1 Scope
NOTE This specification covers the materials, fabrication, installation, stressing, grouting, and field quality control of cast-in-place post-tensioned concrete for buildings and parking structures. (1.1)
1.2 What Post-Tensioning Is
NOTE Post-tensioning is a method of prestressing in which high-strength steel strand is installed in the forms in an unstressed condition, the concrete is placed and cured around it, and the strand is then tensioned (stressed) against the hardened concrete and locked off at anchorages cast into the member. (1.2.1)
NOTE The resulting compressive force counteracts the tensile stresses that service loads would otherwise produce, allowing thinner sections, longer spans, reduced deflection, and controlled cracking. (1.2.2)
1.2.3All work shall conform to the prestressed-concrete provisions of ACI CODE-318-25, Building Code Requirements for Structural Concrete, and to the applicable Post-Tensioning Institute (PTI) and ACI specifications referenced herein.
1.2.4The concrete itself — its materials, mix design, placement, consolidation, and curing — shall conform to Cast In Place Concrete; this standard governs only the requirements unique to post-tensioning. 1.3 How Post-Tensioning Differs from Other Concrete
NOTE Post-tensioning differs fundamentally from both ordinary reinforced concrete and from pretensioned precast. (1.3.1)
NOTE Unlike ordinary reinforcement, the prestressing steel is an actively stressed element carrying a large permanent force — a half-inch strand is routinely tensioned to over 30,000 lbf — so a failure of the strand, anchorage, or its corrosion protection is a release of stored energy, not merely a loss of passive reinforcement. (1.3.2)
NOTE Unlike pretensioned precast, post-tensioning transfers the force into the member through mechanical anchorages bearing on the concrete at the member ends, and the stressing happens in the field on the project, which shifts concrete strength at stressing, anchorage bearing capacity, the stressing sequence, elongation verification, and grouting into the field. (1.3.3)
1.4 Unbonded versus Bonded Systems
NOTE The most important conceptual distinction in this standard is between **unbonded** and **bonded** systems. (1.4.1)
NOTE In an unbonded single-strand system, each seven-wire strand is coated with corrosion-inhibiting grease and encased in a continuous plastic sheath for its full length; the strand remains permanently free to move relative to the concrete, and its force is held only by the end anchorages. (1.4.2)
NOTE This is the dominant system for elevated building floor slabs, beams, and parking decks because it is economical, the tendons are light and flexible, and no grouting operation is required. (1.4.3)
NOTE In a bonded multistrand system, several strands are grouped in a metal or plastic duct cast into the member, tensioned together by a multistrand jack, anchored, and then the duct is filled with cementitious grout; after the grout cures, the strand is bonded to the concrete along its length and is also protected from corrosion by the grout. (1.4.4)
NOTE Bonded systems are used where the higher force per tendon, the structural advantages of bond, and the redundancy of bond (a localized strand failure does not unload the entire tendon) are needed — transfer girders, heavily loaded beams, and longer spans. (1.4.5)
1.4.6A single project frequently contains both unbonded and bonded systems, and each system shall satisfy its own requirements in this standard.
1.5 Corrosion Protection as the Governing Durability Concern
NOTE Corrosion protection is the governing durability concern for both systems and is treated with particular emphasis for parking structures and other chloride-exposed work. (1.5.1)
NOTE Prestressing strand is far more sensitive to corrosion than ordinary reinforcement: it is highly stressed, cold-drawn, and can fail by hydrogen embrittlement or stress-corrosion cracking at a fraction of the section loss that a passive bar would tolerate, and the failure can be sudden and brittle. (1.5.2)
1.5.3For unbonded tendons in aggressive exposure, this standard requires encapsulated tendons — a fully sealed system in which the sheath, the anchorage, and the strand tail are continuously enclosed so that the strand is never exposed to the environment.
1.5.4Where specified, electrically isolated tendons that permit the corrosion protection to be monitored shall be provided.
1.5.5For bonded tendons, the grout is both a structural and a corrosion-protection element, and its quality, completeness of fill, and freedom from bleed voids are critical.
