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Compatibility of Recycled Binder Blends with Recycling Agents: Rheological and Physicochemical Evaluation of Rejuvenation and Aging Processes Lorena Garcia Cucalon, Gayle King, Fawaz Kaseer, Edith ArambulaMercado, Amy Epps Martin, Thomas Frederic Turner, and Charles J. Glover Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01657 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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Industrial & Engineering Chemistry Research

Compatibility of Recycled Binder Blends with Recycling Agents: Rheological and Physicochemical Evaluation of Rejuvenation and Aging Processes Lorena Garcia Cucalon*1†, Gayle King2, Fawaz Kaseer1, Edith Arambula-Mercado1, Amy Epps Martin*3, Thomas F. Turner4, and Charles J. Glover5 * Corresponding author 1 Texas A&M Transportation Institute, College Station, Texas 2 GHK, Inc 3 Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas 4 Western Research Institute, Laramie, Wyoming 5 Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas KEYWORDS: Asphalt binders, recycled blends, compatibility, recycling agents, binder aging. ABSTRACT: Public and private sectors promote increased use of recycled materials in asphalt pavement construction. However, concerns exist about possible premature pavement embrittlement and cracking because of these aged materials. Recycling agents, or rejuvenators, are used to offset these concerns. Rejuvenation mechanisms can be inferred depending on the type of recycling agent, yet there is a lack of supporting experimental evidence, especially related to the recycling agent-asphaltene interaction. This study provides insight on the effects and mechanisms of rejuvenation through extensive rheological and physicochemical characterization by considering experimental variables such as type of recycling agent, type/proportion of recycled materials, and binder type. Rejuvenation was effective based on restoration to PG 70-22, reduction in Glover-Rowe parameter, and reduction in glass transition temperature midpoint and end. Chemical characterization was not conclusive, yet the results were interpreted as strong recycling agent-asphaltene polar interaction. Rheological benefits from rejuvenation were gradually lost with long-term aging.

INTRODUCTION Asphalt binders, largely used in pavements and roofing industries, degrade with time primarily due to oxidation. Upon extreme binder aging, pavements and roofing systems are no longer serviceable and need to be removed and replaced. Reclaimed asphalt pavements (RAP) and recycled asphalt shingles (RAS), either tear-off asphalt shingles (TOAS) or manufacturer waste asphalt shingles (MWAS), can be recycled into asphalt pavements. The economical savings to departments of transportation (DOTs) and pavement contractors, especially when the price of oil is high, drive the rapid implementation and further development of pavement recycling. Environmental benefits are also realized with the use of a large volume of waste products in pavements instead disposing of them in landfills. Private and public sectors are invested in continued research to advance the incorporation of high contents of recycled materials in asphalt pavements by inclusion of recycling agents, also refered to as rejuvenators. Yet, rejuvenation mechanisms have not been fully understood. Asphalt binders can be represented by a colloidal model consisting of a highly polar asphaltene phase dispersed in

a maltene phase. The concept of binder compatibility or stability refers to the balance between soluble and insoluble fractions in the colloid, which controls the flow properties of the colloid. The binder can be separated into four different fractions based on increasing polarity: Saturates, Aromatics, Resins, and Asphaltenes (SARA). The balance between SARA fractions has been related to physical properties, and this balance is a common compatibility indicator1,2. A more compatible asphalt binder has a larger proportion of soluble fractions that results in a softer binder with increased ductility3. Loss of binder compatibility with short- and long-term aging (stiffening and embrittlement) is attributed to volatilization of lighter oils in the asphalt binder and the increased asphaltene content formed upon binder oxidation1. It is important to highlight that binder rheology is not only affected by the total asphaltene content, but also by the size of the asphaltene clusters and the chemistry (or dispersive power) of the maltene phase 1,4. Previous studies suggest that binders with the same asphaltene content can exhibit different rheological characteristics and that the maltenes derived from

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different asphalt binders can have different dispersive power affecting the size of the asphaltene agglomerates1,5. Recycling heavily aged asphalt binders into new asphalt pavements while maintaining sufficient durability is challenging, as recycled binders have a larger asphaltene content as compared to new pavingbinders and are therefore significantly stiffer and more brittle. Depending on the recycled binder ratio (RBR) in a blend of recycled and new binders, the target performance grade (PG) can be met by blending with a softer and less brittle substitute binder and/or by incorporation of recycling agent with both binders. The RBR is the percentage of recycled binder from RAP, MWAS and/or TOAS by weight with respect to the total binder by weight in the mixture. In general, partial rheological restoration of the recycled blends can be accomplished through a combination of the following mechanisms: 

Softening, produced by blending the stiff recycled binder with a softer material (softer substitute binder and/or reycling agent).



