Using Thermal Analytical Techniques To Study Rejuvenators and

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Efficiency and Sustainability

Using thermal analytical techniques to study rejuvenators and rejuvenated reclaimed asphalt pavement (RAP) binders Mohamed Elkashef, David Jones, Liya Jiao, R Christopher Williams, and John Harvey Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03427 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 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

Energy & Fuels

Using thermal analytical techniques to study rejuvenators and rejuvenated reclaimed asphalt pavement (RAP) binders. Mohamed Elkashef*1, David Jones1, Liya Jiao1, R. Christopher Williams2, John Harvey1 1University

of California Pavement Research Center, Civil and Environmental Engineering Department, University of California, Davis, California, 95616.

2Civil,

Construction, and Environmental Engineering Department, Iowa State University, Ames, Iowa, 50011.

KEYWORDS Differential scanning calorimetry; Reclaimed asphalt pavements; Modulated differential scanning calorimetry; Asphalt binders; Rejuvenators ABSTRACT Rejuvenators are added to reclaimed asphalt pavement (RAP) binders to enhance their low temperature and fatigue performance. The effectiveness of a rejuvenator depends on many factors including the rejuvenator’s properties as well as the compatibility between the rejuvenator and the base binder. The thermal properties of a rejuvenator can vary greatly based on its chemical composition. This research focuses on understanding the effects of the thermal properties of

ACS Paragon Plus Environment

1

Energy & Fuels 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

Page 2 of 25

rejuvenators on the rheological properties of the rejuvenated binders, particularly in relation to the critical low temperature. In this research, an extracted and recovered RAP binder was blended with two different rejuvenators at a dosage of 10 percent by weight of the binder and the change in the critical temperatures was assessed using rheological measurements. Differential scanning calorimetry (DSC) was used to obtain the crystallization and melting points of the rejuvenators before and after aging. Modulated DSC (MDSC) was also used to assess the change in glass transition temperature of the control base binder and the rejuvenated RAP binders, as well as to identify crystallization and melting events. The results provided a useful insight into the changes in the properties of rejuvenators with aging through the use of DSC. It also highlighted the effect of the crystallization and low temperature flow properties of the rejuvenators on the performance of rejuvenated binders.

INTRODUCTION The effect of rejuvenators on asphalt binders has been under extensive study. Several studies have concluded that the low temperature cracking and fatigue properties of aged binders are enhanced notably with the use of rejuvenators 1,2. Rejuvenators, however, have the negative effect of lowering the rutting resistance of binders as revealed by the reduction in the critical high temperature 3. Petroleum-based rejuvenators pose serious health concerns because of the presence of polar aromatic constituents which were found to be carcinogenic 4. In this regard, bio-derived rejuvenators offer a safe alternative to petroleum-based rejuvenators. The use of bio-derived rejuvenators based on different sources such as tall oil, cotton seed oil, vegetable oil, and soybean oil has been successfully reported

5–7.

A recent study investigated the effect of six different

rejuvenators on the high and low temperature performance grades of asphalt binders 8. The rejuvenators under study were derived from different sources including tall oil, waste vegetable


2

Page 3 of 25 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

Energy & Fuels

oil as well as waste engine oil. The bio-derived rejuvenators included in the study were shown to cause a significant improvement on the low temperature performance grade compared to the petroleum-based rejuvenators. Rejuvenators vary greatly in terms of their physical and chemical properties. The chemical compatibility between the rejuvenator and the binder is believed to affect the rejuvenator’s performance 9,10. In a recent study, a rejuvenator derived from soybean oil was used to modify neat and polymer-modified asphalt binders, and it was shown that the effect of the rejuvenator was binder dependent 10. It is clear from these studies that investigation of the chemical and thermal properties of rejuvenators is essential to fully characterize the performance of rejuvenated binders. The chemical composition of bio-based rejuvenators is mostly based on fatty acids. Saturated fatty acids typically crystalize at high temperatures. At lower temperatures, other fatty acids start to crystalize in order of their degree of saturation, which means that unsaturated fatty acids crystallize at very low temperatures 11. Due to their low crystallization points, unsaturated fatty acids have better low temperature properties compared to saturated fatty acids. Differential scanning calorimetry (DSC) is a powerful tool which can be used to study rejuvenators and rejuvenated binders. DSC monitors the heat flow into and out of a sample under a controlled temperature program. A reference is used and the difference between the heat flow into and out of the sample and the reference is calculated and used to plot a DSC scan with temperature. DSC scans are used to study the glass transition temperature of asphalt binders as well as any melting or crystallization events of any of the asphalt fractions. A number of properties, such as shear modulus, specific heat, and expansion coefficient of the material, exhibit a distinct change at the glass transition temperature. For asphalt binders, the glass transition temperature has been shown to correlate to the Fraass brittle point at which the binder exhibits brittle behavior 12.


