Friday, October 17, 2014

Ceci n'est pas du béton?

I recently visited a factory that makes prefabricated concrete parts. I was astounded by the amount of handwork and artistry is involved in making the pieces, just as I was at the plywood factory. It turns out that brick - which has the connotation of being close to the hand and human scale - is in fact the construction piece that can be fabricated with the least human contact. (see here)

Concrete, the actual substance of cement mixed with aggregate, is the least of the ingredients. Prefabricated concrete parts are actually made of a lot of carpentry and a lot of metalwork. The density of metal was impressive. Each rod has to be bent to the right shape and tied into place with wire.

The bending of the metal is done mostly by machine, especially for the repetitive pieces. But the wiring is done by hand. I watched a worker deftly twist many pieces using a kind of large crochet hook.

Reinforced concrete is probably the construction technique that has had the most impact on architecture. It seems like an ideal match. Steel is strong in tension but rusts easily, and concrete is strong in compression, starts off as a fluid that can flow around steel bars, and sets to a durable finish. However, there are still some problems that have to be addressed.

The first is getting the concrete to flow around the steel bars without them sinking. They have to almost be suspended within the mold. To solve this issue, there are little plastic caps that are tied to the rebar. The hollow form minimizes the gap within the material and the dome shape allows for a minimal point of contact with the outside surface.

The second problem is concrete's poor insulating capacity. The smooth continuous surface of concrete is a useful finish to have both inside the building and out. But there must be insulation in the middle. They make sandwich panels by pouring two walls of concrete with solid foam insulation between. First, the outside finish is poured, because the nicest surface of the concrete is always the one that was at the bottom touching the mold. Then, the layer of insulation is put down and the second wall is poured using that insulation as the formwork. There are special stainless steel hooks that link the two walls. The pieces, when finished, are airtight and waterproof.
To change the nuance of the outside finish, they can change one of four things- the combinations of these four factors are almost infinite. First is the colour of the cement, grey or white. Next is the colour and size of the aggregate. Next, to put more colour, they can add powdered oxides. Yellows, browns and reds are easy to achieve (and cheapest) because these can all be obtained from iron oxide. To make green, they add copper oxide. Blue, the most expensive and hard to make, is from chromium oxide. Finally, they can change the finish. It can be smooth from the mold, sanded, acid-washed, bush-hammered. For one project they spread sand on the bottom of the each mold and poured the concrete on top to make a finish that was rough and three-dimensional.

The most amazing thing about the factory, though, was its Escher-like stockyard with pieces of buildings lying around- walls with doors to nowhere, spiral staircases. A lot of the pieces are tests and failures - most finished parts are delivered fairly quickly to the construction site, and do not hang around for long. But these failures have an appeal of their own.