Underground Construction - January 2020 - 42

CIGMAT Report
TABLE 5. Model Parameters for Resistivity of Smart Cement Grout
Curing Time (day)

ρmin (1.m)

tmin (min)

p1

q1

to (min)

RMSE (1.m)

R2

Grout (H, w/c = 0.8 only)

1 day

1.04

180

0.286

0.188

63

0.04

0.98

Grout (H, w/c = 0.8 only)

28 days

1.04

180

0.710

0.295

110

0.45

0.97

p2

q2

R2

RMSE
(MPa)

Mix Type

TABLE 6: Peak Stress, Piezoresistivity, Model Parameters for Smart Cement Grout
Strength σf
(MPa)

Curing Time
(day)

Mix Type

Piezoresistivity at peak stress,
(Δρ/ρo )f (%)

Grout (H, w/c = 0.8 only)

1 day

2.96

155

0.031

0.607

0.99

0.08

Grout (H, w/c = 0.8 only)

7 days

9.94

156

0.035

0.642

0.99

0.18

Grout (H, w/c = 0.8 only)

28 days

16.47

179

0.012

0.613

0.99

0.10

TABLE 7: Peak Stress, Piezoresistivity, Model Parameters for Smart Cement Specimens Repaired with Grout
Curing Time
(day)

Mix Type
Initial smart cement
Repaired cement with
grout (H, w/c = 0.8 only)

1 day

Strength σf
(MPa)

Piezoresistivity at peak stress,
(Δρ/ρo )f (%)

p2

q2

R2

RMSE
(MPa)

Strength Regain
(%)

Piezo resistivity
Regain* (%)

12.00

300

0.01

0.693

0.99

0.14

N/A

N/A

10.09

48

0.039

0.601

0.99

0.19

84

21

* At the failure stress of the repaired specimen

12
Vipulanandan p-q curing model
Grout (H, w/c=0.8 only)-experiment

Resistivity (Ohm-m)

10
8
6
4
2
0

0

5

10

15
Time, day

20

25

30

FIGURE 10: Variation of the resistivity with time, for smart cement grouts
with up to 28 days of curing, and using the Vipulanandan Curing model.

After the setting time, hardened cement composite has a complete connected network, leading to formation of a percolated path
of C-S-H, which causes high resistivity due to its continuous gel
micropores. Longer-term, volume fraction of C-S-H will play the
main role in changes of the resistivity of the cement composites:
* After 28 days of curing, the electrical resistivity of smart
cement was 14.14 Ω.m. (FIG. 8).
* After 28 days of curing, the electrical resistivity of the
smart composite (75% gravel) increased by 333% to
61.24 Ω.m (FIG. 8).
Compressive strength was tested after 1 and 28 days of curing.
After 1 day of curing:
* Smart cement compressive strength was 8.6 MPa.
* Smart composite (75% gravel) compressive strength decreased by
29% to 6.1 MPa, compared to the smart cement with no gravel.
After 28 days of curing:
* Smart cement compressive strength was 21.7 MPa.
42

JANUARY 2020 | UConOnline.com

* Smart composite compressive strength decreased by 43%,
to 12.4 MPa.
Changes in compressive strength can be justiﬁed with the
percentage of cement in the concrete.
Piezoresistive behavior of smart cement composites was
evaluated after 1 and 28 days of curing.
After 1 day of curing:
* Piezoresistivity of the smart cement at peak compressive stress
was 375% (FIG. 9 and TABLE 4). Parameters p2 and q2 for the
model were 0.61 and 0.57, respectively.
* Piezoresistivity of the smart composite (75% gravel) decreased
by 57% to 163% compared to the smart cement. Parameters p2
and q2 for the model were 0.40 and 0.80 respectively.
After 28 days of curing:
* Piezoresistivity of the smart cement was 204%. Parameters p2
and q2 for the model were 0.83 and 0.42, respectively.
* Piezoresistivity of the smart composite reduced by 51% to
101%, compared to the smart cement. Parameters p2 and q2
for the model were 0.81 and 0.40, respectively.
The observed trend for resistivity change with time of smart
cement grout was similar to that of the concrete. Minimum
resistivity and time to reach it are summarized in TABLE 5. After
28 days of curing, the resistivity was 9.37 Ωm (FIGURE 10).
Changes in sensing property at failure (Δρ/ρo )f for the
sensing cement grout only after 1, 7 and 28 days of curing were
155%, 156% and 179%, respectively. Using the Vipulanandan
Piezoresistive Model (Eqn. 8), the relationships between
compressive stress and the change in sensing property (Δρ/ρo ),
with and without SS, over the curing periods, were predicted
very well (FIG. 11).
The model parameters q2 and p2 are summarized in TABLE 3. R2
was 0.95 to 0.99. The root mean square of error (RMSE) varied
between 0.04 MPa and 0.64 MPa, as summarized in TABLE 6.
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