Magazine of Concrete Research, 2013, 65(8), 462–474 http://dx.doi.org/10.1680/macr.12.00077 Paper 1200077 Received 30/04/2012; revised 04/12/2012; accepted 11/12/2012 Published online ahead of print 12/03/2013 ICE Publishing: All rights reserved Magazine of Concrete Research Volume 65 Issue 8 Mechanical properties of steel fibre concrete Tadepalli, Mo and Hsu Mechanical properties of steel fibre concrete Padmanabha Rao Tadepalli Engineer, American Global Maritime Inc., Houston, Texas, USA Y. L. Mo Professor, Department of Civil and Environmental Engineering, Director of Thomas T. C. Hsu Structural Research Laboratory, University of Houston, Texas, USA Thomas T. C. Hsu Moores Professor, Department of Civil and Environmental Engineering, University of Houston, Texas, USA Two types of hooked fibres and one type of twisted steel fibre were used as reinforcement in beam specimens of size 150 mm 3 150 mm 3 500 mm. A series of experiments was carried out to investigate the effect of size, shape and amount of steel fibres on the mechanical properties of concrete, such as the compressive strength, first-crack flexural strength and ultimate flexural strength, modulus of elasticity, flexural toughness and ductility. The guidelines of ASTM C1609 were used to carry out the bending tests. The compressive strengths of the concrete mixes were obtained from cylinder tests as per the guidelines of ASTM C39. The experimental findings indicate that addition of steel fibres in concrete can slightly enhance the compressive strength and modulus of elasticity, but remarkably improve flexural strength, flexural toughness and ductility. Results were further extended to find a simple formula for modulus of rupture (MOR) of steel fibre concrete, which is based on ACI 318 code formula for MOR of normal concrete. Notation a distance of the applied load from the support (mm) b width of the beam (mm) D depth of the beam (mm) D f , d diameter of steel fibre F total load applied (N) FF fibre factor f 9 c concrete compressive strength (MPa) f f flexural strength (MPa) L f /D f aspect ratio of fibre l length of fibre V f volume of fibre Introduction Steel fibres of various shapes (i.e. straight, crimped, hooked single, hooked collated, twisted, etc.) are available on the market and intended for structural use. Steel fibre concrete has been studied for more than five decades, but very little literature is available on the performance of different types of steel fibres in concrete with different compressive strengths, that is normal strength and high-strength concretes. The purpose of this experimental study was to determine and compare the structural performance of different types and dosages of steel fibres in normal and high-strength concretes. Based on the test results, the selected steel fibre with the selected dosage will be used to cast full-scale bridge girders to test their shear capacity in a future project. Previous work on fibre-reinforced concrete reported in the literature Steel fibres can be defined as discrete, short lengths of steel having a length to diameter ratio (i.e. aspect ratio) in the range of 20–100. They are sufficiently small to be easily and randomly dispersed in fresh concrete mix using conventional mixing procedures (ACI 318; ACI, 2008). Fibres act as multi-directional, uniformly dispersed micro- reinforcement in the concrete matrix. Their primary function is to bridge across cracks and thereby prevent them from growing by transferring the tension across the cracks. Fibres help to carry and redistribute applied stresses in concrete by undergoing shear strains (Beaudoin, 1990). Thus, shrinkage and thermal cracking during the plastic stage, as well as micro-cracking in the concrete matrix during the loading stage, are controlled by the presence of fibres in concrete. The characteristics of fibres impart post- cracking ductility to fibre-reinforced concrete (FRC). Fibres enhance the mechanical performance of concrete with regard to its tensile and shear strengths, toughness, ductility, durability, fatigue and shrinkage resistance (Shah, 1991). Fibres enhance the mechanical performance of concrete in all failure modes (Gopalaratnam and Shah, 1987). In FRC, an addition of up to 1 . 5% of fibres by volume increases the compressive strength by up to 15% (Dixon and Mayfield, 1971; Johnston, 1974). A gradual slope in the descending portion of the FRC stress–strain curve indicates improved spalling resistance, ductility and toughness (Padmarajaiah and Ramaswamy, 2002). 462