Cesar Tirado

Process to Estimate Permit Costs for Movement of Heavy Trucks on Flexible Pavements

A process based on a mechanistic–empirical (ME) analysis was developed to estimate permit fees on the basis of truck-axle loading and configuration as well as the predicted pavement deterioration that they cause. The process was implemented in a software package, Integrated Pavement Damage Analyzer (IntPave). IntPave is a finite element–based program that calculates pavement responses, uses ME distress models to predict performance under any type of traffic load, is capable of comparing the level of distress caused by a heavy truck relative to a standard truck, and accordingly provides a permit fee. On the basis of a parametric study, it was found that, aside from the truck gross vehicle weight and axle configuration, pavement structure and the damage threshold to rehabilitation also heavily affect the permit fee.

Simulation of lightweight deflectometer measurements considering nonlinear behavior of geomaterials

Lightweight deflectometers (LWDs) are being used more often for modulus-based quality control of earthwork. One of the practical concerns about implementation of LWDs is that the equation used to estimate the LWD modulus is based on elastic half-space theory and does not account for the nonlinear behavior of soil and soil–impact plate interaction. The finite element method can be used to study the effects of nonlinear behaviors of geomaterials and the soil–plate interaction on the measured deflections. This study provides a means for accounting for the impact of these parameters on the measured responses and the depths of influence. A dynamic finite element model that considers the nonlinear behavior of geomaterials was developed to simulate the LWD on a pavement structure. A comprehensive range of single-layer and two-layer systems with a wide range of properties and thicknesses was considered. Transfer functions were developed to adjust the surface deformations and moduli from the responses obtained from a simple layered elastic static model. The study provides practical relationships that minimize the additional effort in implementing time-consuming dynamic finite element methods. The relationships proposed in this study can be used to estimate more representative target LWD modulus values.

Modeling of Slab-Foundation Friction in Jointed Concrete Pavements Under Nonlinear Thermal Gradient or Traffic Loads

The accurate modeling of the thermomechanical response of jointed concrete pavements is of primary importance in the design of pavement sections. From the initial development of pavement analysis software in the early 1970s, it was recognized that the finite element method was the most appropriate modeling tool because of its potential ability to capture all pavement response features. A series of software development efforts have culminated in the production of NYSLAB, an analysis tool for jointed pavement that can predict the complete thermomechanical response, including pavement curling and interactions between slab and foundation. A series of studies were developed in NYSLAB specifically to look into slab–foundation friction generated by nonlinear thermal gradients and traffic loads. Nonlinear temperature gradients can produce slab expansion and contraction that lead to frictional traction between slabs and foundation. The prediction of these friction tractions is complicated by the curling of the slabs that cause some portions of the slabs to lose contact with the foundation. The results of the studies highlight the importance of considering these frictional tractions in the analysis of jointed concrete pavements because they have a significant impact on the bending stresses of portland cement concrete slabs.

Development of NYSLAB

Since the development of ILLI-SLAB software in 1979, researchers have made significant contributions to the capabilities of tools for jointed-pavement analysis. These efforts have culminated in some commercial and noncommercial software packages such as JSLAB and ISLAB2000. Even though JSLAB has undergone several improvements since 1986, the underlying core of the software maintains the characteristics of the initial ILLI-SLAB code. A thorough review of JSLAB2004 revealed that it would be beneficial to redesign the software completely to take advantage of modern computer resources and finite element modeling techniques available today. For this reason, a new analysis tool was developed that significantly improves on the efficiency and capabilities of JSLAB2004. The improvements led to NYSLAB, a tool that (a) has no limitations on the number of jointed slabs and foundation layers analyzed, (b) more accurately models the contact between debonded slabs and between the bottom slab and the top foundation layer, (c) models the foundation layers beyond the edge of the slabs to predict more accurately the edge deflections and stresses, and (d) significantly improves the capabilities for modeling a nonlinear thermal profile on all slab layers to predict more accurately their curling response. A series of parametric studies was conducted to evaluate the performance of NYSLAB and highlight the new and improved capabilities.

Impact of damage on propagation of Lamb waves in plates

A consorted effort is ongoing to utilize spectral analysis of Lamb Waves to rapidly characterize and to detect damage in plates. To optimize test set-up, to understand the limitations of the methodology, and to verify the experimental results, an effort is under way to simulate conditions normally encountered in actual cases. The impact of propagation of Lamb waves in the presence or absence of damage has been simulated using a finite element algorithm, 2D Fourier transform can be used to identify individual modes, and to measure the amplitude and propagation velocity of each mode in a thin plate. Mode conversion due to the presence of a crack can be readily identified with this method. The damage was modeled as notches with different widths and depths. Three different materials were modeled for this purpose: Aluminum 6061-T6, Steel A36, and Graphite- Epoxy; the latter being analyzed in directions parallel nd perpendicular to the fibers. The focus of the work was on low-frequency range where the fundamental symmetric and anti-symmetric modes are dominant. The ratio of notch depth to plate thickness and the absolute notch depth were considered as possible controlling parameters for the sensitivity in the damage detection. The effect of the width of the flaw on the transmissivity of the Lamb waves was also considered and found to be minimal.

