Cementing the Future

Last autumn a 38-year-old retaining wall on my property was not looking fit for winter. Cracks had opened and the parge coat was peeling away.  Repeated freezing of moisture in the cracks surely would lead to more damage. With a bucket of cement and a trowel I hoped to stave off trouble. So far so good: even after the winter we just had, an inspection today shows the wall to be intact.

Cement and its grittier brother concrete (cement plus aggregate – usually stones though it can be other stuff such as fiberglass fibers) are marvelous materials. It is hard to find contemporary structures beyond the most rustic that don’t rely on them – at least for footings and foundations. Multistory commercial buildings are now more often framed with reinforced concrete than with steel. You can pour concrete into any shape, it won’t catch fire, and, while not indestructible, it is pretty hard to damage.

My first significant hands-on experience working with concrete was during my high school years. It was a summer job on a construction site. A basic grunt laborer, I wheeled barrows of it to sections of basement floors that the concrete truck’s chute wouldn’t reach and then rough-leveled it with rakes; the masons smoothed everything afterward. Outside, I did the same on patios after first hacksawing and setting rebar (steel reinforcing rods). Doing that for 8 hours a day doesn’t qualify as fun, but it did give me a better appreciation for the stuff.

Modern concrete dates only to the 18^th century. Making it is not an obvious process. The primary ingredients for cement are limestone (calcium carbonate) and silicate rocks, but, if all you do is crush them to powder and blend them with water, all you will get is mud. When the mud dries it will be powder again. To turn the mixed powder into cement you have to change it at the molecular level; to do this you heat it to an intense 1450 degrees C at which point the powders reform into calcium silicate. Now you’ve got cement. When you mix this with water and gravel you have concrete. Concrete doesn’t harden by drying in the usual sense; water is an ingredient that becomes part of the molecular structure of the hardening material. Internal water molecules continue to form new bonds long after the material feels dry to the touch, so concrete toughens as it cures, often for years.

The process for making Portland cement (so-called for its similar appearance to Portland stone) probably would have been discovered later than the 18^th century had not the folks of that era already known that it was possible. They knew because the ancient Romans had used concrete on a grand scale to build bridges, aqueducts, and buildings, many of which still stood. The Pantheon in Rome (completed 128 AD and still standing) to this day has the largest unreinforced concrete dome ever built. Roman techniques were forgotten in the years after the empire fell, but there obviously was some trick to them that only needed to be uncovered.

So how did the Romans figure it out in the first place? They cheated. More fairly, they got lucky. By geological happenstance, volcanic activity near what would become Naples was amid layers of limestone alternating with ordinary silicates. Superheated volcanic ash from this material formed a natural cement powder that accumulated over millions of years. The Romans needed only to dig it out of the ground. The mix wasn’t identical to modern Portland cement, but it was close enough to work. The Romans improved on the natural mix by adding lime: a single step which they worked out pretty early.

The problem with concrete, as with natural stone, is that it is amazingly strong in compression but very weak in tension. If you support a concrete horizontal beam at each end over an open space, the beam is under compression along the top but under tension along the bottom; so, it is likely to crack at the bottom. The crack will travel to the top and the beam will fail. This is also true of stone, and it is why all pure masonry structures need either arches or a lot of pillars; stone and concrete horizontal beams can’t span large spaces. This problem was solved in the 19^th century by Parisian gardener Joseph Monier. He wasn’t interested in construction. He just wanted flower pots that wouldn’t break as easily as ordinary clay or concrete pots. So, he embedded steel cages in flower pot molds and poured concrete into them. The technique worked perfectly, and the larger applications were immediately obvious. Steel excels in resisting tension, so a concrete horizontal beam reinforced with embedded steel rods can span long distances just fine.

There is very little that can’t be made out of concrete, but there probably are some things that shouldn’t be. More than a century ago, Thomas Edison, saddened by recent fatal fires in the news, decided that the answer was fireproof concrete homes filled with fireproof concrete furniture. He built his own cement plant in Stewartsville, NJ and built some demonstration poured-concrete houses (they still exist). He then produced concrete beds, concrete chairs, concrete sofas, concrete tables, concrete cupboards, concrete bureaus, concrete phonograph cases, and a concrete piano. They didn’t catch on. Go figure.

ZZ Top - Concrete and Steel