Underground Construction - January 2020 - 40

CIGMAT Report
25
20
Electrical resistivity, ⍀m

TABLE 2: Electrical Resistivity Parameters of Smart Cement
Composite Slurries

No gravel
75% gravel
Vipulanandan curing model

ρmin
(1.m)

tmin
(minute)

ρ24
(1.m)

(ρ24 - ρmin)/
ρmin
%

No Gravel

1.02

0.79

90

5.14

550%

75% Gravel

3.74

3.46

60

20.01

478%

Smart Cement
Composite
(by volume)

10
5
0

0

5

10

20

15

25

Time, hrs

FIGURE 7: Development of electrical resistivity of smart cement composites
during the initial 24 hours of curing.
80
No gravel
75% gravel
Vipulanandan curing model

70
Electrical resistivity, ⍀m

60
50

40
30

20
10
0

0

5

10

15
Time, hrs

20

25

30

FIGURE 8: Development of electrical resistivity of smart cement composites
during 28 days of curing.

during the curing time. A two-wire method with AC at 300 kHz
frequency was used in order to minimize the contact resistances
(Vipulanandan et al. 2013). During the initial stage of curing,
both the electrical resistivity (ρ) and electrical resistance (R) were
measured to determine the parameter K, per the following (Eqn.3):
L
R =     = K
 A
where L is the distance between the two points of measuring resistance, and A is the cross-sectional area through which the current
is ﬂowing. The ratio L/A is called the geometry factor (nominal Kn )
that is used for conductive materials.
Electrical resistance (R) and electrical resistivity (ρ) were
measured independently during the initial curing period and the
eﬀective calibration factor (Ke ) for the materials using (insulators) was determined experimentally. For the smart cement and
concrete, parameter Ke became stable (constant) in two to three
hours. Parameter Ke was more than double the arameter Kn for
the specimens tested.
Normalized change in resistivity (Δρ) with the diﬀerent conditions is expressed as follows (Eqn. 4):

R
=

R
40

ρ0
(1.m)

15

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The modiﬁed cement material is represented in terms of
resistivity (ρ) and the changes due to stress are quantiﬁed
to evaluate the sensitivity of the material (Eqn. 4).
The change in resistance was measured using the twoprobe method with the LCR meter. To minimize the contact
resistances, the resistance was measured at 300 kHz using a
two-wire method. This conﬁguration was ﬁrst calibrated using
the same liquid (cement slurry) to determine the parameter
K in Eqn. 1.
The cylindrical specimens (concrete, cement and grout)
were capped and compression tested (ASTM C39) at a
predetermined controlled displacement rate. Tests were
performed in the Tinious Olsun machine, controlling the
displacement rate to 0.125 mm per minute. In order to measure
the strain, a commercially available extensometer (accuracy of
0.001% strain) was used.
During the compression test, the change in resistance was
measured continuously using the LCR meter. The two-probe
method with alternative current (AC) at 300 kHz frequency was
used in order to minimize the contact resistances. The change in
resistance was monitored using the two-probe method, and the
parameter (Eqn. 3) was used to relate the changes in resistivity
to the applied stress.
A commercially available conductivity meter was used to
measure the conductivity (inverse of electrical resistivity).
The conductivity measuring range was from 0.1μS/cm to
1000 mS/cm, representing a resistivity of 100,000 Ω.m. to
0.01 Ω.m, respectively.
The digital resistivity meter measured the resistivity in the
range of 0.01Ω-m to 400 Ω-m.
In order to represent the electrical resistivity development of
the cement, the Vipulanandan Curing Model was used, and the
relationship is as follows (Eqn. 5):

 t + t0 


 t + t 
1 
1
min
o
=
 p1 +q1 
 min 
 t + t0 
 t + t 0   p1 	

 q1 + (1 p1  q1 ) t + t 	 + p1  t + t 	
min
o
min
o












where ρ is the electrical resistivity in Ω.m, ρmin is the minimum
electrical resistivity in Ω.m, tmin is the time corresponding to the
minimum electrical resistivity (ρmin ), t represents the curing time,
t0 is the model parameter inﬂuenced by the initial resistivity, and
p1 and q1 are time-dependent model parameters, as follows:
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