1.6 Coordination
1.6.2Coordinate non-prestressed reinforcement — the conventional mild-steel reinforcing that supplements the tendons at anchorages, in the bursting zone behind anchorages, and as bonded reinforcement required by code — with Concrete Reinforcement. 1.6.3Where post-tensioned members bear on or connect to precast or pretensioned elements, coordinate with Precast Concrete. 1.7 Exclusions
1.7.1This standard does not govern pretensioned precast (see Precast Concrete), post-tensioned slab-on-ground residential foundations, bridge and other infrastructure post-tensioning, or external post-tensioning for repair and strengthening. 2 Referenced Standards
| Standard |
Title |
| ACI CODE-318-25 |
Building Code Requirements for Structural Concrete and Commentary (prestressed concrete provisions) |
| ACI 301-20 |
Specifications for Concrete Construction |
| ACI 117-10 (Reapproved 2015) |
Specification for Tolerances for Concrete Construction and Materials |
| ACI SPEC-423.7-14 |
Specification for Unbonded Single-Strand Tendon Materials |
| ACI 423.3R-17 |
Recommendations for Concrete Members Prestressed with Single-Strand Unbonded Tendons |
| ACI 423.4R |
Corrosion and Repair of Unbonded Single-Strand Tendons |
| ACI 423.6 / ACI SPEC |
Specification for Unbonded Single-Strand Tendons (referenced where adopted) |
| PTI M10.2-17 |
Specification for Unbonded Single-Strand Tendons |
| PTI M10.6-15 |
Specification for Unbonded Single-Strand Tendons Used for Slab-on-Ground Construction (reference only) |
| PTI/ASBI M50.3-19 |
Specification for Multistrand and Grouted Post-Tensioning |
| PTI M55.1-12 |
Specification for Grouting of Post-Tensioned Structures |
| ASTM A416/A416M |
Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete |
| ASTM A722/A722M |
Standard Specification for High-Strength Steel Bars for Prestressing Concrete (post-tensioning bars) |
| ASTM A421/A421M |
Standard Specification for Stress-Relieved Steel Wire for Prestressed Concrete |
| ASTM A615/A615M |
Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (anchorage zone reinforcement) |
| ASTM A706/A706M |
Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement (weldable) |
| ASTM C39/C39M |
Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens |
| ASTM C942 |
Standard Test Method for Compressive Strength of Grouts for Preplaced-Aggregate Concrete (grout cubes) |
| ASTM C939/C939M |
Standard Test Method for Flow of Grout for Preplaced-Aggregate Concrete (Flow Cone Method) |
| ASTM C1741 |
Standard Test Method for Bleed Stability of Cementitious Post-Tensioning Tendon Grout |
| AWS D1.4/D1.4M |
Structural Welding Code — Steel Reinforcing Bars (where anchorage zone reinforcement is welded) |
| IBC |
International Building Code (locally adopted edition) |
2.1.1Materials, fabrication, installation, stressing, grouting, testing, and quality control shall comply with the latest adopted edition of the listed standards and codes.
2.1.2Where the contract documents, the adopted building code, a referenced standard, or the post-tensioning supplier's engineering conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer's review before any post-tensioning materials are fabricated or installed.
☐ Post-tensioning placing (shop) drawings — tendon layout, profiles, forces, elongations
☐ Strand, sheathing, and corrosion-protection system data
☐ Anchorage and coupler product data and qualification test records
☐ Delegated-design calculations (sealed) where design is delegated
☐ Strand and anchorage mill / acceptance certifications
☐ Calibrated jack and pressure-gauge calibration records (within validity period)
☐ Written stressing procedure and sequence
☐ Written grouting procedure per PTI M55.1 (bonded systems)
☐ Concrete mix design with strength-at-stressing requirement
3.1.2No tendon shall be placed and no concrete shall be cast around tendons until the corresponding submittals have been reviewed and accepted.
3.1.3Submittals shall be complete and internally coordinated; piecemeal submissions shall be rejected.
3.1.4The post-tensioning supplier shall prepare and submit placing (shop) drawings showing, for each member, the tendon layout in plan and the tendon profile in elevation, the number and size of strands per tendon, the high and low points of the profile and the support-bar or chair heights that establish the profile, the anchorage and coupler locations and types, the stressing-end and dead-end designations, the calculated jacking force and theoretical elongation for each tendon, the stressing sequence, the bursting and anchorage-zone reinforcement, and the location of all tendon penetrations, blockouts, and pour-strip closures.
3.1.5Placing drawings shall reconcile the tendon profile against the non-prestressed reinforcement and any embedded items so that the tendons can actually be placed at the specified profile without conflict.
3.1.6Product data and the manufacturer's qualification documentation shall be submitted for the strand, the sheathing and corrosion-protection system, the anchorages, the wedges, and any couplers, demonstrating that the anchorage assembly meets the static, fatigue, and (for unbonded systems) the corrosion-protection acceptance tests of ACI CODE-318-25 and the applicable PTI specification.
3.1.7Where the post-tensioning design or any portion of it (for example, the anchorage-zone reinforcement, the tendon profile detailing, or the stressing sequence) is delegated to the supplier, signed and sealed calculations shall be submitted, prepared by a registered professional engineer licensed in the jurisdiction of the project.
3.1.8The delegated-design calculations shall address the effective prestress after all losses, the concrete stresses at transfer (stressing), the stresses in service, and the anchorage-zone (bursting and spalling) forces.
3.1.9A written stressing procedure shall be submitted stating the stressing sequence, the calculated jacking force and gauge pressure for each tendon group, the calibrated jack and gauge identification, the anticipated wedge seating (anchor set) loss, and the elongation-acceptance basis.
3.1.10For bonded systems, a written grouting procedure conforming to PTI M55.1 shall be submitted stating the grout material and mix proportions, the injection and venting sequence, the equipment, and the procedures for verifying complete duct fill.
NOTE Submit the following informational items: (3.2.1)
- Certified mill test reports for the prestressing strand (ASTM A416) or bar (ASTM A722), reporting the actual modulus of elasticity used to compute theoretical elongation.
- Jack and pressure-gauge calibration certificates from an accredited laboratory, dated within the calibration validity period (commonly six months or 1,000 cycles, whichever is sooner), with the jack and gauge calibrated as a unit.
