This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Vaults are sometimes built of a combination of brickwork and concrete, using light brick arches and backing up with concrete. They are also built entirely of concrete, the concrete being rammed on the centers in the same way that concrete footing courses are laid.
271. The ancient Romans adopted a method of constructing composite brick and concrete or masonry arches, which is shown in Figs. 122 and 123. A light center of wood, shown at a, Fig. 122, was used, and on this, brick arches, as shown at b, were built. These brick arches were called armatures, and as they are really the support of the vault, only very light wooden centers were required.
Fig. 123 shows the same form of arch with the spaces between the armatures filled with the concrete.
When vaults intersect each other, they are termed groined vaults, groined being the term applied to the curved intersection or arris of simple vaults crossing each other at any angle.