3

Energy & Fuels 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

Page 4 of 25

The effect of aging on the different asphalt fractions was the topic of a Strategic Highway Research Program (SHRP) study 13. The glass transition temperature was shown to extend over a wide range between -60oC and 0oC. It was also shown that different asphalts exhibit different melting peaks however no correlation could be found between the asphalt composition and the location and intensity of these peaks. The effect of the origin and performance grades of the asphalt binders on their glass transition regions was investigated in an extensive study, where 70 different asphalt binders were analyzed using DSC

14.

For all binders, the glass transition region was observed

between -50oC and -10oC. Crystallization and melting of asphalt fractions was detected following the glass transition temperature. The crystallization upon heating was attributed to species which did not crystalize during cooling due to limited mobility 15. Melting peaks were associated with waxes present in the asphalt binders

16.

transition region

These melting and crystallization peaks were found to perturb the glass

17.

Using modulated DSC (MDSC) could help isolate the melting and

crystallization events and provide an accurate detection of the glass transition temperature. Modulated DSC utilizes a sinusoidal temperature signal that is superimposed on the linear heating or cooling temperature program typically used in regular DSC. The use of the modulated signal provides a varying heating and/or cooling rate which is useful in the measurement of heat capacity and thus helps to better detect glass transition temperatures. In MDSC terminology, the total DSC signal is split into a reversible signal useful to detect glass transition, and a nonreversible signal which is used to detect crystallization and melting 18. Previous studies have used MDSC to study melting and crystallization of waxes and other asphalt constituents and modifiers 19,20.


4

Page 5 of 25 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

Energy & Fuels

Even though the glass transition temperature is considered a fundamental property of the material, the test conditions can greatly influence the measurements. The thermal history of the material, as well as the cooling and heating rates can have a significant impact on the measured value for the glass transition temperature 18. Hence, it is important to erase the thermal history of the asphalt binder prior to taking any measurements by annealing at high temperatures. Annealing is also necessary to remove any polar associations that exist between the molecules at room temperature. The polar associations typically result in exothermic peaks in the DSC scan during heating 21. It is also important to maintain the same cooling and heating rates between the different studied binders. The selected temperature range should cover the main events being detected including the glass transition, melting, and crystallization. MATERIALS AND METHODS The RAP material used in this study was procured from the Sacramento region in California. Extraction of the RAP binder was done according to ASTM D2172-Method A using toluene. Recovery of the RAP binder followed ASTM D5404 using a rotary evaporator. To prevent oxidation of the recovered binder, the recovery process was done under a nitrogen blanket. The performance grade of the RAP binder was determined using two different methods as outlined in AASHTO M323 and the Asphalt Institute (AI) MS-2 manual. The method specified in AASHTO M323 uses Rolling thin film oven (RTFO)-aged binder to test for the intermediate and low temperatures without the need for PAV aging. The AI method, however, uses PAV-aged RAP binder to determine the intermediate and low temperatures. Another difference between the two methods is that the AI method does not test unaged RAP binder. Hence the critical high temperature, using the AI method, is determined as the RTFO-aged high temperature, whereas using the AASHTO M320 method, the high temperature is considered as the minimum temperature between the unaged and RTFO-aged high temperature results.


