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Publication Number: FHWA-RD-02-075
Date: October 2000
The polymer-modified asphalt binders used in this study had continuous high-temperature PG's ranging from 71 to 77. The mixtures were tested by the French PRT at 70°C . This was the highest temperature that can be applied by this tester. The mixtures were tested for cumulative permanent shear strain at 50°C because of testing problems encountered at higher temperatures in previous studies. However, most of the problems were provided by the Frequency Sweep at Constant Height (FSCH) mode of loading. RSCH and FSCH are both applied by the Superpave Shear Tester. It was recommended that the diabase mixtures be retested using RSCH at a temperature closer to the PG's of the asphalt binders.
The objective of phase 2 was to retest the diabase mixtures at 70°C using RSCH to determine which asphalt binders provide high-temperature properties that do not agree with mixture rutting resistance.
All 11 asphalt binders were used. Information on the asphalt binders, aggregates, and mixture design are given elsewhere.(1-2) The reduced asphalt binder content of 4.55 percent by mixture mass was used so that the mixture met the 4.0-percent design air-void level as recommended by Superpave. Prior problems with testing mixtures at temperatures above 50°C contributed to the decision to lower the asphalt binder content. However, it confounded the analysis of the data because the mixtures tested by the French PRT had a 4.85-percent asphalt binder content.
High-temperature asphalt binder properties were measured by a DSR after RTFO aging.(3) Mixture rutting resistance was based on the cumulative permanent shear strains from RSCH and the rut depths from the French PRT.(4-5) All mixtures were subjected to 2 h of STOA at 135°C . Specimens were tested 48 h after compaction.
Cumulative permanent shear strain from RSCH was measured at 7.0-percent air voids, 70°C , and 5,000 cycles. The applied shear stress was 69 ±5 kPa. The loading time was 0.1 s and the rest time was 0.6 s. A minimum of three replicate specimens were tested per mixture. Lower cumulative permanent shear strains indicate more resistance to rutting.
The data at both 50°C and 70°C are given in table 6 and figure 4. The two regression lines suggest that mixtures containing asphalt binders with the highest PG's tended to be highly resistant to rutting at both test temperatures. While this seems reasonable, the difference in asphalt binder content probably decreased the vertical differences between the two lines.
Table 6 shows that the rankings are not the same at 50°C and 70°C . The only certain difference in ranking is that the mixture with PG 70-22 performed relatively worse at 70°C compared to 50°C . Figure 5 shows that the data point for this mixture was furthest from the regression line. Being above the line, this mixture performed worse than expected at 70°C , or better than expected at 50°C . Because of the change in asphalt binder content, no firm conclusions could be made for the other asphalt binders.
The cumulative permanent shear strains in table 6 indicate that grafting did not improve the rutting resistance of EVA at either temperature at a 5 percent level of significance. Grafting and geometry had no significant effect on the rutting resistance of SBS at 50°C . All three SBS mixtures fell into group D. SBS Radial Grafted did perform significantly better than SBS Linear at 70°C . The shear strain for SBS Linear Grafted at 70°C was not significantly different from the shear strains for SBS Radial Grafted or SBS Linear at 70°C .
The coefficients of variation (CV) for cumulative permanent shear strain at 5,000 cycles ranged from 1.7 percent to 36.7 percent. (See table 7.) Several data points were found to be outliers. These outliers were not used when evaluating asphalt binder properties. However, all of the replicate data are included in table 7 because the variability of the data in table 7 is a good representation of the variability typically provided by RSCH. The averages and CV's in the parentheses include the outliers. At 50°C , the CV's ranged from 8.2 to 24.1.(1) Variability appeared to be greater at 70°C , but a paired t-test indicated that it was not greater.
