It appears to be a very common material that is available everywhere, and because it is generally used in only one way we think of it as being very one-dimensional. However, concrete possesses many variables. Every concrete mix and pour has a different character. It is not like steel or glass, which has a more consistent nature. Concrete can vary greatly. Concrete has a depth of expression that changes with every use. Le Corbusier used concrete as if it were clay. He used its plastic quality almost as if he were sculpting. Louis Kahn used concrete as if it were hard steel. The same material—two very different effects.
There is also the rebar—the reinforcing bar—and this is very important. The rebar is like the bones of a human body. The concrete is like the flesh. I think of a poured-concrete building as a metaphor of the human body. If you have a thick bone and not enough muscle and skin, the bone will start to stick out. Or if you have too much skin with no sense of bone, the building will look fat and bloated. It is very important to maintain the precise space, the right amount of space between the rebar... Poured-in-place concrete is a kind of handmade building.
As you know, there are wide variations in color from white to gray to black. Concrete can be very rich in color. I see color there in terms of depth rather than surface. The gradations of color create a sense of depth. If you only look at color in terms of projecting out from the surface, then you fail to see the depth.
I am among a number of architects who use concrete because of the freedom it allows me. I like concrete because you can invent forms, which allows me to create new kinds of spaces... Concrete gives the illusion that it is easy to work with because you can pour a long wall relatively quickly rather than having to build it brick by brick.