混合料低温性能中的改性沥青性能理解

1、Understanding the Performance of Modified AsphaltBinders in Mixtures: Low-Temperature PropertiesFHWA-RD-02-074 by Kevin D. Stuart and John S. Youtcheff FOREWORDThis report documents the effects of polymer-modified asphalt binders on the low-temperature cracking resistances of asphalt mixtures. An em

2、phasis was placed on evaluating the performances of mixtures containing polymer-modified asphalt binders with identical Superpave performance grades, but varied modification chemistries. This study is part of a larger study titled Understanding the Performance of Modified Asphalt Binders in Mixtures

3、, which is partially funded through the National Cooperative Highway Research Program (NCHRP) Project 90-07. The objective of NCHRP Project 90-07 is to determine if asphalt binder performance is correctly captured by the Superpave asphalt binder specification developed under the 1987 through 1993 St

4、rategic Highway Research Program and modified under subsequent studies. This report will be of interest to highway personnel who use polymer-modified asphalt binders and Superpave. The recently developed Superpave critical cracking temperature (Tcr) for asphalt binders agreed with mixture performanc

5、e, except for one asphalt binder that is currently not used in practice. Several aggregate types were included in the study. The addition of hydrated lime to one of the aggregates significantly affected the low-temperature properties of the mixture. The mechanism for this is not clearly understood a

6、nd will be investigated.T. Paul Teng, P.E.Director, Office of InfrastructureResearch and DevelopmentNOTICEThis document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for its contents or use

7、thereof. This report does not constitute a standard, specification, or regulation.The U.S. Government does not endorse products or manufacturers. Trade and manufacturers= names appear in this report only because they are considered essential to the object of the document.ObjectiveThe objective of th

8、is study was to evaluate the cracking temperatures for asphalt binders provided bythe Bending Beam Rheometer (BBR) and Thermal Stress Analysis Routine (TSARTM). Low-temperature mixture properties provided by the Thermal Stress Restrained Specimen Test (TSRST) were used to validate these asphalt bind

9、er tests. An emphasis was placed on evaluating the performances ofmixtures containing polymer-modified asphalt binders with identical Superpave performance grades (PGs) and similar base asphalts, but varied modification chemistries. This would indicate what types of modification provide properties t

10、hat are, or are not, correctly captured by the current Superpave asphalt binder specification. BBR and TSARTMThe BBR provides two cracking temperatures. One temperature is based on creepstiffness (S); the other temperature is based on the m-value. The BBR test isperformed in accordance with American

11、 Association of State Highway and Transportation Officials (AASHTO) TP1, titled Method for Determining the Flexural Creep Stiffness ofAsphalt Binders Using the Bending BeamRheometer (BBR).(1) TSARTM isa computer program from Abatech, Inc., Doylestown, PA, that performs AASHTO PP42-01, Standard Pract

12、ice forDetermination of Low-Temperature Performance Grade (PG) of Asphalt Binders.(2) A critical cracking temperature (Tcr) is computed using data from both the BBR and thedirect tension. The standard test method for the direct tension is AASHTO TP3, titled Determining the Fracture Properties of Asp

13、halt Binder in Direct Tension (DT).(2)TSRSTThe TSRST cools a beam of asphalt mixture at a rate of 10癈elsius/hour (C/h) while restraining itfrom contracting. Stress builds up in the beam until it breaks. The resistance to low-temperature cracking increases as the temperature needed for fracture decre

14、ases. The stress at failure can also be analyzed. Additional information on the TSRST is given in AASHTO TP10-93, titled Method for Thermal Stress Restrained Specimen Tensile Strength.(2) (Note: This test is commonly called the Thermal Stress Restrained Specimen Test.)Asphalt BindersEleven asphalt b

15、inders were tested. This included one air-blown asphalt and eight polymer-modified asphaltbinders: (1) styrene-butadiene-styrene SBS Linear, (2)SBS Linear Grafted, (3) SBS Radial Grafted, (4) ethylene vinyl acetate EVA, (5)EVA Grafted, (6) Elvaloy, (7) ethylene styrene interpolymer ESI, and (8) chem

