Behaviour and design of steel fibre reinforced concrete slabs

Abstract

Using Steel Fibre Reinforced Concrete (SFRC) can bring substantial benefits to the construction industry of which savings in construction time and labour are most significant. In addition, steel fibres enhance crack control particularly when acting in conjunction with reinforcement bars. Despite the aforementioned benefits of SFRC, there is a still a lack of consensus on the principles that should be adopted in its design. Currently, a number of different test methods are used to determine the material properties of SFRC but there is no agreement on which method is best. As a result, steel fibre suppliers claim widely differing properties for similar fibres which leads to confusion amongst designers and in some cases inadequate structural performance. This research considers the design of SFRC slabs with emphasis on pile supported slabs which are frequently designed using proprietary methods due to the absence of codified guidance. Key issues in the design of such slabs are control of cracking in service and the calculation of flexural and punching shear resistances. A fundamental challenge is that SFRC exhibits a strain softening response at the dosages commonly used in slabs. At present, the yield line method is generally considered most suitable for designing such slabs at the ultimate limit state but there is a lack of consensus on the design moment of resistance as the bending moment along the yield lines reduces with increasing crack width. This thesis investigates these matters using a combination of experimental and theoretical work. The experimental work compares material properties derived from notched beam and round plate tests and seeks to determine a relationship between the two. Tests were also carried out on continuous slabs with the same material properties as used in the notched beam and round plate tests. Round plate tests were also carried out to determine the contribution of steel fibres to punching shear resistance. The theoretical work investigates the applicability of yield line analysis to the design of SFRC slabs using a combination of numerical modelling and design oriented analytical models. Design for punching shear and the serviceability limit state of cracking are also considered.