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HOW TO MAKE CONCRETE ITEMS

from TEMPLATES and "SWEEPS"

One way to make concrete birdbaths, concrete bonsai planters, concrete pots and saucers


Finished artificial rock
The finished birdbath

This technique shows a simple and inexpensive way to form concrete for any type of item that has both an inside shape and an outside shape. For instance, a birdbath has a bowl shape on the inside, and another curved shape on the outside.

Other items that can be made by "sweeps" are things like planters, water dishes, dog bowls, bonsai pots, saucers, and so on. All these items could be made by using molds, but they would require two-part molds, plus a backup mold to hold everything together while the mold was used. And if you only wanted one item, it would be a very expensive way to produce the item.

The basic idea is to rotate a template for the inside shape around some material like clay. This forms a clay bowl, upside down. Then a template to form the outside shape is rotated around the clay, but this time with concrete between the clay and the template. After the concrete sets and the clay is removed, you have your concrete item.

All the steps are demonstrated with photos and described below. The item demonstrated is quite a large birdbath, 24" across, 3" deep in the center, and weighing about 50 pounds. We will be showing some custom steps that we did to make this particular birdbath, but that you do NOT need to do for most items. For your first attempt with this technique, it would be much better to try something like a 16" wide, 2" deep birdbath, or a small bonsai pot.

Materials needed:

  • Portland Cement.
  • sand (builder's sand, mason's sand, play sand, "all purpose" sand, etc.) Most "play sand" is easier to use for this technique because it usually has fewer pebbles. Other types of sand may have a lot of pebbles that make working with the concrete more difficult.
  • water base or oil base clay.
  • scrap lumber such as 1x4 or 3/4" plywood.
  • 1/4" metal rod.
  • cement trowel or mason's trowel (steel trowels are much better than plastic)
  • mixing bucket or mortar tub
  • stiff plastic sheet or tar paper or roofing felt
  • powdered concrete pigment if you want any color besides "concrete gray"
  • graph paper to make a template, or use a computer CAD printout if you want
  • any kind of tool that can cut curves in wood - coping saw, scroll saw, jig saw, band saw, etc.
  • a drill or drill press, with 1/4" drill bit
  • hot melt glue gun

Procedure

The first thing to do is make a paper template that represents a cross section of one half of the item. The template for the large birdbath is shown below. Concrete needs to be at least 3/4" thick for strength, so your template should be at least 3/4" wide at all points. Our template is up to 1-1/4" wide in some spots because the finished birdbath will be so wide, deep, and heavy, and the sides will not be supported by the ground. If you make a 16" wide x 2" deep birdbath then 3/4" is plenty.

Underside of the artificial rock
The paper template

Now trace the outline of the inside onto a piece of 3/4" thick wood or plywood. You want to allow an extra couple inches of length in the area marked "A" so that the metal rod that will act as a pivot has a place to pass through the wood template. Later pictures of the template in use show where the pivot will be.

Your template does not need to be all wood if you have other materials available. You could make the main arm from wood and the template that forms the actual shape from thick plastic or sheet metal for instance.

In our template, we have added a "leg" that will rest on the forming base because we are going to attempt to make a curved edge on our birdbath. For your first project, you do not need a leg. The clay and concrete will be formed directly against the base, and make a flat edge instead of a curve.

Applying the plastic shipping tape
The inside shape is traced on wood

You need a flat work surface covered in a waterproof material, larger than the item you are making. A piece of flat stiff plastic is good, or you could use tarpaper. Thin plastic such as garbage bags is not a good material - it will bunch up and get in your way as you rotate the templates.

Your work space should be outdoors but protected from rain, and a space that you do not need for a week or so. A carport floor or garage floor would be ideal. For concrete to cure properly, the temperatures must stay over freezing. Temperatures over 60 degrees F would be preferable.

Next you need a 1/4" metal rod that will act as a pivot, stuck vertically into a 1/4" hole in a block of wood. Take pains to make sure the pivot rod is vertical. Any leaning in the pivot rod results in your template rubbing on one side and missing completely on the other side as you rotate it. Drilling the hole in the block of wood with a drill press is a good idea if you have one.

