Pre-tensioning versus post-tensioning

Both methods put high-strength steel into tension and lock that tension into the concrete; they differ in when the steel is stressed. In pre-tensioning, strands are stretched between fixed abutments at a casting bed before the concrete is placed. The concrete is cast around them, cured, and once it reaches the required transfer strength the strands are cut. They try to shorten, grip the concrete by bond over a transfer length near each end, and put it into compression.

In post-tensioning, ducts are cast into the member. After the concrete has hardened, tendons are threaded through, stressed by hydraulic jacks reacting against the concrete itself, and locked off at anchorages. In bonded systems the ducts are then filled with grout, which protects the steel and bonds it to the concrete; in unbonded systems the tendons are greased and sheathed and can in principle be inspected or replaced.

Pre-tensioning suits factory production of many identical units: hollow-core planks, railway sleepers, poles and bridge girders of moderate span. Tendons are usually straight or deflected at a few points. Post-tensioning suits large, cast-in-place or segmental work: long-span box girders, slabs, tanks, containment structures and continuous bridges, where curved tendon profiles follow the moment diagram.

The two methods lose prestress in different ways. Pre-tensioned members lose stress through elastic shortening at transfer, then creep, shrinkage and relaxation. Post-tensioned members add friction along the duct and wedge draw-in at the anchorage, but elastic shortening affects only the tendons stressed earlier. IS 1343 covers design and construction of both.