Prestressing Steel: Strand, Wire, Bar, and Anchorage Systems
Domain 2 examines how strand, wire, and bar carry force, how each is anchored, and how the inspector confirms that the intended force actually entered the tendon.
Systems and anchorages
- Pretensioned: strand is stressed in a bed before concrete is placed; force transfers by bond when the strand is released.
- Bonded post-tensioned: strand runs in a duct, is stressed after the concrete hardens, then grouted.
- Unbonded post-tensioned: greased and sheathed strand anchored at an anchor plate with wedges. Wedge chucks grip strand; button heads anchor wire; threaded nuts and couplers anchor bar. Each anchorage requires a bearing plate and spiral confinement sized for the system.
Worked example: elongation check
A 100 ft (1,200 in.) tendon of 0.6 in. strand (A = 0.217 in², E = 28,500 ksi) is stressed to 150 ksi. P = 150 × 0.217 = 32.6 kip. Δ = PL/AE = (32.6 × 1,200)/(0.217 × 28,500) = 6.3 in. The field record shows 5.6 in. The difference is (6.3 − 5.6)/6.3 = 11%, beyond the typical ±7% post-tensioning tolerance. Stop, verify gauge calibration, check for excess friction or wedge seating, and resolve before accepting the tendon.
Losses
Wedge seating, friction and wobble, elastic shortening, creep, shrinkage, and relaxation all reduce or redistribute force. Elongation captures the combined short-term effect; it does not measure long-term losses.
Traps
- Comparing elongation to another tendon instead of the calculated value.
- Ignoring seating loss, which appears as a short elongation.
- Mismatching bar couplers to a different bar grade or thread.
- Accepting a gauge without a current calibration record.