Concrete reinforced with steel is an almost magical combination. Each material has a structural quality that compensates for the other’s weakness. Steel has excellent tensile strength, which means that it resists bending, but it is weak in compression. Concrete is strong in compression, meaning it’s good at resisting being crushed or deformed, but it has low tensile strength. When you mold the concrete around the steel, tightly bonding the two together, you get a heroic, super-strong combination.

    Unfortunately, like all super-heroes, there is a kryptonite it can’t resist, and that’s water. If water penetrates the concrete—either through small cracks in the surface or because the steel was placed too close to the surface and it soaks in—when moisture reaches the steel reinforcing bars, they start to rust. Rust is a slow but powerful expansive process. As the rusting steel bars expand, they open up the concrete cracks further, more water gets inside, and the crack-and-rust cycle speeds up. Chunks of concrete start to fall off. This process both weakens the steel and separates it from the concrete, ruining the strength gained by their combination. 

    It’s called “spalling” in the building trades. Reinforced concrete buildings that are close to the ocean are especially vulnerable to spalling because they are being coated 24-hours a day by ocean spray. And the salt in that sea mist further accelerates any steel corrosion.

    We are writing this article just a few days after the collapse of the Champlain Towers South condo in Surfside, Florida. It is too early to determine the exact cause of the structural failure, and it will likely turn out to be a combination of multiple factors. But spalling is the bane of all oceanfront concrete structures and definitely contributed to the disaster.

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Here’s links to some of our other blog posts about CONCRETE AND CONCRETE BLOCK:

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• Do stucco walls mean a house is concrete block?

• What causes stair-step cracks in a block or brick wall?

• Why is my stucco cracking? 

• Why is the concrete window sill cracking?

• What causes a horizontal crack in a block or brick wall? 

• What are the pros and cons of concrete block versus wood frame construction?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize structural problems in a retaining wall? 

• Should I be suspicious about a concrete block house covered with siding? 

• What is fiber reinforced concrete? 

• How can I tell if cracks in the garage floor are a problem or not? 

• How do I recognize serious structural problems in a house?

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What causes a vertical crack in an exterior concrete block or brick wall? 

• Why is there a "WARNING! POST-TENSION SLAB" sticker in my house?

• How can I identify a home as ICF (Insulated Concrete Form) construction?

• How can I tell if cracks in the wall or floor are getting worse or staying the same?

• Why do concrete blocks have holes in them? 

• What are those powdery white areas on my concrete block wall? 

• Is efflorescence a serious problem? 

• What is concrete shrinkage? 

    Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.