A Comprehensive Review of Hot In-Place Recycling Technology: Classification, Factors Affecting Performance of Asphalt Mixtures, and Benefits Analysis

Abstract

The application of hot in-place recycling asphalt mixtures (HIRAMs) is gaining increasing attention in highway maintenance due to its environmental and economic benefits. This paper comprehensively reviews and discusses the state-of-the-art studies in the field of hot in-place recycling (HIR). Firstly, different HIR technologies are introduced, including surface recycling, remixing, and repaving. Then, this paper provides a detailed description of the key factors influencing the road performance of HIRAMs in terms of both materials and production, such as reclaimed asphalt pavement (RAP), rejuvenators, virgin asphalt, virgin asphalt mixtures, preheating temperature, and mixing time. Furthermore, the environmental and economic benefits of HIR are compared with other preventative maintenance and recycling technologies. Finally, some challenges for the investigation of HIR are further discussed, and the corresponding suggestions are recommended for future investigation.

1. Introduction

Asphalt mixtures are the commonly used materials for constructing pavement. However, asphalt pavements experience progressive aging, largely due to environmental and traffic factors, leading to the generation of significant amounts of reclaimed asphalt pavement (RAP). This not only wastes resources but also contributes to various forms of pollution [1,2,3,4,5,6]. To mitigate these issues, the recycling of waste asphalt pavement materials has become progressively more widespread [7,8,9,10,11,12,13,14]. Asphalt pavement recycling technology can be classified into four types based on mixing temperature and recycling site, which are hot central-plant recycling (HCPR), hot in-place recycling (HIR), cold central-plant recycling, and cold in-place recycling (CIR) [15,16,17]. Among these, HIR has gained significant attention due to its notable environmental and economic benefits [18,19,20]. The findings obtained by Ma, et al. [21] showed that the asphalt pavement maintained by HIR achieved the desired performance and economic benefits. Huang [22] conducted a comparative analysis of the economic and environmental benefits of HIR and traditional hot mix asphalt (HMA) technology. The results showed that the total cost of hot in-place recycling asphalt mixtures (HIRAMs) was 34.3% lower than that of HMA. Additionally, HIRAMs saved 24.8% in energy consumption and 35.1% in CO₂ emissions per ton of mixtures produced compared with HMA.

For a quantitative analysis of the recent research status of HIR, 168 papers were found on the Web of Science by using ‘Hot In-Place Recycling’ as the keyword. These 168 papers were statistically analyzed using the ‘HistCite’ software (version 2.1). Notably, the number of papers has increased yearly since 2014, as shown in Figure 1a. Furthermore, nearly 80% of them have been published since 2017, which means that the research on HIR has become a hotspot. Further analysis of these papers reveals that papers published before 2019 have been frequently cited, as shown in Figure 1b, which may be attributed to a huge number of publications during this period. Meanwhile, due to the short available time, fewer citations are found among papers published after 2021. Moreover, it can be seen from Figure 1c that papers in 2013 and 2017 show a high number of citations, indicating that the previous study provides strong support for subsequent research. In addition, Figure 1d demonstrates that China and the United States are the leading countries in terms of the number of published papers and citation frequency.

Currently, HIR is commonly used to treat shallow surface damage on various grades of highways and urban roads. Existing research on HIR mainly focuses on evaluating the road performance of HIRAMs and exploring their specific influencing factors [23,24,25,26,27,28]. Additionally, some scholars also investigate the environmental and economic benefits of HIR. Numerous studies reveal that HIR effectively ensures road performance and concurrently reduces maintenance cost and greenhouse gas emissions generated during the construction process [29,30,31,32,33]. Nonetheless, few articles provide a comprehensive review of the current development of HIR. Therefore, this review summarizes the state-of-the-art development of HIR. Initially, this review introduces the different technologies of HIR. Then, the influencing factors on the performance of HIRAMs are described in detail in terms of materials and production, encompassing RAP materials, rejuvenators, virgin asphalt, VAM, preheating temperature, and mixing time. Subsequently, the environmental and economic benefits of HIR are summarized by comparing with other preventive maintenance and recycling technologies. Finally, this review presents an overview of current studies and challenges related to HIR, along with recommendations for future research directions.

2. HIR Technology Classification

HIR can be divided into surface recycling, remixing, and repaving. The equipment used includes preheating units, hot scarifying units, milling units, mixing units, stacking units, spreading units, and rolling units, which can be arranged for hundreds of feet, thus referred to as the HIR ‘recycling train’. Correspondingly, a complete HIR production process consists of preheating, scarifying, mixing, and rolling, as shown in Figure 2 [34,35]. The preheating process is conducted using 2-3 preheating machines, which can effectively soften the surface layer of asphalt pavement, eliminate the bonding force, and minimize aggregate breakage during the asphalt pavement recycling process [36]. The milling process is performed using a milling machine, which can extract waste pavement materials, remove the damaged sections of the pavement, and ensure a good combination of old and new surfaces. Subsequently, rejuvenators, virgin asphalt, or virgin asphalt mixtures (VAMs) are properly mixed with RAP to produce the recycled asphalt mixtures in the mixer. It is worth noting that the HIR process is classified into surface recycling and remixing based on the different additives introduced during the mixing stage. A paver is employed to complete the paving process of the recycled asphalt mixtures, making the asphalt surface uniform and smooth. Additionally, the process of repaving involves incorporating VAMs on top of the finished recycled asphalt mixtures. The subsequent rolling process, completed by 2-3 traditional road rollers, ensures that the asphalt pavement achieves the required compactness [37].

2.1. Surface Recycling

Surface recycling, a relatively simple process as mentioned above, involves preheating, milling, mixing, paving, and rolling. The milling process can be carried out with spring-turned rake teeth, small-diameter rotary cutter heads, drills, or plows. It should be noted that the mixing process adds rejuvenators as required. Considering that only rejuvenators are added, and the gradation of the RAP material remains unchanged, the performance of the recycled asphalt mixtures is not ideal. Therefore, surface recycling is generally used for road surfaces with less serious road damage. Specifically, the surface recycling technology is used under the following conditions:

(1)

The gradation of RAP material exhibits high uniformity, ensuring controllable post-reconstruction quality of the recycled mixture.

(2)

Pre-existing pavement distress (e.g., ruts and potholes) is shallow, requiring VAM incorporation below 30% of the total blend volume.

(3)

The surface recycling process alone can achieve compliance with design specifications without supplemental treatments.

(4)

The original asphalt binder exhibits moderate aging, enabling effective restoration of its performance through rejuvenation; the recycled asphalt mixture meets wear layer specifications.

2.2. Remixing

Remixing resembles surface recycling, yet during the mixing process, alongside rejuvenators, a specific proportion of virgin asphalt and VAM should be introduced as necessary. Therefore, the recycled mixtures from remixing exhibit superior quality and are better suited for severely damaged road surfaces compared with surface recycling. In particular, remixing can enhance the material performance of aged asphalt pavement, repair the aged wear layer, and improve the pavement strength. Specifically, the remixing technology is used under the following conditions:

(1)

Significant material variability in the original asphalt surface mixture and extensive patched areas preclude achieving wear layer specifications via remixing.

(2)

Severe inherent gradation flaws in the original asphalt mixture prevent remixing from meeting wear layer requirements, though the recycled material remains suitable for middle surface applications.

(3)

The original pavement asphalt is more severely aged, making it difficult to use directly as a wear layer through rejuvenation.

