Effect of Crumb Rubber Particle Size and Content On The Low Temperature Rheological Properties of Binders

Transcription

1 Effect of Crumb Rubber Particle Size and Content On The Low Temperature Rheological Properties of Binders Authors: Venu T. Gopal 1 Peter E. Sebaaly 2 Jon Epps 3 A Paper Submitted for Publication Transportation Research Board Annual Meeting January 13-17, 2002 Washington D.C. 1. Graduate Research Assistant, Pavements/Materials Program, Department of Civil Engineering, University of Nevada, Reno, Nevada, , gopal@unr.edu 2. Professor and Director, Pavements/Materials Program, Department of Civil Engineering, University of Nevada, Reno, Nevada, , Sebaaly@unr.nevada.edu. 3. Materials Engineering Manager, Granite Construction Inc., Sparks, Nevada, , Jepps@granite-net.com

2 ABSTRACT: This paper presents a study to evaluate low temperature rheological properties of crumb rubber modified binders and the impact of the particle size and content on such properties. Three sizes of the crumb rubber (0.18, and 2.0 mm), a control (0%) and two levels of crumb rubber content ( and 24%) were used with four asphalt binders. The measured rheological properties consisted of creep stiffness (St) and logarithmic creep rate (m). Results showed that: increasing the crumb rubber content decreased the creep stiffness which improves thermal cracking resistance. The impact of crumb rubber content on the m-value was very highly inconsistent. The crumb rubber size did not have significant effect on the low temperature properties. The effect of the size of the crumb rubber on the resistance to thermal cracking was dependent on the asphalt binder source. The analysis of the data generated in this experiment showed that some combinations of crumb rubber size and content can either improve or jeopardize the PG low temperature of the asphalt binder. KEY WORDS: crumb rubber, Superpave, rheological properties, creep stiffness, logarithmic creep rate, thermal cracking, low temperature properties. ACKNOWLEDGMENT: The authors would like to thank the Federal Highways Administration for providing the funds for this research and the research team at Oregon State University who were the Principal Investigators for the FHWA Project on Crumb Rubber Technologies.

3 2 INTRODUCTION Crumb rubber modified (CRM) binders have been used as paving materials since the 1960's. The purpose of using CRM binders is to improve the hot mix asphalt (HMA) mixture s resistance to cracking and rutting failures under traffic loading and environmental conditions. Blending crumb rubber into an asphalt binder is believed to improve its elastic and energy absorption properties, which are directly related to the binder s resistance to cracking and rutting failures. Cracking failures of HMA pavements are caused by excessive tensile strains within the HMA layer. The source of excessive tensile strains can be due to bending under traffic loads, shrinkage at freezing temperatures, or reflection of cracks from the old pavement layer beneath the new HMA layer. Low temperature cracking starts at the surface and progresses down with time, because low ambient temperatures chill the road surface first. HMA pavements subjected to high cooling rates and low temperatures develop tensile stresses due to shrinkage, when these stresses exceed the fracture strength of the HMA pavement layer, transverse cracking develops. HMA mixes, which have high stiffness modulus at low temperatures, are very prone to cracking. Because the low temperature cracking of an HMA pavement is purely a tensile stress failure, the resistance of the HMA pavement to such failure is mainly provided by the asphalt binder. Therefore, the low temperature properties of the asphalt binder control the behavior of the HMA mix under freezing environmental conditions. The Superpave performance based (PG) binder grading system uses the rheological properties of the asphalt binder to assess its resistance to low temperature cracking. This paper summarizes the data generated from a research effort to assess the impact of crumb rubber size and content on the low temperature properties of asphalt binders as measured by the Superpave (PG) binder grading system.