Do not permanently fasten the pivot rod into the block of wood. You will need to pull the pivot rod out of wet concrete at the very end of the project, and you will not be able to get to any sort of fastener that is in the block of wood. In other words, the pivot rod should fit into the block of wood firmly, but not so firmly that you cannot pull it out.

A real rock used as a form
The pivot rod in a block of wood

If you are making a large or deep item, you can add something to take up space at this point. That will decrease the amount of clay you need to use. For our large birdbath project, we happened to have an old broken plastic saucer, and the saucer was smaller than the inside of our new birdbath, so we used that to take up space.

First we hot melt glued the wood pivot block to the center of our plastic base, then we drilled a hole in the exact center of the saucer and pushed it over the pivot, and hot melt glued the saucer to the base as well, so the pivot is fixed in place. Gluing the block of wood to the base makes it stay put when you need to pull the pivot rod out of it through the wet concrete later.

Hardware cloth shell
The saucer that just takes up space, over the pivot

Now you need a spacer that fits on the pivot rod, and holds the template at the right height. If you make a lot of things by sweeps, it is handy to get some 1/4" "shaft collars" such as those shown below. They fit over the pivot rod and can be tightened in place at any height by using a set screw. The home centers usually have this item in their specialty hardware section, usually in a drawer.

If you are making your first project, you can just use a piece of wood of the right thickness, with a 1/4" hole so it fits over the pivot rod.

The right concrete consistency
Two shaft collars and an Allen wrench

Now you need to cut the template shape out of the wood, and drill a vertical hole through it so it will fit on the pivot rod and be able to rotate around the pivot.

Put a shaft collar or wooden spacer on the pivot rod, under the template, so that the template has something to "ride on" as you rotate it. Position the spacer so that the template will be level. This will ensure a level bottom to your project, if your template has a flat bottom.

If you are using shaft collars, you can put one above the template also. This will help keep the template from riding up as you form the clay or concrete.

The distance marked "A" on the template is the inside radius of our finished birdbath. You can see that the template needed to be a little longer where the pivot goes, so the pivot had some material to go through.

The area marked "B" on the template is where we will attempt to make a curved edge on the birdbath. Your first project does not need this, and it does not need the "leg" that holds the template off the base either.

After the first row
The template for the inside shape of the birdbath in place on the pivot rod

Now you need the material that you will form to make the inside shape of the item. If you already have enough soft oil base clay, that is an excellent material. If you don't already have some it will add a lot of expense to the project.

For our project, we used some clay that we dug out of our garden. If your part of the country is also "blessed" with heavy clay below the topsoil, then you also have a free source of material. This type of clay can crack and split when it dries out, so you can make a mix of about 50:50 sand and clay to prevent that, but which is still very workable.

Plaster is not really a good material to use because you would need to break it out of the inside of the birdbath or planter when you are finished, and that risks cracking the fresh concrete.

Damp sand alone will be difficult to work with. Even though you can make a perfect shape with it, it will want to move around when it comes time to sweep the concrete over it. You could try forming it and then spraying it with white glue mixed in water so that it stiffens the outer layer, or you could try adding a thin layer of plaster over it to make a hard shell.

After the second row
Clods of garden clay

If you use clay from your garden, it will be easier to mix with sand and wet with water if it is broken up.

After the shell is covered
After breaking and sifting the clay

A heavy-duty 1/2" drill motor and any type of mixing blade will make mixing the clay, and later the concrete, easier. But you can do both by hand with a trowel or similar tool if you need to.

In the picture below, the mixing blade on the left is sold for mixing sheetrock "mud" or tile grout, and works very well, but you need a large and powerful drill motor. The other two mixers are sold for mixing paint and take a little longer to get results, but your drill motor does not need to be as large.

After the top coat
Types of drill mixers you can use for clay or concrete

Now you can add your clay. At first, you do not need to use the template to form the clay, but you can use the template so you can tell when you have enough clay. Then you can sweep the final layer of clay to make the clay take on the shape of the template.

