### Moisture Measurement with GPR  
Besides the mostly negligible conductivity and magnetic permeability, the electric permittivity \(\varepsilon\) is the governing material parameter for moisture measurements with GPR [12; 13]. This gets particularly clear by comparing \(\varepsilon\) for dry concrete and water. Whereas the former lies between 2 to 9 [14], the latter shows values around 81. This difference causes a significant rise for wet concrete (between 10 to 20), which influences various propagation characteristics of the electromagnetic (EM) waves. By analyzing specific time-, amplitude-, or frequency-based features of the received signals, these water-related influences become measurable. A detailed review of those features typically used for moisture measurement with GPR in CE is presented in [11]. However, a short overview is given in the following. First, the velocity \(v\) of an EM wave is directly related to \(\varepsilon\). For non-magnetic conditions, as it is usually the case in building materials, it can simply be calculated as follows [14; 15]:  
\[v=\frac{c}{\sqrt{\varepsilon}}=\frac{2D}{T}, \tag{1}\]  
where \(c\) is the velocity of EM waves in free space, and \(T\) the two-way travel time in a material with the thickness \(D\). Comparing the dry state of a material, sent and reflected pulses are received later for rising moisture content. Furthermore, the intensities and thus the measured amplitudes are reduced due to higher attenuations, caused by generally increased conductivity and more frequently occurring scattering events on water-filled pores. Filled pores also lead to Rayleigh scattering [16], which is one way to explain the observable shift of the received signals to lower frequencies for higher moisture content. Another explanation is given with the presence of dielectric dispersion, presented in the popular models of [PERSON] [17] and [PERSON] [18]. It describes the rising imaginary part of \(\varepsilon\) and the resulting absorption of higher-frequency components close to the relaxation frequency, which is 10 GHz to 20 GHz for free water [15; 19], but can be smaller for porous materials. Another important characteristic of EM waves is the occurrence of reflection and transmission on material boundaries with different permittivities. With \(\varepsilon_{1}\) and \(\varepsilon_{2}\) of two mediums, an EM wave travelling from medium 1 to medium 2 is reflected by the amount of the reflection coefficient \(r\in[-1,1]\), which is calculated as follows [20]:  
\[r=\frac{\sqrt{\varepsilon_{1}}-\sqrt{\varepsilon_{2}}}{\sqrt{\varepsilon_{1}}+ \sqrt{\varepsilon_{2}}} \tag{2}\]Therefore, the amplitude of a reflection wave (RW) is highly influenced by the boundary's permittivity contrast, from which it originates. Figure 1 shows this simplified ray-based principle with an exemplary screed plate above air, forming such a permittivity contrast. It also presents the usually performed collection of multiple reflection signals (A-scans) along a survey line, whereas the offset between the transmitter (T) and receiver (R) stays constant (common-offset configuration). The recorded A-scans can then be combined in a radargram (B-scan) that offers the opportunity to visualize spatial deviations caused by inhomogeneities, like the presence of reinforcements or water-damaged areas.  
The most dominant wave-type in an A-scan is the direct wave (DW), which travels the shortest path between T and R and is therefore recorded first. As shown, it is a superposition of an air and a ground wave and is generally used as a time reference for the following RW, since the moment of emitting the pulse (time zero) is unknown [21].  
Typical signals and their respective features measured on layered floor constructions are discussed in Section 2.5.