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Energy & Fuels 1999, 13, 340-345
Effect of Asphaltenes on SHRP Superpave Specifications Christopher H. Domke, Richard R. Davison, and Charles J. Glover* Department of Chemical Engineering, The Texas Transportation Institute, Texas A&M University, College Station, Texas 77843-3122 Received September 23, 1998. Revised Manuscript Received November 17, 1998
The composition of an asphalt can greatly affect its physical and chemical properties. Increasing the asphaltene content creates an asphalt with superior high-temperature properties while having a minimum impact on the low-temperature properties. Increasing the saturate content improves the low-temperature properties but hurts the high-temperature properties as well. Hardening due to oxidation is dependent on the amount of both asphaltenes and saturates present. At low levels of asphaltenes, saturates help the hardening susceptibility. However, as levels of asphaltenes increase, an increased saturate level can greatly increase the hardening susceptibility.
Introduction Superpave specifications, known as performance grading (PG), have been implemented to ensure proper use of asphalts in a given climatic region. Different parameters are measured after various aging conditions have been applied to the asphalt. G*/(sin δ) is used to measure the high-temperature rutting parameter. It is measured on the original asphalt as well as after the thin film oven (TFO) test. The pressure aging vessel (PAV) is then used to artificially age the asphalt under high temperature and pressure. Following the PAV, the bending beam rheometer (BBR) is used to test the low-temperature thermal cracking by measuring stiffness (S) and the creep parameter (m value). Both G*/(sin δ) and S and the m value must pass a certain value for the asphalt to be considered a passing grade. The grade specification is determined by the temperature, in 6 °C increments, at which the specification is met. Several years ago it was determined that the oxidation of an asphalt binder could be represented accurately by the increase of carbonyl groups.1,2 This increase in carbonyl can be measured by the use of Fourier transform infrared spectroscopy (FT-IR). A spectrum of SHRP AAA-1 at different aging times is seen in Figure 1. Although this carbonyl growth represents only a small portion of the spectral change during aging, the area under the carbonyl peak correlates linearly with the total amount of oxygen present for every asphalt for which the oxygen content was measured3 (Figure 2). Thus, the FT-IR procedure is a good indication of the amount of oxidation the samples undergo while aging. Upon oxidative aging, the viscosity of an asphalt binder also increases. It was determined that the logarithm of viscosity (ln η) varied linearly with the carbonyl area (CA) observed from the FT-IR.1,2 The slope * Corresponding author. (1) Martin, K. L.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Transp. Res. Rec. 1990, 1269, 9-19. (2) Lau, C. K.; Lunsford, K. M.; Glover, C. J.; Davison, R. R.; Bullin, J. A. Transp. Res. Rec. 1992, 1342, 50-57. (3) Liu, M. Ph.D. Dissertation, Texas A&M University, 1996.
Figure 1. FT-IR spectrum of AAA-1.
Figure 2. Percent oxygen versus carbonyl area.
of this line, designated as the hardening susceptibility (HS), has a unique value for each asphalt. The HS has been determined to be independent of oxidation temperature below 373 K (212 °F) (Figure 3). The PG and the HS can be greatly affected by the composition of an asphalt. Asphalts are comprised of three major constituents: saturates, asphaltenes, and
10.1021/ef980194s CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
Effects of Asphaltenes on SHRP Superpave Specifications
Figure 3. Hardening susceptibility at various temperatures.
aromatics. Saturates are light, waxy materials. Asphaltenes are heavy, coal-like materials. Aromatics vary depending on the crude. In particular, how the amount of asphaltenes and saturates affect the physical and chemical properties of an asphalt is studied in this paper. Experimental Section Two asphalts aromatic fractions were used in this study: the bottoms of a vacuum tower distillation process (VTB) and the middle cut (resin) from a residual oil supercritical extraction (ROSE) unit using the VTB as the feed. Thus, the samples contain the same components; however, the resin contains a narrower molecular-weight band than the VTB due to the ROSE fractionation. The Giant Corbett process was utilized to produce large quantities of aromatic fractions and saturates.4 The Giant Corbett is similar to ASTM D-4124, however, it is used on a larger scale. The resin and VTB first had their asphaltenes removed by using a ratio of 20 mL of n-hexane to 1 g of asphalt. The mixtures were heated and stirred for 1 h, and the asphaltenes not dissolved were allowed to settle overnight. The mixtures were then filtered through coffee filters to remove the asphaltenes. The filtrate liquid was evaporated to yield the maltene. The maltene was then dissolved in n-heptane and run through the Giant Corbett. Saturates eluted first until a straw-like color began to elute from the column. The saturate product was then removed, and a 85%/15% (v/v) mixture of TCE/ethanol was used to remove the aromatics from the alumina. The saturates were saved and rerun through the Giant Corbett to collect any light aromatics that may have passed through the column. These light aromatics were combined with the previous collection of aromatics, and this product was considered to be the aromatic fraction. Saturates were collected by dissolving the top cut of the VTB ROSE fractionation (DAO) in a ratio of 20 mL of n-heptane to 1g of DAO. The mixture was heated again and stirred for 1 h before being allowed to settle overnight. The mixture was then filtered through a coffee filter to remove any asphaltenes. The filtrate was then passed through the Giant Corbett. The DAO fraction was used because of its large quantity of saturates, thus saving time to produce the needed amount of saturates for blending. Asphaltenes were collected by dissolving the bottom cut of the VTB ROSE fractionation (asphaltenes) in a ratio of 20 mL of n-hexane to 1 g of asphaltenes. The mixture was heated and stirred for 1 h before being allowed to settle overnight (4) Peterson, G. D.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Transp. Res. Rec. 1994, 1436, 38-46.