Impact of different approaches to modelling rigid pavement base layers on slab curling stresses

The magnitude of stresses induced in jointed concrete slabs due to thermal loads is influenced by the stiffness of the underlying foundation layers (base, sub-base and subgrade). The layer that most significantly affects the Portland cement concrete (PCC) slab responses is the base. Field observations have demonstrated the increase in reflecting cracking of PCC slabs placed over relatively stiff base layers. To predict thermo-mechanical responses of jointed PCC slabs accurately, appropriate idealisation of foundation layers in finite element (FE) analysis is required. Several modelling methods have been proposed to idealise the effect of the base layer. These methods differ in the structural contribution assigned to the base layer in the pavement concrete system. Four approaches for modelling the base layer in FE analysis of jointed concrete pavements are presented in this paper. The first and second approaches involve modelling the base as a plate separate from the other foundation layer(s). In the third and fourth approaches, the base layer is modelled as part of a Winkler or Vlasov foundation, respectively. A series of parametric studies are carried out to evaluate the capability and feasibility of each modelling approach proposed in this study in reflecting the effect of the base course rigidity on the PCC slab responses under thermal loads.

Mechanistic Estimation of Lightweight Deflectometer Target Field Modulus for Construction Quality Control

The lightweight deflectometer (LWD) is an emerging device for evaluating the quality of compacted layers. Establishing the proper field target modulus for the LWD is crucial for judging the quality of the compacted unbound layers. A rigorous numerical approach to determine the project-specific field target values for a LWD device during the pavement design process is presented in this study. The input parameters to the model include the thickness of the layers, as well as the nonlinear stiffness parameters of each layer estimated from the resilient modulus tests. Simple relationships are proposed to estimate the target modulus when the earthwork can be approximated as a single layer. A parametric study was performed to quantify the impact of the input parameters on the estimated target modulus. The applied load and the plate diameter of the LWD along with some of the nonlinear soil parameters significantly affect the target modulus. The processes of the development and validation of these models are presented in this paper. Furthermore, the impact of moisture variation on estimated target LWD modulus is discussed.

Evaluating Influence Depth of Light Weight Deflectometer through Finite Element Modeling

The light-weight deflectometer (LWD), which is the portable version of the common falling weight deflectometer (FWD), has been gaining popularity due to the ease of use and familiarity of pavement community to the deflection-based concepts. LWD imparts a pulse load to the surface of a soil layer through a circular plate to measure the soil surface deflection and to estimate the effective modulus of the underlying system. An extensive finite element modeling was performed to study the mechanism of two common types of LWDs during quality control of unbound pavement materials. The performance of the LWDs was evaluated under various loading conditions, different plate sizes and a diverse range of geomaterial properties. The influence depth was studied using both stress and strain criteria. The nonlinear constitutive model parameters seem to have significant influence on the measured influence depth of both devices. However, different functional properties of each device found to be the source of variations in the results.

Impact of Geospatial Classification Method on Interpretation of Intelligent Compaction Data

Intelligent compaction is an emerging technology in the management of pavement layers, more specifically, of unbound geomaterial layers. Different types of intelligent compaction measurement values (ICMVs) are available on the basis of the configuration of the roller, vibration mechanism, and data collection and reduction algorithms. The spatial distribution of the estimated ICMVs is usually displayed as a color-coded map, with the ICMVs categorized into a number of classes with specific color codes. The number of classes, as well as the values of the breaks between classes, significantly affect the perception of compaction quality during the quality management process. In this study, three sets of ICMV data collected as a part of a field investigation were subjected to geostatistical analyses to evaluate different classification scenarios and their impact on the interpretation of the data. The classification techniques were evaluated on the basis of the information theory concept of minimizing the information loss ratio. The effect of the ICMV distribution on the selection of the classification method was also studied. An optimization technique was developed to find the optimal class breaks that minimize the information loss ratio. The optimization algorithm returned the best results, followed by the natural breaks and quantile methods, which are suited to the skewness of the ICMV distribution. The identification of less-stiff areas by using the methods presented will assist highway agencies to improve process control approaches and further evaluate construction quality criteria. Although the concepts discussed can apply to any compacted geomaterial layer, the conclusions apply to the type of compacted soil in this particular test section.

Explaining Overlay Tester Results with Digital Image Correlation and Finite Element Analysis

The overlay tester (OT) is a relatively new testing procedure that characterizes the fatigue cracking resistance of asphalt mixtures by evaluating the number of cycles to failure of a specimen. The uncertainty of the results has been a concern to some transportation agencies. A careful experimental evaluation of the OT test protocol reveals the complex stress and strain distributions that should be understood before improving the test. Thus, a study was conducted using the digital image correlation (DIC) and finite element analysis (FEA) methods to analyze the OT testing process and results. The DIC technology can be used to visualize the loading responses of the asphalt specimen and correlate them with the experimental results. The FEA, in addition to predicting the behavior of the hot mix asphalt (HMA), can be used to validate the DIC. Thus, the use of these two methods leads to a better understanding of the OT procedure.