Comparative Study of Pavement Rehabilitation Using Hot in-Place Recycling and Hot-Mix Asphalt: Performance Evaluation,Pavement Life Prediction,and Life Cycle Cost Analysis

Abstract

The use of hot in-place recycling (HIR) for pavement surface rehabilitation has gained increasing attention in recent years with the aim of achieving sustainable pavement. For the purpose of cost saving and the consumption of 100% reclaimed asphalt pavement (RAP), an in-place HIR train has been developed to combine various units including preheating, milling, rejuvenating, and compacting. Compared with the conventional hot-mix asphalt (HMA) surface treatment, the HIR technique might result in different pavement performances as a result of the low mixing temperature and insufficient quality control. This study aimed to conduct a comparative analysis of pavement surface rehabilitation using HIR and HMA, including the performance evaluation, pavement life prediction, and life cycle cost analysis (LCCA). HIR and HMA mixes were collected from construction areas and the nearby asphalt plant, respectively. The loose mixes were reheated and compacted for performance testing, including dynamic modulus tests, Superpave IDT tests, and moisture susceptibility tests. The AASHTOWare Pavement ME Design software was adopted to predict the pavement life with two rehabilitation techniques, followed by the LCCA with regard to the prediction results. Test results showed that pavement surface rehabilitation with HIR achieves acceptable performance and economic benefits. However, the HIR mixes are brittle and more susceptible to cracking and, therefore, have shorter pavement service lives than HMA.

Keywords

infrastructure pavements design and rehabilitation of asphalt pavements asphalt

The increasing costs of asphalt binder and aggregates have put pressure on highway maintenance budgets. Pavement managers are seeking alternative cost-effective approaches to rehabilitate roads (1 –4). Currently, there exist several asphalt recycling techniques consisting of two basic approaches: a) inclusion of the milled reclaimed asphalt pavement (RAP) to replace a proportion of aggregates in hot-mix asphalt (HMA) in an asphalt plant; and b) in-place recycling, including cold in-place recycling (CIR) and hot in-place recycling (HIR) ( 5 , 6 ). The in-place recycling technique encourages the consumption of 100% RAP from the existing pavement, which allows the use of suitable recycling agents to rejuvenate the aged asphalt. HIR is one of the pavement rehabilitation techniques used primarily for the surface distresses, limited up to 25 to 50 mm ( 7 ). The pavement temperature before paving is usually around 110°C ( 6 ). The existing pavement is softened with flame heating. This is followed by scarification, rejuvenation, and compaction by an HIR train equipped with all the construction units. The cost-effectiveness of in-place recycling might be attributed to the saving of asphalt binder and virgin aggregates, lower traffic disruption, and lower transportation costs ( 8 ). Repaving with HMA overlay is applied when HIR is not sufficient to restore the required pavement properties ( 9 ).

A series of studies have explored the concept of incorporating RAP (up to 30%) during HMA production in asphalt plants. Results have shown that the incorporation of RAP can increase the stiffness but also reduce the dissipated creep strain energy of the asphalt mixtures ( 5 , 10 , 11 ). The brittleness of the RAP binder is considered a significant factor that would attenuate the cracking resistance of asphalt mixtures and, therefore, the pavement’s durability ( 12 ). The aforementioned cracking problems might be exacerbated during HIR procedures since 100% RAP is commonly used for rehabilitation. Zhong et al. ( 13 ) investigated the binder aging level during HIR of asphalt pavement. They concluded that extra aging caused by HIR was not significant and the aging level of the bottom part is smaller than the surface part. To improve the properties of the HIR mix, recycling agents were used to recover the chemical and mechanical properties of the RAP binder in HIR ( 14 , 15 ). Hesham and Bonaquist ( 16 ) suggested that the addition of recycling agents could restore the recycled binder PG grade close to its original condition. A new additive, named Styrene-butadiene rubber (SBR) latex, was also introduced to enhance the moisture susceptibility and low-temperature cracking resistance of the HIR mix ( 17 , 18 ). Preheating conditions including temperatures and heating times were critical to the performance of HIR mixes ( 19 ). Ma et al. ( 6 , 20 ) proposed that a proper increase in temperature and adequate use of a rejuvenator help improve the mobilization rate of the RAP binder and with it the performance of the HIR mix. Ali and Grzybowski conducted a case study involving a life cycle cost analysis (LCCA) between HIR and conventional pavement rehabilitation. Projections reflected that HIR could result in a reduction of more than 40% in the initial cost ( 21 ). Cao et al. ( 22 ) assessed the cost and environmental concerns between HIR and the conventional milling and filling techniques with assumed service life, indicating that HIR could save 5% of the costs and reduce by 16% the overall environmental impact.

