How to reinforce a stone foundation wall with a new concrete wall

Many older homes have foundations made of field stone that may have sagged or warped over time due to the elements pushing on the stones. Since mortar as we know it today was not used, the stones can be easily moved. If there has been a serious total collapse of the stone wall, it needs to be completely replaced. If the problem is less severe, a new concrete reinforcing wall may be necessary. If you have a dirt basement floor, the job is even easier. Starting on the inside of the basement, you must first excavate a trench along the existing wall to install a new base for the backing wall. I would make the base at least two feet wide, but three feet wide is much better. Make the base a foot deep. The top of the new footing should meet the bottom of the stone wall, and the new flooring will sit on top of the footing. Be careful not to disturb the soil under the stones. You will not be able to jog under a stone wall. Once you’re done digging, form the foundation and install the rebar. Use bars that are at least 5/8′ in diameter. Lay them out along the base and on a three foot wide base there should be three continuous bars. Now take four foot long cut pieces and bend a one foot curve at one end to form a she. These bars shall be tied to the innermost and outermost reinforcing bar of the footing to form two rows of dowels extending the full length of the footing, no more than thirty-two inches apart. When the foundation is poured, you will only see the vertical part of the bars sticking out. Once the concrete for the base is dry, remove all the forms leaving no wood for termites to attach to for dinner.

Now, using rebar of the same size, install horizontal bars starting three inches up from the base, placing one every twelve inches along the entire length of the new wall. Do this on both rows of studs. These, when finished, are called wall mats. Using a few small pieces of rebar, wire tie them between the two mats to keep them apart when you pour the heavy concrete. The new thickness of the wall will depend on how thick the stone wall is and how arched the stone wall is. My last wall was basically twenty four inches thick, but twelve inches thick is the minimum you should build. Your rebar wall mats should be as close to three inches from both sides of the wall as possible. The interior carpet may be a bit longer depending on the amount of arch in the stone wall. I used Symonns concrete form panels from my rental shop to help form the new wall, but you can also build the forms out of wood. The advantage of concrete formwork panels is that they come in all sizes, heights and widths, have clips for easy fastening, have precast anchors, etc. You must remember that this is a one-sided form. The most difficult type to build. You need to brace it all the way and then add a few more reinforcements to keep it from shifting under the weight of the wet concrete. Installing what is called a whaling brace will keep the shapes from bending, but will add a lot of bracing. Specifically per day, you will need extra help. Pouring through a base window is no joke, as those inside cannot see the one outside in the ready mix truck.

A man has to stand outside and shout orders through the window. Faster, slower, more, too wet and so on. Inside, a man doesn’t have to do anything but observe the shapes. If something moves, he must immediately stop the spill. Being buried under wet concrete and shattered form panels is no joke. Another person has to vibrate the forms with a mechanical vibrator to ensure there are no voids in the concrete. Another person can hit the forms with a hammer to make sure there is concrete in all the spaces and look down through the top of the form to make sure they fill as level as possible. This eliminates putting too much pressure on one point of the forms at a time. Too much pressure can cause the forms to fail. Wet concrete exerts what is called hydraulic pressure laterally against the formwork and can amount to tons of pressure. I’ve seen forms that were blown a hundred feet from the job site fail. Depending on the thickness of your new wall, a two-day wait to remove the formwork is more than enough. Twenty four hours is even better. Any spilled concrete will become very hard and make removal of the form much more difficult if you let the concrete sit too long. Again peel all the wood remains of the work. Backfill and level the floor area to prepare it for the new basement floor.

Pete

Your friendly building inspector

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