An asphalt joint is formed when a fresh mix is laid and compacted next to an existing layer, brings about temperature difference during compaction, and therefore requires extra care in quality control and expose to higher cracking risks. Self-healing asphalt aims to stimulate the healing capacity of asphalt mixture and prolong its service life. The main objective of this study is to develop and optimize a calcium alginate capsules healing system for an asphalt joint mix. Capsules following two different self-healing concepts were prepared, namely conventional alginate capsules and conductive alginate capsules. Microscopy, Computed Tomography (CT) and Thermogravimetry analysis (TGA) were used to investigate the performance of alginate capsules. The results show that both types of capsules have a porous structure and a stable performance under high temperature, and therefore potentially survive from the asphalt mixing and production process. These capsules will be implemented and evaluated in full asphalt mix in future research.

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Bitumen is a key constitutive material in asphalt pavements. It binds together the rock scaffolding of a pavement. Bitumen provides asphalt pavement with flexibility and enables it to respond to traffic loading and return to its original condition after the loading, i.e. bitumen restores/repairs the damage. In Porous Asphalt (PA) or Stonemastic Asphalt Mix (SMA) asphalt mixtures, due its open graded structure, bitumen modifiers are used to improve or restore bitumen physical and mechanical performance. Traditionally bitumen modifiers are made with products of crude oil, such as: ethylene vinyl acetate (EVA) copolymers and styrene–butadiene–styrene (SBS). As crude oil production declines and the environmental and financial costs of crude oil extraction increase, there is a need to identify environmentally sustainable alternatives to use in bitumen. Bitumen modifiers generated from biological sources offer an environmentally friendly and economically viable alternative to crude oil based bitumen modifiers. This paper describes how PHBV (poly-3-hydroxybutyrate-co-3-hydroxyvalerate), a bio-based co-polymer, might be used as an alternative bitumen modifier. The effect of PHBV on 70/100pen was investigated for this paper. The chemical and physical effect of the PHBV on the bitumen performance was investigated using Gel Permeation Chromatography, Fourier Transformed Infrared Spectroscopy, Differential Scanning Calorimetry, microscopic imaging and Dynamic Shear Rheometer tests. The results indicate that PHBV has significant potential as a bitumen bio-polymer modifier.

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Bitumen rejuvenators are used to improve or restore the physical and mechanical performance of aged bitumen. Traditional bitumen rejuvenators are a product of crude oil. As crude oil production declines and the environmental and financial costs of crude oil increase, there is an urgent need to identify more environmentally sustainable bitumen rejuvenator alternatives. Bitumen rejuvenators generated from biological sources offer an environmentally friendly and economically viable alternative to the crude oil-based bitumen rejuvenators. This paper describes a study wherein microalgae oil was used as an aged bitumen rejuvenator. The chemical, physical, and mechanical effects of microalgae oil on 70/100pen aged bitumen were investigated. The results indicate that microalgae oil has the potential to be used as an aged bitumen rejuvenator.

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Conductive alginate capsules encapsulating a bitumen rejuvenator (HealRoad capsules) has demonstrated good healing abilities in pure bitumen and mortar mixes. HealRoad capsules can efficiently heal damage via induction heating. They also release the encapsulated rejuvenator, thereby rejuvenating aged bitumen. These findings indicate that HealRoad capsules and induction heating systems combined could represent a possible asphalt pavement maintenance method. This paper investigated the effect of HealRoad capsules on the mechanical performance of the 10 mm stone mastic asphalt mix and measured the damage repair (healing) efficiency of the capsules in an asphalt mix. The results indicate that in small amounts, >1%, HealRoad capsules do not degrade the mix performance (indirect tensile strength and rutting resistance) and in some cases, the HealRoad capsules actually improve mix performance, e.g., in terms of the indirect tensile strength ratio (water sensitivity). However, the HealRoad capsules are unable to stimulate induction healing due to the small volume of capsules within the mix. Further investigation demonstrated that increasing the capsules in the mix to >5% can stimulate induction heating effectively. However, it also indicated that a high content of HealRoad capsules reduces the asphalt mix strength. The study has shown that HealRoad capsules are an effective healing system for high bitumen content mixtures such as mortar mixtures but is an inefficient healing system for a full asphalt mix, such as the 10 mm stone mastic asphalt mix@en

