What is concrete spalling?
Saturday, January 19, 2019
Spalling is the deterioration of a concrete surface over time, with crack lines forming first, then chunks of concrete falling out as the problem advances. It is caused by moisture penetrating the layer of concrete covering the reinforcing steel in the beam, column, or foundation pier. Eventually the steel begins to rust inside the beam and, because rust is slow but very powerful expansive process, cracks appear at the surface over the reinforcing steel. It‘s a progressive deterioration, because the open crack allows more moisture to accelerate the corrosion--formally known as ferrous oxide scale--which opens the crack further. Eventually small chunks of concrete begin falling out and, if left without repair long enough, structural failure follows.
Spalling is more prevalent at homes near the Florida coast, where a salt mist gets blown over the surface of the beam from the ocean breeze. It starts appearing in oceanfront homes in the Florida Keys, for example, as early as 20-years after construction, but can happen eventually at inland homes—just much later at 40 to 50 years.
Any exposed area of steel-reinforced concrete can develop spalling, including precast components like window sills (example shown below), and the concrete shell of a pool.
Oceanfront exposure is actually only one factor that makes spalling more likely to begin. Too little concrete coverage over the steel (less than about one-and-a-half inches), poorly placed concrete with air pockets in it, and an upset of the natural alkalinity of the concrete can all set spalling in motion. In the photo below, corrosion of reinforcing steel that has been set too close to the bottom of a concrete second floor walkway has started to pop off chunks of concrete below it. The cantilevered slab has temporary supports in place while it awaits repair, because of the loss of structural integrity.
Even a concrete slab on the ground can develop spalling if the steel mesh is pulled up too close to the surface, as in the photo below.
The fix is simple but labor-intensive: each crack line has to be jack-hammered open to expose the surfaces of the steel reinforcing bars. The rust is then cleaned away with a chemical solution like Ospho®, shown below, and stiff brush, coated with an anti-corrosion solution, and then a special concrete/mortar mixture is packed into the damaged area and smoothed out along the surface.
Also, see our articles How does concrete spalling cause structural failure if not repaired? and What would cause long horizontal lines of brick mortar to fall out? and How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?
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To learn more about exterior walls and structures, see these other blog posts:
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