To build new stores quickly and meet its expansion goals in the northern United States, The Home Depot needed an alternative construction method for building well-insulated retail outlets in colder climates. When the company built a new 112,000-square-foot store in Walker, Mich., in 1997, it meet the necessary insulation requirements by using a tilt-up sandwich panel system that combines poured-in-place concrete and insulation board.

The Thermomass Building System, manufactured by Composite Technologies, Ames, Iowa, consists of 1 ½-inch-thick Styrofoam insulation board sandwiched between a 7 ¼-inch-thick outer wythe and 3-inch-thick inner wythe of concrete. Fiber-composite connectors tie the two wythes of concrete together, and the insulation, which has an R value of 5 per inch of thickness, extends to the panel edges.

Because the panels are cast onsite, the insulation board is delivered to the jobsite with predrilled holes for the connectors. Workers start by placing the bottom layer of concrete over the required reinforcing steel, then they lay the predrilled insulation on top of the concrete while the mix is still plastic. Next, they insert the connectors, which have an ultimate pullout capacity of 2,500 pounds each, into the predrilled holes. Finally, the top layer of reinforcement is installed and the top wythe of concrete placed.