Underground Construction - January 2019 - 39

Table 3: Failure Model Parameters for Rocks

Figure 12: Vipulanandan failure model compared to Mohr-Coulomb model for limestone

Smart Cement Contaminated with Montmorillonite Clay
Portland cement slurries are not only used in construction, but also in
repair applications related to slurry walls, piles, other foundations, pipelines, tunnels, wells (oil, gas and water), bridges, buildings and highways.
Based on the application, cement slurries are made with additives and
water-to-cement ratios varying from 0.3 to more than 1. Construction
of deep foundations, near surface and underground structures require
drilling in the ground using drilling muds and placing the cementitious
materials in the boreholes, which may result in various types of clay soil
contamination. Clay soil contamination will impact the cement hydration and long-term properties. Unfortunately, there are no real-time
monitoring methods to detect the clay soil contamination of cementitious materials during construction or the effects of clay contaminations
during the service life of the infrastructures.
Electrical resistivity measurement has been used by many researchers
in the last two decades to characterize not only the cement concrete, but
also other cement slurries and composites. Limited studies have used
electrical measurement methods to study the microstructural evolution in hydrating cement-based material systems. However, there is no
information in the literature on electrical resistivity for characterizing
cements contaminated with clays.
Smart cement with high sensing properties developed by Vipulanandan can sense any change in the cement during the curing period and
after cement hardens. The smart cement can sense the changes in the
water-to-cement ratio, different additives, and any pressure applied to
the cement in terms of piezoresistivity. A recent smart cement study was
focused on class H and class G oil well cements with 0.1 percent conductive filler. Hence, if the sensing properties and piezoresistivity properties
of the Type 1 Portland cement can be improved, then Portland cement
can also be used for monitoring during the installation and the entire

service life of various types of civil infrastructures.
The overall objective of this study was to investigate and quantify
the effects of montmorillonite clay soil contamination on the behavior
of smart Portland cement. Specific purposes involved the effect of clay
soil contamination on the piezoresistive behavior of hardened modified
Portland cement and modeling the piezoresistive behavior of the smart
Portland cement.
Materials & Methods
ASTM has designated five types (I - V) of Portland cements. Chemically and physically these types differ in the tri-calcium aluminate (C3A)
content and the particle fineness. Of the five, Type I is very widely used
for many applications. Commercially available Portland cement Type I
was modified with filler (mixture of basaltic and carbon fibers) to make it
piezoresistive material.
Commercially available Wyoming bentonite was used and was characterized using the X-ray diffraction analyses. This clay is used in water-based drilling muds.
The cement samples were prepared by using a table to blender mixing
at a speed of 2,000 rpm for 5 minutes. Modified Portland cement with
water-to-cement ratio of 0.38 was used in this study. Cement slurries
were prepared with and without selected amounts of clay contamination
(0 percent, 1 percent and 5 percent, based on the total weight of cement
slurry mix).
After mixing the cement, specimens were prepared in plastic cylindrical molds with a diameter of 50.8 mm and a height of 101.6 mm. Two
conductive flexible wires were placed about 51 mm apart in all the molds.
After preparing the specimen, based on the weight and volume the unit
weight was determined for the three compositions investigated in this
study.
The specimens were cured under room condition (relative humidity
of 50 percent) and room temperature of 23oC. The cylindrical specimen was capped and tested at a predetermined controlled displacement
rate of 0.01 percent per minute. Compression tests were performed on
cement samples after 28 days of curing using a hydraulic compression
machine.
Piezoresistivity describes the change in electrical resistivity of a
material under pressure. Since cement serves as a load bearing part of
foundations, buildings, bridges, pipelines and wells in real applications, the piezoresistivity of smart cement was investigated under
compressive loading. During the compression test, electrical resistance was measured in the stress axis. To eliminate the polarization
effect, alternating current (AC) resistance measurements were made
using an LCR (L- inductance; C-Capacitance; R-Resistance) meter at
a frequency of 300 kHz. The changes in resistivity were related to the
applied stress.
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Underground Construction - January 2019

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