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  • Development of Laboratory T...
    Jiao, Liya

    01/2023
    Dissertation

    Fatigue cracking is the most common distress in asphalt pavements. Currently, no performance-related laboratory tests exist for fatigue cracking to use in routine asphalt mix design to approve job mix formula (JMF) or quality control and quality assurance (QC/QA) in California. The existing four-point bending (4PB) test was developed to evaluate the fatigue performance of asphalt materials, but it is not necessarily appropriate for use in routine JMF, and it takes too long to complete for QC/QA. The first objective of this doctoral dissertation is to evaluate potential surrogate fatigue performance-related testing methods and identify a test that is simple and easy to perform and also provides a guidance for asphalt mix design on routine projects and for QC/QA on all projects.Potential performance-related tests evaluated in this study included monotonic loading fracture tests: semicircular bend (SCB) test, indirect tensile asphalt cracking test (IDEAL-CT), and repeated loading fatigue testing on fine aggregate matrix (FAM) mixes with linear amplitude sweep (LAS) testing configuration. These tests were conducted on a variety of asphalt materials, and they were assessed based on simplicity, repeatability (variability), and their relationship to flexural stiffness and fatigue life from 4PB tests.Fracture parameters obtained from SCB tests and IDEAL-CT and fatigue parameters from FAM mix fatigue tests were assessed as potential fatigue cracking indicators. Linear regression analysis was performed to correlate these indicators with the initial flexural stiffness and fatigue life from 4PB tests. The regression analysis results demonstrated that the SCB tests and IDEAL-CT provided the similar fracture information. In addition, fracture parameters from SCB tests and IDEAL-CT were found to be well correlated with the initial flexural stiffness from 4PB tests. Meanwhile, the initial flexural stiffness from 4PB tests showed a moderate nonlinear correlation with the fatigue life from 4PB tests.Among all fracture parameters, material strength obtained from IDEAL-CT was found to have low variability, strong correlation with flexural stiffness and a moderate correlation with fatigue life from 4PB tests, therefore, strength was proposed as a surrogate indicator for flexural stiffness and an indication of fatigue performance. The relationship identified in this study between flexural stiffness and flexural fatigue life, and the one between flexural stiffness and material strength from IDEAL-CT were used to develop a preliminary specification for fatigue performance. The strength from IDEAL-CT should meet both upper and lower specification limits to ensure required fatigue performance met for asphalt mixtures. However, since there was no strong relationship found directly between strength from IDEAL-CT and fatigue life from 4PB tests, fracture tests did not provide sufficient information to predict fatigue life performance.The repeated loading FAM mix fatigue test showed promising comparison results with both initial flexural stiffness and fatigue life from 4PB tests. The comparison between master curves of FAM shear stiffness and the ones of full graded hot mix asphalt (HMA) flexural stiffness indicated that FAM mixes were more sensitive to temperature and loading frequency than HMA as expected because of higher binder contents in FAM mixes. Linear correlations with R2 values of 0.63 and 0.59 were found between FAM shear stiffness and HMA flexural stiffness at intermediate frequencies (100 Hz and 10 Hz) at a reference temperature of 20 °C. In addition to the comparison between flexural stiffness of HMA and shear stiffness of FAM mixes, the dynamic compressive stiffness of HMA obtained from the asphalt mixture performance tester (AMPT) was also included to explore the effect of different loading configurations on the relationship between HMA stiffness and FAM mix stiffness. The shear stiffness of FAM and dynamic compressive stiffness of HMA were found to be moderately correlated at frequencies of 1 Hz, 10 Hz, 100 Hz and 1000 Hz. Furthermore, these three different types of stiffness: flexural stiffness of HMA, dynamic compressive modulus of HMA and shear stiffness of FAM mixes also indicated that the addition of rejuvenator to asphalt materials containing up to 50% RAP effectively reduced the stiffnesses almost to the same level of the virgin control mix. Given these findings, an attempt was made to upscale the shear stiffness of FAM mixes to the flexural stiffness and dynamic moduli of HMA with two methods. The comparison between predicted and measured moduli showed that the shear stiffness of FAM mixes provided reasonable estimates of both flexural stiffness and dynamic modulus of HMA at intermediate frequencies (1 to 10 Hz) with the error percentage less than 10%. On the other hand, overprediction was noted from both methods at higher frequencies.The