For the tests at 50°C , the cumulative permanent shear strains were correlated to the G*/sind 's of the asphalt binders at three DSR frequencies: 10.0, 2.0, and 0.125 rad/s.(1) The G*/sind 's are given in table 8. These three frequencies provided r2's of 0.06, 0.55, and 0.89, respectively, using log-log transformations. The correlation depended on DSR frequency. The correlation to high-temperature PG was 0.68 without transformation. (Note: A log-log transformation was used if it provided a higher r2. Relationships between asphalt binder and mixture high-temperature properties are normally curvilinear. Thus, a log-log or power law transformation usually provides a higher r2.)
|Asphalt Binder or Mixture||DSR After RTFO Aging||RSCH After 2 h of STOA|
|High-Temp. PG||G*/sind at 70°C and Three DSR Frequencies (Pa)||Cumulative Permanent Shear Strain at 70°C (mm/m)||Cumulative Permanent Shear Strain, at 50°C (mm/m)|
|10.0 rad/s||2.0 rad/s||0.125 rad/s|
|Elvaloy||77||4 110||1 330||166||13 100||A||14 600||A|
|EVA||75||1 910||434||36||22 220||A||B||13 600||A|
|SBS RG||71||2 680||657||49||23 720||B||21 300||B||C||D|
|SBS LG||72||2 880||746||62||27 600||B||C||23 200||C||D|
|EVA G||74||3 440||823||61||28 200||B||C||15 400||A||B|
|CMCRA||76||4 510||1 150||93||28 900||B||C||19 100||A||B||C|
|ESI||76||4 030||1 040||76||30 400||B||C||22 700||C||D|
|Air-Blown||74||3 870||920||66||33 340||C||21 300||B||C||D|
|SBS L||72||2 710||655||50||35 800||C||26 500||D|
|PG 70-22||71||2 640||568||37||50 850||D||23 900||C||D|
|PG 64-28||67||1 570||330||21||62 700||E||38 600||E|
Figure 5. RSCH cumulative permanent shear strain at 70°C
vs. RSCH cumulative permanent shear strain at 50°C.
Table 7. Replicate cumulative permanent shear strains at 70°C .
|2,500 Cycles||5,000 Cycles|
|Strain (mm/m)||Average Strain||CV1||Strain (mm/m)||Average Strain||CV1|
|PG 70-22||1||41 980||35 820||17.9||64 000||50 850||28.0|
|2||29 490||36 000|
|3||40 620||62 000|
|4||31 170||41 400|
|Elvaloy||1||8 990||11 400||18.3||10 290||13 100||18.4|
|2||12 740||14 570|
|3||12 470||14 330|
|ESI||1||27 230||24 800||11.1||34 560||30 400||15.2|
|2||21 820||25 410|
|3||25 400||31 260|
|EVA||1||(10 260)2||19 110
|(11 470)2||22 220
|2||15 210||17 280|
|3||21 540||26 080|
|4||20 590||23 290|
|5||(25 490)2||(32 590)2|
|SBS LG||1||19 570||22 800||13.2||23 130||27 600||14.2|
|2||23 400||29 420|
|3||25 560||30 320|
|AB||1||(41 510)2||26 740
|(57 680)2||33 340
|2||27 930||34 600|
|3||28 330||36 670|
|4||24 790||30 190|
|5||25 900||31 880|
|SBS L||1||39 930||31 100||27.4||42 500||35 800||22.2|
|2||22 970||27 030|
|3||30 280||37 990|
|EVA G||1||23 710||23 700||17.1||27 880||28 200||19.7|
|2||19 620||22 750|
|3||27 700||33 850|
|SBS RG||1||23 750||20 300
|27 860||23 720
|2||20 610||23 660|
|3||(36 060)2||(39 120)2|
|4||17 800||21 170|
|5||19 050||22 170|
|CMCRA||1||24 160||24 900||12.8||28 050||28 900||13.0|
|2||22 150||25 610|
|3||28 380||32 960|
|PG 64-28||1||52 300||51 400||4.0||62 850||62 700||1.7|
|2||52 810||61 600|
|3||49 010||63 700|
1CV = Coefficient of Variation, percent = (standard deviation
|Asphalt Binder||G*/sind at 50°C After RTFO Aging (Pa)|
|10.0 rad/s||2.0 rad/s||0.125 rad/s|
|EVA||26 300||12 100||2 740|
|EVA Grafted||35 800||14 300||2 310|
|Elvaloy||28 700||10 000||1 600|
|CMCRA||44 300||13 900||1 540|
|Air-Blown||49 100||14 200||1 390|
|SBS Linear Grafted||25 600||8 000||920|
|ESI||32 300||8 900||870|
|SBS Linear||25 400||7 700||810|
|PG 70-22||40 700||10 200||810|
|SBS Radial Grafted||25 100||7 600||800|
|PG 64-28||22 200||5 400||400|
Table 6 gives the G*/sind 's at 70°C . When correlated against cumulative permanent shear strain, frequencies of 10.0, 2.0, and 0.125 rad/s provided r2's of 0.22, 0.37, and 0.59, respectively, using log-log transformations. This correlation also depended on DSR frequency. The correlation to high-temperature PG was 0.63 without transformation. All of the r2's are given in table 9.