5

Energy & Fuels 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

Page 6 of 25

Two different commercially available bio-rejuvenators based on tall oil were used in this study, hereafter to be referred to as EV and HG. The rejuvenators were added to the RAP binder at a dosage of 10 percent by weight. This dosage was selected based on previous experience with other bio-based rejuvenators. Previous work conducted using a soybean-derived rejuvenator showed that a drop in the high temperature grade between 2.1 to 2.8oC was noted per 1 percent dosage of the rejuvenator 22. The RAP binder was excessively stiff and had to be heated to 180oC before being poured and blended with the rejuvenator. The rejuvenator and the binder were manually blended using a glass stirrer for 3-5 minutes. The rejuvenated RAP binders were tested using a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR) to determine their performance grades according to AASHTO T315 and AASHTO T313, respectively. RTFO-aging was done according to ASTM D2872 at a temperature of 163oC for 85 minutes to simulate short term aging. Long term aging was conducted using RTFO-aged binder in a pressure aging vessel (PAV) at a temperature of 100oC and a pressure of 2.1 MPa according to ASTM D6521. In order to assess the effect of aging on the thermal properties of the rejuvenators, RTFO-aging followed by PAV-aging of the rejuvenator were conducted according to ASTM D6521 and ASTM D2872, with minor modifications to account for the fluidity of the rejuvenators. Only 10 g of rejuvenator were used per RTFO-bottle to avoid spilling in the RTFO oven. In the same way, the amount of rejuvenator in the PAV-aging pans was limited to 15 g. The thermal characteristics of the rejuvenators before and after aging were examined using differential scanning calorimetry (DSC). The DSC scans were studied to determine the crystallization and melting events. DSC measurements were made using a DSC Q2000 from TA instruments. The DSC was connected to a cooling accessory allowing measurements to be made at temperatures as low as -90oC. Samples of the rejuvenators weighing between 5-10 mg were placed in hermetically sealed aluminum pans.


6

Page 7 of 25 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

Energy & Fuels

Testing was done under a flow of nitrogen purge gas at a rate of 50 ml/min. The testing method involved cooling the sample at a rate of 3oC/min to -50oC. The temperature was held at -50oC for 10 minutes to equilibrate the sample prior to heating at a rate of 3oC/min up to 30oC. The heat flow into and out of the sample was recorded during both the cooling and heating cycles. The thermal characteristics of the PAV-aged control and rejuvenated binders were examined using modulated differential scanning calorimetry (MDSC). The binder samples were placed in hermetically sealed aluminum pans and annealed by preheating to 150oC for 5 minutes followed by cooling at a rate of 10oC/min to -50oC. The samples were then heated at a rate of 3oC/min to 30oC. A modulated temperature signal of 0.64oC at a period of 40 seconds was added during the heating cycle. Heat flow measurements were reported during the heating cycle only. The DSC heating scans were studied to determine the glass transition temperatures as well as the crystallization and melting events in the control and rejuvenated RAP binders. Temperature modulation was only necessary when analyzing binders in order to provide better accuracy in detection of the glass transition region. RESULTS AND DISCUSSION BINDER PROPERTIES The critical temperatures of the control and rejuvenated RAP binders are listed in Table 1. For the control RAP binder, the numbers in parenthesis represent the critical intermediate and low temperatures obtained using RTFO-aged binder as specified by the AASHTO M323 method. The critical high temperature of the RAP binder clearly indicates that it is in a state at the high end of the aging range observed in previous studies. There was no considerable difference between the unaged and RTFO-aged critical high temperatures hence there was no concern about residual solvents from the extraction process. A notable difference between the unaged and RTFO-aged


7

Energy & Fuels 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

Page 8 of 25

critical temperatures could be indicative of solvents remaining in the binder due to the extraction process, which was not the case here. Table 1. Properties of control and rejuvenated binders Binder

RAP

RAP + EV

RAP + HG

Unaged (High Temperature), oC

109.9

88.7

92.2

RTFO (High Temperature), oC

109.8

89.6

90.8

Intermediate Temperature, oC

51.3 (48.6)*

35.5

37.7

Low Temperature (Stiffness), oC , Ts

-3.7 (-7.2)*

-17.2

-18.4

Low Temperature (m-value), oC , Tm

7.2 (-2.4)*

-15

-13.8

-10.9 (-4.8)*

-2.2

-4.6

0.5

1.7

1.7

ΔTc = Ts-Tm Mass loss (%)