In contrast to HMA mixtures in asphalt plants, HIR mix is produced in situ with 100% RAP asphalt emulsion, using a fire heating method, but a lower mixing and compaction temperature, which might lead to different mixture performances and pavement service life. Furthermore, the quality control and LCCA of the pavements between HIR and HMA are also worth investigating. This study, therefore, aims to conduct comprehensive comparisons between HIR and HMA mixes and pavements, including performance evaluation, pavement life prediction, and LCCA. To achieve this, HIR mixes from three different projects were collected and recompacted in the laboratory. One common HMA surface mix was also obtained from the asphalt plant for comparison. The asphalt mixture performance tester (AMPT), superpave indirect tensile strength (IDT) tests, and tensile strength ratio (TSR) tests were adopted for performance evaluation. Field cores were collected to assess the in situ construction qualities. ME software was used for modeling and pavement life prediction.

Materials Preparation

HIR and HMA surface mixes were collected from three different construction sections and asphalt plants in Tennessee. All the materials came from a certain region, indicating similar aggregate types. The grain size distributions of the raw materials are shown in Figure 1. Both HIR and HMA mixes follow a similar gradation, which generally matches the typical surface mix in Tennessee. One cationic asphalt emulsion was used as a recycling agent to restore the binder properties of RAP. The asphalt contents of HIR and HMA mixes were determined through the centrifuge extraction test. As shown in Table 1, the asphalt contents of HIR mixes are generally higher than HMA since HIR mix contains the asphalt binder in RAP and asphalt emulsion. The incorporation of recycling agents would soften and activate more RAP binders and increase the effective binder contents of HIR mixes ( 20 ). The asphalt contents in three HIR mixes represent the different dosages of asphalt emulsion incorporated during construction. The loose HIR mixes were reheated and recompacted for performance testing. Two replicates of AMPT tests and three replicates of IDT tests were conducted. Twelve field cores with a diameter of 150 mm were also extracted from the pavements before and after HIR procedures in each section, followed by cutting the top 50 mm for further performance testing.

Table 1.

Asphalt Content of HIR and HMA Mixes

Type of mixes	HIR mix 1	HIR mix 2	HIR mix 3	HMA
Asphalt content (%)	6.93	7.32	7.68	6.0

Note: HMA = hot-mix asphalt; HIR = hot in-place recycling.

Experimental Methodology

Mixture Performance Tests

AMPT Tests

Dynamic modulus tests and flow number tests were included in this study. In the dynamic modulus test, the trimmed specimens with a diameter of 100 mm and a height of 150 mm were placed in an environmental chamber and conditioned at 4°C, 20°C, and 40°C. The vertical deformations were captured via three linear variable differential transformers (LVDT) at 120° angles on the side. Tests were performed under an axial haversine load with various frequency sweeps at different temperatures. The dynamic modulus was calculated based on the collected stress and strain.

The flow number test is an efficient approach to characterize the creep properties of asphalt mixtures. The specimens are subjected to a continuous repeated load until the deformation reaches 3,000 cycles or 50,000 microstrains. The flow number is denoted as the cycles in which the rate of the permanent strain rate is achieved, reflecting the rutting potential of the mixtures ( 23 ). In general, a higher flow number indicates less rutting potential of the asphalt pavement.

