Evaluation of performance grading parameters for crumb rubber modified asphalt binders and mixtures

The final version of this document has been published in:

Advanced Testing and Characterization of Bituminous Materials, Two Volume Set

Edited by Andreas Loizos , Manfred N . Partl , Tom Scarpas , and Imad L . Al-Qadi; CRC Press 2009

Print ISBN: 978-0-415-55854-9; eBook ISBN: 978-0-203-09298-9; DOI: 10.1201/9780203092989.ch58

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1 BACKGROUND

Modifying asphalt binders is a common way of improving binder characteristics and perfor-mance. The use of crumb rubber as an asphalt binder modifier has received more attention be-cause of its cost efficiency and environmental benefits. These binders are produced by mixing a percentage of crumb rubber with asphalt cement at an elevated temperature. In practice, a wide range of mixing temperatures and mixing times along with an array of crumb rubber types have been used with varying degrees of success.

From 1987 through 1993, the Strategic Highway Research Program (SHRP) carried out a ma-jor research program to develop the Superpave performance-based specifications and test meth-ods for asphalt binders as well as similar tests and a mix design practice for HMA mixes (Bahia et al. 2001a). The specifications were believed to be “blind to source” and thus suitable for all binders, regardless of source, type and modification. However, the SHRP asphalt research was carried out almost exclusively with unmodified asphalt binders, so the applicability of the Su-perpave specifications and test methods to modified binders was not validated.

Soon after the introduction of the Superpave performance grading specification, a few studies

were carried out on the effect of crumb rubber modifier on the parameters measured using the

Superpave protocol (Bahia et al. 1995, Madapati et al. 1995). The high temperature performance

grade for crumb rubber rose significantly. However, there was little positive effect on the low

temperature performance grade. In these studies, the Superpave protocol was presumed to be

valid for modified binders and, thus, no mixture tests were conducted for comparison. Further studies and practice have shown that modified asphalt binders graded according to

Superpave performance grading specifications show marked improvement in selected perfor-mance characteristics compared with unmodified asphalt binders. However, users and producers

Evaluation of performance grading parameters for crumb rubber modified asphalt binders and mixtures

N. Tabatabaee, H.A. Tabatabaee, M.R. Sabouri & P. Teymourpour Dept. of Civil Engineering, Sharif University of Technology, Tehran, Iran

ABSTRACT: This study examined crumb rubber modified (CRM) binders produced by mixing different percentages of crumb rubber with PG58-22 and compared them with PG64-22 and PG64-16 binders. Superpave performance grading was used to determine the performance grade of these modified binders. The modified and unmodified binders were then used to make asphalt concrete specimens. These specimens showed an increase in indirect tensile strength and a de-crease in Marshall stability with the use of CRM binders. The laboratory performance of the specimens were tested and compared. Rutting susceptibility, selected as indicative of the high temperature performance of the mixes, was compared by conducting an unconfined static creep test at high temperature. Intermediate temperature performances were compared by determining the absorbed energy during the indirect tensile strength test at two temperatures. The results were used to evaluate the suitability of the Superpave performance grading procedure in predict-ing the performance of CRM binders.






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of modified asphalt binders remain concerned that the current specification and test methods do not fully measure the performance enhancement contributed by modification (Bahia et al. 2001a).

NCHRP Project 9-10 recommended changes to Superpave performance grading specifica-tions and supporting test methods to fully characterize modified asphalt binders and validate those recommendations through laboratory performance testing of modified HMA (Bahia et al. 2001a). It was found that the current Superpave performance grading specification does not ad-equately characterize the performance of modified asphalt binders; typically, the binders’ poten-tial performance is underestimated. Many common modifiers were investigated, but the project concentrated on polymer modified binders and thus crumb rubber modifier was excluded from the scope of the project. Kim et al. (2001) carried out a study on crumb rubber modified (CRM) binders using methods developed as part of the NCHRP 9-10 project, but no conclusion was de-rived on the suitability of current Superpave protocols for crumb rubber modified binders.

More countries are taking advantage of CRM pavements (Lee et al. 2007), thus determining the applicability of the Superpave performance grading specification to CRM binders is of great importance. It was for this purpose that the present study was carried out. Several unmodified binders and binders modified with various percentages of crumb rubber were selected. The binders were graded and analyzed according to Superpave performance grading specifications. Asphalt mixture specimens were then fabricated using the binders tested. These specimens were subjected to performance related tests at high, intermediate and low temperatures. The suitabil-ity of the Superpave performance grading protocol was evaluated by the comparison of the re-sults of the mixture tests with the performance grading parameters derived during the Superpave binder tests.

