Kimberly Belli

Effectiveness of 2-D and 2.5-D FDTD Ground-Penetrating Radar Modeling for Bridge-Deck Deterioration Evaluated by 3-D FDTD

Computational modeling effectively analyzes the wave propagation and associated interaction within heterogeneous reinforced concrete bridge decks, providing valuable information for sensor selection and placement. It provides a good basis for the implementation of the inverse problem in defect detection and the reconstruction of subsurface properties, which is beneficial for defect diagnosis. The objective of this study is to evaluate the effectiveness of lower order models in the evaluation of bridge-deck subsurfaces modeled as layered media. The two lower order models considered are a 2-D model and a 2.5-D model that uses the 2-D geometry with a compressed coordinate system to capture wave behavior outside the cross-sectional plane. Both the 2- and 2.5-D models are compared to the results obtained from a full 3-D model. A filter that maps the 3-D excitation signal appropriately for 2- and 2.5-D simulations is presented. The 2.5-D model differs from the 2-D model in that it is capable of capturing 3-D wave behavior interacting with a 2-D geometry. The 2.5-D matches results from the corresponding 3-D model when there is no variation in the third dimension. Computational models for air-launched ground-penetrating radar with 1-GHz central frequency and bandwidth for the detection of bridge-deck delamination are implemented in 2-, 2.5-, and 3-D using FDTD simulations. In all cases, the defect is identifiable in the results. Thus, it is found that in layered media (such as bridge decks) 2- and 2.5-D models are good approximations for modeling bridge-deck deterioration, each with an order of magnitude reduction in computational time.

Comparison of the Accuracy of 2D vs. 3D FDTD Air-Coupled GPR Modeling of Bridge Deck Deterioration

Computational modeling is beneficial in the preparation for nondestructive wave-based sensing. Forward models, which can be implemented through a variety of computational modeling techniques, enable parametric evaluations to assess the functionality of a sensor under different conditions, and are integral to the solution of the inverse problem. The focus of this article is on the comparison of two-dimensional (2D) and three-dimensional (3D) Finite Difference Time Domain (FDTD) models. This article gives a presentation of the accuracy of 2D modeling by comparing FDTD simulations of reinforced bridge deck deterioration in 2D and 3D. Simulations for a healthy and delaminated bridge deck are examined. It is shown that the difference in propaga-tion between the 3D and 2D point sources must be considered and is more pronounced at greater distances from the source location. The effect of scattering from the delamination is visible and, while there is variation in amplitude between the 3D and 2D models, the shapes of the resulting waveforms (including the peak arrival times) are well matched.

Integrated Sensor and Media Modeling Environment Developed and Applied to Ground-Penetrating Radar Investigation of Bridge Decks

Integrated sensor and media modeling environment has been developed to simulate subsurface sensing systems and environmental parameters relevant to the subsurface sensing modalities. The modeling environment is designed to represent complexity in subsurface features, sensor models, and the integration of the sensors with the subsurface environment. The ability to model complex subsurface environments and any potential random distribution of subsurface properties allows for realistic modeling of heterogeneous subsurface environments such as bridged deck/pavement systems. Many applications can benefit from the modeling, simulation, and interpretation capabilities in the new modeling environment that supports improved understanding of system behavior through simulations to evaluate the ability of a particular modality to detect defects. While numerous modeling packages exist to simulate different wave-based modalities, the integrated sensor and media modeling environment is developed to, in a straightforward manner, physically represent the complex subsurface civil infrastructure environment. Physical modeling capabilities enable the object-oriented programming environment facility portability to other application domains as a generic volume serves as the boundaries for internal elements modeled to represent realistic changes in material properties and buried objects. Model development is demonstrated on a realistic bridge deck example.

