Q: I am casting simple forms in concrete, mostly using holes in the ground for molds, and I’m looking for ways to generate different interesting forms and surfaces. Any ideas?
A: You can, of course, get texture in your concrete by taking molds of things that have texture. For instance you can start with a clay original, give it whatever texture you want, then take a (urethane) rubber mold from it, paint in the first layer of concrete mortar, then apply the rest. After it has set two days or so, remove the mold—it will simply peel away, no need for release, and your texture will be there.
If this is too much work, or you want to work more directly, try lining your earth molds with crushed rock of various sorts, or small pebbles, colored sand, etc. I have made columns this way, constructing my molds from plywood, which I lined on the bottom surface with crushed lava, and then laid broken pottery face down in the bed of lava and poured concrete in. When unmolded, the concrete had stuck to the ceramic and the red lava gave it some color. I used the columns to build a wall
Another approach is to mix pebbles, rocks, or other permanent materials (try lime-proof pigments from your concrete supply) into the concrete mix, then unmolding it before it is quite set and with a brush and water eroding the surface until the stones or whatever are revealed. (this is called “exposed aggregate”) Besides making molds, you can build concrete up directly on an armature, usually built of steel. Generally you weld the basic form in reinforcing steel rods about 1 cm. diameter, then cover this with steel mesh tightly wired on. (I like to have these armatures hot-dipped in zinc to inhibit corrosion, which will eventually cause the steel to rust and shed the concrete). Then either use plastic cement with your sand and water or add lime to the mix for stiffness (one part lime for each two parts cement). The texture will depend on the technique of application, which you can experiment with. Try stamping various things into the wet cement for textural variation.
Once the concrete has set, you can also set tile, stone, etc. onto the surface with adhesive compounds (check your local tile supply). You can use cement based compounds (“thinset”) or organic-based glues (mastic)
It is important to properly cure the concrete, keeping the whole piece damp for at least a week after making it. The concrete will be stronger the longer this process lasts. Without it, the concrete will be crumbly and weak. Concrete also has problems with freezing and thawing, so if you are making an outdoor piece for the frozen north, try using air-entrainment agents and anti-porosity admixes, which help with but don’t completely solve this problem.