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.