Superpave IDT Tests

Superpave IDT tests consist of the resilient modulus test and the IDT strength test. As for the resilient modulus test, a specimen 150 mm in diameter and 50 mm in thickness is subjected to a repeated load with a 0.1 s loading and a 0.9 s unloading rest period. Four strain gauges are placed in the middle to record the horizontal and vertical deformations. The resilient modulus (Mr) is calculated by the ratio of the applied stress and recoverable strain.

For the IDT strength test, the load was applied to the same specimen at a constant rate of 50 mm/min until the failure occurs. The stress and strain curves are constructed according to the collected force and deformation. The dissipated creep strain energy threshold (DCSEf) can be determined by the difference between fracture energy (FE) and elastic energy (EE) in the stress-strain curves ( 24 ).

Moisture Susceptibility Tests

Two types of moisture susceptibility tests were conducted to evaluate the moisture resistance of HIR and HMA mixes: the immersion conditioned (IM) test, and the freeze-thaw-conditioned (F-T) test. The asphalt mixtures were compacted into a cylinder with a diameter of 150 mm and a height of 50 mm at 7% ± 0.5% air voids. One group of specimens was immersed in a water bath at 60°C for 24 h, followed by conditioning at 25°C for 2 h, while the other group of specimens was conditioned in the freeze-thaw machine for one cycle before further IDT strength testing. The tensile strength ratio (TSR) between the conditioned and unconditioned specimens was calculated to evaluate the moisture resistance of the HIR and HMA mixes.

Pavement Life Prediction

ME Pavement Design

The ME software was used to predict the performance and life cycle between the pavement after HIR surface treatment and the construction of a new HMA surface layer. The pavement performance models were locally calibrated to ensure the reliability of the prediction ( 25 , 26 ). The traffic, climate, and pavement structures were determined according to the field project condition. The pavement layer and thickness were adopted following the Tennessee DOT pavement design guide. The average two-way annual daily truck traffic (AADTT) was calculated as 2,000. The design life was set as 20 years. The pavement structure models are presented in Figure 2. D-mix, a dense mix of HMA, is applied as the surface layer, while BM2-mix is used as the binder layer in Tennessee. Afterward, the pavement design life was reduced until all the pavement performance satisfied the pavement design criteria, which determined the life cycle of the pavement.

Binder Level-1 Data Inputs

As for binder properties, the binders of HIR mixes before and after rejuvenation were extracted and recovered for dynamic shear rheometer (DSR) frequency sweep tests at different temperatures. Asphalt is assumed to be fully blended with the RAP binder, which is the ideal blending scenario for HIR. One unmodified asphalt PG 64-22 and one SBS modified asphalt PG 76-22 were used for pavement prediction and comparison with the HIR surface mixes. DSR master curves were constructed based on superposition principles ( 27 , 28 ). Figure 3 and Table 2 present the DSR master curves and input parameters, respectively. It can be seen that the stiffness of the RAP binder is much higher than the base asphalt, especially in low frequency (high temperature). The addition of additives could soften the RAP binder and improve the rheological properties of the binder blends.

Table 2.

Binder Properties Design Input at 10 rad/s

PG 64-22				PG 76-22
Temp (F)	Temp (°C)	G* (Pa)	Phase angle (°)	Temp (F)	Temp (°C)	G* (Pa)	Phase angle (°)
59	15	676,435.5	65.3	59	15	1,146,239	61.8
77	25	116,630.4	72.1	77	25	245,153	64.9
95	35	15,805	78.7	95	35	45,049	64.9
HIR mix				HIR RAP
Temp (F)	Temp (°C)	G* (Pa)	Phase angle (°)	Temp (F)	Temp (°C)	G* (Pa)	Phase angle (°)
59	15	14,782,760	43.63	59	15	29,618,406	31.32
77	25	6,162,031	51.34	77	25	11,253,999	38.39
95	35	862,668	58.55	95	35	3,055,880	46.59

Note: HIR = hot in-place recycling; RAP = reclaimed asphalt pavement; PG = performance grade.

Mixture Level-1 Data Inputs

The asphalt mixtures from different pavement layers were used to manufacture the cylinders for dynamic modulus tests at different temperatures. The dynamic modulus inputs for different asphalt mixtures are shown in Table 3.

Table 3.

