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

Tim Carter, Tribune Content Agency on

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 coming out indicate the cause of the collapse might 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 put on by the Portland Cement Association a couple 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 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.

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 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 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.

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