concrete forms: plywood thickness?

Plywood Thickness for Concrete Forms

When forming structures such as pilasters in a gap of a stacked block wall, the thickness of the plywood used for formwork is crucial. A common problem encountered during pours is that the concrete tends to fill up rapidly, resulting in significant hydraulic pressure against the formwork. As the vibrator is used, this pressure can cause the plywood to bow and distort, especially between any bracing. In such cases, the edges of the plywood adjacent to the block wall can lift, and any anchors drilled into the blocks may be compromised.

To ensure stability, it is essential to install through ties as close to the vertical edges of the forms as possible. Additionally, placing more through ties in the field can help, depending on the dimensions of the pilasters. Columns often fail dramatically during a pour, typically resembling a zipper bursting from the bottom up.

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Understanding Concrete Forms

When constructing concrete slabs, it's vital to position the top of the forms at the precise height desired for the finished slab surface. The height of the form determines the final level of the slab, as concrete finishers utilize screed boards—either long straight 2x4s or aluminum pieces—to level the concrete by dragging along the tops of the form boards. This process continues until the concrete aligns with the form height.

To illustrate, if the form boards are set higher than the intended slab height, the resulting slab will also be elevated accordingly. The visual representation should show the top of the gravel approximately four inches lower than the tops of the forms, ensuring a uniform slab thickness, except where it may be thicker in areas such as footings.

In '92, my father and I built the forms for a monolithic slab for the 1,600 square-foot house where I currently reside. We utilized 2x12 lumber, securing them to 2x4 stakes placed every four to five feet. This personal experience confirms the importance of maintaining the top of the forms at the correct elevation to achieve the desired slab thickness.

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