This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
362. The centers for stone, plain concrete, and reinforced-concrete arches are constructed in a similar manner. A reinforced-concrete arch of the same span and designed for the same loading, will not be so heavy as a plain concrete or stone arch, and the centers need not be constructed so strong as for the other types of arches. One essential difference in the centering for stone arches and that for concrete or reinforced-concrete arches, is that centering for the latter types of arches serves as a mould for shaping the soffit of the arch-ring, the face of the arch-ring, and the spandrel walls.
The successful construction of arches depends nearly as much on the centers and their supports as it does on the design of the arch. The centers should be as well constructed and the supports as unyielding as it is possible to make them. When it is necessary to use piles, they should be as well driven as permanent foundation piles, and the load should not generally be heavier than that on permanent piles.