Knowledge Centre

Requirements for efficient installation of hollow-core slabs

Building contractors can facilitate rapid installation of hollow-core slabs by providing clear access to the site and structure. This is in addition to providing a solid working platform around the structure to support the mobile crane that is used to lift and place the hollow-core slabs.

Load-bearing walls need to be level, or a level mortar needs to be placed to allow for a flush ceiling.

Because floor panels are cut to suit each individual project, delays will be avoided by ensuring that the structure has been built strictly to the design.

Moreover, window and door openings need to be supported using props while the slabs are being placed.

Lintels need to be used in openings of up to 2 m, and five courses of brickwork are required with Brickforce in every layer.

In terms of openings that exceed 2 m, refer to the structural engineer for details of the additional steel and support required during the installation of the hollow-core slabs.

Grouting materials are supplied by the customer and need to be available on the day that the installation takes place.

Finishing-off the soffit slabs

The smooth soffit, or underside of the slab provides a high quality finish and, therefore, does not require plaster.

To ensure an even higher quality outcome, the V-joint between the panels should be raked clean after grouting, and a small quantity of ‘Acrylic sealant’ inserted to provide a neat finish.

Closing the joints with plaster is not recommended, but rather apply a coat of bonding liquid to the soffit followed by a layer of textured paint.

Screeding specifications

A screed bonds very well to the top surface of the slabs. If the recommended procedures are followed, it will be impossible to remove the screed after five days of placement.

Use a 50 mm leveling screed inside the structure and refer to the consulting engineer’s design specification for joint movements.

Before screeding, remove all loose material from the surface, and then wet the slabs thoroughly and allow standing water to evaporate.

The ideal screed mix comprises a 1:4 ratio volume of cement and natural concrete sand.

Water is added until the mix is relatively dry, but workable enough to provide a compact finish.

About a 50 mm-thick screed is laid to level out any camber in the slabs and then floated using steel. The screed is kept moist for 48 hours to prevent shrinkage cracks and to allow curing.

Meanwhile, a 4 mm wire is placed in a square pattern at 200 mm centres in the leveling screed of balconies, roofs, walkways and other exposed areas of the structure to prevent cracking due to temperature and moisture variations.

A similar approach must be adopted for slabs that are to be tiled. Allow for expansion joints every three meters in the tiles, especially where the section changes shape, such as at doorways. A flexible tile adhesive is also strongly recommended.

In terms of car parks, a 5,6 mm wire is placed in a square patter at 200 mm centres, before applying a structural topping of at least 50 mm. A rough finish is acceptable in this application.

Where a 50 mm screed depth is required, lay the mesh flat on top of the slab. In cases where it exceeds this depth, lay the mesh 20 mm from the upper surface of the screed.