## 3 Finite Element Simulations  
The present study utilized finite element models to examine all of the nondestructive evaluation (NDE) techniques included in this research. The models were created using COMSOL Multiphysics(r) software, and each NDE technique was given its own unique set of parameters and criteria to account for its distinct properties. The objective was to build a single model that could simulate the three NDE technologies of interest in this study (the half-cell potential, electrical resistivity, and impact echo (IE)) and to examine the effect of different parameters on their measurements. The software was used to integrate properties of interest for all three technologies into a single model, allowing for the simultaneous simulation of measurements by the three NDE technologies. The concrete slab was modeled as a block with dimensions of 1.0 m \(\times\) 1.0 m \(\times\) 0.2 m, with Perfect Match Layers (PMLs) that simulated the absorbing boundaries of the four sides to avoid the reflection of elastic waves from the sides for the impact echo measurement. The model components, including various NDE probes, are illustrated in Figure 1. The model was sliced for meshing purposes, allowing for finer discretization during the solution process. Extra-fine mesh was used around the contact points of the probes of the NDE technologies, such as around the electrodes of the Wenner probe for ER, around the HCP reference electrode, around the transducer and impact area for the impact echo, and around the embedded steel bar. The reason for creating an extra-fine mesh is to achieve precise simulations of the electrical field distribution for ER, wave propagation for the IE, and potential differences for the HCP. The maximum element size for the free tetrahedral element is 0.001 m, with a growth factor equal to 1.2. Additionally, the model components encompass defects, such as the corrosion of steel bars and delamination inside the concrete slab, which were incorporated within the scope of this study. Five parameters were included to investigate their impact on the interpretation of NDE measurements. The five parameters chosen for this study include the degree of saturation (DoS), rebar corrosion length (CL), delamination depth (DL), concrete cover thickness (CC), and delamination moisture content, whether it is air-filled delamination (AFD) or water-filled delamination (WFD). This chapter focuses on changes in the electrical field and resistivity of the concrete deck that occur as a result of these parameters and the corresponding HCP and ER measurements. The results from the simulation will be utilized for the development of machine learning algorithms. The impact of these parameters is determined by conducting several parametric studies. Below is a description of the variations in the five parameters in this study. 1. Degree of saturation (DoS): A variety of saturation levels were selected to investigate the influence of pore saturation on the NDE measurements. These values span different saturation conditions that a structure may encounter. Seven saturation levels were chosen, covering a range from 20% to 80% in increments of 10%. 2. Corrosion length (CL): The term \"corrosion length\" refers to the length of the anode portion of the steel reinforcing bar. A set of eight values was selected to simulate the corrosion length, which ranges from 2.5 cm to 35 cm in increments of 5 cm. Delamination depth (DD): A set of six values was selected to reflect various depths of delamination. These values range from 40 mm to 90 mm below the surface of the concrete deck in increments of 10 mm. * Concrete cover (CC): The concrete cover thickness has a substantial impact on the HCP measurement readings due to the presence of non-uniform corrosion on the steel bars. A set of four values were selected to represent the concrete cover thickness: 38 mm, 51 mm, 63 mm, and 76 mm. * Delamination moisture condition: Two values were selected to simulate the moisture condition of the delamination within the concrete deck for this parameter.