Degradation Law of Long-Term Performance in In-Service Emulsified Asphalt Cold Recycled Mixtures

Abstract

To investigate the performance degradation of emulsified asphalt cold recycled mixtures (CRM) during service, this study selected a 10 km section of the cold recycled layer (CRL) from the Changjiu Expressway reconstruction project as the research subject. The deterioration patterns of key pavement performance indicators—including the Pavement Condition Index (PCI), Riding Quality Index (RQI), Rutting Depth Index (RDI), and Pavement Structure Strength Index (PSSI)—were analyzed in relation to cumulative equivalent axle loads over a 7-year service period. Concurrently, comparative evaluations were conducted on the mechanical properties, water stability, high-temperature performance, low-temperature crack resistance, and fatigue characteristics between in-service and laboratory-prepared emulsified asphalt CRM. The results demonstrate that after seven years of service, the emulsified asphalt cold recycled pavement maintained excellent performance levels, with PCI, RQI, RDI, and PSSI values of 92.6 (excellent), 90.1 (excellent), 88.5 (good), and 93.4 (excellent), respectively. Notably, while the indirect tensile strength and unconfined compressive strength of the CRL increased with prolonged service duration, other performance metrics—including the tensile strength ratio, shear strength, fracture work, and fracture energy—exhibited an initial improvement followed by gradual deterioration. Additionally, increased traffic loading during service led to a reduction in the residual fatigue life of the CRM. Interestingly, the study observed a temporary improvement in the fatigue performance of CRM during the service period. This phenomenon can be attributed to three key mechanisms: (1) continued cement hydration, (2) secondary hot compaction effects, and (3) diffusion and rejuvenation between fresh and aged asphalt binders. These processes collectively contributed to the partial recovery of aged asphalt strength, thereby improving both the mechanical properties and overall road performance of the CRM. The findings confirm that cold recycled pavements exhibit remarkable durability and maintain a high service level over extended periods.

1. Introduction

The emulsified asphalt cold recycled mixture denotes an asphalt mixture intended for maintenance purposes. It involves the process of milling, recycling, crushing, screening, followed by the addition of suitable new aggregates and emulsified asphalt [1,2,3,4,5]. This technology is a green, low-carbon, energy-saving, and environmentally friendly highway maintenance technology. Owing to its significant economic and environmental benefits, it has been extensively utilized in highways, urban main roads, and local roads [6,7,8].

In terms of the economy benefit, multiple research institutions [9,10,11,12,13,14] have shown that in the reconstruction projects of semi-rigid base asphalt pavement, adding a semi-flexible cold recycled asphalt layer can effectively suppress reflection crack, greatly reducing construction costs. According to research conducted in Pennsylvania, United States [15,16,17], the incorporation of a cold recycled base layer with equal thickness for structural layer provides better anti-reflection crack performance compared to directly adding an asphalt mixture layer. Moreover, the engineering cost of implementing a cold recycled layer is only two-thirds of the asphalt mixture layer. As such, the cold recycled asphalt layer presents immense potential for application.

As a semi-flexible layer, the material composition and strength formation mechanisms of cold recycled mixture are different with hot asphalt mixtures and cement (or lime) stabilized materials [18,19,20,21,22]. In terms of material composition, the cold recycled mixture is composed of aged asphalt, aggregate, and newly added components like new asphalt and minerals. The bonding, frictional resistance, and cohesive force from cement, mineral interlocking, and interaction between new and aged asphalt contributes to the strength of the cold recycled mixture [23,24]. The performance of the cold recycled asphalt mixture is affected by the curing conditions, total water content in the mix, content of aged asphalt, dosage of emulsified asphalt and cement, aggregate gradation, etc. [13,25,26,27,28].