2.3. Repaving

Repaving is a process of adding VAMs as an abrasion layer and then rolling the mixtures on the basis of surface recycling or remixing. During this process, the recycled asphalt mixtures form the screed layer, while the added mixtures function as the upper layer. The thermal bonding between these layers results in their integration, thus eliminating the need for a bonding layer in the repaving. It is important to note that in the process of repaving construction, a mixer equipped with double screed plates is needed. Specifically, the recycled asphalt mixtures are applied onto the first screed plate, and the VAMs are simultaneously applied onto the recycled mixtures using the second screed plate. Subsequently, the two layers are rolled together. Compared with surface recycling and remixing, repaving is extensively applied for the repair and renovation of heavily damaged roads as well as the reconstruction of old roads because the repaired pavement has better performance. Specifically, repaving technology is used under the following conditions:

(1)

Significant material variability in the original asphalt surface mixture and extensive patched areas preclude achieving wear layer specifications via remixing.

(2)

The original asphalt surface layer is inadequate for upgraded traffic demands, necessitating an overlay with virgin asphalt mixtures for structural enhancement.

(3)

Severe inherent gradation flaws in the original asphalt mixture prevent repaving from meeting wear layer requirements, though the recycled material remains suitable for middle surface applications.

3. Factors Affecting the Road Performance of HIRAMs

HIRAMs are commonly utilized as a surface layer of pavements and are therefore directly exposed to traffic loads and environmental factors. Specifically, they are subjected to constant vertical and horizontal forces from wheels, which can be exacerbated by the erosive effects of precipitation and fluctuating temperatures. Given these challenges, the identification of the influencing factors on the pavement performance of HIRAMs is vital to guide the design and production processes. This section delves into a comprehensive examination of the impact that material choices and production processes have on the performance of recycled asphalt mixtures, as depicted in Figure 3. It is worth noting that in HCPR asphalt mixtures, RAP content is generally controlled at 30% or less [38], while that in HIRAMs is usually higher than 70% [39,40]. Considering the similarity of factors affecting the performance of hot recycled mixtures, the RAP contents in this section are all higher than 70% to reflect the differences between HCPR and HIR.

3.1. Raw Materials

HIRAMs mainly consist of RAP materials, rejuvenators, virgin asphalt, and VAMs [41,42,43,44].

3.1.1. RAP Materials

The incorporation of a high percentage of RAP in the production of HIRAMs significantly influences the performance characteristics of these mixtures [41,45,46,47,48,49]. High-temperature stability assessment by Yang, et al. [47] on rejuvenated asphalt with 70% and 80% RAP binder content, using the dynamic shear rheometer (DSR) test, indicated that the rutting factor increased with higher RAP content. A similar finding was reported by Dong, et al. [41] This trend was attributed to the fact that the addition of RAP binder was found to increase the hard component in the asphalt, thus improving its deformation resistance at high temperatures. In contrast, Zhu, et al. [48] reported a contrasting outcome when the RAP content escalated to 70%. They thought that the poor blending between aged and virgin asphalt caused by the increase in RAP content weakened the internal bond between asphalt and aggregates. Additionally, the increase in RAP content led to a larger variation in RAP gradation, resulting in uncontrollable mixture performance. With respect to the effect on the water stability, freeze–thaw splitting and immersion Marshall tests were performed by Yang, et al. [47] for mixtures containing a RAP content of 70% and 80%. They reached the conclusion that the residual Marshall stability and tensile strength ratio (TSR) of the mixtures decreased to varying extents with the increase in RAP content, as shown in Figure 4a. They believed that the significant deterioration of the adhesive performance between the aged and virgin asphalt resulted in potential moisture damage. With regard to low-temperature cracking resistance, three-point bending tests conducted by Yang, et al. [47] showed that the tensile strain of the mixtures with 80% RAP content was significantly lower than that of the mixtures with 70% RAP content. The tensile strain of both was less than 2500 με, which did not meet the requirement in China, as shown in Figure 4b. This reduction in strain was mainly linked to the hardness of aged asphalt in RAP, thus increasing the possibility of brittle fracture in recycled asphalt mixtures at low temperatures. Furthermore, fatigue resistance was evaluated by Dong, et al. [41] on recycled asphalt with 60% and 80% RAP binder content using the ultimate fatigue temperature. Contrary to expectations, they found that the mixtures with 80% RAP content displayed superior fatigue resistance. Furthermore, Wang, et al. [49] suggested that this enhancement might be due to the increased proportion of aged asphalt and a corresponding decrease in virgin asphalt. In addition, the incomplete blending between aged asphalt and virgin asphalt in the mixtures was also one of the possible reasons.

The aging degree of aged asphalt in RAP also significantly affects the performance of recycled asphalt mixtures. Specifically, The microstructure of asphalt changes with the increase in aging degree. The asphaltene molecules in the asphalt combine to form larger structures, which exhibit higher shear resistance when subjected to shear forces. This change in structure is manifested at the micro-level as variations in the adhesive performance between asphalt and aggregate, thereby affecting the overall performance of the mixture. The findings of Zhou, et al. [50] showed that an increase in the aging degree of the aged asphalt in RAP correlated with an increase in the rutting factor, fatigue factor, and creep stiffness of the recycled asphalt, while the creep rate decreased. This trend suggests that while high-temperature stability is enhanced, there is a concurrent reduction in low-temperature cracking resistance and fatigue resistance.

3.1.2. Rejuvenators

Rejuvenators play a crucial role in restoring the performance of aged asphalt. On one hand, rejuvenators adjust the chemical composition of aged asphalt by replenishing the lost light components, such as oils and resins, thereby restoring the fluidity and plasticity of the asphalt. On the other hand, rejuvenators can disrupt the interactions between asphaltene molecules in aged asphalt, promoting the redispersion of asphaltene molecules, thus restoring the flexibility and elasticity of the asphalt. Currently, rejuvenators such as mineral oil, bio-oil, and composite oil are commonly employed in the HIR project [51,52,53,54]. In terms of high-temperature stability, some scholars have studied the effects of mineral oil rejuvenators on aged asphalt or aged asphalt mixtures. Specifically, Ren, et al. [55] evaluated the influence of waste engine oil (WEO) on high-temperature stability. The results indicated that adding WEO reduced the rutting factor of aged asphalt. Furthermore, the macroscopic test results of HIRAMs showed consistent findings [25]. However, mineral oil has some limitations, such as the volatility of light components at high temperatures and the poor rejuvenated efficiency. To address these issues, many scholars used bio-oil as an alternative. Bio-oil produced from sawdust was added to aged asphalt by Zhang, et al. [56]. The DSR findings indicated that the rutting factor of bio-rejuvenated asphalt decreased by 75.5% compared with aged asphalt. The authors attributed this to the higher content of light components and aromatics in bio-oil. Furthermore, in order to compare the effect of different types of bio-oil on the high-temperature stability, Zaumanis, et al. [25] designed HIRAMs incorporated with waste vegetable oil (WVO), waste vegetable grease, organic oil, and distilled tall oil, respectively. The results of the Hamburg wheel track test illustrated that the HIRAM prepared by distilled tall oil had the shallowest rut depth, following by organic oil, WVO, and waste vegetable grease. The above results indicated that the addition of mineral oil and bio-oil significantly reduced the high-temperature stability of aged asphalt mixtures. Therefore, some scholars expect to improve the high-temperature stability of HIRAMs by adding polymers to develop composite-modified rejuvenators. For instance, a composite-modified rejuvenator consisting of automatic oil, a plasticizer, and an anti-stripping agent was used to prepare HIRAMs by Zhang, et al. [57], who found that the HIRAMs exhibited better dynamic stability compared with that only added by WEO or vegetable oil.