The global road network spans 64.3million km and is of huge significance for the social and economic development. The level of investment in road construction and maintenance is high, e.g. EU €44billion/year (2019), China €614.7billion/year (2019) and US €94billion/year (2019). Despite the level of investment, there has been minimal investment in the development of new asphalt technologies, particularly when compared with R&D investment in other industries, such as the automotive industry. Despite the limited investment, there have been some innovations in asphalt technology. For the past 20 years, researchers have developed bio‐inspired asphalt technology, self‐healing and bio‐binders and have applied them to asphalt pavements. This research has emerged as a response to global warming and the need to reduce both carbon emissions and reliance on oil in asphalt technology. This paper charts the development of two bio‐inspired technologies and considers their significance in relation to the need to reduce carbon emissions and oil dependence (in line with the UN strategic goals, specifically: SDG 9, 11 and 12). This paper considers the potential benefits of bio‐inspired technologies and outlines the current barriers to their further development. This paper aims to begin a conversation with stakeholders on how to speed up the acceptance of bio‐inspired asphalt technologies and their adoption in road design, construction and maintenance. Or is it the case that we have reached the end of the road for bio‐inspired road construction materials?.

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Self-healing technology, such as encapsulated rejuvenator, induction heating or combination of the above two methods, has been demonstrated to be a promising autonomic crack healing mechanism for porous asphalt. With the advantage of low cost, environmentally friendly and excellent performance, the calcium alginate capsules encapsulating rejuvenator showed promising effect to promote the crack healing in asphalt concrete. However, the healing effect of the calcium alginate capsules on the damage from fatigue loadings is still unknown, therefore became the research focus of this study. To this aim, the calcium alginate capsules were produced and incorporated into the porous asphalt (PA). A laboratory ageing process was performed on the PA to simulate the situation when asphalt pavement prone to cracking and a healing needs to take place. Afterwards, the four-point bending fatigue test (4PBF) was employed to characterize the fatigue resistance of the PA specimens. A 24-hour-rest period was carried out to allow the healing of fatigue damages. Additionally, a 4PBF testing and healing programme was carried out to evaluate the fatigue damage healing efficiency of the calcium alginate capsules healing system. It was found that the capsule healing mix showed a higher fatigue life than the reference mix, but the results were a bit scattered which might be caused by the capsule distribution which determines whether the healing takes place on the damage site or not.

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This paper explores the potential use of conductive alginate capsules encapsulating a bitumen rejuvenator as a new extrinsic self-healing asphalt method. The capsules combine two existing self-healing asphalt technologies: (1) rejuvenator encapsulation and (2) induction heating to create a self-healing system that will provide rapid and effective asphalt pavement repair. The work presents a proof of concept for the encapsulation process, which involves embedding the capsules into the bitumen mortar mixture and the survival rate of the capsules in the asphalt mixture. A drip capsule production process was adopted and scaled up to the production of 20l wet capsules at rate of 0.22 l/min. To prove the effectiveness and its ability to survive asphalt production process, the capsules were prepared and subjected to thermogravimetric analysis (TGA) and uniaxial compression Test (UCT). The test results demonstrated that the capsules had suitable thermal characteristics and mechanical strength to survive the asphalt mixing and compaction process. Scanning electron microscopy (SEM) was employed to investigate physiological properties, such as rejuvenator (oil) and iron particle distribution, within the capsules. The electrical resistance tests proved that the capsules were capable of conducting electrical current. The capsules were also tested for their conductive properties in order to determine whether they are capable of conducting and distributing the heat once subjected to induction heating. The results showed that capsules containing higher amounts of iron (alginate/iron powder in a ratio of 20:80 by weight) can efficiently conduct and distribute heat. To prove its success as an asphalt healing system, conductive alginate capsules encapsulating a bitumen rejuvenator were embedded in a bitumen mortar mix. The samples where then subjected to local damaging and healing events, and the degree of healing was quantified. The research findings indicate that conductive alginate capsules encapsulating a bitumen rejuvenator present a promising new approach for the development of an extrinsic self-healing asphalt pavement systems.

