Roman concrete’s ability to last for millennia puts modern concrete to shame, but this ignores that the overwhelming majority of modern concrete is reinforced concrete, Brian Potter explains, with some type of steel embedded in it:
Usually this is in the form of bars (rebar), but it might also be mesh, or fibers, or steel cable. Steel is stronger than concrete, particularly in tension (reinforcing steel has perhaps 10-15x the compressive strength of concrete, but more than 100x the tensile strength of concrete), and a comparatively small amount of steel can greatly increase the strength of a concrete element. By adding steel, you can make shallow concrete elements (beams, slabs, etc.) that can still span long distances and that wouldn’t be possible if the concrete were unreinforced.
Concrete is also brittle, whereas steel is ductile — if a plain concrete element fails, it’s likely to fail suddenly without warning, whereas a steel element will (generally) stretch and sag significantly before it fails, absorbing a lot of energy in the process. This makes reinforced concrete fundamentally safer than unreinforced concrete — if you have a lot of warning before a structure fails, you have time to safely get out of the building. For this reason, structural concrete is often required by code to have some minimum amount of steel reinforcing in it, and concrete that might experience large sudden loads in unpredictable ways (such as from an earthquake) is required to have a LOT of additional reinforcing. Most buildings built in zones of very high seismicity aren’t actually designed to come through the earthquake undamaged — they’re merely designed to not catastrophically collapse so people can safely get out.
(Earthquake design might seem like something that you only need to worry about in a few places, but most of the US can theoretically see a surprisingly strong earthquake and the buildings must be designed accordingly.)
But while reinforcement provides a lot of benefits, it has drawbacks. The primary one is that, over time, the steel in concrete corrodes. This is the result of two mechanisms – chloride ions making their way through the concrete, and concrete absorbing CO2 over time (though the second one happens much more slowly). As the steel corrodes, it expands, putting internal pressure on the concrete, eventually resulting in cracking and spalls (chunks of concrete that have fallen off).
How quickly this happens depends on a lot of factors. Concrete exposed to weather or water will corrode faster than concrete that isn’t. Concrete where the rebar is farther from the surface of the concrete will last longer than concrete where the steel is closer to the surface. Concrete exposed to harsh chemicals such as salts or sulfates will corrode faster than concrete that isn’t.
The comparatively short lifespan of modern concrete is overwhelmingly the result of corrosion-induced failure. Unchecked, reinforced concrete exposed to the elements will often start to decay in a few decades or even less. Precast concrete parking garages, for instance, are exposed to a lot of weather, since they’re open-air structures and vehicles bring moisture and road salts inside them. And a precast garage will often have many exposed steel elements, since steel plates stitch the pieces of concrete together. A precast garage might have a design life of 50 years, and often need very substantial repairs much earlier. Roman concrete, however, is unreinforced, and doesn’t have this failure mechanism.
This type of failure is exacerbated by the fact that modern concrete is designed to come up to strength very quickly, which results in numerous small cracks caused by shrinkage strains in the hardened concrete. These cracks make it easier for water to reach the steel, accelerating the process of corrosion. They also make the concrete more susceptible to other types of decay like freeze-thaw damage. Roman concrete, on the other hand, cured much more slowly.
If we wanted to build more durable concrete structures, the most important thing would be to remove or minimize this failure mechanism, and structures designed for long lives often do. Buddhist or Hindu temples, for instance, will use unreinforced concrete, or concrete with stainless steel rebar, and often have 1000-year design lives (though whether they will actually survive 1000 years is another question). Stainless steel rebar advocates like to trot out a concrete pier in Mexico built in 1941 with stainless steel rebar, which has needed no major repair work despite being in a highly corrosive environment.
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Using unreinforced concrete dramatically limits the sort of construction you can do — even if the code allows it, you’re basically limited to only using concrete in compression. Without reinforcing, modern concrete buildings and bridges would be largely impossible.
Other methods of reducing reinforcement corrosion also have drawbacks, especially cost. Stainless steel rebar is four to six times as expensive as normal rebar. Epoxy coated rebar (commonly used on bridge construction in the US) is also more expensive, and though it can slow down corrosion, it won’t stop it. Basalt rebar won’t corrode (as far as I know) but can apparently decay in other ways.
Adding cost to a building to potentially extend its lifespan is often tough to make the numbers work for a developer. Well-made reinforced concrete that’s protected from the weather can last over a century, so the net present value of any additional lifespan beyond that is pretty low. It’s much more likely that the building will be torn down for other reasons long before the concrete fails.