*Numbers between parenthesis using Asphalt Institute MS-2 method A comparison between the intermediate and low temperatures of the RAP binder, obtained using the AASHTO M323 and the AI MS-2 methods, clearly shows that PAV aging resulted in notable additional stiffening of the RAP binder. These results indicate that RTFO-aged RAP binder should not be considered sufficiently representative of long-term aging as implied by the AASHTO M323 method. The use of the rejuvenators resulted in lowering the stiffness of the binder and reducing the critical high temperature for both the unaged and the RTFO-aged binders. The effect of the two rejuvenators on the RAP binder was very similar particularly after RTFO-aging. Both rejuvenators enhanced the performance of the binders at low and intermediate temperatures. The critical intermediate and low temperatures were lowered upon the addition of the rejuvenators indicating better fatigue under constant deformation and low-temperature cracking performance, respectively. The impact of the two rejuvenators on the intermediate and low temperatures was


8

Page 9 of 25 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

Energy & Fuels

comparable. The mass loss recorded using the two rejuvenators at 10 percent dosage was equal to 1.7 percent as shown in Table 1, which indicates that the rejuvenators contained a notable percentage of volatile constituents. The parameter ΔTc is used to evaluate the thermal cracking potential of binders. ΔTc is calculated as the difference between the critical low temperatures at a stiffness S=300 MPa and m=0.300 as per ASTM D7643. Aged binders are shown to have highly negative ΔTc values, where the performance is controlled by the m-value. The m-value is the slope of the creep stiffness curve at a time of 60 seconds and serves as an indication of the ability of the binder to relax stresses at low temperatures and slow loading. With aging, the binder loses its stress relaxation ability and the temperature increases at which m=0.300, resulting in a highly negative ΔTc value. The addition of the two rejuvenators improved the ΔTc value of the rejuvenated RAP binder as compared to the control RAP binder, indicating better stress relaxation properties. REJUVENATORS A DSC scan shows the amount of heat flow in and out of a sample with varying temperature. DSC measurements can be taken during heating and cooling of the sample. Figures 1 and 2 show the DSC scan for the cooling cycle of rejuvenators EV and HG, respectively. As the rejuvenator is cooled down, crystallization occurs. An exothermic peak in the DSC cooling scan marks a crystallization event. The DSC cooling scans of the two rejuvenators were distinctly different owing to their different chemical compositions. The DSC cooling scans for both rejuvenators showed more than one crystallization peak, also indicating different chemical constituents within each of the rejuvenators. The width of the peaks can be used as an indication of the purity of the rejuvenators. The broad crystallization peaks noted in the DSC scans indicate that the rejuvenators are composed of a mixture of molecules, with different chemical structures, that crystallize over a wide range of temperatures.


9

Energy & Fuels 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

Page 10 of 25

Figure 1. Cooling scan for rejuvenator EV

Figure 2. Cooling scan for rejuvenator HG Rejuvenator EV shows a number of crystallization peaks. A small peak is noted around -5oC denoting partial crystallization of the rejuvenator. More crystallization peaks appear at low temperatures representing other chemical constituents with higher degrees of unsaturation. The peak intensity and peak areas of the different chemical constituents correspond to their heat


10

Page 11 of 25 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

Energy & Fuels

capacities as well as their percentages within the rejuvenator. Rejuvenator HG starts to partially crystalize at around -15oC with full crystallization taking place near -45oC. The study of crystallization temperature is related to the cold flow properties of the rejuvenator. The flowability of the rejuvenator at low temperature is important particularly in cold climatic regions. The rejuvenator should be sufficiently flowable to ensure proper blending with the binder. The results show the rejuvenators start to considerably lose their flowability, due to onset of crystallization, at temperatures of around -5oC and -15oC for the EV and HG rejuvenators, respectively. Even though it is reasonable to assume that a rejuvenator with low crystallization temperatures should result in better low temperature performance of the rejuvenated binder, it was not possible to find a strong correlation between the DSC scans of the rejuvenators and the critical low temperatures of the rejuvenated binders. The DSC cooling scans of the two rejuvenators indicate that rejuvenator HG has a higher proportion of constituents crystalizing at lower temperatures than rejuvenator EV. According to the binder results, rejuvenator HG had slightly better performance in terms of low temperature (stiffness) as compared to rejuvenator EV. However, rejuvenator EV slightly outperformed HG with regards to the m-value low temperature. In order to better correlate the DSC scans and the low temperature properties of the binders, a rigorous study of the effects of the saturated and unsaturated fatty acids on the rheological properties of the binders, namely stiffness and stress relaxation, is needed. Figures 3 and 4 show the DSC heating scans for rejuvenators EV and HG, respectively. The endothermic peaks noted in the DSC heating scan correspond to different melting events. Similar to crystallization, melting of different constituents within the rejuvenator took place at different temperatures. Generally, the heating scan for each rejuvenator had the same number of peaks as


11

Energy & Fuels 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

Page 12 of 25

its cooling scan. The melting peaks however were broader and appeared at higher temperatures relative to the crystallization peaks.

