Concrete formwork - Advice on shuttering

[Rick]

Going to be casting a large concrete slab floor for a combined garage & car port.. structural calcs done … aprox 6.6m x 8.5m
300mm depp x 450 wide thickened at edges and centre wall, rest is 150mm thick (with a lot of steel)

Anybody done any shuttering recently for something this size …

Assume I use pylwood shuttering with a wood backing frame at top, bottom, edges and presumably at something like 600mm centres.

What thickness shuttering ply ?
What size for the support framing .. 50 x50, 75x50?

Guessing that I will use some 45 degree struts to help brace this in place, and probably some steel pins (15mm diam x 900mm) … driven in at corners and maybe at 2m intervals, to stop form work bowing when the pour is made.

Open to any advice if somebody has done this recently.

[Stuarty]

ive used old joists, which suited the height i required perfectly, ive also used chipboard, with 50x50 legths for framework. i also use threaded rod all the way through it to stop the sides from bowing

[Tony McC]

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.

[Rick]

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
|
V
_
| | raft side
-----------“ | |
“ | | __/




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.

[Tony McC]

"Sprag" is just another word for "brace" but usually refers to a timber strut rather than summat like an acrow.

If you backfill around the base of the shutter, it will need to be welll-compacted to ensure it will actually prevent the shutter moving when the concrete is poured. Loose backfill is a waste of time, and if it is well-compacted, you then have the fun and games involved in digging it all out again when you come to strike the formwork.

Further, the backfill does nothing to support the top of the shutter, so you'd still need some form of bracing against the top waler.

20mm dia pins - I'd buy a few 6m lengths of 20mm rebar (T20s) and saw them up as required. I can't recall the cost, but they really aren't all that expensive, and you can always sell them to a groundworker when you've done with them!