Post-Tensioning Technician Flashcards

What is the fundamental structural difference between bonded and unbonded post-tensioning systems?

Answer: In bonded PT, ducts are grouted after stressing so the tendon becomes integral with the concrete; in unbonded PT, the tendon remains free to move along its entire length

In bonded PT, cementitious grout is injected into the duct after stressing, permanently bonding the tendon to the surrounding concrete and distributing force along the entire length. In unbonded PT, the greased and sheathed tendon can slide relative to the concrete; force is transferred only at the anchorage hardware at each end.

Why is tendon elongation measured and compared to a calculated value during post-tensioning operations?

Answer: To confirm the applied prestress force is consistent with the design and that friction losses are within acceptable limits

Tendon elongation is proportional to the applied force (Hooke's Law: ΔL = PL/AE). Comparing measured elongation to the theoretically calculated value verifies that the correct prestress force was applied and that friction along the duct, wobble, and anchor set losses are within design assumptions. Significant deviations indicate a problem.

'Anchor set loss' (also called 'draw-in' or 'seating loss') occurs when:

Answer: The stressing jack is released and the wedge grips seat into the anchor casting, allowing a small inward movement of the tendon

When the hydraulic jack releases its load, the wedge-type grips must seat firmly into the anchor casting. This seating allows a few millimeters of tendon to draw back into the anchor. This 'draw-in' reduces the final prestress force and must be calculated and accounted for in the elongation acceptance check.

A minimum concrete compressive strength is specified before post-tensioning can begin because:

Answer: The high concentrated stresses at the anchorage zones require adequate concrete strength to resist local crushing and cracking

When PT force is applied, enormous concentrated compressive stresses are introduced at the anchorage zone bearing plates. If concrete has not reached sufficient strength, it can crush, spall, or crack at the anchor. Design specifications establish the minimum concrete strength (often 3,000–4,000 psi) that must be confirmed by cylinder breaks before stressing begins.

A PT stressing jack must have a current calibration certificate because:

Answer: The hydraulic pressure gauge reading is used to calculate the applied tendon force, so gauge accuracy directly determines whether the correct prestress is achieved

The applied PT force is calculated as: Force = Gauge Pressure × Jack Ram Area, adjusted by the calibration factor. An inaccurate gauge produces an incorrect force, leading to under-stressed (structurally deficient) or over-stressed (risk of strand fracture or anchor blowout) tendons. FDOT requires jack calibration within 6 months or after any repair.

If the measured elongation of a post-tensioning tendon differs from the calculated theoretical elongation by more than ±7%, the correct action is to:

Answer: Stop stressing, document the discrepancy, and notify the Engineer of Record before proceeding

FDOT specifications and PTI standards require that measured elongation fall within ±7% of the calculated value. A discrepancy beyond this tolerance indicates a potential problem — excessive friction, duct obstruction, tendon damage, or incorrect jack calibration — that the engineer must evaluate. Proceeding without investigation risks structural deficiency or failure.