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Ask the Builder: Condo collapse, rusty rebar and your home

Pictured is a piece of rusty rebar inside spalling concrete. As the rebar rusts, it expands and fractures the concrete, exposing the rebar to even more moisture.  (Tim Carter/Tribune Content Agency)
Pictured is a piece of rusty rebar inside spalling concrete. As the rebar rusts, it expands and fractures the concrete, exposing the rebar to even more moisture. (Tim Carter/Tribune Content Agency)
By Tim Carter Tribune Content Agency

After seeing news of the tragic, sudden collapse of the 40-year-old condominium building near Miami, my wife had all sorts of questions.

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.

Early reports indicate that the cause of the collapse may have been weakened concrete, as well as unstable soil under the structure. It’s going to take many months to complete a forensic study of this catastrophe, so keep in mind that much of what you see now in the news is speculation.

My nearly 50 years of building experience coupled with an eye-opening editors conference by the Portland Cement Association a couple of decades ago 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 the factor that might have caused the collapse of the condo building.

Let’s first talk about concrete. The condo building was built using concrete and reinforcing steel (rebar) within the concrete. Concrete is manmade 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.

However, this same concrete only has on average one-tenth the amount of strength in tension that 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 and so forth. Tile will crack if you step on it, and there’s a hollow spot under it.

Steel, however, 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 provides the tensile strength missing within the concrete itself.

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 away from the wall. Rusty rebar in concrete can expand so much that it causes chunks of concrete to fall out of an overhead slab or to fall off an important support column.

But wait, it get’s 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 practically next to the ocean. Talk to any builder or remodeler who works near the ocean or a saltwater sea, and they’ll have all sorts of tales about how long poorly protected steel lasts in those conditions. This is not new. Mariners have long known that the sea eats men and iron.

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 difficult to paint it. The paint can be rolled on, sprayed or even brushed. It’s best to use a special metal primer and then a finish coat of paint.

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 and so forth 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 saltwater 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.

Subscribe to Tim Carter’s free newsletter and listen to his new podcasts at askthebuilder.com.

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