Water stability is a critical property for HIRAMs, especially in environments prone to moisture damage. Zaumanis, et al. [25] found that both WEO and aromatic oil improved the water stability of HIRAMs. Similarly, the research conducted by Zaumanis, et al. [25] demonstrated that the addition of organic oil and distilled tall oil also improved the water stability of HIRAMs. However, not all bio-oils had positive effects; the addition of WVO had been reported to be of no benefit in improving water stability, which can be attributed to the increase in hydrophilicity of the binder due to the presence of a large number of free fatty acids in WVO [58].

Regarding the low-temperature cracking resistance, Ren, et al. [55] found through bending beam rheometer test that the addition of WEO decreased the creep stiffness and increased the creep rate of aged asphalt, suggesting an improvement in the asphalt’s flexibility at low temperatures. Furthermore, using the indirect tensile strength test, Zaumanis, et al. [25] observed a significant enhancement in the indirect tensile strength of HIRAM with the addition of WEO and aromatic oil. Additionally, Zhang, et al. [56] studied the low-temperature cracking resistance of virgin asphalt, aged asphalt, and sawdust oil-rejuvenated asphalt using the asphalt binder cracking device, and found that the addition of bio-oil rejuvenators decreased the low-temperature cracking temperature of aged asphalt. This was attributed to the high content of aromatics in the bio-oil, which balanced the chemical composition of the aged asphalt. The bio-oil also exhibited good fluidity, and improved the crack resistance at low temperatures. Furthermore, the study conducted by Zaumanis, et al. [25] demonstrated that the addition of WVO, waste vegetable grease, organic oil, and distilled tall oil increased the low-temperature indirect tensile strength of HIRAM, as shown in Figure 5. In order to further enhance the low-temperature cracking resistance of rejuvenated asphalt, Li, et al. [59] developed a composite-modified rejuvenator by adding styrene–butadiene rubber (SBR) latex into the bio-oil rejuvenators. Bending beam rheometer test results revealed that the use of bio-oil and SBR latex decreased the creep stiffness and increased the creep rate of the rejuvenated asphalt at low temperatures. This was because a mesh structure was formed due to the blending between butadiene chains in SBR and asphalt, thus improving the low-temperature cracking resistance of the asphalt.

The fatigue resistance of asphalt are critical factors affecting the durability and service life of pavements. In recent studies, rejuvenators have been identified as an effective means to enhance these properties in aged asphalt. Ren, et al. [55] found that the addition of WEO reduced the fatigue factor of aged asphalt through DSR test, thereby improving its fatigue resistance. Similarly, Zaumanis, et al. [25] and Zhang, et al. [57] reported that the addition of WEO could restore the fatigue life of HIRAM but it was difficult to achieve similar level as that of the VAM. Zhang, et al. [57] thought that the presence of lubrication components in WEO hindered the viscosity recovery of the aged asphalt, thus affecting the structural strength of the HIRAM. Furthermore, some researchers explored the effect of bio-oil rejuvenators. Elkashef and Williams [60] conducted linear amplitude sweep test using HIRAM with soybean-derived rejuvenators, and found a significant improvement in fatigue life. Similarly, using indirect tensile fatigue test, Ziari, et al. [61] reported that the addition of WVO enhanced the fatigue resistance of HIRAM. Furthermore, Li, et al. [59] combined SBR latex with the bio-oil rejuvenators to rejuvenate aged asphalt. The results illustrated that the composite rejuvenators restored the polymers in the asphalt, and had positive effects on the fatigue resistance of the recycled asphalt. Based on the comprehensive consideration, 10% bio-oil and 3% SBR latex of asphalt by weight exhibited the best effect.

In addition to the type of rejuvenator, the content of rejuvenator is equally crucial for restoring the performance of aged asphalt. An insufficient rejuvenator content may fail to effectively improve the asphalt’s performance, while an excessive amount could lead to a decline in asphalt performance. Zhang, et al. [56] studied the effect of sawdust oil with content of 10%, 15%, and 20% on the high-temperature stability of aged asphalt. The results demonstrated that the rutting factor of rejuvenated asphalt generally decreased with the increase of rejuvenator content. Furthermore, in the study conducted by Ren, et al. [55], quantification results showed that as the content of WEO and WCO increased from 5 wt% to 15 wt%, the rutting failure temperatures gradually decreased. The bending beam rheometer test revealed that the creep stiffness of the rejuvenated asphalt decreased, while the creep rate increased with the increasing content of WEO and WCO. Additionally, the fatigue failure temperatures also showed a decline.

3.1.3. Virgin Asphalt

In the production of HIRAMs, the addition of virgin asphalt primarily serves to adjust the viscosity of aged asphalt. Currently, the types of virgin asphalt primarily employed in the HIR project include base asphalt and modified asphalt. For high-temperature stability, Daryaee, et al. [62] demonstrated that modified asphalt, created from 85/100 penetration asphalt and waste polybutadiene rubber, enhanced the high-temperature stability of aged asphalt more effectively than base asphalt, as evidenced by DSR test results. Similar conclusions were drawn by Zhou, et al. [50] through adding TLA lake asphalt. Regarding water stability, Zhu, et al. [48] found that the addition of base asphalt significantly increased the TSR of the aged asphalt mixtures. In order to further improve the water stability of aged asphalt, modified asphalt has been employed to rejuvenate aged asphalt. Daryaee, et al. [62] found that the increase in TSR of the HIRAM prepared from waste-polybutadiene-rubber-modified asphalt was much greater than that prepared from base asphalt alone. In terms of low-temperature cracking resistance, Zhu, et al. [48] reported that the addition of base asphalt improved the bending tensile strength of the HIRAMs through a bending test. This finding was in agreement with that obtained by Yang, et al. [47], who selected styrene–butadiene–styrene block copolymer (SBS)-modified asphalt as virgin asphalt. As for fatigue resistance, Dong, et al. [41] analyzed the impact of base asphalt on the aged asphalt using the ultimate fatigue temperature. The results illustrated that the addition of base asphalt significantly improved the fatigue resistance of the aged asphalt. Furthermore, Daryaee, et al. [62] added waste polybutadiene rubber modifier to base asphalt to prepare modified asphalt and concluded that waste-polybutadiene-rubber-modified asphalt greatly increased the fatigue life of HIRAMs through a four-point beam fatigue test.

Apart from the type of virgin asphalt, the content of virgin asphalt also affects the performance of the HIRAMs. For high-temperature stability, Yang, et al. [47] prepared HIRAMs with different contents of SBS-modified asphalt and found that the increase in the content of SBS-modified asphalt reduced the dynamic stability of the recycled mixtures through a wheel tracking test. Concerning water stability, Jiang, et al. [63] conducted freeze–thaw splitting and immersion Marshall tests and concluded that the values of residual Marshall stability and TSR of HIRAMs were improved as the base asphalt content increased. In terms of low-temperature cracking resistance, the study conducted by Yang, et al. [47] revealed that the increase in SBS-modified asphalt content resulted in higher tensile strain of HIRAMs. With regard to fatigue resistance, Dong, et al. [41] reported that the fatigue factor of aged asphalt linearly decreased with increasing content of base asphalt, indicating an improvement in the fatigue resistance of aged asphalt. Similarly, the quantitative results obtained by Yang, et al. [47] indicated that as the content of SBS-modified asphalt increased, the fatigue factor of the rejuvenated asphalt decreased.