Energy & Fuels, Vol. 13, No. 2, 1999 341 and was filtered through a coffee filter the next day. The filter cake from the coffee filter was allowed to dry before use. The asphaltene fraction was used because of its large quantity of asphaltenes, thus saving time to produce the needed amount of asphaltenes for blending. The aromatics and asphaltenes were weighed together in a pint can for mixing. The can was wrapped in heating tape and temperature controlled at 204.4 °C (400 °F). The can had a nitrogen-gas blanket over the top of the sample to limit the amount of oxidation that would occur during the mixing process. The samples were mixed using a drill press with a spindle running at 1550 rpm for 1 h. The aromatic/asphaltene sample was immediately transferred to smaller tins, and the appropriate amount of aromatics and saturates were mixed in by hand to cut the sample back to the desired amount of asphaltenes and saturates. The asphalts were aged in a pressure oxygen vessel (POV)s a stainless steel reactor immersed in a temperature-controlled water-triethylene-glycol bathsto determine their aging properties. In this case, the hardening susceptibility was measured. Inside each POV was a rack with a capacity of approximately 80 4-cm × 7-cm aluminum trays. Each tray contained approximately 2.4 g of asphalt, resulting in an asphalt thickness of less than 1 mm. This low thickness minimizes the effect of oxygen diffusion on oxidation throughout the sample thickness.5 At various times, depending on the temperature and pressure of the reactor, samples were removed to determine the carbonyl area and viscosity of the samples. Carbonyl levels were measured with a Mattson Galaxy 5000 FT-IR by the attenuated total reflectance (ATR) method of Jemison et al.6 An integrated peak area in the carbonyl region from 1650 to 1820 cm-1 is defined as the carbonyl area (CA). Viscosities were measured with a Carri-Med CSL-500 controlled stress rheometer by using a 2.5 cm composite plate and a 500 µm gap. The low-frequency limiting dynamic viscosity at 333 K (140 °F) was determined and used for this work. For high-temperature performance grading measurements, the Carri-Med CSL-500 controlled stress rheometer was used. A 2.5 cm composite plate and 500 µm gap were again used. The frequency was set at 10 rad/s, and a torque sweep was run until the sample reached its Newtonian limit, usually around 10% strain. The complex shear modulus, G*, and the phase angle, δ, were measured and G*/(sin δ) was calculated. The asphalt samples were first aged for 5 h at 163 °C in the TFO following ASTM D-1754. A 7.6 g amount of material was weighed into several small round pans to create 3.2-mm thick samples and placed in the oven to age. This procedure was used because 50 g of material was not available. The TFO samples were then placed into our POV. The samples were aged for 20 h under 20 atm of air. These conditions are considered the proper amount of aging to simulate appropriate road aging. These samples approximate the same aging condition as the PAV.5 Following the PAV, the samples were then subjected to BBR analysis (AASHTO TP1). Approximately 12 g of asphalt was weighed into beams with dimensions of 125 mm long, 12.5 mm wide, and 6.25 mm deep. The beams were equilibrated at the operating temperature for 1 h, then placed under a 100 g constant load for 4 min. S and the m value were measured at 60 s and these are the properties considered for the PG specifications.
Results and Discussion The asphaltene content of an asphalt is an important factor in determining the physical properties of an (5) Domke, C. H.; Liu, M.; Davison, R. R.; Bullin, J. A.; Glover, C. J. Transp. Res. Rec. 1997, 1586, 10-15. (6) Jemison, H. B.; Burr, B. L.; Davison, R. R.; Bullin, J. A.; Glover, C. J. Fuel Sci. Technol. Int. 1992, 10, 795-808.
