Farhad Salour Doctoral Thesis

Figure 5
. Pavement layer thicknesses and installation of TDR probes (Erlingsson et al., 
2002). 

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10 
Capacitance measurement 
Capacitive sensors can also be used to measure the moisture content of the pavement 
layers (Dirksen, 1999; Hu et al., 2010; Fredlund et al., 2012). Commonly used 
capacitance sensors consist of two electrodes separated by a dielectric. An oscillator 
applies a high frequency electromagnetic pulse (50 to 150 MHz) to the electrodes which 
in return provides a resonant frequency. A capacitance field is generated between the 
two electrodes of each sensor which is extended into the soil next to the sensors. An 
array of sensors in the access tube which is inserted in the pavement structure logs the 
output frequency. The capacitance technique measures the apparent dielectric constant 
of the soil medium which is further empirically related to the soil moisture content 
through a non-linear calibration. The greater the soil moisture content, the smaller the 
resonance frequency and the apparent dielectric constant (Topp et al., 1980; Fares and 
Polyakov, 2006). The soil relative dielectric constant is related to the unfrozen moisture 
content and frozen moisture has only a minor influence on the dielectric constant 
(Patterson and Smith, 1981). Once a proper calibration is done, the capacitance 
technique can measure the apparent liquid moisture content of the soil with high 
accuracy. Figure 6 shows a capacitance probe and installation of a moisture rod in a test 
site in southern Sweden. Figure 7 represents one-year volumetric moisture content 
measurements at four different depths in a pavement structure (depth 50 cm is in the 
subbase layer and depths 90, 120 and 150 cm are in subgrade). 
Figure 6
. Capacitance moisture probe (left) and installation of a moisture rod in a test 
site in southern Sweden (photos from Klas Hansson). 

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11 
Figure 7
. Volumetric moisture content measurements at Torpsbruk test site along 
county road 126 in southern Sweden. 
Ground
 Penetrating Radar
Ground Penetrating Radar (GPR) is a non-destructive testing technique that is used to 
determine material thickness, frost depth, and groundwater table level as well as the 
moisture content (Saarenketo and Scullion, 2000). This technique is usually used for 
larger investigations such as monitoring the pavement system conditions at network 
level. A GPR device is basically a radar system that uses a transmitter to send 
electromagnetic waves. The electromagnetic wave penetrates the pavement structure 
and the reflected wave is detected by a receiver. Based on the travel time and the 
strength of the reflections, the dielectric constant of the material layers and therefore 
their moisture content can be determined.