The correlations to G*/sind at 50°C and 70°C using a DSR frequency of 0.125 rad/s are shown in figures 6 and 7, respectively. The correlation at 50°C is very good. The largest deviation was provided by Elvaloy, followed by SBS Radial Grafted. Figure 7 shows that the G*/sind 's for EVA and SBS Radial Grafted are low at 70°C . G*/sind underpredicted their resistances to rutting. Because the data for EVA significantly affected the position of the trend line, the trend line in figure 7 was drawn without the data for EVA. The data point for SBS Radial Grafted did not significantly affect the position of the trend line.
Figures 8 and 9 show the data at 70°C using DSR frequencies of 2.0 and 10.0 rad/s, respectively. Both figures show that the G*/sind for PG 70-22 is high, while the G*/sind 's for EVA and Elvaloy are low. The G*/sind for SBS Radial Grafted is low at 2.0 rad/s, but not at 10.0 rad/s based on 95-percent confidence bands.
Figure 10 shows that the correlation using high-temperature PG is poor, although the r2 increased from 0.63 to 0.79 after excluding the data for SBS Radial Grafted. The 95-percent confidence band for cumulative permanent shear strain at the mean PG of 74 is 22 000 to 36 000 mm/m with SBS Radial Grafted and 25 000 to 36 000 mm/m without SBS Radial Grafted.
The higher r2 using G*/sind at 0.125 rad/s compared to G*/sind at 10.0 rad/s suggests that, according to cumulative permanent shear strain, a low DSR frequency might provide a better grading system. If the frequency is changed, then the criterion, which is currently 2200 Pa after RTFO, must also be changed. Figure 11 shows the relationship between cumulative permanent shear strain and temperature if the frequency is changed to 0.125 rad/s, but the criterion is not changed. The temperatures are very low and the correlation is poor. The lack of a known correlation between cumulative permanent shear strain and pavement rutting makes it difficult to choose a criterion. A preliminary recommendation based on the ranking in table 6 is to use a maximum allowable shear strain of around 30 000 mm/m to 40 000 mm/m. Figure 7 shows that a shear strain of 30 000 mm/m (log 30 000 = 4.477) provides a criterion of around 60 Pa (log 60 1.8), although the scatter in figure 7 shows that this will not provide a perfect grading system. Furthermore, the relationship based on 0.125 rad/s in figure 7 is not better than the relationship based on high-temperature PG in figure 10.
Table 10 and figure 12 provide the cumulative permanent shear strains for the asphalt mixtures at 70°C and 5,000 cycles vs. the cumulative permanent shear strains for the asphalt binders from repeated creep at 70°C and 100 cycles. The correlation is poor, having an r2 of 0.58. If the data for the PG 64-28 materials are removed, the r2 drops to 0.21. Based on the mixture test results, the cumulative permanent shear strains for the Elvaloy and EVA Grafted asphalt binders are high, while they are low for the PG 70-22 asphalt binder. The relationship should start at the zero-zero origin, but it does not. Therefore, the relationship must be curvilinear. Figure 13 provides a log-log relationship. This did not improve the correlation. The r2 of 0.38 is poor. The repeated creep is a new asphalt binder test and it is not known if the protocols are the optimal protocols.
The rut depths from the French PRT at 70°C are given in tables 11 and 12.(1) Table 12 shows that the mixture with SBS Radial Grafted had a high coefficient of variation. Tests on the mixtures with EVA and SBS Radial Grafted were repeated. The range in the replicate rut depths for these mixtures is relatively large compared to the range in average rut depth for all modified asphalt binders. The statistical ranking in table 11 shows that the rut depths for all mixtures, except for the mixture with PG 64-28, had rut depths that were not different at a 5-percent level of significance.