16、ically modified crumb rubber asphalt CMCRA. There were two control asphalt binders: an unmodified PG70-22 and an unmodified PG 64-28. The eight polymer-modified asphalt binders include elastomeric and plastomeric modifiers, some with the same modifier but different geometries (linear vs.radial geome

17、tries). The term grafted includes any mode of chemically reacting a polymer with an asphalt binder, for example, vulcanization. The target PG for the polymer-modified asphalt binders was PG 73-28. The PG64-28 asphalt binder, and a PG 52-34 asphalt binder from the same crude source, were modified. Th

18、e air-blown asphalt was originally the PG 52-34 asphalt binder.Experiment Using Diabase AggregateThe mixtures consisted of diabase aggregate and the 11 asphalt binders. A minimum of three replicate beams at 7.0 ?0.5-percent air voids were tested by the TSRST. The mixtures were subjected to 2 h of sh

19、ort-term oven aging (STOA) at 135癈 before compaction. Two hours of STOA was found to provide the average amount of aging for pavements constructed in 1993 for a Federal Highway Administration (FHWA) Superpave validation study.(3) It was based on pavement samples taken 3 months after construction. Mi

20、xture tests that measure low-temperature cracking resistance are often performed on specimens that have been subjected to long-term oven aging (LTOA), such as at 85癈 for 120 h.(4) In this study, it was decided to use 2 h of STOA and then determine from the data whether LTOA was needed. The asphalt b

21、inders were subjected to both rolling thin-film oven (RTFO) aging and pressure aging vessel (PAV) aging before testing, which is considered LTOA. The data are given in tables 1 and 2. These tables provide the same data. The mixtures in table1 are ranked according to the average TSRST fracture temper

22、ature, while the mixtures in table 2 are ranked according to the average TSRST fracture stress. An analysis of variance and Fishers least squares difference (LSD) were usedto rank the mixtures at a 5-percent level of significance. The capital letters in the tables are the statistical rankings. All m

23、ixtures with the same letter have averages that are not significantly different from one another. They are in the same group. All groups are designated by asingle letter. However, the groups can overlap. An average with more than one letterindicates that it falls into more than one group. For exampl

24、e, if an average has the designation A B, it falls into two groups, both A and B. Fishers LSD showed that the average TSRST fracture temperatures for the mixtures in table 1 must differ by at least 4.0癈 for them to be significantly different ata5-percent level of significance. The average TSRST frac

25、ture stresses must differ by approximately 500 kilopascals (kPa) for them to be significantly different. The data for individual beams are given in tables 3 and 4. (Note: The standard deviation s and coefficient ofvariation CV for fracture temperature depend on what unit is used Celsius, Fahrenheit,

26、 or Kelvin. Celsius was used in this study.) Table 1. Low-temperature asphalt binder properties vs. TSRSTwith the materials ranked according to mixture fracture temperature.Asphalt Binder and Mixture DesignationAsphalt Binder Cracking Temperature After RTFO/PAV (癈)?/strongMixture Property After 2 h

27、of STOA at 135癈TSRST Fracture Temperature and Ranking (C)TSRST Fracture Stress (kPa)TcrBBRSBBRmESI-29-31-31-33A2320Elvaloy-34-31-34-33AB2240SBS Linear Grafted-34-33-34-33ABC2310EVA Grafted-33-32-31-31ABCD1860SBS Linear-33-32-31-30ABCDE2110SBS Radial Grafted-34-32-32-30ABCDE2300EVA-31-31-31-29BCDE279

28、0CMCRA-29-29-29-29CDE1095Air-Blown-28-30-28-27DEF1960PG 64-28-28-28-30-26EF1680PG 70-22-27-28-29-24F2120Tcr = Critical Cracking Temperature. S = Creep Stiffness. m = m-value.Table 2. Low-temperature asphalt binder properties vs. TSRST with the materials ranked according to mixture fracture stress.As

29、phalt Binder and Mixture DesignationAsphalt Binder Cracking Temperature After RTFO/PAV (癈)?/strongMixture PropertyAfter 2 h of STOA at 135癈TSRST Fracture Temperature (癈)?/strongTSRST Fracture Stress and Ranking (kPa)TcrBBRSBBR mEVA-31-31-31-292790 AESI-29-31-31-332320 ABSBS Linear Grafted-34-33-34-3