Dynamic Modulus of Asphalt Mixtures in Different Pavement Layers

Temperature (°F)	Frequency (Hz)
	0.1	0.5	1	5	10	25
Dynamic modulus of HIR mix (psi)
14	2,132,499	2,365,736	2,455,186	2,637,494	2,705,520	2,786,409
40	1,274,491	1,577,604	1,705,456	1,987,229	2,099,961	2,239,597
70	465,389	697,235	797,339	1,078,398	1,206,781	1,379,401
100	118,114	202,981	252,804	404,709	486,601	609,898
130	28,435	50,623	65,020	115,253	146,316	198,403
Dynamic modulus of D-mix with PG 64-22 (psi)
14	1,751,298	2,076,674	2,203,310	2,461,892	2,557,819	2,670,907
40	7,922,916	1,135,249	1,290,768	1,650,911	1,799,975	1,987,100
70	169,227.5	323,025.6	412,935.4	676,585.7	810,901.1	1,002,730
100	23,123.3	53,610.5	75,852.6	160,645.3	215,656.8	309,022.5
130	3,542.3	7,965.7	11,464.9	26,980.4	38,839.2	62,101.8
Dynamic modulus of D-mix with PG 76-22 (psi)
14	2,520,385	2,722,695	2,795,313	2,934,413	2,983,130	3,038,682
40	1,348,727	1,709,169	1,858,036	2,175,995	2,298,423	2,445,601
70	256,367	452,466	556,287	847,530	990,656	1,190,588
100	31,996	58,908	77,229	144,498	187,781	261,992
130	7,208	10,729	13,062	21,818	27,808	38,943
Dynamic modulus of BM-2 mix (psi)
14	2,504,737	2,705,062	2,777,478	2,917,199	2,966,520	3,023,069
40	1,347,969	1,699,086	1,844,327	2,155,694	2,276,201	2,421,692
70	271,373	457,767	560,217	845,515	985,071	1,179,726
100	31,920	59,384	77,993	145,757	188,989	262,624
130	6,571	10,052	12,371	21,116	27,111	38,251

Note: HIR = hot in-place recycling; RAP = reclaimed asphalt pavement; BM = bituminous macadam; PG = performance grade; psi = pounds per square inch.

Design Criteria and Thresholds

A series of pavement performances were analyzed at a design life of 20 years, including the international roughness index (IRI), top-down fatigue cracking, bottom-up fatigue cracking, thermal cracking, and pavement deformations. The performance criteria were summarized in Table 4 to achieve a fair pavement condition with Tennessee pavement design criteria (Table 5).

Table 4.

Design Parameters Thresholds in ME Design

Design input	Limit	Reliability
IRI (in./mile)	170	90
AC top-down fatigue cracking (% lane area)	20	90
AC bottom-up fatigue cracking (% lane area)	20	90
AC thermal cracking (ft/mi)	1,000	90
Permanent deformation—total pavement	0.4	90
Permanent deformation AC only (in)	0.25	90

Note: ME = Mechanistic-Empirical; IRI = international roughness index; AC = asphalt content.

Table 5.

Tennessee Pavement Design Criteria

Performance criteria	Good	Fair	Poor
IRI (in./mi)	<95	95–170	>170
Cracking (%), asphalt pavements	<5	5–20	>90
Cracking (%), jointed concrete pavements	<5	5–15	>90
Rutting (in.)	<0.20	0.20–0.40	>0.40
Faulting (in.)	<0.10	0.10–0.15	>0.15

Note: IRI = international roughness index.

LCCA

The estimated uniform annual cost (EUAC) method expresses life cycle costs as an annualized estimation of cash flow instead of a lump-sum estimation of the present value. The EUAC values of HIR and conventional HMA were calculated using the following equation:

EUAC = Cost \frac{i {(1 + i)}^{n}}{{(1 + i)}^{n} - 1}

(3)

where i = discount rate, and n = number of years

Meanwhile, the LCCA of the pavement after 48 years was also plotted to evaluate the cost effectiveness of using an HIR surface treatment compared with the HMA surface layer.

