Comparison of Performance Properties of Terminal Blend Tire Rubber and Polymer Modified Asphalt Mixtures

Transcription

1 Comparison of Performance Properties of Terminal Blend Tire Rubber and Polymer Modified Asphalt Mixtures Alireza Zeinali 1, S.M.ASCE; Phillip B. Blankenship 2, PE; and Kamyar C. Mahboub 3, Ph.D., PE, F.ASCE 1 Doctoral Candidate, Department of Civil Engineering, University of Kentucky, 161 Oliver H. Raymond Bldg., Lexington, KY ; PH (859) ; azein2@uky.edu 2 Senior Research Engineer, Asphalt Institute, 2696 Research Park Drive, Lexington, KY 40511; PH (859) ; pblankenship@asphaltinstitute.org 3 Professor, Department of Civil Engineering, University of Kentucky, 263 Oliver H. Raymond Bldg., Lexington, KY ; PH (859) ; kc.mahboub@uky.edu ABSTRACT Incorporation of used tires in asphalt mixtures has been a major advancement in the using recycled materials in asphalt pavements. Tires contain some of the polymeric components that have been used to modify the asphalt binders for decades, but in a solid form. This paper presents the result of a research to examine whether or not the proper application of the tire rubber in asphalt mixtures can enhance the pavement s mechanical properties to the same degree that the traditional polymeric binder modifiers do. The performance of a terminal blend tire rubber mixture was compared to a polymer-modified mixture through mechanical testing at the Asphalt Institute laboratory. Two mixtures in this study were designed with the same gradation in accordance with the California standard specifications, one with polymer, and the other with terminal blend asphalt. Flexural beam fatigue test, Superpave shear test, and disk-shaped compact tension test were conducted to evaluate the performance of the mixtures with respect to fatigue cracking, rutting, and low temperature cracking, respectively. The results revealed that the terminal blend mixture would have a better rutting performance. Furthermore, the terminal blend mixture exhibited a slightly more ductile behavior at the low temperature of -12 C. The two mixtures showed a similar performance with respect to fatigue cracking at 20 C. INTRODUCTION The technologies involving the production and application of rubberized asphalt have evolved since their early usage in mid 1960s. Although alleviating the environmental issues of storing the scrap tires have been an incentive in using rubberized asphalt, the main driving force has been to enhance the performance properties of asphalt mixtures. Tire rubber contains some of the polymeric components that have been used to modify the asphalt binders for decades. For instance, the popular styrenebutadiene-styrene (SBS), which is an elastomeric asphalt modifier, utilizes the

2 strength of polystyrene and the flexibility of polybutadiene polymers to increase the elasticity and stretchiness of the binder (Kuennan 2012). These styrene-butadiene block polymers abound in the typical scrap tires. It is believed that if properly processed, the tire rubber can improve the asphalt properties to the same extent that the classical (or not -so-classical) modifiers do. Such modifications are particularly performed to increase the binder s stiffness at high temperatures, and make the binder more flexible at low temperatures. Asphalt-rubber is one form of rubberized asphalt which is produced thorough a wet process. As defined by the ASTM D6414 standard, asphalt-rubber binder is a blend of paving grade asphalt cement, ground recycled tire (that is, vulcanized) rubber and other additives, as needed, for use as binder in pavement construction. Typically, rubber is ground to mesh particle size (1 2 mm) to produce asphaltrubber binder. The rubber modifier should be incorporated into the hot asphalt so that the rubber particles swell and interact with the binder. Several research studies have shown that major improvements can be achieved in the binder s properties by using crumb rubber modifiers ( Stroup-Gardiner et al. 1993; Bahia 1995; Huang et al. 2002). However, storage of regular asphalt-rubber binders is a challenge. These binders are normally blended with the mix at hot-mix plants, and using special mixing equipment. A more recent technology of producing rubberized asphalt involves pulverizing the tire rubber into very fine particles ( smaller than 40 mesh or 400 microns), and blending with asphalt at the refinery or terminal. These modified binders are referred to as terminal blends. During the production of terminal blends, the rubber is dissolved into the asphalt to a great extent and a smooth texture, similar to polymer-modified binders, is achieved. As the result, unlike asphalt-rubber, a terminal blend can be reserved in storage tanks with proper agitation. In contrast to the asphalt-rubber binders, which cannot be used in dense graded hot-mix asphalt (HMA), the terminal blends can be utilized in any kind of HMA (Hicks et al. 2010). Furthermore, the terminal blends can be processed and placed in the field by regular mixing and paving equipment. Research have shown that mixtures with terminal blends can exhibit superior performance when compared to conventional HMA (Hicks et al. 2010; Hajj et al. 2011). The performance benefits of using terminal blends, however, have not been adequately quantified, and more research is required to compare their performance to polymer-modified binders. This paper presents the results of an experimental study to evaluate the use of tire rubber as a modifier for asphalt. The primary objective of the study was to investigate whether the terminal blend tire rubber mixture would provide the same performance as the polymer-modified mix. This would assist in optimization of incorporating the recycled tire rubber in asphalt mixtures as well as provide a better insight into the performance improvements which are procured by using terminal blend tire rubber asphalt. MATERIALS AND EXPERIMENTAL PLAN The main goal of material selection in this study was to isolate the effect of using tire rubber and polymer modification from all other variables associated with asphalt