Reduction of the size of asphaltene associations, achieved by adding recycling agent which breaks apart strong polar bonds or aromatic pi-pi interactions.



Improvement in the dispersive power of the continuous maltene phase, achieved by inclusion of the recycling agent.

The problem is further complicated by the fact that recycled binder blends continue to age in service. Depending upon factors such as the size of the pericondensed asphaltenes in the recycled binder and the chemical reactivity of the added recycling agent, aging in blends could result from a combination of the following mechanisms which are not necessarily common to substitute/virgin binders but must be considered for recycled mixtures: 

Formation of pericondensed asphaltenes and related changes in compatibility with maltenes.



Re-agglomeration of asphaltene clusters which may have initially been dispersed during blending.



Chemical changes in the recycling agent with aging may result in reduced dispersive power of the maltene phase. For example, bio-oils used as rejuvenators usually contain double bonds, which themselves react with oxygen. Bio-oils may be triglycerides, or they may be simple fatty acid molecules which are often reacted into esters or amides to enhance behavior.

Ongoing research studies have been oriented towards increasing RBR limits by using recycling agents. The objectives of this study are to: (1) provide fundamental insight on the mechanisms of rejuvenation when a recycling agentis included in recycled blends, with specific interest on the impact of the recycling agent on the asphaltene agglomerates, and (2) evaluate rheological and physicochemical changes upon rejuvenation and aging of

recycled blends considering a wide experimental methods and variables.

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of

EXPERIMENTAL DESIGN Experimental methods and variables were carefully selected in this study in order to satisfy the objectives.

Experimental Methods Rheological characterization was conducted using the dynamic shear rheometer (DSR) and bending beam rheometer (BBR) to characterize the binders and blends in terms of high and low PG temperatures (AASHTO T315, T313, PP42). Physicochemical aspects of recycling and rejuvenation were evaluated by differential scanning calorimeter (DSC) and SAR-ADTM (Saturates, Aromatics, Resins – Asphaltene Determinator). In addition, long-term durability was evaluated by the Glover-Rowe (G-R) DSR function after laboratory short- and long-term aging using the rolling thin film oven (RTFO) per ASTM D2872 plus 20 and 40 hours in the pressure aging vessel (PAV) per ASTM D6521. Tracking of oxidation products was performed using Fourier transform infrared spectroscopy (FT-IR). DSC All DSC analyses were performed using a TA Instruments DSC Q2000 instrument (TA Instruments, New Castle, Delaware) with refrigerated cooling. Samples were sealed in air in hermetic aluminum pans (Tzero Low-Mass Pans). Sample weights ranged from 7 to 10 milligrams, thus occupying a minimum of 70% of the total volume (10L) of the Tzero pans. Samples were equilibrated at 165°C for five minutes and then cooled to -90°C at 2°C per minute with a ± 1.0°C, 80 second modulation. The samples were then heated at 2°C per minute with a ± 1.0°C, 80 second period modulation to 165°C. Oxidation was not expected to be extreme under these conditions, and all samples were treated consistently to enable comparisons between them. Masson and Polomark (2001) indicated that if oxidation occurs during testing the reversing heat capacity curves from heating and cooling cycles would differ significantly6. In this study the reversing heat capacity curves from heating and cooling cycles were comparable, therefore minimizing the concern for oxidation during testing. The DSC analysis software (Universal Analysis 2000 v4.5a, TA Instruments, New Castle, Delaware) was used to determine asymptotes, glass transition parameters, and crystalline fraction areas. For glass transition measurements, the low-temperature analysis start point was -70°C, and the high temperature analysis end point was 100°C. Parameters reported include the location of the glass transition temperature calculated by inflection, Tg(I), and the transition high temperature limit (Tg end) as illustrated in Figure 1. Values obtained from heating cycles are reported in this study, previous research suggest that heating curves produce more repeatable results7. The Tg(I) observed during heating is expected to be slightly higher than that observed during cooling.


Page 3 of 13 Data

Reversing Heat Capacity (J/ C)

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Industrial & Engineering Chemistry Research pavement durability. The DSR function is refered to in recent literature as the Glover-Rowe (G-R) parameter and can be obtained experimentally from strain controlled oscillatory measurements in the linear-viscoelastic region using the DSR at 15°C and 0.005 rad/s as indicated in equation 3 (or shifted to, via time-temperature superposition). G-R parameter thresholds for durability assessment have been estimated based on a previous ductility study11 with field aging in a PG 58-28 climate and used in recent literature for evaluating long-term binder performance12–14. The G-R parameter can also be used as a rheological index of compatibility with aging.

