Performance Evaluation of Aged Asphalt Pavement Binder through Rejuvenators

Abstract

Natural resources are declining due to rising infrastructure, renovation, demolition, and recycling of existing structures that necessitate sustainable development. It urges the researchers to modify the aged asphalt binder in the recycling to enhance the performance life of asphaltic pavements. The aim of this research study is to reutilize recycled materials through rejuvenation. This study utilizes the Cereclor to rejuvenate the aged binder collected from recycling and explore its transformation by comparing it with parent binder of similar grade. Different types of tests, such as basic physical properties, saturates, aromatics, resins, and asphaltenes (SARA) analysis for the fractional composition, bending beam rheometer (BBR), and dynamic shear rheometer (DSR) for rheological properties are applied to investigate these properties and effect on the performance. The results show that rejuvenator improved the fractional composition of the aged asphalt binder. It significantly improved the physical properties of the binder. The asphaltene contents are reduced up to 18% in the fractional composition through the addition of the optimum dosage (7.5%) of the rejuvenator. The colloidal instability index was decreased from 0.74 to 0.43 value by changing its unstable to stable colloidal structure. In addition, rejuvenator improved the rheological properties at a wide range of temperatures. The rejuvenator has the potential to soften the aged binder through optimum dosage (7.5%), as indicated in the results of fractional composition, colloidal structure, and rheological properties. Furthermore, it has been concluded that it can be utilized as a rejuvenator in the recycling industry to resolve the recycled materials disposal issues and lead to promote sustainable development.

1. Introduction

Natural resources are declining due to rapid growth in population globally. Developing countries are facing such challenges due to urbanization. The development process is also in progress in the construction industry. The development of infrastructure and renovation, demolition, and recycling of existing structures is still in progress. A large quantity of material is producing from the recycling process of asphaltic pavements. It is crucial for the relevant departments to handle such wastage due to high transportation charges and scarcity of disposal land to protect the environment [1,2].

The National Highway Authority (NHA) and provisional departments are actively involved in the development, maintenance, rehabilitation, and reconstruction of road networks throughout the country. The entire road network, more than 90%, consists of black-topped bituminous surface pavements. The black pavement typically deteriorates away due to traffic loading and environmental effects before the design life [3,4]. The transition in the pavement industry to a circular way of running development needs effective and sustainable energy resources and recycling techniques. The framework was applied for sustainability of asphaltic pavements through materials circular index. The material circularity index was calculated for the wearing course and base course separately, and the results indicated the circularity in different approaches for both layers [5]. The utilization of recycled materials in pavement construction is quite important to achieve a safe and durable structure able to exhibit the highest performance. In particular, aspects of both mechanical behavior (i.e., stiffness, rutting, or cracking sensitivity) and functional behavior (i.e., skid resistance) need to be considered when investigating the use of recycled materials in asphalt base and wearing courses, respectively, under variable environmental conditions [6], where pavements can be normally exposed to extreme weather and temperature conditions.

The reclaimed asphalt pavement (RAP) application started in the United States in the 1980s and then spread globally [7]. The general framework for adopting the recycling approach was developed in Pakistan a few years ago. Specific guidelines provide a link between local construction practices and design aspects to implement the recycling techniques [6,7]. Previous studies offer significant evidence on reclaimed and recycled materials utilized in Illinois pavement construction [8,9,10]. The quantity of RAP material recommended to be used in asphalt mixture varies and reaches up to 100% [11].

