Development of a Crumb Rubber Modified (CRM) PG binder Specification

Transcription

1 Development of a Crumb Rubber Modified (CRM) PG binder Specification Gaylon Baumgardner Paragon Technical Services A P.O. Box 1639 Jackson, MS g.baumgardner@ptsilab.com John A. D Angelo D Angelo Consulting, LLC 8528 Canterbury Dr. Annandale, Virginia, USA johndangelo@dangeloconsultingllc.com ABSTRACT. Ground-up scrap tires have been used to modify asphalt binder for over 40 years. In that time the primary specification has been of an empirical nature. The primary specs have included particle size with time and temperature of blending. Some crude rotational viscosity measurements have been used to provide some quantification of properties. With the advent of the Superpave asphalt binder specifications efforts have been made to develop similar rheological tests for CRM asphalt binders. In this study the Bending Beam Rheometer and a new geometry using a Searle system for testing large particle size CRM is evaluated to determine if valid test results can be achieved for a PG type specification. Testing using multiple size rubber particles in the new cup and bob geometry is evaluated. Evaluations are made using neat and polymer modified binders in the new geometry to compare to the standard DSR parallel plate geometry. Various sizes of CRM from 0.25 µm up to 1 mm are used to evaluate the new cup and bob geometry to determine if the results can be used for development of a PG type specification system. Full AASHTO M 320 Table 1 and MP 19 testing is done on all CRM binders in the study and from this data a proposed performance based CRM binder Superpave specification will be developed. KEYWORDS: Rubber, Asphalt, PG Specification, Cup and Bob geometry

2 2 Baumgardner and D Angelo 1. BACKGROUND Scrap tire rubber, also known as crumb rubber modifier (CRM), has been used since the 1960 s to modify asphalt binder (1,2). Uses have included stress absorbing membranes, stress absorbing membrane interlayers, crack seals, hot mix asphalt, and open graded friction courses. Historically the specifications for CRM binder in most of these applications have been recipe or method type. Method specs describe very specific processes and amounts of material are described to produce a specific product. In many cases where contractors have experience with these specifications good performance can be achieved (1, 2). However, this makes transfer of these processes and specifications very difficult from one location to another and increases the potential for failures (3). Crude test procedures have been tried to provide some type of quality control for the various processes. The primary device is the hand held rotational viscometer (1). This can provide some indication of viscosity increase from the addition and blending of rubber into the binder but has high variability. Binder testing has been done using the Superpave binder tests on CRM binder but this has been limited to CRM sizes that can be handled in the 1 or 2 mm gap using DSR parallel plate geometries (4,5,6) typically 30 mesh material or smaller. These studies did show the increase in modulus of the binder with the addition of the CRM and that the size, percentage of rubber and base asphalt all had an effect on the binder properties. However, in general use CRM comes in many different sizes and the most used material is typically larger than the 30 mesh. This requires testing of the binder with larger particle sizes using geometries with larger gaps. Many studies have shown that the CRM size, shape, mixing temperature and asphalt binder will all effect the final properties of the CRM binder (3,4,5,6). Without a well defined binder specification adoption of the use of CRM binder by the US highway agencies will be almost impossible to achieve. Test procedures that can evaluate the performance characteristics of CRM binder are crucially needed. 2. OBECTIVE In the mid 1990 s the Superpave Performance Graded Binder specification became the standard used throughout the US. Rheological testing of binders is now a standard practice to evaluate performance characteristics of neat and modified binders. Superpave Specifications do have limitations that restrict materials that can be tested. Existing specifications use the Dynamic Shear Rheometer (D SR) and parallel plate geometry with a 1 mm gap for testing of binders at high and intermediate temperatures (7). This geometry limits the materials that can be tested between the plates. Particulate matter larger than 250 µm cannot be tested due to the

