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4 using a single laboratory that has experience with the test in a shifting of the master curve, but the overall range will not being evaluated, while ASTM D1067 recommends using three change significantly. As shown, the range of dynamic modu- laboratories. Since the data from each laboratory must be lus values can be covered using the following temperature and evaluated separately, the use of multiple laboratories in the frequency combinations: ruggedness testing does not improve the quality of the statis- tical analysis. As stated in ASTM D1067, the primary benefit High modulus, 4C at 10 Hz obtained from the inclusion of multiple laboratories in Intermediate modulus, 20C at 0.1 Hz ruggedness testing is an additional review of the validity of the Low modulus, 40C at 0.01 Hz test method and the need for added clarity in the operating instructions. Two laboratories were included in the rugged- Project 9-19 has suggested that confined tests may be nec- ness experiment. Tests were conducted in AAT's laboratory essary for gap- and open-graded mixtures. It is likely that the using the Interlaken SPT and in the FHWA Mobile Asphalt sensitivity of dynamic modulus measurements to confining Laboratory using the IPC Global SPT. pressure effects will be different for dense compared to gap- and open-graded mixtures. Therefore, two mixtures were used in the ruggedness testing: a 9.5-mm dense-graded mixture 1.3.2 Ruggedness Testing Plan with a PG 64-22 binder, and a 12.5-mm Stone Matrix Asphalt for Dynamic Modulus (SMA) mixture with a PG 76-22 binder. Since, as discussed This section discusses the ruggedness testing plan that was below, one of the factors to be considered in the ruggedness developed for the dynamic modulus test. It discusses the evaluation is air versus water for temperature conditioning, selection of the materials, testing conditions and factors that a moisture sensitive dense-graded mixture was used. Smaller were included evaluation nominal maximum aggregate size mixtures were selected to minimize testing error associated with specimen preparation and thereby accentuate the planned effects. Table 2 presents Materials and Testing Conditions mixture proportions for the mixtures used in the ruggedness Temperature and loading rate are the two factors that most testing. The dense-graded mixture uses a somewhat moisture influence the dynamic modulus of asphalt concrete mixtures. sensitive diabase from Northern Virginia having a typical ten- Figure 1 presents a dynamic modulus master curve generated sile strength ratio of 75 percent in AASHTO T283, Standard using the reduced testing protocol developed in Phase IV Method of Test for Resistance of Compacted Bituminous Mix- of this project (Temperatures of 4, 20, and 40C and loading ture to Moisture Induced Damage. The SMA mixture uses a frequencies of 10, 1, 0.1, and 0.01 Hz). Aggregate type and combination of diabase from Northern Virginia and Lime- gradation, volumetric properties, and binder grade will result stone from West Virginia. Tests were conducted using the 1.0E+07 Selected Testing Conditions 1.0E+06 4C E*, psi 3.0000 20 C 1.0E+05 2.0000 40 C Log Shift Factor 1.0000 Fit 0.0000 -1.0000 1.0E+04 -2.0000 -3.0000 0 10 20 30 40 50 Temperature, C 1.0E+03 1.0E-06 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 1.0E+06 Reduced Frequency Figure 1. Typical dynamic modulus master curve.

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5 Table 2. Composition of the mixtures. sequence of 4, 20, and 40C recommended in the reduced dy- namic modulus testing procedure developed in Phase IV of Property 9.5 mm 12.5 mm Dense SMA this project. However, it is well known that water has better Binder Content, % 5.7 6.5 thermal conductivity than air, and the overall time to com- Sieve Size, mm plete the testing could be substantially reduced if the speci- Gradation, 19 100 100 % passing 12.5 100 97 mens could be equilibrated in water baths set to the testing 9.5 91 81 temperatures. For example, the Marshall stability test, 4.75 68 30 AASHTO T245, Standard Method of Test for Resistance to 2.36 40 19 1.18 31 15 Plastic Flow of Bituminous Mixtures Using Marshall Appara- 0.6 22 13 tus, requires temperature equilibration times of 30 min in 0.3 12 12 water baths and 2 hours in ovens, both set to the specified test 0.15 7 10 0.075 4.8 8.3 temperature. If in the ruggedness testing the dynamic modu- lus is not significantly affected by the use of water as a condi- tioning fluid, it may be possible to complete the testing at all three combinations of temperatures and frequencies listed three temperatures required for master curves in a single day. above. Air versus water as conditioning fluids was, therefore, in- cluded in the ruggedness testing program. Factors and Levels Loading rate. Loading rate has a similar effect as temper- ature on the mechanical properties of asphalt concrete. In fact, The dynamic modulus test includes a number conditions this is the basis of the time-temperature superposition concept that require some level of control in order to minimize test- used in the development of dynamic modulus master curves. ing error. The sections below discuss the selection of factors Although loading rate has a major effect on the mechanical and their levels for the ruggedness testing. properties of asphalt concrete, it will not be included in the Temperature. Temperature is the most important factor ruggedness testing because the load standard error computed affecting the mechanical properties of asphalt concrete mix- by SPT software is very sensitive to variations in the frequency tures and must be carefully controlled to obtain precise test of the applied load. Limiting the load standard error to 10 per- data. For the SPT, the temperature is controlled by first equil- cent or less ensures that the frequency of the applied load will ibrating the specimen in a separate conditioning chamber to be the same as the specified loading frequency. the test temperature. A dummy specimen of the same size Axial strain. Research has shown the dynamic modulus as the