Spalling on an Epic Scale!

by Tim Buell September 1, 2012

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Photo: A huge area of the brick facade of Fort Jefferson spalled away.

Last month, I promised to expand on the tragic design flaw I observed when I visited Fort Jefferson in the Dry Tortugas. (Last month’s article.)

In modern construction, the use of steel reinforcement in concrete has become the norm. It was not always this way. Usually, not too many problems arise as a result of doing this as long as the concrete is of good quality and the steel is not close to the surface. The concrete itself creates an environment that naturally protects the steel from corrosion. If a pathway allows water and other corrosive elements such as salt to find their way to the steel, it can rust. As steel rusts, oxides build on the surface, causing the steel to expand. The expanding steel causes the concrete around the steel to “spall” — to peel away. This exposes the steel to even more water penetration, leading to an even faster rate of corrosion.

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 Photo: Spalling due to rusting rebar.

As my wife and I toured Fort Jefferson, I could see where the surface of a wall had been pushed away, exposing rusting steel. See the photo above.

In brick homes, steel lintels over doors and windows can rust and cause another issue that is mechanically similar to spalling. The rusting can result in lifting the bricks above the lintel and cause diagonal cracks that radiate upward and away from the ends of the steel. See the photo below.

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Photo: A diagonal crack from expansion of rusting steel.

 I saw some of the most extreme examples of this condition at Fort Jefferson in the Dry Tortugas. When the fort was built, tons of iron were used to reinforce areas around the gun ports to provide additional protection to these areas were they to be struck by cannonballs. This was a new design element. Previous to this fort’s design, giant pieces of stone were used instead of steel for this extra protection.
If someone were to sit down and figure out how they could install steel in such a way as to cause its quickest demise, they would come up with the way it was done at Fort Jefferson. None of the steel was truly encased in protective concrete. Instead, it was installed in the porous coral-fill behind the brick veneer and in the brick veneer. It was vulnerable to round-the-clock condensation.

The roof of the ramparts all around the fort were covered with earth, which allowed rain to be absorbed into the structure, where it found its way to the steel. See the photo below for a view from the top of the fort.

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Photo: The view from the top of the fort

These conditions guarantee the steel reinforcement is continually being attacked from within and without.
In most cases, the reinforcement was not small diameter rods like we use in modern construction, but instead was massive iron plates weighing hundreds of pounds.

Three gun ports appear in the photo below. The port on the right has been rebuilt and appears much as intended — minus the gun sticking out. The two ports at the left show extreme spalling and much of the brick and steel is in the moat. If you look carefully, you can see several of the massive steel billets that used to frame the ports. They measured approximately 6″x16″x48″— approximately 1,500 lbs each.

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Photo: The spalling of the brick facade with three gun ports. The gun port on the right was rebuilt
.

 In the close-up photo of a gun port below, the large, heavy iron plates around the openings are clearly visible.
As these metal components rust and expand, they push all the bricks and themselves into the moat.

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Photo: Closeup of the gun port

The complexity of maintaining such a structure is immense. Hopefully, as they rebuild the areas around the ports, they won’t replace the iron, but instead use materials that are more conducive to the environment — given that the gun ports are not likely to come under heavy cannon fire any time soon.


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