Steel Fibres in Reinforced Concrete Structures of Complex Shapes: Structural Behaviour and Design Perspectives

Doctoral thesis, 2014

In concrete structures of complex geometries, the formability of concrete is an asset. However, complex geometries introduce time-demanding form and reinforcement works. By streamlining design and production, utilising the benefits of computational tools as well as modern production technologies, the buildability of complex concrete structures can be increased. Combinined with the use of alternative reinforcement methods, the scope of unique concrete structures could be broadened. Conventionally, steel bars have been used; however, alternative reinforcement methods have been introduced. First, several reinforcement methods were studied and the focus for the remainder of the work were set on the structural behaviour of conventional reinforcement and steel fibre reinforced concrete (SFRC). It may not be possible to apply standard idealisations to concrete structures of complex geometry. Two methods for the design of conventional reinforcement, based on linear finite element analyses, were investigated and both were found to provide a rational approach calculating the amount of reinforcement needed. To investigate the structural behaviour of SFRC, an experimental programme was conducted. Two-way slabs with combinations of conventional reinforcement and SFRC were tested, investigating the effect from steel fibres on load redistribution. To provoke redistribution after cracking, the conventional reinforcement was arranged asymmetrically, forming a weak and a strong direction. As expected, steel fibres increased the load-carrying capacity and the number of cracks. Furthermore, the steel fibres increased the portion of applied load transferred to the supports in the weak direction and contributed to evening out the load of the length of the support. Material characterisation of the SFRC was performed through both uni-axial and three-point bending tests. A numerical approach was successfully utilised to relate the two test methods to each other. Analytical and numerical analyses of both beams and slabs were conducted, and the results were compared with experiments. In these cases, the additional capacity provided by the steel fibres was observed both experimentally and in numerical analysis. Depending on the interpretation of analytical proposal in Model Code 2010, the load-carrying capacity was either underestimated or rather accurately estimated. Through a combination of experimental, numerical and analytical work on both structural and material levels, this thesis contributes to an improved understanding for the structural use of SFRC, especially in structures of complex geometry. Keywords: Concrete structres, Geometrically complex structures, Reinforcement alternatives, Steel fibre reinforcement, Rational design