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How does concrete spalling cause structural failure if not repaired?
Saturday, June 26, 2021
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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