Final answer:
A moisture barrier must be placed between a concrete form and base plate to prevent moisture absorption and related structural damage.
Step-by-step explanation:
The barrier that must be placed between a concrete form and a base plate is a moisture barrier. This barrier is crucial in preventing moisture from the ground or underlying surfaces from being absorbed into the concrete, which can lead to degradation, freeze-thaw damage, and other structural issues over time. The moisture barrier, typically consisting of a durable plastic sheeting or other impermeable materials, is especially important in situations where the concrete will serve as a foundation or floor slab and is in direct contact with soil or other potentially damp surfaces.