2 MATERIALS AND METHODS

PG 58-22, PG 64-22 and PG 64-16 asphalt binders were selected for use in this study. They are identified as binders A, B and C, respectively. Different percentages of crumb rubber modifier were added to the binders. The crumb rubber was produced by Yazd Tire Company using an ambient grinding process. The rubber particle gradation was relatively coarse, with 12% remain-ing on the number 30 sieve and 80% remaining on the number 50 sieve.

The PG 58-22 binder (A) was modified by adding 3%, 6%, 9%, 12% and 15% crumb rubber. These are identified as A3%, A6%, A9%, A12% and A15%, respectively. Unmodified PG 58-22 (A), PG 64-22 (B) and PG 64-16 (C) binders were also tested for comparison with modified binder characteristics.

A densely graded aggregate gradation was selected for this study. A nominal maximum size of 12.5 mm was selected and the gradation was set to meet the ASTM D3515 and the Superpave gradation criteria for dense gradation. To compensate for the expansive effect of the crumb rub-ber, a gradation curve close to the lower limits was used, as shown in Figure 1.

Figure 1. Aggregate gradation curve (12.5 mm nominal maximum size).






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2.1 Binder and mixture fabrication

The crumb rubber was added slowly over 5 min to the PG 58-22 binder and mixed at 350 rpm for 60 min at 165°C.

Cylindrical asphalt concrete specimens of two thicknesses were made. The diameter of all specimens was 100 mm, with thicknesses of 63 and 100 mm. The binder and the aggregate were blended at 165°C and compacted using a Marshall hammer.

3 TESTING PROCEDURE

3.1 Binder characterization

Rotational viscosity was measured at three temperatures (105, 135 and 165 °C) and complex shear modulus (G*) and phase angle (δ) were measured at temperatures ranging between 7°C and 88°C using a dynamic shear rheometer (DSR). Creep stiffness (S) and creep rate (m-value) were measured at low temperatures (-18 to -6 °C) using a bending beam rheometer (BBR). Standard procedure as recommended by Superpave performance grading specifications (AASHTO MP1) were followed except for DSR measurement, where the gap between the test-ing plates was increased to 2.0 mm to comply with gap to maximum particle size ratio require-ments (Bahia et al. 1995 & Troy et al. 1996).

3.2 Mixture characterization

The indirect tensile strength test, Marshall stability test and unconfined static creep test were carried out on the specimens as described below.

3.2.1 Indirect tensile strength test: This test was carried out on 100 mm diameter, 63 mm thick specimens using a 100 kN Dartek Universal Testing Machine. The two test settings were 30°C at a 0.85 mm/s crosshead dis-placement rate and 5°C at a 0.085 mm/s displacement rate. Vertical displacement and indirect tensile strength were recorded for each specimen.

3.2.2 Marshall stability test: The standard Marshall test was conducted on the 100 mm diameter, 63 mm thick specimens. These were placed in a 60°C bath for 30 to 40 min, after which the Marshall stability and flow were determined at a 0.85 mm/s displacement rate.

3.2.3 Unconfined static creep test: The unconfined creep test was used to rank the mixtures by rut susceptibility. The test was per-formed in a temperature-controlled container at a constant temperature of 42 ± 1°C, on 100 mm diameter and 100 mm thick specimens. The test consisted of a 2 min preloading at 0.01 MPa as a conditioning stress. Constant loading stress of 0.1 MPa was applied during the test for 1 h loading, followed by 1 h of unloading (Ahmedzade & Yilmaz 2008). The strains after the load-ing and unloading stages were measured and the permanent strain for each mixture was calcu-lated.

4 BINDER CHARACTERIZATION

Superpave performance grading specification response parameters were used to evaluate the ef-fects of the percentage of crumb rubber modifier on asphalt properties. These parameters were rotational viscosity at temperatures of mixing and compaction as a measure of workability; G*/sinδ at high pavement temperatures as a measure of binder ability to resist rutting at corre-sponding service temperatures; G*·sinδ at intermediate pavement temperatures as a measure of binder ability to resist fatigue cracking at corresponding service temperatures; and S and m-






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value at low temperatures as measures of binder ability to withstand thermal cracking (Bahia & Anderson 1995).

4.1 G* and δ

The G* and δ parameters were measured for all binders at a range of temperatures and aging conditions in accordance with the Superpave performance grading criteria. It was observed that increasing the crumb rubber content increased G* significantly and decreased δ, leading to a more elastic binder behavior.

It can be seen from Figure 2 that G*/sinδ increased as the rubber content increased. Accord-ing to the Superpave performance grading specification, the observed trend indicates a signifi-cant increase in rutting resistance up to a temperature of 82°C, where this parameter reaches its limiting value of 1.0 kPa.

G*∙sinδ decreased as the percentage of crumb rubber increased, as demonstrated in Figure 3. Lower values of G*∙sinδ are taken to relate to higher resistance of the binder to fatigue cracking at intermediate temperatures (Bahia & Anderson 1995).

