## 1 Introduction  
The use of infrared cameras for measuring surface temperature distribution for the evaluation of buildings and structures or parts of them is common practice in civil engineering. The so-called passive approach is popularly used, whereby only one infrared image of the inspected surface under certain environmental conditions is recorded and evaluated. A series of standards describing the procedure for different application cases such as for buildings is available, either generally [1; 2; 3; 4] or for bridge decks [5]. On the contrary, the active approach comprises an intended heating or cooling of the inspected surface and the recording of the surface temperature transient. This procedure requires relatively more equipment, time, and experience for evaluation. However, it can provide more reproducible and reliable information about the inspected component as compared to the passive approach. However, its application in civil engineering is limited, and most of the published results are in research and development work. A standard is not yet available for the field of civil engineering. The vast majority of published reports focus on inner detachments with inner structural features or with inner defects in general. On the other hand, for components with well-known or very large thicknesses, the active approach can be exploited to determine thermophysical properties of the respective material. Thus, active thermography can be used for both, i.e., non-destructive failure detection, as well as material characterization. Furthermore, if the material state or composition differs from its regular state, as in the case of moisture, this could also be detectable. In 2007, [PERSON] and [PERSON] reported the influence of moisture in a cement matrix on the thermal diffusivity of the material and related the results to ultrasound velocities [6]. A similar approach was recently used by [PERSON] et al. to monitor the degree of curing in a polymer from a thermal response [7]. Moisture in building materials is a widespread issue that influences the thermophysical properties. Moisture influences not only the pure material properties but also surface temperature and appearance. The surface temperature can be reduced in relation to the ambient temperature due to evaporative cooling [8]. However, the interaction between water transport and temperature is quite complex and results in a specific surface temperature transient with three different phases [9]. Thus, infrared cameras are often used to study the dynamics of drying processes in building materials (see, for example, [10; 11; 12; 13; 14]), especially in order to determine the moisture retention curve for the respective material [15]. The German standard DIN EN 16682 [16] deals with moisture in cultural heritage and mentions thermography in its appendix (C.8) under \"further relative methods\" as a qualitative method. Recently, a robust measurement method was reported that is able to characterize the thermal behavior of concrete under lab conditions and is convertible into an on-site method [17]. The surface to be investigated is heated shortly by a laser pulse, and the cooling down is recorded by an infrared camera without contact. This photothermal method was applied to three different concrete types and was found to work well on dry material. Since moisture influences the thermal material properties of heat conductivity and heat capacity [18; 19] the photothermal method should be sensitive to moisture. However, it remains unclear whether the expected effect is detectable by the available technical equipment and whether the effect can be described within the framework of existing models. The idea of this study is to investigate the influence of moisture on the measurement results of the photothermal method. Here, the same concrete samples as those used in [17] were soaked in different cycles to obtain different moisture contents and investigated with the same equipment as in [17].