Figure 3. Heating scan for rejuvenator EV

Figure 4. Heating scan for rejuvenator HG The rejuvenators were PAV-aged to study the effect of aging on their thermal characteristics. Both rejuvenators show an increased stiffness with aging. No attempt was made to measure the


12

Page 13 of 25 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

Energy & Fuels

viscosity of the rejuvenators before and after aging, however the PAV-aged rejuvenators appeared to be glue-like and were difficult to pour out of their containers. A study of the heating scans of the PAV-aged rejuvenators shown in Figures 5 and 6, for rejuvenator EV and rejuvenator HG, respectively, clearly show evidence of changes in the chemical composition of both rejuvenators with PAV aging.

Figure 5. Heating scan for PAV-aged rejuvenator EV

Figure 6. Heating Scan for PAV-aged rejuvenator HG


13

Energy & Fuels 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

Page 14 of 25

In both rejuvenators the melting peaks became broader which could be an indication of more chemical constituents being formed as a result of the oxidation process. Second, the melting peaks shifted to higher temperatures. The shift to higher temperatures indicates that the constituents being formed had a lower degree of unsaturation compared to the original constituents. For rejuvenator EV, the melting peak at -35oC became broader and was slightly shifted to a higher temperature, as was the peak at -25oC. Similar changes were noted for rejuvenator HG, as shown by the change in the peak at -15oC. Third, the noted changes were more prominent in the unsaturated constituents represented by the melting peaks occurring at low temperatures. This is expected since the double bonds in the unsaturated constituents are reactive and more prone to oxidation. The saturated constituents represented by the melting peaks at higher temperatures, showed little or no change. For rejuvenator EV, the peak at 15oC did not show any notable change. Similarly, the peak at 10oC for rejuvenator HG did not seem to be significantly affected by aging. It could be argued that the PAV-aging method adopted in this study subjects the rejuvenator to extensive aging compared to what would be expected when the rejuvenator is blended with the binder. Aging of a pure rejuvenator in the absence of the binder may cause a higher rate of oxidation in the rejuvenator. Nevertheless, this method serves as a means to characterize the effect of aging/oxidation on the pure rejuvenators. CONTROL AND REJUVENATED BINDERS Modulated DSC (MDSC) was used to analyze the control and rejuvenated RAP binders. MDSC uses a modulated sinusoidal temperature program that is superimposed on the linear temperature ramp used in non-modulated DSC. The use of a modulated signal results in a variable temperature rate as shown in Figure 7. The temperature rate followed a sinusoidal variation with an average of 3oC/min. As noted previously, the variable temperature rate allows better detection of the glass transition temperature.


14

Page 15 of 25 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

Energy & Fuels

Figure 7. Temperature rate used in Modulated DSC The DSC heating scans showing the total heat flow are shown in Figures 8-10 for the PAV-aged control RAP binder, PAV-aged EV rejuvenated RAP binder, and PAV-aged HG rejuvenated RAP binder, respectively. The shape of the total heat flow curve was similar for all three binders. It was also obvious that for all three binders the glass transition region is broad and is perturbed by other exothermic and endothermic peaks. The presence of these peaks makes it very difficult to accurately determine the glass transition region.


15

Energy & Fuels 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

Page 16 of 25

Figure 8. Total DSC Scan for control PAV-aged RAP binder

Figure 9. Total DSC Scan for PAV-aged EV rejuvenated RAP binder

Figure 10. Total DSC Scan for PAV-aged HG rejuvenated RAP binder Using MDSC, it was possible to split the total heat flow into reversible and non-reversible heat flow signals. This was done using the TA Universal Analysis software. The non-reversible signal


16

Page 17 of 25 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

Energy & Fuels

isolates the exothermic and endothermic peaks. Figures 11-13 shows the non-reversible heat flow for the control binder, EV rejuvenated binder, and HG rejuvenated binder, respectively.

