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The MEPDG (ARA, Inc., "NCHRP 1-37A Final Report: Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures," NCHRP Program 1-37A Project, National Research Council, Washington, D.C., 2004) introduces the dynamic modulus as the material property to characterize asphalt concrete. One of the challenges of acquiring the dynamic modulus from existing pavements is the standard dimensions of the test specimen. The specimen size specified in AASHTO TP62-07 (2007, "Standard Method of Test for Determining Dynamic Modulus of Hot-Mix Asphalt Concrete Mixtures," AASHTO, Washington, D.C.) cannot be obtained from many pavement layers. This study evaluates two other geometries, indirect tension specimens and prismatic specimens, to determine whether the measured dynamic modulus is the same as the modulus obtained from TP62 protocol. This study provides a comparison of the effects of a non-uniform state of stress and anisotropy. These effects are isolated by comparing specimens prepared by Superpave gyratory compaction and vibratory steel-wheel compaction. The comparisons are verified using four 12.5-mm surface course mixtures with different aggregate shapes and binder types, and one 25.0-mm base mixture. The results show that the difference between the dynamic modulus values obtained from different geometries is statistically insignificant. The results provide justification for using alternative methods for acquiring the dynamic modulus experimentally?specifically, for previously constructed pavements.
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This paper presents the dynamic modulus of asphalt mixtures with granite aggregate which are highly common in Korea. Dynamic modulus was determined by the simplified master curve using test data covering a large range of temperatures from ?10°C to 55°C. Four different asphalt mixtures were evaluated in this paper. Four specimens were chosen to evaluate mixtures with two different aggregates (13 mm, 19 mm) except for two different asphalt binders(PG 58-22, PG 64-16). In addition, the mixture was controlled air void (2,4, 6%) and asphalt content based on optimum asphalt binder by a Superpavegyratory compactor. It adopts sigmoidal function and compressive dynamic modulus test data obtained at a matrix combination of different frequencies and test temperatures. The experimental dynamic modulus values were compared against modulus values obtained from the predictive equations proposed by NCHRP 1-37AMEPDG.
This thesis presents results from an experimental study on the dynamic modulus testing of hot mix asphalts (HMAs) commonly used in North Carolina in uniaxial compression mode. Forty two mixtures with varying aggregate sources, aggregate gradations, asphalt sources, asphalt grades, and asphalt contents are included in this study. With the dynamic modulus database developed, several issues are investigated in this research. Effects of confining pressure on the dynamic modulus are evaluated by comparing results from uniaxial and triaxial compression tests. A modified dynamic modulus test protocol is developed by reducing the required testing time using more frequencies and fewer temperatures based on the time-temperature superposition principle. Hirsch and Witczak predictive models are evaluated. During this analysis a case study was conducted to determine how much pavement performance changes due to the predictive errors. Finally, effects of different mixture variables on dynamic modulus of asphalt concrete are evaluated.
The impact resonance (IR) test is a nondestructive test method that is used to characterize the linear viscoelastic behavior of asphalt concrete. This method is preferred over other methods because the setup of the IR test is simpler, more efficient, and less expensive than standard axial compression dynamic modulus (|E*|) tests. Researchers originally developed the IR test method for cylindrical specimens of asphalt mixtures and concluded that this method can serve as an alternative to |E*| tests. However, the geometry (100 mm in diameter by 150 mm in height) of the cylindrical specimens used in these tests prohibits the use of IR tests for field cores. Therefore, researchers began to consider thin disk-shaped specimens for IR testing because thinner geometry of such specimens better represents slices of field cores. In this study, a test procedure was developed to evaluate the use of thin disk-shaped specimens for IR tests in order to determine the |E*| values of asphalt mixtures. The IR test protocol was optimized using 2 IR test methods (referred to as Case 1 and Case 2 in this work) under various test conditions to ensure the highest possible quality of the data. Optimal test methods were proposed based on the repeatability and variability of the resonant frequency and phase angle data and the ability of the different test conditions to provide data that best match the |E*| values obtained from standard axial compression |E*| tests. The results demonstrate that the |E*| values of thin disk-shaped specimens determined from the optimized IR tests are similar to the |E*| values of long cylindrical specimens determined from conventional |E*| AASHTO T 342-11 tests and IR tests.
