EVALUATING CREEP COMPLIANCE OF ASPHALTIC PAVING MIXTURES USING A HOLLOW-CYLINDER TENSILE TESTER

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1 Performance Testing and Evaluation of Bituminous Materials 527 EVALUATING CREEP COMPLIANCE OF ASPHALTIC PAVING MIXTURES USING A HOLLOW-CYLINDER TENSILE TESTER William G. Buttlar University of Illinois at Urbana-Champaign, USA Ghazi G. Al-Khateeb Turner-Fairbanks Highway Research Center, McLean, USA Abstract A novel approach for measuring the creep compliance of viscoelastic solids in tension is presented. A prototype hollow cylinder tensile tester (HCT) has been developed, which applies pressure to the internal cavity of the specimen, and the resulting time-dependant strain is measured. In a previous study, the device was shown to produce reliable measures of complex modulus for asphalt concrete. In this study, the accuracy of creep compliance measurements from the HCT was assessed by conducting parallel tests on identical materials using an indirect tensile test (AASHTO TP-9). The creep compliance obtained with the two test devices were found to be in good agreement across a modest range of materials, test temperatures, and loading times, including nominal maximum aggregate sizes (NMAS) up to 19 mm. These findings suggest that both of the devices investigated are capable of measuring fundamental properties of viscoelastic media, such as asphalt concrete creep compliance. The data suggested a slight tendency for the indirect tension test to underestimate compliance relative to the HCT when measuring at high compliance values. Additional testing is needed to thoroughly examine the accuracy and repeatability of the HCT, particularly at higher compliance levels and for mixtures with NMAS exceeding 9.5 mm. 1. Introduction Significant advances in pavement modeling and monitoring of field sections have occurred over the past decade, which are critical steps towards moving from empirical to performance-based designs of flexible pavements. However, the benefits of these advanced design tools cannot be fully realized unless state highway agencies and other parties involved in the design of bituminous mixtures have widespread access to reliable, repeatable, and operationally simple test equipment, which provide the necessary inputs to these new design tools. In the United States, for thermal cracking performance predictions under the AASHTO 2002 design software, creep compliance and mixture tensile strength at low temperatures are required. The Superpave Indirect Tensile Test (IDT), developed under the Strategic Highway Research Program can be used to obtain these properties (1). A hollow cylinder tensile tester (HCT) was recently developed (Figure 1) and shown to have promise as a suitable surrogate device to measure creep compliance and tensile strength of asphalt concrete at low temperatures (2, 3). The main reason for developing a surrogate test for the IDT was to address the lack of portability, the complexity of operation, and the relatively high cost of IDT equipment. Later, it was found that the hollow cylinder tensile testing mode had other inherent benefits, as described in a later section. Previous studies have investigated the use of

2 528 6th RILEM Symposium PTEBM'03, Zurich, 2003 hollow cylinders to obtain properties of asphalt concrete in torsional shear and compression (4, 5). However, the test method presented herein differs from these approaches in that the primary focus is to obtain tensile properties by applying cavity pressure and measuring cylinder expansion. Portion of HCT cavity pressurized by membrane (portion above and below dotted lines pressurized by seals) 24 mm Strain gauge 115 mm 150 mm a) Specimen Dimensions and Loading b) Apparatus (left) and specimen (right) Figure 1. Hollow Cylinder Tensile Tester Concept, Apparatus, and Specimen Figure 2. Superpave Indirect Tension Test Specimen (AASHTO TP-9) The basic principle of the HCT is to apply internal pressure to the inner cavity of a hollow cylinder specimen, which results in circumferential (hoop) tension. Applied stress is linearly related to applied pressure, and the resulting strain is linearly related to cavity volume change or can be directly measured using strain gages. An advantage of hollow cylinder hoop tension testing is that stress and strain fields are uniform over a large portion of the specimen and that hoop tension is dominant over compression, which occurs only in the radial direction. Furthermore, unlike direct tension testing and indirect tension testing, which have significant end effects (e.g., stress concentrations caused by constraint, and by modulus and thermal coefficient differences between test material and platen material), the hollow cylinder hoop tension mode has relatively minor stress concentrations (1). This is particularly advantageous for strength testing, since stress concentrations can have a large influence on specimen failure. 2. Testing Program A testing program was established to compare the creep compliance of hot-mix asphalt as measured in the Hollow Cylinder Tensile Tester to those obtained using the Superpave IDT. Four different asphalt concrete mixtures were produced and tested at the Advanced Transportation Research and Engineering Laboratory (ATREL), in Rantoul, Illinois, USA, as summarized in Table 1.

