XPost: talk.politics.misc, alt.science

https://techxplore.com/news/2023-01-riddle-roman-concrete-durable.html

The ancient Romans were masters of engineering, constructing
vast networks of roads, aqueducts, ports, and massive buildings,
whose remains have survived for two millennia. Many of these
structures were built with concrete: Rome's famed Pantheon,
which has the world's largest unreinforced concrete dome and
was dedicated in A.D. 128, is still intact, and some ancient
Roman aqueducts still deliver water to Rome today. Meanwhile,
many modern concrete structures have crumbled after a few decades.

Researchers have spent decades trying to figure out the secret
of this ultradurable ancient construction material, particularly
in structures that endured especially harsh conditions, such as
docks, sewers, and seawalls, or those constructed in seismically
active locations.

. . .

Rome built many things out of concrete - a different
formula to that largely used today, containing volcanic
ash. Concrete containing coal fly-ash is similar, but
only a small percentage of modern concrete contains it.

The authors were interested in little flecks of lime found
in Roman concrete. For a very long time they were simply
dismissed as an artifact of poor quality materials. However
it turns out they serve an important function - providing
a means to heal micro-fractures in the concrete.

Romans used a process known as "hot mixing" - where the
main ingredients were all baked together at high temperature.
It was long assumed that lime was dissolved in water and
added to the mix at pour time, but this was not the case.
The tiny chips of lime were added to tbe bake mix and
intended to NOT dissolve.

So, 2000 years later and we STILL can't out-do Roman
concrete tech. THEIR stuff has lasted millenia - OUR
stuff fractures and crumbles in mere decades. Junk.

Of course, given the modern practice of re-cycling
real-estate every few decades ... flattening whatever
is there and building something "better" ... perhaps
our crap concrete IS more appropriate. Thing is, in
terms of material and energy costs, that's a terrible
terrible waste.

The OTHER flaw in modern concrete construction is the
ubiquitous use of steel rebar. It rusts and the rust
creates pressure on the concrete - and splits it. The
catastrophic collapse of that big condo in Miami a
couple of years ago was because of this - salty water
penetrated micro-cracks to the rebar and shattered
the concrete. Just slapping some plaster on it does
NOT fix the damage. just makes it LOOK safe. Stainless
rebar, particularly in oceanside construction, could
alleviate this problem but stainless steel is a LOT
more expensive (and has some unfavorable stress failure
issues that'd become more relevant in taller structures).

Perhaps, especially in hostile climes, it would actually
be better to reproduce the Roman mix and 'heavier' walls -
and skip the rebar entirely ? That won't serve for really
tall (>20 floor) buildings that sway in the breeze, but
for things like those beachfront condos ...

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