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Keywords: Indirect tensile test, Simple performance test, Asphalt concrete, WesTrack, Viscoelastic.
This dissertation presents the viscoelastic characterization of asphalt concrete in indirect tensile testing and the development of a simple performance test for fatigue cracking. The analytical solutions to calculate creep compliance and center strain from displacements measured on the specimen surface were developed based upon the theory of viscoelasticity. These developments were verified by 3-D finite element viscoelastic analysis and tests. A simple performance test was developed based on these solutions and work potential theory. To evaluate its validity, the indirect tensile tests were performed on WesTrack asphalt mixtures varying aggregate gradations, asphalt contents, and air void contents. Fracture energy obtained from indirect tensile strength testing and creep testing was highly correlated with field performance of these mixtures at WesTrack. A combination of indirect tensile creep and strength testing was proposed as a simple performance test for fatigue cracking. Recommendations for expanding the applicability of the simple performance test developed are provided.
Introduction -- Objectives and summary -- Theoretical background -- Test methods -- Materials and specimen fabrication -- Uniaxial testing -- Determination of viscoelastic properties from IDT test -- Development of a simple performance test and validation -- Conclusions and recommendations -- Implementation and technology transfer plan -- References -- Appendices.
Introduction and Research Approach -- Findings -- Interpretation, Appraisal, and Applications -- Conclusions and Recommendations -- References -- Appendixes.
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In this study the fatigue performance of asphalt mixtures was interpreted based on fracture energy density. Recent studies indicate that the fracture energy from indirect tension tests correlates with the field performance of asphalt concrete. The fracture energy density was obtained from indirect tensile strength tests, and the fatigue performance of asphalt mixtures was evaluated with four-point bending beam fatigue tests implemented at three different strain levels. Two different asphalt mixtures with varying binder contents were tested during this study. Test results showed that the fracture energy density could be an appropriate material property in phenomenological fatigue models. Thus a phenomenological fatigue model based on fracture energy density is presented, and this approach could be advantageous because a simple fatigue model based on fracture energy density does not require time-consuming fatigue tests. For comparison purposes, the various types of fatigue models were evaluated, and fatigue models based on fracture energy density and dissipated energy showed rather high prediction accuracy. In general, fatigue models based on the energy concept have the least dependence on material properties and can predict the fatigue lives of asphalt mixtures without changing coefficients.
Fatigue performance modeling is one the major topics in asphalt concrete modeling work. Currently the only standard fatigue test available for asphalt concrete mixtures is the flexural bending fatigue test, AASHTO T-321. There are several issues associated with flexural fatigue testing, the most important of which are the stress state is not uniform but varies over the depth of the specimen and equipment for fabricating beam specimens is not widely available. Viscoelastic continuum damage (VECD) fatigue testing is a promising alternative to flexural fatigue testing. Different researchers have successfully applied the VECD model to asphalt concrete mixtures using constant crosshead rate direct tension test. However, due to the load level limitation of the new coming Asphalt Mixture Performance Tester (AMPT) testing equipment, there is an immediate need to develop a model that can characterize fatigue performance quickly using cyclic test data. In this study, a simplified viscoelastic continuum damage model developed at NCSU is applied to various North Carolina mixtures, which are used in the NCDOT HWY-2007-7 MEPDG local calibration project. It is shown that the simplified VECD model can predict fatigue tests fairly accurately under various temperature conditions and strain levels. It is also shown that the model can be further utilized to simulate both the strain controlled direct tension fatigue test and the traditional beam fatigue test. In this thesis, simulation results are presented. Conclusions regarding the applicability of the new model are advanced as well as suggestions for further work.