4.2 Creep stiffness and m-value

In a study conducted by several agencies (Hansen & Anderton 1993), low temperature perfor-mance tests were carried out on binders and mixtures modified with crumb rubber. The results showed a decrease in the resilient modulus of mixtures made with crumb rubber modified bind-ers at low temperatures, as well as a greater stress relaxation ability that decreased the failure temperature in the constrained specimen tensile stress test. It was concluded that crumb rubber modification could decrease thermal cracking susceptibility and improve low temperature per-formance.

Evidence supporting these results was observed in the results of the BBR tests conducted in this study. Figure 4 shows the variation of S(60) (creep stiffness 60 sec after loading initiation) with rubber content for asphalt A. The data shown indicates that the reduction in S(60) is a line-ar function of rubber content, as has been noted by other researchers (Bahia et al. 1995).

Figure 2. Variation of G*/sinδ with temperature






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Figure 3. Variation of G*∙sinδ with temperature.

Figure 4. Creep stiffness by percent crumb rubber for asphalt A at -12°C.

Analysis shows that increasing the rubber content from 3% to 15% caused a 37% decrease in

the creep stiffness of the binder, but not enough improvement in the m-value at the low tem-peratures to cause a grade change. Since the m-value was the controlling parameter for the low temperature grade, the significant reduction of stiffness did not change the low temperature per-formance grade according to the Superpave protocol. Although more studies are needed to veri-fy the results, this indicates the possibility that the existing Superpave performance grading specifications may not be able to detect improvements caused by crumb rubber modification.

4.3 Performance grade

Table 1 shows the performance grade determined for each of the modified binders. In this paper, the temperature at which the Superpave performance grading criteria is met is referred to as the “true performance grade” of the binder. Binder performance improves as the high temperature performance grade (HT) increases, and/or the intermediate (IT) and low temperature perfor-






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mance grades (LT) decrease. The nominal and true high, intermediate and low temperature per-formance grades have been plotted in Figure 5. It can be seen that the addition of crumb rubber improved the high and intermediate performance grades. The low temperature grade remained unchanged as the rubber content increased. Table 1. Performance grade of the binders. _________________________________________________________________________

True PG Temperature ______________________

Binder Rubber content Performance High Intermediate % Grade °C °C _________________________________________________________________________

A 0 PG 58-22 63.4 14.9 A3% 3 PG 70-22 71.6 13.4 A6% 6 PG 70-22 73.9 12.8 A9% 9 PG 76-22 79.0 11.5 A12% 12 PG 82-22 84.0 8.5 A15% 15 PG 82-22 85.0 8.7 _________________________________________________________________________

Figure 5. High, intermediate and low temperature performance grades for asphalt A at different rubber contents.

5 RESULTS AND DISCUSSION

5.1 High temperature performance grade

One of the most popular tests for evaluating rutting susceptibility is the unconfined static creep test. An integral part of the creep that may be utilized is the rebound portion of the curve after the load is removed. Permanent deformation is the maximum deformation minus the amount of rebound. When comparing mixtures, it is this permanent deformation that is important for esti-mating performance (Roberts et al. 1996). Since the rutting is caused by plastic shear defor-mation of asphalt pavement under traffic loading, rutting potential is best evaluated using this parameter (Kim & Sargand 2003).

Figure 6 shows the percent of permanent deformation out of total deformation occurring in the unconfined static creep test. The results show a decrease in the amount of permanent defor-mation as rubber content increases. This agrees with the results obtained by Hanson & Anderton (1993). They observed an improvement in the permanent deformation performance of crumb rubber modified mixtures compared to mixtures made with unmodified binders when tested






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with the unconfined static creep procedure, as well as in the triaxial confined repeated load compressive creep test at 40°C.

The Marshall stability test was performed on the specimens to measure the strength of the as-phalt mixture. The Marshall quotient, the ratio of Marshall stability to flow, also known as the Marshall stiffness index, is an empirical stiffness value used to evaluate the quality of asphalt mixtures. The higher the value is, the stiffer the material is. Hence, the mixture is more likely to resist permanent deformation (Roberts et al. 1996).

Figure 7 shows the results of the Marshal test. A decrease in Marshall stability and an in-crease in flow was observed as the rubber content increased. This is to be expected, as the crumb rubber seems to decrease the stiffness and increase the flexibility of the binder. Other re-searchers have observed this trend for crumb rubber modified mixtures (Hansen & Anderton 1993).

The results for the Marshall quotient calculations for the specimens are shown in Figure 8. It was observed that the Marshall quotient decreased as the rubber content increased, indicating higher susceptibility for permanent deformation. This is contrary to the results of the unconfined static creep test. This index does not take into consideration the mixture’s rebound upon unload-ing, as done in the unconfined static creep test. On the other hand, rebound is the main reason for increased rutting resistance of crumb rubber modified binders, thus the Marshall quotient is not a good indicator of rutting susceptibility for mixtures made with crumb rubber modified binders. Therefore, the results of the unconfined static creep test are better suited to evaluate rut-ting susceptibility of the tested mixtures.