4 3 BACKGROUND In 1994, Reese evaluated the properties of CRM binders in dense graded and open graded mixtures used on six Caltrans projects (1, 2). The evaluated binders were blended in the laboratory and aged according to the projects locations using the thin film oven, tilted thin film oven and the pressure aging vessel. Reese conducted several evaluations using the rheological properties of the CRM binders as measured by the dynamic shear rheometer (DSR) and bending beam rheometer (BBR) including the resistance of CRM binders to fatigue cracking, rutting and thermal cracking, and the impact of the crumb rubber content. Reese concluded that the use of the slope of the creep stiffness curve (m) and a maximum stiffness value are sufficient in determining the resistance of CRM binders to thermal cracking. In 1995, Bahia and Davis evaluated the impact of crumb rubber types and contents on the rheological properties of asphalt binders (3). Three types of crumb rubbers were evaluated, including: ambient shredding (AS), cryogenic grinding (CG) and special extrusion (TP) with contents ranging between 5 and 20% at 5% intervals blended with two different grades of asphalt binders. The measured rheological properties included creep stiffness (St) and the logarithmic creep rate (m) at low temperatures ranging from -20 to 0 0 C. They concluded that the impact of crumb rubber content (2-20%), on the reduction of stiffness at low temperature (-20 to 0 0 C) is a linear function of the rubber content and was independent of the rubber source. The research also concluded that the stiffness decreases by 4% for every 1 percent increase in rubber content for all three types of crumb rubbers. This trend is highly asphalt specific; the lower the stiffness of the asphalt the less significant the effect of the rubber. The effect of different rubber sources were similar. The effect of rubber on the m-value was not significant with respect to the rubber source or content. In 1996, Troy et al. Evaluated the applicability of using the Superpave binder grading system tests to evaluate the rheological properties of CRM binders (4). The research program covered the use of parallel plate system with 1 and 2mm gaps and the plate and cup system with BBR. The research recommended the use of the plate and cup system in conjunction with the

5 4 BBR to grade CRM binders that contain any rubber particles larger than No The low temperature properties of the CRM mixtures as measured by the Thermal Stress Restrained Specimen Test (TSRST) were highly consistent for the samples tested. The fracture temperature coincided well with the low temperature grade of the CRM binders evaluated with the BBR. OBJECTIVE This research project evaluated the effect of crumb rubber size and content on the resistance of CRM binders to low temperature cracking. The specific objectives of the research are summarized as follows: Evaluate the effect of crumb rubber content on the resistance of CRM binders to low temperature cracking using three levels of crumb rubber contents 0, 12 and 24%. Evaluate the effect of crumb rubber size on the resistance of CRM binders to low temperature cracking using three sizes; 0.18, and 2.00 mm. EXPERIMENTAL PROGRAM Table 1 summarizes the various combinations of crumb rubber sizes and contents that were evaluated. The rheological properties: Stiffness (St) and slope of creep curve (m) were measured for each of the combinations listed in table 1. The St represents the creep stiffness and m represents the logarithmic creep rate after 60 seconds of loading time. The four asphalt binders used in this study came from the AASHTO Materials Reference Laboratory (AMRL). These binders were used in the asphalt research of the Strategic Highway Research Program (SHRP). The SHRP AAM-1 is a PG64-16, the SHRP AAB-1 is a PG58-22, the SHRP ABL-3 is a PG58-28, and the SHRP AAA-2 is a PG The crumb rubber modifiers were blended into the asphalt binders at the university of Nevada s Pavement/Materials laboratory using a low shear blender ( rpm). An oil bath was used to maintain the asphalt binder at the desired temperature throughout the blending process. A 600 grams sample of the asphalt binder is weighed into a quart can which is then