- The qualifications and certification of the field personnel performing stressing and (for bonded systems) grouting.
- The post-tensioning supplier's quality-control plan.
☐ Certified mill test reports for strand (ASTM A416) or bar (ASTM A722) with modulus of elasticity
☐ Jack and pressure-gauge calibration certificates (calibrated as a unit, within validity period)
☐ Qualifications and certification of stressing and grouting personnel
☐ Post-tensioning supplier quality-control plan
3.3 Closeout Submittals
NOTE Provide the following at project closeout: (3.3.1)
- Stressing records for every tendon: the gauge pressure applied, the measured elongation, the calculated theoretical elongation, the percent difference, the wedge seating measured, and the date and personnel — reconciled and accepted by the Engineer.
- For bonded systems: grouting records for every duct, including grout batch, flow-cone time, bleed and set verification, injection pressure, the sequence of vents closed, and confirmation of complete fill.
- Records of any tendon that was re-stressed, replaced, or accepted under an engineered variance, with the supporting evaluation.
- As-built tendon layout noting any field-authorized profile or anchorage modifications.
- Warranty documentation.
☐ Stressing records for every tendon (reconciled and accepted by the Engineer)
☐ Grouting records for every duct (bonded systems) with fill confirmation
☐ Records of re-stressed, replaced, or variance-accepted tendons with evaluation
☐ As-built tendon layout noting field-authorized modifications
☐ Warranty documentation
4 Quality Assurance
4.1 Post-Tensioning Supplier Qualification
4.1.1The post-tensioning materials and the supplier's field services shall be furnished by a supplier regularly engaged in the production and installation of the specified system and able to demonstrate that its anchorage assemblies have passed the acceptance tests required by ACI CODE-318-25 and the applicable PTI specification.
4.1.2The Contractor shall furnish evidence of qualification before any post-tensioning material is delivered.
4.2 Field Personnel Certification
☐ Stressing personnel trained and certified in the specified PT system
☐ Grouting personnel certified (bonded systems) — recognized grouting certification
☐ Special inspector for post-tensioning per IBC Chapter 17
4.2.1Personnel performing stressing operations shall be trained and certified in the specified system, and shall be qualified to read and reconcile elongation against gauge pressure.
4.2.2For bonded systems, personnel performing grouting shall be certified under a recognized grouting certification program (such as the ACI/PTI bonded post-tensioning grouting certification).
4.2.3Certification records shall be available at the site.
4.3 Special Inspection
4.3.1Post-tensioned concrete is a structural element requiring special inspection under IBC Chapter 17.
4.3.2The special inspector shall verify the tendon material and placement against the accepted placing drawings before concrete placement, witness and record stressing operations, verify elongation acceptance, and (for bonded systems) inspect grouting for complete fill.
4.3.3The special inspector is engaged by and reports to the Owner; the inspector's presence does not relieve the Contractor of responsibility for conforming work.
4.4 Pre-Installation Conference
4.4.1A pre-installation conference shall be held before the first post-tensioned element is cast, attended by the Contractor's superintendent, the post-tensioning supplier's field representative, the concrete subcontractor, the testing and special-inspection agencies, and the Engineer.
4.4.2The pre-installation conference agenda shall cover tendon placement and profile control, anchorage-zone reinforcement, the concrete strength required before stressing and how it will be verified, the stressing sequence, elongation acceptance and the response to out-of-tolerance results, protection of tendons before and after stressing, and (for bonded systems) the grouting procedure and schedule.
4.5 Concrete Strength Verification Before Stressing
4.5.1No tendon shall be stressed until field-cured cylinders, cured under the same conditions as the member they represent, confirm that the concrete has attained the specified minimum compressive strength at stressing.
4.5.2Verification shall be by ASTM C39 cylinder breaks, not by maturity or rebound estimates alone unless those methods have been correlated to cylinder data and accepted by the Engineer.
NOTE Concrete strength verification before stressing is the single most important hold point in the work: stressing against concrete that has not reached the required strength crushes the concrete behind the anchorage, fails the anchorage zone in bursting, and can cause progressive failure. (4.5.3)
5 Environmental and Service Conditions
5.1 Service Exposure
NOTE The corrosion protection required for the tendons is governed by the service exposure, and parking structures are the controlling case for buildings because their decks are exposed to chloride-laden water tracked in from deiced roads. (5.1.1)
○ Interior, dry, non-corrosive — typical interior building floor
○ Exterior or humid, no chlorides — exposed but not chloride-laden
○ Chloride exposure — parking structure decks, coastal, deicing-salt environment (aggressive)
○ C0 — dry or protected from moisture in service
○ C1 — exposed to moisture but not to external chlorides
○ C2 — exposed to moisture and an external source of chlorides (parking decks, coastal)
5.1.2ACI CODE-318-25 Chapter 19 exposure categories apply to the concrete, and the post-tensioning system's corrosion protection shall be selected to match.
5.1.3In C0 and C1 exposure, standard greased-and-sheathed unbonded tendons or standard grouted bonded tendons are adequate.
