Sung-Hee Kim

VALIDATED MODEL FOR PREDICTING FIELD PERFORMANCE OF AGGREGATE BASE COURSES

The International Center for Aggregates Research Project 502 focused on pavement layers of unbound aggregate proper representation in mechanistic pavement models. The research team developed models for resilient and permanent deformation behavior from the results of triaxial tests conducted at the Texas Transportation Institute and the University of Illinois. The studies indicate that the unbound aggregate base (UAB) material should be modeled as nonlinear and cross-anisotropic to account for stress sensitivity and the significant differences between vertical and horizontal moduli and Poisson’s ratios. Field validation data were collected from a full-scale pavement test study conducted at Georgia Tech. The validation of the anisotropic modeling approach was accomplished by analyzing conventional flexible pavement test sections using the GT-PAVE finite element program to predict responses to load in the UAB layer and comparing these predicted responses to the measured values. Laboratory testing of the aggregate samples was conducted at the University of Illinois, and characterization models were developed for the stress-sensitive, cross-anisotropic aggregate behavior. With nonlinear anisotropic modeling of the UAB, the resilient behavior of pavement test sections was successfully predicted for a number of response variables. In addition, the stress-sensitive, cross-anisotropic representation of the base was shown to greatly reduce the horizontal tension computed in the granular base compared with a linear isotropic representation.

Simple Methods to Estimate Inherent and Stress-Induced Anisotropy of Aggregate Base

Simple methods to estimate cross-anisotropic properties of unbound aggregate assemblies on the basis of aggregate physical properties are presented. A regression model for the cross-anisotropic material properties was developed from a database consisting of aggregates from six sources. Aggregate specimens from each source were tested with the use of different gradations and compaction moisture contents. The results demonstrate that aggregate shape and gradation influence the level of anisotropy, which has a substantial effect on the pavement responses that affect pavement design. The level of anisotropy, defined as the ratio of the horizontal modulus to the vertical modulus, was calculated from the regression model and compared with the results from a micromechanics model. This micromechanics model accounted for the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles on inherent anisotropy. Horizontal-to-vertical modulus ratios ranging fr...

Use of surface free energy properties to predict moisture damage potential of Asphalt concrete mixture in cyclic loading condition

The synergistic effects of microdamage due to repeated loading in Asphalt-Aggregate system at a high temperature (40°C) and moisture damage analysis based on surface free energy theory are presented in this paper. The introduction of moisture in either a liquid or vapor state during cyclic loading may well be more damaging than simply moisture conditioning an asphalt concrete sample prior to testing. This difference may be due to the presence of a dynamic “network” of adhesive fracture, which potentially provides a channel for moisture movement within the sample. The percentage of the surface area of aggregate that has been exposed to water was used as a significant index to quantify the level of adhesive fracture. This index is calculated with the surface free energies of aggregate and asphalt which are measured by two methods, the universal gas adsorption and the Wilhelmy plate, respectively. The relation between the percentage of the surface area of the aggregate exposed to water and the number of cycles of loading assists in quantifying adhesive fracture in the asphalt-aggregate mixture.

Development of Performance Prediction Models in Flexible Pavement Using Regression Analysis Method

The capability to forecast future pavement condition has been questions of common interest for the economic reason for pavement management systems and the need to develop an intelligent prioritization schedule became ever more important for the sake of efficiency. If the pavement performance prediction model can be developed based on the past pavement performance data, the remaining service lives for pavements can be forecasted. It would help to optimize the scheduling of the rehabilitation activit ies and to determine the funding level required to achieve a predetermined level of performance. However, the results of the previous attempts to develop general pavement condition forecasting models have not been satisfied reliable because of the difficulties of collection pavement performance data, complexity of the pavement construction situation and different properties of pavement materials. The Georgia Department of Transportation (GDOT) has used the Pavement Condition Evaluation System (PACES) to evaluate the pavement conditions for the entire highway system in Georgia annually for the past 15 years. In this paper, the as phalt pavement performance prediction models for the state highways and the interstate highways have been developed applying simple and multiple regression analysis methods using the PACES data and PACES rating. The multiple linear regression model is effective to forecast pavement performance when ratings with various AADT. If this pavement performance prediction model using multiple linear regression analysis is implemented into the Pavement Management System, it could play an important role in the decision making process for the asphalt pavement management system.

Estimation of Level of Anisotropy in Unbound Granular Layers Considering Aggregate Physical Properties

A simple procedure is developed to account for the effects of aggregate physical properties (gradation and shape characteristics) in predicting the cross-anisotropic properties of unbound granular layers. Variable confining pressure repeated triaxial tests were performed on six aggregate sources with three different gradations (coarse, intermediate and fine) and three different moisture contents (wet, optimum and dry). The experimental results were analysed to determine the parameters of a cross-anisotropic non-linear elastic model. Image analysis techniques were utilized to measure aggregate shape characteristics (form, angularity and texture). The distributions of particle size and shape characteristics of the six aggregates were fitted using a non-linear regression model. The cross-anisotropic model parameters of unbound granular bases were in turn related to the coefficients of the regression model. The analysis results indicate that aggregate physical properties significantly affect the anisotropic resilient behaviour of unbound granular bases, and this anisotropic resilient behaviour has substantial effect on the critical pavement responses. Thus, it is valuable to approximate the degree of cross-anisotropy in unbound aggregates based on aggregate physical properties in order to adequately analyse and design pavements.

