## 4 Results and Discussion
### Comparison of Experimental and Simulated Results
The damage was considered as detected by the IRT technique if, after the hourly simulation, the temperature difference between the concrete surface on top of the delaminated area and the concrete surface without delamination was equal or greater than 0.5 \({}^{\circ}\)C.  
In a monthly analysis, December presented a larger percentage of contrast results exceeding the 0.5 \({}^{\circ}\)C thresholds. This month represents the beginning of the summer in Brazil and, together with January and February, compose the summer season in the southern hemisphere. This season had a higher quantity of simulations of thermal gradient values exceeding the ASTM D 4788-03 [46] recommendation. The warmer weather in the summer facilitates the heating propagation by solar radiation, the main heating source for passive IRT inspection, which increases the heating of the concrete sample and, therefore, the identification of the damages [24; 37; 38; 39; 40]. Winter and autumn months also present a high incidence of solar radiation; however, the air temperatures usually are lower than summer and spring, and the presence of humidity and fog are more frequent, which makes heat propagation difficult. In general, it can be observed that the summer and spring represent favorable periods to perform infrared thermographic inspections under passive heating in this case of study, with 17.42% more reliable detections than the autumn and winter period. These findings challenge the observation of [PERSON] et al. [24], which reported no significant effect of seasonal environment on the simulated thermal gradient in Orlando, Florida. One explanation could be the extent of the simulation, where they simulated one sunny day in each season, while this study performed an extended and continuous simulation of the entire seasons. Moreover, the study location probably has a different weather condition, which naturally produces different results. On the other hand, our data \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Season**} & \multirow{2}{*}{**Month**} & \(\mathbf{N}^{\circ}\) **of** & \(\mathbf{N}^{\circ}\) **of** & **Monthly Detection** & **Season Detection** & **6-Months Detection** \\  & & **Detections** & **Simulations** & **Percentage (\%)** & **Percentage (\%)** & **Percentage (\%)** \\ \hline \multirow{3}{*}{Autumn} & March & 367 & 744 & 49.33 & & \\  & April & 275 & 720 & 38.19 & 35.46 & \\  & May & 141 & 744 & 18.95 & & \\ \hline \multirow{3}{*}{Winter} & June & 227 & 720 & 31.53 & & 35.87 \\  & July & 216 & 744 & 29.03 & 36.28 & \\  & August & 358 & 744 & 48.12 & & \\ \hline \multirow{3}{*}{Spring} & September & 326 & 720 & 45.28 & & \\  & October & 325 & 744 & 43.68 & 48.08 & \\  & November & 399 & 720 & 55.42 & & \\ \hline \multirow{3}{*}{Summer} & December & 456 & 744 & 61.29 & & 53.29 \\  & January & 428 & 744 & 57.53 & 58.56 & \\ \cline{1-1}  & February & 381 & 672 & 56.70 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Quantitative results of thermal gradient observations above the threshold recommended by ASTM D4788-03. support the [PERSON] et al. [47] work by showing that the thermograms captured in warmer months tend to have higher contrast results compared to colder months. The heatmap in Figure 8 displays the hourly contrast values resulted from the FEM long-term analysis. The black color indicates the higher negative thermal gradients values (where the delaminated concrete area had lower surface temperature than the surrounding concrete), and yellow means the higher positive thermal gradients values (where the delaminated area presented higher surface temperature than the healthy concrete area). The inappropriate periods for inspection are illustrated in the purple color, when the thermal gradients were between \(-0.5\)\({}^{\circ}\)C and \(0.5\)\({}^{\circ}\)C, i.e., insufficient, according to the ASTM D 4788-03 recommendation. The values of the x-axis start in November following the experimental program.