Q: My question regards the best way to stabilize the perimeter foundation on my 1872 farmhouse. The "concrete" surface is turning to dust in places where the harder finish coat has been chipped or broken away. I know the core of the continuous perimeter foundation is good and hard, because I’ve drilled into it when I bolted the house down and shear-paneled the cripples.
Small gaps are appearing between the mudsill and the concrete and my thought is to stuff in some oakum strands before applying the concrete finish.
What product might stop further spalling? What kind of concrete finish coat do you recommend?
A: Simply stuffing some oakum in the gaps and covering it up with a cement topcoat isn’t enough and will only be a cosmetic fix.
Oakum is an oiled ropelike material used back in the day for joining "hubbed" cast iron drainpipe. It worked because lead is soft, oakum is flexible and the water pressure in drain, water and vent pipes is very low. But the flexibility of oakum makes it unsuitable for foundation repair.
Concrete spalling occurs when moisture is wicked into the foundation from the ground and the lime in the concrete mix leeches out. Spalling looks like powdery pockmarks on the finish of the concrete.
Depending when the foundation was placed under your 1872 house, it may not be structurally sound, never mind meeting current building codes. We doubt that the foundation is of 1872 vintage. But if it is, the concrete was almost certainly hand mixed in a wheelbarrow and does not contain any steel reinforcing bar (rebar). If it’s a retrofit, no telling how it was done. The fact that you had to bolt it suggests that reinforcing steel wasn’t an option when the foundation was placed. …CONTINUED
You’ve got two choices. Tear out the foundation and replace it with a code-compliant new foundation, or "cap and saddle" the existing foundation. The best choice is to tear it out and start anew. But because you believe the interior of the foundation is solid, cap and saddle may also be an option.
Cap and saddle is a description of encasing the existing foundation in newly reinforced concrete. Check with the local building authority to see if this solution is allowed.
"Cap" refers to increasing the height of the foundation wall by pouring new concrete on top of the existing stem wall. "Saddle" is increasing the width of the foundation by pouring concrete on each side of the existing wall. Each side of the "saddle" should extend below grade to the depth of the footing.
Two pieces of No. 4 rebar are placed in the cap and in each side of the saddle. All the rebar runs horizontally. Finally an inverted U-shaped piece of rebar is wired to all the horizontal runs to tie them together. When complete, the foundation wall, viewed in cross section, resembles a horse saddle.
Remove the wood mudsill, but leave the bolts you’ve installed. They will serve to tie the new concrete to the old. The stem wall will be 4 to 6 inches higher and 8 to 12 inches wider.
To prepare, take a wire brush and remove all the loose material. With the wall clean, treat it with a concrete bonding agent. Then cap and saddle away.
If you decide to just go for the cosmetic fix, the preparation is the same but use a moisture-resistant concrete patch.
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