A Ferrocement Bench
and a description of the process of ferrocement

 
 

A ferrocement project usually starts with a Rebar frame. This one is extremely rigid, with a lot of short diagonals criss-crossing throughout the structure - all intersections are welded (wiring is acceptable for some projects, but definitely not this one). Then, the skeleton is covered with mesh - in this case, expanded steel lath. It is wired in place, enough to keep it firm when you're applying the cement. Chicken wire is sometimes used, when there is some backing behind the surface to help the cement stick.
I used 1/2" and 3/8" rebar. I make most of the bends by stepping on the rebar with my boot and pulling the long end with my hands - which is sometimes not possible with the 1/2" rebar, so I use the hole in my anvil or use a hammer to beat on the rebar over a two-pipe tool or a depression in a stump.
 

A good mix for the cement:
in a wheelbarrow, add:

5 or 6 shovels sand

1 shovel Masonary cement

1/2 shovel Portland Cement

Add water as you mix with a hoe, until the mix is loose enough to work EASILY.

Then add: 1 handful of fibermesh fiber and mix some more

 

  Next, the first coat of cement is applied. In a case like this, it is done with your hands - with some tough rubber gloves on. The idea is to push the cement through the lath so that it has the cement on both sides. The cement pushes through the mesh and reattaches on the other side. Then, when you've got the cement well pushed into the mesh, you can trowel on some more to build it up a little more. You want to leave the surface rough, so that the next coat has plenty to stick to. If you're outside, you MUST be in the shade - string up a tarp if you have to. The mix should be almost fluffy, just a little shy of being runny. Too dry, and the cement yill be hard to work and won't stick well. In hot weather, your batch may be starting to stiffen before you've used it all up, so you may have to work quickly.
 

OK, now it goes without saying that the cement is well taken care of after this point. This is ESSENTIAL for good ferrocement, especially in hot weather. As soon as the mix firms up a little (1 or 2 hours depending on temperature), I drape some plastic over the work, to keep the moisture in. Then, as soon as the cement has firmed up to the point where it won't wash away, I spray it with water and wrap it with some wet burlap or other rags and recover it with the plastic. Then, every few hours, I spray the burlap down. The reason for this is that cement gains its strength by incorporating water into its matrix. if you let it dry out too soon, it will be weak and powdery.

Mixes vary. You can use Portland and lime. You can add soil. The clay soil around here makes a nice, silky mix. I've read about people in asian countries adding rice-hulls to their mix. Strength was critical for this bench, so I kept it basic and cured it for nearly a week, but for things where strength is not so critical, I reccommend experimenting. From what I have heard, when soil content exceeds 25% weatherability begins to suffer. Making your mix richer (more cement) than 1 part cement to 2 parts sand is not a good idea. It can crack when setting. White portland seems to set quicker and can kick suddenly in heat. It also hydrates in the bag (gets lumpy) sooner and it's more expensive than grey portland.
 
 
  Normally, one recoats the next day with the same mix as the first coat, although depending on the level of finish you want, you may omit the fibers. And of course, you put your color in this layer. This particular bench was finished with an acrylic additive topping cement called Patchcrete. It's harder to work than normal cement because it is so sticky, and you can't use water to slick it. I found that I got a nice finish by dragging the surface with a dry paintbrush after I had troweled it up to the level I wanted, usually about 1/2" for the patchcrete (more for normal cement)  
 

This bench resides at Terra Studios now, in a lilac grotto. You can go sit on it there if you like. People remarked that it was quite comfortable, for something made of concrete.

More ferrocement benches

 

 

Here's a ferrocement feaure which I did on the side of the new studio.The blocky part was sketched out with pieces of steel lath - it's pretty easy to form and cut - sharp edges though, so gloves are a good thing. Then the shapes are attached to the wall with pieces of wire and deck screws. The flat part has chicken wire in it, which is nice for open stretches because the cement trowels in easier than the lath. The tough part about this piece was gettinng the first coat to stay as I was squishing it up into the mesh. I dropped a lot, so I set up a board on the ground so I could recycle the droppings. This was finished out with a small squared off trowel. The textured part was done by dragging the tip along while pushing it in and out.

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