Collapsed Miami condo had been sinking into Earth as early as the 1990s, researchers say

Video:

That big reinforcing shear wall disintegrated like it was made out of sand.

Several of the big contractors in the area who build these kinds of buildings are saying this looks exactly like corroded rebar failure. The original flimsy construction together with foundation settlement causes seams and cracks to open up at the intersections of the various structural planes allowing water to enter.

really doubt they used stainless steel rebar, and fiber reinforced rebar wasn't available at the time; AFAIK, Then recent research out of Northeastern University shows that because of the high CO2 levels in the air, reinforced concrete structural usable lifetime needs to be derated to 67% estimated. really goes after steel rebar. Rebar is hunky dory but it needs to be sealed up inside the concrete but good, good luck with that.

The fallback excuse relying on "building codes" is a scam. Building codes at their minimums are criminally deficient, the reason being they are written by self-styled regulatory organizations controlled by the builders who fight any and everything that might add cost to the build.

Around 20 years ago at a racetrack in NC there was a walkway over a highway that fell with people on it one night. They blamed it on the rebar and I think there was some kind of chemical that had been put on it over they years to melt ice. Anyway it was constructed of materials that should not have been used.

Every day I park on the Pot Hill side of the freeway and take a footbridge over 101 to get to work. One side is called Fallen Bridge Park, because the previous footbridge did.

What do you expect living in a third world country?

I don't think anyone uses SS rebar, particularly not ca. 1980. They're not shooting for Roman Empire longevity, just enough that it's someone else's problem. Maybe it was galvanized.

There's a construction technique that involves tensioned steel cables run through the concrete to keep it in compression. The cables eventually corrode and have to be replaced (one at a time, obviously). I wonder if that could have been a cause.

Looks like the area with the "flimsy" first story parking garage was one of the few areas that remained standing

Galvanized has a tough time with the alkali in the concrete:

There are other coatings like epoxy, but it requires gentle handling and must be expensive.

I think that's only used when they want the structure to flex without damage, things like floors, walkways and poles.

I don't see any cabling in the rubble. Looks like just carbon steel rebar, and the spacing in the flooring looks pretty sparse.

The remaining building could come down in an instant at any time. Just because the construction is flimsy doesn't mean it won't kill you when it falls on you. The responders have to be on edge, ready to beat feet in an instant.

I'm seeing the high end builders using that cable prestressing in slab on grade foundations. This is an application where voids can form under the slab due to settlement or imperfect drainage causing it to crack. The cable tensioning should help the slab hold up to that. Too expensive to be standard, only the high end builders for whom a $1M is a small job, but they're top quality all the way and their products last.

it is banned in some places because it has turned out to be unrealistic to keep the coating intact and then corrosion just gets concentrated

It's a difficult problem. In the US, what one sees these days is a green plastic coating on rebar being used in new construction. One place being "Jersey Barriers" on highways, where exposure to salt is intense.

I think that the intent is to slow corrosion down. Don't know how well this works.

Joe Gwinn

Also you have to paint the ends whenever you cut it, and sometimes on bends if it cracks.

I really think the fiber reinforced polymers are the way to go. They cost more now because they're not selling it in volume like steel.

The concrete mix needs to be modified. It has the nasty property of snapping before deforming much under strain.

Here is a nutty associate professor in CE talking about the details, if you can tolerate the insanity:

Right now his personal choice is galvanized.

Are steel beams just too expensive?

Our house was built after the '89 earthquake and the subsequent crackdowns. It has two big steel beam frames embedded in the foundation and going up, and plywood shear walls with thousands of nails.

Of course, it's not built on soggy ooze.

lørdag den 26. juni 2021 kl. 01.17.38 UTC+2 skrev John Larkin:

steel takes longer and needs fire protection

For compressive loads (weight-bearing), basically yes.

If the steel-reinforced-concrete walls hadn't been open to the weather (if they were interior structure, with exterior cladding of light materials) the Florida coast wouldn't have salted/corroded the steel, one supposes. The Pantheon is concrete... and how old?

Don't see much of that in Seattle; ductile concrete maybe, or prestressed elements, and some of the old stuff is heavy timber (glue-lam glued-up timbers is the modern variant). I think maybe the Florida clime doesn't rain enough to wash off the salt.

Inspections caught problems with stadium roof, floating bridge, viaduct, and West Seattle span, so far the stadium got recycled, the floating bridge... sank during repairs, the viaduct turned into a tunnel, only the bridge to the west is still flaky. When it reopens (next year) it'll go back to carrying 100k trips per day... we hope.

<snipped>

I wonder about the aggregate. There are cases where pyrites in the aggregate cause concrete to crumble and rebar to corrode. In the UK it's particularly prevalent in Cornwall where tin mine tailings were sometimes used in the aggregate on concrete block houses, they call it 'Mundic'.

You can buy a cheap house in Cornwall if it's made from Mundic Block and you have cash - you won't get a mortgage. There are similar issues in the West of Ireland.

On Saturday, June 26, 2021 at 9:38:32 AM UTC-4, Clive Arthur wrote:

Good point. Aggregate and water contamination is a huge issue. The state of Connecticut is still dealing with a huge pyrrhotite contamination issue. The state set up a special agency using an insurance model to deal with paying for the time consuming and expensive repairs.

In Florida seawater and salt contamination of concrete components a is major issue. They have gone so far as to develop a saltwater concrete which eliminates the problem. They have to use fiber reinforced rebar in this case, nothing else will do. The Ley video about the fiber rebar is total hysteria, they can do things like double the diameter of fiber rebar to get around that strain limitation and big cracks. the sate already has several bridge, walkway and road projects using this material. contamination of aggregate and water used in the Champlain Towers construction most certainly could have accelerated its demise. The major inspection was due 20 years ago. This a real can of worms for the coastal cities in the state because you're looking at many billions of lost real estate value. People in Surfside area are already scared to death to remain in their seaside condos. I know some materials scientist or engineer saw this coming decades ago. But the development and profit fanatics would not be stopped.

The strata council or homeowners may hold some responsibility. As I said, the minutes of the past number of years would make for good reading. If council knew, and had presented it to the owners at the AGM and the owners voted it down or defer...

I hope the council minutes were stored off site, or filed with the state government.

John