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Comprehensive Methodologies for Analysis of Thermal Cracking in Asphalt Concrete Pavements
AuthorAlavi, Seyed M. Z.
AdvisorHajj, Elie Y.
Civil and Environmental Engineering
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The overall objective of this study was to develop a comprehensive model for thermal cracking analysis in asphalt pavements. The model essentially aims to be utilized as a robust tool for selecting thermal cracking resistant mixture(s) based on the predicted performance at the location of interest. In this comprehensive model, several enhancements have been introduced to remedy the recognized limitations in the available thermal cracking models. The proposed model mainly accounts for the changes of asphalt mixture stiffness, strength, and contraction properties with the oxidative aging of asphalt binder over time. Overall, the developed model includes four main components, which are briefly described below:Temperature profile prediction: hourly temperatures at different depths of pavement structure are predicted over time using an enhanced heat transfer model with improved boundary conditions. The numerical calculation with the Finite Control Volume Method (FCVM) in a fully implicit scheme facilitates the consideration of any discontinuity in thermal diffusivity properties of pavement layers as well as significantly optimizes the time of calculation. The required inputs for the model are: (I) hourly climatic and meteorological data (i.e., air temperature, wind speed, and solar radiation) at the pavement location, (II) thickness and thermal diffusivity properties of pavement layers, and (III) monthly variable pavement surface radiation properties (i.e., albedo, emissivity, and absorption coefficient). The Predictions of the model were validated for two Long Term Pavement Performance (LTPP) test sections (i.e., Kingman, Arizona, and Great Falls, Montana).Oxidative aging (carbonyl) prediction: the evolution of carbonyl (CA) with time for the asphalt binder at a specific depth in the pavement, will be predicted using an oxidative aging diffusion-based model, which were developed at the Texas A&M University. The model requires (I) asphalt binder oxidative aging kinetics, (II) asphalt binder hardening parameters, and (III) estimated hourly pavement temperatures, to predict CA over time in the asphalt pavement surface layer. The numerical solution of the model is completed using a fully implicit FCVM. The predicted CA values are used to estimate the evolution of asphalt mixture critical properties (i.e., modulus, contraction, and strength) with time due to aging. Using the model predictions, asphalt mixture laboratory aging temperatures and durations can be suggested to simulate field aging of asphalt materials as occurs over over years of service. Thermal Stress Prediction: the thermal stresses in the asphalt layer are predicted using the 1-D linear viscoelastic constitutive relationship modified to account for the aging of the asphalt material in terms of continuous changes in the relaxation modulus and the coefficient of thermal contraction (CTC) over time. Moreover, the temperature dependency of the asphalt mixture CTC is also taken into consideration. The relaxation modulus is obtained from the continuous relaxation spectrum, which is directly calculated from the dynamic modulus data expressed in complex domain (E^*). Thus, the continuous relaxation spectrum can be defined by few shape function parameters (i.e., complex modulus function coefficients). The spectrum shape parameters for the relaxation modulus can be predicted from the CA level using consistent exponential correlations that were developed for the various evaluated mixtures. The temperature-dependent CTC function is determined from the thermal strain versus temperature measurements under a constant cooling rate. These measurements can be obtained from contraction of an unrestrained specimen during testing by uniaxial thermal stress and strain test (UTSST). UTSST has been developed during this study after enhancing the traditional thermal stress restrained specimen test (TSRST) set up, at the University of Nevada, Reno. The UTSST allows concurrent measurements of thermal stress and thermal strain, respectively, from the restrained and unrestrained asphalt mixture specimens. Mixture specific correlations between the CTC function parameters and the CA level can be used in the model to predict thermal strains and consequently thermal stresses over time. Thermal Cracking Events Prediction: the prediction of the possible cracking events is based on the newly defined crack initiation stress (CIS) limit as obtained from the UTSST results as a function of aging. . By definition, a cracking event is when the predicted hourly thermal stress in the asphalt layer reaches the corresponding CIS value at a given aging level. The predicted thermal stress may be compared with different percentages of CIS in order to account for the thermal fatigue cracking. Thermal fatigue cracking is developed by accumulation of damages caused by thermal stresses over cooling/warming cycles, when thermal stresses do not reach the cracking stress limit of the asphalt mixture in one crirical cooling event.Finally, Thermal Cracking Analysis Package (TCAP), coded in MATLAB, provides a graphical user interface (GUI) that can be used to complete thermal cracking analysis, based on the proposed model. TCAP aims to be used by pavement engineers as a design tool for selecting thermal cracking resistant mixtures based on the predicted performance at the location of interest. Moreover, the output of the software can offer recommendations for appropriate times to apply preventive maintenances on existing pavements. TCAP was used to preliminary analyze thermal cracking resistance of selective asphalt mixtures. The results rationally showed higher potential of thermal cracking events for the mixtures with higher air void contents and the asphalt mixture with the unmodified asphalt binder.Validation of the model output with the observed field performance of asphalt mixtures in thermal cracking is recommended. Additionally, TCAP compoents can be improved in future.