CTQP Post-Tensioning Technician — Questions and Answers
Question 1: What is the fundamental structural difference between bonded and unbonded post-tensioning systems?
- Bonded PT uses multi-strand tendons; unbonded PT uses single-wire monostrands only
- 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 (Correct answer)
- Bonded PT requires stressing from both ends; unbonded PT is always stressed from one end
- Unbonded PT is used only in precast bridge girders; bonded PT is used only in cast-in-place structures
Correct 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.
Question 2: Why is tendon elongation measured and compared to a calculated value during post-tensioning operations?
- To estimate the volume of grout needed to fill the duct after stressing
- To confirm the applied prestress force is consistent with the design and that friction losses are within acceptable limits (Correct answer)
- To determine the concrete compressive strength at the time of stressing
- To calculate the total dead load added to the structure by the PT system
Correct 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.
Question 3: 'Anchor set loss' (also called 'draw-in' or 'seating loss') occurs when:
- Concrete creep causes long-term shortening that reduces prestress over decades
- The stressing jack is released and the wedge grips seat into the anchor casting, allowing a small inward movement of the tendon (Correct answer)
- The tendon slips inside the duct during grouting operations
- The strand cross-section reduces slightly under the applied tensile load
Correct 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.
Question 4: A minimum concrete compressive strength is specified before post-tensioning can begin because:
- Fresh concrete generates heat that can damage tendon coatings during stressing
- The high concentrated stresses at the anchorage zones require adequate concrete strength to resist local crushing and cracking (Correct answer)
- Grout will not properly bond to concrete that has not reached full design strength
- The strand wedges cannot properly grip until the concrete has hardened around the duct
Correct 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.
Question 5: A PT stressing jack must have a current calibration certificate because:
- An uncalibrated jack may damage the concrete formwork or anchor plates
- The hydraulic pressure gauge reading is used to calculate the applied tendon force, so gauge accuracy directly determines whether the correct prestress is achieved (Correct answer)
- Jack calibration is required by OSHA purely for worker safety compliance
- An uncalibrated jack consumes excess hydraulic fluid and reduces pump efficiency
Correct 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.
Question 6: If the measured elongation of a post-tensioning tendon differs from the calculated theoretical elongation by more than ±7%, the correct action is to:
- Accept the result — field variations greater than 7% are common and expected
- Continue stressing until the jack pressure matches the target gauge reading, ignoring elongation
- Stop stressing, document the discrepancy, and notify the Engineer of Record before proceeding (Correct answer)
- Immediately de-stress the tendon and replace the strand
Correct 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.
CTQP Post-Tensioning Technician
What is the fundamental structural difference between bonded and unbonded post-tensioning systems?