The utilization of RAP material has been shown to be an economical option to replace the neat asphalt binder during recycling but there is still a challenge regarding the performance of the pavements [12,13]. It has also been observed that RAP binders are stiffer due to the aging of pavements [12] but enhance the rutting resistance [14,15,16,17]. The design of asphalt mixtures with high RAP contents has been increasing since the last decade and it is still challenging to assess the performance of recycled materials [7,18]. Mixes prepared with 0, 20, and 40% RAP were studied to investigate the effects of RAP on HMA through dynamic modulus testing. It was concluded that the addition of RAP in HMA up to 20% does not change the binder grade. However, an addition of 40% RAP in the hot mix asphalt significantly changes the binder grade. More testing is required to confirm the binder grade at low temperatures to assess the PG grading. In addition, this study recommends RAP fractionation in the preparation of laboratory specimens. An extensive performance testing program is also required to investigate fatigue cracking, rutting, and thermal cracking for HMA with high RAP percentages [19]. RAP binder is utilized in HMA to adopt sustainable practices on the environment through life cycle assessment (LCA) that contains material, construction, maintenance, and rehabilitation phases of pavement. It was concluded that the results presented a remarkable reduction in energy consumption and greenhouse emission through the addition of RAP binder. It also showed the importance of achieving equivalent field performance of HMA with RAP and virgin binder [20].

Since the 1930s, mechanisms of short and long-term aging have been studied. The aging of asphalt binder occurs due to different processes, such as oxidation, volatilization, thixotropy (steric hardening), polymerization due to actinic light, and condensation polymerization due to heat. The first two processes occur due to chemical reactions that take place in asphalt due to environmental factors. The steric hardening is structural reconstruction and reorganization at the intermolecular level produced because of temperature variations during the service life. It is also a complex nonrecoverable process that is produced by ultraviolet (UV) light, oxygen, and heating effect due to temperature changes [21,22,23,24]. Numerous analytical techniques have been developed to investigate and link the physical and rheological properties with the performance characteristics of asphalt binders. For this reason, it is essential to introduce a collaboration among material engineers, pavement specialists, highway agencies, and asphalt industries [24]. Meanwhile, there may be durable adhesion and film between aggregate and asphalt binder in the existence of moisture to decrease stripping that can be achieved through modification of asphalt binders [25,26]. SARA fraction detects the polar and nonpolar components of asphalt binders that vary due to the aging process. The saturate fraction contains the inclusive hydrocarbons in the chain of carbon items. The aromatics cover benzene and its byproducts. The polar groups are longer chains with alkane ends and are connected through resins. Asphaltenes are massively polar fractions that are produced through condensed aromatic and naphthenic rings, which also include heteroatoms. It was detected that asphaltene fraction is interrelated with aging and significantly reduces the aromatics [27].

It has been noted that fractional compositions changed due to aging. Maltenes are viscous in nature and consist of saturates, aromatics, and resins. Asphalt binders show a complex molecular and colloidal structure that governs physical, rheological, chemical, and performance characteristics through the interacting of these fractions [28]. Another technique, high pressure-gel permeation chromatographic (HP-GPC), was utilized to divide it into fractions and deliberated the effects of aging phenomena on constituents individually [29]. Investigations related to the chemical properties of asphalt binder through aging show that as the amount of asphaltenes is raised, resins and aromatics decreased. This pattern indicates the brittle and stiffer binder that shows lower penetration and higher viscosity and softening point [30]. Asphalt pavements are aged due to loading and environmental effects and extend to failure of asphaltic layers [31]. It happens due to short-term and long-term aging of asphaltic pavements [31,32,33,34,35,36,37].

Briefly, several rejuvenators and modifiers are utilized in recycling. Several rejuvenators and emulsions are added to the aged binder to rebalance the asphaltene and maltene fractions that recover performance properties to some extent and are unable to improve the low temperature and long-run performance. The addition of different types of rejuvenators provides options in RAP binder, reversible fluxing in bitumen, and warm mix asphalt to improve the properties. Various polymer-modified binders (PMBs) also improve the rheological, mechanical, aging resistance, temperature sensitivity, morphology, and thermal properties but they also have a few drawbacks. The phase stability and rheological properties at high temperatures improved by reinforcing the nanostructure of asphalt modified through poliflex 60/85-E with aluminum oxide. Modifiers resisted the segregation at the interface of materials and increased the resistance against oxidative aging and thermal heating. They were able to resist permanent deformation. The nanoscale properties of aluminum oxide improved the microstructure of the modified binder. The nanoporosity of the modified binder decreased, which can be related to the infiltration and diffusion of oxygen that prevent the agglomeration of particles [38]. It also indicates the issues like thermal instability and phase segregation at different temperatures [39]. Furthermore, in another study [40] concluded that raw materials (soybean, sunflower, canola, coconut, caster, and rapeseed oils) utilized to produce vegetable oil can be mixed at different percentages as modifiers/additives in the asphalt binder.

