Unlike many illnesses that plague aging humans, there are already cures 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 in them is corroded. It’s a slow but relentless process that can become superfueled by salty sea air and tide floods.
Today’s skyscrapers are built with stricter cords and better waterproofing than decades-old condominiums like the collapsed Champlain Towers South in Surfside, Florida. It also eliminates corrosion of rust-preventive 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 seawalls, bridges and public projects, but the broader building industry 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 that he manages many business decisions in the construction industry: money. increase.
For example, polymer bars can increase initial material costs by about 20% and reduce the profit margins of contractors and developers. Khatib, on the other hand, said that significantly reducing repair costs could save millions of condominium buyers over the useful life of the building. Similarly, ultra-high performance concrete can cost 5 to 20 times more than regular concrete, but usually requires less concrete and has very little long-term maintenance costs.
“Ask someone who has a condo facing the sea,” he said. “Balconies are constantly being repaired. Reason: Corrosion. And they just put back more steel and patch it with concrete. Why? Just use a little more and double the lifespan. [with non-corrodible rebars].. There are so many people in academia who know this, but few others know it. “
Rust is an expensive enemy
Champlain Towers were a flawed design from the beginning, and a recent Miami Herald study found that they were built with columns that were too narrow to accommodate the rebar required by plans and codes. It is not yet clear how the corrosion of the steel affected or played a role in the catastrophic and deadly collapse of the surfside, but the unprecedented failure was made by architects throughout South Florida. It has caused a frenzy of structural inspections and almost certainly keeps many condo owners worried about cracks and rusty dirt on the pillars of the garage downstairs at night.
It’s understandable, but many experts in South Florida and elsewhere emphasize that surfside collapse is considered a tragic anomaly that is likely to be caused by a combination of factors. .. They do not believe it suggests that there is a sudden surge in similar failures.
Michael Kreger, Chairman of the University of Alabama’s Civil Engineering Drummond Donation, said:
He said the developers still designed the building to withstand unexpected damage, even if the building code did not elaborate on 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 can be overloaded.
Florida has been hit by hurricanes repeatedly, and the state has also adopted some of the country’s strictest building codes. Modern skyscrapers have been built to withstand the winds of large hurricanes, and other changes such as impact windows have made the building even more secure.
However, corrosion is unavoidable in steel-reinforced coastal structures. “90-95% of all repair costs for concrete buildings are usually spent on replacing concrete to protect corroded steel,” said Christopher Ferraro, assistant professor of civil engineering and coastal engineering at the University of Florida. I presume.
“People think that there isn’t much science because concrete is portable and cheap,” says Khatib. “But there is. It’s a very complicated material.”
Deterioration of concrete
The concrete itself is very durable — ancient Romans used it and their buildings still stand today. But modern rebar is not. The problem is that when steel is exposed to 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 Ferraro of the University of Florida. Over time, moisture from the air or floods penetrates towards the central rebar. “Think of it as a series of rivers at a very microscopic level.”
Corrosion is inevitable, and builders and inspectors know it. To address this, UM’s Khatib has strengthened the cord to add concrete around the steel, essentially allowing more water to move before starting the rusting process. rice field.
“Miami now needs 3 inches of concrete around the rebar in the new building,” he said. “But how many years have you actually bought it? Maybe 10 years per inch? No matter what you do, concrete is porous and salty. It’s not a long-term solution.”
As corrosion begins and concrete cracks begin to form, a feedback cycle occurs, the cracks allow more salt to enter, the cracks grow larger, and the process from collapsing beams and balconies until the concrete rains regularly. The whole accelerates.
Repairs typically include removing and replacing steel, patching cracks, and reinforcing weakened beams and columns. External reinforcement can also be added to prevent the cracks in the concrete from expanding. 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 provide different strengths to cope with the stress on large buildings.
Concrete, the most widely used building material in the world, can support huge weights, but as soon as it is twisted, pulled, shaken or hit with sufficient force or in the wrong place. Will crack. That’s where the rebar comes in. Rebar is strong, but it’s also what engineers call “ductility.” That is, it is flexible and given a little. Combining the two, a concrete building can withstand subtle changes such as temperature changes and large forces such as hurricane winds.
One of the few options that can significantly reduce the problem of corrosion at an affordable price is ultra-high performance concrete (UHPC). Since the cement is hardly mixed with water, the pores are small and separated. Instead of a microscopic river, moisture is trapped in an isolated puddle.
“This eliminates the cause of corrosion. [salt] Atorod Azizinamini, a 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 directed to UHPC, 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 yet enforced building codes that regulate UHPCs under construction, stagnating its use in high-rise and other buildings.
Instead of better concrete, developers can use better rebar instead.
Galvanized steel with rebar dipped in a layer of zinc is an affordable option with a prepaid cost of only 15% higher than traditional “black steel” rebar.
“Because no maintenance is required, this kind of marginal initial cost will pay off in 10 years,” said Khatib of UM. “And it lasts much longer than black steel,” probably two to four times longer. But in the end 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 or chip during shipping or installation. Stainless steel rebar is even better and is the most expensive metal option. Depending on the grade, stainless steel can significantly reduce repairs and extend the life of structures, but is primarily used in bridges and projects in marine environments.
Both Ferraro and UM’s Kativ at the University of Florida said the developer’s decision to keep initial costs low would prevent them from adopting these scientific advances.
“I spent the heyday of my life developing new technologies,” said Khatib of UM. “And no one was using it. I gave [dozens of talks and several papers], But it’s all just for the people of academia. No one in the industry wants to see what’s really happening. After graduating, he and his partner founded Green Coastal Engineering, a company specializing in the latest research and bridging the concrete industry.
According to Ferraro of the University of Florida, the greatest advances in concrete science are being used in publicly visible 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 have reached 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 take a portrait at the Structures Lab at the FIU College of Engineering & Computing in Miami, Florida. .. 2021.
New materials can make Florida condos more durable
Source link New materials can make Florida condos more durable