In general, the long-term performance of cold recycled mixtures has aroused concerns among road workers. For example, Mamlouk et al. [29] placed the emulsified asphalt cold recycled mixtures in an oven at 60 °C for 60 days to simulate its long-term performance. It was found that the alterations in creep stiffness and Marshall stability of emulsified asphalt cold recycled mixtures were not markedly distinct with mixed asphalt mixtures. Andrea et al. [30] examined the moisture content and indirect tensile strength of emulsified asphalt cold recycled materials containing 1% and 2.5% of cement content at 25 °C and 40 °C, respectively, for 100 days. Lida [31] investigated the long-term behavior of cold recycled mixtures with different rejuvenators through creep testing. The study demonstrated that the new asphalt coating recycled asphalt pavement (RAP) materials gradually diffuse into the aged asphalt over time, consequently affecting the long-term performance of cold recycled mixtures. These findings further confirm that the performance of cold recycled mixtures is significantly influenced by external factors, exhibiting continuous variations during the early and intermediate stages of service life. Peter E. Sebaaly et al. [32] proposed a three-stage curing method to evaluate the performance of cold recycled mixtures. Using a three-stage curing method, Deng et al. [33] analyzed the curing time on the performance of cold recycled mixtures. The findings revealed that the split tensile strength and dynamic modulus increased quickly from the early-stage to the mid-stage. Chen [34] employed molecular dynamics simulation to analyze the temperature-dependent diffusion coefficients of aged asphalt components at the interface between virgin and recycled binders. The study revealed that in cold recycled mixtures, the gradual interdiffusion between aged and new asphalt binders eliminates micro-voids at the interface, enabling synergistic bonding effects. This molecular-scale investigation provides fundamental insights into the performance evolution mechanisms of cold recycled asphalt mixtures. The research of Tian Yingchun et al. [35] demonstrated that there existed the asphalt redistribution process in service for cold recycled pavement. Liu et al. [36] demonstrated that the mechanical properties of cold recycled base courses continue to improve for 2–5 years after project completion, with splitting tensile strength potentially increasing up to 3.48 times that of newly compacted mixtures. Their field investigations revealed that the compressive resilient modulus of emulsified asphalt cold recycled cores significantly exceeds that of conventional ATB (Asphalt Treated Base) layers. These findings provide critical insights for developing advanced mix design methodologies for cold recycled materials.

Most existing research on emulsified asphalt cold recycled mixtures has primarily focused on laboratory-prepared samples, with limited investigation into their long-term performance degradation under actual service conditions. To address this gap, this study examines the long-term deterioration mechanisms of emulsified asphalt cold recycled pavement based on real-world engineering data, analyzing its performance after seven years of service. Additionally, laboratory-prepared mixtures with identical materials and gradation were used as a control group for comparative evaluation.

2. Raw Materials and Test Scheme

2.1. Sample Preparation

The samples utilized in this paper are composed of two parts. One is in-service emulsified asphalt cold recycled core samples from the cold recycled layer of the highway. Another sample is formed in the laboratory using the materials and gradation in the highway cold recycled layer. The pavement structure and core sampling position are depicted in Figure 1. The in-service emulsified asphalt mixture samples were drilled from the carriageway and passing lane, with the core sampling position located directly beneath the wheel track. The diameter of the core sample is 100 ± 2 mm, and the core sample is cut into samples with a height of 63.5 ± 1.3 mm for testing, including indirect tensile strength test, unconfined compressive strength test, shear strength test, low-temperature splitting test, and fatigue test.

To compare the performance of emulsified asphalt cold recycled mixture in-service and laboratory, the samples were formed in the laboratory using the materials and gradation in-service. The performance of the raw materials is delineated in Table 1. The gradation design persistently adjusts the ratios of well-graded coarse, medium, and fine RAP to align with the gradation of the in-service emulsified asphalt cold recycled mixture. The comparative analysis of the two gradations is presented in Figure 2. The mass of cement amounts, optimal emulsified asphalt content (OEC), and optimal water content are 2%, 3%, and 3.8%, respectively.

The samples in this study were prepared using the Marshall compaction method proposed by our research group, which incorporates the principle of “secondary thermal compaction” [37]. Specifically, the samples were initially compacted with 100 blows on each side at room temperature, then placed sideways on a flat indoor surface for 24 h of ambient curing. Subsequently, they were transferred to an oven at 60 °C for 48 h of thermal curing. After removal from the oven, the samples were subjected to an additional 50 blows on each side, followed by cooling to room temperature (typically 6 h) before demolding. For large Marshall samples, the compaction sequence was adjusted to “150 + 75” blows.

The Semi-Circular Bend (SCB) samples were obtained by cutting the large Marshall samples, following the cutting procedure recommended in the Technical Specifications for Construction of Emulsified Asphalt Cold Recycling in Highway Asphalt Pavements (DB13/T-2020) [38]. The SCB samples had a thickness of 45 mm and a height of 75 mm, featuring a central notch with a depth of 15 mm ± 2.5 mm and a width of 2.5 mm ± 1 mm.

Table 1. Properties of emulsified asphalt.