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Microwave heating has been shown to be an effective method of heating asphalt concrete and in turn healing the damage. As such, microwave heating holds great potential in rapid (1–3 min) and effective damage healing, resulting in improvement in the service life, safety, and sustainability of asphalt pavement. This study focused on the microwave healing effect on porous asphalt concrete. Steel wool fibres were incorporated into porous asphalt to improve the microwave heating efficiency, and the optimum microwave heating time was determined. Afterwards, the microwave healing efficiency was evaluated using a semi–circular bending and healing programme. The results show that the microwave healing effect is largely determined by the steel fibre content and the mix design of the porous asphalt concrete.. Besides, the uneven heating effect of microwave contributes to an unstable damage recovery in the asphalt mixture, which makes it less efficient than induction heating. However, microwaves exhibited the ability to penetrate further into the depth of the test specimen and heat beneath the surface, indicating deeper damage recovery prospects.@en

Fatigue damage is one of the main modes of deterioration of bituminous materials that leads to the failure of flexible pavements. However, it is well known that bituminous materials have some capability of recovering inflicted damage to some extent due to their viscoelastic nature. This process has been referred to as “self-healing”. Recently, research on crack-healing of bituminous materials has gained more importance and popularity due to the development of self-healing materials by materials scientists and engineers, that have been used in several applications. The Rilem Technical Committee TC 278-CHA (Crack-Healing of Asphalt Pavement Materials) was created in 2016 to tackle the challenges related to testing and evaluating the crack-healing properties of bituminous materials. However, the committee members noticed that there is no clear agreement within the scientific community about some basic terms to use when it comes to this new area of interest. Hence, this paper aims to clarify some of the concepts related to crack-healing and propose some common terms to the researchers and pavement engineers.@en

Self-healing asphalt, which is designed to achieve autonomic damage repair in asphalt pavement, offers a great life-extension prospect and therefore not only reduces pavement maintenance costs but also saves energy and reduces CO₂ emissions. The combined asphalt self-healing system, incorporating both encapsulated rejuvenator and induction heating, can heal cracks with melted binder and aged binder rejuvenation, and the synergistic effect of the two technologies shows significant advantages in healing efficiency over the single self-healing method. This study explores the fatigue life extension prospect of the combined healing system in porous asphalt. To this aim, porous asphalt (PA) test specimens with various healing systems were prepared, including: (i) the capsule healing system, (ii) the induction healing system, (iii) the combined healing system and (iv) a reference system (without extrinsic healing). The fatigue properties of the PA samples were characterized by an indirect tensile fatigue test and a four-point bending fatigue test. Additionally, a 24-h rest period was designed to activate the built-in self-healing system(s) in the PA. Finally, a damaging and healing programme was employed to evaluate the fatigue damage healing efficiency of these systems. The results indicate that all these self-healing systems can extend the fatigue life of porous asphalt, while in the combined healing system, the gradual healing effect of the released rejuvenator from the capsules may contribute to a better induction healing effect in the damaging and healing cycles.

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Rejuvenator encapsulation technique showed great potential for extrinsic asphalt pavement damage healing. Once the capsules are embedded within asphalt pavement, the healing is activated on-demand via progressing microcrack. When the microcrack encounters the capsule, the fracture energy at the tip opens the capsule and releases the rejuvenator. Then the released rejuvenator wets the crack surfaces, diffuses into and softens the aged bitumen, allowing two broken edges to come in the contact, preventing further asphalt pavement deterioration. The quality and speed of the damage repair process strongly depend on the quality of rejuvenator, thus it is important to choose a proper rejuvenator with good abilities to restore the lost properties of bitumen from ageing and show a sustainable performance after healing. To this aim, three different rejuvenators were studied and ranked based on the performance of their rejuvenated bitumen, including physical properties, rheological properties, chemical properties and the performance after re-ageing. Furthermore, these rejuvenators were encapsulated in calcium alginate capsules and the tests on these capsules indicate the diameter, mechanical resistance and thermal stability of the capsules are influenced by the encapsulated rejuvenator. The findings will benefit the development of rejuvenator encapsulation technique and the optimization of the capsule healing system towards a better healing effect in asphalt pavement.

