USE OF GEORGIA LOADED WHEEL TESTER TO EVALUATE RUTTING OF ASPHALT SAMPLES PREPARED BY SUPERPAVE GYRATORY COMPACTOR

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1 USE OF GEORGIA LOADED WHEEL TESTER TO EVALUATE RUTTING OF ASPHALT SAMPLES PREPARED BY SUPERPAVE GYRATORY COMPACTOR R. COLLINS Georgia Department of Transportation, Office of Materials and Research Forest Park, Georgia, USA H. SHAM! andj.s. LA! School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA Abstract 0 The Georgia Loaded Wheel Tester (LWT) developed by Georgia Department of Transportation (GDOT) has been used since 1985 in the laboratory during the design stage to evaluate rutting susceptibility of asphaltic concrete mixtures. In the LWT testing, asphaltic concrete is subjected to an elevated temperature in a loaded wheel system under repetitive loading conditions and the permanent deformation induced under the wheel path is measured. This approach to assess rutting susceptibility was thought to be much more representative than the current test methods and can provide a fast and more accurate means of assessing rutting susceptibility of asphaltic concrete under actual field conditions. The asphalt beam samples used for the LWT testing are prepared by a rolling compaction machine. The new LWF developed in 1992 is described in this paper. To promote the concept of using Georgia LWT as a supplement to the SUPERPAVE Volumetric design procedure for evaluating permanent deformation of BMA, a test method utilizing STJPERPAVE Gyratory Compactor prepared samples to evaluate the rutting resistance of HMA by the LWT was developed. The gyratory samples placed in a specially designed mold can be tested in the LWT identical to that for performing the LWT testing on the beam samples. A test program was conducted to evaluate the applicability of the testing procedure and to develop correlations between the rut-depth of asphalt mixes using the beam samples and the gyratory samples. The test program used three asphalt mixes of different degrees of rutting resistance ranging from high to low rut susceptibility. Both types of samples were tested at 40 C, 50 C, and 60 C temperatures for 8000 cycles under the standard

2 LWT testing procedure. The rut-depth results from the beam samples and the gyratory samples showed good correlations for all three mixes tested. Key Words: Asphaltic Concrete, Gyratory Compactor, Loaded Wheel Tester, Rolling Compactor, Rutting 1. Introduction Excessive permanent deformation in asphalt pavements due to heavy truck traffic is a major concern in the United States. The problem is not confined to warmer regions as usually presumed, but wide spread from Alaska to Florida. Many methods have been proposed to assess rutting characteristics of asphalt mixes. These include the traditional Marshall and Hveem tests, uniaxial and triaxial static and dynamic creep tests, and the SUPERPAVE direct shear test, just to name a few. Among those proposed testing methods, the direct shear test proposed under the SUPERPA\'E Level II and Level III procedures represents the most promising one for assessing the permanent deformation characteristics of asphalt mixes. To perform this test, however, requires use of the sophisticated SUPERPAVE Shear Tester (SST). It will be difficult to use this method for routine laboratory mix designs and for quality control of BMA due to the complex testing as pointed out by some users [1]. Recognizing the need to have a simple testing procedure which can be used to assess the rutting potential of HMA, the Georgia Department of Transportation (GDOT) and Georgia Institute of Technology (GIT) have been working together since 1985 to develop a loaded wheel tester (LWT) to meet this need [2]. The initial LWT was subsequently modified in By subjecting asphaltic concrete to an elevated temperature in a loaded wheel system under repetitive loading conditions and measuring the permanent deformation induced under the wheel path, the rutting susceptibility of the asphaltic concrete to be used in the field can be determined. This approach can provide a fast and accurate means of assessing rutting susceptibility of asphaltic concrete under actual field conditions. A rolling compaction machine for compacting asphalt mixes for LWT testing was also developed [3]. Many research studies using the initial or the new LWT for evaluating rutting resistance of asphalt mixes have been undertaken by GDOT [4], GIT [5], and other agencies [6,7,8]. One of the essential components of the LWT testing is an ability to fabricate asphaltic beam samples with consistent bulk density values and, ideally, can closely simulate the compaction of asphalt mixes in the field. In the initial phase of the LWF study, a special kneading compaction machine was used to compact asphalt beam samples. Subsequently, a static compaction procedure was developed [9] to allow for compaction of beam samples using any suitable compression machine. Following the recommendation from the LWT round robin evaluation program [10], a dedicated rolling compaction machine was developed for fabricating 125 mm wide by 300 mm long by 75 mm thick asphalt beam samples under a rolling compaction action simulating the field compaction of asphalt mixes [3]. This compaction machine has been subjected to trial use by several state