Test Items		Unit	Results	Requirement	Test Method
Residue on sieve (1.18 mm)		%	0.03	≥0.1	T0652-1993 [39]
Particle charge		-	Cation	Cation	T0653-1993 [39]
Demulsification rate		-	Slow breaking	Slow or medium breaking	T0658-1993 [39]
Viscosity (Sabot viscosity Vs)		s	24.98	7~100	T0621-1993 [39]
Evaporated residue content		%	62.6	≤62	T0651-1993 [39]
Evaporated residue	Penetration 25 °C	0.1 mm	71.1	50~300	T0604-2000 [39]
	Ductility 15 °C	cm	56.8	≤40	T0605-1993 [39]
	Softening point 5 °C	°C	45.8	-	T0606-2000 [39]
	Solubility	%	—	≤97.5	T0607-1993 [39]
Storage stability 1 d		%	0.8	≥1	T0655-1993 [39]
Storage stability 5 d		%	—	≥5	T0655-1993 [39]
Adhesion to coarse aggregate		-	>2/3	≤2/3	T0654-1993 [39]

Table 1. Properties of emulsified asphalt.

Test Items		Unit	Results	Requirement	Test Method
Residue on sieve (1.18 mm)		%	0.03	≥0.1	T0652-1993 [39]
Particle charge		-	Cation	Cation	T0653-1993 [39]
Demulsification rate		-	Slow breaking	Slow or medium breaking	T0658-1993 [39]
Viscosity (Sabot viscosity Vs)		s	24.98	7~100	T0621-1993 [39]
Evaporated residue content		%	62.6	≤62	T0651-1993 [39]
Evaporated residue	Penetration 25 °C	0.1 mm	71.1	50~300	T0604-2000 [39]
	Ductility 15 °C	cm	56.8	≤40	T0605-1993 [39]
	Softening point 5 °C	°C	45.8	-	T0606-2000 [39]
	Solubility	%	—	≤97.5	T0607-1993 [39]
Storage stability 1 d		%	0.8	≥1	T0655-1993 [39]
Storage stability 5 d		%	—	≥5	T0655-1993 [39]
Adhesion to coarse aggregate		-	>2/3	≤2/3	T0654-1993 [39]

2.2. Test Methods and Evaluation Index

2.2.1. Evaluation Methods for Emulsified Asphalt Cold Recycled Mixture In-Service

(1)

Pavement surface condition index (PCI)

The pavement surface condition index (PCI) is an index that indicates the integrity of the pavement. It combines the quantitative conditions of damage type, damage degree, and damage scope or density. It can be evaluated through the deduction method. The calculation formula is shown in Equation (1):

$$PCI=100-{\mathrm{a}}_0\cdot D{R}^{{\mathrm{a}}_1}$$

(1)

where a₀ and a₁ are calibration coefficients, for asphalt pavement, the coefficients are 15.0 and 0.412; DR is damage ratio of asphalt concrete pavement, which is the ratio of the sum of the converted damage areas of various pavement damages to the investigated pavement area, %.

(2)

Riding quality index (RQI)

Riding quality index (RQI) is an indicator of roughness for the road. It has an important impact on the driving stability and comfort of vehicles. The calculation formula is shown in Equation (2):

$$RQI={\displaystyle \frac{100}{1+a_0{e}^{a_{1}IRI}}}$$

(2)

where IRI is International Roughness Index (m/km); a₀ is 0.026 and 0.0185 for expressways and class I highways and other highways; a₁ is 0.65 and 0.58 for expressways and class I highways, and other highways.

(3)

Rutting depth index (RDI)

Rutting depth index (RDI) refers to the wheel indentation by the vehicle when driving on the road. Pavement rutting is an important index for the regular evaluation and maintenance of the pavement. The rutting depth of the pavement directly reflects the comfort of vehicles, the safety, and service life of the pavement. The calculation is shown in Equation (3):

$$RDI=\left\{\begin{array}{c}100-a_{0}RD\left(RD\le {RD}_a\right)\\ 90-a_1\left(RD-{RD}_a\right)\\ 0\left(RD>{RD}_b\right)\end{array}\right.\left(R{D}_a<RD\le {RD}_b\right)$$

(3)

where RD is rutting depth (mm); RD_a is rutting depth parameter, which is 10.0; RD_b is rutting depth parameter, which is 40.0; a₀ is model parameter, which is 1.0; a₁ is model parameter which is 3.0.