3.1.4. Virgin Asphalt Mixtures

The addition of VAMs to the HIR process can restructure the chemical composition and physical performance of the aged asphalt and alter the gradation of the RAP, thereby restoring the performance and extending the service life of HIRAMs. In a study by Pan, et al. [64], various proportions of continuously graded and gap-graded VAMs, ranging from 5% to 25%, were integrated into HIRAMs. Wheel tracking test results showed that the dynamic stability of HIRAMs slightly decreased with increasing continuously graded VAM content. In contrast, the dynamic stabilities of HIRAMs first increased and then decreased with increasing gap-graded VAM content, as shown in Figure 6a. They concluded that the increase in VAM content led to an increase in virgin asphalt content and a decrease in RAP content in HIRAMs, which weakened the high-temperature stability of HIRAMs. Moreover, the immersion Marshall test indicated that water susceptibility of HIRAMs gradually stabilized with increasing continuously graded VAM content. When gap-graded VAMs were adopted, the immersion residual stabilities of HIRAMs fluctuated first and then dropped precipitously, as depicted in Figure 6b, which can be attributed to the fact that the aggregates of the gap-graded VAMs were coarser than those of the continually graded VAMs, resulting in a larger void volume for the HIRAMs. Through the three-point bending test, it can be found that the low-temperature cracking resistance of HIRAMs initially increased and then decreased with the increase in continuously graded VAM content, as shown in Figure 6c. In contrast, the low-temperature cracking resistance of HIRAMs consistently decreased with the increase in gap-graded VAM content. The above results show that continuously graded VAMs provided a more stable effect on the performance of HIRAMs. HIRAMs with gap-graded VAMs can also show excellent performance at suitable content.

3.2. Production

Compared with newly laid asphalt pavement, the road performance of HIR asphalt pavement has great instability. The key to ensuring its stability is whether the high content of RAP can be fully blended with the rejuvenators, virgin asphalt, and other materials during the production process. The specific influence mainly includes preheating and mixing [65].

3.2.1. Preheating

One of the reasons why preheating affects HIRAM performance is that the high temperature softens the aged asphalt in RAP, thus resulting in better blending with virgin asphalt and aggregates [34,66]. Liu, et al. [34] studied the effect of preheating temperature on HIRAM using a wheel tracking test. The results illustrated that as the preheating temperature of RAP increased from 90 °C to 150 °C, the rut depth of HIRAMs decreased, while the dynamic stability increased. As for water stability, Ma, et al. [67] found that higher preheating temperatures improved the water stability of HIRAMs, using indexes such as the resilient modulus ratio, TSR, and dissipated creep strain energy ratio. For low-temperature cracking resistance, Liu, et al. [34] and Liu, et al. [68] reported through low-temperature bending beam and indirect tensile strength tests that the increase in temperature improved the low-temperature cracking resistance. The research conducted by Chen, et al. [69] reached a similar conclusion.

On the other hand, preheating affects the diffusion of rejuvenators in aged asphalt, thereby influencing the performance of the mixtures [27,66,68]. Ma, et al. [27] found that inadequate diffusion of rejuvenators in aged asphalt resulted in poor high-temperature stability, low-temperature cracking resistance, and water stability of the recycled asphalt mixtures. Furthermore, Liu, et al. [68] discovered that higher preheating temperatures promoted the diffusion of rejuvenators in aged asphalt. The diffusion process of rejuvenators in aged asphalt is shown in Figure 7.

3.2.2. Mixing

The performance of HIRAMs is significantly influenced by the homogeneity of the mixtures, particularly among RAP and other materials [71,72,73]. Ma, et al. [74] and Li, et al. [75] have investigated the impact of mixing time on this homogeneity. The research indicated that extending the mixing time contributed to the blending of HIR materials, but excessively long mixing times could not further improve the degree of blending. Ma, et al. [71] reported that changing the temperature during mixing could alter the degree of blending of materials in HIRAMs. Furthermore, in the study conducted by Ma, et al. [72], HIRAMs with different degrees of blending were prepared by adjusting the mixing temperature. The results from asphalt mixture performance tests and Ideal-CT tests indicated that while the incorporation of RAP, rejuvenators, and virgin asphalt at high temperatures can reduce the rutting resistance of HIRAMs, it also significantly enhanced the mixture’s crack resistance. Similar findings were reported by Ma, et al. [67], who attributed this to the fact that the higher proportion of RAP coated the aggregates due to the elevated temperature, thus increasing the likelihood of rutting.

4. Long-Term Performance of HIRAMs

As a preventive maintenance technique, the long-term performance of HIR asphalt pavement significantly influences its service life. Studies report that on highway maintenance projects, pavements treated with HIR exhibit good condition with essentially no distress within the first 1–2 years after construction [76]. From the third year onward, minor cracks gradually appear on the pavement surface, and this condition increases somewhat after 4–5 years. Nevertheless, at the five-year mark, key technical condition indexes—including skid resistance, surface distress, ride quality, and rutting—remain relatively favorable. The demonstrated durability satisfies the demands of high-traffic volumes, heavy loads, and high-temperature regions [76,77,78,79].

5. HIR Benefits

In the HIR production and construction process, the utilization of waste pavement materials not only reduces costs over the entire lifecycle but also brings significant environmental benefits. This section provides a detailed review of the benefits of HIR from both environmental and economic perspectives.

5.1. Environment

The relevant studies demonstrated the substantial potential of HIR for reducing carbon emissions and energy consumption. As the RAP content increased, the emissions of greenhouse gas and energy consumption significantly decreased [31,80,81,82]. Numerous scholars have conducted comprehensive comparisons among HIR and other maintenance technologies from these two perspectives and proposed schemes to optimize environmental benefits.

5.1.1. Comparison with Milling and Filling

In an effort to evaluate the environmental merits of HIR in comparison to milling and filling (M&F), several scholars have conducted studies from a carbon emission standpoint. Chai, et al. [83] divided the construction phase into material production and pavement construction stages through life cycle assessment and established a carbon emission model for quantifying the carbon emissions of HIR and M&F with different proportions of new materials, transport distances of RAP materials, and utilization rates. The results of the calculation showed that carbon emission from HIR decreased by 21% compared with M&F. Expanding on these findings, Cao, et al. [30] introduced the eco-efficiency analysis framework to further compare the carbon footprint of HIR with M&F. The study found that over a hypothetical service life of 15 years, HIR reduced carbon emissions by 28% compared with M&F, as shown in Figure 8. The research conducted by Chen, et al. [84] also achieved the similar viewpoint that the carbon emission of HIR was lower than that of M&F. In a parallel investigation, Zhu, et al. [85] used the benefit-to-greenhouse-gas-emission ratio as the environmental indexes to compare the project of HIR with M&F. Interestingly, their results indicated that the benefits of both M&F and HIR were contingent upon traffic volume. However, they also found that M&F had a higher benefit-to-greenhouse-gas-emission ratio than HIR.

