Which of these materials should you NOT use to form slab edges for a monolithic slab?

In the context of forming slab edges for a monolithic slab, precast concrete is generally not used for this purpose because monolithic slabs are typically poured in a single continuous pour. Precast concrete components are created off-site, cured, and then transported for assembly, which does not align with the requirements of forming edges during the pour. The formation process requires temporary forms that can hold the poured concrete in place until it sets, and these forms must be removable.

Materials like wood forms, metal forms, and plywood forms can all be easily shaped and removed when the concrete has set. They are designed to hold the concrete in the desired shape and can accommodate variations in construction layouts. Wood and plywood are particularly common because they are readily available, easy to work with, and can be adjusted on-site.

Using precast concrete for forming slab edges would complicate the process as it would not allow for the necessary flexibility during the initial stages of pouring the concrete, nor would it provide the capability for easy removal after the mix has cured. This reason underscores why precast concrete is not a suitable option for forming edges in monolithic slab construction.