Hollow-core slab

Diagram of a concrete slab of hollow core construction

Photograph of a hollowcore assembly

Ahollow core slab,also known as avoided slab,hollow core plankor simply aconcrete plankis aprecastslab ofprestressed concretetypically used in the construction offloorsin multi-storyapartment buildings.The slab has been especially popular in countries where the emphasis of home construction has been on precast concrete, including Northern Europe and formersocialist countriesofEastern Europe.Precast concrete popularity is linked with low-seismic zones and more economical constructions because of fast building assembly, lower self weight (less material), etc.

Theprecast concreteslab has tubular voids extending the full length of the slab, typically with a diameter equal to the 2/3–3/4 the thickness of the slab. This makes the slab much lighter than a massive solid concrete floor slab of equal thickness or strength. The reduced weight also lowers material and transportation costs. The slabs are typically 120 cm wide with standard thicknesses normally between 15 cm and 50 cm. Reinforcing steel wire rope provides bending resistance.

Slabs in prestressed concrete are usually produced in lengths of up to 200 meters. The process involvesextrudingwet concrete along with the prestressed steel wire rope from a movingmold.The continuous slab is then cut to required lengths by a large diamond circular saw. Factory production provides the obvious advantages of reduced time, labor and training.

Another fabrication system produces hollow-core floor slabs inreinforced concrete(not prestressed). These are made on carousel production lines, directly to exact length, and as a stock product. However, the length is limited to about 7-8 meters. Especially in Belgium, this method is widely used in private housing.

To meet modern standards (both hollow-core and massive slab) ofsoundproofingthe floor needs to be covered with a softfloor coveringthat is able to dampen the sound of footsteps or a floating floorscreedshould be installed. An alternative is to put a strip of rubber underneath the floor slabs.

Hollow-core slabs and wall elements without prestressed steel wire can be formed by extruders. The size of these elements will typically range in width from 600 to 2400 mm, in thickness from 150 to 500 mm, and can be delivered in lengths of up to 24 m.

The voids of the hollow core can be used asconduitfor installations. The interior of the core can be coated in order to use it as aventilation duct.

Bridges

Hollow-core slabs are also used in the construction ofbridges.In many cases the slab is moldedin situ.

See also

• Hollow structural section