Surfside City Condo Collapse – My Early Guess: Rusty Rebar Fractures and Weakened Concrete Columns
This past weekend, my wife asked me all sorts of questions about the tragic sudden collapse of the 40-year-old condominium building near Miami, Florida. We’ve dated since high school and she knows I’ve got a geology degree instead of one in structural, chemical, or metallurgical engineering. All four of these sciences, and maybe a few more, are in play as more facts become available about the deadly condo calamity.
Why is this important to you? After all, you may not live in a condominium or a high-rise building. The early reports coming out indicate the issue might be weakened concrete as well as unstable soil under the structure. Once again, it’s going to take many months to do a forensic autopsy on this catastrophe. Keep in mind that much of what you see now in the news is speculation and past reports from consultants need to be combed through to pick out what signs there were that pointed to an impending disaster.
My nearly fifty years of building experience coupled with an eye-opening editors conference twenty-plus years ago put on by the Portland Cement Association provide me with an insight that should put your head on a swivel. It’s rare to have a house collapse, but hundreds of thousands of homeowners just like you experience costly damage to your homes because of what might have caused the collapse of the condos.
What is Concrete?
Let’s first talk about concrete. The condo building was built using concrete and reinforcing steel within the concrete. Concrete is man-made rock. When mixed, placed, and cured properly, it can have tremendous compressive strength. This means it requires thousands of pounds per square inch (PSI) of pressure to fracture it. It’s not uncommon for this PSI strength to be in excess of 4,000 pounds.
What is the Tensile Strength of Concrete?
However, this same exact concrete only has on average one-tenth the amount of strength in tension than it has in compression. Tension happens when you stretch something. A concrete slab suspended in between two beams will experience tension on the underside of the slab when you put weight on top of the slab. This same issue holds for ceramic tile, stone countertops, etc. Tile will crack if you step on it and there’s a hollow spot under it.
What is the Tensile Strength of Reinforcing Steel?
Steel, though, is a magical building material. It has tremendous tensile strength. The average rebar you might buy at a building supply house or home center is normally rated at 40,000 PSI. You can order it with a 60,000 PSI, or higher, rating!
This is why reinforcing steel is used in concrete construction. It brings to the party the tensile strength missing within the concrete itself.
What is Steel’s Achilles’ Heel?
But normal reinforcing steel has an Achilles’ heel. It can and does rust. When steel rusts, it expands. The expansion force is slow and considerable and it creates tension within the concrete surrounding the rebar.
Rusty angle irons holding up brick above windows can expand so much they push the brick out away from the wall. Rusty rebar in concrete can expand so much it causes chunks of concrete to fall out of an overhead slab or to fall off an important support column.
Does Salt or Seawater Cause Problems With Steel?
But wait, it gets worse. Salt accelerates the rust or corrosion of unprotected steel. This is why you often see concrete bridge decks and supports under highway bridges in such bad shape in cold climates. The rock salt used for snow and ice control seeps into the concrete as the salty meltwater soaks into the concrete. This brine attacks the steel if it’s not protected.
The Miami condo was just a couple hundred feet the ocean for goodness sake. Talk to any builder or remodeler who works near the ocean or a salt-water sea and they’ll have all sorts of tales about how fast poorly protected steel lasts in those conditions. This is not new. Mariners have long known that the sea eats men and iron!
How Can I Prevent Rebar Rust?
This is why you or your builder should paint rebar before it’s installed in concrete. Yes, this is an extra step, but it’s not that hard to paint it. The paint can be rolled on, sprayed, or even brushed. It’s best to use a special metal primer, then a finish coat of paint.
What About Rust on Deck Steel?
All of the above also applies to simple decks as well. The internet is littered with stories and videos of tragic deck collapses. The nails, bolts, structural hangers, etc. in your deck are subject to rust. The treated lumber contains lots of copper that leaches out with each rainfall. This chemical brew is similar to the salt water that probably is the root cause of the condo collapse. The copper-laced water accelerates the corrosion of all steel that’s part of your deck.
How do you prevent collapse or rust damage at your home? Use the best metal parts for outdoor decks that have a thick galvanized coating. Use hot-dipped galvanized nails or screws, stainless-steel nails or screws, paint all reinforcing steel and do whatever else is required so the iron or steel inside and outside your home doesn’t rust. It’s really that simple.