C20 and C30 classes of concrete are produced each with addition of Dramix RC-80/0.60-BN type of steel fibers (SFs) at dosages of 0 kg/m(3), 30 kg/m(3), 60 kg/m(3), and their compressive strengths, split tensile strength, moduli of elasticity and toughnesses are measured. Nine reinforced concrete (RC) beams of 300 x 300 x 2000 mm outer dimensions, designed as tension failure and all having the same steel reinforcement, having SFs at dosages of 0 kg/m3, 30 kg/m3, 60 kg/m3 with C20 class concrete, and nine other RC beams of the same peculiarities with C30 class concrete again designed as tension failure and all having the same reinforcement are produced and tested under simple bending. The load versus mid-span deflection relationships of all these RC and steel-fiber-added RC (SFARC) beams under simple bending are recorded. First, the mechanical properties of C20 and C30 classes of concrete with no SFs and with SFs at dosages of 30 kg/m(3) and 60 kg/m(3) are determined in a comparative way. The flexural behaviours and toughnesses of RC and SFARC beams for C20 and C30 classes of concrete are also determined in a comparative way. The experimentally determined (mid-section load)-(SFs dosage) and (toughness)-(SFs dosage) relationships are given to reveal the quantitative effects of concrete class and SFs dosage on these crucial properties. (c) 2005 Elsevier Ltd. All rights reserved.