CEMENT and CONCRETE RESEARCH, Vol. 20, pp. 665-676, 1990. Printed in the USA. 0008-8846/90. $3.00+00. Copyright (c) 1990 Pergamon Press plc. HICROCRACKINC in FIBER KEI1TFORCED CONCRETE B. Mobasher Graduate Research Assistant, Dept. of Civil Engineering Northwestern University, Evanston, IL 60208 H. Stang Research Associate, Dept. of Structural Engineering Building 118, Technical University of Denmark, Lyngby, Denmark S.P. Shah Director, NSF Science Technological Center for Advanced Cement-Based Materials Northwestern University, Evanston, IL 60208 (Communicated by A.J. Majumdar) (Received Jan. 12, 1990) ABSTRACT Micromechanisms of matrix fracture in Portland cement based fiber composites are studied by means of quantitative image analysis and acoustic emission technique. An experimental investigation has been conducted with different volume concentrations of polypropylene fibers. Uniaxial tensile specimens were loaded under constant strain rate and the acoustic emission response was monitored. Thin sectioned epoxy- dye impregnated samples were also prepared from specimens loaded to specified strain magnitudes. Fluorescence microscopy was used to quantitatively examine the thin sections for crack density, length, and spacing. Introduction The current use of fiber reinforced concrete is limited to applications of low fiber contents, where the fiber contribution is primarily in the post- peak region of the response of the composite. As the volume fraction of fibers increases, and they become more uniformly distributed, the chance that they can hinder the growth of microcracks through an arrest mechanism increases. Hence the matrix fracture toughness can be enhanced. The following study investigates the response of aligned polypropylene fiber-cement matrix composites subjected to uniaxial tension. Composites with different volume fractions of fibers were studied within a strain range of up to 1% strain. Evolution of microcracks were studied by means of acoustic emission technique and quantitative optical microscopy. 665