Structural concrete - Unformed surfaces
Published: 17 August 2011 / BCA 2011
Direct-stick timber flooring is a system where 8-14mm thick tongue and grooved boards are glue fixed direct to the surface of a concrete floor slab. The system has four critical components: concrete surface, moisture vapour barrier, adhesive, and timber boards.
This is a dynamic system that responds to the room environment after it is installed..
If an evaporative airconditioning system is installed the relative humidity will increase, the boards will absorb the moisture and swell. This creates and/or adds to the potentially destructive stresses described above.
If a reverse cycle system is installed it tends to dehumidify the air, causing the boards to shrink, resulting in a widening of the gaps between boards. This change is generally aesthetic. It typically does not result in destructive stresses.
The direct-stick system does not have a substrate layer. The timber is fixed to the surface of the structural concrete. When the timber expands the force is expressed at the timber-to-concrete joint through the adhesive. Given that the tensile strength of commonly used timber species is 100-180 MPa, adhesives have a tensile strength of 2-3Mpa, and that the tensile strength of concrete is only 10% of the compressive strength (25MPa = 2-2.5MPa tensile strength), failure at the concrete surface is likely.
The machine trowelling (or helicopter) used to finish the unformed surface (top) of residential floor slabs achieves a “reasonable” surface that is intended to be covered. It may show trowelling marks and it is only suitable for standard floor coverings (carpet, plank-on-ply, standard ceramic tiles) because it may not achieve a consistently strong surface and may have variations in level up to 10mm in any 3 metre length.
The “specific” finish appropriate for a direct-stick system includes using +30MPa concrete to achieve sufficient surface tensile strength (typically exceeding that required for structural performance), then grinding the surface to remove the construction curing agent (this is incompatible with floor system moisture membrane-adhesive systems) and to remove any surface laitance (a weaker layer of cement and fines) that forms on the surface with machine trowelling.
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