3 Performance Testing and Evaluation of Bituminous Materials 529 The nominal maximum aggregate sizes considered in this study were: 4.75, 9.5, and 19-mm. This range in top aggregate size allowed for investigation of potential factors influencing HCT and IDT results, including non-homogeneity of gyratory-compacted specimens, spalling at specimen ends, and particle-size-to-gage-length ratio. Table 1: Asphalt Mixtures Used in the Study Mixture Type Description Aggregate Type Binder Type Dense-Graded Little- ATREL 19-mm Max PG Rock Binder Course Mix Laboratory Aggregate Size [AC-20] (LR19 PG 64-22) Standard Limestone Polymer-Modified Sand- Asphalt Mixture (SANDAC PG 70-34) Rantoul Traditional Overlay Mix (RNAC PG 58-22) Rantoul Polymer- Modified Mix (RNAC PG 64-28) Peoria Airport Interlayer Mix Rantoul NAC Demonstration Project Rantoul NAC Demonstration Project 4.75-mm Max Aggregate Size Sand 9.5-mm Max Aggregate Size Limestone 9.5-mm Max Aggregate Size Limestone PG (SBS - Modified Binder) PG [AC-10] PG (SBS Polymer-Modified Binder) SBS: Styrene-Butadiene-Styrene PG: Denotes Binder Grade Designation from the Superpave Performance-Graded Binder Specification (AASHTO MP-1). Where possible, closest ASTM D3381 grade (AC) is given in [ ]. Finally, different performance-graded (PG) asphalt binders were used in the mixes prepared. Modified asphalt binders as well as unmodified binders were included. Superpave Performance- Graded, PG 58-22, PG 64-22, PG (styrene-butadiene-styrene [SBS] polymer-modified), PG (SBS polymer-modified), were all used for the preparation of asphalt mixtures. A Superpave gyratory compactor (600 kpa compression, 1.25 degree external mold gyration angle, and 30 rotations per minute) was used to produce 150-mm diameter by 115-mm tall cylinders, which were cored to produce hollow-cylinder specimens with 102-mm diameter cavities (Figure 1a). Indirect tension specimens were produced by sawing gyratory specimens to produce 50-mm thick by 150- mm diameter specimens. By trial and error, the number of gyrations needed to produce indirect tension and hollow cylinder specimens at 4 +/- 0.4% air voids was determined. In general, a slightly higher number of gyrations were required for hollow cylinder specimens, since they comprise the outer portion of the gyratory specimen, which is more highly voided. Bulk specific gravities were determined using saturated-surface dry techniques (AASHTO T-166) in this study. Recent testing suggests that higher void levels are obtained when using paraffin-coated specimens and other similar measurement techniques, and will be addressed in future publications. Creep tests were conducted using the HCT and IDT tests at three test temperatures (-20 o C, -10 o C, and 0 o C), with the exception of the LR19 PG mixture, which was tested at -10 o C only. Creep tests were conducted for a time period of 100 seconds, following Superpave IDT test protocols outlined in AASHTO TP-9 specifications. Creep loads were applied rapidly (generally less than 0.1 seconds), creating strains in the linear viscoelastic range (less than 500 microstrains). Recovery effects were not evaluated in this study. Piston movement and applied pressure were monitored in the HCT mode. In the creep tests, a constant internal pressure was applied to the inner wall of the hollow cylinder using a controlled pressure mode, and the tensile strain using strain gages was monitored throughout the 100-second test. Two-inch strain gages for testing of asphalt mixtures were used as a measure of the circumferential (tangential) tensile strains at the mid-height of the inner wall of the specimen (Figure 1a).