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Self-healing asphalt, aimed to produce a sustainable asphalt pavement using green technology, has been studied in the past two decades. Technologies including encapsulated rejuvenator and induction heating have been proposed, demonstrated in the laboratory, and gradually evaluated in field application. This paper looks into the synergy effect of the above two technologies, where induction heating serves as the asphalt damage repair mechanism, requiring just 2 min heating time and encapsulated rejuvenator will replenish (rejuvenate) aged asphalt binder and reinstate bitumen's healing ability. Moreover, the increased temperature from induction heating could in turn accelerate the diffusion process of rejuvenator into aged bitumen. In this paper, the healing efficiency of the combined healing system was tested in comparison with autonomous asphalt healing system, induction healing system and capsule healing system. Porous asphalt concrete with various healing systems were prepared and a laboratory ageing procedure was followed to simulate the condition when healing was needed (after years of serving). X-ray computed tomography was employed to visualize the material composition and distribution inside of each healing systems. The properties of binder extracted from the porous asphalt samples were examined by dynamic shear rheometer. Indirect tensile strength and indirect tensile stiffness modulus tests were employed to characterize the mechanical properties of the porous asphalt samples with various healing systems. Finally, the cracking resistance of these healing systems was investigated by semi-circular bending test, and the healing efficiency was evaluated using a bending and healing programme. The results indicated that the combined healing system, with synergistic effects of aged binder rejuvenation and crack healing, shows a longer life extension prospect over the other healing systems.

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This paper presents a unique self-healing system for asphalt pavement which employs compartmented calcium-alginate fibres encapsulating an asphalt binder healing agent (rejuvenator). This system presents a novel method of incorporating rejuvenators into asphalt pavement mixtures. The compartmented fibres are used to distribute the rejuvenator throughout the pavement mixture, thereby overcoming some of the problems associated with alternate asphalt pavement healing methods, i.e., spherical capsules and hollow fibres. The healing system performance, when embedded in Porous Asphalt (PA) mix was tested by employing: (i) Indirect Tensile Stiffness and Strength test (ii) 4 Point Bending Fatigue test. The Semi Circular Bend (SCB) test was adopted to study crack propagation and its closure (healing) in an asphalt mix. The findings demonstrate that compartmented alginate fibres have capacity to survive asphalt mixing and compaction process. The fibres can efficiently repair damage (close the cracks), increase asphalt mix stiffness and strength. However, when the asphalt mix is subjected to fatigue loading the system does not significantly improve healing properties of the asphalt mix. Nevertheless, the findings indicate that, with further enhancement, compartmented calcium alginate fibres may present a promising new approach for the development of self-healing asphalt pavement systems.@en

Self-healing within asphalt pavements is the process whereby road cracks can be repaired automatically when thermal and mechanical conditions are met. To accelerate and improve this healing process, metal particles are added to asphalt mixtures. However, this approach is costly both in economic and environmental terms due to the use of virgin metallic particles. So, even though the self-healing of asphalt mixtures has been widely addressed in experimental terms over the years, there is a lack of research aimed at modelling this phenomenon, especially with the purpose of optimizing the use of metal particles through the valorization of industrial by-products. As such, the goal of this study was to develop a statistical methodology to model the healing capacity of asphalt concrete mixtures (AC-16) from the characteristics of the metal particles added and the time and intensity used for magnetic induction. Five metal particles were used as heating inductors, including four types of industrial by-products aimed at transforming waste products into material for use in the road sector. The proposed approach consisted of a combination of cluster algorithms, multiple regression analysis and response optimization, which were applied to model laboratory data obtained after testing asphalt concrete mixtures containing these inductors. The results proved the accuracy of the statistical methods used to reproduce the experimental behaviour of the asphalt mixtures, which enabled the authors to determine the optimal amount of industrial by-products and time needed to make the self-healing process as efficient as possible.

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This repot reviews the EARN project, which was undertaken under CEDR Call2012 in order to investigate the effects of using reclaimed asphalt (RA) and/or lower temperature asphalt on the road network. The work consisted of a review of existing data on service lifetime and availability of road materials and structures, a site trial to evaluate varying proportions of RA, experimental evaluation of moisture damage and ageing in asphalt mixtures and development of an impact assessment model. The site trial involved four different mixtures containing varying proportions of RA and warm mix additive and was monitored for international roughness index, mean profile depth, corrected SCRIM Coefficient, indirect stiffness modulus, water sensitivity and indirect tensile strength, the latter with and without artificial ageing. The monitoring was extended to 40 months with two extensions to the project, when monitoring of the binder mechanical and other properties were also made.