5.1.4In C2 exposure — the parking-structure case — this standard requires encapsulated unbonded tendons (or, for bonded systems, a fully grouted plastic-duct system with sealed anchorages) so that the strand is continuously enclosed and isolated from the chloride source.
5.1.5The cover over the tendons and the watertightness of the deck (membrane or traffic-bearing coating, specified separately) are additional controls in C2 exposure.
6 Prestressing Steel
6.1 Seven-Wire Strand
○ Grade 270, low-relaxation, 7-wire (ASTM A416) — standard
○ Grade 250, low-relaxation, 7-wire (ASTM A416) — where designed
3/8 in. — light tendons, slab-on-ground
1/2 in. — most common in unbonded building and parking slabs
0.6 in. — bonded multistrand and high-force tendons
6.1.1Prestressing strand for both unbonded and bonded systems shall be uncoated, low-relaxation, seven-wire steel strand conforming to ASTM A416/A416M, Grade 270 (270,000 psi specified minimum tensile strength), unless the design requires a different grade.
6.1.2Low-relaxation strand shall be used in lieu of stress-relieved (normal-relaxation) strand, with relaxation not more than 2.5% at 1,000 hours from an initial load of 70% of specified minimum breaking strength.
6.1.3Strand shall be free of loose rust, pitting, and bond-reducing contaminants; tightly adhered surface rust without pitting is acceptable.
6.1.4The strand diameter, the number of strands per tendon, and the tendon force are determined by the structural design and shown on the contract drawings.
NOTE Low-relaxation strand is required because its much lower long-term relaxation preserves more of the effective prestress over the life of the structure and is the current production standard. (6.1.5)
NOTE The half-inch (0.5 in.) diameter strand is the standard for unbonded building floor and parking deck tendons; the 0.6 in. strand is common in bonded multistrand tendons where higher force per strand reduces the number of strands and ducts. (6.1.6)
6.2 High-Strength Bars
○ Seven-wire strand (ASTM A416) — standard for building and parking PT
○ High-strength bar (ASTM A722) — short or straight tendons, special connections
6.2.1Where post-tensioning bars are used (for short, straight tendons, for anchorage to existing construction, or for transverse stressing), they shall conform to ASTM A722/A722M.
6.2.2Bar systems shall use threaded ends and nut anchorages rather than wedges.
6.3 Jacking Force and Stressing Limits
2545
253033354044
Default: 33 kip
6.3.1The jacking force and the resulting effective prestress are structural design values shown on the contract drawings and the accepted placing drawings.
6.3.2The temporary stress in the strand during jacking shall not exceed the limit of ACI CODE-318-25 (commonly 0.80 times the specified minimum breaking strength at jacking, reducing to 0.74 immediately after anchorage seating).
6.3.3The contractor shall not increase the jacking force above the design value in an attempt to compensate for low measured elongation without the Engineer's evaluation.
7 Unbonded Tendon System
7.1 Sheathing and Corrosion-Inhibiting Coating
7.1.1In an unbonded single-strand tendon, the strand shall be continuously coated with a corrosion-inhibiting grease and encased in a seamless, watertight plastic (typically high-density polyethylene or polypropylene) sheath conforming to the materials requirements of ACI SPEC-423.7 and PTI M10.2.
7.1.2The grease shall fill the annular space between the strand and the sheath for the full length, providing both corrosion protection and lubrication so the strand can elongate freely within the sheath during stressing.
7.1.3The sheath shall have a minimum wall thickness adequate to resist damage during handling and concrete placement.
7.1.4Damaged sheathing shall be repaired with a sealed, waterproof wrap before concrete placement.
NOTE A sheath punctured during placement leaves the strand exposed to the concrete and to any moisture that reaches it, and is the most common origin of unbonded-tendon corrosion. (7.1.5)
7.2 Standard versus Encapsulated Tendons
NOTE A standard (non-encapsulated) tendon protects the strand along its length with grease and sheath but does not fully seal the anchorage and the strand tail. (7.2.1)
NOTE An encapsulated tendon extends the watertight enclosure continuously through the anchorage: the anchorage casting is a sealed plastic or plastic-coated unit, a watertight connection joins the sheath to the anchorage, and a sealed cap encloses the stressed strand tail and the wedges, with the cap filled with corrosion-inhibiting grease. (7.2.2)
NOTE The encapsulated system creates a continuous, sealed envelope around the strand from end to end, with no point at which the strand is exposed to the environment. (7.2.3)
○ Standard greased-and-sheathed tendon — interior dry (C0/C1) where permitted
○ Encapsulated tendon — sealed anchorage and capped tail (required for parking and C2 exposure)
○ Encapsulated and electrically isolated — for impressed-current or monitored corrosion protection
7.2.4The level of corrosion protection of an unbonded tendon shall match the service exposure.
7.2.5Encapsulated tendons shall be used for all tendons in C2 (chloride) exposure — most importantly parking structures — and are recommended wherever the tendon is exposed to moisture.
7.2.6For interior, dry building floors (C0/C1), standard tendons are acceptable, except that the specifier shall verify the current ACI CODE-318 requirement for the project and default to encapsulated tendons where there is any doubt about future exposure.
