Finishing the formwork

The rebar was in place and we could indulge in playing with plywood sheets for the foundation wall formwork.

I had taken great care to assure the footing, the footing plates and plywood strips were all level. While we were installing the plywood sheets for the formwork, everything turned out plumb and square. Our previous due diligence paid off and I had a huge weight lifted off my shoulders!

But that took only care of half of my worries. How to keep the formwork from buckling under the pressure of the freshly poured concrete was the next big thing on my mind.

I had purchased a box of snap ties that would help us tying the formwork together. My plan was to space them 24 inches horizontally and 16 inches vertically across the formwork. That meant that I had to pre-drill the plywood sheets in the 24 by 16 inch pattern while setting up the formwork. It was critical to keep the holes on the outside lined up with those on the inside to make sure the snap ties fit across the formwork.

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Although I haven’t even ordered the concrete yet, it was already time to think about the finished wall and how to remove the formwork once the concrete had cured. To prevent the concrete from stubbornly sticking to the plywood, I sprayed the formwork interior with an oil based release agent, which makes it relatively easy to remove the formwork.

All the plywood sheets were up, and the job turned counter-intuitive. We had to take the interior formwork back down and lay it flat so that we could install the snap ties through the outside formwork. To keep the ties horizontal and in place, we installed the brackets and wales and tightened them up.

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We knew that if we did a good job on the pre-drilling and lined up all the holes, we should be able to lift the inner formwork back up and in the process slip the snap ties though the pre-drillled holes. Bingo! Everything lined up again and we finished the job by installing the brackets and wales on the inside, and braced everything at critical points along the inside and outside.

Getting artful

Everything on this formwork is straight, with the exception of the existing limestone foundation wall to which I have to connect. If I don’t want the concrete to pour all over the place but rather keep it in the formwork, I need to produce a tight fit to that rather fancy limestone wall face.

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The old scrib method came in handy again. I ran a simple compass along the limestone face and thus translated that profile onto an adjacent plywood sheet. To cut the profile, I used a jigsaw, and after a little bit of fine tuning here and there, I got the tight fit I needed.

My helpers were rather impressed by the simplicity and ease of this method.

Future proofing

I had given some thought to what utilities may need to pass through the concrete foundation wall. There is for instance the sump discharge line that would connect to a future cistern. But there are also other items such as electrical lines that may lead from the house to the back porch and into the yard.

Rather than pouring the foundation wall and core-drilling through the concrete later to accommodate those utilities, I decided to preemptively install sleeves into the wall through which I can feed those utilities and avoid the whole core-drilling issue.

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Just one of those little details that may pay off one day…

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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