Figure 11. Non-reversible DSC Scan for PAV-aged control RAP binder

Figure 12. Non-reversible DSC Scan for PAV-aged EV rejuvenated RAP binder


17

Energy & Fuels 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

Page 18 of 25

Figure 13. Non-reversible DSC Scan for PAV-aged HG rejuvenated RAP binder The general shape of the non-reversible heat flow scan for all three binders was very similar. No additional peaks were noted in the rejuvenated binders as compared to the control binder. In other words, the melting peaks due to the rejuvenator were not evident in the rejuvenated binders. This could possible mean that the rejuvenator was undergoing a physical or chemical interaction with the binder that was affecting its melting and crystallization behavior. The non-reversible heat flow DSC scans showed two peaks; an exothermic and an endothermic peak. The exothermic peak occurred during the glass transition region around -35oC. There are two possible explanations for the exothermic peak. One explanation is that it could be due to the crystallization of any asphalt fractions which did not crystallize upon cooling due to limited molecular mobility. Another explanation would be that the sample is giving off heat as the molecules start to arrange themselves as they gain more mobility as the material transition from a brittle state to a rubbery state. An endothermic peak is observed around 0oC denoting a melting event. This could be due to the melting of asphalt fractions which crystalized upon heating. Additionally, enthalpic recovery could occur following the glass transition region leading to an endothermic peak. Enthalpic recovery


18

Page 19 of 25 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

Energy & Fuels

takes place as the material gains energy to replace the energy it lost while in a glassy state trying to relieve stresses and attain an equilibrium stage. The glass transition temperature is defined as the half vitrification temperature

23.

The glass

transition does not typically occur at a certain temperature but rather over a wide range of temperatures. There are different ways to calculate a glass transition temperature that defines the glass transition region. In this study, the glass transition temperature was calculated as the midpoint heat capacity between the glassy and amorphous phases. Another temperature which can also be used to define the glass transition region is the glass transition onset temperature. The glass transition onset temperature marks the start of the glass transition region and is estimated as the intersection between the tangent line below the glass transition and the tangent line at the steepest point in the glass transition region 18. The glass transition feature in the TA Universal Analysis software was used to determine the glass transition temperature and the onset glass transition temperature for the PAV-aged control RAP binder as well as the PAV-aged rejuvenated RAP binders using the reversible heat flow DSC measurements. Figure 14 shows an example of how the glass transition temperature and the onset temperature were determined using the TA Universal analysis software for the PAV-aged RAP binder. Two points are selected before and after the glass transition region and the software calculates the glass transition point and the onset temperature. The glass transition point can be calculated as the inflection point or the midpoint between the heat capacity before and after the glass transition region. In this study, the midpoint heat capacity was used to determine the glass transition point.


19

Energy & Fuels 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

Page 20 of 25

Figure 14. Glass transition region for the PAV-aged RAP binder showing glass onset temperature and glass transition temperature. The glass transition temperature and onset temperature for all three binders are shown in Table 2. It is clear that the addition of the rejuvenator to the RAP binder slightly shifted the glass transition point and the onset temperature to a lower temperature. The effect of the two rejuvenators on the glass transition properties were very comparable, which is in line with the results of the low temperature performance grades.

Table 2. Glass transition properties of control and rejuvenated PAV-aged RAP binders Binder

RAP

Glass Transition Temperature, Tg (oC) -34.5 Onset Temperature, Tonset (oC) -31.9

RAP + EV

RAP + HG

-36.9 -33.2

-36.0 -34.6

SUMMARY AND CONCLUSIONS In this study, an extracted and recovered RAP binder was rejuvenated with two different rejuvenators at a dosage of 10 percent by weight. It was shown that the critical high, intermediate,