comparison of fatigue performance between HMA and FAM mix was further investigated based on damage curves. The viscoelastic continuum damage (VECD) model, which depicts the reduction of material integrity under repeated loading as a function of damage accumulated in asphalt materials, was used to formulate damage curves based on the FAM LAS testing results and HMA 4PB fatigue testing results. Comparison results demonstrated that similar damage characteristics were observed between HMA and FAM mixes. The FAM mixes also showed lower material integrity at failure compared to the values of HMA mixtures, which indicated that FAM mixes were more damage tolerant than HMA. In addition to the VECD model, the FAM mix fatigue testing results also showed a good fitting result on the damage model implemented in the California Mechanistic Empirical pavement design software (CalME). Similar ranking result among the CalME damage curves of different material types was found between FAM mix and HMA.Based on this study, it seems promising that FAM mix fatigue testing can be developed to supplement/replace 4PB fatigue testing on HMA due to its relatively more economical, faster and simpler procedure than conventional 4PB tests. More importantly, linear regression analyses on the selected fatigue parameters from FAM mix LAS fatigue test results and HMA 4PB fatigue results indicated that there was a strong correlation between the shear strain value at failure of FAM mixes and the strain value corresponding to one million cycles of fatigue life of HMA. The shear strain value at the failure of FAM mixes also showed a low variability with a coefficient of variation (COV) of 11.2%, therefore, the FAM mix LAS fatigue testing with the fatigue parameter of shear strain value at failure was recommended as a promising surrogate test for 4PB tests on HMA.Fatigue performance was then studied in the context of pavement structure, which is the second objective of this dissertation: develop numerical models using finite element method (FEM) with the software ABAQUSTM to estimate the pavement responses under traffic loading and daily thermal variation. Specifically, composite pavements containing of an asphalt concrete (AC) overlay on top of portland cement concrete (PCC) slabs was taken into consideration to investigate both traffic loading-induced and thermal loading-induced fatigue cracking or reflective cracking performance in this study. As this study only focused on the damage and crack initiation stage of reflective cracking, terms of fatigue cracking and reflective cracking were used in an interchangeable manner.FEM was firstly applied to investigate the impacts from the pavement bonding condition between AC overlays and PCC slabs, tire loading location, pavement material properties and joint properties between PCC slabs on the pavement response under traffic loading. The tensile strain value at the bottom of the AC overlay was considered as the primary fatigue damage parameter. A preliminary simulation study showed that the critical strain type that causes damage in the AC layer was dependent on the bonding condition between the AC overlay and the PCC layer. When the AC overlay is fully bonded with the PCC slabs, debonding between the AC and PCC layers will firstly take place due to the separating tension, and the damage is expected to initiate at the bottom of the AC layer above the joint corner between two PCC slabs. When the debonding area forms and starts to expand between the AC and PCC layers, damage in the AC overlay will then be primarily caused by the bending tensile strain at the bottom of the AC overlay.A full factorial with 2,700 simulation cases was then carried out with varying AC thickness, AC stiffness, bonding condition, stiffness of base layers (k-value), load transfer efficiency (LTE) between PCC slabs, and traffic loading value. Due to the different damage mechanisms of fully-bonded pavement and partially-bonded pavement, two separate regression models were established based on the simulation results to predict the maximum principal tensile strain. The comparison between the predicted strain value from these two models and the value obtained from FEM simulations demonstrated the accuracy of the regression models.In addition to traffic induced reflective cracking, the daily temperature variation induced reflective cracking was also investigated. In contrast to extreme cold temperatures which cause one time fracture cracking, moderate temperatures can induce repeated tensile strain and stress in the AC overlay all year around due to daily temperature variation, which is a more common situation in California. To address potential thermal reflective cracking under moderate temperatures, composite pavement structures under only thermal loading were simulated with FEM, and the critical thermal stress and strain values were calculated. Among the selected six climate regions, the yearly temperature parameters (average yearly maximum, average yearly minimum and average seasonal change) and daily