Construction in in-situ cast flat slabs using steel fibre reinforced concrete

[摘要] ENGLISH ABSTRACT: Fibre reinforced concrete (FRC) transforms concrete from a characteristically brittle material to one with a post-crack tensile residual capacity. Its application in industry has varied over the past of which the tensile properties have generally been used in the form of crack mitigation. More recently, the introduction of steel fibres has broadened this scope to structural applications in which the resisting tensile stresses that develop within a steel FRC (SFRC) element can be rather significant. This thesis reviews the existing practices and design models associated with SFRC and the suitability of its implementation as the sole form of reinforcement in in-situ cast flat slab systems. As a material SFRC is dependent on a number of factors which include the fibre type and volume, fibre distributions, element size, as well as the support and applied load conditions. Thus, its performance can be considered rather variable in comparison to conventional concrete should the incorrect practices be implemented. In order to adequately define the material characteristics, it is necessary to use test procedures that accurately reflect on the intended structural application. As a result a number of test procedures have been developed. In addition to this, the post-crack material performance is associated with a non-linear behaviour. This attribute makes the design of structural SFRC elements rather difficult. In an attempt to simplify this, existing design models define stress-strain or stress-crack width relations in which assumptions are made regarding the cross-sectional stress distribution at specified load states. This thesis takes on two parts in defining the suitability of SFRC as the sole form of reinforcement in flat slab systems. The first is a theoretical investigation regarding the micro and macro scale material performance of SFRC, the practices that exist in defining the material properties and its application in structural systems (particularly suspended slab systems), and a breakdown of the existing design models applicable to strain softening deflection hardening SFRC materials. The second part is an experimental program in which the fresh state and hardened state material properties of specified SFRC mix designs definedthrough flow and beam testing respectively. These properties are then implemented in thedesign and construction of full scale flexural and punching shear test slabs in an attempt toverify the theory applied.The investigation reveals that the use of SFRC significantly improves the ductility ofconcrete systems in the post-crack state through fibre crack bridging. This ductility can resultin deflection hardening of flat slab systems in which the redistribution of stresses increasesthe load carrying capacity once cracking has taken place. However, the performance of largescale test specimens is significantly influenced by the construction practices implemented inwhich the material variability increases as a result of non-uniform fibre distributions. Theresults indicate that the load prediction models applied have potential to adequately predictthe ultimate failure loads of SFRC flat slab systems but however cannot account for possiblenon-uniform fibre distributions which could result in premature failure of the system.

[发布机构] Stellenbosch University