3 mixtures. To do so, a hot-mix asphalt with warm-mix additive (HMA with WMA) was selected which was designed in accordance with the California state specifications. HMA with WMA mixtures typically contain a small amount of warmmix additive and are mixed at temperatures higher than 135 C (275 F). Table 1 presents the design properties of the mixture. For the purpose of this study, this mixture was made with two different modifications as follows: 1- Binder was mixed with finely ground tire rubber at the asphalt terminal to produce a PG 64-28TR binder. The mixture made with this binder is referred to as the TR mix in this paper. 2- SBS modifier was added to the mixture at the mixing plant to produce a PG 64-28PM binder. This mixture is referred to as the PM mix in this paper. It should be noted that the performance grade of the binders were previously verified through binder testing. Table 1. Mix Design and Aggregate Properties. Property Design Value/Type Binder Grade PG (TR and PM) Asphalt Binder Content (AC) 5.1% RAP Content 15% Anti Strip Morelife 5000, 0.5% Warm Mix Additive Advera, 0.25% by DWA Voids Filled with Asphalt (VFA) 69.5% Voids in Mineral Aggregate (VMA) 14.5% Dust to Asphalt Ratio 0.8 G mm Coarse Aggregate Angularity 95/100 Fine Aggregate Angularity 48.5% Flat and Elongated Particles 0.3% G sb G se Sieve No. Sieve Size, mm Percent Passing 3/ / / # # # # # # # Mix samples were collected at the paving site for laboratory performance testing. Conforming to the AASHTO R 30 (2010) standard practice, the mixtures did not need any additional short-term conditioning for mechanical testing since they had already undergone the mixing and hauling processes in the field. Moreover, the focus of the study was to compare the performance factors at the initial stages of the pavement life; thus, no long-term aging was required as well. To make the test specimens, the HMA samples were spread into metal pans at cm height. Then, the samples were placed in a forced draft oven to be heated to the compaction temperature of 149 C (300 F), while the pans were covered with aluminum foil to

4 avoid further mix aging. Then, the samples were removed from the oven and used for compacting the test specimens. Both tire rubber mix (TR) and polymer-modified mix (PM) were subjected to similar mechanical tests at the Asphalt Institute laboratory. Particular emphasis was placed on fatigue, rutting and low-temperature fracture of the mixtures. Four-point flexural beam fatigue test, Superpave shear test (SST), and disk -shaped compact tension [DC(t)] test were conducted to examine the performance of the mixtures with respect to fatigue cracking, rutting, and low-temperature cracking, respectively. This suit of tests would enable an acceptable material characterization with respect to major distresses of asphalt pavements. All the tests were performed on triplicate specimens and using calibrated equipment. The configuration of the three performance tests used in this study is conceptually depicted in Figure 1. a) b) c) Figure 1. Configuration of the: a) four-point flexural beam fatigue test, b) Superpave shear test, and c) disk-shaped compact tension test. MIXTURE PERFORMANCE CHARACTERIZATION Flexural Beam Fatigue Test. Fatigue failure is a major distress in asphalt pavements which is manifested in the form of alligator cracking under repeated traffic load. The four-point flexural beam fatigue test has proven to be capable of simulating the repetitive bending of asphalt pavements under traffic loads. The beam fatigue test examines not only the fatigue resistance of the mixture but also indicates the brittleness of the asphalt-aggregate mix. In this study, a set of constant-strain flexural beam fatigue tests were conducted on the mixtures at the intermediate temperature of 20 C, 10 Hz loading frequency, and three strain levels: 800, 1000, 1200 microstrains. These relatively high test strains were chosen after some preliminary evaluation to achieve an actual fatigue failure in a reasonable amount of time, and avoid extensive extrapolations on the fatigue life data. To manufacture the test specimens, the mix samples were reheated to the compaction temperature. Then, beam specimens were compacted to a standard height using a kneading compactor. The sample weights were adjusted to obtain a final air void content of 7.0±0.5 percent in all specimens. The compacted samples were trimmed using an automatic saw machine and all the prepared specimens were tested for bulk specific gravity to assure that the air voids are within the acceptable range. The failure points of the specimens was determined using the ASTM D7460 (2010) standard method, which employs the complex modulus (flexural stiffness cycle number) of the specimen as an indication of failure. When polymer-modified

