Steel fibre reinforced concrete plates: structural response and reliability of design methods

One of the most intriguing applications of Steel Fibre Reinforced Concrete (SFRC), is in design and construction of slabs. High stress redistribution capacity of these structural elements leads to the creation of large crack areas which allows for exploitation of the benefits that come from the presence of fibres. With the advent of design codes and guidelines, designers are now equipped with tensile laws that enable them to introduce tensile resistance for a cracked concrete. Therefore, it is of utmost importance to investigate the benefits and the shortcomings of application of steel fibres as partial or complete replacement of reinforcing bars in slabs, and to study the reliability of the available tools and methods for design of these elements. A normal design procedure is based on the characteristic properties of the material, i.e. the 5% fractile of its distribution. The tensile properties of SFRC which are derived from tests on small specimens show a large scatter, which results in small characteristic values. On the contrary, the structural resistance of SFRC redundant slabs exhibits a narrow dispersion, very different from the material properties by which they are designed. Corrective factors are suggested in order to fill the gap between the highly dispersed tensile properties and narrowly dispersed structural strength of SFRC slabs. In this thesis, the behaviour of SFRC plate elements are studied and their combination with rebars is investigated and compared to RC plates. A yield line approach is adopted to predict the load bearing capacity and design resistance of the plates. Proper direct tensile law are derived and implemented in a Non Linear Finite Element Model (NLFEM) to predict the structural response of the plates. Furthermore, a relatively large sample of identical SFRC plates are tested and the scatter of their bearing capacity is compared to the results of the standard characterization tests, in order to find corrective coefficients. Shallow beams are tested in order to highlight the advantages and shortcomings of application of fibres in statically determinate structural schemes. Finally the cracking behaviour of plates and beams are investigated and compared to the available formulations on crack spacing. The obtained results highlight the significant advantages of applications of fibres. Fibres substantially reduce deflection and crack openings and considerably increase the load bearing capacity of the plates. However, without the presence of rebars, ductility of these elements might be compromised. If so, application of limit analysis which entails considerable rotation capacity at the plastic hinges could be jeopardized. Based on experimental evidence for the ultimate crack opening of the SFRC plates, modified tensile laws are proposed. With Implementation of the modified tensile laws, and with the average residual tensile strength parameters of the SFRC material, the load bearing capacity of the SFRC plates are predicted with satisfactory results. Afterwards, the behaviour of the plates are modeled in a NLFEM, and the importance of the choice of the internal parameter for regularization is highlighted. Some challenges in modelling of the SFRC material which shows hardening after an initial softening in its tensile behaviour is shown. It is discussed that application of fibres in determinate beams can be unreliable and controlled by the heterogeneity of the SFRC, and addition of fibres to RC beams leads to a reduction of ductility. The narrow distribution of structural resistance for the SFRC redundant plates are depicted and proper magnification factors, which compensate the small characteristic values of tensile properties of the SFRC, are derived. Finally the limitations and strength of formulas adopted to predict crack spacing in beams and slabs that incorporated fibres are shown and some suggestions are given.