ICC Prestressed Concrete

Prestressed concrete works by introducing a compressive force into the concrete element before service loads are applied. When loads are applied, they must first overcome this pre-compression before the concrete sees any net tension. Since concrete is strong in compression but weak in tension, this precompression dramatically improves the element's ability to carry bending loads without cracking. The result is that prestressed elements can span farther, carry more load, and use less material than comparable conventionally reinforced concrete.

Prestressed concrete is produced by one of two methods. Pre-tensioning stresses the strands before the concrete is placed - the strands are pulled tight between abutments in a casting bed, concrete is placed around them, and when the concrete hardens sufficiently, the strands are released. The force transfers from the strand to the concrete through bond. Post-tensioning places tendons in ducts within the concrete element; after the concrete hardens, the tendons are stressed using hydraulic jacks and anchored against the member ends. Post-tensioning is performed in the field and is where the special inspector plays the most active role.

The most common prestressing tendon is seven-wire strand - six wires helically wound around a center wire. The strand's high tensile strength, typically 270,000 psi for low-relaxation strand, allows it to be stressed to the high levels required for effective prestressing. ASTM A416 governs the properties and testing requirements for prestressing strand. The inspector must verify that delivered strand is accompanied by mill certifications conforming to ASTM A416 and that the certifications reference the specific heat and coil numbers being used on the project.

Post-tensioned tendons may be unbonded or bonded. Unbonded tendons are encased in a plastic sheath pre-coated with corrosion-inhibiting grease; after stressing, they rely on the anchor hardware to transfer force to the concrete. Bonded tendons are placed in metal or plastic ducts; after stressing, the ducts are grouted with a cement grout that bonds the strand to the surrounding concrete. Each type has different inspection requirements. Unbonded tendon inspection focuses on sheath integrity, grease coverage, and anchor encapsulation. Bonded tendon inspection adds grout injection verification.

The profile of a post-tensioning tendon - its vertical position along the length of the member - is as important as its material properties. Tendons are typically draped in a parabolic or harped profile that places the strand low at midspan (where sagging moments are greatest) and high at supports (where hogging moments occur). This profile allows the prestress force to work most efficiently against the applied loads. A tendon installed at the wrong height at any point alters the moment balance the designer assumed.

Tendons are held in their correct profile by support chairs or tie wire attachments to conventional reinforcing bars. The inspector verifies that supports are placed at the intervals specified in the shop drawings and that the tendon elevation at critical control points matches the specified profile. Profile tolerances are typically plus or minus 1/4 inch at support points and 3/8 inch at midspan, though project specifications may vary. Significant profile deviations require notification of the engineer before concrete is placed.

The tendon profile controls the distribution of prestress force along the span. A draped (parabolic) tendon profile in a post-tensioned slab applies an upward distributed force that balances a portion of the gravity load. The profile is established using high chairs (at the support locations), low chairs (at midspan), and support bars or rails that hold the tendon at the specified elevation. AASHTO and PTI specifications define the tolerance on tendon elevation: typically +/- 3/8 inch at any point and +/- 1/4 inch at high/low chair locations. The inspector must verify chair heights against the PT shop drawings before placing the tendon.

Unbonded single-strand tendons in slabs are factory-assembled with a continuous plastic sheathing filled with corrosion-inhibiting grease. The sheathing provides the unbonded condition and protects the strand from corrosion. Any puncture, tear, or unsealed joint in the sheathing compromises both the unbonded behavior and the corrosion protection. The inspector must verify that tendons are handled without puncture during placement, that tails are sealed at penetrations, and that any damaged sheathing is repaired per PTI requirements before concrete is placed. Sheathing is a structural component - not a simple packaging material.

Post-tensioning stressing is performed after the concrete has reached a specified minimum compressive strength - typically a percentage of f'c that is sufficient to resist the prestress forces without crushing or splitting the anchorage zone. The inspector must verify, from cylinder break results or maturity method data, that the required strength has been achieved before stressing begins. Stressing before adequate concrete strength can cause local crushing at the anchor bearing plates or splitting cracks radiating from the anchorage zone.

The hydraulic jacks used to stress post-tensioning tendons must be calibrated and the calibration must be current at the time of use. The calibration certificate establishes the relationship between gauge pressure and actual jacking force for a specific jack and gauge combination. Using a jack with an expired calibration, or using the wrong gauge on a calibrated jack, produces stressing forces that may differ significantly from the intended design force. Verify calibration certificates before stressing begins.

