**1. Introduction**

For the past two decades, researchers have been intensively developing self-healing asphalt technology as a cheaper, safer and more sustainable method of maintaining and repairing asphalt roads [1–4]. The idea of the self-healing road stemmed from the concept of the "forever open road", motivated by the need to avoid the traffic disruption and

**Citation:** Tabakovi´c, A.; Faloon, C.; O'Prey, D. The Effect of Conductive Alginate Capsules Encapsulating Rejuvenator (HealRoad Capsules) on the Healing Properties of 10 mm Stone Mastic Asphalt Mix. *Appl. Sci.* **2022**, *12*, 3648. https://doi.org/ 10.3390/app12073648

Academic Editor: Luís Picado Santos

Received: 7 March 2022 Accepted: 30 March 2022 Published: 5 April 2022

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associated risk of accidents caused by road maintenance activities on busy roads. The challenges for the road industry are to improve road material performance (make roads more durable), improve the sustainability of road construction methods (by introducing green technology) and improve the safety of the roads for both road users and road construction workers [5–7]. Self-healing asphalt technology has the potential to address several of the technical, economic, environmental and safety challenges currently facing the road industry. For example, self-healing asphalt technology has the potential to reduce the need for maintenance-related road closures, thereby reducing the risk to road maintenance workers, while simultaneously reducing the environmental impact of road construction and maintenance. It is estimated that self-healing asphalt technology could reduce energy consumption and CO2 emissions by 30% over the lifetime of a road [8].

To date, researchers have tested three extrinsic self-healing methods for asphalt pavements, which are [1]: induction heating [1,9–12], microwave heating [4,13,14], rejuvenation (rejuvenator encapsulation) [2,15–18].

The rejuvenator encapsulation approach represents a more favourable method of asphalt self-healing as it is a passive healing system and does not require external action in order to initiate healing. The rejuvenation enables the aged bitumen to return to its original chemical, physical and mechanical properties. However, there are limitations with this method of self-healing because the healing rate is slow and healing efficiency low [2,18]. Researchers have demonstrated that the induction and microwave heating methods are efficient methods for healing asphalt damage (3 to 5 min) in comparison to rejuvenation [2,19]. However, a challenge with both methods is that the heating process (both induction and microwave) ages the bitumen, causing binder brittleness and the premature failure of the asphalt [9,20].

Hybrid self-healing systems provide a solution to the challenges associated with induction, microwave and encapsulated rejuvenation. The hybrid self-healing technology combines induction heating and encapsulated rejuvenation [2]. In this process, induction heating is used to repair the asphalt damage (and the cracks), while rejuvenation is used to replenish the aged asphalt binder (bitumen). Xu et al. [2] demonstrated that a hybrid healing system improves healing efficiency allowing for rapid damage repair and a simultaneous rejuvenation of the aged asphalt. However, a challenge with this approach is that the bituminous mix must be adjusted to accommodate both capsules and steel fibres. A solution to this challenge lies in combining the induction and rejuvenation into one product. Wan et al. [4,21] developed calcium alginate/nano-Fe3O4 composite capsules for controlled rejuvenator oil release using microwave heating. Wan et al. showed that the capsules demonstrated superior levels of healing with microwave heating due to the combined effect of thermal induction and rejuvenator healing. Wen et al. showed that hybrid Fe3O4– calcium alginate capsules encapsulating a rejuvenator have a higher self-healing capacity (8.6%) when compared to the mix containing pure calcium alginate capsules encapsulating a rejuvenator and 19.3% in comparison to a standard asphalt mix. The asphalt mix containing 2% of the capsules achieved up to 87.2% healing after 40 s of healing time.

The author (Tabakovi´c et al. [3]) has developed a novel extrinsic self-healing asphalt technology: conductive alginate capsules encapsulating a bitumen rejuvenator (HealRoad capsules). The HealRoad technology combines two existing self-healing asphalt methods: (i) rejuvenator encapsulation and (ii) induction heating. Initial results demonstrated that the HealRoad capsules had sufficient thermal and mechanical strength to survive the asphalt mixing process and also that they sufficiently healed the asphalt bitumen damage [3]. Initial results also demonstrated that the capsules can improve the mechanical properties of the asphalt mix in terms of improving tensile strength and stiffness. With a capsule content of 20% in pure bitumen, the mix strength of the material increased to 118%, whereas for mortar bituminous mixtures (bitumen + fine aggregates − sand), the mix strength of the material increased by up to 67%.

