Page 372 - McD_6118-AEW-XX-XX-NBS-A-001_Neat

260 FLOATING CONSTRUCTION
• Insulation: site specific.
- Installation: Loose lay on substrate.
- Joints: Butted.
• Separating layer: Polyethylene sheet minimum 125 micrometres thick (500 gauge).
- Installation: Loose lay over insulation.
- Joints: Lap and seal with tape to prevent penetration of self smoothing screed.
• Edge strips at perimeter abutments: 5 mm thick resilient polyethylene foam.
- Installation: To full depth of screed.
- Joints in strips: Lap with separating layer and seal with tape.

BATCHING AND MIXING

303 AGGREGATES
• Sand: To BS EN 13139.
- Grading limits: In accordance with BS 8204-1, table B.1.

305 BATCHING AGGREGATES
• Mix proportions: Specified by weight.
• Batching: Select from:
- Batch by weight.
- Batch by volume: Permitted on the basis of previously established weight:volume
relationships of the particular materials.
- Gauge boxes: Accurate. Allow for bulking of damp sand.

310 MIXING TROWELLED SCREEDS
• Mixing:
- Mechanical mixer: Suitable forced action. Do not use a free fall drum mixer.
• Materials: Mix thoroughly to uniform consistence.
• Admixtures: Do not use other than the basic accelerator.
• Water content: Minimum necessary to achieve crystallisation of synthetic anhydrite
and full compaction.

315 MIXING SELF SMOOTHING SCREEDS
• Dry preblended materials: Batch using completely full bags only.
• Materials: Mix thoroughly to uniform consistence.
• Water content: Sufficient to achieve correct flow rate for screed.
• Flow rate test of mix batches:
- Test method: Flow ring test.
- Timing: Before pumping screed and at strategic intervals during screeding.
- Criteria: Screed manufacturer's recommended values.
- Results and frequency of testing: Record.

LAYING

325 LEVEL OF SCREED SURFACES
• Permissible deviation (allowing for thickness of coverings): ± 5 mm from datum.

326 SCREEDING TO FALLS 6118-AEW-XX-XX-NBS-A-01
• Screed cover: Maintain minimum thickness at lowest point.
• Falls: Gradual and consistent.
- Gradient (minimum): site specific .

RotF generic (intermediate) NBS 2014