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Concrete can be shaped with forms, molds or by applying it to an armature.
“DIY Stone Lantern” shows how to work with hardware cloth which can be made into an armature for pretty much anything small scale you can imagine.
“Concrete Spheres” shows how to build a round armature out of metal lath and use a simple form for creating hollow spheres.
This page will go into further detail for building large scale armatures including preperations for hanging concrete vertical for stamping & sculpting as well as how to pour molds.
Building Large Armatures
Armatures can be built into any shape imaginable. Ponds, statuary, pillars, walls, boulders, caves, furniture, you just dream it, design it and build it.
Building an armature for large projects is usually done with rebar bent into a skeleton. ½” is ideal as it can be easily bent into the desired shape but when several are tied together the structure soon becomes strong enough to climb on.
Rebar can be nasty stuff to work with. It is often rusty and if you've ever tried to pull one of these ribbed bars out of the ground you will clearly understand why you want to be careful not to loose your footing around their freshly cut ends.
If you’re just doing a small project, rebar can be cut with a metal blade on a circular saw or grinder. If you plan on doing multiple projects or just on big project, a special purpose cutter is well worth the investment. their are several different kinds of rebar cutters to choose from. I use one of the lower priced hydraulic cutters and have made hundreds of cuts with it. They are highly maneuverable and effortlessly cut through rebar of the size they are rated for.
Rebar armatures can be built freestanding atop the ground or you can build over sculpted earth for waterfalls, ponds, cliffs or retaining walls and the likes following the newly made curves of the excavation. It can also be shaped over foam, cardboard, old concrete, rocks, debris or any other junk to create a basic shape.
You will want to wear gloves and safety goggles when handling and bending because it throws out fragments as you bend it. You can bend it over your knee or under your foot but there are special rebar bending tools available for the purpose.
The armature bars need to be secured together everywhere they cross for the strength of the structure. They can be welded or tied with steel tie wire. The wire can be cut into 4” to 6” lengths and twisted with a good pair of pliers. To loose and there's little strength, too tight and the wire snaps causing you to have to retry. Doubling the wire over will help prevent breaking. Just snug is what you want.
There is a pull tool made for tieing wire with special purpose wire which has loops at each end to accommodate the tool but it can take some practice to make it work properly up in the air where much of your work will be done.
Start by bending a rebar into the desired shape of a two dimensional silhouette. Then run the next one the other direction outlining the shape of its width tieing the two together where they cross one another with tie wire.
Continue running bars criss-cross and tieing them together until you can clearly see your shape. At this point it gets a bit trickier. You need to run horizontal bars now at about 12 to 16” apart. A good method is to securely tie the end of a bar at the desired level and bend the bar over to the next vertical bar. Tie it there and continue. If the bar has to be pulled under tension and tied to hold it (which will often happen) this is a good thing. This is similar to how they make extremely strong concrete parts for bridges and such.
Once your skeleton is complete it's advisable to keep it off of the ground. It is said that if a bar is shoved into the earth, it will rot right up the bar and throughout the structure causing eventual deterioration of the structure. I don't know how true that is but I always make sure my bars are completely encased in concrete when I build something for a client.
If you are building a large free standing boulder or a concrete pond or such, you can just go around the perimeter and lift it onto flat rocks, bricks or chunks of scrap concrete. For a pond continue placing chunks under bars until the whole thing is an inch or so off of the ground. For a freestanding structure or pond you will want to cover it with chicken wire discussed in the next step before lifting it onto the blocks because it's going to get a good shaking during the process.
If you need to secure your structure to the earth such as for a cliff, a tall garden wall, pillar or fence, dig a hole and pour concrete in the hole, then stand a rebar length up out of the concrete. Once the concrete cures, this creates a foundation bar to which you can tie bars off of to continue your project above ground.
Welded wire mesh or chicken wire is used to create a multilayered skin. It comes in an octagonal pattern of different sizes. The smallest and most common being made up of 1” octagons. Smaller would work much better but to get smaller you would need to switch to hardware cloth which is much more expensive and less flexible. The answer is to fold it in half with the octagons unaligned to make the holes smaller.
