| Author: | Cheng, Huailei |
| Title: | Determination of the stiffness moduli and fatigue endurance limits of asphalt pavements for perpetual pavement design |
| Advisors: | Wang, Yuhong (CEE) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Pavements, Asphalt concrete Asphalt concrete Pavements -- Design and construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 351 pages : color illustrations |
| Language: | English |
| Abstract: | Perpetual pavement offers a lifespan of more than 40~50 years before structural failure occurs. Perpetual pavement has the advantages of lowering the life-cycle cost, reducing the consumption of non-renewable pavement materials, and reducing traffic delays caused by reconstruction activities. In designing perpetual pavement, the stiffness modulus and fatigue endurance limit (FEL) of the asphalt mixture layer are two key parameters. The stiffness modulus parameter is conventionally determined by the dynamic modulus test using a uniaxial compressive loading mode, or the resilient modulus test using an indirect tensile loading mode. The asphalt mixture's modulus is dependent on the stress states it undergoes (tensile or compressive). The above laboratory tests, however, fail to simulate the complicated tensile-compressive loading modes within the asphalt mixture layers in field pavements, thus are difficult to accurately characterize the real modulus properties of the field asphalt layers. The FEL parameter is also obtained from the laboratory fatigue test that uses sinusoidal or haversine loading waves. However, field measurements reveal that the stress/strain waveforms within asphalt mixture layers induced by various axle configurations (especially tandem axle and tridem axle) are apparently different from the sinusoidal/haversine wave. Loading waveforms affect the fatigue behaviors of the asphalt mixture. Therefore, the FEL of the mixture obtained under sinusoidal/haversine waves may not reflect the real one under field loading waveforms. A valid and reliable design for perpetual pavement requires an accurate assessment of the asphalt mixture layer's modulus and FEL. This research aims to evaluate the modulus and FEL of the asphalt mixture layer based on field loading mode. The findings are expected to provide valuable references for perpetual pavement design. To achieve this goal, four research tasks have been conducted and the results are presented in this dissertation, shown as follows: 1. Development of the modulus master curve of the asphalt mixture layer under field loading mode. A comprehensive method is established to determine the modulus master curve of the asphalt layer under field traffic loading mode. Firstly, a full-scale experimental pavement is instrumented with strain gauges to measure strain responses of the asphalt mixture layer under different vehicular loading conditions. Secondly, the loading frequencies of the asphalt layer under vehicular loads are determined based on the strain pulse durations. Thirdly, the measured strain data (except for one set of data reserved for validation purposes) are used in a finite element (FE) model to back-calculate the asphalt layer's moduli, which are further used with the calculated loading frequency data to develop the modulus master curve of the field asphalt layer. Finally, the accuracy of the developed master curve is further verified by using the strain data reserved for validation. Based on the developed method, the modulus master curves of the asphalt layers in two experimental pavements are established under field vehicular loading mode. The obtained master curves are confirmed to be practical via the validation data and reflect the actual modulus properties of asphalt mixtures that constructed in field pavement. These master curves and corresponding temperature shift factors can be utilized to calculate the asphalt layer moduli at any frequency and temperature, providing reliable inputs for perpetual pavement design. 2. Identification of bimodular properties of asphalt mixture under laboratory loading mode. The moduli of asphalt mixture under three different laboratory loading modes are measured and compared, including the uniaxial compression (UC) loading mode, the indirect tension (IDT) loading mode, and the four-point bending (4PB) loading mode. The comparison results indicate that asphalt mixture modulus is greatly affected by the different compressive–tensile loading modes. To quantitively investigate the asphalt mixture's modulus under compressive and tensile modes, the bimodular theory is further applied to analyze the stress states of IDT and 4PB specimens. Based on the bimodular theory, methods are developed to differentiate the tensile and compressive moduli of asphalt mixture specimens in IDT and 4PB tests. Accordingly, the compressive and tensile moduli of the asphalt mixture are determined and compared. It is found that the tensile moduli of asphalt mixture specimens are lower than the compressive ones over the entire frequency domain. The compressive-tensile modulus ratios reach about 2.5 at relatively low to intermediate temperatures (4.4℃ and 21.1℃), and it rises obviously as the temperature elevates to the high-temperature domain. Therefore, in perpetual pavement design, it is necessary to pay more attention to the difference between the compressive and tensile modulus properties of asphalt mixture layers. 3. Relationships between the moduli of asphalt mixture under laboratory and field loading modes. Combining the research findings made from laboratory and field studies as discussed above, the moduli of asphalt mixture under laboratory and field loading conditions are compared in the frequency domain. The modulus master curve constructed in the laboratory 4PB mode is observed to closely match that obtained from the field vehicular loading mode. Thus, the modulus measured in the 4PB test is preferred to characterize the modulus properties of the asphalt layers in perpetual pavement design. By contrast, the laboratory compressive (UC) and IDT moduli overestimate the asphalt layer moduli under vehicular loading mode, and tend to result in a non-conservative pavement design result. The asphalt mixture modulus under tensile mode also deviates from that under vehicular loading. The respective adjustment factors for UC/IDT/Tensile moduli are calculated in this research in order to facilitate the applications of those moduli in field conditions. In addition, the developed modulus master curve under vehicular loading is proven to accurately estimate the asphalt layer modulus under another field loading mode, i.e., the Falling Weight Deflectometer (FWD) mode, with the FWD-induced frequency (i.e., 33.33Hz) as input. As a research outcome, a comprehensive framework is developed to link asphalt layer moduli under three laboratory modes with the moduli determined by the two field load modes. In the framework, a properly determined loading frequency is used as an intermediary parameter for relationship analysis. Based on this parameter, the asphalt layer moduli obtained from any loading mode are well related to the moduli obtained from other loading modes. In addition, the developed framework is proven to be compatible with a mature pavement design procedure—the Mechanistic-Empirical Pavement Design Guide (MEPDG). 4. Assessments of fatigue responses and fatigue endurance limits (FEL) of compacted asphalt mixtures under actual loading waveforms. The actual loading waveforms induced by different axle configurations are simulated in laboratory uniaxial tension-compression (UTC) fatigue tests and four-point bending (4PB) fatigue tests in order to evaluate the fatigue responses and FELs of the compacted asphalt mixture samples under more realistic loading waves. It is observed that the fatigue response data (fatigue lives, phase angles, stiffness modulus attenuation curves, dissipated energy) of the mixture samples are affected by the loading waveforms induced by different axle configurations. Based on the fatigue response data, three approaches are used to derive the fatigue endurance limits (FELs) of asphalt mixtures, including fatigue life extrapolation, SR (stiffness ratio) extrapolation, and dissipated pseudo-strain energy. The mixture samples' FELs are affected by the loading waveforms, test temperatures, rest periods as well as their stress states. Specifically, the FELs under multi-axle loading waveforms are lower than those under the single-axle wave or the currently used haversine wave. Increases in temperature and rest period lead to the rises in FEL values. Moreover, the FELs obtained from UTC tests are generally lower than those obtained from 4PB tests. These findings indicate that it is necessary to differentiate the FELs of asphalt mixture under various axle configurations, temperatures, rest periods and stress states in perpetual pavement design, rather than just using a representative FEL value. The findings of this study are expected to help improve the determinations of two critical factors in perpetual pavement design: the asphalt mixture's modulus and FEL. Moreover, the newly developed theories, methods, and tools in this research are expected to assist researchers and practitioners in analyzing the mechanistic behaviours of asphalt mixtures and asphalt pavements. |
| Rights: | All rights reserved |
| Access: | open access |
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