The literature indicates that waste cooking oil (WCO) utilized for modification of asphalt binder enhanced the low-temperature properties to some extent. It can change the fractional composition and produces high antioxidant molecules as exposed to higher temperatures [41]. These perform well at the start of their application and would not perform well in later stages. Exploration of new rejuvenators, like Cereclor, in addition to conventional emulsifiers that will perform well in the long-run is still absent. There are several types of rejuvenators that have been utilized to improve the properties of aged binders. They balance the interactions of molecules to some extent, improving the rheological aspects but they have adverse effects on fatigue. Thus, in this research study, a rejuvenator is utilized in recycling to overcome such adverse effects. It would be able to rebalance the fractional composition, stable the colloidal structure, and perform well in the service life of the pavement. It will be able to enhance the fractional components of the aged binder and improve the linkage of carbon chains. Chlorinated paraffins (CPs) or polychlorinated n-alkanes (PCAs), commonly known as “Cereclor”, contain the general formula C_nH_2n+2−zCl_z and are composite blends comprising thousands of various isomers, diastereomers, and enantiomers. These characteristics are directly correlated with the carbon chain and quantity of chlorination [42]. The aims and objectives of this research study are as follows:

• To characterize the physical properties of virgin and rejuvenated aged binder.
• To measure the colloidal instability of aged binder through fractional compositions.
• To investigate the effect of rejuvenator on rheological properties of the aged binder.

2. Materials and Methods

Temperatures vary in Pakistan due to changes in the climate condition in the different regions [43,44]. Mostly asphaltic pavements have been constructed with asphalt binders produced in the locally available oil refineries. Asphalt binder of Pen. grade 60/70 utilized for Motorway M-2 connecting Islamabad and Lahore with a total length of 476 km. Routine rehabilitation is continued periodically to get better serviceability. RAP material was collected during the rehabilitation process. It was produced two weeks after, through the milling action during the rehabilitation process. Aged binder was extracted and recovered through centrifuge and rotavapor by considering ASTM D2172 and ASTM D5404, respectively. After that, aged binder was rejuvenated with Cereclor to improve its serviceable life by enhancing the physical and fractional compositions, and rheological properties. The optimal dosage of Cereclor was chosen during initial laboratory testing. Different dosages were checked by performing physical tests by the hit and trial procedure. The optimum dosage was selected and utilized in the final results. The entire methodology consists of three phases. In the 1st phase, rejuvenator was added to the RAP binder. The rejuvenator with adhesive nature and chemical function similar to asphalt binder was chosen to ensure compatibility. Both materials are polar due to oxygen atoms and sulfur with trace metals. The polarity was induced due to chlorine atoms, while C-H bonds indicate non-polar nature. Polar atoms attract each other and make bonds as they are mixed with asphalt binder. A similar concept was introduced to induce polarity through chlorine atoms in the rejuvenation of asphalt binder. It was considered that it might be able to rebalance the asphaltene and maltene fractions of asphalt binder. The physical and chemical properties of the rejuvenator are provided in

Table 1. Physical and Chemical Properties of binder and Rejuvenator.

Description of Test Values	Asphalt Binder		Properties
	Virgin Binder	RAP Binder	Cereclor
Penetration (0.1 mm)	66	48.7	Chlorine Content (%): 70
Softening Point (°C)	50	65	Viscosity at 100 °C (Cp): 86.2
Ductility at 25 °C (cm)	100	28	Pour Index Approx. (°C): +20
Viscosity at 135 °C (mPas)	317	613	Stability after 4 h 175 °C: 0.2

below.

