On the identification of SFRC costitutive law in uniaxial tension

The identification of the constitutive law in uniaxial tension represents the critical ring in the design of steel fibre reinforced concrete (SFRC) structures. Despite the large amount of literature on the specific topic, some basic still open questions feed some doubts on the methods proposed by various Technical Committees. The main purpose of this paper is to present a critical review of the hypotheses generically assumed in the recommendations in order to specify the most reliable way to identify a proper constitutive law. Taking advantage of a large experimental program, two Finite Element approaches, discrete and smeared, are compared with a plane-section model: in the smeared F.E. approach a suitable characteristic length is used, while in the plane-section approach a structural characteristic length related to the depth of the cross-section is introduced. Two identification test procedures are compared: the first adopts a notched specimen (critical cross section 105 x 150 mm), while the second one considers unnotched specimens with different cross sections (105 x 150 mm and 60 x 150 mm). Two materials are considered: they are characterised by different matrix strengths (fc = 40, 65 Mpa) and reinforced with two fibre contents (35, 50 kg/m3 respectively). Steel fibres are low-carbon hooked-end in both cases.