The first layer (inside the rebar skeleton or cage) is the most difficult because to encase the rebar in concrete, it needs to go on the inside of the structure. Accomplish this by doing it in manageable lengths of chicken wire. It also helps to buy the narrower 24" rolls rather than the wider ones. push it inside the structure and pull it tight against the rebar and then wire it in place. This, as well as the outer layer needs to be tight. If it is loose the concrete will excessively fall through as you apply it causing a huge waste of concrete. If you have a loose area, make a long tie wire and loop it across several octagons then around a rebar. Twist and twist to bunch up and pull the chicken wire against the rebar stretching it tight as you twist and being careful not to over tighten and snap the wire.
To get the tie wires in, around the rebar and back out again can be tricky. The best way is to cut your length of wire and bend it into a check mark shape. Hold the end of the long part of the check mark tight with the plier and push the point of the check mark through. Steer it around a rebar and bull it back guiding the short end of the check mark back to you where you can twist the ends together.
The next layer is much easier with the exception of the tie wires being more difficult to push through. This layer simply gets stretched over the outside. For the tops areas it can be single layered. For vertical surfaces it needs to be doubled to hold the concrete in place while it cures.
EXTRA REINFORCEMENT FOR PONDS
For concrete ponds, use a layer of welded wire ran diagonally over the rebar which should be kept at a 6” grid. Then apply a doubled chicken wire layer to the sides and steep parts of the pond. It’s not necessary to do the layer underneath the rebar.
Once your armature is complete you will need to apply a concrete base coat followed by a mortar texture coat.
Pouring Concrete Molds
Garden statuary with great detail and stepping stones or plaques with intricate artwork can be created in concrete by the use of molds.
Molds can be purchased or built yourself using liquid rubbers combined with a rigid shell.
WHO WOULD HAVE THOUGHT?
imagine the expansion of possibilities when you add the ability of detailed works to concrete.
For example, a garden trolls body could be built with an armature but noses, eyes, ears, toes and fingers would be all but impossible to form with wet mud into any kind of quality detail.
The face hands and feet for the troll could be sculpted in clay then a mold poured in rubber and the parts reproduced in concrete. Now you can attach the detailed concrete parts right onto your steel body armature and sculpt the concrete body around the finished pieces.
Whats even more exciting is that the trolls original clay parts can be easily altered after the rubber molds have been made. Turn the trolls frown into a smile and pour new rubber molds. Build another body armature with a happier body language and you're on a roll building unique trolls!
When pouring a mold the concrete is mixed much runnier (a slurry) than for other purposes so that it gets into all of the details of the mold and so that air bubbles can be floated out by shaking, tapping or vibrating.
I have found using ad mixes such as acrylic mortar admix or liquid polymers with or in addition to the water helps to create strong detail. These products can be found in tile stores used for thin set mortars.
To pour a mold:
1) Coat your mold with a non-stick release agent such as vegetable oil or pam cooking spray. Don't’ miss any details.
2) Try not to entrap air in your slurry when you mix it. I like tap the bubbles out of the mix and let it sit for a while then carefully mix again to pull the solids off the bottom just before pouring the mold.
3) pour in a bit of your slurry mix then tilt and tip the mold to get the slurry to run all over and into all of the detail.
4) Fill the mould with slurry being careful not to create any bubbles. Stop occasionally as you fill to tilt, tip and tap as you go.
5) When the mold is full, spend some time making sure you’ve tapped out all of the bubbles.
6) Concrete cures by chemical reaction. The slower it cures the more strength it will have. If you have delicate shapes or details that could likely break when demolding, it’s good to submerge it completely underwater and let it cure there. At the very least, cover the exposed pour hole with a wet cloth and then the whole thing in plastic. Weigh down the edges of the plastic to seal in the humidity.
7) If your piece needs added strength such as for a stepping stone, you can add steel. You may have to pour in two parts allowing the first pour to set up just enough to float the hardware cloth or rebar. Be sure the steel doesn't lean on the mold or it may eventually allow rust to bleed through your piece. You can also paint the steel to prevent this from happening.