Sungho Mun

Determination of Time-Domain Viscoelastic Functions Using Optimized Interconversion Techniques

ABSTRACT Several viscoelastic response functions are available to characterize the LVE behavior of asphalt concrete, some in time domain such as relaxation modulus E(t) and creep compliance D(t) and other such as complex modulus E* in frequency domain. The use of the complex modulus test has risen sharply after it has been incorporated in the M-E Pavement Design Guide and in the Superpave Simple Performance Test. With the availability of E* data it becomes advantageous to use mathematical interconversion techniques to obtain time-domain functions E(t) and D(t) which are typically used for constitutive modeling and other applications. This paper addresses the steps involved in conducting the interconversion between frequency-domain and time-domain functions. Issues considered include: a) presmoothing of raw data, b) refinement of phase angle data, c) Prony series representation of the fitted data including determination and sign-control of the Prony series coefficients, and d) interconversion techniques: approximate vs. exact. Finally, interconversion methods are evaluated by comparing D(t) data converted from E* to that measured in the lab.

Application of Generalized J-Integral to Crack Propagation Modeling of Asphalt Concrete Under Repeated Loading

An energy-based crack-growth model was developed in this study to simulate the propagation of top-down cracking in asphalt pavements. A viscoelastic fracture mechanics approach, the generalized J-integral, was employed to model the crack growth of asphalt concrete. Laboratory fatigue crack propagation tests for three asphalt mixtures were performed at various load levels, frequencies, and temperatures. Disk-shaped specimens with a proper loading fixture and crack growth monitoring system were selected for the tests. It was observed from the tests that the crack-propagation model based on the generalized J-integral was independent of load levels and frequencies, while the traditional Paris law model based on stress intensity factor was dependent on loading frequencies. However, both models were unable to deal with the temperature dependence of the mixtures. The fatigue crack propagation model proposed in this study had a good agreement between experimental and predicted crack-growth lives, an indicator that the energy-based J-integral could be a better parameter to describe fatigue crack propagation of viscoelastic materials such as asphalt mixtures.

Modified harmony search optimization for constrained design problems

Constrained optimization is a major real-world problem. Constrained optimization problems consist of an objective function subjected to both linear and nonlinear constraints. Here a constraint handling procedure based on the fitness priority-based ranking method (FPBRM) is proposed. It is embedded into a harmony search (HS) algorithm that allows it to satisfy constraints. The HS algorithm is conceptualized using the musical process of searching for a perfect state of harmony. Here, the original heuristic HS was improved by combining both improved and global-best methods along with the FPBRM. The resulting modified harmony search (MHS) was then compared with the original HS technique and other optimization methods for several test problems.

Fatigue Cracking Mechanisms in Asphalt Pavements with Viscoelastic Continuum Damage Finite-Element Program

A study of fatigue-cracking mechanisms in asphalt pavements used the finite-element program VECD-FEP++. This program employs the viscoelastic continuum damage model for the asphalt layer and a nonlinear elastic model for unbound layers. Both bottom-up and top-down cracks are investigated by taking several important variables, such as asphalt layer thickness, layer stiffness, pressure distribution under loading, and load level applied on the pavement surface, into account. The cracking mechanisms in various pavement structures under different loading conditions are studied by monitoring a damage contour. Preferred conditions for top-down cracking were identified with the results from this parametric study. The conjoined damage contours in thicker pavements suggest that a through-the-thickness crack may develop as the bottom-up and top-down cracks propagate simultaneously and coalesce; that idea supports observations from field cores and raises the question of the validity of traditional fatigue performance models that account for the growth of the bottom-up cracking only.

Identification of Viscoelastic Functions for Hot-Mix Asphalt Mixtures Using a Modified Harmony Search Algorithm

This study proposes a modified harmony search (MHS) algorithm for determining the time-domain viscoelastic function of hot-mix asphalt (HMA) concrete materials. This MHS technique, employing a global optimization technique as well as a Wiechert model for the relaxation function, substantially enhances accuracy and consistency in the determination of viscoelastic functions of several HMA mixtures. In addition, this study shows how to determine a time-domain Prony series representation from the complex modulus in the frequency domain using the MHS algorithm. This can be efficiently used for numerical analysis with techniques such as the finite-element method. The results from lab frequency sweep tests of unmodified and lime-modified HMA at various asphalt contents were consistent with the functions obtained from the MHS algorithm.

