Concrete is becoming steadily more applicable as a basic structural material in American zoos, and the zoo in San Diego, California, is particularly outstanding. Concrete was used in seven basic ways at the San Diego Zoo: imitation rocks and cliffs for outside display of animals; walking surfaces; moats; floors and decks; pools; grout; and free forms. Every animal in residence and every visitor to the zoo comes in constant contact with concrete in many diverse- and frequently imaginative- ways. Here are some of them: Since the natural habitat of many of the zoo animals is rocky terrain, almost all of the open air exhibits at the zoo are flanked by ersatz rock formations built of concrete. To make them look more realistic, regular stucco colors were sprayed on the formations to simulate the gradations of rock. Walks and moats are an important part of the design of the zoo. Today, the zoo has more than 10 miles of concrete walks and a number of these have been built with special effects. Near the restaurant, for example, rock salt was scattered on the surface. The salt dissolved and left small, irregular holes in the surface. A great deal of integrally colored concrete was also used for sidewalks. The floors and decks of all of the cages and dens in the zoo are built of concrete unless the animals being caged require some sort of special treatment. Some of the cage floors are radiant heated because of the special health needs of the animals. Pools are very much in evidence at the zoo. All the pools are constructed as simply as possible. The desired shape of the pool is excavated, reinforcing is laid directly on the dirt, and gunite is added to the thickness required by the size and aggressiveness of the animal being contained. Finally, the zoo is dotted with free forms combining attractive design with some sort of functional use by the animal.