Post-Tensioned Concrete Slab Tendon Forensics: Rupture, Corrosion & Splicing
A definitive structural engineering protocol for diagnosing unbonded monostrand tendon failures in two-way flat plates: covering forensic lift-off testing, high-frequency GPR scanning, wedge seat slippage, and PTI DC80.3 dog-bone splice repairs.
Executive Forensic Summary
Unbonded post-tensioned (PT) concrete slabs rely upon continuous internal tensile precompression to resist gravity shears and flexural moments. Unlike bonded rebar, an unbonded tendon acts as an independent externalized spring anchored solely at building slab edges. A single wire fracture or anchorage failure releases strain energy along the tendon's entire length, immediately shedding load to adjacent spans. Structural forensics requires systematic non-destructive imaging (GPR), anchor pocket unmasking, calibrated hydraulic lift-off load measurements, and precision dog-bone pocket splicing compliant with PTI DC80.3 and ACI 423.4R.
01. Mechanics of Unbonded Monostrand Post-Tensioning Systems
Typical unbonded post-tensioned floor systems utilize 0.50-inch (12.7 mm) or 0.60-inch (15.2 mm) diameter seven-wire high-strength low-relaxation steel strands conforming to ASTM A416 Grade 270, possessing an ultimate tensile strength (\(f_{pu}\)) of \(270\text{ ksi}\) (\(1,860\text{ MPa}\)) and an initial jacking stress limit of \(0.80 f_{pu}\) (\(216\text{ ksi}\)).
The tendon strand is coated with an extruded water-resistant lithium- or polyurea-based corrosion inhibitor grease and encased in a continuous high-density polyethylene (HDPE) or polypropylene extruded plastic sheath with a minimum wall thickness of \(0.050\text{ inches}\) (\(1.25\text{ mm}\) per ACI 423.7).
The system provides gravity load resistance via two distinct structural actions:
- Direct Axial Precompression (\(P/A\)): Uniform axial compressive stress, typically maintained between \(125\text{ psi}\) and \(300\text{ psi}\) (\(0.86\) to \(2.07\text{ MPa}\)), counteracts service tensile stresses and suppresses two-way flexural and punching shear micro-cracking.
- Equivalent Upward Balancing Load (\(w_b\)): Tendons are draped into a parabolic profile with high points over columns and low points at midspan. The tensioned tendon exerts a uniform upward vertical balancing pressure counteracting gravity dead loads:
Where \(P\) is the effective final tendon prestressing force after all instantaneous (friction, seating) and long-term (creep, shrinkage, steel relaxation) losses, \(e\) is the tendon sag profile drape eccentricity from centroidal axis, and \(L\) is the span length. When a tendon breaks, \(w_b\) instantly drops to zero across all spans traversed by that strand, causing sudden midspan deflection spikes, bottom-mat cracking, and severe punching shear redistribution at slab-column interfaces.
Left: Structural cross-section of unbonded monostrand tendon. Right: Elevation schematic of PTI DC80.3 dog-bone pocket excavation, double-ended splice chuck installation, and non-shrink repair grout encasement.
02. Forensic Failure Modes: Corrosion, Embrittlement & Anchorage Blowout
Unbonded PT systems are susceptible to specific failure mechanisms distinct from standard mild-reinforced concrete structures:
Paper-Wrapped Tendon Moisture Infiltration (Pre-1985 Structures)
Prior to modern extruded HDPE sheathing standards introduced in the late 1980s, tendons were frequently encased in spirally wound kraft paper or loose PVC tubing with petroleum waxes. In coastal or deicing-salt parking structures, chlorides migrate through poorly grouted edge pocket blockouts, emulsifying the grease and attacking the high-strength steel wires.
Stress Corrosion Cracking & Hydrogen Embrittlement
Because cold-drawn ASTM A416 prestressing wire sustains sustained tensile stresses exceeding \(175\text{ ksi}\) (\(1,200\text{ MPa}\)), it is highly vulnerable to cathodic hydrogen atom absorption. Under acidic micro-environments caused by bacterial grease degradation or sulfur-bearing admixtures, micro-cracks propagate along grain boundaries with zero macro-plastic elongation, causing instantaneous brittle severance without prior sagging warnings.
Anchorage Zone Wedge Slippage & Spalling Blowout
At anchor heads, high-pressure wedge teeth grip the strand. Poor concrete consolidation beneath anchor castings produces honeycomb voids. When loaded, concentrated bursting stresses split the anchor zone concrete. If the anchor seat fractures, the tendon shoots violently out of the building edge (tendon blowout), posing extreme life-safety hazards.
03. Non-Destructive Investigation: GPR, Radiography & Lift-Off Testing
Forensic investigation of PT floor plates must never begin with blind exploratory chipping, which risks severing active high-energy strands. A non-destructive sequence is strictly required:
2.0 to 2.7 GHz High-Frequency GPR
High-frequency Ground Penetrating Radar establishes exact horizontal tendon locations and vertical parabolic profiles. GPR easily differentiates between straight mild-steel bottom mats and draped PT tendons by tracking hyperbolic reflection apex depth across multiple scans.