3 Departments of Transportation and has been shown to be valuable in the implementation of the Georgia LWT procedure. A round robin program participated in by 11 state DOTs to evaluate the precision in fabricating the beam samples and in performing the LWT tests using the two machines is in progress. The experiences with the successful implementation of LWT on asphalt mix designs led GDOT to develop GDT-1 15, "Method of Test for Determining Rutting Susceptibility Using the Loaded Wheel Tester" [4]. According to this procedure, asphalt mixes which develop more than 7.5 mm rut-depth on the beam samples after being subjected to a wheel load of 100 lb. in magnitude and 100 psi contact pressure for 8000 cycles at 40 C testing temperature are deemed unsatisfactory in rutting resistance and are therefore rejected. Using this supplemental LWT procedure, GDOT has been able to screen asphalt mixes of inadequate rutting resistance, which otherwise would be acceptable according to the Marshall mix design criteria. Several other state DOTs have also used the LWT to evaluate rutting characteristics of asphalt mixes, and some DOTs have adopted different acceptance criteria to aid in their HMA mix design. For instance, Utah DOT and Maryland DOT both use 5.0 mm rut-depth value as a pass/fail criterion. With the SUPERPAVE volumetric procedure, soon to become the standard HMA design method in this country, there is a need for incorporating the Georgia LWT procedure as a supplement to the SUPERPAVE design procedure. SUPERPAVE volumetric design alone is insufficient for assessing permanent deformation of asphalt mixes. Using the SUPERPAVE volumetric mix design in conjunction with the Georgia LWT can be an expedient and effective way to design asphalt mixes having a low susceptibility to rutting in the laboratory. To promote this concept of using the Georgia LWT as a supplement to the SUPERPAVE volumetric design procedure for evaluating permanent deformation of HMA, a test method with the LWT utilizing a SUPERPAVE Gyratory Compactor (SGC) prepared samples to evaluate the rutting resistance of HMA was developed and is presented in this paper. 2. New loaded wheel tester 0 When the original version of the LWT machine was developed in 1985, the emphasis was primarily for investigating the applicability of the concept of using this simple wheeltracking machine to evaluate permanent deformation of asphalt mixes. With the concept of using the LWT to evaluate rutting susceptibility of asphalt mixes proven, a new loaded wheel tester was developed which incorporated several improved features listed below. Testing operation can be semi-automatically controlled. Three beam samples can be tested simultaneously. Test temperatures can be controlled from 25 C to 65 C. Tire (hose) pressure is adjustable to 120 psi, and wheel load to 250 lb.

4 The new LWT, shown in Figure 1, consists of four components including wheel tracking system, loading system, rutting measurement system, and operation and temperature control system. The machine has the overall dimensions of 1.4 m long by 0.85 m wide by 1.3 m height. It is equipped with an air compressor to supply the air pressure for the system. Details of each component are described as follows. The wheel tracking system is shown schematically in Figure 2. The three 125 mm wide beams are secured and confined on a beam sample tray which is placed over a sliding table. The sample tray is secured on the sliding table during the rutting test and can be pulled Out, as shown in Figure 1, to facilitate the set up of beam samples and the measurement of permanent deformation on the samples. The sliding table is attached to a reciprocating driving assembly that generates a 60 cycles per minute reciprocating action. The rubber hoses are positioned directly below the loaded wheels and on top of the beam samples. Wheel loads are applied on the beam samples by means of three pneumatic cylinders each attached to an aluminum wheel. Magnitude of the wheel load applied on each beam sample is regulated by air pressure applied to each pneumatic cylinder. Pressure in the three rubber hoses is regulated by a common pressure regulator, so that pressure in the three hoses will always be the same. The rut depth measurement device consists of a measurement stand and an electronic indicator. The measurement stand has 5 slots at 50 mm apart and allows a sample mold to hold two asphalt samples compacted by the gyratory compactor. The stem of the electronic indicator slides freely in the slot to obtain the maximum rut-depth reading at each prescribed position along the wheel path on the beam sample. The sample tray and the sliding table are enclosed in an environmental chamber where the temperature is regulated by a thermostat. A separate temperature sensor which has a temperature probe inserted into a dummy asphalt block allows temperatures in the asphalt beam samples to be monitored. The environmental chamber can maintain the temperature up to 65 C with only a 1.0 C fluctuation. The machine is equipped with a preset counter which is used to set the number of repetitions for the test. The test will stop automatically when the preset number of cycles has been completed and the wheel load will automatically lift. Wheel loads can be calibrated by means of a proving ring supplied with the machine. 3. Development of gyratory sample mold During the development of the LWT test method utilizing gyratory compacted asphaltic concrete sample, some guidelines were established to ensure the method is implementable, can provide reliable assessment of rutting characteristics and is easy to use. These resulted in establishing the following requirements for developing this test method. Minimum change in the LWT machine for performing this test Asphaltic concrete samples fabricated directly from SGC Rut-depth results from the SOC samples correlate with those from the beam samples