6 5 placed inside an oil bath and maintained at 175±5 0 C. The blender s blade is inserted into the asphalt binder sample and operated at rpm while adding the appropriate amount of crumb rubber. The blending process is then continued for one hour. ANALYSIS OF EXPERIMENTAL DATA The Superpave technology was used to assess the resistance of the crumb rubber modified binders to low temperature cracking in terms of the creep stiffness (St) and the logarithmic creep rate (m-value). Each of the crumb rubber modified binders was tested at three temperatures: the first temperature corresponds to the low temperature grade of the base asphalt binder and two additional increments of 6 and 12 degrees Celsius below the first temperature. It should noted here that the BBR testing temperature is 10 o C warmer than the low temperature grade of the binder. The two additional temperature increments were selected to assess the ability of the crumb rubber to improve the low temperature properties of the binders. Therefore, the three testing temperatures are different for each binder. The Superpave technology specifies that the creep stiffness (St) must not exceed 300 MPa at 60 seconds loading time and the m-value which is the slope of the logarithmic relationship between the creep stiffness and loading time must be greater than or equal to when measured at the 60 seconds loading time. Following the Superpave requirements, the tested binders were aged through the RFTO and PAV prior to the evaluation of their low temperature rheological properties in the bending beam rheometer (BBR). The evaluation of the rheological properties of the modified and unmodified binders followed the Superpave technology according to AASHTO standard procedures. The current Superpave binder grading system calls for the use of the BBR and the direct tension (DT) device to fully assess the resistance of asphalt binders to low temperature cracking (5). The research presented in this paper was initiated prior to this latest modification of the grading system and at that time the DT device was not yet perfected, therefore, only the BBR test was used to assess the low temperature behavior of the modified binders.

7 6 The data generated from this research were analyzed to meet the two objectives of the study: a) evaluate the effect of crumb rubber content and b) evaluate the effect of crumb rubber size. As indicated in table 1, three crumb rubber sizes: 0.180, 0.425, and mm were used at three different contents: 0, 12 and 24 percent with four asphalt binders. The 0% content was used as the control. Each of the combinations was tested at three temperatures. Replicate measurements were made at each test condition. The analysis of variance (ANOVA) statistical tools were used to evaluate the level of significance of each parameter, i.e. rubber content and size. Mean comparisons were conducted at a 95% confidence level (" = 0.05). Figures 1 through 8 graphically present the impact of rubber content and size on the St and m properties for the four asphalt binders evaluated in this study. Each graph includes a horizontal thick black solid line indicating the suggested Superpave criteria for St and m. In the case of St the criteria line represents the maximum stiffness value while in the case of m the criteria line represents the minimum slope. By visually comparing the values of St and m with the criteria line, it can be observed whether or not the addition of crumb rubber has improved the low temperature properties of the asphalt binder. Tables 2 through 9 summarize the statistically-based comparisons for the various testing conditions. An entry of S indicates that the two treatment being compared generated the same property, an entry of L indicates that the treatment generated a lower property than the treatment being compared with, and an entry of H indicates that the treatment generated a higher property than the treatment being compared with. For example, in table 2 an entry of L is placed across the St at -6 o C, under the columns of 2.00 mm rubber size and vs 0%. This entry indicates that the addition of of the 2.00 mm crumb rubber generated a St at -6 o C that is lower than the St at -6 o C of the binder with 0% rubber (control). Effect of Crumb Rubber Content on the Stiffness An examination of the graphical presentations in figures 1, 3, 5, and 7 and the statistical comparisons in tables 2 through 5 indicates the following:

8 7 For asphalt binders AAM-1 and AAB-1 (tables 2 and 3), the addition of crumb rubber significantly reduced the stiffness at all three temperatures for all three crumb rubber sizes. In all cases, as the crumb rubber content increased, the stiffness decreased (an entry of L is shown all across the St values). For asphalt binder ABL-3, the addition of crumb rubber at the content significantly increased the stiffness as compared to the control and the 24% content. This is shown in table 4 by the H entries when the is compared with the 0% and by the L entries when the 24% is compared with the. The 24% crumb rubber either reduced or maintained the same St as compared to the control. This is shown in table 4 by either L or S entries when the 24% is compared with the control. For asphalt binder AAA-2, the addition of crumb rubber reduced the stiffness except in the case of the 24% content of the 2.00 mm crumb rubber. This is shown in table 5 by the L entries under all cases except when the 24% of the 2.00 mm rubber is compared with the control and the rubber content. Following the Superpave directive which indicates that decreasing the stiffness means improving the binder s resistance to low temperature cracking, it can be concluded that the addition of crumb rubber improves the resistance of asphalt binders to low temperature cracking. However, an optimum crumb rubber content must be determined for each crumb rubber size and asphalt binder. Effect of Crumb Rubber Content on the m-value An examination of the graphical presentations in figures 2, 4, 6, and 8 and the statistical comparisons in tables 2 through 5 indicates the following: For the AAM-1 binder (table 2), the addition of the rubber significantly increased the m-value at the -12 and -18 o C while the addition of the 24% rubber significantly increased the m-value at all three temperatures. For the AAB-1 and AAA-2 (tables 3 and 5) binders, the addition of and 24% crumb rubber significantly reduced the m-value in the majority of the cases. This is shown in tables 3 and 5 as more L entries are present than both S and H entries combined (28 L, 12 S, and 14 H ). For the ABL-3 binder, the addition of crumb rubber significantly reduced the m- value for all crumb rubber sizes and all three temperatures. The addition of 24% rubber showed an improvement over the but still significnaly below the control (0%).

