System Components

The components of a Post-Tension system are few in number, but large in purpose.

Above are five cut Post-Tensioned cables. The cables are one of the most important components of the entire PT system. The cables are composed of 7 wires, one center wire surrounded by six helically-wound wires. These 1/2" cables are greased and encased in an extruded plastic protective sheathing to form a tendon.

Above is an anchor that is used on the stressing end of the cable. It is composed of high strength steel and it must bear the entire stress that the cable puts onto the slab when it is tensioned. Inside the anchor are the wedges that actually grip the strand and prevent its catastrophic release..

These same anchors are cast into the side of the slab and are reinforced with a large amount of rebar in the anchorage area. The cone-shaped device on the end of the anchor is a pocket former.

When the concrete is poured it is the pocket former that creates the characteristic hole in the side of the slab where the PT cable protrudes. This is the stressing end of the PT system. A hydraulic jack has already stressed these cables. The orange paint on the cables indicates just how much these cables have been stretched. Prior to stressing the orange marks on the cable was at the edge of the pocket former. These cables have been stretched over 8 inches.

Above are the dead end anchorages for three PT cables. Anchorage areas of the slab are heavily reinforced as they bear the entire stress load of the PT system.

But between the anchors lies the real story of Post-Tensioned concrete. For it is here where all the parts of the system come together to create the structure.

Banded cables are the first group of cables to be placed on the concrete slab formwork. Banded cables typically occur over column lines. Above is a large amount of banded cables that will soon be placed into their slab anchorage zone.

Above a worker is placing single cable. Cables such as these are placed only after the banded cables have been finished. This direction is often called the transverse, uniform, or distributed direction. This photo also illustrates the utility of the PT system by allowing the threading of reinforcement under, over, and around the already completed layout to achieve the desired structural outcome.

Above we can see slab reinforcement that is nearing completion. Soon the concrete will be poured. This worker is making final adjustments to a very complicated and densely packed Post-Tensioned slab.

Here are the unplaced sections of PT cables at their intermediate lock-off points. To the right the cables have already been placed, but to the left formwork for the rest of the slab has yet to be built, so the cables will wait in this rolled state.