New Materials Can Make Florida Condos More Durable

Unlike many diseases that plague aging humans, there are already remedies for the most common, serious, and costly threats to thousands of dilapidated buildings along the Florida coast. This is a disease that structural engineers call “concrete cancer”.

Symptoms first appear as rusty columns or cracked balconies, a sign that the rebar is corroded. It is a slow but relentless process that can become overfed by the salty sea air and tides.

Today’s skyscrapers are built with tighter ropes and better waterproofing than decades-old condominiums like the collapsed Champlain Towers South in Surfside, Florida. It also removes corrosion from anti-rust polymer reinforcing rods and ultra-high performance concrete that does not allow salt water to pass through. They are already used in some special constructions, especially dikes, bridges and public projects, but the building industry at large is lagging behind.

Morteza Khatib, a structural engineer with a PhD in Sustainable Concrete Science from the University of Miami, says it’s the same reason he handles many business decisions in the construction industry: money . to augment.

For example, polymer bars can increase initial material costs by around 20% and reduce profit margins for contractors and developers. Khatib, on the other hand, said that a significant reduction in repair costs could save millions of condominium buyers over the useful life of the building. Likewise, ultra-high performance concrete can cost 5-20 times more than regular concrete, but generally requires less concrete and has very low long-term maintenance costs.

“Ask someone who has a condo facing the sea,” he said. “The balconies are constantly being repaired. Reason: corrosion. And they just put more steel back in and fixed it with concrete. Why? Just use a little more and double the life. [with non-corrodible rebars].. There are so many people in academia who know this, but few others know it. “

Rust is an expensive enemy

The Champlain Towers were a flawed design from the start, and a recent Miami Herald study found that they were built with columns too narrow to accommodate the rebar required by blueprints and codes. It is not yet clear how the corrosion of the steel affected or played a role in the catastrophic and deadly surfside collapse, but the unprecedented failure was committed by architects all over South Florida. This has caused a frenzy of structural inspections and almost certainly worries many condo owners with cracks and rusty dirt on the pillars of the garage downstairs at night.

It’s understandable, but many experts in South Florida and beyond point out that the surfside collapse is seen as a tragic anomaly that could be caused by a combination of factors. .. They don’t believe this suggests that there is a sudden increase in similar failures.

Michael Kreger, president of the Civil Engineering Drummond Donation at the University of Alabama, said:

He said developers had always designed the building to withstand unexpected damage, even though the building code did not explain specific requirements decades ago. For example, each column is designed to hold several times the expected weight, so if one column is damaged, the remaining columns may be overloaded.

Florida has been hit by hurricanes on several occasions and the state has also adopted some of the strictest building codes in the country. Modern skyscrapers were built to withstand the winds of major hurricanes, and other changes such as impact windows made the building even safer.

However, corrosion is inevitable in steel-reinforced coastal structures. “90-95% of all concrete building repair costs are typically spent replacing concrete to protect corroded steel,” said Christopher Ferraro, assistant professor of civil and coastal engineering at the University of Florida . I suppose.

“People think there isn’t a lot of science because concrete is portable and cheap,” says Khatib. “But there is. It’s a very complicated material.

Concrete deterioration

Concrete itself is very durable – the ancient Romans used it and their buildings still stand today. But modern rebar is not. The problem is, when steel is exposed to salt, water, and oxygen, it rusts and swells several times its normal size, eventually cracking the concrete due to internal pressure like an explosion at idle.

“Concrete is really like a hardened sponge,” University of Florida Ferraro said. Over time, moisture from air or flooding penetrates towards the center rebar. “Think of it as a series of rivers on a very microscopic level.”

Corrosion is inevitable, and builders and inspectors know it. To solve this problem, UM’s Khatib reinforced the rope to add concrete around the steel, essentially allowing more water to move around before starting the rusting process. paddy field.

“Miami now needs 3 inches of concrete around the rebar in the new building,” he said. “But how many years ago have you actually bought it?” Maybe 10 years per inch? Whatever you do, concrete is porous and salty. It is not a long term solution.

When corrosion begins and cracks in concrete begin to form, a feedback cycle occurs, the cracks allow more salt to enter, the cracks enlarge, and the process goes from collapsing beams and balconies until the concrete rains regularly. The whole is accelerating.

Repairs typically include removing and replacing steel, patching cracks, and reinforcing weakened beams and columns. External reinforcements can also be added to prevent expansion of cracks in concrete. These techniques and regular maintenance maintain the structural integrity of the building decades ago.

But sooner or later the steel will rust.

Steel and concrete offer different strengths to cope with the stresses on large buildings.

Concrete, the most widely used building material in the world, can withstand enormous weights, but as soon as it is twisted, pulled, shaken or hit with sufficient force or in the wrong place. will crack. This is where the rebar comes in. The rebar is strong, but it is also what engineers call “ductility”. That is to say, it is flexible and a bit given. By combining the two, a concrete building can withstand subtle changes such as changes in temperature and large forces such as hurricane winds.

One of the few options that can dramatically reduce the corrosion problem at an affordable price is Ultra High Performance Concrete (UHPC). As the cement is hardly mixed with water, the pores are small and separate. Instead of a microscopic river, moisture is trapped in an isolated puddle.

“It eliminates the cause of the corrosion. [salt] Atorod Azizinamini, professor of civil engineering at Florida International University and director of the Moss School of Construction, Infrastructure and Sustainability, said:

Public projects such as bridges are increasingly oriented towards the BUHP, including two bridges, Deerfield Beach and West Palm Beach, built in 2017 and 2019 respectively. However, according to Ferraro and Kreger, the United States has not still enforced building codes that regulate UHPCs under construction, stagnating their use in high-rise buildings and the like.

Advanced rebar

Instead of better concrete, developers can use better rebar instead.

Galvanized steel with zinc dipped rebar is an affordable option with a prepaid cost only 15% higher than traditional “black steel” rebar.

“Because no maintenance is required, this kind of marginal up-front cost will pay off in 10 years,” said UM’s Khatib. “And it lasts a lot longer than black steel,” probably two to four times longer. But in the end the zinc gives way and the threat of corrosion reappears.

There are also epoxy coated rebar. This is an upgrade that costs about 25% more, but can wear out or chip off during shipping or installation. Stainless steel rebar is even better and is the more expensive metal option. Depending on the quality, stainless steel can significantly reduce repairs and extend the life of structures, but it is mainly used in bridges and marine projects.

Tenacious industry

Ferraro and UM’s Ktiv at the University of Florida said the developer’s decision to keep upfront costs low would prevent them from embracing these scientific advances.

“I spent the prime 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 just for academics. No one in the industry wants to see what’s really going on. After graduating, he and his partner founded Green Coastal Engineering, a company specializing in the latest research and connecting the concrete industry.

According to Ferraro of the University of Florida, the greatest advances in concrete science are being used in public buildings, which he calls “wow” structures. Burj Khalifa, Dubai, for example, is the tallest building in the world, a luxurious skyscraper that opened in 2010, and was built with a combination of high-performance concrete, among other advanced technologies.

“These builders are at the cutting edge of concrete,” Ferraro said. “But for suburbs and school buildings, industry adoption has been delayed.”

On Tuesday, August 10, Atorod Azizinamini, professor of civil engineering at Florida International University and director of the Moss School of Construction, Infrastructure and Sustainability, will be doing a portrait at the Structures Lab of the FIU College of Engineering & Computing in Miami, Florida. . .. 2021.

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