Analysis

0.02

0.015

Tg End - High temperature Tg limit. Tg - Inflection

0.01 -100

-50

0 50 100 Temperature (C)

150

Figure 1. Modulated DSC Analysis – Tg (I) and Tg end The kinetic (not heat capacity related heat flow) data obtained during the experiment were used to measure wax crystallization/precipitation and melting/dissolution onset temperatures and to estimate a crystalline fraction. An assumed heat of crystallization/melting of 180 J/g was used to convert areas to crystalline fraction values. The derivative of heat flow with respect to temperature was used to assist selection of endpoints. The curve overlay capability of the Universal Analysis software was used to display subtle changes in thermal profiles due to additives.

SAR-AD SAR-AD is the latest advancement in SARA characterization developed at WRI8,9 which divides the binder into eight fractions based on polarity, including three asphaltene categories. The Asphaltene Determinator incorporated into the SAR-AD consists of an automated asphaltene separation method based on solubility. During the elution time, data is recorded simultaneously by two detectors: 1) an evaporative light scattering (ELS) detector and 2) a variable wavelength absorbance detector set at 500nm. It is assumed that the relative peak areas obtained from the ELS detector are proportional to the relative percent of the fractions. The traditional indicator of binder compatibility, the colloidal instability index (CII), can be calculated from SAR-AD results by equation 1. A larger CII indicates a less compatible binder. The total pericondensed aromatics (TPA) described by equation 2 is a SAR-AD index which represents the total amount of asphaltenes present in the binder8,9. Asphaltenes ELS and 500nm correspond respectively to the asphaltene content detected by the ELS and by the variable wavelength absorbance detector set at 500nm. 𝐶𝐼𝐼 =

𝑆𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑠 + 𝐴𝑠𝑝ℎ𝑎𝑙𝑡𝑒𝑛𝑒𝑠 𝐴𝑟𝑜𝑚𝑎𝑡𝑖𝑐𝑠 + 𝑅𝑒𝑠𝑖𝑛𝑠

𝑇𝑃𝐴 =

𝐴𝑠𝑝ℎ𝑎𝑙𝑡𝑒𝑛𝑒𝑠 𝐸𝐿𝑆 𝐴𝑠𝑝ℎ𝑎𝑙𝑡𝑒𝑛𝑒𝑠 500𝑛𝑚

1

𝐺−𝑅 =

|𝐺 ∗ |(cos 𝛿)2 𝐺′ = ′ ′ 𝜔𝜂 ⁄𝐺 sin 𝛿

3

in which  is the test frequency, 𝐺 ′ is the storage modulus, |𝐺 ∗ | is the complex modulus, 𝜂′ is the dynamic viscosity, and 𝛿 is the phase angle.

FT-IR Qualitative chemical composition of substitute binders and recycled blends was characterized using an attenuated total reflectance (ATR) FT-IR Bruker Tensor 27 with a diamond crystal Absorbance data were collected over a frequency range of 600 cm–1 to 4000 cm–1, and changes with aging were monitored with emphasis on the carbonyl and sulfoxide regions.

Materials Materials from the rehabilitation of State Highway (SH 31) in Texas were utilized in this study including a PG 64-22 substitute binder, recycled materials, and two types of recycling agent (Table 1). A PG 70-22 target binder was required to satisfy climate and traffic requirements for Texas. Current TxDOT specifications for hot-mix asphalt (HMA) allow the use of up to 30% binder replacement (0.28 RBR) from inclusion of RAP, MWAS, or TOAS when blending with a softer substitute (PG 64-22) and using a warm-mix asphalt (WMA) technology. In the laboratory, recycled binder blends were prepared using a variety of materials at different proportions (Table 1), including two different substitute binders (PG 64-22 from TX and PG 64-28 from NH), three types of recycled materials (RAP, MWAS, and TOAS), and two types of recycling agents at different dosages by weight of total binder. The tall oil (labeled T1), is a bio-based oil while the aromatic extract (labeled A1)is a petroleum derivative. The DOT control blend (0.28 control) used in the field includes RAP with a high-temperature PG (PGH) of 107 and MWAS with a PGH of 133. Alternatives to the DOT control blend included consideration of the following: effect of type of recycling agent (T1 and A1), inclusion of higher RBR (0.5), employing a more oxidized TOAS (PGH of 178), and using a better quality substitute binder (NH PG64-28).