An accurate measurement or estimation of dynamic modulus (|E*|) of a hot mix asphalt (HMA) mix is important to understand stress-strain behavior of flexible pavements under loading and unloading conditions. The Mechanistic Empirical Pavement Design Guide (MEPDG) (Transportation Research Board, Washington, D.C., 2004) recommends that |E*| be used in all three levels of design (i.e., Level 1, Level 2, and Level 3). For Level 1, |E*| is measured in the laboratory, while it is estimated using the Witczak models for Level 2 and Level 3 designs. The measurement of |E*| in the laboratory is not always feasible because it requires costly equipment and skilled personnel. Consequently, use of empirical models seems to be a reasonable approach to estimate |E*|. Several researchers have reported that the accuracy of the Witczak models varies with local materials and volumetric properties. The present study was undertaken to compare the measured and the estimated |E*| for some commonly used mixes in Oklahoma. Specifically, |E*| of five different HMA mixes, comprised of aggregates from several sources and sizes, binder grades, and air voids, were measured in the laboratory. The Witczak 1999 model (Andrei , 1999) was used to estimate |E*| for each of these mixes. A comparison was made between the measured and the estimated |E*| at four different levels of air voids, namely, 6%, 8%, 10%, and 12%. It was observed that the Witczak 1999 model overestimates |E*| at all four levels of air voids. To address these overestimates, the Witczak 1999 model was calibrated. The calibrated model was similar in form to the Witczak 1999 model but having different numerical coefficients. Verification of this model was done using a mix that was not used in the calibration process. Furthermore, two full depth field cores were obtained to further verify the accuracy of the calibrated model. Three different criteria, namely, goodness-of-fit statistics, matching the measured and the estimated |E*|, and average relative error (%), revealed that the calibrated model exhibits much better performance compared to the Witczak 1999 model. It is expected that the calibrated model would be useful in estimating |E*| for the Level 2 and Level 3 designs for the implementation of the MEPDG in Oklahoma.
The dynamic modulus test is widely accepted by pavement agencies as the critical parameter for the recently proposed mechanistic empirical design procedure and the candidate of the simple performance test to accompany the Superpave volumetric mix design process. However, the specified dynamic modulus test procedure is time-consuming and costly. State pavement agencies are seeking a more practical test protocol. This paper presents a method for identifying a practical dynamic modulus testing procedure. The currently well-adopted method of calculating the dynamic method is discussed and compared to the more fundamental dynamic modulus calculation method by using actual experimental data from two different asphalt mixtures. It was found that the NCHRP report proposed method produces higher modulus values, but the difference is less than 10 %, as indicates that the simple peak to peak method can be used in the calculation without compromising accuracy. A comprehensive dynamic modulus test, which incorporates strain level, test temperature, and frequency, was performed on one asphalt mixture. Experimental data were analyzed with t-test at the 95 % level of confidence. The analysis results show no statistical difference between the dynamic modulus for the two studied strain levels and no permanent damage was found on tested specimens for all three test temperatures. Comparison of the master curves built by different temperature and frequency combinations illustrates redundancy in test temperature and frequency. A more practical dynamic modulus test procedure is proposed based upon the evaluation. This research shows that three test temperatures, 4.4°C, 21.1°C, and 37.8°C, and six frequencies, 25, 10, 5, 1, 0.5 and 0.1 Hz, plus one additional frequency of 0.01 Hz at 37.8°C are adequate to build a smooth master curve to satisfactorily characterize asphalt mixtures.
This research study aimed to determine the dynamic modulus, bending stiffness and fatigue properties of four representative Superpave Hot Mix Asphalt (HMA) mixtures used in the construction of base layers of Kansas flexible pavements and to compare the measured values with those predicted by the National Cooperative Highway Research Program (NCHRP) Design Guide. To achieve these objectives, asphalt concrete beams were tested in third point-bending at constant strain, at four temperatures and four levels of strain. Dynamic resilient modulus tests were performed on asphalt cylindrical specimens at five temperatures and five loading frequencies. Multi-linear regression analysis was performed to develop a linear relationship between the bending stiffness and the fatigue life for the asphalt mixes tested.