A Fine Line

That’s an old picture of the Cleft Rock Bridge in Prospect Park, built in 1872 as one of the picturesque bridges that Olmsted and Vaux liked to use for grade separations in park designs. It’s a nice looking bridge, although not remarkable within O&V’s work. What makes it remarkable is that it’s not cut stone or brick like most of the other bridges in Prospect and Central Parks. It’s “coignet stone” which is a fancy and early form of precast concrete. It’s pretty much the same material that fifty years later would be commonly referred to as cast stone.

(In passing, I want to mention that the cast stone for the bridge was manufactured by the New York and Long Island Coignet Stone Company. We are proud to have worked on the restoration of the headquarters building of that company, which has exterior walls built of their product.)

The bridge, and the NYLICSC output in general raise a question that has been batted around for some 150 years: where’s the dividing line between masonry and concrete? At one end of that spectrum, you have ornate carved stone with physical properties that are significantly different from those of concrete; at the other you have structural reinforced concrete that is used in forms that masonry cannot match. In the middle, you have structures like the bridge, which uses precast concrete in forms that could be reproduced in stone but were easier to build in concrete, or the Pantheon, which uses mass concrete in a manner similar to that of unrefined stone.

If the defining characteristic of concrete is that it’s placed in situ, than the 2000-year-old Pantheon is concrete and the 140-year-old bridge is not but rather a form of masonry. If the defining characteristic is reinforcing, than neither is concrete. If the defining characteristic is industrial production, than the bridge is concrete and the temple is not. In other words, the answer you get depends on how you phrase the question, as so often seems to be true when we’re looking at the history of building technology.