And Now for Something ‘Concretely’ Different

Have you ever wondered how numerous Roman structures have lasted for centuries? If you love concrete and are tired of hearing about inflation, we have some exciting news for you—researchers from MIT, Harvard, Switzerland, and Italy have gained a deeper understanding of Roman concrete’s durability.

Researchers explored the origins of lime clasts, or white mineral features, found in ancient Roman concrete. Mixing water with lime, in a process called slaking, could not account for the lime clasts. Closer analysis suggested that lime clasts were formed at very high temperatures, meaning that quick lime may have been used along with (or instead of) slaked lime. As a further bonus, when researchers mixed concrete with quick lime, and created concrete with lime clasts, they found that the concrete samples displayed self-healing properties, as water that entered into a crack would react with the lime clasts and ultimately form calcium carbonate to fill the crack, or react with other materials to strengthen the concrete.

This story caught a fair amount of attention in the press, especially for an obscure topic, perhaps since the contrast is striking between the endurance of the Pantheon and our own seemingly always-in-need-of-repair infrastructure. The story of Roman construction also shows a great reverence for the past, conveying an apparent wisdom of the ancients which we are only now rediscovering, so the subsequent media coverage is as much about how we see the world as how we construct the world.

Alas, Brian Potter has repudiated the nostalgic argument that craftsmanship was superior in the past. Potter doesn’t argue against the research itself, but provides greater context for the manufacturing of modern concrete. Modern concrete is usually reinforced with steel rebar, which provides greater compressive and tensile strength. Unreinforced concrete is also brittle, meaning that if a plain concrete element fails, it is likely to do so suddenly and without warning, which could lead to catastrophe. In contrast, reinforced concrete typically bends and sags prior to failure, which should give people time to vacate the premises before any structural collapse. Building codes often require structural concrete to be reinforced with steel.

Unfortunately, the steel in concrete corrodes over time, and Potter argues that this corrosion is the primary reason modern concrete has a comparatively short lifespan. Could concrete be made to last longer? Sure, if builders used stainless steel rebar—but stainless steel rebar is 4-6 times more expensive than regular rebar. So stainless steel rebar may not make economic sense for a facility whose concrete is largely protected from the weather, as such a structure with regular rebar may already last over 100 years. Potter points out as well that modern concrete also has some self-healing capabilities, though perhaps not as strong as those of Roman concrete.

Perhaps lime clasts will be incorporated into modern construction as a way to improve the durability of concrete. But learning from the past doesn’t mean the past is superior, or that present methods are lacking. Like many decisions, the choice of building materials and techniques involves tradeoffs, and modern society has valued less costly infrastructure whose lifespan is more likely to be numbered in decades than in centuries.

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