3 3 Baumgardner and D Angelo possible interaction of particulate matter on torque and strain response of the binder (7). Typical CRM binder has rubber particles much larger than 250 µm. Rubber particles may range in size from 0.5 mm up to over 1 mm in size. A 1 mm particle tested in a DSR with 1 mm gap parallel plate geometry would be touching both top and bottom plates at the same time so that test results would represent the rubber particle not a rubber modified binder. Performing PG testing on CRM binders with larger particles will require using new geometries that will provide larger gap sizes that can accommodate those particle sizes. One approach that has been used in the food industries has been testing with concentric cylinder geometries. DSR s currently used for asphalt testing can be adapted to use a Searle system. This is one where the center cylinder or bob rotates and the outside cylinder or cup is stationary (8,9). This type of system can perform all the same type of testing that is currently used for asphalt binder grading. The advantage is that the cup and bob geometry can easily handle larger gaps up to 4 to 7 mm and therefore larger CRM particles. One disadvantage of the system is that it does require a much larger sample for testing. Graphics and pictures of the geometry are shown in Figure 1. R b R c 1a 1b Figure 1. a) graphic showing the bob submerged into the cup with CRM binder. b) Photograph of a cup and bob geometry with the bob extended above the cup.

4 4 Baumgardner and D Angelo The Bending Beam Rheometer (BBR) is another primary piece of testing equipment used in the Superpave PG grading system. The BBR is used to measure the low temperature stiffness and relaxation properties of the binder. The testing is done on a beam of asphalt binder 6.4 X 12.7 X 127 mm. Since the beam has a cross section of 6.4 X 12.7 mm it can actually accommodate CRM particles of about 1 mm. Because of this size no changes should be needed to test CRM binder in the BBR with the larger particle sizes. The dimensions of the beam is shown in Figure 2. Figure 2. Dimensions of the beam of asphalt binder for use in testing in the Bending Beam Rheometer.

5 5 Baumgardner and D Angelo The objective of this study was to perform the initial evaluation of the cup and bob and Bending Beam Rheometer for suitability of CRM binder testing. This was performed by testing neat, polymer modified and CRM binders using the new cup and bob in addition to the BBR. Several different concentrations and sizes of CRM particles were used to evaluate the differences in test results and suitability for inclusion into a PG type specification on CRM binders. 3. EXPERIMENTAL PLAN Testing of the cup and bob geometry was conducted on several different rubber sizes and one base binder. Comparison was made between standard PG grading using AASHTO M320 and MP19 of the different control and CRM binders using parallel plate (PP) geometry with 1 mm and 2 mm gaps and the cup and bob geometry with a 6.5 mm gap. Intermediate DSR testing was done only on the neat, polymer modified and the 30 mesh and smaller CRM binders. Standard BBR testing was run on all asphalt binder samples. AASHTO MP 19, MSCR testing was evaluated using the same materials and DSR geometries as the standard PG high temperature properties. To produce the CRM blends a PG was blended with 20, 30 and 60 mesh rubber particles at different percentages. The full testing plan is shown in Table 1. CRM used was all ambient grind and the gradations are shown in Table 2. It was originally believed that there was a larger difference between the 30 and 60 mesh CRM, however, after running a sieve analysis of the rubber it was discovered that the 60 mesh was only slightly finer than the 30mesh. Binder type Geometry and Gap Size PP1 PP2 CB6.5 PG M 320, MP19 M 320, MP19 M 320, MP19 PG M 320, MP19 M 320, MP19 M 320, MP19 PG M 320, MP19 M 320, MP19 M 320, MP % 60M M 320, MP19 M 320, MP19 M 320, MP % 60M M 320, MP19 M 320, MP19 M 320, MP % 30M M 320, MP19 M 320, MP19 M 320, MP % 30M M 320, MP19 M 320, MP % 20M M 320, MP % 20M M 320, MP19 PG MP19 MP19