20

Page 21 of 25 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

Energy & Fuels

and low temperatures of the RAP binder were lowered by the rejuvenation process. The two rejuvenators derived from tall oil had similar effects on the RAP binder. At this dosage however, the RTFO mass loss recorded was 1.7 percent, indicating the presence of highly volatile constituents in the rejuvenators. The crystallization and melting behavior of the rejuvenators before and after PAV-aging was studied using DSC. It was shown that the DSC scans were able to capture the chemical changes occurring in the rejuvenators due to PAV-aging. The effect of aging was more pronounced on the unsaturated constituents compared to the saturated constituents, which could be attributed to the fact that unsaturated constituents are more prone to oxidation. The DSC scans show that the unsaturated constituents did not appear to be notably affected by the aging process. The control and rejuvenated RAP binders were studied using modulated DSC. It was shown that the modulated DSC was very efficient in isolating the crystallization and melting events, and detecting the glass transition region. Two distinct crystallization and melting peaks were noted for both the control and rejuvenated binders. The DSC peaks seen in the pure rejuvenator were not observed in the rejuvenator/RAP blended binders, which could serve as an indication that the rejuvenator is interacting physically and/or chemically with the binder in a way which is obscuring these peaks. It was also noted that the glass transition temperature of the RAP binder was shifted on average 2℃ lower with the addition of the rejuvenators. The effect of the two rejuvenators on the glass transition temperature was comparable, which coincides with their effect on the critical low temperature of the binder. Based on the results of this study, it is concluded that a study of the pure rejuvenators can provide valuable information in relation to their purity, composition and thermal properties which could be used as a start to assess their effect on binders, and could provide a basis for future specification


21

Energy & Fuels 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

Page 22 of 25

development. Techniques such as DSC could be used to engineer rejuvenators with desirable properties, i.e. a balance between saturated and unsaturated constituents where low temperature improvement is attained while maintaining the durability of the rejuvenator. However, the analysis of the rejuvenators alone, even though it provides some useful insight, does not fully explain their effect on binders. Physical and/or chemical interactions are likely to occur between the rejuvenators and the binder as indicated by the DSC scans, hence the analysis of the pure rejuvenators do not fully explain these interactions without a complete study of the performance of the rejuvenated binders. Accordingly, future specifications addressing the use of rejuvenators should not be solely based on characterizing the rejuvenators without assessing their effect on the binder. ACKNOWLEDGEMENTS This work was undertaken with funding from the California Department of Transportation (Caltrans), which is greatly appreciated. The opinions and conclusions expressed in this paper are those of the authors and do not necessarily represent those of the State of California or the Federal Highway Administration. The authors would also like to express sincere thanks to Paul Ledtje and Nacu Hernandez from Iowa State University for their help with binder extraction and testing. REFERENCES (1)

Elseifi, M. A.; Mohammad, L. N.; Cooper III, S. B. Laboratory Evaluation of Asphalt Mixtures Containing Sustainable Technologies. J. Assoc. Asph. Paving Technol. 2011, 80, 227–254.

(2)

Elkashef, M.; Williams, R. C. Improving Fatigue and Low Temperature Performance of 100% RAP Mixtures Using a Soybean-Derived Rejuvenator. Constr. Build. Mater. 2017, 151, 345–352.


22

Page 23 of 25 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

Energy & Fuels

(3)

Shen, J.; Amirkhanian, S.; Aune-Miller, J. Effects of Rejuvenating Agents on Superpave Mixtures Containing Reclaimed Asphalt Pavement. J. Mater. Civ. Eng. 2007, 19 (5), 376– 384.

(4)

Hajj, E.; Souliman, M.; Alavi, M.; Loría Salazar, L. Influence of Hydrogreen Bioasphalt on Viscoelastic Properties of Reclaimed Asphalt Mixtures. Transp. Res. Rec. J. Transp. Res. Board 2013, No. 2371, 13–22.

(5)

Chen, M.; Xiao, F.; Putman, B.; Leng, B.; Wu, S. High Temperature Properties of Rejuvenating Recovered Binder with Rejuvenator, Waste Cooking and Cotton Seed Oils. Constr. Build. Mater. 2014, 59, 10–16.

(6)

Zaumanis, M.; Mallick, R. B.; Frank, R. Evaluation of Different Recycling Agents for Restoring Aged Asphalt Binder and Performance of 100% Recycled Asphalt. Mater. Struct. 2015, 48 (8), 2475–2488.

(7)

Elkashef, M.; Podolsky, J.; Williams, R. C.; Cochran, E. W. Introducing a Soybean OilDerived Material as a Potential Rejuvenator of Asphalt through Rheology, Mix Characterisation and Fourier Transform Infrared Analysis. Road Mater. Pavement Des. 2018, 19 (8), 1750–1770.

