Q: I’m interested in experimenting with concrete to get different surface effects than the usual boring cast concrete look. Any suggestions?
A: You can, of course, get texture in your concrete by taking molds of things that have textures of their own. For instance you can start with a clay original, give it whatever texture you want, then take a urethane or silicone rubber mold from it, paint in the first layer of concrete mortar, then apply the rest after this has set. After two days or so, remove the mold and your texture will be positive again. If this is too much work, or you want to work more directly, try making open-faced molds—this can be as simple as a depression in the ground lined with plastic film—and lining them 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. I then laid broken pottery face down in the bed of lava and poured concrete over this. When unmolded, the concrete had stuck to the ceramic and the red lava gave it some color.
Another approach is to incorporate 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”. Concrete can also serve as a substrate for mosaic work. It is best to set ceramic or glass tile onto concrete that has not fully cured, but if you need to set onto old concrete, prime the surface with admix before applying the thinset mortar with a notched trowel. If you mix concrete with crushed marble as aggregate, you can later grind away the surface and even polish the resulting “cast stone” sculpture. This technique is also called “terrazzo”, and is (or was) commonly employed in making floors and other functional items.
Besides casting it into molds, you can build concrete up directly on an armature, which is 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. For permanent outdoor sculpture, 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). Lime makes concrete even more caustic than it normally is, so be sure to wear protective gloves and other safety gear. The texture will depend on the technique of application, which you can experiment with. You can try stamping various things into the wet concrete for textural variation, or scratch and carve on it after it sets but before it hardens completely.
Whichever technique you use, it is most 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.