## 5 Experimental Results  
### Results from Dry Concrete  
In the following part, thermograms only from the type 1 sample are presented exemplarily; more data were previously reported in [17]. Figure 8 shows the very first thermogram after the pulse had started (within the 25 ms preceding period). It is displayed as difference thermogram, i.e., the initial temperature distribution before the pulse was subtracted to obtain a temperature difference as contained in Equation (1). The thermogram clearly shows the difference between the coated and the uncoated part (the upper-right quarter is uncoated). Investigating both cases is important for a later on-site application, where different kinds of coatings may occur. Figure 9 shows the thermogram after 20 s; the surface temperatures decreased, but a difference between the coated and the uncoated sections remains distinctly visible.  
Figure 8: First difference thermogram of the type 1 sample after the pulse had started (heating duration \(<\) 25 ms). The white dashed rectangle includes the measurement region for the temperature transients.  
For the determination of temperature transients, the part of the surface enclosed by the rectangle in Figure 8 within the uncoated region was selected to achieve an averaging over the grains. The obtained curve is displayed in Figure 10. As introduced in the concept section (Section 2), the 0.5 s pulse is very short in comparison to the observation period. The obtained transient shows exactly the expected behavior from Equation (1): the surface temperature curve forms a line in the double log plot after 0.6 s (Figure 11). After 10 s, the temperature decrease slows down as a result of the small sample thickness of 1 cm. Here, the third simplification mentioned in the concept section (the material is semi-infinitely thick) is no longer valid. Therefore, evaluation of the effusivity according to Equation (1) should be performed within a certain period (in this example, 10 s).  
A comparison of all obtained effusivity values is presented in Figures 12 and 13, together with the requirement of the standard (see Table 2). Details of the uncertainty limits were reported in [17].  
Figure 10: Averaged surface temperature transient recorded for the type 1 sample during and after the 0.5 s laser pulse (averaged over the white dashed rectangle in Figure 8).  
Figure 9: Difference thermogram of the type 1 sample after 20 s cooling down.  
Figure 12 demonstrates that the effusivity can be determined on both uncoated and coated surfaces. Additionally, type 2 sample differs from the requirement of the standard, as also shown in Table 2. It is important to note that this material was classified as repair mortar; therefore, it is technically not concrete and differs from the other two materials in terms of the value of effusivity. However, type 1 and type 3 cannot be distinguished within the given uncertainties.  
Figure 11: Same dataset same dataset as in Figure 10 but as a log–log plot to enable a comparison with Equation (1).  
Figure 12: Overview about the effusivities of the three investigated concrete samples with their uncertainties and the value required by the standard [23].  
It is important to highlight that the reported uncertainties of the photothermal method were estimated from first investigations; thus, there is potential for further improvements. The possibility of detecting effusivity differences is the basis of the proposed approach. In Figure 13, the estimated influence of moisture on the effusivity (numbers from Section 4.3) is displayed in relation to measurement values and their uncertainties (the blue striped rectangle in comparison to the error bars). One can conclude that saturated moisture detection should be relatively straightforward, and the detection of partially dried material might be possible. Thus, it is interesting to study the experimental results obtained for material(s) with different water contents.