a research hotspot for its good application prospect in the field of engineering materials including polymer, concrete, asphalt, etc. (Bergman and Wudl, 2008; Billiet et al., 2013; Castro and Sanchez, 2006; White et al., 2001; Yuan et al., 2008). It has been found that tiny cracks generated in the asphalt pavement can be healed autonomously if a suitable environment condition is given, indicating that the asphalt mixture is a natural self-healing material (Bazin and Saunier, 1967; Lee and Kim, 1998a; 1998b; Little and Bhasin, 2007). The existence of self-healing property of asphalt binder has also been demonstrated by laboratory investigations (Kim et al., 2003), which is attributed to the viscoelastic property of asphalt (García, 2012). When a rest period is applied between two loading cycles, the asphalt tends to heal the existing cracks by wetting and interdiffusion on the damaged areas, and its strength and stiffness are also recovered gradually (Anderson et al., 1994; Carpenter and Shen, 2006). Self-healing property of asphalt mixture has a promising prospect in recovering the performance of asphalt pavement actively, which is in line with the principle that cracks should be rectified at source (Li et al., 1998).

However, due to the aging of asphalt pavement and the recurrent traffic loading during the service life, the intrinsic self-healing capacity of asphalt mixture is severely limited at normal working environment, making it practically difficult to autonomously heal the cracks (Qiu, 2008). Therefore, it is urgent to enhance the self-healing ability of asphalt mixture by extrinsic methods. Several novel measures have been proposed to accelerate the healing process, mainly including the induction heating method (García et al., 2013) and the rejuvenator encapsulation method (García et al., 2010a). The induction heating method works by heating the conductive or microwave-absorbing material in the asphalt pavement and increasing the temperature of asphalt to improve the mobility of asphalt molecules, finally leading to the healing of cracks. This method needs the specified materials mixed in the asphalt mixture and human interventions during the service life, which also produces harmful gases and accelerates the rate of asphalt aging due to the heating process (Liu et al., 2011; Norambuena-Contreras and García, 2016). In comparison, rejuvenator encapsulation method requires no external assistance after the capsules containing rejuvenator are embedded in the asphalt pavement. The capsule is designed to resist the mechanical and thermal condition during construction, and to be sensitive enough to automatically respond to the cracks by releasing the encapsulated rejuvenator, which is very promising in the development of smart asphalt pavement (García et al., 2010a; Karlsson and Isacsson, 2006). Nevertheless, the capsule must be carefully produced and tested for its successful application in asphalt pavement. Additionally, the penetration ability of rejuvenator is of great importance to the healing efficiency, and needs to be paid special attention.

The fabrication process plays a crucial role in the quality control of the capsule containing rejuvenator. The characterization and assessment of the prepared capsules are essential to identify the feasibility of capsules when incorporated into asphalt materials. This review introduces the self-healing phenomenon, mechanism and enhancement method, and elaborates the prevalent rejuvenator encapsulation methods concerning the encapsulation mechanism and the fabrication process. Then, the performances of the capsules prepared by different methods are characterized and compared from the perspective of surface morphology, internal structure, chemical composition, thermal stability and mechanical strength. Finally, the assessment of capsules when applied in asphalt is made in view of their effects on the self-healing property and road performance.