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Figure 4. High-temperature performance grade of resin aromatic blends.
Figure 5. High-temperature performance grade of VTB aromatic blends.
Table 1. Passing Temperatures for Resin Aromatic Blendsa high temperature low temperature % saturates % in blend asphaltenes SHRP continuous SHRP continuous 0%
5%
10%
15%
a
0% 5% 10% 20% 0% 5% 10% 20% 0% 5% 10% 20% 0% 5% 10% 20%
58 76 76 88 52 64 70 82 46 58 64 76 40 52 58 76
62 76 80 89 57 69 75 86 51 63 67 81 45 55 61 77
-4 -4 -4 NA -10 NA NA -4 -16 NA NA -10 -22 -22 -16 NA
-10 -7 -4 NA -16 NA NA -6 -20 NA NA -12 -24 -23 -21 NA
NA: not available. Table 2. Passing Temperatures for VTB Aromatic Blendsa
high temperature low temperature % saturates % in blend asphaltenes SHRP continuous SHRP continuous 0%
5%
10%
15%
a
0% 5% 10% 20% 0% 5% 10% 20% 0% 5% 10% 20% 0% 5% 10% 20%
70 76 82 94 64 70 76 88 58 70 70 82 52 64 70 82
75 81 85 96 66 75 80 90 61 70 74 87 54 64 70 82
2 2 2 NA -10 NA NA 2 -16 NA NA -10 -22 -16 -16 NA
-3 -2 -1 NA -11 NA NA -3 -18 NA NA -10 -23 -20 -19 NA
NA: not available.
asphalt. In particular, the performance grade of an asphalt can be manipulated by controlling the amount of asphaltenes in a given asphalt (Tables 1 and 2). As seen from Figures 4 and 5, the PG high-temperature property G*/(sin δ) is manipulated for different maltene compositions. The numbers are the high-temperature passing temperatures at 6 °C increments, while the
Figure 6. Low-temperature performance grade of resin aromatic blends.
numbers in parentheses are the “continuous” passing grade. The “continuous” passing grade is the interpolated temperature at which the specification would be met. In all cases, both the resin-blended maltenes and the VTB-blended maltenes have an increase in the hightemperature performance grade by adding asphaltenes. This would be beneficial in hot climates where high G*/(sin δ) parameters are required. The low-temperature performance grade is also affected by the addition of asphaltenes. As seen in Figures 6 and 7, the BBR passing temperature slowly increases as more asphaltenes are added. Again, both the Superpave grade and the “continuous” grade are given as before. It can be seen that adding asphaltenes affects the resin blends bending beam results more than the VTB blends. It is speculated that the VTB, having a wider molecular-weight span than the resin, contains heavier molecules that affect the low-temperature stiffness and relaxation parameter in a manner similar to asphaltenes. Combining the high- and low-temperature performance grades of Figures 4 and 6 into Figure 8 and Figures 5 and 7 into Figure 9 allows an interesting
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Energy & Fuels, Vol. 13, No. 2, 1999 343
Figure 7. Low-temperature performance grade of VTB aromatic blends.
Figure 10. Passing temperatures versus percent asphaltenes.
Figure 8. Performance grade of resin aromatic blends.
Figure 9. Performance grade of VTB aromatic blends.
observation. The asphaltenes actually help the hightemperature performance grade more than hindering the low-temperature performance grade. A drastic change can be seen in Figure 8. From point A to point B, the high-temperature performance grade increases
by 24 °C while the low-temperature performance grade remains unchanged (the PG goes from a 52-16 to a 7616). Thus, one could start with a very good lowtemperature performing asphalt and add asphaltenes in order to boost the high-temperature performance while maintaining the asphalt’s good low-temperature properties. The asphaltenes actually produce an increase in the span of the PG (Figure 10). For this representative sample, 15% saturates are kept constant in the blend. The Superpave passing temperature for both the highand low-temperature properties are plotted as the asphaltene content increases. As the asphaltene content increases, both S and G*/(sin δ) increase. However, Figure 10 shows that G*/(sin δ) increases at a much faster rate than S, providing the increase in PG span. The addition of asphaltenes seems contradictory to what is wanted for a long-lasting road, however. Asphaltenes are produced in oxidative aging of asphalts. By adding asphaltenes to an asphalt, one is artificially aging (hardening) the asphalt before it is even placed on the road, which thus creates an asphalt that will likely fail much earlier than it otherwise would. The saturate content of an asphalt is also an important factor in the determination of physical properties. The saturate content shifts the performance grade of an asphalt, allowing for better low-temperature properties. It appears that the effect of the saturate content is equal on both the high-temperature and the lowtemperature performance grade. Looking at Figures 4-7, it can be shown that the increase of saturates appears to lower the continuous passing grade of G*/ (sin δ) while improving the low-temperature continuous passing grade by approximately the same amount. Thus, the saturates shift the performance grade but do not increase the span. (Figure 11). A representative sample is shown keeping 10% asphaltenes constant. Here, both S and G*/(sin δ) decrease with the addition of saturates. However, the PG for both high and low temperature decrease by approximately 6 °C with an additional 5% saturates added. The equal decrease in passing temperature allows for the PG to be shifted to lower temperatures by adding saturates, without affecting the PG span.