Figures 14 and 15 show the relationships between rut depth and G*/sind using DSR frequencies of 0.9 and 10.0 rad/s, respectively. The G*/sind 's for EVA may be low. If so, G*/sind underpredicted the relative rutting resistance provided by EVA. Without EVA, frequencies of 0.9 and 10.0 rad/s provided r2's of 0.83 and 0.91, respectively, compared to 0.54 and 0.56 with EVA. Without both EVA and PG 64-28, frequencies of 0.9 and 10.0 rad/s provided r2's of 0.69 and 0.67, respectively. The data point for PG 64-28 increases the upward curvature of the relationship, while the data point for EVA tends to flatten the relationship. If the 9.9-percent rut depth for SBS Radial Grafted in table 12 were to be eliminated, G*/sind would also underpredict the relative rutting resistance of this asphalt binder.
Figure 16 provides the correlation with high-temperature PG. The PG of EVA agrees with mixture performance. This means that the G*/sind for EVA is not increasing as rapidly as it should at temperatures immediately below its high-temperature PG. EVA was found to have the lowest slope (G*/sind divided by temperature) around the grading temperatures. The r2 of 0.89 drops to 0.57 without the data for PG 64-28. Even so, no data point is more than 1.5°C away from the regression line.
Unlike cumulative permanent shear strain, low and high DSR frequencies provided approximately the same degree of correlation with rut depth, even though a cursory review of the G*/sind 's in table 11 showed that the two frequencies did not provide identical rankings for the asphalt binders. To examine this in more detail, the G*/sind 's were linearly regressed. The r2 of 0.81 in figure 17 shows that the relationship was good. Without Elvaloy, the r2 is 0.97. Based on the rut depths in table 11, the G*/sind of 4110 Pa for Elvaloy at 10 rad/s is low relative to the other asphalt binders. It should have the highest G*/sind .
|RSCH at 5,000 Cycles||Coefficient of Determination, r2|
|High-Temp. PG||G*/sind at the RSCH Test Temperature
and Three Frequencies
|10.0 rad/s||2.0 rad/s||0.125 rad/s|
Shear Strain at 50°C
Shear Strain at 70°C
Figure 11. RSCH cumulative permanent shear strain
at 70°C vs. PG temperature based on 0.125 rad/s.
|Asphalt Binder or Mixture||Asphalt Binder Cumulative Permanent Shear Strain at 100 Cycles (mm/m)||Asphalt Mixture Cumulative Permanent Shear Strain at 5,000 Cycles (mm/m)|
|Elvaloy||3 418||13 100|
|EVA||3 667||22 220|
|SBS Linear Grafted||4 752||27 600|
|EVA Grafted||10 400||28 200|
|CMCRA||4 242||28 900|
|SBS Radial Grafted||4 948||23 720|
|ESI||1 751||30 400|
|SBS Linear||6 609||35 800|
|Air-Blown||5 051||33 340|
|PG 70-22||8 040||50 850|
|PG 64-28||20 082||62 700|
Figure 13. Comparison of cumulative permanent
shear strain using a log-log transformation.
|Asphalt Binder or Mixture
|French PRT After
2 h of STOA
|High-Temp. PG||G*/sind at 70°C (Pa)||Rut Depth at 70°C (percent)|
|10.0 rad/s||0.9 rad/s||6,000 Passes||20,000 Passes|
|Elvaloy||77||4 110||753||6.5 A||7.9 A|
|Air-Blown||74||3 870||439||6.8 A||9.0 A|
|CMCRA||76||4 510||566||6.8 A||9.7 A|
|EVA||75||1 910||203||7.1 A||9.4 A|
|SBS Radial Grafted||71||2 680||312||7.4 A||8.9 A|
|EVA Grafted||74||3 440||394||7.5 A||10.4 A|
|ESI||76||4 030||500||7.6 A||9.2 A|
|SBS Linear Grafted||72||2 880||361||8.2 A||10.3 A|
|PG 70-22||71||2 640||260||8.3 A||10.6 A|
|SBS Linear||72||2 710||309||8.5 A||10.5 A|
|PG 64-28||67||1 570||151||12.1 B||16.0 B|
Table 12. Replicate data for the French PRT at 6,000 wheel passes.
|Asphalt Mixture||Rut Depth at 6,000 Wheel Passes and 70°C (percent)||CV1|
|Specimen No. 1||Specimen No. 2||Average|
|SBS Radial Grafted||6.5||9.9||7.4||22.6|
|SBS Radial Grafted (Repeat)||6.3||7.0|
|SBS Linear Grafted||8.6||7.7||8.2||7.8|
1CV = Coefficient of Variation, percent = (standard deviation ÷ average)*100.
