It is well known that the Romans were great engineers and their structures have withstood the test of time. Through a millennia of battles to the erosive footsteps at the center of worldtourism, the roadways, temples, and ancient super-structures of the far reaching empire are holding up remarkably well.
In a recent study researchers from Italy and the United States analyzed each Mediterranean port constructed of Opus Caementicium (…or Roman concrete, whichever you prefer). All around 2,000 years old, the material strength and corrosion level of these ports, despite the coastal wind salts and the lapping the sea water, were nearly uncompromised!
Nowadays Portland cement is most commonly used in construction, and with an approximate sub-marine shelf life of 50 years, it is the Woburn High JV basketball team against the Celtics when compared to the Roman stuff.
A sample of concrete breakwater extracted from the Bay of Napes dates back to 37 B.C.. The mineral components were analyzed and the secret to the strength of Roman concrete lies in its unique mineral formulation and production technique – The Romans made concrete by mixing lime and volcanic rock.
Like any concrete, Roman concrete consists of an aggregate and hydraulic mortar (a binder mixed with water that hardens over time). They mixed the lime and volcanic ash for underwater coastal structures to form mortar. Then they poured mortar into the sea. The seawater instantly triggered a hot chemical reaction. The lime was hydrated and reacted with the ash to cement the whole mixture. Using volcanic rock and lime as a binder made the impressive difference.
For above ground applications the volcanic ash and lime combination proved just as effective. It inhibits cracks from spreading and provides just the right amount of seismic flex – a very important characteristic considering two fault lines run through Italy making it especially prone to earthquakes!
Without this apparent foresight of Roman scientists and engineers, many or all of the great structures we still know today may have fallen to rubble centuries ago.
How did they create seismic flex? Between larger limestone blocks, interruptions and internal constructions within walls and domes Roman builders intenitionally created discontinuities in the concrete mass. Portions of the buildings could then shift slightly when there was movement of the earth to accommodate such stresses, enhancing the overall strength of the structure.
In this sense bricks and concrete were flexible. It may have been precisely for this reason that, although many buildings sustained serious cracking from a variety of causes, they continue to stand to this day.
The Portland cement formula crucially lacks volcanic ash. As a result, it does not bind quite as well when compared with the Roman concrete, and that is the reason why cement that we use today lasts decades not centuries.