344 Energy & Fuels, Vol. 13, No. 2, 1999
Figure 11. Passing temperatures versus percent saturates.
Figure 12. Hardening susceptibility of resin aromatic blends.
The combination of adding asphaltenes and saturates to an asphalt can improve the performance grade of an asphalt. The asphaltenes create a broadening of the PG span, while the saturates allow for a shift in the grade. Thus, one could theoretically create an asphalt with high quantities of asphaltenes and saturates to produce an asphalt with an excellent performance-grade span. However, there is a problem with adding large amounts of asphaltenes and saturates with respect to the oxidative aging of the asphalts. Figures 12 and 13 show the hardening susceptibility of the resin and VTB blends, respectively. At low asphaltene content, the addition of saturate lowers the HS. A low HS is desirable, as an asphalt with a low HS hardens less than an asphalt with a high HS for a given amount of oxidation. Because the hardening of an asphalt creates an early mode of failure, a lower amount of hardening is desirable. However, Figures 12 and 13 show that as more asphaltenes are added to the blends, the addition of saturates create an increase in the HS. This is likely due to the incompatibility of the asphalt. Corbett7 showed that an asphalt with high amounts of saturates (7) Corbett, L. W. Assoc. Asphalt Paving Technol. 1970, 39, 481491.
Domke et al.
Figure 13. Hardening susceptibility of VTB aromatic blends.
and asphaltenes is incompatible. The oxidation creates asphaltenes from the aromatic fraction8 that would be incompatible if high amounts of saturates were present. This would cause the increase in the HS seen at high asphaltene and saturate contents. There appears to be an optimum composition for the HS. In Figures 12 and 13, at low levels of asphaltenes, the saturates decrease the HS. However, at higher levels of asphaltenes, the saturates hurt the HS. For the resin aromatic blends (Figure 12), the optimum would appear to be at 3% asphaltenes and 17% saturates for those points that are measured. However, this point is the original resin before the Giant Corbett was run to collect the aromatic fraction and appears to be inconsistent with the other blends. It is possible that the aromatic fraction has changed slightly between the original and that used for blending. This original composition aside, it would appear that the optimum for the blended aromatics would occur around 3% asphaltenes and 5% saturates. This composition would create a 64-10 (6713) PG, not a good asphalt by grade, but a great hardening asphalt. The VTB aromatic blends (Figure 13) show the same type of optimum point for the HS. Excluding the 9% asphaltene axis, the optimum appears to be around 5% asphaltenes, and 10% saturates. This composition would create a 70-10 (70-15) PG, a fair grade span but a great hardening asphalt. Conclusions The composition of an asphalt can greatly affect its physical and chemical properties. Increasing the asphaltene content creates an asphalt with superior hightemperature properties while not hurting the lowtemperature properties. Increasing the saturate content improves the low-temperature properties but hurts the high-temperature properties. The performance-grade span increases as asphaltenes are added. However, increasing the saturate levels shifts the performance grade, with little to no PG span change. Hardening due to oxidation is dependent on the amount of both asphaltenes and saturates present. At (8) Corbett, L. W.; Schweyer, H. E. Assoc. Asphalt Paving Technol. 1981, 50, 571-604.
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Energy & Fuels, Vol. 13, No. 2, 1999 345
low levels of asphaltenes, saturates help to lower the hardening susceptibility. However, as levels of asphaltenes increase, an increased saturate level can greatly increase the hardening susceptibility. There appears to be an asphalt-dependent composition for achieving a minimum in the HS. It would be ideal to find the minimum in the HS and then use that composition for the PG, but the optimum composition does not always yield a good PG.
greatly appreciated. The support of Murphy Oil is also greatly appreciated. Disclaimer. The contents of the paper reflects the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration or the Texas Department of Transportation. This report does not constitute a standard, specification, or regulation. This report is not intended for construction, bidding, or permit purposes.
Acknowledgment. Support for this work by the Texas Department of Transportation (TxDOT), in cooperation with the U.S. Department of Transportation, and the Federal Highway Administration (FHWA) is
EF980194S