A basic granite look
Most of the clay is added without using the template

 

Too much paint on this rock
The template makes the final shape in the clay

For our particular project, we found that the wet clay would not hold the shape of the rounded edge like we wanted. So we raised our template slightly by putting a washer on top of the lower shaft collar, and added about a 1/8" layer of plaster to make a shell.

We found that the template would still not make a nice rounded edge, so we made a form from plastic strips, added more plaster around the edge, and customized the sweep to form just the rounded edge by using an old measuring spoon as shown.

You do not need to do any of this. We just show it here to explain the white interior form you will be seeing after this point.

Too much paint on this rock
The customized template to form a rounded edge

Now you should have the interior shape all swept out, and it is time to make the template that will form the concrete. Do this by tracing the outside of your paper template onto wood, and cutting out the curve, allowing some space for the pivot rod as before. Drill the hole for the pivot, add spacers below the template to keep it the correct distance away from the interior form, and make sure the template is level.

It is not obvious in the picture, but we used two threaded rod coupling nuts under the template to keep it the correct distance from the interior form. You could use a wood spacer or a stack of large nuts or washers to do the same thing. You want to use a spacer that you can pull out of wet concrete. The shaft collars we used previously lock on to the pivot rod so they would not be a good choice.

The piece of wood we used for the template was not wide enough, so we attached a wooden paint mixer stick to the outer edge of the template, so that the stick rode on the base and made the template level as we rotated it.

If your interior form is clay or clay/sand, you do not need any release agent. If you use a plaster shell you should coat it with motor oil or petroleum jelly to keep the concrete from sticking.

Too much paint on this rock
The template that will form the concrete

Now you are ready to form the concrete. The concrete mix you need is:

  • 3 parts sand
  • 1 part Portland Cement
  • liquid or powered concrete color if you wish. Follow directions that come with it for the amount.
  • the least amount of water you can use and still mix the concrete thoroughly

Mix the sand and cement first, then add water very slowly until the concrete is just wet enough to mix. You do not want the mix to be soupy or have any excess water. You cannot sweep soupy concrete. The concrete must be rather stiff.

You also cannot sweep anything sold as "concrete mix" because it has gravel in it. You must make your own concrete mix as described, and it is best if you use "play sand" or another fine sand that does not have a lot of little pebbles in it.

As with the clay, you can add concrete using a trowel until you have enough to start being affected by the sweep, then start using the sweep and make the final shape. A trowel also comes in handy for some final smoothing after the shape is made with the template. A wood template like this leaves a rough surface in concrete.

For our particular project we wanted to add some reinforcement since the birdbath was going to be quite large and the sides would not get any support from the ground. We put about a 1/2" layer of concrete over the interior form, then used some poultry cloth for reinforcement, and then swept the outer layer of concrete over that, so the wire was completely embedded in the concrete.

For smaller projects, you do not need to use any reinforcement as long as the concrete thickness is at least 3/4".

Too much paint on this rock
Concrete being formed after adding some poultry wire reinforcement

After the concrete is completely swept and you have your finished shape, the pivot rod and any spacers need to be pulled out of the concrete. This will leave a hole that you can patch by forcing concrete mix down into it and smoothing it off with a trowel.

For our particular project, we did not add color to the concrete mix. We had some colored sand though, and pressed it into the wet concrete, after the concrete had stiffened up for a couple hours. (Technically, it was 3M™ Colorquartz™ Ceramic-Coated Crystals) If you add something like colored sand too soon, it will just sink into the concrete and be invisible. If you try to add it too late, it won't go in at all.

When you are finished with the concrete, cover your project with a plastic garbage bag, weighted down around the edges. Let the concrete cure undisturbed for one week to allow it to attain maximum strength. You can check on the concrete every couple of days, and if any part of it seems to be drying out, spray or pour water on it and replace the plastic. Concrete needs to stay damp in order to cure properly.

Too much paint on this rock
Colored sand added to outside of concrete after it has stiffened a bit

You can see the finished colored sand effect in the picture at the top of this page - a pleasing sort of "granite" look, not nearly as garish as the above photo would suggest.

You can also see the slightly rounded edge we worked so hard to get.... Probably not worth the time and effort.

 


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