(8)

Zaumanis, M.; Mallick, R. B.; Poulikakos, L.; Frank, R. Influence of Six Rejuvenators on the Performance Properties of Reclaimed Asphalt Pavement (RAP) Binder and 100% Recycled Asphalt Mixtures. Constr. Build. Mater. 2014, 71, 538–550.

(9)

Alavi, Z.; He, Y.; Harvey, J.; Jones, D. Evaluation of the Combined Effects of Reclaimed Asphalt Pavement (RAP), Reclaimed Asphalt Shingles (RAS), and Different Virgin Binder


23

Energy & Fuels 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

Page 24 of 25

Sources on the Performance of Blended Binders for Mixes with Higher Percentages of RAP and RAS; 2015. (10)

Elkashef, M.; Podolsky, J.; Williams, R. C.; Cochran, E. Preliminary Examination of Soybean Oil Derived Material as a Potential Rejuvenator through Superpave Criteria and Asphalt Bitumen Rheology. Constr. Build. Mater. 2017, 149, 826–836.

(11)

Elkashef, M.; Williams, R. C.; Cochran, E. W. Thermal and Cold Flow Properties of BioDerived Rejuvenators and Their Impact on the Properties of Rejuvenated Asphalt Binders. Thermochim. Acta 2019, 671, 48–53.

(12)

Claudy, P.; Letoffe, J. M.; King, G. N.; Plancke, J. P. Characterization of Asphalts Cements by Thermomicroscopy and Differential Scanning Calorimetry: Correlation to Classic Physical Properties. Fuel Sci. Technol. Int. 1992, 10 (4–6), 735–765.

(13)

Harrison, I. R.; Wang, G.; Hsu, T. C. A Differential Scanning Calorimetry Study of Asphalt Binders; Washington, DC, 1992.

(14)

Planche, J. P.; Claudy, P. M.; Létoffé, J. M.; Martin, D. Using Thermal Analysis Methods to Better Understand Asphalt Rheology. Thermochim. Acta 1998, 324 (1), 223–227.

(15)

Claudy, P. M.; Létoffé, J. M.; Martin, D.; Planche, J. P. Thermal Behavior of Asphalt Cements. Thermochim. Acta 1998, 324 (1), 203–213.

(16)

Lu, X.; Redelius, P. Effect of Bitumen Wax on Asphalt Mixture Performance. Constr. Build. Mater. 2007, 21 (11), 1961–1970.

(17)

Gill, P. S.; Sauerbrunn, S. R.; Reading, M. Modulated Differential Scanning Calorimetry. J. Therm. Anal. 1993, 40 (3), 931–939.


24

Page 25 of 25 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

Energy & Fuels

(18)

Turner, T. F.; Branthaver, J. F. DSC Studies of Asphalts and Asphalt Components. In Asphalt science and technology; Usmani, A., Ed.; CRC Press, 1997; pp 59–102.

(19)

Lei, Z.; Bahia, H.; Yi-qiu, T. Effect of Bio-Based and Refined Waste Oil Modifiers on Low Temperature Performance of Asphalt Binders. Constr. Build. Mater. 2015, 86, 95–100.

(20)

Qin, Q.; Farrar, M. J.; Pauli, A. T.; Adams, J. J. Morphology, Thermal Analysis and Rheology of Sasobit Modified Warm Mix Asphalt Binders. Fuel 2014, 115, 416–425.

(21)

Wei, J. B.; Shull, J. C.; Lee, Y.-J.; Hawley, M. C. Characterization of Asphalt Binders Based on Chemical and Physical Properties. Int. J. Polym. Anal. Charact. 1996, 3 (1), 33–58.

(22)

Elkashef, M.; Williams, R. C.; Cochran, E. Effect of Asphalt Binder Grade and Source on the Extent of Rheological Changes in Rejuvenated Binders. J. Mater. Civ. Eng. 2018, 30 (12), 4018319.

(23)

Baur, H.; Wunderlich, B. About Complex Heat Capacities and Temperature-Modulated Calorimetry. J. Therm. Anal. Calorim. 1998, 54 (2), 437–465.


25

Using Thermal Analytical Techniques To Study Rejuvenators and

Recommend Documents