The use of a structural topping on precast prestressed planks enhances the structural capacity of the floor due to composite action between the topping and the planks. Typically, a structural topping comprises a 75 mm thick layer of C25/30 normal weight concrete reinforced with fibre or fabric reinforcement.
As a result, the structural topping should not be confused with some types of screed that do not contribute any structural enhancement to the precast concrete planks.
Precast concrete floors in steel framed buildings 2 Steel Construction Institute (SCI) Report P401 [2] indicates that if a structural topping is used typically, it is possible to increase the resistance of hollowcore planks by between 20% and 60%.
Where a structural concrete topping is used, opening of cores for tying purposes may not be required. The reinforcement in the topping is usually sufficient to comply with the tying requirements. However, the specific design must be checked by the frame designer.
Common problems
- Continuity in programme
- Camber in planks when calculating topping depth
Solutions
Camber
Prestressed hollowcore slabs will exhibit a degree of camber. The extent of which will depend upon the span and the amount of prestress within the design. Due allowance must therefore be made for this in determining finishes and the overall floor thickness. The specified thickness of structural topping must be provided at the point of maximum camber.
The amount of camber will usually depend on the length, depth and strand pattern of the plank. For example, where the topping thickness is detailed to be 75mm at the supports, if there is a 25mm camber on the planks, at mid-span the topping thickness will be only be 50mm. Therefore, to achieve a minimum topping thickness of 75mm at mid-span the topping thickness at the supports needs to be the minimum thickness plus the maximum camber.
Correct Specification, design and coordination with principal, engineers, fabricators and precast designers Is essential to ensure an efficient solution is provided.
Programme continuity – end to end
Once the planks are installed and prior to concrete topping shuttering is required to the perimeter of the structure and all internal voids usually in the form of steel edge trims. This would usually be a return visit for fabricators and further coordination for the main contractor. PSS offer the continuity of lifting and placing precast planks followed by the grouting, stud welding, installation of edge trim shuttering, shuttering to columns, reinforcement and concrete topping all in one contract.
Sources and Links:
Concrete Society –
https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=924
MPA Precast –