What timber to use for formwork or shuttering concrete base

What type of timber would you use to construct the formwork (shuttering). There seems to be 4 different types that I could use.

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1. Planed
2. Sawn Untreated
3. Sawn Treated
4. Carcassing

My particular project is for a garage slab that will have a 100 mm sub-base with 100 mm of concrete ontop e.g. The image below shows the sub-base and the proposed formwork.

Post: # Post Rick
Tue Jun 20, 11:47 am

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Tony McC wrote:

18mm ply, liberally daubed with soap oil, and reinforced with 100x50 walers top and bottom and at 300mm centres.

For your bracing, 2 metre centres is awfully risky. You can never over-brace this type of job, but it's almost impossible to fix a brace that's collapsed under pressure, so use 1m centres as a minimum. One 'trick' we'd often use would be to have long beam, usually a 200x150 timber or an RS:laugh: , and have that positioned parallel to the formwork, set back by a metre or so, and pinned to the ground with stakes or tirfors at 500mm centres. We'd then sprag off this at 500-750mm centres. The advanatge is that such a large beam is unlikely to give and the point loads imposed from the formwork braces is spread over a longer beam and so there's less risk of individual failure.

OK .. getting there .... many days of toil.
Have the shuttering initially built ... 18mm ply with 75x50 (on edge) longitudinal support well nailed top and bottom, and vertical 75x50 supports at 600 c/s.
A lot of wood !

Gives me a 8.6 x 6.6m box ... 300mm high
On one side the shuttering runs 50mm away and parallel with a fence where I have concrete H posts concreted in at 1.8m centres .. I'll pack off that, to provide bracing for that side.

As to the other sides ...

You say 'sprag' off the shuttering, what does sprag mean? I assume you mean some form of diagonal brace ?

I looked at your pictures on shuttering, it would be difficult to achieve the diagonal braces as the shuttering is 'dug down' and to dig out sufficient to put in the 60 degree struts every 600mm would mean a huge amount of material removal ..
I have thought of backfilling against the shuttering ... this would get me at least half way up the 300mm.
Not sure if following Ascii Art will help or not :


Could backfill here
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Then would then need to bracing the top ...
A colleague mentioned getting hold of mini acrows ... but can't find any under 1m in length, he seemed to think there are 400mm versions.


Another option would be to sledge in 20mm diam pins maybe 600mm long every 1.2m, but can you hire these ? it would be damn expensive to buy them to waste after this one job.

Welcome any suggestions.

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