2

G-R DSR Function A DSR function was developed with strong correlation to the superseded ductility test at 15°C10 for assessment of



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Table 1. Materials

Label

1 2

Blend Proportions1

Recycling Agent

Continuous PG

Type

%2

High (PGH)

Low (PGL)

TX PG 64-22

Substitute Binder

-

-

69.4

m -24.6 S -29.2

NH PG 64-28

Substitute Binder

-

-

66.9

m -29.2 S -28

0.28 Control

0.1 RAP 0.18 MWAS 0.7 TX PG 64-22

-

-

81

m -15.6 S -26.6

0.28 + 4.5% T1

0.1 RAP 0.18 MWAS 0.7 TX PG 64-22

T1

4.5

73

m -22.0 S -31

0.28 + 5.5% A1

0.1 RAP 0.18 MWAS 0.7 TX PG 64-22

A1

5.5

71

m -22.3 S -30.6

0.5 Control MWAS

0.25 RAP 0.25 MWAS 0.5 TX PG 64-22

-

-

88

m -10.6 S -26.6

0.5 MWAS + 7.5% T1

0.25 RAP 0.25 MWAS 0.5 TX PG 64-22

T1

7.5

73

m -21.7 S -31.2

0.5 Control TOAS

0.25 RAP 0.25 TOAS 0.5 TX PG 64-22

-

-

102

m -2.1 S -22.7

0.5 TOAS + 11.5% T1

0.25 RAP 0.25 TOAS 0.5 TX PG 64-22

T1

11.5

74

m -25.8 S -34.7

0.5 Control TOAS 64-28

0.25 RAP 0.25 TOAS 0.5 NH PG 64-28

-

-

101

m -12.7 S -25.6

0.5 TOAS 64-28 + 12.5% T1

0.25 RAP 0.25 TOAS 0.5 NH PG 64-28

T1

12.5

75

m -27.1 S -32.4

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centrifuge extraction). The recycled binders were then recovered in accordance with ASTM D 5404 using the rotary evaporator. To prepare the recycled binder blends, the preheated substitute binder was combined with the recycling agent at the selected dosage, and then blended with the recycled binders from the RAP and RAS at the selected RBR. Considering the high stiffness and viscosity of the recycled binders, the blending process was performed at an elevated temperature of up to 200°C. After combining all components, the recycled binder blends were aged in the RTFO, followed by 20 or 40 hours at 100°C in the PAV, prior to being characterized to determine chemical and rheological properties.

RESULTS AND DISCUSSION

With respect to the total blend (by weight) By weight of total binder in the asphalt mixture

Blend Preparation The recycled binders from RAP, MWAS, and TOAS were extracted in accordance with ASTM D 2172 (test method A:

Rejuvenation was evaluated by characterizing the recylced blends with different proportions of recycled materials and recycling agents, while laboratory-simulated long-term aging was conducted to assess durability of these recycled blends. Compatibility restoration is discussed in terms of rheological, physical, and chemical characterization. Additionally, changes in rheology and chemistry with long-term aging are presented.

Recycled Blends and Rejuvenation The low temperature PG (PGL) of the target binder (PG 7022 ) was used to select the dosage of recycling agent for the binder blend, and then the resulting PGH of the binder blend at the selected dosage was verified against the PGH of the target binder and adjusted (increased) if needed, while still maintaining the PGL of the target binder. PG results presented in Table 1 show that the incorporation of recycling agents enabled restoration of rheological characteristics of recycled asphalt binders to meet the PG 70-22 requirement for TX. The G-R intermediate temperature parameter was evaluated as a surrogate parameter for ductility. A value of 180 kPa (measured or referenced to 15°C, 0.005 rad/s) corresponds to a ductility of 5 cm (at 15°C, 1 cm/min) and 600 kPa corresponds to 3 cm. The values of 180 and 600 kPa are used as approximate indicators of the ductility range for the onset of fatigue in pavements and extensive cracking respestively.10,11 Results presented in Figure 2 correpond to binders and blends in the original condition, prior any short- or long-term aging. Figure 2 shows that the inclusion of recycling agents restored binder ductility below the cracking thresholds, and sometimes reaching comparable G-R values as compared to the substitute binders. However, additional physical and chemical evaluations were conducted to further evaluate compatibility.


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Industrial & Engineering Chemistry Research 4

G-R @ 15 C, 0.005 rad/s (kPa)

10 1.E+04

3

10 1.E+03

600kPa 180kPa

2 1.E+02 10

1 1.E+01 10

0 1.E+00 10

TX PG 6422

0.28 Control

0.28 + 4.5% T1

0.28 + 0.5 Control 0.5 MWAS 0.5 Control 0.5 TOAS NH PG 64- 0.5 Control 0.5 TOAS 5.5% A1 MWAS + 7.5% T1 TOAS + 11.5% 28 64-28 + TOAS T1 12.5% T1 64‐28

