The Fiber-Matrix Interface in Fiber Reinforced Concrete Studied by Contact Electrical Resistivity Measurement
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XULI FU AND D.D.L. CHUNG Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 14260-4400 ABSTRACT
The contact electrical resistivity of the fiber-matrix interface was found to correlate strongly with the shear debonding strength. In the case of the interface between stainless steel fiber and cement paste, the contact resistivity increased linearly with increasing debonding strength and interfacial phase(s) of volume resistivity higher than that of cement paste enhanced the bonding. INTRODUCTION
The fiber-matrix interface is critical to the properties of fibrous composites. Interface evaluation has been conventionally conducted by single fiber pull-out testing [1], as this test provides a measure of the debonding shear strength. Detailed microstructural examination of the interface by transmission electron microscopy and surface analysis is also valuable, but it is tedious and does not provide a measure of the debonding strength. This paper provides a new method of interface evaluation. This method involves the measurement of the fiber-matrix contact electrical resistivity. The measurement is fast, sensitive and involves inexpensive equipment. It also provides an indirect measure of the debonding shear strength, due to the
excellent correlation between the debonding shear strength and the contact resistivity for a given combination of fiber and matrix. The technique is demonstrated in this work by studying the interface between stainless steel fiber and cement paste (concrete without coarse or fine aggregate). EXPERIMENTAL
The stainless steel fibers were of diameter 60 jim, volume electrical resistivity 6 x 10.' Q.cm, tensile strength 970 MPa and elongation at break 3.2%, as provided by International Steel Wool Corp., Springfield, Ohio. Portland cement (Type I) from Lafarge Corp., Southfield, MI, was used. The water/cement ratio was 0.35. The water reducing agent used in the amount of 0.5% by weight of cement was TAMOL SN (Rohm and Haas Co., Philadelphia, PA), which contained 93-96% sodium salt of a condensed naphthalenesulfonic acid. The volume electrical resistivity of the cement paste was 1.4 x 10' Q.cm at 1 day of curing, as measured by the fourprobe method using silver paint for electrical contacts. The contact electrical resistivity between the fiber and the cement paste was measured at 1 day of curing using the four-probe method and silver paint as electrical contacts, as 559 Mat. Res. Soc. Symp. Proc. Vol. 370 ©1995Materials Research Society
Current contact Voltage contact Fiber Voltage contact Current contact-___ Cement paste Fig. 1
Sample configuration for measuring the contact electrical resistivity of the
interface between a fiber and cement paste.
illustrated in Fig. 1. One current contact and one voltage contact were on the fiber, while the other voltage and current contacts were on the cement paste embedding the fiber to a distance of 1cm. The cement paste thickness was 1.5 mm on each side sandwiching the fiber. The fiber length was 5 cm. The current w
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