Substrate Preparation – Screeding
Bonded Screed
ARDEX A 35 MIX can be applied as a bonded screed by first preparing a grouting slurry using an appropriate mixture of ARDEX P 51 Concentrated Water Based Primer & Bonding Agent diluted with an equal volume of water. To this, add ARDEX A 35 MIX Powder until a creamy consistency is achieved. This grouting slurry should then be applied to a suitably prepared concrete base, which must be hard, sound, and free from any materials that could inhibit adhesion. While the slurry is still wet and workable, the ARDEX A 35 MIX must be placed and compacted onto the base in a ‘fresh in fresh’ manner.
Unbonded Screed
For unbonded screeds, ensure the concrete slab is relatively flat and true before laying a proprietary damp proof/slip membrane. If leveling or filling is needed for uneven areas, refer to the ARDEX A 46 datasheet for localized areas and the ARDITEX NA datasheet for larger areas.
Floating Screed
For floating screeds, place a suitable damp proof or separating membrane over the insulation before applying the screed mortar. In cases where the concrete base has insufficient drying (above 75% relative humidity), is directly on the ground without a proper damp proof membrane, or is exposed to rising damp, ARDEX A 35 MIX should be installed over an effective damp proof membrane. For both unbonded and floating screeds, a proprietary damp proof/slip membrane should be installed according to BS 8204-1: 2003+A1: 2009 and BS 5385-3: 2014.
For bonded screeds, first apply ARDEX DPM 1 C or ARDEX DPM 1 C R and allow it to cure. Follow this by applying ARDEX R 3 E Moisture Tolerant Epoxy Primer, fully sand-blinding while wet, and allowing it to cure. Afterward, the surface should be swept and vacuumed clean before applying the ARDEX A 35 MIX bonding slurry. Alternatively, ARDEX A 38 MIX Pre-Blended Ultra Rapid Drying Cement may be applied directly to damp concrete using ARDEX E 100 Additive as the bonding slurry. For more technical information, refer to the ARDEX A 38 MIX datasheet or consult Technical Services.
Substrate Preparation – Screed Repairs
When repairing dry internal screeds, cut back to sound material with vertical edges. Treat the cut edges with a bonding coat made from ARDEX A 35 MIX powder mixed with ARDEX P 51, diluted with an equal volume of water until it reaches a creamy consistency. Apply ARDEX A 35 MIX onto this bonding coat while it remains wet and workable. Ensure that the mix is workable by adding up to a maximum of 2 liters of water per 25 kg bag of ARDEX A 35 MIX. To promote rapid drying and strength, do not exceed the recommended maximum water content.
Mixing
Mix ARDEX A 35 MIX to achieve a normal screed mortar consistency. When squeezed in hand, the mortar should retain its shape and leave the hand slightly moist, without forming a water film on the surface when compacted. Mixing should be done using a pan, trough, or another forced action type of mixer, as normal ‘free-fall’ mixers are unsuitable for semi-dry screed mortars. Always use clean equipment and avoid adding other cements, lime, or screed additives.
Application
The working time of the mixed mortar is approximately 1 hour at 20°C, so all steps—mixing, placing, compacting, and trowelling—must be completed without delay. Limit the amount of mortar mixed and the area to be screeded so that finishing can occur within this timeframe. For new bays laid against hardened screeds, ensure that day work joints are vertical and treated with the grouting slurry. Apply ARDEX A 35 MIX only when temperatures are above 5°C.
Application Over Underfloor Heating
After ARDEX A 35 MIX has been applied over a hot water floor system, wait three days before gradually increasing the water temperature to a flow temperature of 25°C, maintaining it for an additional three days. The maximum floor temperature can then be used and sustained for a further four days, while avoiding draughts across the screed. After this period, allow the floor to cool to room temperature (above 15°C) before installing floor coverings. Note that ARDEX A 35 MIX screeds can withstand thermal loading of up to 65°C (water temperature).
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