test specimen with a thermocouple installed at the to be sensitive to the applied axial strain, particularly at high middle and exposed to the same thermal history as the test temperatures or low frequencies of loading (4). AASHTO specimen is used to determine when temperature equilibrium TP 62 has a very wide tolerance of 50 to 150 strain for the is achieved. Once the specimen is equilibrated at the test axial strain, which may be partially responsible for the poor test temperature, a maximum time limit has been specified to in- precision reported in the recently completed interlaboratory strument the specimen, install it in the test chamber, and have study for the dynamic modulus test (3). In the SPT, a control the test chamber return to the test temperature. The current loop has been specified with a tolerance of 75 to 125 strain, tolerance on the temperature in the equilibration chamber and in the Phase II evaluation the axial strains were controlled is 0.5C from the target temperature. The time limit for within 80 to 110 strain. Axial strain level was a factor in the transfer is 3 min. Both of these were successfully met in the ruggedness testing with the factor levels set at 75 and 125 strain Phase II evaluation testing that resulted in an acceptable as specified in the equipment specifications for the SPT. coefficient of variation of 13 percent. In the ruggedness test- ing, the effect of increasing the equilibration tolerance and Confining pressure. Research has also shown the dynamic the specimen transfer time were evaluated, since less stringent modulus at high temperatures and low frequencies of loading control on these factors may reduce the overall testing time. to be sensitive to confining pressure (4). Neither the Proj- A test temperature tolerance of 1.0C and specimen transfer ect 9-19 test methods (1) nor AASHTO TP 62 address confined times of 3 and 5 min were investigated. dynamic modulus testing. Currently the SPT requires control A related factor that will be investigated in the ruggedness of confining pressure to 2 percent of the specified value. The testing is the fluid for conditioning the test specimens. maximum confining pressure available in the SPT is 210 kPa; Currently air is specified as the fluid in the conditioning therefore, the maximum deviation from the target is 4.2 kPa. chamber, and the specimen equilibration time at each tem- In the Phase II evaluation, this level of control was easily main- perature may be as long as 4 hours for the temperature tained by the two devices. The ruggedness testing included

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6 confined tests with confining pressures of 135 and 140 kPa to friction reducer, greased latex versus TeflonTM was included verify that the current level of confining pressure control is in the ruggedness evaluation to verify that either approach is adequate. Unconfined tests were performed with and without acceptable. the membrane to determine if the level of confinement pro- vided by the membrane is significant. If tests at multiple con- Specimen properties. Air void content and end paral- fining pressures are desired, the procedure will be simplified lelism are two specimen properties that must be controlled. if the membrane can remain in place during the unconfined With available specimen fabrication techniques, an air void testing. tolerance of 0.5 percent of the target is obtainable with care- ful control. It is desirable to increase the air void tolerance End friction reducer. A major assumption in the dy- to minimize the number of specimens rejected. The Hirsch namic modulus test is that the stresses are distributed uni- model, which was developed to estimate the effect of volu- formly over the specimen. Friction between the loading platen metric properties on the dynamic modulus can be used to and the specimen produces shear stresses which result in a assess the effect of air voids on the dynamic modulus (6). Fig- deviation from this assumption. The effects of friction can be ure 2 shows the potential error caused by a 1.0 percent change minimized by using long specimens and making measurements in air voids. As shown the error is dependent on the modulus of near the middle. The test specimen size for the simple perfor- the mixture and varies from about 3 percent for low and high mance tests was determined in an extensive specimen size and modulus values to 9 percent for intermediate modulus values. geometry study conducted in Project 9-19 (5). The specimen This analysis shows that variability in specimen air voids is a sig- diameter of 100 mm was selected to provide flow data that are nificant contributor to the overall test variability and that a high independent of specimen size. The height to diameter ratio of degree of control over air void content is needed. However, the 1.5 was selected to provide dynamic modulus and flow data current tolerance of 0.5 percent is probably the tightest con- that are independent of specimen height. In the Project 9-19 trol obtainable using current specimen fabrication techniques. specimen size and geometry study, an end friction reducing Therefore, air void content was not considered in the rugged- element consisting of two latex sheets separated by silicon ness testing. The current tolerance of 0.5 percent should be grease was used. The reduction of end friction in these tests used until specimen fabrication equipment is improved. was probably a significant factor in the conclusions concern- Like end friction, the degree of parallelism of the specimen ing specimen size. The greased latex sheets are not conducive ends affects the distribution of stresses in the specimen. The to production testing; therefore, in Project 9-29 TeflonTM uniform stress distribution assumed in the analysis of the sheets were used in the evaluation testing. The type of end dynamic modulus data requires smooth, parallel ends. Sawed 10 9 % Error Due to 1% Change in Air Voids 8 7 6 4 % Air Voids 5 7 % Air Voids 4 3 2 1 0 1.0E+04 1.0E+05 1.0E+06 1.0E+07 Dynamic Modulus, psi Figure 2. Estimated testing error for current air void tolerance.