7.3 Electrically Isolated Tendons
7.3.1Where the corrosion protection is to be monitored or where an impressed-current cathodic protection strategy is anticipated, electrically isolated tendons shall be used, in which the strand and its anchorage are electrically isolated from the surrounding concrete and reinforcement by non-conductive components so that the integrity of the encapsulation can be verified by an electrical resistance measurement and monitored over the life of the structure.
7.3.2Electrically isolated tendons are a specialized requirement and shall be specified only where the design calls for them.
7.4 Anchorages — Unbonded
7.4.1Unbonded tendons shall be anchored at each stressing end by a casting that bears on the concrete and grips the strand with hardened steel wedges, and at the dead end by a pre-seated wedge anchorage or a swaged/button-headed dead-end fitting.
7.4.2The anchorage assembly (casting, wedges, and, for encapsulated systems, the sealing components) shall be a qualified assembly that has passed the static and fatigue acceptance tests of ACI CODE-318-25 and PTI M10.2.
7.4.3The anchorage casting shall be installed with the manufacturer's anchorage-zone (bursting) reinforcement detailed on the placing drawings.
NOTE Omitting or mislocating the bursting reinforcement causes the concrete behind the anchorage to split when the tendon is stressed. (7.4.4)
8 Bonded Multistrand Tendon System
8.1 Ducts
○ Corrugated galvanized steel duct — interior / non-aggressive
○ Corrugated plastic duct (PE/PP) — parking, coastal, and aggressive exposure (preferred for C2)
8.1.1In a bonded multistrand system, the strands of each tendon shall be installed in a duct cast into the member, sized to allow the strands to be placed and later to allow grout to surround every strand.
8.1.2Ducts shall be of corrugated galvanized steel or, for aggressive exposure and where specified, of corrugated plastic (polyethylene or polypropylene) conforming to PTI/ASBI M50.3.
8.1.3Plastic ducts shall be used for parking and coastal structures, providing an additional barrier against chloride ingress and electrical isolation.
8.1.4Ducts shall be mortar-tight at all joints and connections so that cement paste from the surrounding concrete cannot enter and block the duct.
8.1.5Ducts shall be supported at the spacing required to hold the tendon profile and to resist flotation and displacement during concrete placement.
8.2 Anchorages and Couplers — Bonded
8.2.1Bonded multistrand tendons shall be anchored by a bearing-plate anchorage with a wedge plate gripping each strand, qualified by the static, fatigue, and load-transfer acceptance tests of ACI CODE-318-25 and PTI/ASBI M50.3.
8.2.2Couplers, where used to splice tendons at construction joints or stage-stressing locations, shall develop the specified breaking strength of the tendon and shall be located and detailed on the placing drawings.
8.2.3Couplers concentrate force and require their own anchorage-zone reinforcement.
8.2.4For aggressive exposure, the anchorage shall be a sealed (encapsulated) anchorage compatible with the plastic-duct system so that the corrosion-protection envelope is continuous through the anchorage.
8.3 Grout Vents and Inlets
8.3.1Each duct shall be provided with grout inlets and with vents at all high points, at anchorages, and at any location where air or bleed water could be trapped, so that the duct can be completely filled and air and water displaced during grouting.
8.3.2The location of inlets and vents shall be shown on the placing drawings and shall reflect the tendon profile.
NOTE An un-vented high point traps a void that leaves strand ungrouted and unprotected. (8.3.3)
9 Tendon Profile and Placement
9.1 Profile Control
○ As shown on the structural and placing drawings (high/low points and support heights)
NOTE The tendon high points, low points, and support-bar heights are
as indicated on the post-tensioning placing drawings and structural drawings.
(9.1.1) 9.1.2Tendons shall be supported at the heights shown on the placing drawings using support bars, chairs, or bolster strips at the specified spacing so that the as-placed profile matches the design profile within tolerance.
NOTE The tendon profile is the geometric basis of the entire post-tensioning design: it determines the eccentricity of the prestress force and therefore the balancing load the tendons apply to the member. (9.1.3)
NOTE A tendon placed too low at midspan, or too high over a support, does not balance the load the design relied on and can produce cracking and excessive deflection that are difficult to remedy after stressing. (9.1.4)
9.2 Placing Tolerances
± 1/4 in. — members 8 in. thick or less (thin slabs)
± 3/8 in. — members over 8 in. to 24 in. thick
± 1/2 in. — members over 24 in. thick (beams, girders)
9.2.1The vertical placing tolerance for tendons shall conform to ACI CODE-318-25 and the placing drawings, and is tighter for thin members where a small vertical deviation is a large fraction of the available depth.
9.3 Concrete Cover Over Tendons
1-1/2 in. — typical interior or non-aggressive
2 in. — exposed to weather / parking deck top
Per design and ACI 318 for the exposure category
9.3.1Concrete cover over tendons, ducts, and anchorages shall conform to ACI CODE-318-25 for the service exposure and shall not be reduced below the value the design requires for the C exposure category.
9.3.2In parking and chloride exposure, cover is a primary corrosion-protection control and is frequently increased above the code minimum.
9.4 Protection of Tendons Before Stressing
9.4.1Tendons and their sheathing or ducts shall be protected from damage during placement of concrete and from heat, sparks, and weld spatter during adjacent work.