Figure 2. DSR results – G-R after rejuvenation (no aging) DSC evaluation was conducted to characterize glass transition temperature (calculated by inflection point) and the high-end temperature end the glass transition from heating and cooling experiments; and to quantify the wax content of the binders and blends. The wax content was found to be around 1% for all samples. The accuracy and reproducibility of the wax determination is questionable at such concentrations, thus no further comparisons were made. Changes in the glass transition temperature (Tg) caused by blending recycled binders and/or recycling agents with virgin/substitute binders are presented in Figure 3. For the TX PG 64-22 substitute binder, the inclusion of recycled materials (0.28 Control, 0.5 Control MWAS, and 0.5 Control TOAS) resulted in lower Tg. Comparing within the control blends increasing RBR (0.28 to 0.5) and inclusion of more severely aged binder (TOAS versus MWAS) produced an increase in Tg. For the NH PG 64-28 substitute binder, the inclusion of recycled materials (0.5 Control TOAS 64-28) increased Tg. Conversely, the inclusion of recycling agents drastically reduced Tg, except for blend 0.28+5.5%A1. It is important to note that for all

these cases, the Tg of the recycled blends with recycling agent is much lower than that for the corresponding substitute binders without recycled materials. It was observed for most cases that increasing RBR and/or inclusion of more severely aged materials (TOAS) increased Tg, while inclusion of recycling agents result in a much more pronounced reduction of Tg. Softening/rejuvenating additives generally solidify at much lower temperatures as compared to asphalt binders, therefore the resulting blend can have a significantly lower Tg. Moreover, recent research suggests that incompatible rejuvenated binder blends may exhibit two distinct glass transition temperatures, one corresponding to the additive and the other to the binder, whereas compatible blends behave more like a homogeneous material, exhibiting one glass transitiontemperature15. Also, the reduction in Tg from blending binder with oil-based additives (similar to oil-based recycling agents) correlates to a reduction in PGL16.

-20

-25

Tg (I), C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-30

-35

-40 TX PG 6422

0.28 Control

0.28 + 4.5%0.28 + 5.5% 0.5 Control 0.5 MWAS 0.5 Control 0.5 TOAS + NH PG 64- 0.5 Control 0.5 TOAS T1 A1 MWAS + 7.5% T1 TOAS 11.5% T1 28 64-28 + TOAS 12.5% T1 64‐28

Figure 3. DSC results - Glass transition temperature by inflection


Industrial & Engineering Chemistry Research Numerous research studies on virgin binders have compared Tg (mid-point of the glass transition range by inflection) to various measures of performance with mixed results17,18. For example, binder oxidation significantly extends the glass transition temperature range between onset and completion, but without significantly changing Tg. The relatively small changes observed in Tg from inclusion of recycled materials is comparable in magnitude to the changes in Tg reported in literature provoked by binder oxidation. Recent studies at WRI revealed that the high temperature end of the glass transition (Tg end) observed by heating experiments changes more dramatically with aging, therefore more relevant for correlations with rheological behavior related to cracking9,19. During cooling, the molecules typically cannot

associate fast enough to form glass at the high temperature end of the transition. During heating, there is time to associate and extend the high temperature end of the transition. An increased Tg end indicates the material becomes brittle at higher temperatures, which is undesirable. Changes in Tg end caused by blending recycled binders and/or recycling agents with virgin/substitute binders are presented in Figure 4. Inclusion of recycled materials resulted in increased Tg end as compared to the substitute binders while a clear benefit from employing an RA was observed by a significant reduction in Tg end. Recycled binder blends with higherdosages of recycling agents exhibited greater improvement in Tg end.

10

5

Tg End, C

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0

-5

-10

-15 TX PG 6422

0.28 Control

0.28 + 4.5% 0.28 + 5.5% 0.5 Control 0.5 MWAS 0.5 Control 0.5 TOAS + NH PG 64- 0.5 Control 0.5 TOAS T1 A1 MWAS + 7.5% T1 TOAS 11.5% T1 28 64-28 + TOAS 12.5% T1 64‐28

Figure 4. DSC results - Glass transition high temperature end SAR-AD results are discussed in terms of CII and TPA indices presented in Figure 5. In general, results indicated a detrimental effect on compatibility (CII) and pericondensed aromatics (TPA) when incorporating recycled materials in binder blend. Interpreting results from inclusion of the recycling agent is problematic because the chemistry of the recycling agent itself controls the fraction in which it elutes during the SAR-AD analysis. For example, aromatic oils largely elute in aromatic and saturate fractions, whereas the much more polar fatty acids and triglycerides from bio-oils likely elute with polar or asphaltene fractions. It is important to highlight that the amounts of recycling agent added to the recycled blends may not be sufficient to re-balance the SARA fractions to proportions comparable to those of substitute binders. This is especially true for bio-oils, which cannot fully

