Farhad Salour Doctoral Thesis

Figure 4
. Conceptual schematic of the regions of surface water in pavement structures. 
Pavement moisture monitoring 
It is well known in pavement engineering that moisture can have a significant 
detrimental effect on the performance and bearing capacity of pavement structures. 
Excess moisture presence in pavement structures usually results in accelerated 
pavement distresses, particularly when combined with heavy axle loads. Both field 
observations and laboratory based research studies have shown that proper drainage can 
greatly improve pavement performance and increase its service life. Then, 
understanding moisture variation and movement in pavement structures and 
moisture-related distresses have become of great interest among the pavement research 
community. This has resulted in development of non-destructive techniques for in situ 
real-time measurement of moisture content within the pavement systems. The 
commonly used methods for moisture measurements in pavement engineering 
applications are Time Domain Reflectometry (TDR) technique, capacitance 
measurement and Ground Penetrating Radar (GPR). 

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9 
Time Domain Reflectometry 
Time Domain Reflectometry (TDR) is a very practical technique for measuring the 
in situ moisture content of unbound materials and subgrade soils. TDR technique 
applies electromagnetic technology to measure the relative dielectric constant or relative 
permittivity of a medium. In a multi-phase medium, the overall dielectric number 
𝜀
𝑚
is 
highly dependent on the dielectric numbers of the constituents. The volume fraction of 
the constituents is governing the overall dielectric number. In unbound granular 
material and subgrade soils the multi phases mainly consist of solid constituents, water 
and air. The relative dielectric number of soil constituents and air is much smaller than 
that of water 
8
2


soil

,
1

air

and 
88
77


water

. Therefore, the volumetric moisture 
content of the soil can be determined by measuring its overall dielectric number (Topp 
and Davis, 1985; Erlingsson et al., 2009a; Fredlund et al., 2012). 
In the TDR technique, a very short electric pulse is sent through a waveguide which 
consists of 2 or 3 metallic rods. The impedance discontinuity at the end of the 
waveguide results in backward reflection of the electric pulse which is then detected by 
a receiver. By measuring the travel time of the electric pulse and knowing the length of 
the waveguide, the propagation velocity of the electromagnetic wave can be measured. 
The dielectric constant of the soil medium is thereafter determined as follows: 
2
0
)
2
(
l
t
c
m


[1] 
where 
𝜀
𝑚
is the dielectric constant, 
0

is the light velocity in vacuum and 
t
is the travel 
time of the electromagnetic pulse and 
l
is the length of the metallic rods. TDR probe 
readings are usually reported as the volumetric moisture content through an empirical 
regression which is derived from analyses based on a variety of soil types (Topp et al., 
1980; Hu et al., 2010). However, the moisture content can also be reported as the 
gravietric mositure content or degree of saturation using volume-mass relations. 
Figure 5 shows installation of TDR probes in a thin flexible test road section in Iceland 
(Erlingsson et al., 2002).