4 530 6th RILEM Symposium PTEBM'03, Zurich, 2003 Creep compliance values were computed using simple formulas developed by Buttlar et al. (2), based upon 3D finite element analysis. These formulas account for effects of eccentric coring of test specimens, which cannot be accounted for using traditional axisymmetric thick-walled cylinder solutions. Since a sealing system is required to keep the inflatable membrane from escaping out specimen ends, the internal pressure in the hollow cylinder is not perfectly uniform. Less pressure is generally applied in the seal region (top and bottom) as compared to the test pressure applied by the membrane (Figure 1a). Although the effects of this partial cavity loading on total specimen response are almost negligible, the aforementioned formulas incorporate these effects. In the IDT test, surface mounted sensors are attached to the center of the flat faces of the specimen (Figure 2) in an attempt to reduce end effects caused by loading platens (1). Vertical and horizontal displacement transducers with 37.5-mm gage length were used. 3. Results and Discussion Typical creep compliance versus time curves at three temperatures (0, -10, and -20 o C) obtained in HCT and IDT testing are presented in Figure 3, while more detailed test results can be found in Table 2. Figure 4 presents HCT creep compliance data at 100 second loading time plotted against IDT results at the same loading time and test temperature. In general, creep compliances obtained with the two devices were found to be in good agreement. Given the considerable difference in testing modes (indirect tension versus hollow cylinder tension), the similarity in measured compliance suggests that each test is capable of capturing fundamental properties of asphalt concrete at low temperatures, such as creep compliance. In a related study, favorable agreement was also observed between the complex modulus of asphalt concrete (E*) measured using the HCT in tension and the standard uniaxial E* test in compression (ASTM D3497) for test temperatures of 0 o C and 20 o C (6). A discrepancy which seems to follow a repeatable trend can be observed in Figures 3 and 4. It appears that the measured creep compliance values obtained with the two devices diverge slightly at the warmest test temperature (0 o C) and at longer loading times. At higher creep compliances, HCT values tend to be larger than those obtained in the IDT. It is hypothesized that as the mixture becomes more compliant, end effects in the IDT test become more significant. As the loading heads penetrate into the specimen, localized damage can lead to stress redistribution, possibly reducing horizontal deflections (perpendicular to the direction of load) measured on the IDT specimen. Lower horizontal deflections result in lower estimated creep compliance (1). In addition to the discrepancy noted above at higher creep compliances, significant differences were noted in the creep compliances obtained for the SAND AC PG mixture, particularly at 1 and 10 second loading times (Table 2). While these discrepancies are still under investigation, it is currently believed that errors in procedures for tuning of closed-loop control system parameters for mixtures of varying stiffness in the two test devices (IDT and HCT) may have been largely responsible. Non-optimal tuning of control system parameters can lead to slow rise times and/or temporary overshoot of target load levels, which particularly affects short loading time test results. Finally, based upon the results presented in Figure 5, it appears that aggregate size does not have a significant effect on creep compliance measurement in either the IDT or HCT, for mixtures with nominal maximum sized gradations ranging from 4.75 mm to 19 mm. Even though the 19 mm mixture has aggregates that approach the wall thickness of the HCT specimens, which is 24 mm, good agreement was still observed between IDT and HCT test results. Although promising, this finding is considered as tentative since only one 19-mm nominal maximum size mixture has been

