Q: How do you go about making concrete sculpture like the kind you can buy for your garden?
A: Generally, this type of sculpture is done by making a full-sized model in clay, usually plastiline, then taking a mold from the model and pouring in the concrete mix. Reinforcing metal rods can be used or not, depending on the configuration. Though this used to be done with plaster piece-molds, now flexible urethane rubber molds are generally used, with a rigid shell, either in plaster or fiberglass, to hold its shape. A mold release will increase the life of the mold and facilitate removing the piece without losing fine surface details. A concrete vibrator is helpful for removing air-bubbles before they set, and for compacting the mix. Various admixes can be used, including super-plasticizers to reduce the water and increase the final strength of the concrete, as well as combat water separation and weeping. Air-entrainment agents will help the sculpture resist freeze-thaw cycles outdoors. Wait two days before unmolding, and wet-cure the concrete cast for at least a week.