5 HMA is tested, the complex modulus method has proven to produce more accurate and consistent results as compared to the AASHTO T321 (2007) method, which considers reaching to 50 percent of initial stiffness as the failure point. Figure 2 displays the relationship between the test strain and cycles to failure for both polymer-modified (PM) and terminal blend tire rubber (TR) mixtures. Typically, a linear model satisfactorily fits the beam fatigue test data when plotted in a log-log form (log of cycles to failure versus log of test strain in microstrains). Such linear models were developed using a linear regression analysis for both mixtures as presented in Figure 2. As a part of the National Cooperative Highway Research Program (NCHRP) Project 9-38, a stochastic method was proposed to estimate the endurance limit of asphalt mixtures based on beam fatigue test data at three strain levels (Prowell 2010). Using this method, the lower limit of the 95-percent confidence intervals for the TR and PM mixtures were estimated to be 396 and 355 microstrains, respectively. Based on the endurance limit values and considering the logarithmic nature of the models and the extrapolations used to estimate the endurance limit values, the mixtures are expected to exhibit very similar performances when constructed in thick surface layers. Moreover, the two mixtures showed similar fatigue resistances at high strain levels, which is equivalent to the conditions of very thin asphalt layers or reflective cracking. As illustrated in Figure 3, the average cycles to failure of the mixtures were close at all three tested strain levels. The statistical t-test, which was conducted on the cycles to failure data, did not indicate a significance difference between the fatigue resistances at any of the strain levels. In short, the TR and PM mixtures exhibited very similar fatigue performances at the moderate temperature of 20 C. Figure 2. Flexural beam fatigue test results.

6 Figure 3. Average cycles to fatigue failure at various strains. Superpave Shear Test. At higher pavement temperatures, when the asphalt exhibits a more viscous behavior, permanent deformation of the mixture becomes the primary concern. If the mixture does not provide adequate shear strength the repeated heavy loads will accumulate small, but permanent deformations, which lead to formation of a longitudinal rut along the wheel paths ( Asphalt Institute 2001). The original Superpave mix design did not fully account for this permanent deformation, and to address the load-related performance of HMA, the Superpave shear tester (SST) was developed during the Strategic Highway Research Program (SHRP). In this study, the mixtures were subjected to the SST repeated shear at constant height (RSCH) to be examined for permanent deformation potential. To make the test specimens, the mixtures were compacted into 150-mm diameter in a Superpave gyratory machine to the constant height of 150 mm. Then, two 50-mm thick specimens were fabricated from the center part of the compacted specimens. The final air void content of all the specimens was 7.0±0.5%. The RSCH test was conducted at 54 C and according to the AASHTO T320 (2007) standard method. The test was continued to 10,000 cycles and at each cycle, a haversine shear stress of 69 kpa was applied for 0.1s followed by a 0.6s rest period. Tables 2 and 3 contain the accumulated permanent deformation of the specimens of TR and PM mixtures, respectively. By fitting a logarithmic function to the permanent strain data, the number of load cycles required to reach 5% permanent strain was estimated for each specimen (Tables 2 and 3). Based upon this number of cycles, Sousa (1994) developed a model to predict the number of ESALs that would lead to a rut depth of 12.5 mm on the pavement. As presented in Tables 2 and 3, the analysis indicated that the TR mix would take a higher traffic to show a 12.5-mm rut depth; however, it should be noted that both mixtures were extremely rut resistant. Moreover, as Figure 4 demonstrates, the progressive permanent strain of the TR mix was lower than the PM mix during the RSCH test. This also implies that the TR mix would exhibit a higher rutting resistance in the field.

