What timber to use for formwork or shuttering concrete base

When it comes to constructing formwork (shuttering) for concrete projects, selecting the appropriate type of timber is crucial. Recently, I discovered there are four primary options to consider:

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

For my current project, which involves pouring a garage slab with a 100 mm sub-base and an additional 100 mm of concrete on top, the choice of timber becomes especially important. The image below illustrates the sub-base and the proposed formwork design.

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

If you are seeking further insights, please visit concrete wall shuttering.

Tony McC wrote:

18mm ply, generously coated with soap oil, reinforced with 100x50 walers both at the top and bottom, spaced at 300mm intervals.

For bracing, spacing at 2 meters is overly risky. It’s crucial to over-brace this kind of construction, as fixing a brace that has collapsed can be nearly impossible. Therefore, I recommend a minimum of 1-meter spacing. A useful trick is to use a long beam, typically a 200x150 timber or an RS, positioned parallel to the formwork and set back by about a meter, anchored to the ground with stakes or tirfors at 500mm centers. Sprag off this beam at 500-750mm intervals. This approach distributes the point loads from the formwork braces over a longer beam, reducing the risk of individual failures.

Progress is being made after many days of hard work.
I have the shuttering constructed with 18mm ply and 75x50 (on edge) longitudinal supports, well fastened at both the top and bottom, along with vertical 75x50 supports at 600 c/s.
That requires a significant amount of timber!

The structure is designed to be an 8.6 x 6.6m box with a height of 300mm.
On one side, the shuttering runs 50mm away from a fence where concrete H posts are set in at 1.8m centers. I will pack that side for additional bracing.

For the other sides...

You mentioned 'spragging' off the shuttering—does that mean using some form of diagonal brace?

I viewed your shuttering images, and it seems challenging to implement diagonal braces since the shuttering is 'dug down.' Digging out enough space to install 60-degree struts every 600mm would involve significant material removal.
I'm considering backfilling against the shuttering, which might effectively reach half the height of the 300mm.
I'm uncertain if the following ASCII art will assist:


Could backfill here
|
V
_
| | raft side
-----------â'œ | |
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Then, I will need to brace the top.
A colleague suggested mini acrows, but I haven't found any shorter than 1m; he thought there might be 400mm versions available.

Another option would be to drive in 20mm diameter pins around 600mm long every 1.2m, but are those available for rental? Purchasing them would be costly, especially for one-time use.

I welcome any suggestions.

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