9 8 The above observations indicate that the impact of crumb rubber content on the m-value is not as predictable as its impact on the stiffness. The addition of crumb rubber significantly increases the m-value only for very limited and very specific combinations of asphalt binder and crumb rubber. Materials engineers should be very cautions in using crumb rubber to improve the m-value of asphalt binders. Effect of Crumb Rubber Size on Stiffness An examination of the graphical presentations in figures 1, 3, 5, and 7 and the statistical comparisons in tables 6 through 9 indicates the following: For the AAM-1 binder (table 6), increasing the size of the crumb rubber significantly increased the stiffness at -6 o C while it had no significant impact at the colder temperatures of -12 and -18 o C. For the AAB-1 binder (table 7), increasing the size of the crumb rubber had no significant impact on the stiffness. For the ABL-3 binder (table 8), the impact of increasing the size of the crumb rubber on the stiffness was inconsistent. In some cases it significantly increased the stiffness while in other cases, it significantly reduced the stiffness. For the AAA-2 binder (table 9), increasing the crumb rubber size did not significantly impact the stiffness at the rubber content while it significantly increased the stiffness at the 24% rubber content. In general, increasing the crumb rubber size can either increase or maintain the low temperature stiffness property of an asphalt binder. According to the Superpave criteria this does not constitute an improvement in the resistance of binders to low temperature cracking. Effect of Crumb Rubber Size on m-value An examination of the graphical presentations in figures 2, 4, 6, and 8 and the statistical comparisons in tables 6 through 9 indicates the following: For all four binders evaluated in this study, increasing the size of the crumb rubber either

10 9 maintains the m-value or significantly reduces it except in the case of the ABL-3 binder where increasing the rubber size increased the m-value when used at 24% and at -30 o C temperature. Similar to the case of the impact of rubber size on the stiffness, this type of impact does not constitute an improvement in the resistance of of binder to low temperature cracking. Impact of Crumb rubber on the PG Grade This analysis examines the cases where the addition of certain combination of crumb rubber size and content would impact the PG low temperature grade of a binder. In other words, this analysis will identify the cases where by using crumb rubber the PG low temperature grade of a binder has been improved or jeopardized. This analysis identified the St and m-value for each CRM binder and checks them against the Superpave criteria of St being less or equal than 300 MPa and m being greater or equal to at the low temperature grade. AAM-1 Binder The unmodified AAM-1 binder has a PG low temperature grade of -16. The data generated in this research showed that the following combinations of crumb rubber sizes and contents would improve the PG low temperature grade of the AAM-1 binder to -22: mm at mm at mm at 24% mm at 24% And the following combinations of crumb rubber sizes and contents would improve the PG low temperature grade of the AAM-1 binder to -28: mm at 24% mm at 24% The data generated in this research also showed that none of the evaluated combinations of crumb rubber sizes and contents would jeopardize the current PG low temperature grade of

