They look dull and, in fact, if you've seen one concrete masonry unit—or concrete block or CMU—you may think you've seen the lot of them: Usually they're rough, gray oblong blocks formed from water, sand, crushed stone, and cement, with two big holes in them. Many, in fact, are more hole than concrete.
Yet concrete blocks are the potatoes of common architecture—sort of ugly and endlessly useful. They're laid and stacked in bonds like bricks from the lowest basements to the highest floors of a building, as foundations, inner and outer walls, mechanical core linings, and, not least, the main line of safety around fire stairs. Even without a fire, they protect us more than we appreciate. Architects and contractors rely on concrete blocks—8 billion of them produced in North America in 2007 alone—for their compressive muscle. Blocks can carry weight like other, more plastic incarnations of concrete, often with steel rebar and poured concrete inside the cavities. But, given their déclassé look, as often as not they're hidden like underwear behind brick veneers, ceramic tiles, modular panels, or exterior finishing systems. Or they're simply glazed or slapped with coats of paint.
Concrete block's value lies in its versatility—certainly not in portability. Because concrete is heavy, it is generally made close to where it is used; otherwise, it costs more to ship than it's worth (which is currently about $1.30 per unit). The Atlanta metro area has seven concrete block production plants. One of them, in Woodstock, near a highway where the suburbs turn not quite rural, sends most of the nearly 3 million units of concrete block it makes each year to building sites within a 50-mile radius.
On a November visit to the Woodstock plant, the morning's work was narrated by the plant's manager, Ray Spurling, when it was possible to hear him over the noise of the machinery. The visit was arranged by the North American division of Lafarge, the France-based cement and aggregates company that owned the plant. (Coincidentally, the same week, Lafarge sold the Woodstock plant, along with several other Southeast assets, to Oldcastle, the Atlanta-based building materials company.)
Spurling says the Woodstock plant's six employees make about 1,125 blocks an hour during nine-hour weekday shifts (except on Fridays, when the plant closes). An individual block takes about 6.5 seconds to mold, but the entire cycle takes 24 hours before the block is fully done; other plants may operateon 18-hour or 36-hour cycles. At any given time, there are up to 20,000 blocks—or what Spurling calls "8-inch equivalents" if they are other than standard size—in some stage of production.
"It's a continuous cycle," Spurling says. "Once you're making one, you're pulling one" off the production line. "If one side shuts down, both sides shut down."