From the perspective of energy consumption, Chen, et al. [84] analyzed the energy consumption during the construction phase, including raw material production, transportation, mixture production, and mixture construction. It was found that the energy consumption of HIR accounted for only 35.6% of that of M&F, which was attributed to a smaller proportion of newly added materials. Similarly, Yang [86] calculated the total energy consumption of HIR and M&F at each stage and reached the conclusion that HIR reduced energy consumption by 45.57% compared to M&F. However, over a hypothetical service life of 15 years, Cao, et al. [30] discovered that M&F saved 7% in energy consumption compared to HIR. Furthermore, based on the characteristics of asphalt pavement maintenance, Zhu, et al. [85] used the benefit-to-energy-consumption ratio to compare the energy consumption of HIR with the M&F project. The calculated results indicated that M&F had a higher benefit-to-energy-consumption ratio than HIR. Various scholars’ results regarding the environmental benefits of HIR and M&F are not identical. This is mainly related to the different assumed service life as well as the evaluation phase. Additionally, the different analysis models may also be a reason for the discrepancy in their conclusions.

5.1.2. Comparison with Thin HMA Overlay

Some scholars have also assessed the environmental benefits of HIR compared to thin HMA overlay (THO). A model for calculating carbon emissions in HIR and THO was developed by Chai, et al. [83], who found that the carbon emissions of HIR were reduced by 28% compared to THO. To further compare the carbon emissions at each stage, Huang [22] calculated the emissions by multiplying the material consumption of the two mixtures by their respective carbon emission factors. The results showed that HIR emitted 14.80 kg less CO₂ per ton of mixture than THO during the production stage. Similarly, in the construction stage, HIR emitted 1.49 kg less CO₂ per ton of mixture, with a reduction rate of 5.6%.

With respect to energy consumption, Mo [87] evaluated the energy consumption of HIR and THO using indexes such as asphalt, aggregates, and fuel consumption. It was illustrated that HIR, when adopted for maintenance, could save 97.97 kg/m² in virgin aggregates and 3.71 kg/m² in virgin asphalt, with the average fuel consumption of HIR units being only 0.7 kg/m². Furthermore, Huang [22] found that HIR could save 230.82 MJ/t of energy, with a saving rate of 73.78% compared with THO during the production stage. In contrast, during the construction stage, HIR consumed more fuel, thus leading to higher energy consumption per ton of asphalt mixture than THO. Moreover, when considering the overall energy consumption, HIR still exhibited lower energy consumption than THO. Specifically, the energy consumption for producing HMA was 564.83 MJ/t, while HIRAMs’ energy consumption was 424.83 MJ/t, resulting in a saving rate of 24.8%.

5.1.3. Comparison with Other Recycling Technology

When evaluating the environmental benefits of different pavement recycling technologies, it is essential to consider the specific applications and layers in which they are utilized. Cold central-plant recycling, which is typically reserved for the construction of base and subbase layers, and HIR, predominantly applied to surface layers, are not directly comparable due to their distinct application layer within the pavement structure. Therefore, most scholars focus on comparing the environmental benefits of HIR with HCPR and CIR. Specifically, Yang [86] calculated the carbon emissions during the production, transportation, and construction stages of HIR and HCPR, and summed them up to obtain the total carbon emissions. The results demonstrated that HIR had 2.01 kg CO₂/m² lower total carbon emissions than HCPR. Also, the study conducted by Cao [88] reached a similar conclusion using the environmental factor method. To further compare the environmental benefits, the findings obtained by Miliutenko, et al. [32] indicated that HIR gave more global warming potential savings than HCPR, as shown in Figure 9a. Chai, et al. [83] further refined this analysis by asserting that the carbon emissions of HIR and HCPR are contingent upon the transportation distance and the utilization rate of RAP materials. When the HCPR and HIR were prepared with 100% RAP materials, HIR emitted less carbon than HCPR only if the transport distance of the materials used exceeded 42 km. When RAP material utilization was lower than 75%, HIR emitted less carbon than HCPR. Additionally, regarding energy consumption, Cao [88] calculated the total energy consumption of HIR and HCPR, which included the stages of old road treatment, new material production, and mixture production and construction. The results illustrated that HIR reduced energy consumption by 53.95% compared with HCPR. The research conducted by Miliutenko, et al. [32] showed that HIR provided more cumulative energy demand savings than HCPR, as shown in Figure 9b.

For HIR and CIR, in order to obtain the comprehensive carbon emissions, Yang [86] summed up the carbon emissions during the stages of raw material production, construction, and transportation. The results showed that HIR’s comprehensive carbon emissions were 0.531 kg CO₂/m² higher than those of CIR. This finding was consistent with that obtained by Cao [88], who used an environmental factor method to calculate and compare the carbon emissions of HIR and CIR. Additionally, from the perspective of energy consumption, Yang [86] used a comprehensive energy consumption calculation model to calculate the total energy consumption during the stages of raw material production, construction, and transportation of HIR and CIR. It was found that HIR had 19.987 MJ/m² higher energy consumption than CIR. A similar research methodology was conducted by Cao [88], who found that HIR had 4.20% higher energy consumption than CIR.

5.1.4. Comparison Between Different HIR Technologies

In order to further understand the environmental benefits of HIR, scholars have compared the environmental benefits of different HIR technologies. It should be noted that mixtures obtained through surface recycling are rarely used due to unsatisfactory performance. Therefore, this section does not include surface recycling. Based on the greenhouse gas emission factors provided in the Intergovernmental Panel on Climate Change National Inventory Guidelines, Hu [89] obtained the carbon emission coefficients for the production and construction stages of remixing and repaving. The results revealed that the carbon emissions from repaving were approximately 19.98% higher than those from remixing. Furthermore, Yu, et al. [90] employed the discrete event simulation method and found that under identical paving mass conditions, the air pollutant emissions from remixing were 19–21% higher than those from repaving. Conversely, as illustrated in Figure 10a,b, under the same paving length or paving duration conditions, the air pollutant emissions from remixing were lower than those from repaving. This result was also confirmed in a study by Liu [91].

With regard to energy consumption, Hu [89] determined the energy consumption coefficients for both remixing and repaving, drawing on the net calorific values of energy from the Intergovernmental Panel on Climate Change National Inventory Guidelines. The result revealed that the energy consumption of repaving was 10.38% higher than that of remixing. Building upon this foundation, Liu [91] delved deeper into the relative energy consumption of these two HIR technologies. The study found that the energy consumption of remixing was 10.2% higher than that of repaving under the same paving mass, whereas the energy consumption of repaving was 12.4% higher than that of remixing under the same paving length.

5.1.5. Optimization of HIR Environmental Benefits

In order to reduce carbon emissions and asphalt fumes of HIR, some scholars have focused on optimizing the HIR production process. For instance, Yu, et al. [92] developed a multi-objective optimization model to help decision-makers optimize and customize the construction plan for the HIR project. They quantified the environmental impact through discrete event simulation and employed non-dominated sorting genetic algorithm II to solve the multi-objective optimization problem. The results demonstrated that there was a potential to reduce the environmental impact by 3.5% compared to the baseline. To further optimize the carbon emissions of HIR, Yao, et al. [33] established a model to optimize the construction parameters, as shown in Equation (1). The results revealed that the optimization of the construction parameters could reduce the project’s carbon emissions by 16.81% and result in a decrease of 199.7 tons in carbon emissions compared with the pre-optimized situation, as depicted in Figure 11.

$$minF\left(x\right)=min\left[y,Total cost or Total C{O}_{2 }emissions\right]$$

(1)

$$s.t.\left\{\begin{array}{c}y\ge y_{limit}\\ \underset{\_}{x_i}\le x_i\le \overline{x_i}\end{array}\right.$$

where $x_i$ is the design and construction parameter, $\underset{\_}{{ x}_i}$ refers to the lower limit value of the design parameters, $\overline{{ x}_i}$ refers to the upper limit value of design parameters, and $y$ is the performance of HIRAMs.