A variety of testing methods are employed by researchers to characterize the fatigue performance of asphalt concrete. These testing methods need to be evaluated based on their performance to characterize the fundamental properties and field performance of mixtures. In this study, the indirect tensile tests and the uniaxial tests were investigated. The indirect tensile creep tests and the uniaxial tensile creep tests were performed on a North Carolina mix to study the characterization of the fundamental properties and on WesTrack mixes to study the characterization of field performance. It was found that that the values of creep compliance from the indirect tensile creep and the unaxial tensile creep tests on North Carolina mix cannot be compared favorably. Fracture energies from the indirect tensile strength tests on WesTrack mixes highly correlate with the field performances, while those from uniaxial direct tension tests did not match field performances. It is believed that anisotropicity could be the cause of differences in performance of the two test methods. It is disconcerting when researchers attempt to establish a constitutive relationship between the property parameters of mixtures and laboratory fatigue life using different testing methods, without knowing if the laboratory fatigue life from these testing methods really reflects field performance. Further research is needed to obtain a better understanding of different testing methods.
The main objective of this study was to develop a new test that would be simple to conduct and analyze for evaluating resistance of asphalt mixtures to fatigue cracking. For this purpose, a piece of equipment known as the Hamburg Wheel Tracking Test (HWTT) was used. HWTT has been widely accepted as a reasonable and reliable test to evaluate the rutting and moisture damage performance of asphalt mixtures but its use for evaluating fatigue resistance of asphalt mixtures is relatively a new concept. Its use as a fatigue test is advantages over current laboratory fatigue tests because of the possibility of considering various underlying support layers when the specimen is subjected to repeated loading. Ability to assess the asphalt mixture fatigue resistance in a layered system under repeated loading and in a laboratory environment provides the opportunity to integrate material design and pavement structural design for optimum performance. In addition, this test is practically feasible as a routine test method in terms of reliability, equipment availability, and data processing efficiency. The tests were conducted on a two-layer structure with the asphalt concrete slab as the top layer and the neoprene or an unbound aggregate base as the underlying layer. Based on the numerical analysis, a width of 6 inches was selected as the optimum width for this study. However, the experiment also included the 4-inch width for comparison. The thickness of the slab was selected at three levels: 1.0, 1.5, and 2.0 inches, with the 1.5-inch thickness being the main thickness applied to most of the slabs. In an asphalt pavement structure, the surface layer is often designed at a thickness of 1.5 to 2.0 inches. These tests were based on strain amplitude growth corresponding to three defined stages during the test (Early, Middle, Late). The impact of several factors on the test results was investigated. Those factors included the slab width and thickness, the underlying support type and condition, temperature of the test, the speed of tracking, and the type of asphalt mixture. The experiment showed that as the slab thickness become smaller, the strain amplitude and its corresponding growth rate become larger, exhibiting a relatively higher rate of fatigue damage. Using the unbound aggregate base proved to be challenging because of difficulty in achieving a smooth, even surface. A smooth surface was required to ensure proper bond of the gauges and collection of the data. Using synthetic rubber neoprene provided a smooth base and delivered a more reliable dataset compared to the aggregate base in this study. As expected, temperature played a significant role in the experiment, as a higher initial strain amplitude was observed at elevated temperatures. A higher strain and increased strain growth rate were observed at lower loading speeds. After determining the appropriate dimensions, temperature, and underlying support, tests were conducted on six different asphalt mixes. Strain development was monitored at the bottom of the slab for each mix. From the data collected, a new fatigue life model was developed to evaluate the fatigue performance of asphalt pavements when tested under the conditions explored in this research. The fatigue life in this model is defined as the number of HWTT load cycles at which the strain amplitude doubles, or equivalently, when the stiffness is reduced to half. This determination is made under the premise that the test operates in stress control mode. It was found that the rate of growth in strain amplitude during the test increased considerably when the binder content in the mix was decreased, validating the importance of ensuring adequate binder content in the asphalt mix. The SMA mixture demonstrated a higher rate of strain amplitude increase compared to the dense-graded mixtures. The mix with 35% RAP content and a PG 64-22 binder delivered lower growth in strain amplitude compared with other mixes. The IDEAL-CT test was also performed to draw comparisons. However, based on the outcomes, it showed no correlation with the results of the newly developed fatigue test. Based on the growth of strain amplitude throughout the test, a fatigue life model was developed. The fatigue life can be determined using this model, based on the number of cycles, with a criterion of either a 50% reduction in stiffness or a doubling of the strain amplitude. Finally, a numerical model was developed, and the observed strain response demonstrated that this model is capable of predicting both the type of response and the magnitude of strain amplitude with relative accuracy.