Not surprisingly, construction was a topic at the recent COP27, UN Climate Change Conference in Egypt last November.
According to experts, the construction and real estate sectors, by themselves, are responsible for 40% of global greenhouse gas (GHG) emissions worldwide. The production of cement, alone, a key component of most construction projects, emits 7% of global CO2.
Adding insult to injury, we recently learned why modern concrete (a composite of cement bonded to aggregate) doesn’t last as long as the concrete used by the Roman Empire in the era "before Christ" (B.C.). Both the Colosseum and the Pantheon – which boasts the world’s largest reinforced dome – have survived for more than 2,000 years, while here in Florida we require periodic inspections, beginning at 25 or 30 years, of concrete buildings due to structural failures caused by concrete breakdown. (Full disclosure: SPS, my consulting firm, owns a 50% interest in a general contractor that specializes in concrete restoration projects).
Cracking and spalling in concrete are caused primarily by carbonation and chloride contamination of the concrete. Both of these lead to corrosion of the embedded rebar which expands and exerts pressure on the concrete. Here in coastal South Florida, with our salt-laden air and hurricane events, chloride corrosion is a very real problem.
So why was the Roman concrete so superior to ours, even in wet and earthquake-prone locations?
Now a team of researchers from the U.S., Italy, and Switzerland believe they have found the answer. They analyzed samples from the city wall at the Italian archeological dig site in Privernum, a city near the Appian Way that was conquered and occupied by the Romans in the late 4th century B.C. Those samples, typical of concrete found throughout the Roman Empire, had white fragments in them. The fragments, or clasts, were limestone and apparently gave the concrete the ability to self-repair its cracks by dissolving and recrystallizing after becoming wet.
Lime clasts had been previously identified in ancient Roman concrete, but had been attributed to poor workmanship or materials, not to intentional compounding.
Today, if our concrete lasted longer, we would arguably need to produce less, since it would be primarily for new construction and not for repair projects. Less concrete would equate to less cement production, and a resultant decrease in GHG emissions.
For those inclined to eliminate concrete entirely, some architects and builders are turning to alternatives such as compressed earth blocks. These blocks are composed of clay, sand, and a small amount of cement. In addition to a smaller carbon footprint during production, earth blocks have natural cooling thermal properties, absorbing heat during the day and releasing it at night. Of course, mud bricks have been used for thousands of years throughout the world and sun-dried brick, known as adobe in North America, was used by indigenous Americans for centuries.
With all that in mind, perhaps we should study the texts of ancient engineers as carefully as we read today’s peer-reviewed papers.
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A regular contributor to HPAC Engineering and a member of its editorial advisory board, the author is a principal at Sustainable Performance Solutions (SPS), a south Florida-based engineering firm focusing on energy and sustainability. He can be reached at [email protected].