Characterizing fiber dispersion in cement composites using AC-Impedance Spectroscopy

https://doi.org/10.1016/j.cemconcomp.2004.06.003Get rights and content

Abstract

A novel method is developed to evaluate fiber dispersion issues in short (or discontinuous) fiber-reinforced cement composites. Fiber orientation, global segregation (i.e., as a result of gravitational settling or improper mixing), and local aggregation (i.e., small fiber agglomerates distributed uniformly throughout the matrix) are quantified using an electrical measurement technique. The method is based on AC-Impedance Spectroscopy (AC-IS) and uses the intrinsic conductivity approach to characterize fiber dispersion through a process that is able to isolate some of the effects. A flow chart is developed to describe the method, which consists of 3D AC-IS measurements, a point probe technique, and a dispersion factor (DF) analysis.

Introduction

The use of fiber reinforcement in cement-based composites has led to significant improvements in the mechanical properties of an otherwise brittle matrix, especially under tensile loads. Already used for various non- or semi-structural purposes, fiber-reinforced cement composites (FRCs) could be extended to more load-bearing applications as a result of the increase in ductility, toughness, and strength with the fiber addition [1]. To achieve this goal, a critical factor in determining the effectiveness of fibers in improving mechanical performance is fiber dispersion, which includes both how the fibers are oriented and their location and arrangement within the cement matrix.

Non-uniform fiber dispersion may severely limit the ability of the fibers to improve the composite properties [2], [3], [4]. In the case of oriented fibers, improvements in performance vary in different directions, which may or may not be desirable depending on the application. For fibers with random orientations but non-uniform distribution in the matrix, crack growth would be easier than in a matrix with uniform fiber dispersion. The easier crack growth results from the increase in fiber-free areas. Mechanical failure from cracking begins first with the formation of fine discontinuous microcracks throughout the matrix, which then coalesce to form larger cracks that eventually cause the material to fail. The main role of fibers in increasing the strength of the brittle matrix is in controlling the failure mechanisms by bridging the cracks and making crack formation and coalescence more difficult [5]. The fiber-free areas in poorly dispersed composites act as flaws where crack initiation and propagation occur more easily [6].

In developing FRCs for structural applications, controlling and characterizing fiber dispersion is vital to maximizing the mechanical properties and performance. Established techniques to investigate fiber dispersion using image analysis [6], [7], [8] can be destructive and time consuming. An electrical measurement technique may alleviate these concerns. Extensive research has been conducted examining the electrical properties of FRCs either for electrical purposes (e.g., electromagnetic shielding and thermistors), but more often for monitoring strain and damage in the composites [9], [10]. Chung [9], [10] have investigated DC electrical properties of FRCs and attributed changes in resistance to microstructural changes such as fiber pull-out, fiber realignment, and/or changes in fiber spacing when the composite is under static or dynamic loading.

The effectiveness of AC electrical properties to investigate FRCs has also been demonstrated [11], [12], [13], [14], [15]. AC-Impedance Spectroscopy (AC-IS) of composites with conductive fibers (e.g., steel and carbon) results in a frequency-dependent behavior [15]. The response appears as a dual cusp in Nyquist plots (real vs. negative imaginary impedance). The dual-cusp behavior is caused by fibers behaving insulating at low frequencies (analogous to DC measurements) and conducting at high frequencies [13], [14], [15]. The change in behavior results from the existence of a high impedance interface on the conductive fibers, which shorts due to displacement currents through the interface at high frequencies. For example, in cement composites with steel fibers, the high impedance interface is an oxide that forms due to the high pH environment of the cement. Therefore, for composites with high impedance interfaces on the fibers, AC-IS provides additional information not available from DC measurements.

Previous work has resulted in the development of a universal equivalent circuit model to understand the “frequency-switchable” behavior [16]. The work demonstrates how AC-IS can be used to investigate fiber properties such as fiber loading and aspect ratio (length divided by diameter) in non-percolating systems (when the conductive fibers do not form a continuous path through the material, which would otherwise result in a dramatic change in resistance over a small fiber-loading range). In this work, electrical measurements with AC-IS are developed to monitor fiber dispersion in FRCs. The issues examined include fiber orientation, global segregation of fibers (e.g., settling of large fibers due to gravity), and local aggregation due to the formation of small fiber agglomerates distributed uniformly throughout the matrix.

Section snippets

Experimental procedures

Cement composite samples were produced with type I ordinary Portland cement (OPC) in cubic geometries (86 mm on a side) in polycarbonate molds. Samples were stored at 100% relative humidity, and electrical measurements were made at 7 days. The mixing procedures varied slightly for the experiments and are described in detail below.

The electrodes for the 2-point AC-IS measurements were achieved with a “wet” technique where large 0.5 mm thick steel plates were placed in reservoirs of 1 M NaCl

The AC-IS and intrinsic conductivity approach

AC-IS involves the application of a low-amplitude AC excitation by surface electrodes over a range of frequencies and then measurement of the current response (i.e., gain and phase angle) by the impedance analyzer. Each frequency generates a single datum (current response) that has both real and imaginary components and can be shown in a Nyquist plot, which is negative imaginary impedance vs. real impedance. Fig. 2 shows data for samples used in the orientation study. Both samples (with and

Conclusions

Electrical measurements using AC-Impedance Spectroscopy (AC-IS) are non-destructive and effective in characterizing the orientation and arrangement of conductive fibers in FRCs. The AC-IS method makes use of the intrinsic conductivity approach, which dictates how the conductivity of the composite should behave for fibers of a known aspect ratio (AR) and volume fraction in a randomly aligned and well-dispersed FRC. The deviation from theoretical behavior serves as the basis for quantifying fiber

Acknowledgments

The authors gratefully acknowledge the assistance of Edward J. Garboczi of the National Institute of Standards and Technology, Materials and Construction Research Division. This work was supported in part by the National Science Foundation under grant no. DMR-00-73197.

References (22)

  • S.P. Shah

    Do fibers increase the tensile strength of cement-based matrices?

    ACI Mater J

    (1991)
  • Cited by (90)

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