Concrete slabs to accommodate masonry walls and tiled floors - Electronic Blueprint
Structural Engineers are required to design and specify suspended concrete
slabs and concrete slab-on-ground construction that is both structurally adequate and
serviceable, including:
- Limiting deflections so supported walls (particularly masonry) are not subject to cracking;
- Limiting cracks that disrupt floor coverings such as tiles.
Insulation of Tiled Patio Slabs Functioning as a Roof to Rooms Underneath
Construction of this type is covered by AS 3958.1 "Guide to the installation of ceramic tiles"
Clause 3.3.2.1 for "Exterior floors, decks and roofs". This specifies ceramic tiles, cement-based
adhesive, waterproof membrane and concrete (with a statement that "Slab should be sloped for
complete drainage). The clause covers two cases. One is without a membrane, but with positive
drainage below the slab. The other is for a suspended slab with a maximum deflection of 1 in
360 [and with a membrane].
However, there appears to be a problem with this specification - In most cases, the top
surface of most suspended slabs will be constructed level, and the falls will be taken up in a
screed immediately under the tiles. If the membrane is above the screed it will drain, but if it
is below the screed (on the level slab) it will not drain. This may not be a problem, provided
large quantities of moisture are not being accumulated. The fact that the standard does not address
this question gives cause for concern.
Notwithstanding the above-mentioned failure to address falls, screeds and whether membranes must
slope, AS 3958.1 Clause 3.3.2.1 does not address the inclusion of insulation. Nor do any of the
other diagrams (including Figure 1.1) address the question of insulation. In contrast, Figure
1.1 of AS 3958.2 "Guide to the selection of a ceramic tiling system" does address this question,
showing an insulation layer sandwiched between two impermeable layers. Clause 5.6.2 provides some
limited specification. When an insulation layer is incorporated within a tiled floor system, it must
have sufficient compressive strength and rigidity to prevent movement and subsequent cracking of
the tiles or grout. The properties of extruded polystyrene board are shown in AS 1366.3. As a
starting point, it is suggested that one of the higher strength boards e.g. Class VH or H be
specified (165 kPa or 135 kPa at 10% compressive strain). Although these strengths are very
high compared to the actual load likely to be applied, so is the 10% strain. 0.5% strain in 100mm
of board represents 0.5 mm of movement (still enough to cause a crack) and would be caused by 8.2
or 6.7 kPa respectively. It is strongly recommended that the manufacturer's recommendations be sought.
Suspended Slab Deflection
- If masonry walls are to be constructed over suspended concrete slabs, the deflection is limited by
AS 3600 Table 2.4.2 to span/800 for spans and cantilever length/400.
- If tiles are fixed to floors or walls that are particularly flexible, they will become loose and
drummy. Ensure that all substrates (e.g. concrete slabs, timber joists, flooring, studs and wall
cladding) have sufficient stiffness to avoid excessive deflection. In the absence of manufactures'
recommendations to the contrary, the same criteria as recommended in AS 3600 Table 2.4.2 for masonry
walls would be reasonable.
Substrate Curing
- AS 2870 Clause 5.3.7 has provisions for the detailing of slabs to minimise shrinkage in tiled areas.
Tiles should not be fixed to concrete slabs until the concrete has cured for at least 6 weeks. If
the slab does not include additional reinforcement to inhibit shrinkage, the tiling should be delayed
at least 12 weeks.
- Tiles should not be fixed to concrete masonry until it has cured for at least 28 days.
- For clay masonry substrates, a sand-cement render coat should be applied and allowed to cure for
at least seven days before tiles are applied. Recesses in Concrete Slabs In order to achieve falls
in tiled floors and to reduce the step between tiled and untiled rooms, it is preferable to recess the
concrete slabs. Particular attention is required to ensure that the correct cover to steel reinforcement
is achieved. The effect of reduced structural slab thickness, to account for the recess, should also be
considered when assessing the slabs for both strength and deflection.
For further information on this topic, or for relevant Continuing Professional Development Distance
Learning Packages, please contact Electronic Blueprint at info@electronicblueprint.com.au.
For further information contact:
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