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How to fix holes in your home's concrete foundation

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By Tim Carter
Friday, February 11, 2011; 1:12 PM

Q. I have to make an unexpected repair to my poured concrete foundation. A water leak led me to remove drywall in my basement, and to my surprise, I discovered rows of holes 5/8-inch diameter in my concrete walls. Some of the holes have water dripping through them. These perfectly drilled holes are not random and are as deep as the foundation is thick. What created them? How can I patch them so they don't leak? What's the best material to use when doing concrete repair for cracks or holes like this? - Bryan R., Cincinnati

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A. The holes in your foundation were not drilled; they were created by smooth steel rods that were part of the foundation-form panels used to create your concrete foundation.

These rods passed through the concrete forms. Slots at each end of the rod held a steel pin that prevented the forms from expanding outward under the enormous pressure of the liquid concrete that filled the forms to create your foundation walls. Once the concrete set and hardened, the form panels were removed and the rods tapped out with a hammer, leaving these holes with the smooth bore.

I have to believe that the foundation contractor used a concrete repair product at the very least on the outside face of your foundation that's now covered with dirt. Unfortunately, he may have used the wrong product for basement concrete repair.

There are many concrete repair products out there, but my personal favorite for this situation is a powder that contains Portland cement, bentonite clay and some other ingredients that cause the patching material to harden and expand at the same time. Bentonite is a fine clay that expands when it gets wet.

Many homeowners might try to repair and patch these holes with bricklayer's mortar or a mixture of Portland cement and sand. Or they may think that an epoxy concrete repair is even stronger. The trouble with regular mortar and Portland cement is that they shrink ever so slightly as they harden and cure. This creates a tiny pathway for water to enter.

You need a product that actually expands as it cures, much like you see with spray foam insulation. If you've ever used this foam, you know it goes into a crack one size, but hours later, it's much bigger, as the foam has hardened.

These expanding hydraulic concrete-repair cement products are available at hardware stores, building supply businesses and home centers. They come as a dry powder in a can and will clearly say on the label that they expand as they harden. This is mission critical. Look for that on the label.

These products often contain ingredients that cause them to harden pretty quickly. It's not uncommon to have work time measured in just a few minutes. I'd only mix up what I could use in 10 minutes. You can sometimes extend the work time by refrigerating the powder to get it cold and using very cold water as you mix it.

Be sure to vacuum out the holes and remove all debris. If you can insert a small bottlebrush to get out any dust or silt that's on the concrete, this will really help ensure patching success. You want the concrete surface in each hole to be perfectly clean.

Just before filling each hole with the expanding concrete-repair product, use an old spray bottle and spritz the hole with a spray of clean water. You want the surface of the inside of the hole to be slightly damp. This will really help the patching material bond with the dry concrete.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site, www.askthebuilder.com .


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