Solid As A Rock?

Solid As A Rock?

   Solid as a rock, yet often ignored.  The pillar of strength, yet often stepped on.  Made of earth from which it came, yet usually covered.  As ancient as Rome, yet still waiting to be fully discovered.  One speaks, of course, of concrete, something that has fascinated me since 1980 when I first began keeping notes on it, watching it evolve and being discovered in its different forms.  And admittedly, something I knew little about, despite it being almost everywhere.

   Concrete is now so prevalent that it we have entire bridges made of it, bridges with spans taller than the Eiffel Tower (concrete is now used for more bridge building than any other material), and is classically remembered as the material for the largest unreinforced dome in the world, now two thousand years old, the Pantheon in Rome.  Which is not so ironic since Rome led so many nations in innovation (one burial site uncovered by archeologists digging in Scotland found not gold but a Roman hoard of nearly 900,000 nails, a valuable commodity at the time and hundreds of years ahead of the knowledge of their warring enemies)...but Romans were blessed by a natural site, a volcanic mountain of almost natural concrete ("There were no concrete structures for more than a thousand years after the Romans stopped making it," says Mark Miodownik in his book, Stuff Matters).

   Cement is the basic ingredient, a result of high heat chemically melding aluminum- and iron-rich rocks, say calcium carbonate (limestone) and silicate (clay), and the heat has to be nearly twice that of the hottest wood fire.  Add water and there you have it...well, not quite.  Add too little or too much water and you have a bad and weakened batch...homes collapse (as you witness in third world sites hit by an earthquake).  The problem is that concrete doesn't "dry," per se; the water merely begins the chemical bonding (or "unbonding" as author Miodownik would say).  Indeed, this bonding or hardening continues for years. 

   But erosion and crumbling does occur, so engineers are working with adding bacteria and starch to concrete and creating concrete that repairs itself.  From Miodownik's book:  It was discovered that one particular type (of alkali-loving bacteria) called B. pasteurii could excrete the mineral calcite, a constituent of concrete.  These bacteria were also found to be extremely tough and able to survive dormant, encased in rock, for decades.  Self-healing concrete has these bacteria embedded inside it along with a form of starch, which acts as food for the bacteria.  Under normal circumstances these bacteria remain dormant, encased by the calcium silicate hydrate fibrils.  But if a crack forms, the bacteria are released from their bonds, and in the presence of water they wake up and start to look around for food.  They find the starch that has been added to the concrete, and this allows them to grow and replicate.  In the process they excrete calcite, a form of calcium carbonate.  This calcite bonds to the concrete and starts to build up a mineral structure that spans the crack, stopping further growth of the crack and sealing it up.

   New discoveries have concrete that acts as a filter, lightweight and porous and now being used on some streets and sidewalks (the water is purified as it filters through as well, cleansing out oils and other toxic materials);  And there is also now a textile version of concrete called concrete cloth.  This material comes in a roll and needs only water to be added for it to harden into any shape you like...its biggest application may be in disaster zones, where tents made in situ from rolls of concrete dropped from the air can create a temporary city in a matter of days, one that will keep out rain, wind, and sun for years while rebuilding efforts continue.  There even exists concrete that cleans itself, leaving its white or whatever color you make it free of dirt simply by its reaction with sunlight (it also cleans the air in the process).

   All in all, kudos to Mark Miodownik's book for reviving my fascination with concrete, for letting me know that the material continues to evolve and be rediscovered in ever newer ways.  Next time you pass an "ugly" concrete wall, just think of the possibilities to come.  Nothing really is as it seems, whether that's a fully-tattooed person or a stray cat in a field.  Behind that opinionated somewhat scruffy exterior may rest a slew of possibilities and potential...and now, that even applies to concrete!