15.0 screeding to level

A floor ‘screeded to level’ is precisely that. The top most point of the supporting structure is chosen, (usually at a buildings columns) as the datum and the minimum specified slab thickness is measured from this point. The slab is installed by laser levelling and the top surface can be constructed flat with relative ease. The tradeoff however, is significant extra weight due to ‘ponding’ where supporting beams and the tray itself have deflected under the weight of wet concrete.

Where a floor system is ‘screeded to level’ and is supported on a series of rigid permanent supports along with rigid temporary propping, ‘ponding effects’ occur only within the slab span itself. In this case the max slab thickness is limited to the slab thickness, plus the allowable maximum soffit deflection of the slab. For a proposed span of 3000mm this equates to a maximum increase in thickness of 3000/130 = 23mm or approximately 0.40 kPa of uniform loading. On low level construction of one or two levels the positive effects of minimizing ponding of ’screeded to level’ floors by using rigid temporary propping allows the most economical use of primary /secondary beams.

Where a floor is ‘unpropped’ an is ‘screeded to level’, slabs are generally supported on flexible structural steel primary and secondary beams. HERA REPORT R4-107:2005 specifies the following maximum deflection limits for primary and secondary beams. (section 1.3.3):
- All primary beams                                 10mm
- Internal Secondary Beams                     20mm
- Spandrel Secondary Beams                   10mm

While the actual deflection of primary and secondary beams depends on each particular project’s loadings, configuration and beam sizes, the imposition of maximum limits allows manufacturers to incorporate ponding effects based on the maximum limits. Based on these, at the worst location, at slab centre span of the secondary beams an additional 30mm of concrete must be accommodated for in the dead loading of tray flooring. The consequence of using ‘Unpropped’ slabs which are ‘screeded to level’ and supported on flexible beams are increased floor weights and shorter tray floor spans but compensate with minimal cost of propping and shorter construction times. In this situation, at the worst location, the total maximum slab thickness is therefore 53mm (being 23mm+10mm + 20mm) and can constitute a significant proportion of the slab dead load.

15.1 ‘normal’ degree of supervision

The advantage of trayflooring systems is their simplicity of installation and speed of construction. it is however important to ensure that the desired features associated with the floor are also assured. It is critical that the degree of ‘flatness’ and ‘levelness’ that a floor has been specified to have, is verified. Along with this the amount of weight which is carried by the supporting needs to also be checked. All floors therefore need to have a degree of supervision applied to them.

15.1.1 ‘normal’ supervision requirements for all floors

1. Where beams have been propped, or precambered, a string line shall be placed at an offset to the beams top flange and midspan deflections recorded for beams in place. No floor is to be poured where beams do not meet the tolerances of the project designer, and all beams failing in this fashion are to be referred to the designer. A record of all measurements along with any correspondence on such issues are to be kept as part of the contract.
2. Inspection of the surface flatness of all floor areas in accordance with part 3 inspection aspects of composite construction HERA REPORT R4-107:2005 are to be carried out and the records kept as a part of the contract. It is critical that the contract documents specify what classification in terms of clause 3.2.2 each section of floor is to comply with. These classifications are restated here for HERAS REPORT.

classification (1) – Non critical
Mechanical rooms, non public areas, surfaces to have raised computer flooring, surfaces to have thick set tiles and slabs in car parking buildings.

classification (2) Critical:
Carpeted areas of commercial retail and residential buildings

classification (3) Critical:
Surfaces to receive thin – set flooring (eg vinyl).