The asphalt fumes generated during the operation of the HIR ‘recycling train’ were widely recognized as an environmental hazard [93]. To reduce asphalt fumes, Zhang, et al. [94] invented an HIR asphalt fume treatment device comprising a gas hood, pipeline components, fume filters, and fans, as depicted in Figure 12. The device was proven to effectively treat the asphalt fumes generated during the HIR pavement recycling process. Similarly, Zhou [95] proposed a clean HIR technology that utilized negative pressure circulation and secondary combustion technology to reuse the waste heat and exhaust gas generated during pavement heating. The results showed that compared with traditional HIR, clean HIR avoided the negative impact of asphalt fumes and high-temperature air on the surrounding environment.

5.2. Economy

Considering the reduction in cost associated with stone storage, asphalt materials, and transportation, HIR could effectively lower the maintenance expenses of asphalt pavement and thus bring better economic benefits [96,97,98,99,100]. Numerous scholars not only conducted comprehensive comparisons between HIR and other maintenance technologies based on cost but also further analyzed the economic benefits of different HIR technologies and then optimized the HIR process.

5.2.1. Comparison with Milling and Filling

In the realm of pavement maintenance, the economic viability of HIR compared to traditional M&F has been a subject of considerable interest among researchers and practitioners. Ali and Grzybowski [29] concluded that the construction costs of HIR were less than half of those of M&F based on a project from the Florida Department of Transportation. Furthermore, Pan, et al. [101] evaluated the cost-effectiveness of HIR and M&F using the benefit–cost ratio (BCR). The results indicated that both in short-term and long-term maintenance, HIR had a higher BCR than M&F, as shown in Figure 13. Furthermore, the research conducted by Cao, et al. [30] illustrated that HIR saved 5% more cost than M&F in a 15-year service life. In contrast, some scholars believed that under certain conditions, M&F had advantages over HIR. According to the Equations (2)–(4), Zhu, et al. [85] found that the benefits of M&F and HIR were associated with traffic volume. When the traffic volume was low, the $BCR$ of M&F was higher than HIR. Moreover, a sensitivity analysis conducted by Cao, et al. [30] indicated that when the life extension ratio (HIR/M&F) reached 12/15, M&F had more advantages in economic terms.

$$S={\int }_{t_1}^{t_2}\left[y_2\left(t\right)-y_1\left(t\right)\right]dt+{\int }_{t_2}^{t_3}\left[y_2\left(t\right)-y_0\right]dt$$

(2)

$$C=C_i+\sum _{k=1}^N{C}_m\left[{\displaystyle \frac{1}{{\left(1+i\right)}^{n_k}}}\right]+\sum _{k=1}^N{C}_u\left[{\displaystyle \frac{1}{{\left(1+i\right)}^{n_k}}}\right]-C_s\left[{\displaystyle \frac{1}{{\left(1+i\right)}^n}}\right]$$

(3)

$$BCR={\displaystyle \frac{S}{C}}$$

(4)

where $S$ is the pavement maintenance benefit, $y_1$ is the pavement performance equation, $y_2$ is the pavement performance equation after maintenance, $t_1$ is the maintenance time, $t_2$ is the time when the pavement performance reduces to the minimum acceptable level, $t_3$ is the time when the pavement performance reduces to an acceptable level again, $C$ is life cycle cost, $C_i$ is construction cost, $C_m$ is maintenance cost, $C_u$ is the user cost including fuel cost and delay cost, $C_s$ is the salvage of the pavement, $N$ is the total number of maintenance activities over the life cycle, and $n_k$ is the time point (in years) at which the k-th maintenance activity occurs.

5.2.2. Comparison with Thin HMA Overlay

Some scholars also compared the economic benefits of HIR and THO. Specifically, Pan, et al. [101] evaluated the cost-effectiveness of HIR and THO by calculating the BCR. The results indicated that HIR had a higher BCR than THO, as shown in Figure 13. Through analyzing the cost of materials, fuel, and equipment usage, Huang [22] found that the overall cost of HIR was 34.3% lower than THO. Similarly, in the research of Mo [87], the equivalent annual cost of HIR including direct cost, indirect cost, profits, and taxes was approximately 11.25 CNY/m², which was lower than that of THO.

5.2.3. Comparison with Other Recycling Technology

In the quest to optimize pavement recycling technologies for economic efficiency and environmental sustainability, a comprehensive analysis of the cost-effectiveness of different methods is essential. Using a 10-year dataset from highway maintenance, Xu, et al. [102] compared the unit price of recycled materials and the overall return rates of HIR, HCPR, and CIR. The results indicated that HIR had the best economic benefits. Specifically, the unit price of HIR was 102.66 CNY/t and 52.94 CNY/t lower than that of HCPR and CIR, respectively. To further compare the cost savings among different recycling technologies, Cao [88] calculated the material cost by multiplying the unit price of materials by their consumption. The findings revealed that HIRAMs had a higher cost savings rate in raw materials compared to HCPR mixtures and CIR mixtures because of higher content of RAP. Specifically, HIR could provide a cost saving of 63.11% and 24.28% compared to HCPR and CIR, respectively.

5.2.4. Comparison Between Different HIR Technology

In the project of HIR, choosing between repaving and remixing is a critical decision that carries significant economic implications. Through a developed discrete event simulation model, Liu [91] found that remixing had better economic benefits than repaving. This finding was further corroborated by Hu [89], who conducted an equivalent annual cost analysis to compare the actual economic benefits of both methods over a 15-year period. The results revealed that the equivalent annual cost of repaving was notably higher than remixing.

5.2.5. Optimization of HIR Economic Benefits

To maximize the economic benefits of HIR, some scholars optimized the construction process through modeling. Specifically, a multi-objective optimization model constructed by Yu, et al. [92] quantified the economic benefits through discrete event simulation. The model is designed to optimize costs, including material input, fuel consumption, and equipment usage. To further optimize HIR, Yao, et al. [103] developed a cost model and a multi-objective optimization model for HIR, which effectively reduced construction costs. They found that construction costs were closely related to the road surface temperature during construction. When the pavement surface temperature was 60 °C, construction costs were significantly reduced by 19.0% to 22.5% compared to when it was 38 °C. Additionally, the construction parameters optimized through the model not only improved the uniformity of compaction but also led to a reduction in construction cost per unit area by 8.7% at 38 °C and 24.2% at 60 °C, thereby significantly enhancing economic benefits. On this basis, Yao, et al. [33] further optimized HIR construction parameters with the model shown in Equation (1). The study demonstrated that the optimized parameters reduced the total construction cost of the project by 6.67%, as depicted in Figure 14. In addition, an intelligent control measure for construction temperatures was proposed, as shown in Figure 15, which not only maximized the benefits of HIR but also ensured construction quality.

6. Technical Challenges and Future Research Recommendations

As of 2023, HIR has had a history of more than 50 years. Throughout this period, researchers have devoted considerable effort to investigating the effects of various factors on the road performance of HIRAMs, as well as the environmental and economic benefits of HIR. Though these studies provide a lot of support in advancing the development of HIR, there are still many challenges that need to be further addressed.