9.4.2Sheathing damaged before concrete placement shall be repaired and sealed.
9.4.3Anchorage pockets and stressing-end hardware shall be kept clean and free of concrete that would prevent the jack from seating.
NOTE A tendon with a torn sheath cast into the concrete is a corrosion liability that cannot be remedied without removing and replacing the tendon. (9.4.4)
10 Concrete for Post-Tensioned Members
10.1 General
NOTE Two requirements specific to post-tensioning, the specified compressive strength and the concrete strength at stressing, are added in this standard. (10.1.2)
10.2 Specified Compressive Strength
4000 psi
5000 psi — common for PT slabs and beams
6000 psi
7000 psi — heavily loaded girders and transfer members
10.2.1The specified 28-day compressive strength for post-tensioned members shall be as shown on the contract drawings.
10.3 Concrete Strength at Stressing
25004500
25003000350040004500
Default: 3000 psi
10.3.1The concrete shall attain a specified minimum compressive strength before any tendon is stressed, verified by field-cured cylinder breaks.
10.3.2The required strength at stressing is set by the design for the anchorage bearing and the member stresses at transfer, and is commonly in the range of 3,000 to 4,000 psi for slabs but shall be taken from the contract documents where stated.
10.3.3Where the design permits partial stressing at a lower strength followed by full stressing once the concrete gains strength, the partial- and full-stressing strengths and the corresponding forces shall be stated on the placing drawings and reflected in the stressing procedure.
NOTE The strength-at-stressing requirement governs the construction schedule, because the member cannot be stressed — and therefore cannot be partially de-shored and put to work — until it is reached, and stressing against under-strength concrete crushes the concrete at the anchorage and fails the anchorage zone. (10.3.4)
11 Stressing Operations
11.1 Stressing Equipment
○ Calibrated as a unit by accredited lab, within validity period (standard)
○ Per supplier's quality-control plan and applicable PTI specification
11.1.1Tendons shall be stressed with hydraulic jacks calibrated as a unit with their pressure gauges by an accredited laboratory within the calibration validity period.
11.1.2Calibration records shall be at the site and the calibration shall remain valid for the duration of stressing.
NOTE The calibration relates gauge pressure to the actual force delivered by the jack; an out-of-calibration jack or gauge is the most common cause of the entire tendon population being over- or under-stressed without the elongation check revealing it, because the gauge reading is the contractor's primary force control. (11.1.3)
11.2 Stressing Sequence
○ Per the accepted placing drawings and stressing procedure (standard)
○ Engineer-directed sequence for staged or transfer construction
11.2.1Tendons shall be stressed in the sequence shown on the placing drawings.
11.2.2The member shall be free to shorten and camber as it is stressed — forms and shoring that restrain the movement shall be loosened as the stressing procedure requires — so that the prestress goes into the member rather than into the formwork.
NOTE The sequence matters because stressing one tendon shortens and cambers the member and changes the force in tendons already stressed, and an out-of-sequence stressing operation can overload the member, the shoring, or the supporting structure, and can crack the member. (11.2.3)
11.3 Elongation Measurement and Acceptance
○ ± 7% of calculated theoretical elongation (ACI 318 — building tendons)
○ ± 7%, with absolute-value assessment for short tendons per PTI
○ Computed from the strand modulus on the certified mill report, the jacking force, and the assumed friction and wobble coefficients on the placing drawings (standard)
11.3.1The delivered prestress force shall be verified at every tendon by two independent means: the jack gauge pressure (giving the applied force from the calibration) and the measured strand elongation (giving the force from the strand modulus and the stretch).
11.3.2The measured elongation shall agree with the calculated theoretical elongation within ±7% for tendons in buildings, per ACI CODE-318-25.
11.3.3For short tendons, where a small absolute discrepancy is a large percentage, the tolerance may be assessed in absolute terms per the applicable PTI specification rather than as a percentage.
11.3.4Where the difference exceeds the tolerance, the cause shall be investigated and resolved with the Engineer before the tendon is accepted.
11.3.5The tendon shall not be over-stressed beyond the code jacking limit to force the elongation into tolerance.
NOTE Reconciling gauge pressure and elongation is the fundamental quality check of post-tensioning: a gauge reading alone can be wrong because of a faulty gauge, jack friction, or a blocked tendon, and an elongation reading alone can be wrong because of anchorage slip, a misread reference mark, or an incorrect assumed modulus. (11.3.6)
11.4 Wedge Seating (Anchor Set)
0.06250.375
0.06250.1250.18750.250.375
Default: 0.25 in
11.4.1The expected seating loss shall be accounted for in the design and in the calculated elongation, and the measured draw-in shall be within the supplier's expected range.
11.4.2A tendon with excessive draw-in shall be evaluated and, if required, re-stressed or replaced.
NOTE When the jack releases the strand and the wedges seat into the anchorage, the strand draws back a small distance — the anchor set or wedge seating, commonly on the order of 1/4 in. — and a corresponding loss of force occurs at the anchorage; excessive draw-in indicates a wedge that did not seat properly and a tendon that has lost more force than the design assumed. (11.4.3)
11.5 Stressing Records
11.5.1A stressing record shall be completed for every tendon at the time of stressing, documenting the gauge pressure, the corresponding force, the measured elongation, the calculated theoretical elongation, the percent difference, the measured wedge seating, the jack and gauge identification, and the date and operator.