5 Performance Testing and Evaluation of Bituminous Materials 531 tested to date. Testing on a wider variety of mixtures and across a broader range of temperatures is underway to further validate these findings. Creep Compliance (1/GPa) HCT 0 o C IDT -10 o C -20 o C Time (sec) Figure 3: Creep Compliance Comparison: Rantoul National Aviation Center (RNAC) PG Mixture (Average of Three Tests) Table 2: Comparison of Creep Compliance (1/GPa) at Various Loading Times 100-Second Loading Time 10-Second Loading Time 1-Second Loading Time Mixture HCT IDT HCT IDT HCT IDT LR o C RNAC -20 o C RNAC -10 o C RNAC 0 o C RNAC -20 o C RNAC -10 o C RNAC 0 o C SAND-AC -20 o C SAND-AC -10 o C SAND-AC 0 o C

6 532 6th RILEM Symposium PTEBM'03, Zurich, IDT Creep Compliance (1/GPa) Test temperature, in o C LR RNAC -20 RNAC -10 RNAC 0 RNAC -20 RNAC -10 RNAC 0 SAND-AC -20 SAND-AC -10 SAND-AC HCT Creep Compliance (1/GPa) Figure 4: Creep Compliance Comparison for All Mixtures at 100-Second Loading Time Creep Compliance (1/GPa) mm mm HCT IDT mm Figure 5: Creep Compliance at 100-Second Loading Time and -10 o C for Mixtures with Varied Nominal Maximum Top Aggregate Size A system to measure specimen strain by measuring the volume change of the internal cavity (2) is currently under development. This system is a highly desirable alternative to strain gages, since it eliminates specimen instrumentation. Another advantage of the volume-based strain measuring system is the ability to assess average strain across the entire inner surface of the hollow cylinder, which will tend to increase test repeatability.

7 Performance Testing and Evaluation of Bituminous Materials Summary A new approach for measuring the creep compliance of viscoelastic solids in tension was presented, involving a hollow cylinder tensile tester. Compliances measured using the HCT were in good agreement with the Superpave indirect tension test. These findings indicate that both of these devices are capable of measuring fundamental properties of viscoelastic media, including asphalt concrete. The data suggested a slight tendency for the indirect tension test to underestimate creep compliance relative to the HCT at high compliance values. Although promising, additional testing is needed to thoroughly examine the accuracy and repeatability of the HCT, particularly at higher compliance levels and for mixtures with nominal maximum aggregate sizes larger than 9.5 mm. 5. References 1. Buttlar, W. G., and R. Roque, Experimental Development and Evaluation of the New SHRP Measurement and Analysis System for Indirect Tensile Testing of Asphalt Mixtures at Low Temperatures, Transportation Research Record, No. 1454, National Academy Press, Washington, D. C., pp , Buttlar, W.G., G. G. Al-Khateeb, and D. Bozkurt, Development of a Hollow-Cylinder Tensile Tester to Obtain Mechanical Properties of Bituminous Paving Mixtures, J. Assoc. of Asph. Pav. Tech., Vol. 68, pp , Al-Khateeb, G. G., Buttlar, W. G., and J. M. Bauer, Evaluating Tensile Strength of Asphaltic Paving Mixtures Using a Hollow Cylinder Tensile Tester, Ninth International Conference on Asphalt Pavements, Copenhagen, Denmark, Alavi, S. H., and C. L. Monismith, Time and Temperature Dependent Properties of Asphalt Concrete Mixes Tested as Hollow Cylinders and Subjected to Dynamic Axial and Shear Loads, J. Assoc. of Asph. Pav. Tech., Vol. 63, pp , Richardson, I. R., D. N. Chapman, and S. Brown, Relating Failure Tests Performed in Hollow Cylinder Apparatus to Inherent Anisotropy, Transportation Research Record, No. 1526, National Academy Press, Washington, D. C., pp , Buttlar, W. G., Bauer, J. M. and, Sherman, D. S., Dynamic Modulus of Asphalt Concrete with a Hollow Cylinder Tensile Tester, J. Trans. Res. Bd, No. 1789, National Academy Press, Washington, D.C., pp , 2002.