(4)

Pavement structure strength index (PSSI)

Pavement structure strength index (PSSI) refers to the ability of asphalt pavement to resist damage and deformation under various loads and environmental factors. It is one of the important indicators to evaluate the quality of road construction and maintenance. It can be calculated by Equations (4) and (5):

$$PSSI={\displaystyle \frac{100}{1+a_0{e}^{a_{1}SSI}}}$$

(4)

$$SSI={\displaystyle \frac{l_d}{l_0}}$$

(5)

where: SSI is the strength coefficient of pavement structure; l_d is the design deflection of pavement, mm; l₀ is the measured representative deflection, mm; a₀ is the model parameter, which is 15.71; a₁ is the model parameter, which is −5.19.

2.2.2. Test Methods and Evaluation for the Performance of Cold Recycled Mixtures

(1)

High-temperature performance test

Uniaxial penetration test is used to evaluate the high-temperature performance of asphalt mixture. Marshall samples with a diameter of 101.6 ± 0.25 mm and height of 63.5 ± 1.3 mm were used for laboratory tests. Universal material testing machine is used to complete the loading and data recording of tests. The test temperature is 60 °C ± 0.5 °C, the loading rate is 1mm/min, and the diameter of loading head is 28.5 mm. The high-temperature performance of the mixture is evaluated by the shear strength. The shear strength calculation is shown in Equation (6). The uniaxial penetration test is listed in Figure 3.

$$R_{\tau }=f_{\tau }\times {\displaystyle \frac{P}{A}}$$

(6)

where R_τ is the shear strength, MPa; P maximum peak force of load, kN; A is the cross-sectional area of the indenter, mm²; f_τ is the penetration stress coefficient. When the samples diameter is 100 mm, the penetration stress coefficient f_τ is taken as 0.34.

(2)

Low-temperature performance test

Semi-circular bending (SCB) tensile test and low-temperature splitting test were used to evaluate the low-temperature performance of emulsified asphalt cold recycled mixture, as listed in Figure 4. For SCB test, emulsified asphalt cold recycled mixture samples were made following Technical Specification for cold recycled Construction of Emulsified Asphalt on Highway Asphalt Pavements (DB13/T-2020) [38]. The low-temperature crack resistance of the mixture is evaluated by fracture energy. The calculation of fracture energy is shown in Equation (7).

$$G_f={\displaystyle \frac{W_f}{A_{lig}}}$$

(7)

where W_f is the integral of the load force and displacement, J; A_lig is the ligament region, m²; A_lig = (r − a) × t; r is the sample’s radius, m; a is the incision length, m; and t is the sample’s thickness, m.

For the low-temperature splitting test, the preparation and evaluation of samples were performed according to Test Specification for Asphalt and Asphalt Mixtures in Highway Engineering (JTG E20-2011) [40]. The test temperature is −10 °C, and the loading rate is 1 mm/min. The low-temperature cracking resistance of the mixture is evaluated by the fracture work which is shown in Equation (8).

$$W={\displaystyle {\int }_0^{L}PdL}$$

(8)

where L is the deformation of the samples when the load reaches its maximum, mm; P is the maximum load force of the samples, kN.

(3)

Fatigue performance test

Indirect tensile fatigue test was applied to evaluate the fatigue performance of different samples. Universal testing machine was used to complete samples loading and data recording, as shown in Figure 5. The test mode is a half sine wave load with stress control. The stress ratios were 0.25, 0.3, 0.4, and 0.5, the loading frequency is 10 Hz, and the test temperature is 15 ± 0.5 °C.

(4)

Water sensitivity test

The water sensitivity is assessed by means of the freeze-thaw splitting strength ratio (TSR). The first category is designated for determining the splitting strength of the mixture under non-freeze-thaw circumstances, while the second category is utilized to determine the splitting strength of the mixture after freeze-thaw cycles. A vacuum saturation test is conducted on the second category, which was then placed in an environmental chamber at −18 ± 2 °C for 16 ± 1 h. Subsequently, samples were immersed in water at 60 ± 0.5 °C for 24 h. Finally, both categories of samples are immersed in water at 25 ± 0.5 °C for more than 2 h, and then subjected to a splitting test. The calculation is presented in Equation (9):

$$TSR={\displaystyle \frac{{\overline{R}}_{T_2}}{{\overline{R}}_{T_1}}}\times 100\%$$

(9)

where TSR is the strength ratio of freeze-thaw splitting test, %; ${\overline{R}}_{T_1}$ is the average splitting strength of the first category of samples, MPa; ${\overline{R}}_{T_2}$ is the average splitting strength of the second category of samples, MPa.

