Sand, gravel, rebar, cement, our kind of stuff.
From Wired, July 2:
Overreliance on the world’s favorite building material will lead to more crumbling towers, dams, bridges, and roads. And way more greenhouse gases.
It will likely be many months before we know for sure what caused the catastrophic collapse of the Champlain Towers South building in Surfside, Florida, last week, which killed at least 18 people. But it’s already clear that at least one culprit was failing concrete. In 2018 an engineering firm warned that concrete beneath the building’s pool and entrance drive showed "major structural damage,” and it found “abundant cracking” in the underground parking garage. Just a few months ago, the president of the building’s condominium association wrote that “concrete deterioration is accelerating.”
While this kind of sudden, wholesale building collapse is very rare, the problem of crumbling concrete isn’t at all. It’s a slow-moving crisis that affects much of the world. Billions of tons of concrete in the form of buildings, roads, bridges, and dams may need to be replaced in the coming decades. That will cost trillions of dollars—and generate staggering amounts of climate-change-fueling carbon emissions.
Concrete, which is essentially just sand and gravel glued together with cement, is by far the most widely used building material on earth. We pour enough each year to build a wall 88 feet high and 88 feet wide right around the equator. That’s largely because the number and size of cities is exploding. The number of urban dwellers has more than quadrupled since 1960 to more than 4 billion, and it’s still rising. We’re adding the equivalent of 10 New York Cities to the planet every single year.
There’s no way cities could grow this fast without concrete. It’s an almost magically cheap, easy way to quickly create roads, bridges, dams, and relatively sturdy, sanitary housing for huge numbers of people. An estimated 70 percent of the world’s population now live in structures made at least partly out of concrete.
But none of those structures will last forever. Concrete fails and fractures in dozens of ways. Heat, cold, chemicals, salt, and moisture all attack that seemingly solid artificial rock, working to weaken and shatter it from within. (Rising temperatures and atmospheric carbon levels are expected to make things worse.)
That threatens not just apartment towers, but our concrete-based infrastructure. A 2021 report by the American Society of Civil Engineers found that more than 20,000 concrete bridges across the US are structurally deficient and nearly half the nation’s public roadways are in “poor” or “mediocre” condition.
Things are far worse in many developing nations, where building standards are low and regulations often ignored. To cut costs, builders often use unwashed sea sand to make concrete. Those grains are cheaper, but they are coated with salt that dangerously corrodes rebar. Concrete buildings made with sea sand pancaked by the dozens in Haiti’s 2010 earthquake. Shoddy concrete was also likely a key reason for the collapse of a factory in Bangladesh in 2013 that killed more than 1,000 people. According to The Financial Times, as much as 30 percent of Chinese cement is so low-grade that it produces dangerously flimsy structures known as “tofu buildings.” Cheaply made concrete is one of the reasons so many schools collapsed in China’s 2008 Sichuan earthquake, killing thousands.....
....MUCH MORE
If interested in more detail see 2016's "The Trouble With Reinforced Concrete"
