A falsework is a auxiliary construction, which was used in the building industry to brick up arches. Today it is used foremost in concrete bridge building in order to form the material and the systems.
The wooden falsework is the basis of the brick arch. The single bricks are layed directly on the falsework. After achieving the right stability the       

falsework can be rebuild carefully. The arch is now stable by itself.

Falseworks are used since the acient world and are still important in todays contructions.
Depending on the form and the weight of the arch, the falsework is constructed and build.