In the 2nd phase, physical and fractional composition analysis was performed to characterize the colloidal instability. Finally, rheological properties were measured in the 3rd phase to explore the rejuvenation effect of Cereclor on the RAP binder. The entire methodology is described in Figure 1.

Physical tests were performed to measure the basic properties of asphalt binder and their effect on the performance during the service life of the pavement. Penetration grade is an empirical method to check the consistency or hardness of asphalt binder with a standard needle of 100 g weight applied on the surface of the sample for 5 s at 25 °C. Ring and ball softening points are typically utilized to measure the suitable temperature at which the binder acquires specific softness. This temperature can also be used along with the penetration value to quantify the temperature susceptibility of asphalt binders. The flash point test indicates the temperature at which asphalt binder can be carefully heated in the direct flame. The ductility test measures the distance of the asphalt binder specimen that can be elongated without breaking under 5 cm/min at 25 °C. Asphalt binder showing lower ductility value is an indicator of weaak adhesive characteristics and finally poor performance in service life. The flow properties of asphalt binder were measured through (ASTM D 4402-02).

The SARA fraction was performed by following ASTM D 4124-09. The SUPERPAVE was developed for the performance of asphalt binder specification. It covers a broad range of temperatures, from freezing to high temperatures. Furthermore, it relates various aspects of different asphalt binders to field performance. It normally characterizes the binders at different temperature ranges.

The Bending Beam Rheometer (BBR) Test according to AASHTO M320 was conducted to measure creep stiffness (S) and performance grading at low temperatures to resist cracking. Dynamic Shear Rheometer (DSR) Test enabled simultaneous evaluation of time and temperature variations effects on asphalt binders. It was conducted according to AASHTO M320 for assessing upper-temperature grades of asphalt binder. Two basics geometric arrangements are normally utilized, such as plate-to-plate and cone-to-plate arrangements, relying upon the desired findings. When used for examining asphalt binders, it generally measures the basic rheological characteristics, such as complex shear modulus and phase angle at medium to high temperatures. In this research, DSR was utilized with a frequency level of 10 radian/second and 10% strain. The output parameters were calculated through the sinusoidal stress of each sample.

3. Results and Discussion

3.1. Conventional Properties of Asphalt Binder

The effect of the rejuvenator and emulsion is presented in

Table 2. Basic Conventional Properties.

Test and Sample	RAP Binder	Emulsified	RAP+ 2.5% C	RAP+ 5% C	RAP+ 7.5% C	Virgin 60/70
Penetration (0.1 mm)	48.7	62.5	55.4	61	68	66
Softening point (°C)	65	52	64	60	52	50
Ductility at 25 °C (cm)	28	42	39	67	101	100
Flash and Fire Point (°C)	245, 278	240, 270	243, 269	235, 266	231, 258	232, 265
Viscosity at 135 °C (mPas)	613	441	535	417	312	317

, that illustrates the conventional properties of different samples.

From all physical testing, it can be indicated from Table 2 that asphalt binder becomes stiffer through aging during the service life. The penetration and ductility are decreasing, and softening point and flash point are increasing with aging. The rejuvenator has been added to the aged asphalt binder to check how it performs changes in the properties. It can be noted from Table 2 that as the rejuvenator is added to the aged binder, it changes from hard to softer. At optimum value of 7.5%, the ductility is reached at 101 cm. Higher dosages were also applied, and it was noted that they make it softer and the value of ductility increases. It is also in line with SARA analysis and rheological results.

The rejuvenator has softened the aged binder significantly by changing its liquid, semi-solid, and solid fractions. It changed due to variations in the fractional composition of asphalt binder mixture, as indicated in the literature [45]. Normally, minor components in the asphalt binder are decreased due to physical absorption as the rejuvenator is added, thus, it hardens the mixture and ultimately lowers the penetration and ductility. The addition of a rejuvenator indicates resistance to loading and deformation, as provided in the literature [46]. Figure 2 demonstrates the penetration index (PI) for different samples.

