Punching shear strength of steel fibre reinforced lightweight concrete slabs.

One of the problems in slab-column connections is the punching shear failure at over loads. Such failures are sudden and catastrophic, and are undesirable since they do not allow an overall yield mechanism to develop. Fibre reinforcement restrains cracking, and increases the tensile strength of concrete and bond resistance of steel reinforcement. Therefore, it should be possible to use steel fibres as shear reinforcement. This investigation is a study of the structural behaviour of fibre reinforced lightweight concrete flat slabs in punching shear. Twenty full scale connections were tested simply supported on all four sides and loaded centrally through a column stub. The mix consisted of Lytag, sand and fly ash as partial replacement of cement. The main variables studied were the fibre volume, fibre type, column size, amount of reinforcement and concrete strength. Extensive measurements of deformations were made throughout the tests. Fibre reinforcement reduced all the deformations of the plain concrete slab at all stages of loading. For a given serviceability criterion, the presence of fibres increased the service load of the corresponding plain concrete slab by 15-50%. Fibres also increased the post-yield ductility and energy absorption characteristics of the slabs by. 125-260% and 240-270% respectively. The presence of fibres improved the load at first crack, punching shear strength and the residual resistance after punching by about 35%, 40% and 150-400% respectively. Fibres also produced gradual punching failures and sometimes changed the mode of failure into flexure. Empirical and theoretical equations have been proposed to predict both ultimate flexural and punching shear strength of steel fibre reinforced concrete slab-column connections and they show good agreement with data from other investigations. It is concluded that fly-ash can be successfully used in structural lightweight concrete mixes. The addition of fibres in lightweight concrete connections reduces deformations in general, delays the formation of flexural and inclined shear cracking, and increases the service load, ultimate strength, ductility and energy absorption characteristics.