Results and Analysis

Performance Tests

AMPT Test Results

Figure 4, a to c , plots the dynamic modulus test results of HIR, and HMA mixes at different temperatures. The former study reflects that the dynamic modulus of asphalt mixtures is sensitive to the change of asphalt binders ( 29 ). The mixtures with higher asphalt content present a lower dynamic modulus at all temperatures and frequency sweeps in HIR mixes. A larger proportion of HIR binder generates a softer asphalt mixture, indicating the pavements’ greater rutting potential. Compared with HIR mixes, HMA mixes present a larger dynamic modulus value at low temperatures (4°C) and intermediate temperatures (20°C) but a lower value at high temperatures (40°C). It can also be noticed that HMA mix has less asphalt content than HIR mixes but provides stiffer performance by the asphalt mixtures, especially at low temperatures. Such differences might be attributable to the better adhesion and coating capability between the virgin asphalt and the aggregates. As the temperature increases, the virgin binder tends to be softened more severely than the RAP binder, indicating more rutting issues with the asphalt mixtures. A lower flow number value of HMA, obtained at 54.4°C in Figure 4d, also validates more rutting potential of HMA pavements at high service temperatures.

Superpave IDT Results

Figure 5, a to c , illustrates the resilient modulus, IDT strength, and DCSE_f test results of HIR and HMA mixes at room temperatures. The resilient modulus test is applied to characterize the elastic behavior of asphalt mixtures. As for HIR mixes, it is shown that HIR mix with higher asphalt content has a lower resilient modulus, indicating the increasing ductility of the asphalt mixtures. HMA mixes exhibit the lowest resilient modulus compared with HIR mixes as a result of the better ductility of virgin binder than the aged binder.

DCSE_f reflects a threshold for cracking initiation and propagation of the asphalt mixtures. In general, asphalt mixtures with a higher DCSE_f value have improved cracking resistance ( 30 ). As shown in Figure 5, b and c , the IDT strength and DCSE_f of the HMA mix are much higher than the HIR mixes, even though it contains the lowest asphalt content. Therefore, the HMA mix tends to have better cracking resistance than the HIR mix. The high production temperature of HMA with virgin asphalt would contribute to an improved coating and adhesion between the binder and aggregates. However, for the HIR mix, the incomplete activation of RAP binder through fire heating, the low mixing, compaction temperature, and the large proportion of RAP binder would result in a weaker coating with aggregates and, therefore, compromised cracking resistance in the asphalt mixtures. In addition, the major concern of the HIR mix is the cracking issue, which can be potentially improved by using more additives and increasing the mixing temperatures.

Moisture Susceptibility Test Results

The immersion conditioned test and freeze-thaw conditioned test were adopted to evaluate the moisture resistance of the asphalt mixtures, denoted as TSR (IM) and TSR (F-T), respectively, in Figure 6. Freeze-thaw conditioning has more severe moisture damage than the immersion conditioning method. It can be seen that the HMA mix has better moisture resistance than the HIR mix with regard to both moisture damage scenarios. The TSR values for HIR mix with different asphalt content did not vary a lot. The TSR values for HIR mix underwater immersion conditions could satisfy the minimum criteria for Superpave asphalt mixture design (TSR ≥ 0.80). Further strategies should be considered to improve the moisture resistance of HIR mixes, especially for areas with large temperature differences.

Field Cores Evaluations

Field cores from one HIR section were collected from the pavement before and after HIR surface treatment. The air voids of old and new cores are around 4% and 7%, respectively. The cores were trimmed from the top 50 mm for further Superpave IDT tests. As a comparison, the collected HIR loose mixes were also compacted at 7% and 4% air voids in the lab to simulate the pavement condition after construction and long service life. Figure 7, a and b , illustrates the indirect tensile strength and DCSE_f test results of the cores and the lab compacted specimens. It can be noticed that the IDT strength and dissipated creep strain energy of the old pavement decrease by around 40%, corresponding to their initial conditions. This means that the quality of the pavement surface was mitigated by the cracking issues during the service life, which required surface rehabilitation. The new cores and lab compacted HIR mix present similar values for the IDT strength and DCSE_f, indicating a good quality control of the pavement surface treatment through HIR procedures.