Measuring the elongation of a tendon as it is stressed is the primary method for verifying that the intended prestress force has been applied. The relationship between gauge pressure and tendon elongation is calculated in advance from the tendon length, cross-sectional area, modulus of elasticity, and estimated friction losses. If the measured elongation matches the calculated value within the acceptance tolerance - typically plus or minus 7% per ACI 318 - the stressing operation is considered successful.

When measured elongation falls outside the tolerance, the cause must be identified before the tendon is locked off. Under-elongation may indicate a duct obstruction, a kinked tendon, or incorrect jack calibration. Over-elongation may indicate tendon slippage at the dead-end anchorage or incorrect friction assumptions. Both conditions require evaluation by the post-tensioning engineer before stressing continues. The inspector must document all measured elongations for every tendon stressed.

After stressing, the protruding strand tail must be cut to a specific length and the anchor pocket must be cleaned and prepared for encapsulation. The chuck and bearing plate assembly within the pocket must be confirmed to have seated properly, with the wedges fully engaged around the strand and no evidence of slippage. A bearing plate that has rocked or tilted may not provide adequate bearing against the concrete, and a chuck that has not fully engaged may allow the tendon to slip under load.

For unbonded monostrand tendons, the live-end anchorage must be fully encapsulated to prevent moisture and chlorides from reaching the bearing plate and strand. PTI DC10.5 describes the encapsulation requirements - typically a proprietary cap or grease-filled pocket former system that isolates the metal anchor hardware from the environment. In parking structures and exterior slabs, encapsulation failure over time is one of the primary causes of post-tensioned slab deterioration. The inspector verifies that the encapsulation material is applied to the full pocket before patching.

Post-tensioning anchors in unbonded slab systems are located at the slab edge. A pocket former (a preformed plastic or foam block) is placed over each anchor casting before concrete is placed to create a recess in the slab edge. After stressing, the tail is cut inside the pocket, and the pocket is filled with dry-pack mortar or a specified patch material. The pocket former must be positioned so the bearing plate will be fully surrounded by concrete and the recess will be deep enough for the cut tail and patch. Pocket formers that are omitted, incorrectly positioned, or poorly secured shift during placement and compromise the anchor pocket geometry.

After stressing and acceptance of elongation, the protruding strand tail must be cut and the pocket filled with a patch material that provides the same concrete protection and appearance as the surrounding slab edge. Tails are typically cut with a cold chisel and hammer or an abrasive blade. Tails must be recessed a minimum of 3/4 inch below the slab face before patching to provide corrosion protection over the wedge and anchor casting. Patch material must be approved by the PT supplier - not any mortar will do. Shrinkage-compensating or polymer-modified patches are typically specified to prevent patch delamination.

For bonded post-tensioned systems, the duct must be grouted after stressing to bond the strand to the surrounding concrete and to protect the strand from corrosion. Grouting is typically performed using a cementitious grout formulated to have low bleed, good flowability, and adequate strength. Voids in the grout within the duct leave sections of strand unprotected and unbonded - a corrosion risk and a structural deficiency that cannot be corrected without removing the concrete.

Grout is injected at one end of the duct under pressure and forced through the full duct length until it exits at the vent or the opposite end at the same consistency as the grout being injected. Venting at intermediate high points allows trapped air to escape. The inspector must verify that grout actually exits at each vent before that vent is closed, and that the exit grout consistency is similar to the injected grout - thin, water-rich grout exiting at the vent indicates segregation within the duct.

Post-tensioning duct grout for bonded systems must meet PTI DC10.5 requirements for flowability, bleed water, expansion, and compressive strength. The grout is typically a combination of Portland cement, water, and approved admixtures (plasticizers, expansion agents, and anti-bleed agents). The water-cement ratio is critical: higher W/C produces segregation and bleed water that leave voids in the duct; lower W/C reduces flowability and risks blockage. The PTI pre-qualification test involves pumping grout through a simulated duct geometry to demonstrate flowability without bleed-water segregation. The inspector must witness the preparation and testing of each grout batch before pumping begins.

Grout injection in bonded PT systems follows a strict sequence: duct air venting, forward pressure injection, vent-by-vent monitoring of grout outflow to verify duct is filling, and final pressurization to force grout into dead-end zones. Injection proceeds from the low end of the duct to the high end. Vents at high points must be left open until grout flows freely without air. The injection pressure must not exceed PTI DC10.5 maximums (typically 150 psi) to avoid cracking the surrounding concrete. The inspector must witness the complete injection sequence and document outflow consistency and timing at each vent.