replace the aromatic fractions which upon oxidation have become more polar and elute with resins or asphaltenes. However, even though the bio-oils elute with polar or asphaltene fractions, they are highly mobile molecules that have the potential to both soften and restore binder phase angle. Historical data on virgin binders (withouth polymer modification) indicate that embrittlement with aging correlates well to loss in phase angle10. Furthermore, the large pericondensed asphaltene clusters formed during oxidation may not be dissociated by any type of recycling agent. Hence, the colloidal instability index may be a good way to follow the evolution of compatibility as a binder is oxidized, but it may not have equivalent relevance as a tool to evaluate the effectiveness of different recycling agents in restoring binder rheology.


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CII

TPA 50

0.8

40

0.6

30

0.4

20

0.2

10

CII

TPA

1.0

0.0

TX PG 64-22

0.28 Control

0.28 0.28 + 4.5% T1 + 5.5% A1

0.5 Control MWAS

0.5 MWAS + 7.5% T1

0.5 0.5 TOAS Control + 11.5% T1 TOAS

NH PG 64-28

0.5 Control 0.5 TOAS 64-28 TOAS 64‐28 + 12.5% T1

0

Figure 5. Colloidal instability index and total pericondensed aromatics A possible mechanism of rejuvenation consists of dissociation of asphaltene agglomerates by inclusion of recycling agents20. The asphaltene determinator in the SAR-AD test was used specifically to evaluate the effect of the recycling agent on the total asphaltenes and asphaltene sub fractions. The proportional asphaltene peak areas detected by ELS and 500nm are shown in Figure 6a and Figure 6b, respectively, in terms of fractions soluble in cyclohexane (CyC6), toluene, and methylene chloride:methanol (CH2Cl12:MeOH). The results show an increased asphaltene content resulting from incorporation of recycled materials (by both detectors). However, SARAD only captures a very minor decrease in asphaltene content when T1 is included and even an increased asphaltene content for blends with TOAS including 11.5 and 12.5% T1 (Figure 6a and b). Such an observation was unexpected, considering that T1 does not contain any of the polycyclic aromatic molecules thought to create the molecular associations called asphaltenes. Upon further CyC6

consideration, the fatty acid group on the tall oil molecule must have enough polarity to bond with other polar molecules in the asphaltenes (such as polar carbonyl groups) when dissolved in heptane solvent. This is supported by the fact that the much less polar aromatic rejuvenator (A1) does not appear to elute with asphaltenes in the SAR-AD analysis. In general, a recycling agent can elute in different fractions of the recycled binder blends, therefore it is not possible to recommend compatibility indices applicable to all situations when each recycling agent may rejuvenate the blend by a different mechanism. In fact, a strong fatty acid-asphaltene polar interaction may be the mechanism by which the recycling agent provides improved molecular mobility of the large asphaltene agglomerates. This might be achieved by interacting with individual asphaltene molecules to prevent them from associating with other asphaltene molecules.

Toluene

CH2Cl2:MeOH

40

30

ELS Detector

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

10

0 TX PG 6422

0.28 Control

0.28 + 4.5% T1

0.28 + 0.5 Control 0.5 MWAS 0.5 Control 0.5 TOAS + NH PG 64- 0.5 Control 0.5 TOAS 5.5% A1 MWAS + 7.5% T1 TOAS 11.5% T1 28 64-28 + TOAS 12.5% T1 64‐28

(a) ELS detector


Industrial & Engineering Chemistry Research CyC6

Toluene

CH2Cl2:MeOH

80

60

500nm Detector

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

0 TX PG 6422

0.28 Control

0.28 + 4.5% T1

0.28 + 5.5% A1

0.5 Control 0.5 MWAS 0.5 Control 0.5 TOAS + NH PG 64- 0.5 Control 0.5 TOAS MWAS + 7.5% T1 TOAS 11.5% T1 28 64-28 + TOAS 12.5% T1 64‐28

(b) Optical absorbance at 500nm Figure 6. Asphaltene determinator A major limitation of the SARA or SAR-AD analyses is that the compatibility indices were developed for virgin binders (or crude oils) and may not be capable of characterizing the recycled binder blends with recycling agents accurately. For example, the saturates fraction of binders may include crystallized waxes. Saturate molecules are highly mobile with many rotational and vibrational degrees of freedom, so long as they remain mobile in the maltene phase. This motion is critical to increasing phase angle and the molecular mobility needed to maintain fluidlike behavior (higher phase angles). However, when saturates solidify into microcrystalline waxes, they are completely immobilized, resulting in a phenomenon characterized as low temperature reversible physical hardening. The compatibility index (CII) places all saturates in the numerator as a negative for compatibility, when in fact they are essential for maintaining ductility. However, when these same saturates crystallize as solids, their impact on asphalt rheology is highly detrimental. Understanding how a high CII might contribute to a more ‘gel’ type behavior of the colloid is valuable, but CII alone is not sufficient to predict critical rheological changes introduced by a recycling agent. In fact, T1 in this study includes straight chain fatty acids with one or more double bonds which greatly inhibit wax crystallization. The question that remains is the stability of these double bonds with oxidation. The asphaltene determinator could possibly be further developed/adjusted to accurately evaluate recycled blends with recycling agents, most specifically to better assess the effect of the recycling agent on the asphaltene agglomerates.