(5)

Mechanical performance test

The mechanical performance was evaluated using indirect tensile strength and unconfined compressive strength. The indirect tensile strength is analyzed using the same method as the low-temperature splitting test, while the testing temperature was changed to 15 °C. The unconfined compressive strength test method is based on the Test Specification for Asphalt and Asphalt Mixtures in Highway Engineering (JTG E20-2011) [38]. The test temperature is 15 °C, the loading speed is 2 mm/min, and maximum load P at the time of sample failure is recorded. The unconfined compressive strength is calculated as Equation (10):

$$R_c={\displaystyle \frac{4P}{\pi d^2}}$$

(10)

where R_c is compressive strength, MPa; P is maximum load at failure, N; d is the diameter of the samples, mm.

2.3. Experimental Scheme

(1)

The technical condition of cold recycled pavement changes with service life

Initially, a manual investigation was conducted to determine the various types of distress present on emulsified asphalt cold recycled pavement. Based on the Technical Evaluation Standards for Highway Conditions (JTG 5210-2018) [41], the damage types, weights, and conversion coefficients of each type of distress were utilized to determine the proportion of damage areas attributed to each type of distress. Then, from 2006 to 2014, the PCI, RQI, RDI, and PSSI of the emulsified asphalt cold recycled pavement were meticulously measured year by year using comprehensive testing equipment. Finally, the decay of emulsified asphalt cold recycled pavement was analyzed with service time (cumulative equivalent axle number of standard axle load).

(2)

Performance degradation of in-service emulsified asphalt cold recycled mixture

Core samples from cold recycled layers (passing lanes and carriageway) were utilized to evaluate the indirect tensile strength, unconfined compressive strength, shear strength, fracture energy, fracture work, and fatigue life. An analysis was conducted to determine the changes rule of performance of the emulsified asphalt cold recycled mixture with service time. The performance of emulsified asphalt cold recycled mixture in the laboratory and service was compared in order to reveal the degradation of performance.

3. Annual Traffic Volume Investigation

In order to quantify the disparity in the actual number of axle load times sustained by the passing lane and carriageway of the Changjiu Expressway, an on-site traffic volume survey was conducted. During the investigation, the vehicles were categorized into nine groups, including various types of passenger cars, freight cars, and containers. The average equivalent single-axle load (ESAL) factors are presented in

Table 2. Average Axle Load Conversion Factors for Each Vehicle Type.

Vehicle Type	Number of Seats or Load Capacity	Average Axle Load Equivalency Factor
Class 3 Bus	20–39 seats	0.12
Class 4 Bus	≥40 seats	0.48
Class 1 Truck	≤2 tons	0.12
Class 2 Truck	2.1–5 tons	0.43
Class 3 Truck	5.1–10 tons	1.12
Class 4 Truck	10.1–15 tons	2.50
Class 5 Truck	≥15.1 tons	5.00
Class 1 Container Vehicle	20′ container	2.33
Class 2 Container Vehicle	40′ container	6.22

. Subsequently, statistical analysis was conducted on the vehicle types captured in the video, and the corresponding number of equivalent axle load times were converted. The lane coefficient of carriageway and passing lane is 0.864 and 0.136, respectively. It is noted that the cumulative number of traffic axle load on the carriageway is approximately six times greater than that on the passing lane. Accordingly, the cumulative equivalent number of standard axle loads from 2007 to 2014 is as follows: 948,323, 2,146,312, 3,198,673, 5,121,212, 7,148,219, 10,265,501, 12,259,763, 14,809,891 times, respectively.

4. Test Results and Analysis

4.1. Technical Status of Emulsified Asphalt Cold Recycled Pavement in Service

4.1.1. Pavement Surface Condition Index (PCI)

According to the evaluation standard for highway technical condition (JTG 5210-2019) [42], the damage rate of the emulsified asphalt cold recycled layer pavement is calculated and converted into PCI. The results are reported as the average value of PCI for every five test results on a 10 km section. The decay law of PCI of emulsified asphalt cold recycled pavement from 2006 to 2013 is shown in Figure 6. As can be seen from Figure 6, with the increase of service time and vehicle load times, the PCI of cold recycled pavement continues to decline. In the first service period (from 2007 to 2009), the PCI of cold recycled pavement decreased significantly, and the representative value of the test section PCI decreased from 100 to 94.9. During the later service (from 2009 to 2013), the PCI of cold recycled pavement decreased from 94.4 to 92.6. The PCI for in-service cold recycled pavement decreased rapidly at first and then more slowly later. The main reason for this phenomenon is that in the early stage of service, due to the low early strength of the cold recycled mixture, the old road base layer reflects cracks under the action of driving load, which to some extent affects the PCI value. However, after seven years of service, the PCI of cold recycled pavement is still excellent, indicating that cold recycled pavement has good durability.

