RC beams with steel fibres - Towards better determination of their minimum conventional reinforcement ratio

Minimum longitudinal reinforcement ratio in reinforced concrete (RC) beams is determined by criteria that refer to flexural capacity. However, relatively recent experiments have shown that RC beams with steel fibres (RFRC beams) with the above-determined minimum reinforcement, exhibit a pronounced reduction in their flexural ductility, compared with similar plain RC beams. This phenomenon was characterized by cracking localization, when only one or two cracks widened significantly more than the other cracks did. This led to increase of the steel plastic strains, even up to its rupture, in the rebars that crossed these cracks. The ductility reduction in this case impairs the structural response of RFRC beams compared with RC beams without fibres. Therefore, this effect of the fibres must also be considered when determining the minimum longitudinal reinforcement ratio in RFRC beams. The paper presents analysis of test results that pertain to flexural ductility in RFRC beams and its relation with the cracking localization phenomenon. This phenomenon may be quantified by the ratio between the number of the significantly wide cracks and the total number of flexural cracks. It is shown that RFRC beams with smaller reinforcement ratios had higher cracking localization. It is also shown that as the reinforcement ratio decreases below a certain value, which is considerably larger than the minimum required by the codes, the flexural ductility decreases as well. Within this range of small reinforcement ratio, these two trends are related, whereas for larger reinforcement ratios, there is no effect of cracking localization on the flexural ductility.