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Stresscrete 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. Hollowcore may be designed for other uses such as wall cladding and retaining wall structures. The recommended cast in-situ topping thickness is a minimum of 75mm.

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.

Sound Transmission

One of the major features of a concrete floor is the low sound transmission.

Props

End props must be provided where they are required for stability of edge loaded beams. Other props are not normally required but can be used to enhance load capacity and reduce deflections.

Fastening and suspension

Light fastenings can be fixed in the area between strands by means of different anchors, bolts and screws. Fastenings must not 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 using a hook or welded steel piece on the end of the rod.

Topping strength = 25 MPa minimum at 28 days (as re- quired by NZS 3101:2006).

Unit strength = 45 MPa at 28 days minimum.

Fire resistance rating

Standard fire resistance ratings of Hollowcore units in the load span tables is 1.5 hours. Fire resistance ratings are unrestrained ratings and are based on minimum strand cover and equivalent concrete thickness requirements.

Hollowcore section properties

Section properties are based on a 1200 mm wide section of floor with a 75 mm thick concrete topping. The modular ratio for 25 MPa topping concrete is assumed to be 0.75, for calculation of composite section properties, A’, Yb’, I’, Zb’ and Zt’.

Shear capacity

The shear capacity of extruded floor slabs is adequate for the uniformly distributed loads given in the load/span tables. 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 shear loads.

Penetrations

Small holes and recesses between strands at the position of the voids can be accommodated, contact your nearest Stresscrete branch for advice.

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.