CTQP Post-Tensioning Technician

Question 1

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

Accepted 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

Answer

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?

Accepted Answer

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

Answer

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:

Accepted Answer

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

Answer

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:

Accepted Answer

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

Answer

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:

Accepted 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

Answer

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.

CTQP - Construction Training & Qualification Program Practice Test

CTQP - Construction Training & Qualification Program Practice Test

CTQP Post-Tensioning Technician