After seeing news of the tragic, sudden collapse of a 40-year-old condominium building near Miami, I had all sorts of questions.
Early reports have indicated 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 and hear now is only speculation.
While it’s rare to have a house collapse, many owners do experience costly damage to their homes caused by the same factors reported at the collapsed condo building.
The building was built using concrete and reinforcing steel (rebar) within the concrete. Concrete is a human-made rock. When mixed, cured and placed 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 has, on average, one-tenth the amount of strength in tension that it has in compression. A concrete slab suspended between two beams will experience tension on the underside of the slab when weight is placed on top. This is also true 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 typical rebar you might buy at a building-supply store or home center is normally rated at 40,000 psi. You can order it with a 60,000 psi or higher rating.
That 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 rust. When steel rusts, it expands. The expansion force is slow and considerable, and it creates tension within the concrete surrounding the rebar.
Angle irons supporting brick above windows can expand so much when they’re rusty that they push the brick out from the wall. Rusty rebar in concrete can also cause chunks of concrete to fall out of an overhead slab, or to fall off an important support column.
In addition, salt accelerates the rust or corrosion of unprotected steel. This is why concrete bridge decks and supports under highway bridges in cold climates can appear to be in bad shape. 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.
To protect rebar from rust or corrosion, you or your builder should paint it before it’s installed in concrete. The paint can be rolled, sprayed or brushed on. It’s best to use a special metal primer and then a finish coat of paint.
All of the above applies to simple decks, as well. Tragically, stories and videos of deck collapses are all-too common. The nails, bolts and structural hangers used on decks are subject to rust. Treated lumber contains lots of copper that leaches out with each rainfall, accelerating the corrosion of any steel that’s part of your deck.
How can you prevent collapse or rust damage at your home? Use the best metal parts for outdoor decks and be sure they include a galvanized coating. Use hot-dipped, galvanized nails or screws (or 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.
Tim Carter has worked as a home improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.