7 Table 2. SST Data for the Terminally-Blended Tire Rubber Mix Specimen Air Permanent Shear Strain at Cycles to ESALs to 12.5 Voids cycles 5% Strain mm Rut Depth (%) cycles cycles TR-S % 1.09% 1.33% 8.70E E+15 TR-S % 1.73% 2.20% 6.23E E+10 TR-S % 2.72% 3.05% 5.31E E+08 Average % 1.85% 2.19% 2.90E E+15 Std. Dev E E+15 Table 3. SST Data for the Polymer-Modified Mix Specimen Air Permanent Shear Strain at Cycles to ESALs to 12.5 Voids % Strain mm Rut Depth (%) cycles cycles cycles PM-S % 2.50% 2.73% 1.25E E+09 PM-S % 2.49% 2.71% 1.42E E+09 PM-S % 2.39% 2.62% 2.95E E+09 Average % 2.46% 2.69% 1.87E E+09 Std. Dev E E+08 Figure 4. Accumulation of permanent deformation by repeated shear loading. Disk-shaped Compact Tension Fracture Test. A fundamental advantage of asphalt pavements is in their ability to relax the thermally-induced stresses. By lowering the temperature, however, the asphalt material becomes more brittle and as the result, the mix becomes more susceptible to cracking. The resistance of an HMA to crack growth at low temperature can be quantified by the disk-shaped compact tension fracture test (Wagoner et al. 2005). In this test, a tensile load is applied at a constant displacement rate on a pre-notched disk-shaped specimen (Figure 1c). By increasing the load, the notch propagates and eventually the specimen fails. The fracture energy of the specimen, which is defined as the area under the load versus crack mouth opening displacement (CMOD) curve, indicates the cohesion that the mixture would provide against crack growth. In this study, the DC(t) test was executed on the TR and PM mixtures using the ASTM D7313 (2007) standard procedure. After reheating the mix samples in covered pans, they were compacted to 150-mm height and 7.0±0.5 percent air voids

8 using a Superpave gyratory machine. Two 50-mm thick disk-shaped specimens were cut and trimmed from each gyratory compacted specimen. After testing for bulk specific gravity, the loading grip holes and the initial notch were cut in the specimen. All the trimmings were performed by automatic cutting tools to fabricate uniform specimens. To condition the specimens for testing, they were placed in an environmental chamber for at least 3 hours before testing. The test temperature and the conditioning temperature was selected to be -12 C, which is 10 C warmer than the pavements low temperature (at 98% reliability) in central California. Table 4 presents the calculated fracture energy of the mixtures. Generally, the higher fracture energy value indicates a more ductile mixture and as the result, a better performance against low-temperature cracking. Asphalt Institute has recommended a minimum limit of 400 J/m 2 for the fracture energy of HMA to prevent excessive low-temperature cracking (Blankenship et al. 2010). Considering this threshold, both mixtures are expected to perform satisfactorily in terms of thermal cracking. The average fracture energy of the mixtures, as shown in Table 4, is not significantly different, and a similar low-temperature cracking could be expected from the TR and PM mixtures. Nonetheless, more variation was observed in the fracture energy of the TR mix. Figures 5 and 6 illustrate the data plots obtained from the DC(t) testing of the terminal blend and the polymer-modified mixtures, respectively. The TR mix seemed to have a slightly lower peak load and a higher CMOD value at the failure point. This implies that the TR mix exhibited a slightly more ductile behavior in the test; however, the differences were minuscule and the mixtures performed similarly. Table 4. DC(t) Fracture Test Data Air Voids Fracture Energy (J/m 2 ) Peak Load Time of Peak Mixture Specimen (%) Test Data Average CV (kn) Load (s) TR-D Terminally- TR-D % Blend Rubber TR-D PM-D Polymer- PM-D % Modified PM-D Note: CV= coefficient of variation Figure 5. DC(t) data plots for the tire rubber mix.

