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| Healing Type | References | Materials Used | Diameter (μm) | Length (mm) | Temperature Heating (°C) | Duration of Heating (s) | Relevant Conclusion |
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| Induction Heating | Liu et al. [23] | Steel fibre type 1 | 29.6–191.1 | <1 | — | 180 | (i) The porous asphalt concrete with the best performance, i.e., higher resistance to particle loss and water damage, was 8% by volume of bitumen of steel wool (type 00) of the length of 9.5 mm
(ii) Optimum content for the steel fibre type 1, 00, and 000 are 20%, 12%, and 10%, respectively, while the maximum temperatures reached by each types of steel fibre are 207°C, 169°C, and 137°C, respectively. |
| Steel wool type 00 | 8.89–12.7 | 3.2/6.4/9.5 |
| Steel wool type 000 | 6.38–8.89 | 6 |
| Induction Heating | García et al. [24] | Steel wool type 000 | 6.35–8.89 | 3–22, 30 | — | 60, 120, 180 | (i) At an optimal volume of fibre addition, the graphite can increase and stabilize the conductivity of the mastic.
(ii) The maximum temperature for the 6% volume of fibre after 60 s, 120 s, 180 s are approximately 100°C, 150°C, and 200°C, respectively.
(iii) Increasing the volume of steel fibre by more than 6% does not increase the maximum temperature further. The resistivity graph proves that the increase of steel fibre content by more than 6% will improve conductivity by approximately 10%. |
| Graphite filler | <20 | — |
| Induction Heating | García et al. [51] | Steel wool type 00 | 8.89–12.7 | 1–15 | — | 180 | (i) Based on the Marshall tests, the mechanical resistance of the asphalt concrete remains unaffected with any change in the volume of fibers and the mixing time. The response remained constant even though the parameters were changed.
(ii) At the initial period of mixing, the mixing time had a negative effect on the length of the fibres.
(iii) The temperature increased linearly for the increase in the volume of fibres.
(iv) The maximum temperature reached was lower when the fibre was added in the first place to the mixer than the fibre added in the last place after aggregate and bitumen. This is due to the length reduction of the fibre when it is mixed with the aggregate. |
| Induction Heating | Liu et al. [52] | Steel wool type 00 | 8.89–12.7 | 10 | 30, 50, 70, 85, 100 | — | (i) The mortar of porous asphalt concrete exhibits a higher nanoindentation modulus when strengthened with steel wool.
(ii) Heating of the beam fracture at 20°C does not result in any healing. The fracture recovery increased from 14.9 to 78.8% when the heating time is from 30 to 85°C. |
| Induction Heating | Liu et al. [53] | Steel wool type 00 | 8.89–12.7 | 10 | 70, 85, 100 | 180 | (i) The best outcome of healing effects is obtained for an optimal heating temperature of 85°C. Overheating may result in problems like swelling and binder drainage, inhibiting healing.
(ii) Performing induction heating multiple times can extend the fatigue life of porous asphalt concrete.
(iii) The fatigue life extension ratio (healing rate) of the fatigue damaged beams can be improved by induction heating, wherein the rate of healing is dependent on the higher level of microstrain developed. |
| Induction Heating | García et al. [54] | Steel wool type 3 | 154.98 | Length for each steel type.
Short fibres: 2.5, Long fibres: 7 | — | 60 | (i) Steel wool fibers might negatively impact the abrasion loss performance of the asphalt concrete mixture when they are not homogeneously distributed, characterized by the appearance of fibre clusters.
(ii) The mechanical resistance of the test samples could be recovered up to 60% when the material was damaged. This happened at around 100°C. |
| Steel wool type 1 | 83.89 |
| Steel wool type 00 | 36.42 |
| Steel wool type 0000 | 28.55 |
| Induction Heating | Liu et al. [11] | Steel fibres | 70–130 | 4.2 | 80 | — | (i) The induction heating rate of the asphalt mixture can be increased with the suitable composition of steel fibres and steel slag. The order of heating rate for different asphalt mixtures is SF + SS > SF > SS. The selection of the right composition can facilitate homogenous heating and the efficiency of induction healing.
(ii) Incorporating steel slag or steel fibres may also improve the water stability, Marshall stability, and residual Marshall stability ratio. |
| Steel slag | Replace partially of coarse aggregate | — |
| Microwave Heating | Tabaković et al. [55] | Grade 3 coarse-grained Trollull Steel Wool | 90 | 10 | — | 180 (at 300 W) | (i) Busy airports that face problems of runway closure can benefit from the microwave healing process using steel fibres. The addition of steel fibres with 3 min of microwave healing initiates a rapid healing process.
(ii) Optimum steel fibre content for the Porous Friction Course (PFC) mix is 5% with 300 W heating power. |
| Microwave Heating | Wang et al. [56] | IM8 Carbon fibres | 5.2 | 6.35 | — | 120 (at 1100 W) | (i) The electrical resistivity of the mastic can be decreased with an optimal addition of carbon fibres. It can even absorb more microwave energy for healing. Thus, less energy is required to heal cracks on using carbon fibres.
(ii) 3% of AS4 and 2% of IM8 carbon fibres by bitumen weight have the best healing performance.
(iii) The microwave energy was absorbed by the carbon fibre and heal the crack when the surface temperature went up to around 100°C. |
| AS4 Carbon fibres | 7.1 | 6.35 |
| Microwave Heating | Hassan et al. [57] | Kitchen wash steel wool | 97.54 | 3–7 | — | 60, 90 (at 700 W) | (i) Steel wool content of 3 and 5% by volume of bitumen have an excellent bonding between the fractured surfaces 24 hours after heating.
(ii) Modified asphalt with steel wool causes the bulk density to reduce while the air void percentage increases. Thus, to overcome this problem, higher compaction energy is required. |
| Microwave Heating | González et al. [58] | Steel shavings | 1.310 | 3–21 | — | 40 (at 700 W) | (i) After microwave radiation, the average surface temperature of the asphalt reached 71.7°C.
(ii) The number of air voids in the mixture increased with the addition of shavings which was observed in SEM analysis. The air voids are due to the complex geometry and rough surface of the shavings. |
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