Spray a shell structure onto the inside of an inflated balloon--first some layers of urethane foam, then some layers of shotcrete. Some call it stratiform, a generic term that refers to its being built up in several layers. Domes spanning up to 230 feet have been built in this way. Although spherical and ellipsoid domes are the most widely used, the method works with irregular or free-form shells whose shapes can be matched by a skilled air-form maker.


Structures built with these layers of urethane and concrete can be efficiently heated or cooled because of the layer of insulation that is seamless and free of cracks and air leaks, and because of thermal lag afforded by the inner concrete layer. Stratiform domes are used today for storage of grain and other commodities; for private residences; for institutional buildings such as schools, churches, and animal shelters; and for water tanks and cold storage units.


Construction of a stratiform shell involves several basic steps. Generally, a foundation perimeter tension ring must be built first. The air bag form, typically hemispherical, is spread out and attached at its perimeter to the foundation. The balloon becomes the form for polyurethane foam insulation which is sprayed on the inside. The next step calls for reinforcement to be placed, usually attached to the foam layer. After the reinforcement is satisfactorily positioned, several thin layers of shotcrete are applied, building up the necessary thickness. After the shotcrete has gained sufficient strength, the air form may be peeled off.