Figure 6. Percent permanent deformation at different rubber contents.






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Figure 7. Marshall stability and flow number at different rubber contents.

Figure 8. Marshall quotient at different rubber contents.

As can be seen in Figure 6, as the rubber content increased, the true high temperature perfor-

mance grade also increased, indicating an improvement in rutting resistance. This agrees with the results of the unconfined static creep test, also shown in Figure 6.

5.2 Intermediate temperature performance grade

It can be seen from Figure 9 that the introduction of a higher dose of crumb rubber into the binder has lowered the temperature at which G*∙sinδ has reached the 5000 kPa maximum al-lowable value, because of the higher G*∙sinδ at lower intermediate temperature grades where fa-tigue cracking is prevalent. This means that the binder has become more flexible at lower inter-mediate temperatures and more fatigue resistant.

To compare the performance of the modified and unmodified binders at intermediate temper-atures, indirect tensile strength testing was carried out at 5 and 30 °C. These temperatures are the two extreme ends of the observed intermediate temperature range in the binders tested. The results were plotted with their corresponding intermediate temperature performance grades for different binders, as shown in Figure 9. It was observed that the addition of 3% rubber caused a 42% increase in the indirect tensile strength at 30°C and a 30% increase at 5°C. As the rubber content increased from 3% to 15%, little variation was observed in the tensile strength at both test temperatures.






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The more energy a pavement can absorb as it deflects before cracking, the lower the likeli-

hood of fatigue cracking or other damage to occur (Roberts et al. 1996). This is an important

factor when considering pavement resistance to fatigue cracking. Figure 10 compares the

amount of absorbed energy up to the point of failure for the indirect tensile strength tests.

Figure 3 shows that increasing rubber content causes a significant reduction in G*∙sinδ,

which is taken to signify an improvement in the fatigue performance. It may be observed from

Figure 10 that the addition of 3% crumb rubber to binder A increased the absorbed energy by

32% and 14% for the tests conducted at 30 and 5 °C, respectively. Further increasing the rubber

content to 15% doubled the energy absorption at 30°C and caused a 37% increase in energy ab-

sorption when tested at 5°C.

However, although the intermediate temperature for binder A is less than binders B and C,

the introduction of crumb rubber into binder A further improves (reduces) the intermediate tem-

perature. This trend is not seen in the unmodified mixture’s absorbed energy during the indirect

tensile test, as is apparent from the sudden drop in energy absorption from binder B to binder A.

This suggests that G*∙sinδ is not a good fatigue performance indicator. Bahia et al. (2001) also

questioned the validity of using G*∙sinδ as an indicator of fatigue performance.

Figure 9. Variation in indirect tensile strength with different binders.






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Figure 10. Absorbed energy to failure in indirect tensile strength tests.

6 FINDINGS AND CONCLUSIONS

Based on the test results and the analysis presented, the main findings of this study are:

1. Characterizing the studied binders in accordance to the Superpave performance grading

specification showed that crumb rubber modification increased the high temperature

performance grade while reducing the intermediate temperature grade. Both changes are

favorable.

2. Crumb rubber modification caused a significant reduction in binder stiffness at low

temperature. The effect of crumb rubber modification could not be seen using the Su-

perpave performance grading criteria. Some revision in the criteria may be required.

3. The results of the unconfined static creep test showed that crumb rubber modified bind-

ers are more resistant to permanent deformation compared to unmodified base binders.

Higher rubber content results in more rebound deformation during unloading.

4. As the rubber content of the binder increased, the Marshall stability decreased and the

flow increased. This seems to be due to the decrease in stiffness and the increase in

flexibility caused by the addition of rubber.

5. The Marshall quotient is not a suitable index for evaluating rutting susceptibility as it

does not consider rebound during unloading and merely ranks mixtures by stiffness.

6. Crumb rubber modified mixtures had higher energy absorption before failure during the

indirect tensile strength test at both 30 and 5 °C. This could lead to better resistance to

both fatigue and low temperature cracking.

7. Tensile strength values measured in the indirect tensile test were unaffected by the

change in binder rubber content. This was true regardless of the test temperature.

8. The change in the Superpave performance grade for high temperatures as the rubber

content increased seemed to correctly reflect the behavior observed in the laboratory

performance tests. 9. The intermediate temperature performance grade trend for the crumb rubber modified

binders followed the observed behavior in energy absorption of the mixture in the indi-rect tensile test, but did not correctly reflect the decreasing energy absorption for the unmodified binders.






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Evaluation of performance grading parameters for crumb rubber modified asphalt binders and mixtures

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