Rib and Block Slabs

Rib and Block Slabs from SPANCON

By: SPANCON  09-06-2010
Keywords: Building Contractors, Building & Construction, Housing Construction


Rib and block slabs are made up of precast concrete rectangular shaped ribs spaced 660mm apart. Non-structural hollow concrete rebated filler blocks are placed between these ribs. The size of the block determines the beam spacing and provides a flush soffit. 

A structural concrete topping should have a minimum strength of 25 MPa at 28 days and a minimum thickness of 40 mm, or 1/10 x clear distance between the ribs. Welded mesh reinforcement is placed in this topping to control possible shrinkage cracks. The filler blocks are available in different heights ranging from 110 mm to 285 mm which produces an overall depth of slab from 170 mm to 340 mm, or more if double blocks are used. This type of slab requires temporary supports at approximately 1,5 m to 1,8 m centres.

The advantages of rib and block slabs are as follows:

  • They provide an economical, versatile lightweight monolithic slab system. Components are relatively light and no mechanical handling is necessary.
  • Slabs may be designed as either simply supported or fully continuous.
  • They are ideal for soffit plaster but fixing of suspended ceilings is also easy and simple.
  • Electrical and plumbing services are readily catered for by omitting hollow blocks at specific locations.

Rib and block slabs are designed as a series of 'T' sections with the in situ cast concrete providing the compression flange and the precast beam the tension reinforcement. The ribs and the composite slab are designed for specific spans and loads and are reinforced accordingly, (complying with the relevant Code of Practice.) Two or more ribs may be placed together to accommodate concentrated line loads parallel to the span. If necessary, blocks may be omitted over the support to increase the shear capacity.


Holes can be made on site in the blocks between ribs to accommodate horizontal services. For larger holes, whole blocks can be omitted with shuttering fixed to adjacent blocks to restrain the concrete topping at the time of pouring. Holes larger than a single block must be referred to the supplier for approval, preferably at the initial design stage.

Rib and block floors have a good key on the soffit for plastering.

Rib and block slabs can be used in many applications, from domestic housing to industrial units.

Rib and block slabs are particularly suitable for irregular shaped buildings or where multiple cantilevers are present, but can also be employed in buildings of cellular construction, up to four stories and where uninterrupted long spans are appropriate.

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