Short communication: Improving a model for the dynamic modulus of asphalt using the modified harmony search algorithm

We report the improvement of a dynamic modulus model using a modified harmony search (MHS) algorithm to describe the resistance to rutting and fatigue cracking of asphalt concrete mixtures. The MHS algorithm was reformulated to improve the harmony search (HS) algorithm by introducing minimum and maximum bandwidths. Using the MHS algorithm, model parameters for lime-modified asphalt concrete mixtures were extracted and a good fit to the dynamic modulus data obtained from laboratory tests was achieved.

Continuum Damage Finite Element Modeling of Asphalt Concrete

This paper presents the research into the implementation of the viscoelastic continuum damage model (VCDM) of asphalt concrete into a finite element analysis. The VCDM is based on: (1) the elastic-viscoelastic correspondence principle using pseudostrains; (2) the work potential theory for damage modeling; and (3) the time-temperature superposition principle with growing damage. With the aid of Schaperys preceding work, the VCDM is implemented in a three-dimensional setting. The resulting material model was implemented in the commercially available finite element program, ABAQUS. Results from constant crosshead rate monotonic tension tests at varying temperatures and strain rates were used to verify the finite element model. Verification results are promising when the viscoelastic response dominates the behavior. However, at high temperatures and/or slow strain rates, the effect of the viscoplastic response in the mixture was found to be important and warrants the incorporation of the viscoplastic model for an accurate response prediction.

Study on the Functional Evaluation of Permeable Asphalt Concrete Pavement in Seoul City

The functional evaluation of constructed permeable pavements was conducted in terms of water permeable performance and noise reduction measurements in Seoul city. The field measurements of noise was based on two methods such as pass-by and novel close proximity(NCPX). The pass-by test and NCPX method are related to noise propagation and tire/pavement interaction noise measurement, respectively. For the water permeable tests, five sections were chosen; furthermore, the measurements were conducted for both of wheel path and non-wheel path area. For the pass-by measurement, three sections were chosen; furthermore, two different locations, which were near measurement point to traffic noise and far measurement point inside park or hosing complex, were selected for each section. Finally, tire/pavement interaction noise measurements were carried out at four locations. The results show that the functional performance of water permeability and noise reduction was well remained within 2 or 3 years after permeable pavement construction.

A Comparison Study of Friction Measurements for Chip Seal

This paper presents a development in measuring skid resistance in a laboratory performance test for chip seal. It is important to develop a relationship between the British pendulum test (BPT) and the locked-wheel skid test (LWST), or grip tester (GT). The chip seal is a typical pavement preservation treatment used by the North Carolina Dept. of Transportation (NCDOT). In North America, loss of skid resistance is a common road condition that indicates the need for a chip seal, as one of the major advantages of chip seal is an increase in skid resistance. Most agencies have a specified cycle in which skid resistance is measured as a part of their pavement-management system. These skid resistance measurements are invaluable when deciding which roads require chip seal [Gransberg, D. D. and James, D. M. B., “Chip Seal Best Practices,” NCHRP Synthesis of Highway Practice 342, Transportation Research Board of the National Academies, Washington, D.C., 2005]. In this study, skid resistance was evaluated on 14 selected chip seals using three different tests: the BPT, LWST, and GT. The correlation between British pendulum number (BPN) and skid number (SN) was relatively strong with an R2 value of 0.74. This finding indicated that the BPN measured in the laboratory could be utilized for predicting the SN, which cannot be measured in the laboratory.

Software Development of the Traffic Noise Prediction Based on the Frictional Interaction between Pavement Surface and Tire

Domestic economic development, industrialization, and urbanization have brought along not only increased highway traffic but also elevated traffic noise levels. Thus, it is necessary to accurately predict the traffic noise levels in order to address the public demand of alleviating the noise levels in urban areas. In this study, the method of evaluating the sound power level of road traffic was investigated in terms of considering the types of road surface and vehicle, based on previous researches. Regarding CPX (Close Proximity Test) and Pass-by test, the measured noise data of Test Road of Korea Highway Corporation were utilized in order to construct the database of sound power levels of various vehicles. Specifically, the 38 noise measurement and analysis in 1/1-octave band frequencies at 12 pre-selected sites were carried out, considering topography and road surface. Finally, the comparison study was conducted between predicted and measured data in terms of traffic noise. The traffic noise prediction was based on the KRON (Korea Road Noise) program, which was developed being equipped wit 3-dimensional GUI. In addition, the traffic noise characteristics were evaluated in terms of vehicle types and pavement surface conditions.