Correctly built, earth-sheltered homes are most energy efficient. Incorrectly built, they are energy disasters. Many also have grave structural problems that require tremendous costs to rectify. In conventional reinforced concrete the reinforcing bar begins to truly function only after the concrete fails. After the basement wall cracks, the reinforcing bar takes hold and prevents further failure of the concrete. However, the crack that has formed can leak.

When post-tensioning is used the reinforcing bars are replaced by a series of «-inch 7-strand cables (tendons). As supplied, these cables are encased in a thin covering with a lubricant inside that allows the cables to slide. One end of the cable is permanently secured in an anchor, approximately 3 by 5 inches. After the concrete has been cast around the cable and has gained enough strength the tendons are pulled to the required tension. The tensioning force, applied to the «-inch tendon by a hydraulic jack, is approximately 33,000 pounds, or 80 percent of the tensile load capacity of the tendon. When the jack is released the cable pulls against two wedges that have been inserted into the tapered hole of the stressing anchor. This locks up the cable and keeps the concrete under compression. Tensioning of the cables places a dynamic force in the structure before the earth load is applied.

In our operations the entire building is post-tensioned, beginning with the floor slab. The cables are placed every 4 feet in each direction in the floor slab and tensioned when the slab reaches sufficient strength. Tendons are also set vertically in the footings at 4-foot intervals around the perimeter of the slab before the concrete floor is cast. After the floor is finished, standard tendons are run horizontally within the wall forms. The walls are then cast around the tendons and the horizontal tendons are tensioned. In the roof deck the tendons are placed much closer, averaging 12 to 14 inches apart, depending on the span desired.