6.1. Applicability of HIR Technology

HIR, as delineated by the Asphalt Recycling & Reclaiming Association, includes surface recycling, remixing, and repaving. These methods are capable of addressing various levels of damage in the surface layer, as summarized in

Table 1. Characteristics and applicability of the three processes.

Recycling Process	Characteristics	Features and Applicability
Surface recycling	Only rejuvenators need to be added.	Surface recycling is suitable for roads with little road damage and small damaged areas and can eliminate cracks and ruts in the original pavement.
Remixing	Rejuvenators, virgin asphalt (if necessary), and asphalt mixtures are added.	Remixing is suitable for moderately damaged pavement and can improve the material performance of old asphalt pavement, repair aging and unstable wear layers, and improve pavement strength.
Repaving	On the basis of the surface recycling and remixing, an abrasion layer is added.	Repaving is applicable to severely damaged roads. The rehabilitated asphalt pavement has good skid resistance, improved cross slope, and increased pavement strength.

. In the surface recycling process, only rejuvenators need to be added. The gradation of RAP materials remains unchanged, resulting in the non-ideal performance of recycled asphalt mixtures. Currently, the application scopes of the three recycling technologies are solely determined by the degree of surface damage. To enhance their utility, it is advisable to associate HIR with evaluation indexes of road condition, thereby refining their application criteria. In addition, no matter what kind of HIR is used, it can only deal with the shallow layer disease with the depth of 19–50 mm. Therefore, before utilizing HIR, pre-treatment is necessary for addressing the disease of deep-layer disease. Currently, some organizations have advanced technologies such as double-layer recycling and multi-step recycling to achieve a recycled depth of approximately 100 mm. However, these technologies have not been widely adopted. Furthermore, whether different HIR technologies demonstrate the same applicability under varying climatic conditions, and whether distinct HIR equipment delivers different outcomes, remains unclear. Consequently, further research should be conducted to investigate these questions, with subsequent efforts focused on promoting the application of these findings in future practice.

6.2. Key Factors Affecting the Road Performance of HIRAMs

6.2.1. Raw Materials

The performance of HIRAMs is significantly influenced by the quality of raw materials, which include RAP materials, rejuvenators, virgin asphalt, and VAMs.

Table 2. Factors affecting the performance of HIRAMs.

Influence Factor			Performance
			High Temperature	Moisture Susceptibility	Low Temperature	Fatigue
RAP		Content	✓	✓	❖	✓
			❖	❖
		Aging degree	✓	N/A	❖	N/A
Rejuvenators	Types	Mineral oil	❖	✓	✓	✓
						❖
		Bio-oil	❖	✓	✓	✓
				❖
		Composite oil	❖	✓	✓	✓
	Content	Mineral oil	❖	✓	✓	✓
		Bio-oil	❖	✓	✓	✓
				❖
		Composite oil	N/A	N/A	N/A	N/A
Virgin asphalt	Types	Base asphalt	❖	✓	✓	✓
		Modified asphalt	❖	✓	✓	✓
	Content	Base asphalt	❖	✓	✓	✓
		Modified asphalt	❖	✓	✓	✓
VAM	Types	Continuously graded	❖	✓	✓	N/A
		Gap-graded	❖	✓	✓	N/A
	Content	Continuously graded	❖	✓	✓	N/A
					❖
		Gap-graded	✓	✓	❖	N/A
			❖	❖
Production		Preheating temperature	✓	✓	✓	✓
			❖
		Mixing temperature and time	❖	N/A	✓	✓

✓: Improved. ❖: Declined. N/A: Not available.

presents the effects of these variables on the road performance of HIRAMs. Relevant research indicated that the rise in RAP content increased the hard components in the rejuvenated asphalt, making HIRAMs harder [41,47,48,49]. In other words, this enhances high-temperature stability of HIRAMs but concurrently reduces water stability, low-temperature cracking resistance, and fatigue resistance. However, there is not a consensus in the literature regarding this conclusion. For instance, Zhu, et al. [48] reported that the dynamic stability of HIRAMs first increases and then decreases with the increase in RAP content. This trend was attributed to the poor blending of aged and virgin asphalt with increased RAP content. Moreover, the increase in RAP content led to greater variance in RAP gradation, resulting in uncontrollable mixture performance, which was also one of the reasons. Therefore, due to the notable impact of RAP content on the performance of recycled mixtures, it is advisable to establish corresponding standards to improve the service life of HIR pavement. On the other hand, the increase in the aging degree of aged asphalt in RAP detrimentally impacts the low-temperature cracking resistance and fatigue resistance of HIRAMs, while it positively affects high-temperature stability. However, there are few studies about the effect of the aging degree of aged asphalt in RAP on the water stability of HIRAMs. Additionally, the majority of current studies concentrate solely on the impact of a single factor on the road performance of recycled mixtures, which ignores the interaction between RAP content and the aging degree of aged asphalt. This oversight may limit the full understanding of how these factors work together to affect HIRAM performance. Researchers should consider the complex interactions between these variables to provide a more nuanced assessment of HIRAM performance.

Mineral oil and bio-oil rejuvenators effectively restore high-temperature stability and low-temperature cracking resistance of recycled mixtures. The incorporation of polymers within these rejuvenators can further amplify these beneficial effects, so it is recommended to develop composite-modified rejuvenators in the future to enhance the service life and performance of recycled asphalt pavement. Furthermore, while traditional rejuvenators are primarily designed for unmodified asphalt, the prevalence of polymer-modified asphalt in current pavements necessitates the development of reactive rejuvenators. These reactive rejuvenators should possess the capability to repair the degraded polymer structures in aged asphalt. Additionally, the content of rejuvenators is also an important influencing factor. The increased content of rejuvenators improves the low-temperature cracking resistance and fatigue resistance of rejuvenated materials. However, this increase may come at the expense of reduced high-temperature stability. Furthermore, considering that there is limited research on the relation between composite-modified rejuvenators and water stability, further studies are recommended to address this gap.

Currently, virgin asphalt used in HIR projects can be classified into base asphalt and modified asphalt, both of which are capable of restoring the road performance of recycled mixtures. Nevertheless, there is limited research on how virgin asphalt interacts with aged asphalt and on how different types of virgin asphalt affect the road performance of recycled mixtures, which hinders the advancement of HIR. Therefore, it is necessary to conduct targeted research to bridge this gap. For VAMs, the effect of continuously graded VAMs on HIRAMs is relatively more stable. In contrast, when gap-graded VAMs are used, the performance of HIRAMs is prone to drastic fluctuations with changes in VAM content, which may be caused by the difference between the gradation of virgin aggregates in VAMs and the gradation of RAP aggregates. Nonetheless, an appropriate content of gap-graded VAMs can lead to better performance of HIRAMs. It is recommended that relevant models be developed in the future to determine the appropriate content of VAMs from the perspective of pavement distress.

The range factors mentioned in the table are all increases.