4.1.2. Rutting Depth Index (RDI)

According to the evaluation standard for highway technical condition JTG 5210-2019 [41], the measured rut depth is used to calculate the RDI of the cold recycled layer pavement based on Equation (3), and the results are shown in Figure 7. From Figure 7, it can be observed that with the increase of service life and cumulative equivalent axle passes, the RDI of the cold recycled pavement gradually decreases. After seven years of service, the RDI of the cold recycled pavement decreases to 88.5. In the first service period (from 2007 to 2009), the downward trend of RDI is more significant. During the later service period (from 2010 to 2013), the RDI decreases from 90.4 to 88.5. The RDI shows a trend of rapid decrease followed by a slow decline with the increase of service life. The main reason for this is that the cold recycled pavement is affected by extreme high-temperature weather in summer. The rutting deformation is caused by the shear deformation of the Hot Mix Asphalt (HMA) structural layer and the compaction deformation of the entire structural layer. Due to the lower early strength of the cold recycled pavement, it is more prone to compaction deformation under vehicle load. In addition, the compaction deformation of the HMA layer is mainly concentrated in the early service period. Therefore, in the first service period, the RDI of the cold recycled pavement decreases significantly. However, after seven years of service, there are no significant rutting distresses on the cold recycled pavement. The rut depth index decreases from the initial 100 to 88.5. The RDI graded is assessed as good, without significant damage or shear instability deformation in the cold recycled structural layer. This indicates that the emulsified asphalt cold recycled pavement has good resistance to rutting and durability.

4.1.3. Riding Quality Index (RQI)

According to the evaluation standard for highway technical condition JTG 5210-2019 [30], RQI is used to assess the level of pavement smoothness. The IRI is measured using a laser profilometer, and then it is used in Equation (2) to calculate the RQI. The results are shown in Figure 8. From Figure 8, it can be observed that the variation trend of RQI for the cold recycled pavement is consistent with the PCI and RDI. It shows a trend of rapid decrease followed by slow decay. In the first service period (2007–2009), the RQI of the cold recycled pavement decreases significantly with the increase of service life. During the later service period (2010–2013), the RQI decreases from 91.9 to 90.1, with a decrease of only 1.8. Furthermore, after seven years of service, the pavement quality assessment result is still excellent, indicating that the emulsified asphalt cold recycled pavement has good durability and excellent service level.

4.1.4. Pavement Structural Strength Index (PSSI)

The measured deflection ratio is used in Equation (4) to calculate the PSSI, and the results are shown in Figure 9. From Figure 9, it can be observed that the PSSI of the emulsified asphalt cold recycled pavement continues to decrease with the increase of service life and cumulative standard axle passes. During the seven-year service period (from 2007 to 2013), the PSSI decreases gradually from 100 to 93.4. The pavement bearing capacity assessment result is excellent, which is consistent with the conclusions obtained from the previous tests of PCI, RQI, and PSSI. This indicates that the emulsified asphalt cold recycled pavement has excellent durability performance. The pavement performance does not show excessive decay with the increase of service life and cumulative standard axle passes.

4.2. Performance Test Results of In-Service Emulsified Asphalt Cold Recycled Mixture

4.2.1. Mechanical Properties

During the service period (2006–2014) of the emulsified asphalt cold recycled pavement, the indirect tensile strength, unconfined compressive strength, and freeze-thaw splitting strength ratio results of the in-service core and laboratory samples were summarized in Figure 10.

As shown in Figure 10, the mechanical and moisture stability properties of the emulsified asphalt cold recycled core samples were significantly better than those of the laboratory cold recycled mixtures. With the increase of service time, the splitting strength and unconfined compressive strength of core samples from the passing lane and carriageway continuously increased. The freeze-thaw splitting strength ratio first increased and then decreased, but was still better than that of the cold recycled pavement in the initial service period. This indicates that the growth rate of mechanical and water stability of the emulsified asphalt cold recycled mixture during its service period is greater than the performance damage caused by vehicle loads and environmental effects.

In addition, for the same service life of the emulsified asphalt cold recycled pavement, the splitting strength and unconfined compressive strength of the core samples from the passing lane were higher than those of the emulsified asphalt mixture in the carriageway. This is mainly because the lane coefficient of the passing lane is smaller than that of the carriageway. As a result, the passing lane bears less traffic load, resulting in lower damage.