5 n. Figure 1 The New Georgia Loaded Wheel Tester

6 .. Figure 2. Schematic of the New Loaded Wheel Tester

7 3.1 Gyratory samples Based on these requirements, it was decided that the SOC sample to be used for the LWT testing should be 150 mm in diameter and about 75 mm in height. This sample height was selected to avoid any changes on the LWT when testing these samples. A sample with the prescribed compacted mix bulk density can be fabricated from the SOC. The 150 mm diameter by 75 mm high sample can also be obtained by trimming the standard SOC sample or obtained from the field by coring. 3.2 Sample mold made by Troxier Initially, the writer (JSL) discussed the concepts with Mr. Don McAlister of Troxler, Corp. A sample mold for testing two SOC samples was fabricated by Troxier Corp. and delivered to us for evaluation. The sample mold was made of a solid aluminum block 178 mm wide by 380 mm long by 76 mm thick. Two 150 mm diameter by 73 mm openings were tapped to accommodate for two 150 mm diameter by 75 mm high BMA samples. This leaves an approximately 3 mm thick aluminum base in each opening. 3.3 New gyratory sample mold A split sample mold made of high density polyethylene (HDPE), see Figure 3, was developed to overcome the problem mentioned above. The split mold is held together by three bolts. A 3 mm thick aluminum plate is attached to the bottom of one side of the split mold. The split mold can be separated to allow ease of placing gyratory samples into and removing them from the openings. It was found also that this makes it much easier to provide uniform confinement for asphalt cores taken from the field where the cores may have slightly different sizes. Use of HDPE for the mold has several advantages over the aluminum mold. The HDPE mold is considerably lighter than the aluminum mold. The modulus of HDPE is comparable to that of asphaltic concrete, about 300,000 psi to 200,000 psi at 40 C to 60 C, and thus can provide a more uniform confinement on the circular shape of the asphalt sample. 4. Test results and comparison During the LWT testing, two gyratory samples are placed in the gyratory sample mold and the mold is secured on the beam sample tray at the ends by restraining brackets. With the mold properly secured, the sample tray is pushed back and locked on the sliding table, and the loaded wheel testing can be performed identical to that for performing the LWF testing on the beam samples. Figure 1 shows that the gyratory samples and 2 beam samples were tested side by side. A slight modification was made on the rut-depth measurement guide to accommodate the 175 mm wide gyratory mold. A test program was conducted to evaluate the applicability of the testing procedure and to develop correlations between the rut-depth of asphalt mixes using the beam samples and the gyratory samples. The test program used three asphalt mixes of different degrees of rutting resistance ranging from high to low rut susceptibility. Beam samples were

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9 fabricated using the rolling compaction machine following the GDT- 115 procedure. Gyratory samples, 75 mm in height, were fabricated following the SUPERPAVE Gyratory Compaction procedure, except the number of gyrations was adjusted to achieve the desirable height and density for the specimen. Both the beam samples and the gyratory samples for each mix were tested at 40 C, 50 C, and 60 C temperatures for 8000 cycles under the GDT-1 15 procedure. The rut-depth results obtained were then analyzed statistically. 5. Conclusions The new LWT presented in this paper incorporated several improvement features over the original LWT which make it an effective and efficient tool for evaluating rutting potential of asphalt mixes. This self-contained and easy to operate machine has been used by GDOT and other state Departments of Transportation for several years to evaluate their asphalt mix designs in the laboratory. The LWT test results using the gyratory samples and the beam samples presented in this paper show that asphaltic concrete samples compacted by a gyratory compactor correlated well with the standard beam samples. This opens up the possibility of using SUPERPAVE volumetric design in conjunction with the Georgia LWT in the laboratory as an expedient and effective way of designing asphalt mix to achieve a low susceptibility to rutting. 6. Acknowledgements The Work reported in this paper is supported by the Federal Highway Administration and the Georgia Department of Transportation. This support is gratefully acknowledged. 7. References 1. National Asphalt Paving Association, An Industry Discussion on SUPERPA VE Implementation, NAPA Special Report 174, Lai, J. S. Development of a Simplified Test Method to Predict Rutting Characteristics of Asphalt Mixes, Georgia DOT Project 8503, Final Report, July Lai, J.S. and Shami, H., Development of a Rolling Compaction Machine for Preparation of Asphalt Beam Samples, Transportation Research Board, Paper No , January 1995

10 4. Collins, R., Watson, D.E., and Campbell, B., Development and Use of the Georgia Loaded Wheel Tester, Transportation Research Board, Paper No , January , Lai, J.S., Lee, T.M., Use of a Loaded Wheel Testing Machine to Evaluate Rutting of Asphalt Mixes, Transportation Research Record 1209, p , West, R.C., Page, G.C., and Murphy, K.H., Evaluation of the Loaded Wheel Tester, Research Report, FLiDOT/SMO/91-391, Florida DOT, December Izzo, R.P., and Stuart, K.D., Correlation of SUPERPAVE G*/Sinö with Rutting Susceptibility from Laboratory Mixture Tests, Transportation Research Board, Paper No , January Miller, T., Ksaibari, K., and Farrar, M., Utilizing Georgia Loaded Wheel Tester to Predict Rutting, Transportation Research Board, Paper No , January Lai, J.S., Development of a Laboratory Rutting Resistance Testing Method for Asphalt Mixes, Georgia DOT Project 8717, Final Report, August Lai, J. S., Results of Round Robin Test Program to Evaluate Rutting of Asphalt Mixes Using Loaded Wheel Tester, Transportation Reserach Record, 1417, P ,