Characterisation of unbound aggregate materials considering physical and morphological properties

Abstract The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.

Testing and modelling of friction characteristics between concrete slab and subbase layers

This paper presents the results of an analysis of the interlayer friction characteristics between a concrete slab and various types of subbases, which are commonly used in concrete pavement construction. The interlayer friction characteristics were examined by conducting push-off tests on concrete slabs placed on top of lean concrete, crushed stone, or asphalt subbase, with and without separation membranes, under a range of saturated conditions. Throughout the tests, the friction resistance force and horizontal displacement, along with different types of subbases, were measured during the first application of a load, steady conditions, and saturated conditions. The measured interlayer friction resistance force and horizontal displacement after applying the first load showed results similar to other studies. To incorporate the test results into an analysis, a simple method was developed to estimate the maximum friction resistance force, initial slip displacement, and friction stiffness based on bilinearisation from the field-measured nonlinear friction resistance force, displacement relationship using the energy method. The finite element programs, ABAQUS and EverFE, were incorporated to analyse a concrete pavement. The interlayer friction behaviour was considered both with nonlinear and bilinear models in finite element programs and the interlayer friction characteristics were modelled with connector elements. The maximum tensile stress and horizontal displacement under temperature loading conditions were calculated using analytical models and the results were compared with field data. The results of the bilinear model based on the energy method matched the field data.

Mesoscale Fracture Analysis of Multiphase Cementitious Composites Using Peridynamics

Concrete is a complex heterogeneous material, and thus, it is important to develop numerical modeling methods to enhance the prediction accuracy of the fracture mechanism. In this study, a two-dimensional mesoscale model is developed using a non-ordinary state-based peridynamic (NOSBPD) method. Fracture in a concrete cube specimen subjected to pure tension is studied. The presence of heterogeneous materials consisting of coarse aggregates, interfacial transition zones, air voids and cementitious matrix is characterized as particle points in a two-dimensional mesoscale model. Coarse aggregates and voids are generated using uniform probability distributions, while a statistical study is provided to comprise the effect of random distributions of constituent materials. In obtaining the steady-state response, an incremental and iterative solver is adopted for the dynamic relaxation method. Load-displacement curves and damage patterns are compared with available experimental and finite element analysis (FEA) results. Although the proposed model uses much simpler material damage models and discretization schemes, the load-displacement curves show no difference from the FEA results. Furthermore, no mesh refinement is necessary, as fracture is inherently characterized by bond breakages. Finally, a sensitivity study is conducted to understand the effect of aggregate volume fraction and porosity on the load capacity of the proposed mesoscale model.

Influence of Box Culverts on Behavior of Jointed Concrete Pavements

Structures underneath pavements commonly reduce the bearing capacity of the ground because of poor compaction and long-term settlement at sites around these structures. When a load is applied at these sites, the stress on the concrete slab increases and may exceed the design strength, inducing distress and shortening the lifespan of the pavement. In this study, we investigated the transverse cracking of concrete pavement slabs above box culverts in a test road of the Korea Expressway Corporation. Transverse cracking in the slabs was surveyed in both the north- and southbound lanes with different cover depths over the box culverts. To examine the transverse cracking, the jointed concrete pavement of the test road containing the culvert was modeled and analyzed by the finite-element method. Wheel loading was applied after taking into account the self-weight of the pavement and the temperature gradient within the concrete slab. The positions and magnitudes of the maximum tensile stress and the corresponding positions of wheel loading were investigated for each loading combination. In this manner, we identified the optimal joint positions for which the maximum tensile stress in the pavement slab is minimized. We also analyzed the behavior of the pavement in relation to the cover depth and reinforced concrete transition slab length. Accordingly, we proposed an appropriate length for the reinforced concrete transition slab to ensure minimized tensile stress in the pavement slab.

Investigating the performance of as-built and overlaid pavements: a competing risks approach

Overlay has been a commonly used rehabilitation measure for flexible pavements. Properly designed and constructed overlays restore pavement service conditions and economically extended pavement service lives. In the light of different failure modes that could be experienced by a flexible pavement, such as cracking, rutting and roughness, and complex interactions among them, this paper considers these failure modes as competing risks and evaluates performance differences between overlaid pavements and as-built pavements in terms of hazard function, cause-specific cumulative incidence function and cause-specific conditional cumulative incidence function. A case study was undertaken using the pavement condition survey data in Florida. It was found that the overlaid pavements generally have a higher failure risk compared with the originally built pavements across all failure modes and exhibit three distinct phases of failure hazard change: increasing, stabilising and decreasing. For as-built pavements, the failure hazard increases dramatically after exceeding the design lives of pavements.