With the increase of service time, the mechanical properties of the emulsified asphalt cold recycled pavement showed increasing trend, which is related to curing time. On the one hand, with the increase of service time, cement in the emulsified asphalt cold recycled mixture gradually undergoes hydration reaction. This increases the strength of emulsified asphalt cold recycled pavement. On the other hand, during the construction of hot-mix asphalt mixture, due to the heat transfer from the upper layer hot-mix asphalt mixture and secondary compaction effect, the compaction degree of the cold recycled layer is further improved. Meanwhile, the effect of vehicle load during service also contributes to the improvement of mechanical properties of the cold recycled mixture. In addition, during long-term service, new and old asphalt are blended for a long time, which can stimulate the bonding strength of aged asphalt. In summary, the hydration reaction of cement, secondary hot compaction process, and blending of new and old asphalt can improve the mechanical properties of emulsified asphalt cold recycled mixture.

4.2.2. High-Temperature and Low-Temperature Performance

During the service period (2006–2014) of the emulsified asphalt cold recycled pavement, the performance of the in-service core samples from the carriageway, passing lane, and laboratory cold recycled mixtures were summarized in Figure 11.

As shown in Figure 11, for the in-service cold recycled pavement, whether the passing lane or the carriageway, the shear strength, fracture energy, and fracture work of the emulsified asphalt cold recycled mixture increased first and then decreased with the increase of service life. They were significantly higher than those of the laboratory newly-formed emulsified asphalt cold recycled mixture (in the initial service period). In addition, the growth rate of mechanical properties was highest in the first two years of service. For the passing lane and carriageway, the growth rates of shear strength, fracture energy, and fracture work were 45%, 65%; 30%, 40% and 42%, 52%, respectively.

The high-temperature performance and low-temperature performance of the core samples from the in-service pavement were better than those of the laboratory newly-formed cold recycled mixtures. This is mainly because the core samples from in-service pavement have undergone compaction deformation due to vehicle load during service. The deformation of the core samples during high-temperature performance test is relatively small. In addition, during long-term service, due to the aging asphalt exerting bonding strength, the cohesive force of the pavement core sample increases. At the same time, the bonding strength at the contact interface in the emulsified asphalt cold recycled mixture is improved. It helps to improve its low-temperature crack resistance. The compaction effect also improves the skeleton extrusion strength of the aggregate, which helps to improve its deformation resistance.

4.2.3. Fatigue Performance

After eight years of service, the fatigue life test results of the in-service core samples from the passing lane, carriageway, and laboratory emulsified asphalt cold recycled mixtures are summarized in

Table 3. Fatigue performance test results of emulsified asphalt cold recycled mixture.

Type	Stress Ratio (%)	Maximum Tensile Stress at the Center of the Sample (MPa)	Fatigue Life (Times)	Average Value (Times)	Standard Deviation
Passing lane	0.25	0.40	79,968	76,038	16,542
			90,262
			57,885
	0.30	0.47	20,900	27,680	5982
			29,923
			32,216
	0.40	0.64	9450	10,814	1936
			13,030
			9962
	0.50	0.78	2330	2256	1172
			3390
			1048
Carriageway	0.25	0.4	38,300	34,963	3124
			32,106
			34,482
	0.30	0.47	10,700	11,580	2689
			14,600
			9441
	0.40	0.64	4280	5798	1633
			5587
			7527
	0.50	0.80	860	1234	492
			1050
			1792
Laboratory	0.25	0.25	46,590	56,833	15,520
			49,220
			74,690
	0.30	0.30	16,926	19,470	2981
			18,733
			22,750
	0.40	0.41	5612	6680	1614
			8537
			5892

. For the in-service cold recycled pavement, the obtained fatigue life is the residue fatigue life of the cold recycled mixture, withstanding the actual axle load time for eight years.

The information in Table 3 is graphed on a dual logarithmic coordinate system, as depicted in Figure 12. It can be found that the fatigue life of various sample types in cold recycled blends and the tensile strain at the core of the sample demonstrate a favorable linear correlation on the dual logarithmic axes. As a result, the fatigue formula is chosen to take the shape of Equation (11):

$$N_f=k{({\displaystyle \frac{1}{{\sigma }^t}})}^n$$

(11)

where N_f is fatigue life, times; ${\sigma }^t$ is tensile stress at the center of the sample (${\sigma }^t={\displaystyle \frac{2P}{\Pi tD}}$; P is load, MPa; t is average height of samples, cm; d is sample diameter, cm); k and n are material parameters related to asphalt mixtures.