This study evaluated the effect of HealRoad capsules in a 10 mm stone mastic asphalt (SMA) mix. The aim of the work was to investigate whether the HealRoad capsules can efficiently repair the damage in an asphalt mix (close cracks and restore asphalt mix strength). A second aim of the research was to ascertain how the inclusion of HealRoad capsules would affect the mechanical performance of the asphalt mix. Indirect tensile strength (ITS), water sensitivity and resistance to rutting tests were conducted to evaluate the effect of HealRoad capsules in the 10 mm stone mastic asphalt (SMA) mix.

### **2. Materials and Methods**

*2.1. Materials*

### 2.1.1. The 10 mm Stone Matic Asphalt Mix

The 10 mm Stone Mastic Asphalt (SMA) mix was prepared in accordance with the Transport Infrastructure Ireland (TII) Specification for Road Works Series 900 guidance. The aggregate was an Irish Metasandstone PSV60. The filler was an imported limestone filler to EN. The bitumen was a polymer-modified 65/105-60 bitumen. Figure 1 shows the mix grading curve.

**Figure 1.** The 10 mm SMA grading curve.

Table 1 summarises the SMA mix constituents and shows their proportions in the mix, both with and without capsules. The HealRoad capsules were added in amount of 0%, 0.31%, 0.44%, 0.64%, 1.02% and 1.45%. These proportions reflect the following proportions of capsules in the capsule and bitumen portion of the mix: 0%, 5%, 7%, 10%, 15% and 20%. The HealRoad capsules are assumed to comprise the bitumen portion of the mix as their purpose is to initiate healing and rejuvenate the bitumen.

### 2.1.2. Capsule Design

The HealRoad capsules were prepared using a drop process [16,22] from an emulsion of rejuvenator suspended in a water solution of sodium alginate. To this aim a 6 wt % solution of sodium alginate in deionized water was prepared. At the same time, a 2.5 wt % poly(ethylene-alt-maleic-anhydride) (PEMA) polymeric surfactant solution was prepared by dissolving the copolymer in water at 70 ◦C and mixing it for 60 min. After the PEMA was dissolved in water, it was allowed to cool to room temperature (20 ± 2 ◦C) and was then combined with the rejuvenator. For this study, a vegetable oil of 0.9 g/cm<sup>3</sup> density at room temperature (20 ± 3 ◦C) was used, forming a bitumen healing agen<sup>t</sup> solution, in a PEMA/rejuvenator with a 1/1.5 proportion by weight. The sodium alginate solution was mixed with iron powder (40 μm particle size), at 700 rpm for 1 h to allow the uniform dispersal of iron particles within the alginate mix. Following the initial investigation [3],

an Alginate (Alg):iron powder (Fe) mix ratio was prepared in proportion by weight of dry constituents ratios: 20:80. After the sodium alginate and iron solution was fully mixed, the PEMA and rejuvenator (oil) solution was added to the alginate—iron powder solution mix in a ratio of 70% rejuvenator:30% sodium alginate [23]. The full capsule solution (20 L) was then mixed at 700 rpm for 20 min. More detail on capsule production can be found elsewhere [3]. All chemicals used in the production of the HealRoad capsules were purchased from the Merck Group—Sigma Aldrich, Wicklow, Ireland, except for the rejuvenator (vegetable oil) which was purchased from a local supermarket.


**Table 1.** Mix constituents proportions.

### 2.1.3. Test Specimen Compaction

Cylindrical test specimens were compacted following EN 12697-31:2019. A Coopers Gyratory CRT-GYR-EN was used to compact the test specimens. Test specimens were compacted in dimensions of 100 mm diameter and 70 mm height. After 12 h of curing, specimens were cut, using a masonry saw to a diameter of 100 mm and a height of 32 mm.