11.5.2The records shall be reviewed and accepted by the special inspector and the Engineer as a hold point before the tendons are cut and the pockets are sealed.
12 Tendon Cutting and Anchorage Pocket Treatment
12.1 Tendon Cutting
○ Abrasive saw or hydraulic shear, no heat (standard)
○ Oxy-fuel / flame cutting — prohibited
12.1.1The stressed strand tails projecting beyond the anchorage shall not be cut until the stressing records for those tendons have been reviewed and accepted, because once cut the tendon cannot be re-stressed.
12.1.2Tails shall be cut by an abrasive saw or shear, leaving the minimum projection the anchorage requires.
12.1.3Tails shall not be cut by an oxy-fuel torch or by any method that heats the strand, because heat can release the wedges or anneal and weaken the strand at the anchorage.
12.2 Anchorage Pocket Treatment
○ Greased sealed cap over wedges, pocket filled with non-shrink low-permeability mortar (encapsulated / parking standard)
○ Pocket filled with non-shrink patching mortar (interior, non-encapsulated)
12.2.1After the tails are cut, the anchorage pocket shall be cleaned and sealed to protect the anchorage and the strand tail from corrosion.
12.2.2For encapsulated unbonded tendons, the sealed cap shall be installed over the wedges and tail and filled with corrosion-inhibiting grease, and the pocket shall be filled with a non-shrink, low-permeability patching mortar that bonds to the surrounding concrete.
12.2.3For bonded systems, the anchorage shall be capped and the pocket filled after grouting.
NOTE The pocket treatment completes the corrosion-protection envelope at the most vulnerable point of the tendon — the anchorage — and an unsealed or poorly patched pocket is a frequent origin of anchorage corrosion and of unsightly rust staining on parking-structure faces. (12.2.4)
13 Grouting of Bonded Tendons
13.1 General
NOTE Grouting applies only to bonded multistrand systems. (13.1.1)
13.1.2After the tendons are stressed and accepted, each duct shall be filled with cementitious grout that bonds the strand to the surrounding concrete and provides corrosion protection.
13.1.3Grouting shall conform to PTI M55.1, Specification for Grouting of Post-Tensioned Structures.
NOTE Incomplete grouting — leaving voids where air or bleed water was trapped — is the principal durability failure of bonded post-tensioning, because the ungrouted strand has no corrosion protection and the bond the design relied on is absent, and the voids are hidden inside the duct and cannot be detected after the fact without specialized testing. (13.1.4)
13.2 Grout Material
○ Prepackaged non-bleed thixotropic PT grout (PTI M55.1) — standard, required for aggressive exposure
○ Job-proportioned grout qualified to PTI M55.1 — non-aggressive interior only, with Engineer approval
☐ Flow cone time (ASTM C939)
☐ Bleed / volume stability (ASTM C1741)
☐ Compressive strength cubes (ASTM C942)
☐ Wick-induced bleed test (inclined-tube) per PTI M55.1
13.2.1The grout shall be a prepackaged or job-proportioned cementitious grout that is non-bleed and thixotropic, with low permeability and minimal shrinkage, conforming to PTI M55.1.
13.2.2A non-bleed, thixotropic grout shall be used for all but non-aggressive interior exposure.
13.2.3The grout shall be tested for flow (ASTM C939 flow cone), bleed and volume stability (ASTM C1741), and compressive strength (ASTM C942 cubes).
NOTE A non-bleed, thixotropic formulation is required because ordinary cement grout bleeds water that rises to high points and leaves voids when the bleed water is later reabsorbed or evaporates; the anti-bleed formulation holds the water in the mix and fills the duct completely. (13.2.4)
13.3 Grouting Operation
○ Inject from low end, continuous pumping, close vents low-to-high as consistent grout appears, record each vent (PTI M55.1 standard)
13.3.1Grout shall be injected from the lowest inlet of each duct and shall be pumped continuously until grout of the same consistency as that injected flows from each vent in sequence from low to high, at which point each vent is closed.
13.3.2The grout shall be injected at a controlled rate and pressure that fills the duct without segregating the grout or rupturing the duct.
13.3.3Grouting shall not be performed when the duct or the ambient temperature is below the minimum stated in the grouting procedure, and the grout shall be protected from freezing until it has set and gained strength.
13.3.4The grouting record shall confirm, vent by vent, that the duct was completely filled.
13.4 Grouting Schedule
13.4.1Bonded tendons shall be grouted as soon as practicable after stressing and acceptance, within the maximum time the grouting procedure and PTI M55.1 allow, to limit the period during which the stressed but ungrouted strand is exposed to moisture and corrosion inside the duct.
13.4.2Where grouting cannot follow promptly, the ducts shall be protected against moisture entry in the interim.