6 6 Baumgardner and D Angelo Table 1. Test for the various binders and geometries. An Anton Paar Smart Pave 301 DSR was used for high and intermediate testing required for AASHTO M 320 and MP 19. RTFOT testing of the binders was done with standard 25 mm plates using a 1mm and 2mm gap. The cup and bob geometry used a 27 mm cup and a 14 mm bob, which produced a 6.5 mm gap. Intermediate temperature testing was done using 8 mm plates with a 2 mm gap as per standard T315 test procedures. Testing of larger CRM at intermediate temperatures will be part of future evaluations to determine if the cup and bob geometry can be used at intermediate temperatures. Standard BBR molds were used for all low temperature testing. Standard BBR molds are 6.4 mm by 12.7 mm and it was felt that they could accommodate the CRM up to 1 mm in size without adversely affecting the test results. Percent Passing Liberty 20 Mesh PolyVulc 30 Mesh Table 2. Sieve analysis of the various CRM sizes. PolyVulc PolyVulc 0080 PLB2B5044 PLB5E5250 PLB4D4861 PLB4D (2000 micron) (850 Micron) (600 Micron) (425 Micron) (300 Micron) (180 Micron) (150 Micron) (7.5 Micron) 7.4 The initial evaluation was done by comparing the results of the parallel plate geometry to the cup and bob geometry. Replicate tests were not run for the initial evaluation. The results for the cup and bob were compared to the parallel plate geometry by evaluating any difference in results against the single operator precision 2Ds of the current AASHTO T 315 procedure. This was used as the initial evaluation to determine if the new geometry would provide results that are currently accepted in the asphalt industry. The future development of full test procedures will include a thorough statistical evaluation of the reputability and reproducibility of the new geometries.

7 7 Baumgardner and D Angelo The shear stress and shear strain calculations are slightly different for the cup and bob as compared to the parallel plate calculations. These are shown in equations 1 and 2. Shear Stress = = 1 Shear Strain = = ( ) 2 T = torque R b = radius of the bob R c = radius of the cup = angular rotation of the bob The cup and bob does not require trimming which is an advantage with CRM binders. The actual surfaces that control the geometry are defined by the surface of the bob and the walls of the cup. Any binder that is at the bottom of the cup and overtops the bob can be ignored. Similarly to the standard parallel plate geometry where the stress and strain is controlled by the outside edge, the cup and bob is controlled by the surface area of the bob and the radius of the bob and cup. Any binder at the bottom of the cup and any that overflows the bob have an insignificant effect on the stress. 4. RESULTS The initial evaluation was to compare the test results from standard parallel plate geometry to results from cup and bob for asphalts binders that can be tested meeting all the current specifications. Three binder were evaluated; a neat PG from Lion Oil produced from a light Saudi crude stock, a PG produced from the Lion plus Polyphosphoric Acid and a PG produced from the plus 2.25% SBS polymer. The M 320 test results for both the standard 25 mm parallel plate 1 mm gap and 2 mm gap geometry and the 6.5 mm gap cup and bob geometry indicated almost no difference in results. A detailed discussion of the comparison of parallel plate and cup and bob geometries is covered in a separate paper (10). The comparison of high temperature continuous grades, are shown graphically in Figure 2. All the M 320 test results are shown in Table 3 and 4. The CRM binders were then tested in the 3 DSR geometries as appropriate to the particle size. The 15% 30 Mesh CRM was not tested in the 1 mm gape parallel plate geometry and the 20 Mesh CRM was only tested in the cup and bob 6.5 mm

8 8 Baumgardner and D Angelo geometry. The cub and bob results from the CRM binders provided a slightly higher variation from the 1 mm parallel results than the neat or polymer modified binders. A more detailed discussion is provided under a separate paper (10). In general the data indicate that the 1 mm gap parallel plate and cup and bob will provide similar results. Based on this a PG type specification can be developed for CRM binders using larger size rubber. The variability between parallel plate and cup and bob geometries was slightly higher for the CRM binder as compared to the neat and polymer modified binders. This increase in testing variability may be due to the variability of the CRM binders not the test procedure. This will be evaluated in further development of the test procedure. Continous Grade temp C PP1 PP2 CB6.5 Figure 2. Bar graph of the continuous high temperature grade for three typical binders tested in various geometries indicating the high repeatability of the results. Binder Samples Geometry PP1 PP2 PG grade PG grade PG grade

