ON THE USE OF STEEL FIBERS AS COMPLEMENTA RY REINFORCEMENT OF REINFORCED-CONCRETE BEAMS IN BENDING

This paper reports the results of experimental research focusing on tw o possible methods to enhance the service load behaviour of ordinary r einforced concrete structural elements, i.e. the use of steel-fibre co ncrete and the use of high-strength concrete. The programme involved t he testing of nine reinforced concrete beams with rectangular cross-se ction (b = 25 cm, h = 15 cm) and span L = 140 cm. The reinforcement ra tios considered (rho = 0.33, 0.52 and 0.75%) are usual for slabs,for w hich the deflection limit state may often become a design factor. For each reinforcement ratio, we tested a beam in ordinary concrete (BN; 3 2 MPa), a beam in high-strength concrete (BHP; 60 MPa) and a beam in f ibre-reinforced concrete (BF; 38 MPa; fibre content 0.4 vol%). We also tested a fibre-reinforced concrete beam with no bar reinforcement (BR F; rho = 0) to determine the material properties of fibre concrete tha t we needed to compute the carrying capacity of the BF beams. Test res ults show that the use of high-strength concrete seems more effective than the use of fibre concrete to reduce the deflections at service lo ad level. BF beams are slightly stiffer than the BHP beams for higher loading levels (stabilized cracking phase of BN and BHP beams)for the two lowest reinforcement ratios. The carrying capacity of the BF beams is greater than the carrying capacity of the BN beams; the difference in carrying capacity may be estimated on the safe side with a simple prediction model, and is significant (experimentally 32%) for the lowe st reinforcement ratio. However, the ultimate load behaviour of the BF beams is far less satisfactory than the ultimate load behaviour of th e BN and BHP beams. For the BF beams, we observed the concentration of all inelastic deformations at failure in a single cracked section and the ductility factor varied between 1 and 2.8 compared with 4.5-6.3 f or the BN beams and 8.7-10.6 for the BHP beams. The ultimate load beha viour of the BHP beams was quite satisfactory, with the largest number of cracks equally spaced and equally open and the largest deformation capacity. We conclude from these tests that the tensile properties of cracked fibre-reinforced concrete are too limited to reach the level of deformation that is required in safe reinforced concrete design and that the use of steel fibres as complementary reinforcement of rebars should be avoided.