Pavement Life Prediction

The ME software was adopted to predict the pavement’s service life through HIR and HMA surface rehabilitation considering different pavement performance criteria. The predictions for pavement performances after 20 years are presented in Table 6. Compared with the pavements constructed by the new HMA surface layer, the pavements after HIR procedures present similar permanent deformations but a higher fatigue cracking area and thermal cracking length. Both the bottom-up fatigue cracking area and the thermal cracking length fail to satisfy the criteria. The pavement after HIR procedures is, therefore, not susceptible to rutting issues, whereas it tends to encounter bottom-up fatigue cracking problems, especially thermal cracking. The pavement prediction results agree with the laboratory performance test results.

Table 6.

Pavement Performance After 20 Years

Performance criteria	PG 64-22	PG 76-22	HIR	Limit	Reliability
IRI (in./mile)	158.24	158.46	172.87	170	90
AC top-down fatigue cracking (% lane area)	4.69	12.91	14.13	20	90
AC bottom-up fatigue cracking (% lane area)	58.44	58.44	64.47	20	90
AC thermal cracking (ft/mile)	934.59	485.08	1,727.52	1,000	90
Permanent deformation—AC only (in.)	0.22	0.12	0.20	0.25	90

Note: PG = performance grade; HIR = hot in-place recycling; IRI = international roughness index; AC = asphalt content.

Table 7 presents the life cycle of pavements using different rehabilitation approaches. Based on the combination of fatigue cracking and thermal cracking prediction, a pavement’s life cycle after the HIR technique is around eight years, four years shorter than the pavement with the new HMA surface layer.

Table 7.

Life Cycles of the Pavement After Different Rehabilitation Approaches

Reference	Types
Reference	PG 64-22	PG 76-22	HIR
Based on fatigue cracking	12 years	12 years	10 years
Based on thermal cracking	20 years	Over 20 years	8 years

Note: HIR = hot in-place recycling; PG = performance grade.

Life Cycle Cost Analysis

The EUAC method was applied to evaluate the economic value of different alternatives, reflecting the life cycle cost of the pavement. As for the 2-in. surface layer, the average cost of the HIR section is 4.02 $/yd² (19,778.1$/lane mile), while that of the convention section via HMA milling and filling is $12.8/yd² ($62,975.05/lane mile) in Tennessee. Correspondingly, assuming a discount rate of 4%, the EUAC of HIR over 8 years and the conventional pavement over 12 years are $ 0.60/yd² ($2,937.60/lane mile), and $1.36/yd² ($6,710.13/lane mile), respectively. Pavement rehabilitated through HIR could, therefore, offer up to 56% of the initial cost saving. The LCCA of 48 years between pavement after HIR construction and HMA built surface layer construction is plotted in Figure 8a. Assuming two new pavements after rehabilitation with HIR and HMA at the beginning, the pavements would last 8 years and 12 years for using HIR and HMA surface rehabilitation, respectively. The construction periods for the two techniques were assumed to be 1 year. The pavements were, therefore, rehabilitated at the end of the 7th and 11th years, while the construction cost started to increase afterward. It can be shown that, at the end of 47 years, the HIR techniques could save around $ 90,000 compared with the new HMA surface layer.

As mentioned in the input parameters, not all of the RAP binder in the HIR mix can be activated to coat the aggregates. The inputs of the binder properties in the HIR mix, therefore, tend to be the ideal condition. In the real scenario, a thin overlay was sometimes repaved to achieve a better performance of the pavement, which might alter the cost-effectiveness of the pavement. Assuming the pavement life is also 8 years with thin overlay and additional surface treatment, the LCCA of 48 years for the pavement is also presented in Figure 8b. It can be seen that HIR and HMA treatments perform similarly in the first two life cycles (20 years), while the HIR technique could save around 20% of the cost over the 48-year life cycle. The HIR surface treatment was, therefore, shown to be more cost-effective than the new HMA surface layer with regard to the long service life of the pavements.