9 SUMMARY AND CONCLUSIONS An experimental study was performed to compare the laboratory performance of a terminal blend tire rubber asphalt mixture (TR) to a polymer -modified asphalt mixture (PM). The mixtures were made with a similar job mix formula and mix design to restrain the test results from all material-related factors except for binder modifications. The binders used in both mixtures were both confirmed to have a Superpave grade of PG Mix samples were collected from paving sites in California and shipped to Asphalt Institute laboratory for testing. Four-point flexural beam fatigue test, Superpave shear test, and disk-shaped compact tension fracture test were executed to evaluate the performance-related properties of the mixtures with regard to fatigue cracking, rutting, and low-temperature cracking, respectively. The beam fatigue test was conducted at three strain levels and the moderate temperature of 20 C. The results indicated that the mixtures have similar fatigue performances and they both provide a high resistance against fatigue cracking. Additionally, the endurance limits of the mixtures were estimated to be very close. This implies that the mixtures could have similar fatigue performances at lower strain levels. The repeated shear test at constant height (RSCH) test was conducted at 54 C. The analysis of the results indicated that the terminal blend mix was more resistant to permanent deformation as compared to the polymer-modified mix. However, both mixtures were estimated to be extremely rut resistant. The fracture energy of the mixtures was determined at -12 C using the DC(t) test. No significant difference was observed between the average fracture energy of the TR and PM mixtures. Based on the DC(t) test result, a similar low-temperature performance could be expected from both mixtures. In summary, this study confirmed that blending the tire rubber into the asphalt binder by the terminal blend technique could improve the performance of the HMA. When properly made, a terminal blend tire rubber asphalt mix can exhibit a comparable, or even better performance than a polymer-modified mixture. REFERENCES AASHTO R30 (2010). Standard Practice for Mixture Conditioning of Hot Mix Asphalt (HMA). Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 33rd Edition and AASHTO Provisional Standards, Washington, D.C. AASHTO T320 (2007). Standard Method of Test for Determining the Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester (SST). Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 33rd Edition and AASHTO Provisional Standards, Washington, D.C. AASHTO T321 (2007). Standard Method of Test for Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending. Standard Specifications for Transportation Materials and Methods

10 of Sampling and Testing, 33rd Edition and AASHTO Provisional Standards, Washington, D.C. Asphalt Institute (2001). Superpave Mix Design, SP-2, 3rd Edition, Lexington, KY. ASTM D6414 / 6114M (2009). Standard Specification for Asphalt-Rubber Binder. ASTM Annual Book of Standards Vol , West Conshohocken, PA. ASTM D7313 (2007). Standard Test Method for Determining Fracture Ener gy of Asphalt-Aggregate Mixtures Using the Disk-Shaped Compact Tension Geometry. ASTM Annual Book of Standards Vol , West Conshohocken, PA. ASTM D7460 (2010). Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending. ASTM Annual Book of Standards Vol , West Conshohocken, PA. Bahia, H. (1995). Critical evaluation of asphalt modification using strategic highway research program concepts. Transp. Res. Rec., 1488, Blankenship, P., Anderson R. M., King, G. N., and Hanson, D. I. (2010). A Laboratory and Field Investigation to Develop Test Procedures for Predicting Non-Load Associated Cracking of Airfield HMA Pavements, AAPTP Project Report, Airfied Asphalt Pavement Technology Program. Hajj, E. Y., Sebaaly, P. E., Edgard, H., and Borroel, C. (2011). Performance Evaluation of Terminal Blend Tire Rubber HMA and WMA Mixtures- Case Studies. Asphalt Paving Technol., 80, Hicks, R. G., Cheng, D., and Duffy, T. (2010). Evaluation of Terminal Blend Rubberized Asphalt in Paving Applications, Report CP2C TM, California Pavement Preservation Center. Huang, B., Mohammad, L. N., Graves, P. S., and Abadie, C. (2002). Louisiana Experence with Crumb Rubber-Modified Hot-Mix Asphalt Pavement. Transp. Res. Rec., 1789, Kuennan, T. (2012). The Chemistry of Asphalt Modifiers. Better Roads, 82(4), Prowell, B. D., Brown, E. R., Anderson, R. M., Daniel, J. S., Swamy, A. K., Quintus, H. V., Shen, S., Carpenter, S. H., Bhattacharjee, S., and Maghsoodloo, S. (2010). Validating the Fatigue Endurance Limit for Hot Mix Asphalt, NCHRP Report 646, Transportation Research Board of the National Academies, Washington, D.C. Sousa, J. B. (1994). Asphalt Aggregate Mix Design Using the Repetitive Simple Shear Test (Constant Height). Asphalt Paving Technol., 63, Stroup-Gardiner, M., Newcomb, D. E., and Tanquist, B. (1993). Asphalt -Rubber Interactions. Transp. Res Rec., 1417, Wagoner, M. P., Buttlar, W. G., Paulino, G. H., and Blankenship, P. (2005). Investigation of the Fracture Resistance of Hot-Mix Asphalt Concrete Using a Disk-shaped Compact Tension Test. Transp. Res. Rec., 1929,