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A problem increasingly faced by airport authorities is the maintenance of runways. Due to their large aircraft loadings associated with take-off and landing operations, runways experience surface deterioration. Poor quality runway surfaces cannot be tolerated in such an environment. Maintenance issues must be carried out to maximise safety and minimise the risk of aircraft damage. A recent development has been the introduction of self-healing technologies such as rejuvenator encapsulation, induction and microwave heating to address these issues. This paper summarises a laboratory investigation to determine the effectiveness of microwave self-healing for crack repair of Porous Friction Course (PFC) used for airfields. Four mixtures containing varying percentages of conductive steel fibre were tested. Their relative performance was assessed using the Indirect Tensile Stiffness Modulus (ITSM) and Indirect Tensile Strength (ITS) test methods. The results show that the addition of conductive steel fibre increases initial stiffness and strength of the mix. A combination of micro-wave heating and steel fibre addition to the mix indicates that it is possible to significantly improve asphalt performance by making it self-healing to structural problems such as cracking.@en

It has been demonstrated that calcium alginate capsules can be used as an asphalt healing system by pre-placing rejuvenator (healing agent) into the asphalt mix and releasing the rejuvenator on demand (upon cracking). This healing mechanism relies on the properties of capsules which are determined by the capsule preparation process. In this study, to optimize the calcium alginate capsules, capsules are prepared using varying Alginate/Rejuvenator (A/R) ratios. Light microscope microscopy and Environmental Scanning Electron Microscope (ESEM) are employed to characterize the morphology and microstructure of these capsules. Thermal stability and mechanical property are investigated by thermogravimetric analysis (TGA) and compressive tests. The testing results indicate that higher alginate content results in smaller diameter and lower thermal resistance, but higher compressive strength. The optimum A/R ratio of calcium alginate capsules is found to be 30/70. To prove the effectiveness of the optimized capsules, the capsules are embedded in asphalt mortar beams and a bending and healing program is carried out. The effect of capsule shell material on the mechanical response of asphalt mixture is evaluated through three-point bending on the mortar beams embedded with blank capsules (without the healing agent). Aged mortar beams containing alginate capsules encapsulating rejuvenator demonstrate a higher strength recovery after bending tests, which indicates effective healing due to the release of the rejuvenators from the capsules.

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This paper presents a unique self-healing system for asphalt pavement which employs compartmented calcium-alginate fibres encapsulating an asphalt binder healing agent (rejuvenator). This system presents a novel method of incorporating rejuvenators into asphalt pavement mixtures. The compartmented fibres are used to distribute the rejuvenator throughout the pavement mixture, thereby overcoming some of the problems associated with alternate asphalt pavement healing methods, i.e., spherical capsules and hollow fibres. The healing system performance, when embedded in Porous Asphalt (PA) mix was tested by employing: (i) Indirect Tensile Stiffness and Strength test (ii) 4 Point Bending Fatigue test. The Semi Circular Bend (SCB) test was adopted to study crack propagation and its closure (healing) in an asphalt mix. The findings demonstrate that compartmented alginate fibres have capacity to survive asphalt mixing and compaction process. The fibres can efficiently repair damage (close the cracks), increase asphalt mix stiffness and strength. However, when the asphalt mix is subjected to fatigue loading the system does not significantly improve healing properties of the asphalt mix. Nevertheless, the findings indicate that, with further enhancement, compartmented calcium alginate fibres may present a promising new approach for the development of self-healing asphalt pavement systems. @en

In recent decades, researchers have revealed the great healing potential of asphalt and proposed various novel methods to inspire and improve the self-healing capacity of asphalt aimed to prolong the service life of asphalt pavement. In this review, up to date research progresses in induction healing and embedded rejuvenator encapsulation are presented, respectively. Meanwhile, the trial section applications of induction healing and capsule healing are highlighted, which show promising results. Finally, some recommendations for the future development of self-healing asphalt are proposed.@en