It is worth remembering that the cost data were mainly considered to be the direct construction cost. However, the total cost includes some indirect costs such as user costs and environmental costs that were not included in this study. It also needs to be pointed out that the AADTT was adopted based on the traffic conditions of the low-volume road. The increase in traffic volume or load capacity would damage the HIR pavement surface more severely, which might lead to a shorter pavement life compared with the HMA surface treatment. In addition, the extended service life of the HIR surface layer would further reduce the quality of existing RAP aggregates and cause more aging of the RAP binder, which might affect the reliability of the LCCA. Lacking adequate pavement performance data in the field, future test sections and pavement monitoring should be conducted to validate the pavement life and performance between the HIR and HMA pavement surface rehabilitation techniques.

Conclusions

In this study, a comprehensive comparison of pavement surface rehabilitation using HIR and HMA was conducted, including the performance evaluation, pavement service life prediction, and LCCA. HIR mix from three different sections and the plant mix from the same region were collected, while field cores were also obtained to assess the pavement condition after HIR surface treatment. Table 8 summarized the major comparison results of the asphalt mixtures and pavement performances with HIR and HMA surface rehabilitation techniques. The main conclusions can be summarized as follows:

HIR mixes showed acceptable rutting and moisture resistance. Cracking resistance is the main issue that HIR mixes would encounter. HMA has a stronger coating between asphalt and aggregates than the HIR mix even with the lower asphalt binder content, indicated by higher IDT strength and DCSE_f.

The incorporation of recycling agents in the HIR mix would soften the RAP binder and increase the effective binder content of HIR mixes, which improve the ductility and cracking resistance of the asphalt mixtures.

The DCSE_f of field cores reflected the decrease of more than 40% in the cracking resistance of the existing pavement surface before HIR rehabilitation. The HIR technique showed consistent construction qualities as laboratory mixes, which could restore the cracking resistance of existing pavement.

ME prediction results indicated that pavement after HIR surface treatment would yield a larger value of roughness index and encounter severe fatigue cracking as well as low-temperature cracking issues.

LCCA results reflect the ability of HIR surface rehabilitation to achieve a saving of over 50% of the initial cost compared with the conventional HMA milling and filling technique. Along with the overlay, HIR surface rehabilitation is expected to save the construction cost for the whole life cycle. Various traffic volumes or load conditions and further pavement monitoring should be considered for LCCA validation.

For future studies, pavement monitoring is promoted to investigate the detailed differences between the HIR and HMA pavement conditions. Innovative approaches, including rejuvenators or heating techniques, should be developed to improve the cracking resistance of the HIR mix. In addition, detailed energy consumption, emissions, and toxicity potential data need to be collected to evaluate the environmental effect of using HIR for pavement rehabilitation.

Table 8.

A Summary of Asphalt Mixtures and Pavement Performance With Two Rehabilitation Techniques

Mixture performances and pavement response	HIR	HMA milling and filling
Mixtures
Flow number (cycles)	High	NA
Resilient modulus at 25°C (psi)	High	NA
IDT strength at 25°C (psi)	NA	High
DCSE_f at 25°C (KJ/m³)	NA	High
TSR(I-M) & (F-T)	NA	High
Pavement
IRI (in./mile)	High	NA
Fatigue cracking (% lane area)	High	NA
Thermal cracking (ft/mile)	High	NA
Permanent deformation (in)	NA	Little high
Predicted life (years)	8	12
Initial cost saving	Up to 50%	NA
Cost saving of 48 years	Up to 20%	NA

Note: NA = Not available.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Baoshan Huang, Yuetan Ma; data collection: Yuetan Ma, Pawel Polaczyk; analysis and interpretation of results: Yuetan Ma Miaomiao Zhang; draft manuscript preparation: Rui Xiao, Xi Jiang. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The author would like to thank the Tennessee Department of Transportation (DOT) for offering financial support and conducting the field project (RES2019-03).

ORCID iDs

Yuetan Ma

Pawel Polaczyk

Miaomiao Zhang

Rui Xiao

Baoshan Huang

The views reflect within this manuscript are those of the authors and do not necessarily represent the views of Tennessee DOT.

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