30、32310 ABSBS Radial Grafted-34-32-32-302300 ABElvaloy-34-31-34-332240 ABPG 70-22-27-28-29-242120 BSBS Linear-33-32-31-302110 BCAir-Blown-28-30-28-271960 BCEVA Grafted-33-32-31-311860 BCPG 64-28-28-28-30-261680 CCMCRA-29-29-29-291095 DTcr = Critical Cracking Temperature. S = Creep Stiffness. m = m-val

31、ue.Table 3. TSRST fracture temperatures for individual beams with diabase.Asphalt Mixture DesignationTSRST Fracture Temperature (癈)?/strongTest #1Test #2Test #3Test #4sCVSTOA = 2 h ESI-32.7-31.9-33.2-37.32.47.3Elvaloy-31.3-35.4-34.32.16.4SBS Linear Grafted-29.6-34.0-35.53.19.4EVA Grafted-31.1-25.8-3

32、6.3-31.24.313.9 SBS Linear-32.2-30.1-29.11.65.3SBS Radial Grafted-31.3-26.1-33.03.612.0 EVA-29.6-29.0-30.40.72.4CMCRA-28.6-29.7-28.40.72.4Air-Blown-26.3-27.8-27.30.83.0PG 64-28-25.0-27.1-27.01.24.6PG 70-22-24.0-24.0-26.7-23.01.66.7STOA = 8 h ESI-35.0-30.7-32.52.26.9Elvaloy-29.0-27.6-30.21.34.6SBS Ra

33、dial Grafted-28.3-30.9-28.41.55.0PG 70-22-20.4-20.2-22.41.25.7STOA = 24 h ESI-23.4-23.7-24.50.72.3Elvaloy-25.8-27.1-26.80.72.6SBS Radial Grafted-22.9-23.5-25.11.14.8PG 70-22-19.3-26.7-21.43.816.9 s = Standard Deviation of Fracture Temperature, C.CV = Coefficient of Variation, percent = (s ?average)*

34、100.Table 4. TSRST fracture stresses for individual beams with diabase.Asphalt Mixture DesignationTSRST Fracture Stress (kPa) (STOA = 2 h)Test #1Test #2Test #3Test #4sCVEVA2830265028901204.3ESI22602370248021701305.6SBS Linear Grafted2450221022701205.2SBS Radial Grafted29402080187057024.8Elvaloy26702

35、000205037016.5PG 70-22207019102470203024011.3SBS Linear8401202022101205.7Air-Blown17701920219021010.7EVA Grafted192024401220610 32.9PG 64-2815401940156023013.7CMCRA1290105094018016.41Outlier.s = Standard Deviation of Fracture Temperature, C.CV = Coefficient of Variation, percent = (s ?average)*100.T

36、ables 1 and 2 show that the ranges in the TSRST fracture temperatures and stresses provided by the polymer-modified asphalt binders were narrow. Table 1 shows that most of the polymer-modified binders provided relatively close fracture temperatures, and six of these binders fell into group A. Furthe

37、rmore, the fracture temperature for SBS Linear and SBS Radial Grafted are not significantly different fromthe temperature forany other mixture except for the mixture with the unmodified PG70-22 asphalt binder. The closeness of the fracture temperatures was expected because the asphalt binders were p

38、roduced to have close low-temperature PGs. Table 2 shows that the TSRST fracture stresses for 8 of the 11mixtures werenot significantly different. Most mixtures fell into group B. The TSRST tests on the mixtures with EVA and SBS showed that grafting and polymer geometry generally had no significant

39、effect on fracture temperature or stress. The only exception is that the mixture with EVA Grafted had a significantly lower fracture stress than the mixture with EVA. The correlation between TSRST fracture temperature and Tcr is shown in figure 1. The r-squared (r2) was low at 0.54, but it increased

