###### Abstract  
One of the concerns about the use of passive Infrared Thermography (IRT) for structural health monitoring (SHM) is the determination of a favorable period to conduct the inspections. This paper investigates the use of numerical simulations to find appropriate periods for IRT-based detection of subsurface damages in concrete bridge slabs under passive heating along a 1 year of time span. A model was built using the Finite Element Method (FEM) and calibrated using the results of a set of thermographic field inspections on a concrete slab sample. The results showed that the numerical simulation properly reproduced the experimental thermographic measurements of the concrete structure under passive heating, allowing the analysis to be extended for a longer testing period. The long-term FEM results demonstrated that the months of spring and summer are the most suitable for passive IRT inspections in this study, with around 17% more detections compared to the autumn and winter periods in Brazil. By enhancing the possibility of using FEM beyond the design stage, we demonstrate that this computation tool can provide support to long-term SHM. infrared thermography; concrete bridges; non-destructive test; delamination; finite element method +
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related to the design and construction of new bridges and assessing the condition of existing structures, the inspections provide chronological data about the identification and characterization of potential defects and their evolution [5]. As the performance of a structure depends on the integrity of the elements that compose it, the number of interventions in an operating structure must be set to a minimum. Therefore, the use of non-destructive tests (NDT) has become attractive, representing a fast, harmless, and accurate approach to monitor the structure without impairing its operation [6,7]. Infrared vision (IR) has gained interest as an NDT method due to its capability of examination beyond the visible spectrum [8]. Infrared thermography (IRT) is one of the IR applications, comprising the measurements performed in the thermal mid-wave (MWIR) and longwave (LWIR) infrared bands [9]. The inspection is based on the thermal contrast between different materials, given a heat flux variation. Undamaged elements normally present uniform heat distribution on their surface, while internal or surface defects change the heat propagation by providing resistance or increasing the heat dissipation through the element. As a result, the inspection reveals defective areas with high or low-temperature patterns in the surface thermograms [10]. Moreover, the IRT approach is being constantly improved by the combination with other NDT methods and the use of computational techniques to advance the collection, storage, visualization, and analysis of the thermographic data [11, 12, 13, 14, 15, 16, 17, 18]. Finite Element Method (FEM) constitutes one method of numerical simulation, where the domain of a complex problem is divided into sub-regions of simple geometry, i.e., finite elements [19]. This method could provide similar outputs to the realistic infrared thermography inspection, including the information about the temperature over time and the thermal map of the inspected surfaces [20,21]. Thus, this tool offers the possibility of reducing the time and cost required to make test samples and the need for numerous tests in the target structures. FEM has been used in the NDT studies and practice to predict test results, to examine experimental configurations, to perform parametric and optimization studies, and to develop the field of inspections [22]. In addition, it could be a useful method for predicting favorable time windows for inspection [23,24]. Numerical analysis and IRT have been integrated to detect or examine damages in concrete structures [25, 26, 27, 28, 29, 30, 31]. However, only a few researchers have studied the combination of passive IRT and FEM to improve inspection of concrete bridges.