**3. Results**

### *3.1. Binder Drainage Test*

Table 2 summarizes the binder drainage test results of six 10 mm SMA mixes containing varying amounts of HealRoad capsules. The results show that the HealRoad capsules slightly increase binder drainage, but that they remain within acceptable limits. The mix containing 0.44% of capsules shows the same drainage level as the control mix (0% of capsules), whereas the mixes containing 0.64% and 1.45% of HealRoad capsules demonstrate an increased binder drainage level. This is probably due to the release of the rejuvenating oils caused by exposure to high temperatures of 180 ◦C.


**Table 2.** Binder Drainage.

### *3.2. Indirect Tensile Strenght Test*

Figure 3 shows the effect of the HealRoad capsule content in the 10 mm SMA mix. The results indicate that the mixes containing 0.44% and 0.64% of HealRoad capsules are the only mixes performing as well as the control mix. The results also indicate that increasing the capsule content above 1% results in a decreased ITS. These results indicate that in terms of 10 mm SMA mix strength performance, the optimal HealRoad capsule content is between 0.44% and 0.64%.

### *3.3. Water Sensitivity Test*

Following the ITS test, it was decided to focus on mixtures containing 0.44% and 0.64% of HealRoad capsules to further investigate the effect on the 10 mm SMA mix. Figure 4 shows the indirect tensile strength ratio of the mixes. The results show that all mixes outperform the standard ITSR requirement of 80%. Furthermore, the results demonstrate that the HealRoad capsules have a positive effect on the water sensitivity of the 10 mm SMA mix. An increase of HealRoad capsule content in the mix, up to 0.64%, results in an increased ITSR. The ITSR value for both mixes containing HealRoad capsules is higher than 90%.

**Figure 3.** Indirect tensile strength test results.

**Figure 4.** Water sensitivity test results—ITSR.

### *3.4. Wheel Tracking Test*

The wheel tracking tests results are presented in Figure 5. The results show a very slight increase in rut depth with 0.4% of HealRoad capsules in the mix. The results also show that by increasing the HealRoad capsule content to 0.64%, the rut depth within the mix decreases by 15%, to below the control mix. These results indicate that the HealRoad capsules can improve the mix resistance to rut deformation.

**Figure 5.** Wheel tracking results—resistance to rut deformation.

### *3.5. Extrinsic Healing*

3.5.1. SMA Mix with Low Capsule Content

Following the ITS tests (see Section 3.2), cylindrical test specimens were subjcted to induction heating. The aim was to determine whether the HealRoad capsules within an asphalt mix can conduct induction energy and initiate asphalt healing. Unfortunately, none of the asphalt mixtures demonstrated healing capacity. This is because of the low volume and high dispersion rate of the HealRoad capsules within the mix, which resulted in poor conductivity of the induction energy. Figure 6 shows a cross section of test specimen containing the highest volume of HealRoad capsules—1.45%. The image shows that the HealRoad capsules are well embedded in the test specimen, however, such capsule dispersion causes the HealRoad capsules to have poor induction energy conductivity.

**Figure 6.** Image of 10 mm SMA—mix 6 containing 1.45% capsules in the mix.

### 3.5.2. High Capsule Content SMA Mix

In order to test the HealRoad capsules for induction heating in an asphalt mix, the amount of HealRoad capsules was increased to 5%, 10% and 20% (when capsule content is higher than 20%, the HealRoad capsules do not disperse evenly in the mix). The percentages of constituent material in each mix are summarized in Table 3. These mixtures were not investigated for standard mix performance, the aim was only to test whether the increased volume of HealRoad capsules had the capacity to initiate induction heating and self-heal. Figure 7 shows the images of 10 mm SMA mix containing varying percentages of HealRoad capsules, 0–20%. From the Figure 7c,d, it is evident that the mixes containing 10% and 20% of HealRoad capsules show evidence of corrosion. Another finding is that the mixes containing 20% of HealRoad capsules had fewer large aggregates in comparison to mixtures

without HealRoad capsules. This could be an issue for the material strength as large angular aggregates, which determine the load carrying performance of a mix, are replaced by softer, round HealRoad capsules.


**Table 3.** The 10 mm SMA mix designs.

**Figure 7.** The 10 mm SMA mix containing varying amounts of capsules: (**a**) 0% capsules, (**b**) 5% capsules, (**c**) 10% capsules and (**d**) 20% capsules.

The specimens were subjected to the ITS test protocol described in Section 2.2.4 and test specimen healing was performed following the protocol described in Section 2.2.5. After the ITS test, the test specimens were tied with cable ties to join two test specimen parts and to allow test specimens without HealRoad capsules to heal. Figure 8 shows the tied test specimens.

**Figure 8.** The 10 mm SMA test specimen containing 5% capsules ready for induction healing.

The healing results show a positive response to induction heating with a test specimen temperature increase. As expected, test specimens containing higher amounts of HealRoad capsules were able to reach high temperatures, while test specimens containing 20% of HealRoad capsules reached a temperature of 80 ◦C. Figure 9 shows the temperature readings (measured using an infrared laser temperature gauge).

