Hollowcore is a 1200mm wide extruded, prestressed, voided
slab unit with a reinforced concrete topping. Standard unit
depths are 200, 300 and 400mm. Units are cut to a customised
length and may have raking ends. Hollowcore is ideally suited
for large floor spans with commercial loading.
One of the major features of a concrete floor is the low
sound transmission. The table below shows sound transmission
ratings achieved by Hollowcore floors.
LIFTING AND HANDLING
Hollowcore floor slabs must be handled and supported near
their ends at all times. Fabric strops, purpose made clamps or
lifting forks are recommended for installation. Chains or wire
strops can be used but may cause some edge damage. Safety chains
must always be used under units where clamps are used.
A seating length of 75mm is recommended. Top surface of
support should be packed using either damp mortar or a plastic
bearing strip. Slabs must be positioned in contact with
neighbouring units (unless otherwise noted). It is recommended
to start placement working from the centre of the building out
(where possible) as any construction tolerance can be spread
over both sides of the slab area.
End props must be provided where they are required for
stability of edge loaded beams.
FIRE RESISTANCE RATING
Standard fire resistance ratings of Hollowcore units in the
load span tables is 2 hours. Fire resistance ratings are
unrestrained ratings and are based on minimum strand cover and
equivalent concrete thickness requirements.
The shear capacity of extruded floor slabs is adequate for
the uniformly distributed loads given in the load/span graphs.
Concentrated loads near supports may result in high shear or
strand bond stresses. Extruded slabs are not recommended for
highway loadings, in truck docks or similar areas with high
FASTENINGS AND SUSPENSIONS
Light fastenings can be fixed in the area between strands by
means of different anchors, bolts and screws. No fastenings must
be attached within a 30mm radius of the prestressing strands.
Heavier fastenings can be attached either in the joint between
slabs or through the slab itself. The extra load due to
suspension must be taken into account in the design
calculations. A suspension point can be made at the joint
between slabs by anchoring a steel rod into the joint concrete
using a hook or welded steel piece on the end of the rod.
Small holes and recesses between strands at the position of
the voids are usually made on the building site. Holes may be
circular or rectangular, and up to three are permitted in the
same cross section (two for 300mm and 400mm units). Holes are
considered to be in the same cross-section if they are less than
750mm apart in the longitudinal slab direction. When making
holes, great care must be taken not to damage the slab. It is
particularly important that the prestressing strands are not cut
Fastenings & Suspensions
WATER IN CORES
Some construction practices and weather conditions can result
in water being trapped in the cores. Holes may be drilled in the
ends of all units to drain this water.
Strand - Stress relieved 7 wire strand to BS 5896 Topping
strength = 20 MPa minimum at 28 days.
Unit strength = 42 MPa at 28 days minimum.
Hollowcore may be designed for other uses such as wall
cladding, and retaining wall structures.
FURTHER TECHNICAL INFORMATION
Specifications for the manufacture, transport and erection of
Hollowcore units, and standard details for typical situations,
are available from your nearest Stresscrete branch. Experienced
and qualified staff will be pleased to discuss design and fixing
details. Refer general notes