A Time Domain Equivalent Source Model of an Impulse GPR Antenna Based on Measured Radiation Fields

Ground Penetrating Radar (GPR) is a valuable tool for determining bridge deck health. The ability to simulate scattering from bridge deck elements and the complex interactions between them, as well as from changes due to the presence and relative location of defects is important for understanding observed responses. These simulations can be performed using electromagnetic computational modeling techniques such as Finite-Difference Time Domain (FDTD). In order to accurately model the GPR investigation, it is necessary to have a time domain equivalent source model that can launch and receive electromagnetic waves into the computational space that replicates the signals transmitted and received by the physical GPR antenna. However, due to complexity of design and proprietary information, the GPR unit is typically very difficult, or even impossible, to fully model with sufficient detail. For bridge deck applications, simulation in two-dimensions adequately captures much of the three-dimensional scattering. Two-dimensional simulations have significant computational savings over three-dimensions, and are more feasible to be iteratively implemented to solve inverse problems. The work presented here uses experimental results and presents a computational approach to determine the characteristics suitable for excitation of a two-dimensional FDTD model.

Forward Time Domain Ground Penetrating Radar Modeling of Scattering from Anomalies in the Presence of Steel Reinforcements

Ground penetrating radar (GPR) for nondestructive testing is applied to civil infrastructure such as bridges and roadways. Conventional methods of processing and analyzing GPR data for civil infrastructure are often qualitative, using relative reflection amplitude from subsurface boundaries or reinforcing steel (rebars) as an indicator of health. A Finite Difference Time Domain (FDTD) simulation of GPR is used to generate a bridge deck response of a heterogeneous model of a healthy bridge deck. The result is a healthy deck background that can be removed from measured data to bring anomalies to attention. For the purpose of this article, the measured data is also simulated. In lieu of modeling the identified rebars as perfect electrical conductors (PECs), they are modeled as hard point source excitations to allow for examination of the effect that the scattered waves from the rebar can have on an anomaly. This is an important consideration for application of many inversion methods.

Analysis of Simulated Variation in Dielectric Properties of Reinforced Concrete Systems

Heath monitoring systems that monitor location, characterization, and quantification of damage can benefit from model-based assessment to relate better the measured response to physical indicators of damage. Subsurface sensing technologies, one of the many sensing tools available for health monitoring systems, produce wave-based responses that transverse the subsurface and can be converted to a subsurface image for further interpretation. An integrated modeling environment is under development. This paper uses this modeling environment to simulate response from a ground penetrating radar investigation of a reinforced concrete bridge deck. The focus of this paper is to analyze the differences in the simulated responses due to variability in the dielectric properties of concrete and the presence of air voids. Calculated dielectric properties can vary 10 to 15%. Quantification of the dielectric variability is necessary to develop robust solutions of the inverse problem that will characterize subsurface conditions.

Use of 2D FDTD simulation and the determination of the GPR travel path angle for oblique B-scans of 2D geometries

Scattering from a subsurface point object, such as a reinforcing steel bar embedded concrete or a tunnel buried in sand, results in a B-scan contour that is essentially hyperbolic as the Ground Penetrating Radar (GPR) passes over the object. The shape of the hyperbola can be used to determine the angle at which the GPR traveled over the point object. This information is very useful in determining the orientation and size of an object such as reinforcing steel, buried utilities, and subsurface tunnels. A 2D Finite Difference Time-Domain (FDTD) method can be used to simulate the GPR B-scan when the geometry is invariant in the third dimension and the sensors are appropriately located. The shape of the hyperbola extracted from 3D simulation, analogous to field-collected data, can be compared to a library of hyperbolas extracted from 2D simulations and used to determine the angle of the GPR travel path from the cross-sectional plane.

2−1/2 Dimensional bi-static GPR propagation and scattering modeling of roadways and tunnels with projected 2D FDTD

Subsurface sensing modalities such as Ground Penetrating Radar (GPR) are increasingly being used to assess the condition of aging civil infrastructure by evaluating deterioration within roadways and bridges, and to monitor the security of national borders by the detection of underground tunnels. The need to address these issues is intensifying and, while valuable data are collected using nondestructive evaluation there is urgency for improved understanding and analysis. Simulation of GPR investigations to search for defects in bridges and the presence of underground tunnels can help to understand and analyze real world data. Three-dimensional simulations consider the full geometry of an area. When the geometry is relatively invariant in the third dimension, 2−1/2D simulations can capture most of the 3D scattering and account for bi-static transmitters and receivers located out of the cross-sectional plane. Additionally, comparison of 3D simulation results to a library of 2D results may help to indicate the angle of GPR travel from the cross-sectional plane.