9 9 Baumgardner and D Angelo CB6.5 PG grade PG grade PG grade %diff CB/PP PAV DSR PG grade BBR PG grade BBR S PG grade BBR m PG grade Diff S m C Table 3. Test results for the high temperature Superpave M320 specification on neat and polymer modified binders using the three different test geometries. Geometry PP1 PP2 CB6.5 10% 60M 15% 60m Binder Samples 10% 15% 30M 30M 15% 20M 20% 20M PG grade PG grade PG grade PG grade PG grade PG grade %diff CB/PP PAV DSR PG grade BBR PG grade BBR S PG grade BBR m PG grade Diff S m C

10 10 Baumgardner and D Angelo Table 4. Test results for the complete Superpave M320 specification on CRM binder blends and the three different high temperature test geometries. PG testing of the CRM binders clearly shows the changes that occur to the base with CRM size and percentage. Figure 3 shows a comparison of the complete continuous grading of the base PG binder to the base plus 10% 60 mesh blend. The 10% 60 mesh CRM increases the high temperature stiffness of the binder, as expected, but it also lowers the intermediate DSR stiffness and BBR low temperature properties. The 10 % 60 mesh CRM changed the PAV DSR continuous grade from 24.6 C down to 20.4 C. The addition of the CRM also lowered the low temperature continuous determined from the BBR from -22 to C PG Grade PP PP CB PAV DSR BBR 10% 60M PP1 10% 60M PP2 10% 60M CB 10% 60M PAV DSR 10% 60M BBR Figure 3. Bar graph of the continuous PG grading of the base and base + 10% 60 mesh CRM. Each of the CRM blends shows differences in the intermediate and low temperature properties compared to the base PG Figure 4 shows the intermediate DSR continuous grade for the 10% and 15% of the 30 and 60 mesh CRM. In this case since the 30 and 60 mesh materials were very similar materials they can be considered replicates instead of different sizes. The 10% CRM reduces the intermediate DSR continuous grade by 4 C and the 15% CRM reduces it by an additional 4 C. The results are similar for both of the 10 and 15% blends of CRM. This indicates that there is an interaction of the CRM with the asphalt binder which

11 11 Baumgardner and D Angelo will affect not just high temperature properties of the binder, but also intermediate and low temperature properties. The BBR data shows similar reductions in the continuous grade as was seen in the intermediate DSR. Figure 5 shows the continuous low temperature grade of the base binder and the 10 and 15 percent blends of CRM. The 10 percent blends reduced the grade by 3 to 3.5 C and the 15 percent blends reduced it by 5 C. Again there is clearly interaction between the base binder and the CRM that is reducing the low temperature properties. 30 PAV DSR Grade C % 60M 15% 60m 10% 30M 15% 30M Binder Blend Figure 4. PAV DSR continuous grades for the base and blends with different sizes and percentages of CRM.

12 12 Baumgardner and D Angelo % 60M 15% 60m 10% 30M 15% 30M Low temperature grade C Binder type Figure 5. PAV BBR continuous grades for the base and blends with different sizes and percentages of CRM. The BBR data indicates that the CRM affects different properties of the binder to different degrees. The BBR specification has two parameters; stiffness or S value and relaxation or m value. The actual temperature where the binder meets the low temperature specifications is typically different for the S and m values of the binder. Most typical neat binders will have a difference of about 2 C between the S and m grade temperatures. CRM appears to have a large effect on this difference in the S and m values. Figure 6 shows a plot of the difference in the continuous grade temperature of the S and m values for several binders used in this study. The PG which was used as the base asphalt for the CRM blends has a difference in S and m grade temperatures of 2 C. When PPA was added to this binder the difference in S and m grade temperatures stayed about the same. The PG used in this study to evaluate the cup and bob geometry had a difference in S and m grade temperatures of about 4 C. The CRM blends had differences of about 4 C for the 10 percent blends and over 7 C for the 15 percent blends. It appears the CRM has a much larger affect on the S value as compared to the m value of the base binder. The CRM significantly reduced the stiffness of the binder, but had less effect on the relaxation properties.

