Unlike many diseases that afflict aging humans, there are already cures for the most common, serious and expensive threat to thousands of aging buildings along Florida’s coast, a malady that structural engineers have dubbed “concrete cancer.”
The symptoms show up first as rust-stained columns or cracked balconies — tell-tale signs the steel reinforcement inside them is corroding. It’s a slow but relentless process that can be super-fueled by salty sea air and tidal flooding.
Today’s high-rises are constructed with tougher codes and better waterproofing than decades-old condominiums like the collapsed Champlain Towers South in Surfside, Fla. But some research engineers believe there are even better ways to build along the beach with improved materials that can resist or even eliminate corrosion — such as rustproof polymer reinforcing rods and ultra high performance concrete that is largely impervious to saltwater. They’re already in use in some specialty construction, most notably in seawalls, bridges and public projects, but the broader building industry lags behind.
One reason, said Morteza Khatib, a structural engineer who earned a doctoral degree studying sustainable concrete science at the University of Miami, is the same one that controls many business decisions in the construction industry — money.
Polymer bars, for instance, might increase upfront material costs by around 20 percent and cut into profit margins for contractors and developers. On the flip side, Khatib said, it could save condo buyers millions over a building’s lifetime by dramatically reducing repair costs. Similarly, ultra-high performance concrete can range from five to 20 times more expensive than normal concrete, but less of it is typically needed and it also eliminates most long-term maintenance costs.
“Ask anyone with a condo facing the ocean,” he said. “The balconies are constantly getting repaired. The reason: corrosion. And then they just put more steel back in, and patch it with concrete. Why? Just spend a bit more and double its lifespan [with non-corrodible rebars]. There are so many people in academia that know this, but almost no one outside it does.”
RUST IS AN EXPENSIVE ENEMY
Champlain Towers was a flawed design from the start, a recent Miami Herald investigation found, constructed with columns too narrow to accommodate the steel reinforcement called for in plans and codes. It’s not yet clear how or if corrosion of that steel played a role in the catastrophic and deadly collapse in Surfside, but the unprecedented failure has set off a frenzy of structural inspections by building officials across South Florida and almost certainly has kept many condo owners up at night worrying about cracked columns and rusty stains in the downstairs garage.
While that’s understandable, many experts in South Florida and elsewhere stress that they consider the Surfside collapse a tragic anomaly likely caused by a combination of factors. They don’t believe it suggests there will be a sudden surge of similar failures.
“Most old concrete buildings in this country, even ones from the ‘70s, are completely reliable,” said Michael Kreger, Drummond Endowed Chair in Civil Engineering at the University of Alabama.
Even if building codes didn’t spell out specific requirements decades ago, developers still designed buildings to be resilient to unexpected damage, he said. For example, each column would be designed to hold several times the weight it was expected to, so if one column was damaged the rest could shoulder the extra burden.
With Florida repeatedly whacked by hurricanes, the state also has adopted some of the country’s toughest building codes. Most recent high-rises are built to withstand major hurricane winds and other changes, such as impact windows, have further improved building safety.
But corrosion is inevitable for a steel-reinforced coastal structure. Christopher Ferraro, an assistant professor at the University of Florida Department of Civil & Coastal Engineering, estimates that “90 to 95% of all repair costs for concrete buildings usually deal with replacing concrete to protect corroding steel.”
“People think that because concrete is portable and cheap, there’s not much science,” said Khatib. “But there is. It’s a very complicated material.”
Concrete itself is extraordinarily durable — the ancient Romans used it and their buildings still stand today. But modern steel rebars are not. The problem is that when steel bathes in salt, water, and oxygen, it rusts and swells several times its normal size, eventually cracking the concrete from internal pressure like a slow-motion explosion.
“Concrete is really like a hardened sponge,” said UF’s Ferraro. Over time, moisture from the air or flooding seeps towards the steel rebars at the center. “Think of it as a series of rivers at a very microscopic level.”
Corrosion is inevitable and builders and inspectors know that. To address it, codes have been toughened to add more concrete around the steel, essentially giving the moisture farther to travel before starting the rust process, said UM’s Khatib.
“Miami now requires 3 inches of concrete around steel rebars in new buildings,” he said. “But how many years are you actually buying with that? Maybe 10 years per inch? No matter what you do, concrete is porous, and salt will get in. It’s not a long-term solution.”
Once corrosion starts and concrete cracks begin to form, it creates a feedback cycle where more salt can get in through the cracks, making them bigger and accelerating the whole process until concrete periodically rains from a crumbling beam or balcony.
Repairs usually involve removing and replacing steel, patching cracks and shoring up weakened beams or columns. Exterior reinforcements to keep concrete cracks from growing also can be added. Those techniques and regular maintenance preserve the structural integrity of decades-old buildings.
But sooner or later, steel will rust.
Steel and concrete bring different strengths to deal with stresses exerted on a big building.
Concrete, the most widely used building material in the world, can hold up colossal weights but quickly cracks when twisted, pulled, shaken or struck with enough force or in the wrong place. That’s where steel rebars come in. They’re strong but also what engineers call “ductile,” meaning they’re pliable — they give a bit. With the two bonded together, a concrete building can weather subtle shifts like temperature changes and massive forces like hurricane winds.
One of few alternatives that is both affordable and can greatly reduce the corrosion problem is ultra-high performance concrete (UHPC). The cement is mixed with barely any water so that pores are tiny and disconnected. Instead of microscopic rivers, moisture gets trapped in isolated puddles.
“This eliminates the source of corrosion: [salt] ingress,” said Atorod Azizinamini, Vasant Surti professor of civil engineering and director of Moss School of Construction, Infrastructure and Sustainability at Florida International University.
Public projects like bridges are increasingly turning towards UHPC, such as two bridges in Deerfield Beach and West Palm Beach built in 2017 and 2019, respectively. However, the United States has yet to implement building codes that regulate UHPC in construction, stalling its use in high rises and other buildings, according to Ferraro and Kreger.
In lieu of better concrete, a developer can instead use better rebars.
Galvanized steel, which is rebar dipped in a layer of zinc, is an affordable option that costs only 15% more upfront compared to traditional “black steel” rebars.
“Since it doesn’t require maintenance, that kind of marginal initial cost pays for itself in 10 years,” said UM’s Khatib. “And it lasts way longer than black steel,” perhaps with a lifespan two to four times greater. But eventually the zinc gives way and the corrosion threat reappears.
There’s also steel rebar coated in epoxy, an upgrade that adds about 25 percent to costs, but can wear or chip off during transportation or installation. Stainless rebar is even better and the most expensive metal option. Depending on the grade, stainless can also greatly reduce repairs and increase the lifespan of structures — but it’s mainly been employed in bridges and projects in marine environments.
A STUBBORN INDUSTRY
Both UF’s Ferraro and UM’s Khatib said that decisions by developers to keep initial costs low hampers the adoption of these scientific advances.
“I spent the prime years of my life developing new technology,” said UM’s Khatib, “and no one was using it. I gave [dozens of talks and several papers], but it’s all just for people in academia. No one from the industry wants to see what’s really going on.” After graduating, he and his partner founded Green Coastal Engineering, a firm dedicated to bridging the newest research with the concrete industry.
According to UF’s Ferraro, the biggest advances in concrete science are utilized in buildings with high public visibility, what he calls “wow” structures. For example, the Burj Khalifa in Dubai, the tallest building in the world and a luxurious skyscraper that opened in 2010, was constructed with a high performance concrete mix among other advanced techniques.
“Those builders are getting to the cutting edge limits of concrete,” said Ferraro. “But for the buildings in suburbia or schools, the industry is slow to adopt.”