Pre-tensioned precast concrete elements are manufactured in a certified plant under quality control procedures governed by PCI (Precast/Prestressed Concrete Institute) plant certification requirements. The special inspector for pre-tensioned concrete may be required to visit the plant during production of critical elements to verify strand placement, tensioning operations, concrete placement, and strand release procedures. The nature and frequency of plant inspection depends on the project specification and statement of special inspections.

Strand release in pre-tensioned beds must occur at the specified concrete strength to avoid cracking or damage. After the strands are released, the prestress force transfers from the strand to the concrete through bond over the development length - the region at each end where the bond builds from zero to full prestress. End zone cracking during release indicates that the release strength was insufficient or that the release procedure was too abrupt. End zone reinforcement - typically hairpin bars or spiral reinforcement - is provided in the design to control this cracking.

Pre-tensioned concrete is cast in a stressing bed where strands are tensioned against abutments before concrete is placed. Hold-down devices (depressions in the strand profile) create a draped strand path to improve shear capacity near supports. The inspector witnessing plant production must verify the applied stress per strand against the calibrated jack data and the strand stress record sheet. Each wire in a strand must be checked for proper seating in the wedge chucks. Strands that show visible broken wires or pop-outs must not be tensioned. The approved strand pattern and spacing from the shop drawings must match the abutment layout.

Transfer of prestress occurs when the strands are released from the abutments after the concrete reaches the minimum transfer strength (typically 3,500-4,000 psi as specified). Release method matters: gradual detensioning (hydraulic pressure release) is preferred over flame-cutting, which can cause dynamic stress transfer. After transfer, the member shortens and cambers (deflects upward). The inspector should document the member's camber at transfer, compare it to the design camber, and note any unexpected cracking at the transfer zone. Excessive cracking at the ends of pre-tensioned members indicates transfer occurred too early or the strand was over-stressed.

Post-tensioning shop drawings are submitted by the specialty post-tensioning contractor and show the tendon layout plan, tendon profile elevations, anchor hardware details, stressing sequence, and elongation calculations. These drawings are approved by the engineer of record before construction begins. The inspector uses them as the primary reference document throughout the installation and stressing operations.

Post-tensioning shop drawings are prepared by the PT specialty contractor and approved by the EOR. They show the tendon layout plan, tendon profile elevation (dimensions from soffit to tendon centerline at each support and midspan), anchor location plan, and stressing schedule. The stressing schedule specifies the order in which tendons are stressed (typically alternating banded and distributed tendons, then the other direction) and any minimum concrete strength before stressing. Inspectors must carry approved shop drawings during stressing operations to verify each tendon is stressed in the correct sequence and to the correct elongation.

The stressing record documents the applied load and measured elongation for each tendon. Acceptance requires that the measured elongation falls within the tolerance of the calculated theoretical elongation, typically +/- 7% per PTI DC80.3 and ACI 318 Section R26.10. Elongation that is consistently low across multiple tendons in the same zone suggests friction losses greater than assumed in design. Elongation consistently high may indicate that the strand is not fully bonded to the concrete at anchor zones or that the friction coefficient used in design is too high. Both conditions require EOR review before additional stressing proceeds.

Tendon breakage during stressing produces a violent event - the sudden release of all stored energy in the strand. This is a serious safety hazard and a structural deficiency. The immediate response is to stop all stressing in the area, ensure worker safety, and notify the engineer. The engineer evaluates whether the structural design can be modified to account for the missing tendon or whether a replacement tendon can be installed. Tendon breakage typically results from a kink or notch in the strand, inadequate concrete strength at the time of stressing, or mechanical damage to the strand during installation.

Low elongation results - more than 7% below the calculated value - are a frequent problem requiring investigation before lock-off. Common causes include duct or sheath obstruction from concrete intrusion, excessive friction from kinking or improper installation, and dead-end anchor failure where the anchor block has pulled through the concrete rather than holding against it. The inspector documents all anomalous elongation results and does not allow the stressing operation to proceed until the cause is identified and the engineer has directed a course of action.

