Time-Domain Reflectometry of
Water Content in Portland Cement Concrete
CHARLES J. KORHONEN, VINCENT C. JANOO, AND CHRISTOPHER M. BERINI
parts: part I tested both concrete and mortar, and
INTRODUCTION
part II tested only concrete. We pretested the
probes in concrete and related materials before
During construction of the new Denver Inter-
starting the main experimental program.
national Airport, portions of the portland cement
In addition to studying the relationship be-
concrete pavement were instrumented with time-
tween water content and dielectric constant of
domain reflectometry (TDR) probes to monitor
concrete, we looked at shrinkage as a function of
water content within the concrete. Though in-
moisture content. Preliminary results are shown
stalling the probes into the concrete was easy,
in Appendix B.
data taken with the probes were confusing. At
times the probes were indicating that the concrete
TDR overview
was wetter than it could possibly be. A likely rea-
The TDR method as applied to the measure-
son for this problem seemed to be with the
ment of dielectric properties is given by Fellner-
method used to analyze the data rather than with
Feldegg (1969) and Topp et al. (1980). Briefly, the
the probes themselves. Data analysis is necessary
TDR method calculates a material's relative dielec-
because the probes do not measure water content;
tric constant by comparing the velocity of an elec-
instead, they measure the dielectric constant of a
tromagnetic signal propagated through that
material from which the water content must then
material to one propagated through free space.
be derived.
The TDR system used in this study was a Tek-
At the time of their installation, little informa-
tronix model 1502B metallic cable wave generator
tion existed on the use of TDR probes in concrete.
controlled by a Campbell Scientific CR10 datalog-
The best information came from studies of soils.
ger. This system operates by sending electromag-
Notably, Topp et al. (1980) developed a mathe-
netic signals into a coaxial cable to a probe (Fig. 1)
matical relation between the dielectric constant
at the end of the cable. The instrumentation meas-
and volumetric water content for soils. Since both
ures the time that it takes the signal to travel
soil and concrete are porous and are geologic
down the length of the probe and to reflect back
materials, it was reasoned that Topp's relations
to its source, which gives signal velocity:
might be applicable to concrete. They were not.
Thus, the purpose of this study was to establish in
ν = 2L t
(1)
the laboratory a relationship between water con-
tent and dielectric constant of concrete.
where ν = signal velocity
L = probe length
t = transit time.
EXPERIMENTAL PROGRAM
The experimental program consisted of em-
This velocity is related to the dielectric constant
bedding TDR probes in specimens of fresh con-
of the material in which the probe is embedded:
crete and mortar, curing the specimens for a min-
ν= c
imum of 28 days in water, and then measuring
(2)
ε 0.5
their dielectric constants at known water contents
where c is speed of light in free space, and ε is
(see App. A). The program was divided into two
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