How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
There's cracks running along the home's concrete tie beam. What's wrong?
Wednesday, July 11, 2018
You likely have concrete “spalling,” which is caused by moisture penetrating the layer of concrete covering the reinforcing steel in the beam, column, or foundation pier. Over time, 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 along the coast, where a salt mist gets blown over the surface of the beam from the ocean breeze. It starts appearing in oceanfront homes in Cedar Key as early as 20-years after construction, but occurs eventually at inland homes--just much later.
Any exposed area of steel-reinforced concrete can develop spalling, including floor slabs and precast components like window sills (example shown below).
Oceanfront exposure is actually only one factor that allows spalling 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 blog post What would cause long horizontal lines of brick mortar to fall out?
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