Long-Term Durability of Rejuvenated Blends All substitute binders (and recycled blends) degrade with aging, and eventually will reach an undesirable level of embrittlement for pavements. Long-term aging evaluation was conducted for the substitute binders, DOT control

blend (0.28 control), and the additional recycled binder blends with recycling agents that were considered. G-R is presented in Figure 7 with the corresponding durability thresholds related to onset of cracking (180kPa) and significant cracking (600kPa) on asphalt pavements9,10. The DOT control blend (0.28 Control) may be excessively brittle based on the two damage thresholds after RTFO and PAV aging. Comparing both substitute binders, the TX PG 64-22 reaches the durability thresholds significantly faster than the NH PG 64-28. All the rejuvenated recycled blends after short-term aging (RTFO) ranked between both substitute binders. Better or worse, rejuvenated recycled blends at this aging state were similar to substitute binders, and demonstrated improved cracking resistance as compared to the DOT control blend (0.28 control). After 20 hours of PAV aging, all the rejuvenated recycled blends exceeded the threshold for onset of pavement cracking (180kPa), and were similar or slightly worse than the TX PG 64-22 substitute binder. After 40 hours of PAV aging, all rejuvenated binder blends may have reached severe embrittlement levels based on G-R thresholds. This indicates a diminished effectiveness of the recycling agent with long-term aging. It is important to recall that aging (stiffening/ embrittlement) of rejuvenated recycled blends may be related to chemical changes in addition to those common to virgin binders. In addition to formation of common oxidation products, it is possible that with time the binder blend loses compatibility due to re-agglomeration of asphaltene clusters which may have initially been dispersed during blending with recycling agent, and/or chemical changes in the recycling agent with aging which may affect rheology. FT-IR spectra for all binders and recycled blends with aging are shown in Figure 8 from (a) through (h). Experimental data were recorded in a range of 600cm–1 to 4000cm–1. Since no changes were observed with aging above 2000 cm–1 ,


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these data are not presented. Carbonyl containing groups and sulfoxides are the typically observed oxidation products in asphalt binders, which are identified in FT-IR with peaks at 1700cm-1 (ketones, carboxylic acids) and 1032cm-1 (sulfoxides). For all the substitute binders and rejuvenated recycled blends, these peaks were observed to increase with aging. Other functional groups overlap in the carbonyl band with binder oxidation products including aldehydes, acid, anhydride, amides, and esters, which are common in bio-oils used as asphalt modifiers. For all the rejuvenated blends with T1, a significanly increased peak at 1700cm-1 is observed and attributed to the fatty acids in the tall oil (Figure 8 (f) and (h)). Additionally, a peak at 1743cmRTFOT

1

was observed for all recycled blends with T1 which can be attributed to esters present in the recycling agent (Figure 8 (f) and (h)). The same peak was observed in the TX PG 64-22 substitute binder, suggesting that this substitute binder was modified with a bio-based additive to meet PG specifications. The peak at 1743cm-1 is also observed to grow with aging, and for some cases the growth in this peak is more evident than that at 1700cm-1. Such observations can be attributed to changes in T1 with aging, and further studies are needed to evaluate chemical changes in the recycling agents with aging and their impact on the recycled blends. PAV 20