11 10 the AAM-1 binder of -16. AAB-1 Binder The unmodified AAB-1 binder has a PG low temperature grade of -22. The data generated in this research showed that none of the evaluated combinations of crumb rubber sizes and contents would improve the PG low temperature grade of the AAM-1 binder to -28. The data generated in this research also showed that the following combination of crumb rubber sizes and contents would jeopardize the current PG low temperature grade of the AAB-1 binder of -22 by failing the m-value criterion (figure 4): 2.00 mm at 24% ABL-3 Binder The unmodified AAB-1 binder has a PG low temperature grade of -28. The data generated in this research showed that none of the evaluated combinations of crumb rubber sizes and contents would improve the PG low temperature grade of the ABL-3 binder to -34. The data generated in this research also showed that the following combinations of crumb rubber sizes and contents would jeopardize the current PG low temperature grade of the ABL-3 binder of -28 by failing the St and m-value criteria (figures 5 and 6): 2.00 mm at 0.425mm at 0.18 mm at AAA-2 Binder The unmodified AAA-2 binder has a PG low temperature grade of -34. The data generated in this research showed that none of the evaluated combinations of crumb rubber sizes and contents would improve the PG low temperature grade of the AAA-2 binder to -40. The data generated in this research also showed that the following combinations of crumb

12 11 rubber sizes and contents would jeopardize the current PG low temperature grade of the AAA-2 binder of -34 by failing the m-value criterion (figure 8): 2.00 mm at 24% CONCLUSIONS AND RECOMMENDATIONS This experimental program evaluated the impact of crumb rubber contents and sizes on the low temperature properties of four asphalt binders. A total of 108 combinations were tested in the bending beam rheometer to evaluate their St and m-value. All of the evaluated binders were aged in the RTFO and PAV according to applicable AASHTO procedures. Based on the analysis of the data generated from this experiment, the following conclusions and recommendations can be made: In general crumb, rubbers can be used within a wide range of contents and sizes to reduce the low temperature stiffness of asphalt binders. However, an optimum crumb rubber content must be determined for each crumb rubber size and asphalt binder. The impact of crumb rubber on the m-value is highly dependent on the properties of the asphalt binder. This experiment showed that crumb rubbers are not effective in increasing the m-value of asphalt binders. In general, crumb rubbers can be used over a wide rage of contents and sizes without affecting the PG low temperature grade of the asphalt binder. However, in some limited cases, the use of certain combinations of crumb rubber size and content can either improve or jeopardize the PG low temperature grade of the asphalt binder. It is highly recommended that when crumb rubbers are used to improve the resistance of asphalt binders to permanent deformation and fatigue failures, the impact of these modifications on the low temperature properties of the binder should be carefully assessed. The results presented in this paper show that crumb rubbers can be used to improve the low temperature properties of asphalt binders but only if carefully designed and evaluated.

13 12 REFERENCES 1. Reese, R., Development of a Physical Property Specification for Asphalt-Rubber Binder, Journal of the Association of Asphalt Paving Technologists, vol. 63, 1994, p Reese, R., Properties of Aged Asphalt Binder Related to Asphalt Concrete Fatigue Life, Journal of the Association of Asphalt Paving Technologists, vol. 66, 1997, p Bahia, H. and Davies, R., Role of Crumb Rubber Content and Type in Changing Critical Properties of Asphalt Binders, Journal of the Association of Asphalt Paving Technologists, vol. 64, 1995, p Troy, K., Sebaaly, P.E., Epps, J., Evaluation Systems for Crumb Rubber Modified Binders and Mixtures, Transportation Research Record, no.1530, 1996, p AASHTO Provisional Standards, 2001, MP1a, Specifications for Performance Garded Asphalt Binder, Washington, D.C., 2001.