In summary, numerous studies have shown the significant effects of RAP materials, rejuvenators, virgin asphalt, and VAMs on the road performance of recycled mixture. However, the relative importance of these factors within the context of HIRAMs remains to be fully understood. Specifically, Jing, et al. [23] reported that RAP and rejuvenators had a more pronounced influence on the road performance of HIRAMs compared to virgin asphalt, but the degree of importance in relation to VAMs remains unknown. Therefore, analyzing the degree of importance of RAP materials, rejuvenators, virgin asphalt, and VAMs on the performance of HIRAMs is a critical direction for future research. Additionally, existing research neglects the interactions among these factors affecting the performance of HIRAMs. Also, the current research does not distinguish the impacts of these factors on the performance of HIRAMs prepared by different HIR technologies. Furthermore, while most studies focus on RAP application and rejuvenator mechanisms, a key technical challenge remains: the inherent high variability of RAP is insufficiently addressed in current research. Reclaimed RAP exhibits significant variations in critical properties due to its complex service history and exposure. This intrinsic heterogeneity is largely overlooked, as studies frequently employ standardized or idealized RAP samples under laboratory conditions. Consequently, they fail to adequately reveal how this variability fundamentally impacts recycled mixture performance. This oversimplification limits the applicability of findings to real-world conditions. Effectively identifying, quantifying, and mitigating this variability’s impact on HIR processes and performance—and developing corresponding predictive models and design methods—thus remains a paramount challenge for future research.

Compounding this, standardized testing protocols for rejuvenator efficacy and long-term performance under varied field conditions remain underdeveloped, hindering mix design optimization. Without robust methods to predict field-scale behavior from laboratory results, achieving universally reliable mix designs is challenging. Further, fragmented policy frameworks and inconsistent specification requirements across regions create adoption barriers, limiting technology transfer. Addressing these interconnected gaps—through harmonized standards, validated accelerated testing, adaptive design methodologies, and proactive policy alignment—is essential.

6.2.2. Production

In the production stage of HIR, preheating is a crucial factor that impacts the road performance of recycled mixtures. Firstly, the high temperature softens the aged asphalt, which enhances its blending with the virgin asphalt and aggregates. Secondly, the preheating temperature influences the diffusion of rejuvenators in the aged asphalt. It has been proven that increasing the preheating temperature improves water stability, low-temperature cracking resistance, and fatigue resistance of recycled mixtures. However, the relationship between preheating temperature and high-temperature stability is more complex and remains a subject of debate. For instance, Ma, et al. [67] argued that high temperature activated RAP binders to coat aggregates, increasing the possibility of rutting. Additionally, it is important to acknowledge that the high-temperature heating during the HIR construction process may cause additional aging of the treated pavement [104]. This should also be considered as a factor in the design of HIRAMs.

During the mixing stage, prolonging the mixing time and raising the mixing temperature can enhance the crack resistance of HIRAMs. This is because mixing is critical for achieving a uniform distribution of the materials, which directly influences the performance of the recycled mixtures. Moreover, studies on process-related factors, such as the addition order of materials and the mixing process, are lacking. In the future, it is recommended to investigate the effects of changing these processes on the performance of recycled mixtures. Additionally, there is a lack of studies on the paving and compaction stage, which may be due to the fact that HIR is primarily carried out in the field using specialized “HIR trains”. Considering the challenges of artificially controlling the construction process indoors, it is suggested that research be conducted on different types of “HIR trains” used in the industry.

6.3. Long-Term Performance of HIRAMs

HIR asphalt pavement demonstrates favorable skid resistance, surface integrity, ride quality, and rutting resistance during the critical five-year service period. However, research extending beyond this five-year timeframe remains notably limited and lacks validation under special climatic conditions. Future work should address these gaps to achieve reliable lifecycle prediction.

6.4. HIR Benefits

As delineated in Section 5.1, HIR offers significant advantages over THO in terms of carbon emissions and energy consumption. In contrast, the benefits and drawbacks of HIR compared to M&F remain contentious, largely due to the varying life cycles and maintenance areas considered in different studies. Furthermore, while the environmental benefits of HIR surpass those of HCPR, they are inferior to those of CIR. This discrepancy may be attributed to the on-site heating during HIR construction, which results in higher carbon emissions and energy consumption. Currently, scholars have considered the application of warm mix recycling technology in HIR project [105]. However, their research primarily focuses on road performance while lacks investigation of environmental benefits. Therefore, further research on the application of warm mix recycling technology in HIR projects is warranted to mitigate the environmental impact of HIR.

With respect to economic benefits, HIR is superior to THO, HCPR, and CIR. However, there are still uncertainties compared with M&F due to variations in analysis periods and methods across different studies. Additionally, most studies focus on the cost of HIR materials, often overlooking the costs associated with the HIR construction phase. In the future, researchers should develop models to analyze the cost of HIR from different perspectives to further enhance its cost-effectiveness. For instance, Yao, et al. [103] found that the construction cost of HIR increased with greater compactness. This finding helps project managers in accurately controlling construction cost, although the correlation with factors like traffic volume and lane type is still unclear. Similarly, Bouraima, et al. [106] extracted the recycled asphalt and recycled asphalt mixtures from slow lanes and emergency lanes of HIR asphalt pavement, respectively. They found that there was a large difference in their performances. Therefore, whether different designs for different lanes can reduce construction costs under the premise of ensuring road performance is a question for scholars to further consider.

7. Conclusions

HIR is an effective technology for reclaiming aged asphalt pavement, allowing for the conservation of virgin asphalt binders and aggregates. Additionally, it addresses minor surface distress in highways and urban roads. This paper provides a comprehensive review of the classification of HIR, the factors affecting pavement performance, and environmental and economic benefits. The following conclusions can be drawn:

(1) HIR includes surface recycling, remixing, and repaving, which are suitable for asphalt pavement with different degrees of damage. Surface recycling suits minor damage but yields suboptimal performance due to unchanged RAP gradation. Remixing effectively restores moderately damaged pavements. Repaving is optimal for severe damage. Furthermore, HIR is mainly used to address superficial damage. Therefore, future work must advance deep-layer recycling (>100 mm) via multi-step techniques.

(2) Increasing RAP content and asphalt aging enhances HIRAMs’ high-temperature stability but reduces their low-temperature cracking resistance, moisture resistance, and fatigue resistance. Bio-oil and mineral oil rejuvenators improve low-temperature cracking resistance while compromising high-temperature stability. Composite-modified rejuvenators outperform conventional rejuvenators in enhancing mixture performance, whereas VAM incorporation causes significant performance fluctuations.

(3) Increased preheating temperature enhances HIRAMs’ water stability, low-temperature cracking resistance, and fatigue resistance, though its impact on high-temperature stability remains contentious. Extended mixing time improves cracking resistance but adversely affects high-temperature stability.

(4) Current research predominantly focuses on single-factor impacts while ignoring factor interactions. Future studies should adopt reliable evaluation methods to systematically investigate how various factors affect HIRAM pavement performance. Furthermore, existing work inadequately addresses the impact of RAP’s inherent variability on recycled mixtures and lacks standardized field testing methods.

(5) HIR asphalt pavement demonstrates excellent durability over the critical five-year period, maintaining favorable key performance indicators and reliably meeting demanding conditions in high-traffic, heavy-load, and high-temperature environments.

(6) HIR reduces CO₂ emissions and energy use versus THO and HCPR, though CIR has superior environmental performance. Remixing emits significantly fewer air pollutants than repaving for fixed paving lengths. Asphalt fume emissions during HIR train construction present major environmental hazards, indicating potential for further optimization by addressing this issue.

(7) In terms of economic benefits, HIR reduces most material costs compared to CIR/HCPR and achieves higher benefit–cost ratios than THO. Moreover, in view of the significant differences in the performance requirements of asphalt mixtures for different lanes, it is worthwhile to explore the differentiated design method based on lane functional partitioning to optimize the construction cost under the premise of safeguarding road performance, which is a subject of in-depth research.