40、 to 0.85 after eliminating the data fromESI. Table 3 shows that the individual TSRST fracture temperatures for ESI were not highly variable. High variability could decrease the confidence in the average temperature. Tables 3 and 4 show that EVA Grafted and SBS Radial Grafted provided the highest var

41、iability using 2h of STOA. The slope for the regression line without ESI is 0.96 and the offset is 2癈, with Tcr providing the lower temperature. Therefore, the TSRST fracture temperature isequal to Tcr plus 2癈 when the mixture STOA period is 2h. This difference in temperature may be related to diffe

42、rences in age-hardening or to the absorption of asphalt light ends into the aggregate. LTOA would increase the fracture temperatures of the mixtures, which would increase the offset.Figure 2 shows that the correlation between the TSRST fracture temperatures and the cracking temperatures provided by

43、creep stiffness was poor, although the trend is correct. Ther2was 0.66. The slope is1.5. The data point for ESI is not an obvious outlier as in figure 1. Therefore, creep stiffness alone cannot explain why ESI is an outlier based on Tcr. Furthermore, the polymer in ESI is not prone to separate from

44、the base asphalt during use. Figure 3 shows that the correlation between the TSRST fracture temperatures and the cracking temperatures provided by the m-value was poor, although the trend is correct. Ther2was 0.59. The slope is 1.2. For a given asphalt binder, the BBRprovides two temperatures. One t

45、emperature is based on creep stiffness while the other temperature is based on the m-value. The higher of the two temperatures is the limiting cracking temperature. Figure 4 shows that the correlation between the TSRST fracture temperatures and the limiting cracking temperatures was weak. Ther2was 0

46、.71. The slope is1.5, which means that when the temperature based on the BBR changes by 6癈 (onePG), the change in TSRST fracture temperature is 9癈. A 9癈 change in fracture temperature is large. This suggests that the increment between PGs should be less than 6癈. Figure 5 shows that the relationship

47、between Tcr and the limiting cracking temperature from the BBR was fair. Ther2s were 0.77 and 0.89, with and without ESI, respectively. Although the two tests may correlate to each other, they are not identical. They provided different slopes when correlated to the TSRST fracture temperature. There

48、was no correlation between the TSRST fracture temperature and the TSRST fracture stress. A linear regression provided an r2 of 0.09. There was no correlation between the TSRST fracture stress and Tcr, creep stiffness, m-value, or the limiting cracking temperature based on both creep stiffness and m-

49、value. The r2s were 0.13, 0.24, 0.23, and 0.23, respectively. The relationship using creep stiffness is shown in figure 6. Experiment Using Diabase, Granite, and Limestone AggregatesThe ESI, Elvaloy, SBS Radial Grafted, and air-blown asphalt binders were tested with granite and limestone aggregates

50、to determine the effect of aggregate type on TSRST fracture temperature. Table 5 shows that the effect of aggregate type was relatively small, except for the mixtures with Elvaloy. The fracture temperature of -24癈 for Elvaloy with granite is high compared to the temperature of -33癈 for Elvaloy with

51、either diabase or limestone. It was hypothesized that the adhesive strength between Elvaloy and granite may be relatively poor. Therefore, 1.0-percent hydrated lime was added to the granite aggregate to determine if this would decrease (improve) the fracture temperature. The average temperature did

52、decrease from -24癈 to -36癈, although the fractured surfaces of the beams showed no visual differences. Granite with hydrated lime was then evaluated as a fourth aggregate type. Table 5 shows that creep stiffness provided cracking temperatures closest to the average TSRST fracture temperatures. Howev

53、er, most of the temperatures are very close to each other, so a firm conclusion regarding which asphalt binder test correlates the best with TSRST fracture temperature cannot be made. Note that 2 h of STOA were used for the mixtures. The effect of aggregate type on the fracture temperature of each a

54、sphalt binder is shown in table 6. Aggregate type had no significant effect on the mixture with the air-blown asphalt binder. Although the hydrated lime decreased the fracture temperature for Elvaloy with granite, it did not decrease the fracture temperatures for the other three asphalt binders. In fact, the hydrated lime significantly increased the fracture temperatures for ESI and SBS Radial Grafted with granite. Table 7 presents the same data grouped to show th