It is noted from Figure 2 that RAP binder showed the highest value of PI, which initiated the thermal cracking. With the addition of a rejuvenator, it lowered the PI value and minimized this cracking. It made it softer and weather-resistant, as indicated in the literature [46]. A detailed explanation of the chemical changes during the multiscale aging of asphalt binders challenged the oxidation phenomena. It is known that because of oxidation, polar fractions, such as asphaltene, intensify. The asphaltene fraction affects the stiffening behavior of asphalt binders and its quantity increases in the RAP more than the virgin binders. Non-polar constituents, such as aromatics, start decreasing after aging. The fractional composition of the binder is calculated and shown in Figure 3.

It can be noted from Figure 3 that saturate and resin contents are slightly changed and resistant to the aging process. The aromatic contents decrease significantly, breaking the internal bonds and enhancing the solidified asphaltene contents, which makes the asphalt binder harder and stiffer. Such changes can occur during the aging process. In such a way, ultraviolet radiations hold enough energy to ionize the atoms or create the free radicals that are reactive and accelerate molecular fracture (cracking). After the addition of a rejuvenator, it rearranged the components by changing the SARA fractions. These modified fractions indicate it a softer binder and will be able to perform in the service life of pavement. Its higher values indicate that stiffer binders, such as asphaltenes, are more peptized through the resins, which governs sol structures, while lower values indicate softer binders found in gel structures [47].

The colloidal instability index (CII) was calculated from the fractional composition of the binder. It is the ratio of dispersed fraction to the flocculated fraction as indicated in Equation (1) and described in Figure 4. It represents the sol or gel structure of the binder. The higher value of CII indicates a stiffer binder due to the highest asphaltene contents and the lower value describes the softer binder, as mentioned in the literature [42].

$$Collidal Instability Index \left(CII\right)=\frac{Floculated}{Dispersed}=\frac{Saturates+Asphaltenes}{Resins+Aromatics}$$

(1)

It can be noted from Figure 4 that the colloidal instability index (CII) indicates the stable colloidal structure for the virgin binder and stiffer for the RAP binder. It indicates an increase in asphaltene content due to aging. CII value is increased, which makes the binder stiffer, brittle, and indicates a sol structure. The fractional contents of RAP binder have been restored considerably through rejuvenation with different quantities of Cereclor. This improves the entire structure of the binder and creates a softer, stable binder that leads to gel structure, as indicated in the literature [45,46]. It will perform better in the service life of the pavement.

3.2. Effect of Rheological Characterization on Asphalt Binder

BBR was performed to measure the thermal cracking.

Table 3. Low-Temperature Stiffness Properties of Virgin, Aged and Modified Asphalt Binder.

Sample Code	Stiffness (MPA)				m-Value
	−4 °C	−10 °C	−16 °C	−22 °C	−4 °C	−10 °C	−16 °C	−22 °C
VB Pen 60/70	89	147	157	312	0.372	0.323	0.281	0.267
Emulsified	92	168	273	337	0.367	0.317	0.305	0.245
RAP	107	213	397	349	0.319	0.268	0.228	0.236
RAP + 2.5% S	83	144	235	343	0.347	0.331	0.307	0.281
RAP + 5% S	76	114	191	283	0.356	0.341	0.315	0.293
RAP + 7.5% S	89	156	247	349	0.341	0.327	0.313	0.282

describes the results measured at −4 °C, −10 °C, −16 °C, and −22 °C low temperatures.

