For thousands of years masons have used temporary wooden forms (falsework) to support arches and other shapes that are not self-supporting until they’re complete (concrete forms are a type of falsework). But not every mason uses forms when building complex curved shapes. The mason in the video below is building a Catalan vault in Querétaro, Mexico and it’s just him, a trowel, mortar, and brick.
This type of vault has been built for hundreds of years—with and without using falsework. According to Michael Ramage, lecturer in architecture at Sidney Sussex College in the UK:
Catalan vaulting is a masonry technique perfected in Catalonia, Spain which relies on fast setting mortar for construction and structural form for strength. The origins of the technique are obscure, but likely come from the varied cultures of the Mediterranean.1 The technique was brought to the United States in the 1880s by Raphael Guastavino, and perfected as a monumental construction method used in hundreds of buildings throughout America. Among many well-known examples are the Boston Public Library, The hall of Ellis Island, Grand Central Terminal, and the Dome of St John the Divine in New York City.
When Ramage refers to “structural form” he’s talking about the arch-like shape rather than the form that might have been used during construction.
The mason in the video, Hugo Martinez, uses mortar made from cement lime and sand in a 1:1:8 ratio. As you can see, it is a loose mix but the brick is dry so it absorbs enough moisture to cause it to grab. When used as a roof, the top of the vault is overlaid with rebar and a layer of mortar or concrete.