Building a concrete wall using slip-forms

Previously on The Havens, I mentioned in passing that we were getting set to build a strawberry bed.

Work is well underway on this project.   In fact, we began the initial work yesterday in between me writing my Garden Bloggers Bloom Day post and continuing the cleaning out of the day lily garden at the north edge of the Stroll Garden.  Today I helped build this wall in the morning before I had any massage clients.

I believe I have also mentioned previously how difficult it is to get an award winning picture in this garden, due to the work in progress.   For example, if I am trying to get a long view of the Stroll Garden from the back door, there is a heck of a lot of stuff in the way that a professional photographer would view with a jaundiced eye.

That is, unless (s)he were intending to do a photo essay about how to build a concrete wall using slip-forms.   Then all that stuff would be an object lesson.

The first picture would illustrate that in order to build a rock and concrete wall without using too much of the expensive concrete, you need a good collection of rocks of all sizes.   The more rocks you have, the less concrete you need.  These rocks came off our place, emerging from the ground during the vineyard preparation five years ago. For a long time we used them as a rock mulch for the grape vines, but that became a logistical nightmare for weed and fungus control, and so Jim removed them from the vineyard and piled them back in the savanna for future reference.   This is the future.

The second photo illustrates the panoply of equipment that you will need for the job.   The little cement mixer is electric, and was an impulse purchase that occurred once when Jim went to a tool show unaccompanied.   When he brought it home, I had no idea why we needed a cement mixer, but it turns out that we need one on a very regular basis, especially since we put the concrete block bed borders into the vegetable garden.

The picture below illustrates why it is necessary to have at least two wheel barrows.   We have two wheel barrows in use at the same time on a regular basis.  Today there was sand in one wheel barrow and the portland cement in the other.   That way when the evening falls we can move any unused cement into the carport easily where it will be safe from the dew fall.   (Damp cement is counterproductive.)  The sand arrived here in the back of the pickup truck, we buy it a half yard at a time from our local cement block purveyor.

Making the mortar requires a judgment call.   You start with about a gallon of water in the cement mixer, add one shovel of cement and one shovel of sand and start the machine turning.   As the cement mixer continues to turn, you add three more shovels full of sand and sufficient water to make the mortar the right consistency.   It can’t be too runny, and it can’t be too stiff or it will be hard to work the mortar down into the interstices between the rocks.   It seems like the proper consistency is sort of like a rather thick muffin batter.

The next shot is a view of the trough Jim made to pour the portland cement mortar into once it is “right”.   We scoop the mortar out of the trough, which in this picture is resting on our venerable garden cart.   That item is loaded with rocks for the wall.   The buckets contain little rocks and there is one bucket that has water in it for keeping our tools clean.

Finding a bucket that will hold water is starting to be quite a challenge at The Havens.   So many of them have been sitting around in the sun they have become brittle.   Then the second you do something the slightest bit stressful, like throw a few rocks in it, the bucket cracks.   Good bye water holding ability.    Today I about decided that it was time to acquire some metal buckets that will last.

In addition to rocks, sand, cement, a water source, wheelbarrows and buckets, you need a hoe, a couple of things to schloop the cement out of the trough, some old butter knives for pointing the mortar, and the slip forms.  I found rubber kitchen gloves to be invaluable hand wear, giving me enough sensitivity to work the cement in between the rocks, but remaining stout enough to stand up to handling rocks.

The next picture shows the site once it was prepped for the wall to be built.   First the trench was excavated to 6 inches depth and backfilled with road base.  Jim spent a couple of days tamping the road base by tapping it with the big sledge hammer, since we don’t have a mechanical compactor.   Then he pounded rebar into the trench through the road base and down into the undisturbed dirt beneath in order to anchor the wall.   He doesn’t want it to go anywhere later on.   Also, he established the lines that indicate the straight and level course for the top of the wall.

The next picture shows the slip forms in use.   These very useful pieces of equipment are designed to clamp onto the part of the wall you have already built, thereby making it possible for you to create nice high and straight walls.   Notice the turnbuckle located down at the top of the form.   That is what clamps the form to the wall beneath it.  We are only going up 16 to 18 inches, but we have friends that used these very forms to make the walls of their house over 10 feet tall.

You can see the progression of the wall in the above photo.   First you schloop in a layer of cement maybe one to two inches deep.   Then you start placing rocks.   This is where having a varied selection of rocks available is a real asset.  Ideally, you will have rocks that have flat sides that you can lay in right next to the forms.   Then you throw mud (mortar) at those rocks and wedge them into place using the odd shaped rocks and smaller pebbles you can find in your pile.   Remember, the more rocks you have in the forms, the less cement you will have to make.   But there has to be enough mortar to hold the rocks in place.  It is important to have mortar contacting the surfaces of the rocks and filling the holes in between them so your wall will be strong once the mortar sets.

Once you have the form filled with rocks and cement almost to the top of the form, you let it sit for about a half hour to let the cement start to set up.   Then you loosen the turnbuckle and lift the form up and away from your setting wall — carefully, so you don’t dislodge the rocks.  Then you use the butter knife to point in the mortar on the outside of the wall before the cement has completely set up.  That is what Jim is doing in the following picture.  He is catching the excess mortar, then he throws it down on the work surface where we are going to work next.

Later, after the cement sets for an hour or so, he begins working over the surface using a wet stiff bristled brush, so the excess mortar comes off the rocks.   The next day he will use a wire brush to remove even more.   This is the way the wall looks after it has sat over night, but before the wire brush cleaning.

After the first course was built up about a foot or so, Jim laid in horizontal rebar for more reinforcement, and tied it to the verticals.   We raised the forms and continued filling them with rocks and mortar.   When we had the wall built to the height we wanted it to be less about half an inch, we stopped adding rocks and Jim added a top layer of mortar which he smoothed off.

Below is the completed wall after the top layer has been added and the pointing has been done.   At this point there had been no brushing done, it was still too wet.   We will be doing quite a bit of “cleaning up” of the front of this wall, because the flat rocks that were against the forms were chosen for the excellence of the crystal formations and striping in the karst limestone that “grows” out of the fields here.

You can see the horizontal rebar emerging from the unfinished end of the wall at the right edge of the picture.

We figure in about a week, we will have the wall completed for the new strawberry beds.   Pretty cool, huh?