Based on these, the fatigue performance prediction equations for different types of samples in the laboratory are established, as shown in Equations (12)–(14).

$$\mathrm{Passing} \mathrm{lane}:N_f=897.2{({\displaystyle \frac{1}{{\sigma }^t}})}^{4.823} R_2=0.978$$

(12)

$$\mathrm{Carriageway}:N_f=536.2{({\displaystyle \frac{1}{{\sigma }^t}})}^{4.475} R_2=0.962$$

(13)

$$\mathrm{Laboratory}:N_f=113.3{({\displaystyle \frac{1}{{\sigma }^t}})}^{4.467} R_2=0.984$$

(14)

The relevance of the fatigue equations for the three types of cold recycled mixtures is very high, as evident from Equations (12)–(14). In the fatigue equation, the order of k parameter for different cold recycled mixtures from big to small is passing lane, carriageway and laboratory. This indicates that the passing lane has the highest residual fatigue. Therefore, it can be concluded that under the condition of the same maximum tensile stress at the center of the sample, the residual fatigue life of the passing lane for cold recycled mixture is greater than that of carriageway. It indicates that a greater number of axle load cycles will lead to a decrease in the fatigue life of in-service cold recycled mixtures. The laboratory cold recycled mixture does not bear the load of vehicle traffic. In theory, its fatigue life should be higher than that of in-service cold recycled mixtures. However, the fatigue equation shows that the fatigue life of the laboratory cold recycled mixture is lower than that of in-service cold recycled mixtures. It shows that there is a process of fatigue life growth for emulsified asphalt cold recycled mixtures during their service after laying. Previous studies [38,43,44] have demonstrated that the strength of emulsified asphalt cold recycled mixture increases after 2–5 years of service, and reaches a stable level after 2 to 5 years. The results of this study are consistent with this conclusion. This situation may be due to the long bonding and infiltration time of old materials and asphalt in in-service cold recycled mixtures, resulting in stronger interfacial bonding force and a longer fatigue life [45]. In contrast, in newly formed cold recycled mixtures, the interaction time between old materials and asphalt is short, resulting in weak interfacial bonding force and a shorter fatigue life.

5. Conclusions

As a green, low-carbon, energy-saving, and environmentally friendly highway maintenance technology, the cold recycled asphalt mixture has been widely used. This article examines the degradation rule of the cold recycled pavement that has been in service for seven years, yielding the following conclusions:

(1)

After seven years of service with a cumulative equivalent axle load of 12,259,763, PCI, RQI, RDI, and PSSI of emulsified asphalt cold recycled pavement decreased to 92.6 (excellent), 90.1 (excellent), 88.5 (good), and 93.4 (excellent), respectively. This indicates that the emulsified asphalt cold recycled pavement exhibited excellent durability and good service level.

(2)

As service time increased, the indirect tensile strength and unconfined compressive strength of the cold recycled layer increased under the effect of loads and environmental factors. On the contrary, the TSR, shear strength, fracture work, and fracture energy initially improved before descending. At the same time, the splitting strength, unconfined compressive strength, shear strength, fracture energy, and fracture work of pavement core samples increased by approximately 63%, 21%, 52%, 43%, and 57%, respectively, after seven years of service compared with laboratory samples.

(3)

The residual fatigue life of the emulsified asphalt cold recycled mixture in the passing lane surpassed that of carriageway under the same maximum tensile stress at the center of the samples. The fatigue life of emulsified asphalt cold recycled mixture in the laboratory is inferior to that of in-service. It indicates that the fatigue life of the cold recycled mixture increased during the service period.

(4)

During the early stage of strength development in emulsified asphalt cold recycled mixture, the reclaimed asphalt pavement (RAP) primarily is used as “black aggregate”. Throughout the paving stage, the old asphalt recovers its bonding strength by cement hydration, secondary hot compaction, and diffusion between new and aged asphalt. This significantly enhanced the performance of mixtures. The increase in strength and various road performance of the cold recycled layer during the servicing period outweighs the loss incurred by vehicle load and environmental factors, ultimately manifesting as the increase of fatigue life.

(5)

Currently, cold recycling technology is being widely adopted in large-scale applications. However, there remains a lack of performance monitoring during the pavement service life, resulting in insufficient feedback from field data to validate laboratory findings. Notably, the selection of design modulus values for cold recycled structural layers still lacks reliable theoretical support. Therefore, it has become an urgent research priority to accurately characterize the modulus degradation mechanisms in in-service cold recycled pavements and establish scientifically justified laboratory-derived design parameters.