14 13 Table 1.Laboratory Experimental Program. Asphalt Binder Crumb Rubber Size Crumb Rubber Test Temperature (mm) Content (%) (C) AAM-1 (PG64-16) 0.18, 0.425, , 12, 24-6, -12, -18 AAB-1 (PG58-22) 0.18, 0.425, , 12, 24-12, -18, -24 ABL-3 (PG58-28) 0.18, 0.425, , 12, 24-18, -24, -30 AAA-2 (PG64-34) 0.18, 0.425, , 12, 24-24, -30, -33

15 Table 2. Statistical comparison of St and m at various rubber contents for asphalt binder AAM mm mm 0.18 mm Property vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs -6 o C L L L L L L L L L -12 o C L L L L L L L L L -18 o C L L L L L L L L L -6 o C S S S S H H S S S -12 o C S H S H H H H H H -18 o C S H H H H S H H H Table 3. Statistical comparison of St and m at various rubber contents for asphalt binder AAB mm mm 0.18 mm Property vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs -12 o C L L L L L L L L L -18 o C L L L L L L L L L -24 o C L L L L L L L L L C L L L L L L L L L -18 o C L L S S S S S S S -24 o C S H H H H H H H H L: Lower S: Same H: Higher

16 Table 4. Statistical comparison of St and m at various rubber contents for asphalt binder ABL mm mm 0.18 mm Property vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs -18 o C H L L H L L H S L -24 o C H S L H S L H S L -30 o C H S L H S L H S L -18 o C L L H L S H L S H -24 o C L L H L S H L L H -30 o C L L H L L H L L H Table 5. Statistical comparison of St and m at various rubber contents for asphalt binder AAA mm mm 0.18 mm Property vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs vs 0% 24% vs 0% 24% vs -24 o C L H H L L L L L L -30 o C L S H L L L L L L -33 o C L H H L L L L L L -24 o C L L L L L L S L L -30 o C L L L S L L S L L -33 o C S L L H H H H H H

17 Table 6. Statistical comparison of St and m a various rubber sizes for asphalt binder AAM % 24% Property 2.00 vs vs vs vs vs vs o C H H H H H S -12 o C H S S H S L -18 o C S S S S S S -6 o C S S S L L S -12 o C L L S L L S -18 o C S S S L L S Table 7. Statistical comparison of St and m a various rubber sizes for asphalt binder AAB % 24% Property 2.00 vs vs vs vs vs vs o C S S S S S S -18 o C S S S S S S -24 o C S S S L L S C L L L L L S -18 o C L L S L L S -24 o C L L S L L S

18 Table 8. Statistical comparison of St and m a various rubber sizes for asphalt binder ABL % 24% Property 2.00 vs vs vs vs vs vs o C L L L S L L -24 o C L S H S S S -30 o C L L S L L S -18 o C S S S L S H -24 o C S S S L S H -30 o C L L L S H H Table 9. Statistical comparison of St and m a various rubber sizes for asphalt binder AAA % 24% Property 2.00 vs vs vs vs vs vs o C S S S H H S -30 o C S S S H H S -33 o C S S S H H S -24 o C S S S L L S -30 o C S S S L L S -33 o C S S S L L S

19 Control C C C C % C % C % Stiffness (MPa) Temperature (C) Figure 1: Effect of crumb rubber size and content on stiffness for the AAM-1 binder Control C C C C % C % C % m-value Temperature (C) Figure 2: Effect of crumb rubber size and content on m-value for the AAM-1 binder

20 Control C C C C % C % C % Stiffness (MPa) Temperature (C) Figure 3: Effect of crumb rubber size and content on stiffness for the AAB-1 binder Control C C C C % C % C % 0.35 m-value Temperature (C) Figure 4: Effect of crumb rubber size and content on m-value for the AAB-1 binder

21 Control C C C C % C % C % Stiffness (MPa) Temperature (C) Figure 5: Effect of crumb rubber size and conte nt on stiffness for the ABL-3 binder Control C C C C % C % C % 0.35 m-value Temperature (C) Figure 6: Effect of crumb rubber size and content on m-value for the ABL-3 binder

22 Control C C C C % C % C % Stiffness (MPa) Temperature (C) Figure 7: Effect of crumb rubber size and conte nt on stiffness for the AAA-2 binder Control C C C C % C % C % m-value Temperature (C) Figure 8: Effect of crumb rubber size and content on m-value for the AAA-2 binder