Concrete sculpture may be done by carving, direct modeling, casting in a mold, or casting on a bed of sand. Brian Adddis has tried them all. The sculptor of "Abstract Form," a 15-foot-high creation standing on the grounds of the Portland Cement Institute of South Africa in Johannesburg, is devoted to concrete as a sculptor's medium. His devotion is due to the strength, low materials cost, and adaptability to almost any size and shape, as well as the variety of colors and textures available.
Addis prepared the original concept in wax and enlarged it to a full-size model in plaster of paris. He hired the firm A. Radaelli Terrazzo to make the mold pieces and do the actual casting. The sculpture was cast as an unreinforced shell with a wall thickness of about 8 inches. Its core was filled with compacted moist sand. Sheet metal strips, bent to shape and placed on edge, separated the core sand from the concrete shell. The strips were moved up and rebent as concreting progressed. The mold was also built up as concreting moved along. The sculpture remained in the mold for 5 days after casting. After roughening the surface by waterblasting, the concrete surface was washed with a 4 percent solution of hydrochloric acid in water.