BUILDING YOUR OWN ROCK WALL USING SLIP FORM part 2

It was a long couple of days as I tried to contain my curiosity to see how our first effort at slip-form masonry had turned out. (See Tuesday’s  post Building a rock wall using slip form part 1 ) We placed the stones into the form and filled in behind them with concrete and smaller rocks.  But it’s the nature of a slip-form technique that you can not see the finished effect until the concrete is set, and you remove the form.

I had to do some phone interviews for a writing assignment and by the time I got out to the house site Monday morning, our mason Tom Spicer was already hard at work.  The frames were removed and stacked and Tom was working on the next step.

Stone wall unveiled!

Though the mortar mixture is pretty thick, quite a bit of it does ooze through to the front.  Some of that can be avoided by placing small chips of rock here and there to block the flow, which we did to some extent, but obviously not enough.  And you can also fit chips of rock into depressions on the front of the rocks that otherwise might fill with cement.  Those can easily be chipped away at this point.

My first assignment was to take a hammer and start knocking off all the concrete goobers.  It was a long, hard job.  Some of them chipped away with very little resistance.  Others took a lot of convincing, and pounding on rock with a hammer for several hours will communicate a lot of shock up your arm.  A LOT of shock.

Tom mixes the mortar. You use as little water as possible, and end up with something the consistency of crumbly pie dough, which like pie dough holds together when compressed.

The next step was to fill in all the openings between the face stones with mortar.  The goal is to eliminate even the smallest cavities where water can do it’s freeze/thaw thing and gradually start to crack the wall.

For mortar we mixed up a thick paste of water and the same Portland cement used with gravel and sand for filling the wall.  But this time we left out the gravel so it would make a smooth, tight filling for every nook and cranny.

There is more than one way to mortar.  You can just fill in the deep cavities, but leave a depression around each stone and have a lot of rock jutting out.  As dramatic as that would have been, we chose to take a more fully-mortared approach to make sure that the surface was as solid as possible.

Tuck pointing.

It turns out that I like this look.  It reminds me of the sturdy stone foundation of a classic  old barn.  It’s a solid, authentic look.  I like that our house is sitting on this kind of wall.  We will use this same stone wall technique to make a raised foundation around the upper floor of the house for the straw bales tosit on.

I am a novice with a trowel.  I found myself using my fingers as much as the trowel to push the mortar deep into the cracks, and I just pressed and smoothed with the trowel as a final finish.  Tom recommended that I wear my rubberized gardening gloves for this step to protect my skin from the caustic and drying affects of the mortar.

I mortared this part all by myself.

I appreciated that the hawks (metal trays you hold your supply of mortar on) were small.  That made it easy to get right up to the cavity and scoop it in.  And it wasn’t too heavy to hold for hours.

The next day, while the concrete was still a little malleable, we did the final cleaning up process.  Tom said this is sometimes done with a brick, but we didn’t have any bricks, so we used small pieces of the same stone the wall is made of.  I felt very Neanderthal hunting around for just the right rock to use as a tool.  I ended up with a collection of several that were shaped just right for various tasks.

There is something very elemental about holding the perfect-shaped stone in your hand for a serious task.

Cleaned on the left. Not yet cleaned on the right.

We used our stone tools to break off all the ragged edges of the mortar at the edges of the stones and to smooth the mortar between the stones.  It was a fatiguing and yet relaxing and satisfying to crouch in front of yesterday’s handiwork  and pound, scrape and grind it into final form.

Too bad there was no one I was frustrated with.  I could have worked off a lot of hostility these past two days knocking off concrete goobers and pounding the mortar into shape.

I’m very pleased with this wall.  And I’m even more please to have had a hand in the making of it.  This is indeed a very satisfying way for a novice to set stone, and as I said in the previous post, Tom calculated that because of the locally-sourced stone in the wall, he only used only a small fraction of the Portland cement it would have taken to simply pour a solid concrete wall.

Tom and Bryan get ready to set a plate of treated lumbar on the stone to connect the stone and the framed wall above it.  The mortar becomes much lighter as it dries.

In a perfect world, I would have preferred to build the section of the upper floor foundation around the back of the house first in order to gain some experience on a less front-and-center section of our slip-form walls.  After all, the choice of neighboring rock faces is a bit of an art form, and some practice would have been nice.  But you build a house from the basement up, so this section was the place to start.  And as I said, I’m very happy with the result.

Now it’s on to the building the basement walls, including the first un-milled timbers.  Also, the well drilling was started today!

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9 replies

    • You could build a wall, Lorijo. It helps to build it in a group to spread around the heavy lifting. The second day when we finished laying the stone with six people all working on different sections of the wall, things went really fast. The first day with just Tom and I had a much slower pace. I was a little concerned that the wall would end up with 6 different patterns in it, but the rock kind of demanded a certain regularity.
      I am sure getting a sense of accomplishment every time I look at that wall.

  1. It looks good, it has a lovely warm finish to it. I think I shall have to post some pictures of the type of walls they do here in Latvia so you can see the difference. You might have to wait a while though!

  2. Thanks, Joanna
    I’d be very interested in Latvian stone walls.
    I think natural walls are very indicative of their place because the rock is different and the mortars can be too. (although the dreaded but durable Portland cement is pretty universal, I suspect.)

    • I suppose the question is how long has Portland cement been available relative to the ages of the houses around here. Not sure how old they are really.

  3. I agree: working with stone is deeply satisfying. I’ve never tried a project with mortar, but I think I’d be right there with you, using my fingers to push it in. I think your end result is lovely and quite deserves front and center.
    Eleanor

  4. Thank you, Eleanor. Building this house is even more of a constantly engaging journey than I imagined it would be.
    One of the things I could not have imagined is how the personalities of the crew would color every day.
    Working with Tom was really an education. He really knows his stone and concrete and has made many of these walls, but he has a very easy-going, play-it-as-it-lays way of working and supervising that gives a novice assistant a lot of ownership.
    So that wall not only looks good to me, it has good associations.

  5. I think the guy in charge of that operation didn’t know enough about slip forming at all. He used too much gravel. Causing too many small gaps. Thus weakening the whole wall. Well. Considering that slip forming this way creates major cold joints, the shear strength of the wall relies on the rebar and the solidity of the concrete. Also. You could have placed sand on the front faces of the stone to keep concrete from running to the front. Pointing is an illusion of a strong all. Underneath it is the truth. And as you can see in the photos, that wall is full of air.

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