Hollow-core slab

  A hollow core slab , also known as a voided slab (German: Hohlkörperplatte, Swedish: hålplatta, Finnish: ontelolaatta) is a prefabricated slab of prestressed concrete typically used in the construction of floors in multi-storey apartment buildings. The slab has been especially popular in countries, where the emphasis on home construction has been on affordable housing, including socialist countries of Eastern Europe and the nordic countries.

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The concrete slab has tubular voids extending the full length of the slab, typically with a diameter nearly equal to the thickness of the slab. This makes the slab much lighter than a massive floor of equal thickness or strength. The slabs are typically 120cm wide with standard thicknesses between 15cm and 50cm. The concrete I-beams between the holes contain the steel rods that provide the majority of the tensile stress that holds the slab together under heavy loads.

The slabs are usually produced in a factory, typically in lengths of about 100 meters. The process involves extruding wet concrete along with the steel rods from a moving mold. The continuous slab is then cut according to the lengths required on the construction site. Factory production provides the obvious advantages of reduced time, labour and training.

Hollow-core slabs are poor sound insulators. To meet modern standards of soundproofing the floor needs to be covered with a soft floor covering able to dampen the sound of steps. An alternative is to use a thin "floating" slab of concrete insulated from the voided slabs.

However, as the hollow-core slabs are mainly stationary air inside, they are excellent thermal insulators, and they require very little building insulation. This is a contributory factor especially in countries and climates, where annual temperature variation can be great, such as in Scandinavia.

Bridges

Voided slabs are also used in the construction of bridges. In many cases the slab in molded in situ.