Youngsoo Richard Kim

Toward a micromechanics-based procedure to characterize fatigue performance of asphalt concrete

A lattice-based micromechanics approach is proposed to characterize the cracking performance of asphalt concrete. A random truss lattice model was introduced and investigated for simulating the following: (a) linear elastic and viscoelastic deformation of homogeneous materials in axial compression and shear loading experiments, (b) linear elastic deformation and the stress field in heterogeneous materials in an axial compression loading experiment, and (c) damage evolution in elastic solids under an indirect tensile test. The simulation results match well with the theoretical solutions and show excellent promise in predicting cracking patterns in the indirect tensile test. A brief discussion about ongoing work is also presented.

Local Calibration of Mechanistic–Empirical Pavement Design Guide for Flexible Pavement Design

This paper presents the calibration of the Mechanistic-Empirical Pavement Design Guide (MEPDG) for flexible pavements located in North Carolina. Two distress models, permanent deformation and bottom-up fatigue cracking, were used for this effort. A total of 53 pavement sections were selected from the Long-Term Pavement Performance (LTPP) program and the North Carolina Department of Transportation databases for the calibration and validation process. The verification runs for the LTPP sections performed with the parameters developed during the national calibration effort under NCHRP Project 1-37A showed promising results. The Microsoft Excel Solver program was used to fit the predicted rut depth values to the measured values by changing the coefficients in the permanent deformation models for hot-mix asphalt (HMA) and unbound materials. In this process, the sum of the squared errors was minimized for each of the permanent deformation models separately. For the alligator cracking model, the only possibility of reducing the standard error and bias was through the transfer function. Again, the Microsoft Excel Solver program was used to minimize the sum of the squared errors of the measured and the predicted cracking by varying the C1 and C2 parameters of the transfer function. The standard error for the HMA permanent deformation model, as well as that for the alligator cracking model, was found to be significantly less than the global standard error after the calibration. It was decided that both models would be kept for a more robust calibration in the future that would increase the number of sections and include more detailed inputs (mostly Level 1 inputs).

Determination of Asphalt Concrete Complex Modulus with Impact Resonance Test

A methodology is presented to determine the dynamic modulus and phase angle of hot-mix asphalt (HMA) with much simpler, nondestructive impact resonance (IR) test. Thirty-five HMA mixtures with varying aggregate and binder characteristics were tested with the IR and conventional dynamic modulus tests. A comparison of the dynamic modulus and phase angle obtained from the IR tests and those measured from the conventional dynamic modulus tests demonstrates the reliability of the proposed procedure. However, it was found that the dynamic moduli from the IR tests at multiple temperatures constitute only the upper half of the master curve (i.e., the high frequencies). A combination of the IR test and the dynamic modulus test at 54.4°C or the IR test and Witczaks predictive equation at 54.4°C was found to provide accurate dynamic modulus master curves. The main advantage of these combinations of testing methods over conventional dynamic modulus testing is that they are much simpler and require less expensive tes...

Predicting the Resilient Modulus of Asphalt Concrete from the Dynamic Modulus

The NCHRP 1-37A Guide for Mechanistic-Empirical Design of New and Rehabilitated Design Structures introduces the dynamic modulus as the material property to characterize asphalt concrete. This is a significant change from the resilient modulus used in the previous AASHTO pavement design guide. This paper presents an analytical method of calculating the resilient modulus from the dynamic modulus. It involves the application of multiaxial linear viscoelastic theory to linear elastic solutions for the indirect tension test developed by Hondros. The prediction method is verified by using three 12.5-mm surface course mixtures with different aggregate shapes and binder types and one 25.0-mm base mixture. Results show that the predicted and measured resilient modulus values are in close agreement. The results provide a forward model for the potential back-calculation of the dynamic modulus from resilient modulus databases already available in highway agencies, such as the Long-Term Pavement Performance Materials Database.

Performance Evaluation of Bituminous Surface Treatment Using Third-Scale Model Mobile Loading Simulator

This paper presents a new test protocol for the performance evaluation of bituminous surface treatments (BSTs) using the third-scale Model Mobile Loading Simulator (MMLS3). The BST performance characteristics covered by this protocol include aggregate retention, bleeding, skid resistance, aggregate embedment depth, cracking, and rutting. In this study, the new MMLS3 BST performance test method was applied to evaluate effects of various mix parameters on aggregate retention and bleeding; these parameters include aggregate and emulsion application rates, fines content, and aggregate gradation. Results demonstrate that the BST performance test method developed in this study is an excellent means of supporting current BST design and evaluating effects of various factors on performance of BSTs.