Figure 17. G*/sind at 0.9 rad/s vs. G*/sind at 10.0 rad/s.
Figure 18 provides the correlation with the cumulative permanent shear strains from the asphalt binder repeated creep test. The r2 of 0.83 drops to 0.19 without the data for PG 64-28. The narrow range in rut depth provided by the French PRT makes it difficult to make a firm conclusion.
Figure 19 provides the relationship between the cumulative permanent shear strain from RSCH at 70°C and the French PRT rut depth at 70°C . If the data for the mixture with the PG 64-28 asphalt binder are excluded, the remaining data indicate that the French PRT provided a narrower range in performance compared to RSCH. The statistical rankings in tables 6 and 11 support this finding. Table 6 shows that shear strain provided five statistical groups (A through E), while table 11 shows that only the mixture with the PG 64-28 asphalt binder had a significantly different resistance to rutting according to the French PRT. In a previous FHWA study, both tests agreed with full-scale pavement rutting tests, although only five asphalt binders were evaluated and only two of these binders were polymer-modified asphalt binders.(5)
The mixtures with EVA, EVA Grafted, and SBS Linear at an asphalt binder content of 4.85 percent were tested using RSCH at 70°C to determine if the reduction in asphalt binder content contributed to the differences between RSCH and the French PRT. Table 13 shows that the cumulative permanent shear strains for EVA Grafted and SBS Linear at an asphalt binder content of 4.85 percent were not repeatable, so a conclusion could not be made.
The cumulative permanent shear strains from RSCH at 70°C were correlated to the G*/sind 's of the asphalt binders at 70°C and three DSR frequencies: 10.0, 2.0, and 0.125 rad/s. The best correlation was provided by a frequency of 0.125 rad/s. At 0.125 rad/s, G*/sind underpredicted the relative rutting resistance provided by EVA and SBS Radial Grafted. G*/sind at the standard frequency of 10.0 rad/s underpredicted the rutting resistances provided by EVA and Elvaloy, and overpredicted the relative rutting resistance provided by the unmodified PG 70-22 asphalt binder. High-temperature PG underpredicted the relative rutting resistance provided by SBS Radial Grafted.
Based on the French PRT at 70°C , G*/sind underpredicted the relative rutting resistance provided by EVA at both high and low DSR frequencies. However, the high-temperature PG of EVA agreed with mixture performance. This means that the G*/sind for this asphalt binder did not increase as rapidly as it should have at temperatures immediately below its high-temperature PG of 75°C . The correlation between high-temperature PG and the French PRT provided no obvious outliers.
Grafting did not improve the rutting resistance of EVA. Grafting and geometry had no effect on the rutting resistances of the SBS-modified asphalt binders at 50°C . The effect at 70°C was marginal.
Figure 19. Cumulative permanent shear strain of the asphalt
mixture vs. French PRT rut depth of the asphalt mixture.
|Average Strain||CV1||Strain (mm/m)||Average Strain||CV1|
|EVA||1||33 640||35 040||9.4||11 470||22 140||36.7|
|2||38 800||17 280|
|3||32 680||26 080|
|EVA G||1||37 860||25 040||44.8||27 880||28 200||19.7|
|2||17 040||22 750|
|3||20 220||33 850|
|SBS LG||1||30 280||44 000||33.2||23 130||27 600||14.2|
|2||32 530||29 420|
|3||55 500||30 320|
1CV = Coefficient of Variation, percent = (standard deviation ÷ average)*100.
Topics: research, infrastructure, pavements and materials
Keywords: research, infrastructure, pavements and materials, Superpave, asphalt binder specification, permanent deformation, Superpave Shear Tester, SST, French Pavement Rutting Tester, polymer-modified asphalt binders.
TRT Terms: research, facilities, transportation, highway facilities, roads, parts of roads, pavements