**Figure 9.** Temperature reading of the induction healing.

Figure 10 shows the test samples after the ITS test and before healing. The damage in the mixture containing 20% of HealRoad capsules was significant. We believe that this was caused by the high proportion of capsules in the mix, which replaced larger aggregates. Large aggregates (angular, flaky and elongated aggregates) in an asphalt mix form a load carrying skeleton which is responsible for pavement strength and for resistance to rutting [26]. Figure 11 shows the images of the same samples after healing. It is evident that mixtures with a high HealRoad capsule content—Mix 3 with 10% of capsules and Mix 4 with 20% of capsules—more efficiently repaired the damage, i.e., closed the cracks, in comparison to the control mix—Mix 1 containing 0% of capsules and Mix 2 containing 5% of capsules.

**Figure 10.** Images of test specimens after ITS test: (**a**) control mix—Mix 1, (**b**) Mix 2–5% capsules, (**c**) Mix 3–10% capsules and (**d**) Mix 4–20% capsules.

**Figure 11.** Images of test specimens after induction healing: (**a**) control mix—Mix 1, (**b**) Mix 2–5% capsules, (**c**) Mix 3–10% capsules and (**d**) Mix 4–20% capsules.

Figure 12 shows the initial ITS result for mixtures with a HealRoad capsule content of ≥5%. When compared with data from the initial tests, for mixes containing <1.45% of HealRoad capsules, the material strength was reduced as capsule content increased. In mixes containing 20% of HealRoad capsules, the ITS was reduced by 55%. There are two possible reasons for this:


**Figure 12.** Initial ITS results.

Figure 13 illustrates the ITS results after healing. The results demonstrate that mixes containing HealRoad capsules did not achieve the expected healing. Figure 14 shows the healing index of each mix indicating that HealRoad capsules did not improve material strength recovery, whereas the control mix achieved up to 32.5% of strength recovery and the mix containing 20% of HealRoad capsules achieved up to 35.07% of strength recovery. However, looking at the initial material strength (Figure 12) and recovered material strength (Figure 13), it is evident that the control mix has the highest strength level both before and after healing. This indicates that there are no benefits to be accrued from including the HealRoad capsules in the asphalt mix.

**Figure 13.** ITS results after healing.

**Figure 14.** Healing index of the 10 mm SMA mix with and without capsules.

### **4. Conclusions**

The asphalt mix performance test results indicate that mixes containing low amounts of HealRoad capsules (0.44% and 0.64%) have as good and better mix performance as the control mix in terms of strength, water sensitivity and rutting. Unfortunately, mixes containing a low amount of HealRoad capsules, 0–1.45%, do not conduct inductive energy and therefore do not initiate healing. This is due to the poor conductivity properties of iron powder and the high dispersion of the HealRoad capsules in the mix. However, increasing the HealRoad capsule content of the mix to >5% demonstrates increased temperatures when test specimens are subjected to the induction heating. The results show that mixtures containing 20% of HealRoad capsules can reach a temperature >80 ◦C and can recover up to 35% of its initial ITS. However, mixes without HealRoad capsules recovered 32.5% of their initial strength, without increased temperature. This indicates that although HealRoad capsules are capable of heating the mix and efficiently initiate self-healing, they do not assist the mix in recovering its material strength. Furthermore, increasing the HealRoad capsule content in the mix above 5% gradually decreases the mix strength, whereas the mix containing 20% of HealRoad capsules has a 55% lower initial ITS in comparison to the control mix, i.e., mix without capsules. These results show that HealRoad is not an efficient asphalt damage repair system. The results of our initial investigation [3], where HealRoad capsules were tested in pure bitumen and bitumen mortar mixes, indicated that HealRoad capsules may be suitable for use in asphalt mixtures with high bitumen and low aggregate content. Further studies will therefore focus on testing HealRoad capsules as a healing system in asphalt plug joint mixes. To enhance the conductivity of HealRoad capsules, the pure iron powder (Fe) will be replaced with magnetite (Fe3O4) as a conductive material. Wan et al. [4,21] have demonstrated that alginate capsules containing magnetite as a conductive material have good conductive properties. The assumption is that a more conductive material can improve energy conduction and perhaps also the healing efficiency of the system. Despite weak healing performance, HealRoad capsules may still have the potential to improve asphalt mix performance, paving the way for further improvement and development of the self-healing systems for asphalt pavement mixes.

### **5. Patents**

An intellectual property application titled: "A Conductive Alginate Capsule Encapsulating a Healing Agent", was submitted by the TU Dublin and Amir Tabakovi´c to the European Patent Office, international patent application number: PCT/EP2021/075254. The objective of the IP application is to ring-fence specific capsule formulation for potential production and licensees' particular applications.

**Author Contributions:** Conceptualization, A.T.; methodology, A.T. and D.O.; validation, A.T., C.F. and D.O.; formal analysis, A.T.; investigation, A.T.; resources, A.T.; data curation, A.T. and C.F.; writing—original draft preparation, A.T.; writing—review and editing, A.T., C.F. and D.O.; visualization, A.T.; project administration, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Enterprise Ireland, gran<sup>t</sup> number CF 20191063P.

**Acknowledgments:** The Authors would like to acknowledge Technological University Dublin (TU Dublin) for hosting the project between 2019 and 2021 and TU Dublin CREST research Centre for providing the space and equipment for production of the capsules.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
