Less on-site labor, a shorter construction time and improved quality control have made precast concrete an attractive construction technique in conventional and now earth-sheltered construction. One patented multipurpose precast system has been used in both earth-sheltered and above-ground construction. The basic component of the system is an 8x12-foot waffle-shape panel that is normally cast in a contractor's central yard, though a mobile casting yard may be feasible. The panel consists of a 2-inch-thick skin and a grid of 6-inch-deep ribs, which together make the total thickness of the panel 8 inches.
Depending on whether the panels are used horizontally or vertically, the ribs of the panels act as beams or columns, respectively. Steel mesh reinforcement is cast within the skin of each panel and the ribs are reinforced with steel reinforcing bars, or for special uses, with prestressing strands. To assure proper cover of concrete, the steel bars are supported on custom-designed polyurethane bar chairs. After the panels have been erected, these bar chairs double as nailing blocks for wood furring strips.
When a panel is set into place, it is bolted to the footings and adjacent panel through holes precast in the ribs. This bolting permits quick erection with a minimum of temporary bracing. In addition, the panels are connected to the footings by being seated in a bead of epoxy that serves as a structural bonding agent and as a seal against water penetration. Panels are erected with skin facing the exterior and waffle grid facing the interior. Then the electrical wiring and copper tubing are installed by running them through the 1x3/4-inch notches precast into each panel rib. The 6-inch-deep voided areas of the panels provide ample space for electrical boxes or other hardware. The notches do not provide enough clearance for ductwork in the walls, but conventionally built interior partitions should provide adequate space for the small heating, ventilating and air conditioning requirements of an earth-sheltered home.