13 13 Baumgardner and D Angelo 8.00 Diff between S & m grade temp C % 60M 15% 60m 10% 30M 15% 30M Binder Figure 6. Bar graph of the difference in temperature between the continuous BBR S and m grade values. A comparison of the 20 mesh blends to the 30 and 60 mesh blends shows a distinct difference in the affect of the rubber size to the PG binder properties. Figure 7 shows a comparison of the continuous high temperature grade measured in the cup and bob geometry of the 30 and 20 mesh CRM. It only takes 10% of the 30 mesh CRM to increase the base up to 72.4 C while it takes 15% of the 20 mesh CRM to create the same change. The same affect can be seen by comparing the 15% concentration of 30 mesh to the 20% concentration of the 20 mesh CRM. The same affect is also seen in the low temperature BBR properties.

14 14 Baumgardner and D Angelo High PG Temp C % 30M 15% 30M 15% 20M 20% 20M CRM Size & Percentage Figure 7. Comparison of high temperature continuous grades of CRM binders from different mesh sizes and concentrations. Further evaluation of the new cup and bob testing geometry was to run samples using AASTHO MP 19 TP 70 MSCR testing. The same binders were used in the MSCR test as in the standard PG testing. Control binders in this case provided mixed results as to the acceptability of the cub and bob to replace the 1 mm parallel plate. There was only a 4% difference in the MSCR results for the PG64-22 binder, however, the difference between the results for the PG70-22 and PG76-22 binders were significantly higher at 26 and 34% respectively. To determine if this was a geometry problem or just possible testing error an additional binder was tested. This binder was a PG 64-34, which was selected because it was a highly modified material prepared from a soft base asphalt. The PG binder provided MSCR in both geometries very similar to the neat PG Comparison of results for the PG is well within the single operator 2Ds for the MSCR test as shown in Figure 8. Further testing will be performed on stiffer materials to determine if issues with equipment compliance may be the cause of variation, specifically when high torque is needed. The MSCR test is also more sensitive to temperature and polymer modification which can be seen in higher variability (11). A more detailed discussion of the MSCR testing of CRM binders is covered in a separate paper (10).

15 15 Baumgardner and D Angelo J nr kpa % diff 9.5% diff Jnr.1 Jnr 3.2 PP1 CB Figure 8. Statistical difference in results between the 1 mm plate geometry and cup and bob for MSCR compliance results at 0.1 kpa and 3.2 kpa for a highly modified PG binder.

16 16 Baumgardner and D Angelo 5. CONCLUSIONS Crumb rubber modifier (CRM) has been used for many years in asphalt binder to provide improved field performance. Historically the increase in viscosity of the CRM binder was measured using crude vane viscometers to quantify its performance characteristics. With the introduction of the Superpave system far more accurate tools were introduced in the asphalt binder testing system to measure performance characteristics. Due to testing geometry limitations CRM binders generally have not been able to be tested using the Superpave binder tests specifically the high temperature testing. This inability to fully test the material has limited the use and adoption of CRM binders. There are existing geometries well known in the rheology field, specifically coaxial cylinder geometries or cup and bob that can handle larger particle sizes typically used as CRM. However, these geometries are not familiar in the asphalt industry. This study investigated the ability of the cup and bob geometry to test neat, polymer modified and CRM binders. The initial evaluation indicates that the cup and bob geometry can provide similar results for the Superpave binder parameter G*/sin for the neat, polymer modified and CRM binders. The test results from the cup and bob were within the single operator allowable variability of the parallel plate geometry for neat and polymer modified binder. Variability of the CRM binder was higher; however, this may be due to the variability of the binder not the testing geometry. This will be evaluated in future studies. The cup and bob geometry can provide similar results for the Multiple Stress Creep and Recovery Test for both neat and polymer modified binders. Standard BBR testing can be done on CRM binders even with larger CRM size particle up to 1.5 mm in size and provide results that should relate to the typical PG low temperature testing. The size and concentration of rubber will affect the high temperature properties of the CRM binder with finer rubber material having a larger effect than coarser rubber. The CRM will also positively affect the intermediate and low temperature properties of the binder. The size and concentration of rubber particles both affect low temperature properties as with high temperature.

