## 4 Results and Discussion
### Comparison of Experimental and Simulated Results
The computational modeling proposed in this study showed the possibility of identifying subsurface damages in reinforced concrete bridge slabs at different times of the year and under different environmental conditions, including temperature, wind speed, and solar radiation. Although the simulated thermal gradient results present an average difference of 0.39 \({}^{\circ}\)C compared to the experimental inspection, the correlation between the model results and the experimental study was equal or above 0.96 in all the thermographic tests. Consequently, the reasonable accuracy of the simulation supports the extrapolation of the model analyses to a long period. Figure 6: Comparison between thermal gradients obtained from the IRT experiment and simulated by the FEM. Discussion on Appropriate Periods for IRT Inspection of Concrete Bridge Slabs Based on Long-Term Fem Analysis  
Numerical simulation of 1 year of the inspection was performed using the FEM model of the concrete slab with fabricated delaminations and the weather conditions data available in the Brazilian meteorological database [41] near the study site. All the information about the environmental conditions used in the long-term simulation is available anytime in the referred database, for open consultation or download. In addition, all the modeling data are available for interested readers through email requests. The results of the long-term simulation are presented in Figure 7. The period of simulations started on 1 November 2018, the same month that the experimental program started, and ended on 31 October 2019. The analysis was performed 24 h per day, with hourly time steps. Each daily simulation took an average time of 152 s, which reduced an entire year's analysis to approximately 16 h. The simulations were performed using a desktop computer with 16 GB RAM, 2.3 GHz Intel Core i7 CPU, and an NVIDIA GeForce MX250 GPU. The daily varying input parameters of the model were presented in Table 1. Conventionally, the year was divided into the local seasons: Autumn (March, April, and May), winter (June, July, and August), spring (September, October, and November), and summer (December, January, and February). The incidence of the rain was plotted in Figure 7, and the numerical model correctly simulated the low thermal gradients values in these events, mainly between the \(\pm\)0.5 \({}^{\circ}\)C thresholds. The solar radiation parameter allows this prediction, as rainy days usually have low solar radiation levels, generating a model outcome that correctly shows these days as not favorable for inspections. ASTM D 4788-03 [46], which is the American standard that regulates the thermographic inspections in bridge decks, recommends that the bridge deck should remain dry for at least 24 h hours before an IRT inspection. In addition, the regulation prescribes a minimum thermal gradient of 0.5 \({}^{\circ}\)C between healthy areas and areas with suspected delamination to identify subsurface damage. Figure 7: Thermal gradient results from the FEM simulation of one year of IRT inspection. The direct observation from Figure 7 allows us to percept the difference in the thermal gradient behavior along the seasons, where months with warmer weather have greater thermal gradient values and a larger number of simulations with reliable thermal gradients (out of the ASTM D4788-03 boundaries). The greater thermal gradient between damaged and undamaged areas facilitates subsurface damage identification [9; 10; 24; 37] because the contrast in the thermogram colors becomes more noticeable with the increase of the temperature difference. Table 4 presents the number and percentage of IRT simulations that exceed the ASTM D4788-03 recommendation of a minimum thermal gradient, according to the different seasons considered in the simulation. The detection percentage was calculated by dividing the number of hourly detections by the number of hourly simulations.