14 Anchorage Zone Reinforcement
14.1 Reinforcement Requirements
○ Per supplier's qualified anchorage detail and placing drawings (standard)
○ Per Engineer of Record's anchorage zone design
NOTE The concrete immediately behind each anchorage carries a large concentrated force that spreads into the member, producing bursting tension transverse to the tendon and spalling tension at the loaded face. (14.1.1)
14.1.2The anchorage zone shall be reinforced with the anchorage-zone (bursting and spalling) reinforcement detailed on the placing drawings, in conformance with ACI CODE-318-25.
14.1.3The anchorage-zone reinforcement shall be in place and inspected before concrete placement.
14.1.4Where the anchorage-zone reinforcement is welded, welding shall conform to AWS D1.4.
NOTE The anchorage-zone reinforcement is what prevents the concrete from splitting when the tendon is stressed, and it cannot be added after the fact. (14.1.5)
15 Tolerances
15.1 Member and Anchorage Tolerances
○ Per supplier's installation tolerance and placing drawings (standard)
15.1.2The anchorage location and angularity tolerance shall be held so that the jack can be aligned with the tendon and seat squarely on the anchorage.
NOTE The tendon vertical placing tolerance is addressed under Tendon Profile and Placement. (15.1.3)
NOTE An anchorage cast out of square forces the strand over an edge and produces a friction and seating anomaly at the most highly stressed point. (15.1.4)
16 Field Testing and Quality Control
16.1 Concrete Testing
○ Field-cured cylinders (ASTM C39) cured with the member, broken before stressing (standard)
○ Maturity or in-place method correlated to cylinders and accepted by Engineer
16.1.2Field-cured cylinders cured alongside each post-tensioned member shall be tested to confirm the concrete has reached the specified strength at stressing before any tendon in that member is stressed.
NOTE The field-cured cylinders reflect the actual in-place strength under the member's temperature history, which is the strength that matters for the stressing decision. (16.1.3)
16.2 Stressing Quality Control
16.2.1Every tendon's stressing operation shall be witnessed and recorded by the special inspector, and the elongation reconciled against the gauge force within tolerance before acceptance.
16.2.2The aggregate elongation results shall be reviewed for systematic bias, and where the entire population of tendons reads consistently high or low against theoretical, the systematic cause shall be found before the results are accepted.
NOTE A systematic high or low bias across the whole population is more likely a systematic error (an out-of-calibration jack, a wrong assumed modulus, or a calculation error) than a series of individual tendon problems. (16.2.3)
16.3 Grouting Quality Control
16.3.1For bonded systems, the grout shall be tested for flow, bleed, and strength as specified, and each duct's grouting shall be recorded and confirmed complete.
16.3.2Where the contract documents or the building code require, post-grouting verification of fill (such as impact-echo or borescope inspection at vents) shall be performed on a sampling of ducts to confirm the absence of voids.
17 Protection and Repair
17.1 Protection of Stressed Tendons
○ No coring, drilling, or cutting of PT members without Engineer review of tendon layout and locating (standard)
17.1.1After stressing and before cutting and pocket sealing, the projecting tails and anchorages shall be protected from damage and from moisture.
17.1.2After pocket sealing (and, for bonded systems, after grouting), the corrosion-protection envelope shall be complete.
17.1.3Any subsequent coring, drilling, or cutting of a post-tensioned member shall not be performed without the Engineer's review of the tendon layout.
17.1.4The locations of tendons shall be recorded on the as-built drawings precisely so that later coring, drilling, or cutting can avoid them.
NOTE Cutting a stressed tendon releases its force suddenly and can injure personnel and damage the structure. (17.1.5)
17.2 Repair of Damaged Tendons
17.2.1A tendon with a damaged sheath discovered before stressing shall have the sheath repaired and sealed.
17.2.2A strand found broken, corroded, or with a damaged anchorage shall be evaluated by the Engineer.
17.2.3Depending on the system, the remedy may be re-stressing, replacement of the tendon, or an engineered supplemental tendon, and the evaluation shall account for the redistribution of force to adjacent tendons.
17.2.4Repairs to unbonded tendons shall follow the principles of ACI 423.4R.
17.3 Repair of Concrete Defects
17.3.1Concrete defects in post-tensioned members — honeycombing, anchorage-zone cracking, or low-strength concrete — shall be evaluated and repaired in accordance with Cast In Place Concrete and the Engineer's direction, with particular attention to anchorage-zone defects. NOTE Anchorage-zone defects compromise the ability of the concrete to carry the anchorage force and may preclude stressing until repaired. (17.3.2)
18 Warranty
18.1 Warranty Requirements
1 year from substantial completion
2 years from substantial completion
18.1.1The Contractor shall warrant the post-tensioned concrete work against defects in materials and workmanship — including anchorage failure, corrosion of strand or anchorage attributable to a defective or incomplete corrosion-protection system, incomplete grouting of bonded tendons, anchorage-zone cracking, and loss of prestress attributable to construction defect — for a period of not less than one year from substantial completion, or for the period stated in the contract documents if longer.
18.1.2Material warranties provided by the post-tensioning supplier and any deck-membrane or coating manufacturer shall be passed through to the Owner and included in the closeout submittals.
18.1.3The warranty shall not limit the Engineer's right to require corrective work for nonconforming conditions discovered during the warranty period.