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Where a tray floor slab is specified to be ‘screeded to thickness’,
concrete is poured to a constant thickness over the full extent of the
slab. As a consequence, the top surface of the slab follows the deflected
shape of the primary and secondary beams, as well as the tray decking.
Screeding to thickness is generally specified to eliminate ‘ponding’
effects and to maximize allowable propping span lengths. The benefit to
a project is that concrete quantities are minimized, as well as the tota
l building weight, resulting in smaller supporting beam, column and
foundations sizes. The benefits accrue not only from vertical load
analysis, but also from seismic load analysis.
Two alternative construction procedures are possible, depending on flatness requirements:
14.1 significant variation in surface flatness permitted
Where a significant variation of floor level is permitted, such as industrial buildings, storage or car parking buildings, screeding to thickness using height indicators provides the most economical solution. Designs based on this procedure will:
Ensure minimum concrete quantities.
Allow primary/ secondary beam deflection requirements to be independent of slab thickness and ponding limitations. This will permit the most economical beam sizes and minimize any need for pre-cambering or propping of beams.
14.2 little variation in surface flatness permitted
Where all the advantages in (i) above are desired but where no significant variation in floor flatness is permitted, construction procedures must be most stringent.
Primary/ secondary beam deflections must be minimized and can be achieved by:
Propping secondary or both secondary and primary beam lines.
Pre-cambering beams so that wet concrete loading will bring beams back to level upon pouring
Trayfloor deflections must be minimized by either:
Decreasing slab spans to ensure that deflections are small.
Installing prop lines at midspan of slabs
14.3 ‘realistic’ assessment required
Before designers utilize ‘screed to thickness’ a realistic assessment of deflections should be carried out for all designed members to determine the practicality of achieving assumed conditions on site. If ‘extensive’ supervision is not available the ‘screed of thickness’ procedure should not be used.
14.4 extensive” degree of supervision
While it is desirable to reduce concrete quantities and beam/ column member sizes this can only be accomplished if strict supervision procedures are observed. The following ‘extensive’ supervision procedure must be used to ensure that greater / lesser thicknesses are not placed on floors being poured. All parties involved in a project should be preferably involved at the design stage to ensure that the objectives of the ‘screed to thickness’ procedure can be applied to a particular project. If a contractor is unable to confidently apply the ‘extensive’ supervision required below, then either the ‘screed to level’ procedure should be used right from the design stage, or an alternative contractor who is used to this type of procedure and who has stated that they are able to comply with the supervision requirements should be used for the construction.
14.1.1 extensive’ supervision requirements for floors ‘screeded to thickness’
A minimum of one (1) person is to be designated to oversee the construction and to be responsible for all aspects of the ‘screed to thickness’ technique, and record keeping.
Thickness indicators constructed from a material which will not affect the performance of the tray floor in any manner, nor downgrade in any fashion over time within the finished floor are to be installed over any floor ready to be poured. The distance in either direction between adjacent indicators is to be limited to a distance which enables a competent concrete worker to screed between indicators comfortably with a straightedge. In any case indicators should not be placed further apart than 3.0 meters. A photographic record of the placement of all indicators is to be kept of all floor areas, clearly showing the dimensions between indicators, as a part of the building contract.
Indicators are to be positioned accurately with permissible tolerances for placement of height of +3mm/ - 0mm.
It is at the contractors discretion whether or not to remove indicators from the slab, but if they are, removal of individual indicators must not occur until that area has been completely levelled by straightedge and screeding is not taking place between that indicator and the adjacent indicator.
The extent of floor covered by each concrete truck is to be recorded accurately on a floor plan of the area being poured. At the end of that trucks pour, a calculation of the thickness of the floor is to be carried out based on the volume of load from the truck and the area of the floor covered. As with the indicators, the tolerance for such calculations is to be +3mm/-0mm
If the calculated thicknesses do not fall within the specified tolerances, then immediate steps are to be taken to rectify the thickness, both in regard to the work just checked and also for the procedures to be employed for the next truck. Any steps taken must ensure that the slab thicknesses any area fall within the tolerances.
A check of the total pour carried out in one procedure is checked for the total truck volume and total area of pour, in the same manner as for that carried out for each truck, as detailed in (e) and (f) above.
All records of truck volumes, extent of each ‘truck pour’, total pour and thickness calculations are to be kept as a part of the building contract.
If the records show that the thickness has either been exceeded, or not met, the contractor shall provide alternative proposals, in regard to how the contractor shall rectify the defective areas.
14.4.2 additional requirements for slabs permitting significant variation in surface flatness
The rules for ‘normal’ supervision, detailed in clause 15.1.1 are to be carried out, but with limits which have been agreed between the client, the designer and the contractor. It is to be noted that where significant variations are permitted the classifications stated within clause 15.1.1 will not be realistic.
14.4.3 additional requirements for slabs requiring little variation in surface flatness
Where little variation in surface flatness is permitted, the rules for ‘normal’ supervision detailed in clause 15.1.1 are to be followed without exception, it is reiterated that work carried out to this type of specification is the most exacting and requires the greatest level of care. It would be expected that contractors carrying out work under this specification would have a sizeable history using trayflooring previously.