17 17 Baumgardner and D Angelo Testing on the cup and bob geometries verified the effect of CRM size and concentration on the stiffening effect of the base binder. The cup and bob is now able to do this testing on much larger mesh size materials than could be done with a 2 mm gap for parallel plate testing. This study has indicated that the cup and bob can be used in PG testing and will allow for binder classification in the PG system of CRM binders with larger size rubber particles. This will allow for the development of a PG or MSCR specification of CRM binders. Further work is needed to refine the test procedures and establish precision and bias of the new geometry. ACKNOWLEDGMENTS The authors would like to thank Dr. Isaac Howard, Mississippi State University Department of Civil and Environmental Engineering for his support and guidance. Special appreciation is given Andy Menapace and Judge Brown of Paragon Technical services for their efforts in the laboratory testing the many binder samples used in the study.

18 18 Baumgardner and D Angelo 6. Bibliography 1. Epps, J. Uses of Recycled Rubber Tires in Highways, Synthesis of Highway Practice 198, National Cooperative Highway Program, National Academy Press, Heitzman, M. State of the Practice Design and Construction of Asphalt Paving Materials With Crumb Rubber Modifier, Publication FHWA-SA , FHWA, U.S. Department of Transportation, May Use of Scrap Tire Rubber, State of the Technology and Best Practices, California Department of Transportation, Materials Engineering and Test Services, Office of Flexible Pavement Materials, 5900 Folsom Blvd. Sacramento, CA , February Abdelrahman, M. Carpenter, S. Mechanism of Interaction of Asphalt Cement with Crum rubber Modifier, Transportation Research Record Journal of the Transportation Research Board, No. 1661, Washington D.C. 1999, pg Loh, S., Kim, S., Bahia,H., Characterization of Simple and Complex Crum Rubber Modified Binders, Wisconsin Department of Transportation, Report WI/PR-07-01, July Baumgardner, G., Anderson, D., Trans-Polyoctenamer Reactive Polymer/Recycled Tire Rubber Modified Asphalt: Processing, Compatibility and Binder Properties, 5 th International Transport Conference, Wuppertal Germany, Petersen, J.C., et all, Binder Characterization and Evaluation Volume 4: test Methods, SHRP-A-370, Strategic Highway Research Program, National Research Council, Steffe, J. F. Rheological Methods in Food Process Engineering, Freeman Press 2807 Still Valley Dr. East Lansing, MI, Copyright 1996, pg Schramm, G. A Practical Approach to Rheology and Rheometry. Copyright 1994 by Gegrueder HAAKE GmbH, D Karlsruhe, Dieslstrasse 4, Federal Republic of Germany, Pg Baumgardner, G., D Angelo, J., Evaluation of New DSR Testing Geometry for Performance Testing of Crumb Rubber Modified (CRM) Binder. Preprint Transportation Research Board annual meeting D'Angelo, J, Dongre, R; Practical Use of Multiple Stress Creep and Recovery Test: Characterization of Styrene-Butadiene-Styrene Dispersion and there Additives in Polymer Modified Asphalt Binders, Transportation Research Record Journal of the Transportation Research Board, No. 2126, Washington D.C. 2009, pg