It can be noted from Table 3 that in the first section, creep stiffness (S) increases with a decrease in temperature from virgin to RAP binder. The RAP binder fails and is susceptible to thermal cracking as its stiffness rises to over 300 MPa. Besides, m-value reduces as temperature decreases in a similar pattern and fails at −22 °C in both criteria. The increase in stiffness and decrease in m-value (declines in the rate of stress relaxation) facilitate the propagation of thermal cracking. As Rejuvenator (Cereclor) is added to the RAP binder, it softens the stiffer binder and restores its properties. It can be noted that stiffness and m-value are dropped significantly through the variation in the low temperature properties. The rejuvenated RAP binder satisfies both criteria to resist thermal cracking. Low-temperature grades were also measured and shown in Figure 5. The performance grade of the binder is presented in Figure 5.

It can be noted from Figure 5 that PG grade of virgin, RAP, and rejuvenated binder samples indicates that upper and lower temperature are changing and shifting towards stiffer binder with aging. With the addition of rejuvenator to the RAP binder, it significantly shifts the PG grade from PG-82-4 to PG-76-4, PG-70-10 and PG-64-16 with 2.5%, 5%, and 7.5% doses of Cereclor, respectively. It significantly reduces the upper and lower temperature by reducing the stiffness of the aged binder. Additionally, rejuvenation has been shown to have a major effect on PG grades. The rejuvenator has the potential to soften the binder and these results are indicated in the fractional composition and colloidal instability index in the previous section. A frequency sweep test was performed and master curves were drawn at 58 °C by providing a horizontal shift of the data, as shown in Figure 6.

Figure 6 shows the master curves between complex shear modulus and reduced frequency for virgin, RAP, and rejuvenated biner samples. It can be noted that initially, there is a linear trend that indicates RAP binder has maximum stiffness in upper, medium, and lower temperatures. A similar result trend is observed in the fractional composition of RAP binder due to severe aging. It makes it stiffer and increases the asphaltene content and reduces the maltenes content. However, this effect drops when the temperature decreases. The rejuvenator reduces the stiffness of RAP binder in both upper and lower frequencies in different patterns. At the low frequency, it is observed that stiffness is reduced, which indicates projection in the medium to high temperature. Normally, these regions are utilized in selecting the parent grade of the binder. This governs the brittle and stiffer binder that indicates the change in the rheological properties and the results are in line with the literature [30,31,47].

4. Conclusions

The following conclusions are observed from this research study:

• The physical properties penetration and ductility are increased while the softening point and flash point are decreased through the addition of rejuvenator. The penetration index (PI) indicates resistance to temperature susceptibility. It would be able to perform in different environmental changes.
• The asphaltene contents are reduced up to 18% in the fractional composition through the addition of the optimum dosage (7.5%) of the rejuvenator. The colloidal instability index has decreased from 0.74 to 0.43 by changing its unstable to a stable colloidal structure. It indicates a softer binder that shows the gel structure. This structure may provide stability in the matrix of the modified asphalt blend.
• The rejuvenator shifts the PG grade from PG-82-4 to PG-76-4, PG-70-10, and PG-64-16 with 2.5%, 5%, and 7.5% doses of Cereclor, respectively. It reduces the upper and lower temperature through reducing the stiffness of aged binder. The stiffness and m-value have been decreased through changing the low-temperature properties. It would be resistant at upper and lower temperatures that resist rutting and cracking, respectively.
• In addition, rejuvenator has improved the rheological properties, it reduces the stiffness of aged binder in both upper and lower frequencies in different patterns. At low frequencies, a significant reduction in stiffness is observed that reflects the moderate to high-temperature regions.
• It is concluded that the rejuvenator has the potential to alter the fractional components, and hence, it can enhance the rheological properties of the RAP binder.
• The findings of this research study indicate that the rejuvenator optimum dosage, i.e., 7.5%, has the potential to change the fractional composition of the aged binder. It has improved the colloidal structure, indicating the softer binder and gel structure. It has also improved the performance grades and rheological properties of the aged binder. It can be utilized in recycled materials during the recycling technique of asphaltic pavements. By utilizing it, recycled waste materials would be useful during the rehabilitation and reconstruction of the existing pavement. It will be useful as a recycled material and it can help save energy.