Designing real-time H.264 decoders with dataflow architectures

High performance microprocessors are designed with general-purpose applications in mind. When it comes to embedded applications, these architectures typically perform control-intensive tasks in a System-on-Chip (SoC) design. But they are significantly inefficient for data-intensive tasks such as video encoding/decoding. Although configurable processors fill this gap by complementing the existing functional units with instruction extensions, their performance lags behind the needs of real-time embedded tasks. In this paper, we evaluate the performance potential of a dataflow processor for H.264 video decoding. We first profile the H.264 application to capture the amount of data traffic among modules. We use this information to guide the placement of H.264 modules in the WaveScalar dataflow architecture. A simulated annealing based placement algorithm produces the final placement aiming to optimize the communication costs between the modules in the dataflow architecture. In addition to outperforming contemporary embedded and customized processors, our simulated annealing guided design shows a speedup of 13% in execution time over the original WaveScalar architecture. With our dataflow design methodology, emerging embedded applications requiring several GOPS to meet real-time constraints can be drafted within a reasonable amount of design time.

Backcalculation of Dynamic Modulus from Resilient Modulus of Asphalt Concrete with an Artificial Neural Network

The NCHRP Project 1-37A Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures introduces the dynamic modulus (|E*|) as the material property for the characterization of hot-mix asphalt mixtures. This is a significant change from the resilient modulus used in the previous AASHTO Guide for the Design of Pavement Structures. One of the challenges of changing the material characterization is that databases, such as the Long-Term Pavement Performance Materials Database, contain older material characterization information. Thus, such databases must convert their data to the currently accepted standard (i.e., |E*|). Other investigators have presented evidence that the resilient modulus can be predicted from the dynamic modulus by using the theory of viscoelasticity. By using their prediction method, this study proposes the population of a database of measured dynamic moduli with the corresponding predicted resilient moduli to train an artificial neural network (ANN). The ANN model was verified with four 12.5-mm surface course mixtures with different aggregate types and binder types and one 25.0-mm base mixture. The dynamic moduli predicted from the measured resilient moduli with the trained ANN were found to be reasonable compared with the measured dynamic moduli.

TEMPERATURE CORRECTION OF BACKCALCULATED MODULI AND DEFLECTIONS USING LINEAR VISCOELASTICITY AND TIME-TEMPERATURE SUPERPOSITION

New analytical procedures for temperature correction of backcalculated asphalt concrete moduli and surface deflections were developed based on the theory of linear viscoelasticity and the time-temperature superposition principle and verified using falling weight deflectometer data and field temperature measurements. The new correction procedures explicitly utilize the thermorheological properties of the asphalt mixture. The resulting temperature-modulus correction factors depend only on the relaxation modulus and time-temperature shift factor of the mixture. The temperature-deflection correction factor depends on both the material properties and the layer thicknesses of the pavement section. Emphasis has been placed on the analytical description of the mixture’s thermoviscoelasticity responsible for temperature effects on mixture modulus and pavement deflection. A mechanistic framework for dealing with temperature correction problems for asphalt pavement has been introduced.

Determination of the Appropriate Representative Elastic Modulus for Asphalt Concrete

The National Cooperative Highway Research Program (NCHRP) 1-37A mechanistic-empirical pavement design guide (MEPDG) utilises the dynamic modulus of asphalt concrete in a multilayered elastic analysis to determine the primary responses in asphalt pavements. The dynamic modulus depends on temperature and loading frequency. In the MEPDG, the pulse time is used to determine the loading frequency. This methodology has been under scrutiny due to the large modulus it estimates. This paper evaluates several approximation methods, including the current MEPDG analysis, for the calculation of stresses and strains in linear viscoelastic materials by comparing analysis results with the solutions from the exact linear viscoelastic analysis. Sources of the errors that result from various approximation methods are discussed. Also, an alternative method to that of the MEPDG is proposed for determining the appropriate representative elastic modulus of asphalt concrete.

Understanding the Effects of Aggregate and Emulsion Application Rates on Performance of Asphalt Surface Treatments

Presented are the effects of aggregate and emulsion application rates on the performance of asphalt surface treatments (ASTs), as evaluated by the third-scale Model Mobile Loading Simulator (MMLS3) and digital image processing. Performance characteristics evaluated in this study include aggregate loss and bleeding. Lightweight aggregate and granite aggregate with different gradations were used with CRS-2 emulsion to design the various combinations of aggregate and emulsion application rates. The MMLS3 test protocol was applied to these AST specimens, and performance measurements were analyzed as a function of the application rates. Results demonstrated that the new AST performance test method introduced in this study was an excellent means of evaluating AST performance. It was found that the major factor affecting AST performance was aggregate gradation. Also, a method was developed to determine the optimum aggregate and emulsion application rates from the MMLS3 test results. The optimum rates determined from this method were confirmed by a blind test performed by bituminous supervisors and road maintenance unit engineers at the North Carolina Department of Transportation. Finally, results from this study were used to study the voids concept, which serves as the foundation for AST mix design. It was concluded that the reference voids (the compacted voids resulting from MMLS3 trafficking) depend on the voids in the loose aggregate that are, in turn, affected by the aggregate gradation type.