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Composites with spherical inclusions, steel ball bearings or glass beads, were prepared from type I ordinary Portland cement (OPC) with a water-to-cement ratio of 0.4 by weight. The dry powder and water were mixed by hand for approximately 3 min prior to blending at high speed in a commercial blender for 2 min to achieve homogeneity. The slurries were allowed to set for an appropriate time (≈3 h) to achieve a viscosity sufficient to prevent subsequent settling by the steel ball bearings (3.18 mm diameter, Bearing Distributors, Wheeling, IL) or glass beads (3.0 mm diameter, Fox Industries, Fairfield, NJ), which were mixed in by hand.1 The viscous mixtures were then cast into rectangular polycarbonate molds (25 mm x 25 mm x 100 mm), as in Fig. 2.
Stainless steel electrodes (20 mm x 30 mm x 0.5 mm) were cast in place 5 mm from each end, yielding an inter-electrode spacing of 90 mm. These embedded electrodes served as 2-point ac impedance electrodes, as well as the outer current electrodes for dc measurements. Samples were stored at 100% RH; they were removed from the molds after 24 h, and impedance measurements were made at seven days. Voltage contacts for 4-point dc measurements were made by tightly wrapping 0.25 mm diameter steel wire loops around the samples, as shown in Fig. 2. Silver paste was painted over the wire loops in 2 mm wide strips to ensure electrical contact with the specimens. An enamel sealant was then used to seal the silver paste to prevent water (and contact) loss at these electrodes.
For 2-point impedance measurements, a Solartron 1260 impedance/gain-phase
analyzer was employed with Z-60 personal computer software for data acquisition
(Schlumberger, Houston, TX).1 The excitation
voltage was 1 V and scans were performed from 11 MHz to 5 Hz, with data
collected at 20 steps per frequency
decade. The 4-point dc resistance measurements were carried out with a
programmable current source and digital multimeter using LabVIEW1 personal
computer software for data acquisition (Keithley, Models 220 and 2000,
Cleveland, OH).1 For resistance measurements, current was applied to the outer electrodes of Fig.
2 in increments of 1 mA
from 10 mA to −10 mA. In certain instances current was increased to ±50 mA. The
voltage drop between the center electrodes was recorded at each current level.
Specimen resistance was calculated from the slope at the origin in current vs. voltage plots.
Fig. 2. Experimental set-up for impedance and 4-point dc resistance
measurements.
1. Certain commercial equipment is identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the equipment used is necessarily the best available for the purpose.
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Discussion Up: Main Previous:
Introduction