How to lay Mandoval Vermiculite Lightweight Aggregate screeds
The information contained in this article is confined to roof screed applications.
For details on use in the horticultural industry click here.
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Vermiculite Roof Screeds . . .
* Are one fifth of the weight of normal concrete
Correct Proportions for roof screeds
* Keep buildings cool in summer and reduce structural movement in the roof caused by solar heat
* Conserve heat in winter
* Are easily mixed and laid without special equipment
* Can be easily laid to falls (1 in 10)
* Do not require expansion joints
* Are totally incombustible
* Provide a permanent non-warping base for all surfacing materials
2 x 8kg bags Medium grade Mandoval Vermiculite
1 x 50kg Bag Portland Cement
About 40Lt water.
Quantities required for a cubic metre
14 x 8kg bags Medium grade Mandoval Vermiculite
7 x 50kg bags Portland Cement
About 280 – 300Lt water.
1. Screeds placed on wooden surfaces will need a plastic undersheet.
2. Screeds are porous and need a waterproofing membrane.
Mixing Instructions :
To test for a suitable consistency, a handful of the mix when gripped firmly should just release a few drops of water and no more. The mixture should NOT be sloppy.
The following method is recommended:
BY MACHINE – Pour measured volume of water into mixer. Add cement and mix thoroughly to a consistent slurry.
Add vermiculite and mix until uniform distribution has been achieved, about one and a half minutes. DO NOT OVERMIX.
The Vermiculite concrete should be spread and leveled off, and then consolidated to the desired depth (minimum 50mm)
The Vermiculite screed should be covered while still wet with a sand/cement topping, finished level and smooth to receive the weatherproofing.
The thickness of the topping must be 10mm for normal purposes.
Precautions to be taken against rain, frost and heat.
Curing and Drying :
Since the principle of all insulation by means of lightweight concrete is the existence of air space, the screeds must be properly dried out before the weatherproofing finish is applied.
Drying time will vary according to weather and site conditions, and the usual precautions observed when laying normal concrete should be observed for Vermiculite concrete.
Properties of screed
1. Mass of dried screed 450 – 510kg/m³
2. Thermal conductivity K = 0.120 W/mºC
3. Compressive strength typically 0.5 mpa.
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