Tendon profile deviations - whether caused by displaced chairs, improper support spacing, or construction traffic on the form - alter the load-balancing distribution along the span. A kink in the tendon profile creates a concentrated transverse force at the kink location rather than the distributed force that balancing requires. Even a 1-inch elevation deviation at a single chair can shift the peak balanced load several feet from the design location. If profile deviations are discovered after concrete is placed, they must be documented and the EOR must recalculate the actual load-balancing condition to determine whether the element still meets design requirements.

The anchor zone in post-tensioned concrete is a highly stressed region where the concentrated prestress force from the tendon anchor must spread into the full cross-section. Splitting cracks (perpendicular to the tendon axis) and spalling cracks at the anchor face are common field problems caused by insufficient confining reinforcement, anchor hardware misalignment, or premature stressing before concrete reaches sufficient strength. ACI 318 Section 25.8 requires bursting and spalling reinforcement in anchor zones. The inspector must verify that this reinforcement is placed per the drawings and must not allow stressing to proceed if splitting cracks appear before the full stressing force is reached.

Prestressed concrete elements typically require higher concrete strength than conventional reinforced concrete - commonly 5,000 psi or above for post-tensioned building slabs, and higher still for precast elements designed for long spans. The higher strength provides greater bearing capacity at anchor zones, reduces camber, and improves the efficiency of the prestress. The concrete mix design must achieve not just the 28-day f'c but also the specified release strength or minimum stressing strength, which may be as little as 2,500 psi or as high as 3,500 psi depending on the design.

Chloride content in concrete for prestressed elements must be strictly controlled because even small concentrations of chlorides can initiate pitting corrosion on prestressing strand, which is catastrophically notch-sensitive. ACI 318 limits the chloride ion content of concrete for prestressed elements to 0.06% by mass of cement - significantly lower than the limit for non-prestressed concrete. Admixtures containing chloride are prohibited, and water sources must be verified to be within chloride content limits.

Prestressed concrete typically requires higher-strength concrete than conventionally reinforced sections because prestress losses increase with creep and shrinkage (both of which are more favorable in higher-strength mixes), and higher concrete strength improves anchorage zone capacity. Minimum specified compressive strengths of 4,000-6,000 psi are common for PT slabs; precast pretensioned members may require 5,000-8,000 psi at 28 days with 3,000-4,000 psi minimum at transfer. The inspector must verify that the mix design is approved for the specified strengths and that early-age strength testing (at transfer age, typically 1-3 days for pre-tensioned, 5-10 days for PT slabs) is performed before transfer or stressing proceeds.

Post-tensioned slabs and beams contain PT sheaths, mild reinforcement, and anchorage hardware that collectively create congested zones where concrete consolidation is challenging. Inadequate consolidation in the anchor zone creates honeycombing that severely reduces the confined concrete strength needed to resist bursting forces. The inspector must observe concrete placement to verify that vibration is performed with a small enough diameter vibrator to penetrate between PT sheaths and that vibration duration is sufficient without over-vibration that causes segregation.

The stressing record is the central documentation artifact for post-tensioned concrete inspection. It records, for every tendon stressed on the project, the date and time of stressing, the gauge pressure at lock-off, the theoretical elongation from the calculations, the measured elongation, and the percent deviation. This record demonstrates that the required prestress force was applied to every tendon and that the measured results were within the acceptance tolerance.

The final documentation package for prestressed concrete inspection includes the complete stressing records, the grouting records for bonded systems, the concrete strength test results confirming that stressing was performed at the correct concrete strength, the records of any anomalies and their resolution, and the final inspection report certifying that all required inspection was performed. This package is submitted to the engineer of record and building official.

Post-tensioning stressing records are permanent structural documents, not routine inspection notes. ACI 318 Section 26.10 requires records to include: the date of stressing, the tendon identification number, the required jacking force, the measured jacking force from the gauge, the calculated theoretical elongation, the measured actual elongation, and the inspector's signature. These records must be submitted to the EOR and the building official and retained permanently with the project record. IBC Section 1705.3 identifies PT stressing as a required special inspection activity, and the stressing log is the primary evidence that the inspection occurred and was satisfactory.

Post-tensioning systems require a comprehensive close-out package that includes: all stressing records signed by the inspector, the PT shop drawing approval record (showing EOR approval stamp and date), grout injection records for bonded systems, anchor pocket patching records with materials used, photographs of the installed tendon layout before concrete placement, and the Statement of Special Inspections completion certificate. This package must be delivered to the building official and the owner as a permanent building record. In post-occupancy renovations or demolitions, these records are critical to identify tendon locations and prevent accidental strand severance.