In-Situ Hydraulic Lift-Off Testing
To measure residual effective prestress, the anchor pocket grout is carefully chipped away. A calibrated hollow-core hydraulic jack and dial gauge are installed onto the exposed strand tail. Jacking force is applied until the wedges just unseat (observed via micro-strain sensor or dial gauge break point), yielding exact residual tension (\(P_{eff}\)).
04. PTI DC80.3 Dog-Bone Pocket Splice Repair Protocol
When a severed or heavily corroded tendon strand is detected, full-length tendon replacement is often architecturally impossible due to inaccessible building facades. PTI DC80.3 (Guide for Repair and Strengthening of Post-Tensioned Concrete Structures) establishes the definitive dog-bone cavity splice repair protocol:
- Shoring Installation: Prior to concrete excavation, install structural screw-jack shores directly beneath the repair bay across two spans to carry dead plus construction live loads.
- Dog-Bone Pocket Excavation: Sawcut the slab top or soffit perimeter to a maximum depth of \(0.75\text{ inches}\) (avoiding mild steel and adjacent tendons). Mechanically chip a pocket measuring approximately \(48\text{ inches}\) long, tapered at the ends ("dog-bone" geometry) to prevent stress concentration.
- Strand De-tensioning & Cutback: Secure existing tendon tails with temporary clamp grips. Cut out corroded strand segment back to sound, unpitted steel.
- Coupler Installation: Install a specialized double-ended mechanical post-tensioning splice coupler (barrel chuck) compliant with PTI Level 3 certification. Couple the existing strand to a new replacement length of ASTM A416 Grade 270 strand.
- Hydraulic Restressing: Attach a monostrand center-hole stressing jack to the splice assembly. Apply tension up to \(0.70 f_{pu}\) (\(28.9\text{ kips}\) for \(0.5\text{ in}\) strand), measuring strand elongation to verify friction behavior. Lock wedges into place with positive power seating.
- Grease Sealing & High-Strength Grouting: Inject PTI-approved corrosion inhibitor grease into the splice sleeve. Apply an epoxy bonding agent (ASTM C881 Type V) to substrate concrete and fill the dog-bone cavity with ASTM C1107 Class C non-shrink cementitious grout achieving minimum \(6,000\text{ psi}\) (\(41.4\text{ MPa}\)) compressive strength at 28 days.
05. Safety Protocols: Core Drilling & Saw Cutting in PT Structures
The most frequent cause of catastrophic PT tendon failure is accidental cutting by electrical, plumbing, or HVAC core drillers during tenant fit-outs. To eliminate coring catastrophes:
- Mandatory 3D GPR Scanning: Every slab penetration must undergo bi-directional GPR scanning over a minimum \(3\text{ ft} \times 3\text{ ft}\) square centered on the core location.
- Clearance Zone Requirements: Maintain a minimum clear distance of \(4.0\text{ inches}\) (\(100\text{ mm}\)) from the outer boundary of any detected tendon hyperbola to account for HDPE sheath thickness and drill bit wandering.
- Top/Bottom Marking: Scan both top and bottom surfaces in slabs with double curvature profiles or dense mechanical inserts.
06. Structural Capacity Recalculation Following Loss of Prestress
When one or more tendons cannot be spliced and must be abandoned, structural engineers must re-evaluate the slab system under ACI 318-19 Chapters 8 and 22 to determine whether supplementary carbon fiber reinforced polymer (CFRP) or secondary steel beam support is required:
- Serviceability Limit State (Extreme Fiber Tensile Stress): Check tensile stress in precompressed tensile zone: \(f_t = -\frac{P}{A} + \frac{M_{serv} y}{I_g} \le 6\sqrt{f'_c}\). If lost tendons cause \(f_t\) to exceed \(7.5\sqrt{f'_c}\) or \(12\sqrt{f'_c}\), cracking will occur under service dead plus live load.
- Ultimate Flexural Strength (\(\phi M_n\)): Evaluate nominal flexural strength with reduced effective prestress: \(f_{ps} = f_{se} + 10,000 + \frac{f'_c}{100 \rho_p}\) (for unbonded tendons with span-to-depth ratio \(\le 35\)). Supplemental externally bonded CFRP laminates on the tension face can replace the missing ultimate moment capacity.
- Two-Way Shear Perimeter Re-Verification: Re-calculate nominal punching shear \(v_c\) with reduced effective precompression \(f_{pc}\) in the slab core per ACI 318-19 Section 22.6.5.
Assess Post-Tensioned Slab Integrity
Input structural span dimensions, prestress levels, tendon drape eccentricities, and observed cracks into StructForensic Pro to compute residual capacity and code-compliant retrofit specifications.