## 6 Discussion  
The primary goal of the performed experiments was to investigate whether an expected change of the thermal effusivity of moist concrete is detectable by means of the photothermal method. The obtained results on moist concrete presented in Figures 17-20 demonstrate a clear influence of moisture on the respective surface temperature transient after pulsed heating. The size of the effect was even larger than estimated (considering the vertical extent of the blue rectangle in Figure 19) by a simplified model (Equation (1)), which worked well in the case of wet concrete. However, the estimated size of the effect is based on the listed assumptions about the influence of moisture on physical properties of concrete, mainly the linear relationship between moisture mass concentration and thermal conductivity with a slope value of 4. Selecting an higher value of 8, as reported for concrete in [25], would double the effect. It should be mentioned here that higher values of slope above 5 also have a large impact on the thermal diffusivity of moist concrete and could explain an increasing thermal diffusivity with increasing moisture as reported by [PERSON] [6].  
Two further points have to be considered when the quantitative effect is discussed: the variable reflectivity and an (probably) inhomogeneous moisture and temperature distribution in depth.  
* Since the exact reflectivity values were not measured for every photothermal measurement, Figures 17-20 show only raw data, i.e., measured temperature transients. The (additionally) observed moisture influence on the surface reflectivity (see Figure 16) would cause an additional effect when estimating the effusivity using Equation (1). Here, moist concrete tends to have a lower surface reflectivity, resulting in increased energy absorption (see Table 1). This would further (slightly) increase the differences between the estimated effusivities.
* Due to the dynamics of the drying process, inhomogeneous depth profiles for moisture and temperature are expected. The moisture profile is a violation of the first simplification for Equation (1) (isotropic material properties), and a temperature profile would violate the second simplification, as pointed out before. Therefore, Equation (1) can probably not be applied to moist materials, and the real effusivity cannot be  
Figure 20: Surface temperature transients recorded in the type 3 sample in different trials with different moisture contents. The dotted line represents a curve according to Equation (1).  
calculated--in contrast to dry materials. This conclusion also fits the observation that an investigation of moist samples with the transient plane source (TPS) revealed completely incomprehensible results. Since the detection principle of TSP is also based on thermal diffusion processes at and below the surface in a homogeneous material with isotropic temperature distribution, the underlying quantitative analysis might fail in the case of drying moist surfaces.  
To achieve the goal of a quantitative measurement of moisture by the photothermal method, the influence of a spatially (in depth) varying effusivity profile and of an inhomogeneous temperature distribution have to be validated, for example, by thermal FEM simulations. Depending on the results, Equation (1) could be modified with a correction term or a new evaluation method could be deduced. The use of thermal effusivity as indicator for moisture may not useful, and another \"thermal response parameter\" may be better-suited. The reduced surface temperature (as shown in Figure 15) would probably be included in an extended model description of the photothermal response. Eventually, a systematic study of the named points is urgently needed to evaluate the performance and limits of the photothermal method for on-site applications.