PAV 40

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Figure 7. G-R with aging


0.5 TOAS + 11.5% T1

NH PG 64-28 0.5 TOAS 64-28 + 12.5% T1

Industrial & Engineering Chemistry Research RTFOT

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Ketone Carboxylic Acid C=O

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Figure 8. FT-IR with aging


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CONCLUSIONS AND FUTURE WORK This study provided fundamental insight on the mechanisms of rejuvenation and aging of recycled binders with recycling agents through rheological, physical, and chemical characterization. Rheological and physical measurements reported in this study confirm that there is a rejuventation effect upon inclusion of recycling agents. The recycled binder blend PG can be restored to meet a Texas 70-22 climatic requirement, and the glass transition is significantly lowered when blending with the recycling agent. The chemical analysis by SAR-AD was not conclusive, and it did not confirm nor deny the hyphothesis regarding the reduction of asphaltene agglomerates, but did provide evidence of a strong polar interaction between asphaltenes and T1 which may contribute to increased molecular mobility and restoration of rheological properties. Depending upon their aromaticity and polarity, recycling agents may elute in different SAR-AD fractions. This, plus questions regarding wax crystallization, make it difficult to use chemical indices to predict effectiveness of the recycling agent. It is also unwise to restrict recycling to specific petroleum streams by continuing to specify chemical fractions using Rostler or SARA methods. Upon long-term aging, rejuventation effectiveness is diminished, especially after 40 hours of PAV aging. Nevertheless, all rejuvenated binder blends continue to show improved performance as compared to the DOT control blend without recycling agent, corroborating the added value of using recycling agents to increase the amount of recycled materials (RBR). While it appears possible to increase the RBR in asphalt blends and achieve desired performance by inclusion of recycling agents based on the results of this study, this statement may or may not hold true for asphalt mixtures where other variables should be considered such as blending efficency that may introduce an additonal challenge. It is recommended to conduct further studies on recycled binder blends and corresponding mixtures considering various sources and types of recycled materials. It is also necessary to evaluate increased dosages of recycling agent for improved long-term perfomance without over softening the substitute binder and thus compromising rutting performance at early life or moisture susceptibility on the asphalt mixtures. The impact of the recycling agent on the asphaltene associations and the effect of aging on the dispersive power provided by the recycling agent remain unclear. Considering the large number of recycling agents available on the market, it is important to understand the chemical changes typically observed in the different types of recycling agents which may compromise durability of the recycled blends. Furthermore, it can not be emphasized strongly enough that the chemical and rheological properties of the recycled binder vary markedly, and make a huge contribution to ultimate performance. Highly oxidized materials have very low relative phase angles. recycling

agents can soften them to a target binder moduli, but restoring phase angles to recreate binder relaxation properties may not be possible. Additional to the tools presented in this study, several techniques have been utilized with the purpose of studying binder compatibility, chemical, physical, and microstructural changes occurring during binder oxidation (e.g. high performance gel permeation chromatography – HP-GPC, nuclear magnetic resonance – NMR, Heithaus Titration, ion-exchange chromatography - IEC, Raman spectroscopy, atomic force microscope - AFM, X-ray photoelectron spectroscopy – XPS, mass spectroscopy). The limitations encountered in the application of SAR-AD to the study of asphaltene dissociations (explained by a strong polar interaction between recycling agent and asphaltenes) could also present a limitation in the possible application of some of the techniques listed above to study rejuvenation. It is recommended to continue to study this topic by a variety of available techniques and continued development/adjustment of experimental techniques aimed to characterize rejuvenation is encouraged. Rheological characterization remains the most reliable tool to evaluate effects of rejuvenation and effectiveness with aging given the complexities associated to chemical characterization techniques to date.

AUTHOR INFORMATION Corresponding Authors * Tel.: +1-800-641-4691 ext. 2857. E-mail: [email protected] * Tel.: +1-979-862-1750. E-mail: [email protected]

ORCID Lorena Garcia Cucalon: 0000-0003-3139-1457 Gayle King: 0000-0002-7690-0748 Fawaz Kaseer: 0000-0002-9443-4429 Edith Arámbula-Mercado: 0000-0002-1435-3662 Amy Epps Martin: 0000-0001-7207-5368 Thomas F. Turner: 0000-0002-2752-8302 Charles J. Glover: 0000-0001-8851-277X

Present Address †Lorena Garcia Cucalon TAMKO Building Products, Joplin, MO

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources This work was made possible by the financial support provided by the National Cooperative Highway Research Program (NCHRP) through project NCHRP 9-58: “The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios”.

ACKNOWLEDGMENT The authors are thankful to Dr. Jean Pascal Planche who provided guidance during experimental design and interpretation of results. Gratitude is extended to Mr. Juan S. Carvajal



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Munoz, Ms. Katherine Klentzman, Mr. Thomas A. Henz, and Dr. Fan Yin for their support in laboratory evaluations.

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ABBREVIATIONS DSC, Differential Scanning Calorimeter; G-R, Glover-Rowe; MWAS, Manufacturer Waste Asphalt Shingles; RA, Recycling Agent; RAP, Reclaimed Asphalt Pavement; RAS, Recycled Asphalt Shingles; RBR, Recycled Binder Ratio; SAR-AD, Saturates Aromatics Resins and Asphaltene Determinator; TOAS, Tear-off Asphalt Shingles; TxDOT, Texas Department of Transportation.

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