Post-tensioned concrete construction is widely used in Texas commercial buildings, parking garages, and multistory structures because it allows longer spans, thinner slabs, and more efficient use of materials. However, when the high-strength steel tendons inside post-tensioned concrete corrode or fail, the consequences can be severe — including sudden loss of structural capacity, slab deflection, and in extreme cases, progressive collapse. This guide explains how post-tensioning works, what causes failures, and how repairs are performed.
How Post-Tensioned Concrete Works
Post-tensioned (PT) concrete uses high-strength steel cables (tendons) that are tensioned after the concrete has been placed and cured. The tension in these cables compresses the concrete, which counteracts the tensile forces that concrete handles poorly. This compression allows PT slabs and beams to span longer distances with less material than conventional reinforced concrete.
There are two main types of post-tensioning systems:
- Unbonded tendons: Individual steel strands coated with grease and enclosed in a plastic sheath. The strand is free to move within the sheath, and the force is transferred to the concrete only at the anchor points. This is the most common system in building construction in Texas.
- Bonded tendons: Steel strands placed in metal or plastic ducts that are filled with cementitious grout after tensioning. The grout bonds the strand to the surrounding concrete along its entire length. This system is more common in bridges and heavy infrastructure.
What Causes Post-Tensioning Failures
Corrosion
Corrosion is the primary cause of post-tensioning tendon failure. In unbonded systems, the protective grease and plastic sheath can be damaged during construction or deteriorate over time, allowing moisture and chlorides to reach the high-strength steel. Once corrosion begins, the reduced cross-section of the strand can lead to sudden failure under normal loading conditions. In bonded systems, incomplete grouting leaves voids where water can accumulate and cause corrosion.
Anchor Head Failure
The anchor heads at each end of a tendon transfer the cable force to the concrete. Corrosion of anchor components, inadequate concrete strength at the anchorage zone, or construction defects can cause anchor failures. These are particularly common at building edges and expansion joints where water exposure is highest.
Construction Defects
Improper stressing (too much or too little tension), damaged sheaths during concrete placement, and incorrect tendon profiles are construction defects that may not become apparent for years. These defects reduce the long-term performance of the post-tensioning system and can lead to premature failures.
Overloading
When building use changes — such as converting office space to storage, adding heavy equipment, or increasing vehicle loads in a parking garage — the post-tensioning system may be subjected to loads beyond its design capacity. This can cause tendon stress to exceed safe limits, leading to strand breakage.
Signs of Post-Tensioning Problems
Building owners and facility managers should watch for these indicators of potential post-tensioning issues:
- Rust staining on concrete surfaces: Brown or orange stains on the underside of slabs or along beams may indicate corroding tendons or anchor heads.
- Concrete spalling at slab edges: Deterioration near anchor locations is a warning sign of anchor head corrosion. See our guide on understanding concrete spalling for more on this issue.
- Visible tendon ends protruding from concrete: Exposed tendon ends at slab edges indicate failed or deteriorated pocket covers that should be protecting the anchors.
- Slab deflection or sagging: Noticeable deflection in a PT slab may indicate loss of prestress from tendon failures.
- Popping or snapping sounds: Sudden tendon failures can produce audible sounds. Any unexplained noises from a PT structure should be investigated immediately.
- Cracking patterns: Longitudinal cracks along tendon lines or transverse cracks at midspan may indicate loss of prestress force.
Post-Tensioning Repair Methods
Tendon Splice Repair
When an individual tendon has failed at a specific location, a splice repair can restore the tendon force. This involves exposing the broken tendon, installing a mechanical coupler or splice chuck, re-stressing the tendon to the design force, and patching the concrete. Splice repairs are effective for isolated failures but require careful engineering to ensure the repaired tendon performs as designed.
Tendon Replacement
When a tendon is too corroded for splice repair, full replacement may be necessary. For unbonded tendons, the old strand can sometimes be pulled out through the anchor pocket and a new strand installed in the existing sheath. When the sheath is also damaged, a new tendon path may need to be created.
External Post-Tensioning
When internal tendon repair is impractical, external post-tensioning can be installed to restore or increase the structural capacity. External tendons are mounted on the outside of the concrete element using steel brackets and deviators, then stressed to provide the required compression force. This method is particularly useful when multiple tendons have failed in the same area.
CFRP Strengthening
In some cases, CFRP (Carbon Fiber Reinforced Polymer) strengthening can supplement or replace lost post-tensioning capacity. CFRP sheets bonded to the tension face of a slab or beam provide additional flexural capacity without the complexity of tendon repair. This approach is often faster and less disruptive than traditional PT repair methods.
Anchor Head Protection and Repair
Corroded anchor heads are cleaned, treated with corrosion inhibitors, and encapsulated with protective coatings or concrete repair mortar. Pocket covers are replaced to prevent future water intrusion. This preventive repair can extend the life of the entire tendon system.
When to Call a Structural Engineer
Post-tensioning repairs are not DIY or general contractor work. A structural engineer experienced with PT systems should be engaged whenever:
- Any tendon failure is suspected or confirmed
- Rust staining or spalling is observed near tendon or anchor locations
- Building use is changing in a way that increases loads on PT elements
- The structure is more than 20 years old and has never had a PT condition assessment
- Water intrusion is occurring through PT slab cracks or at anchor locations
Texas Structural Concrete provides post-tensioning assessment, repair, and CFRP strengthening services for commercial buildings throughout Texas. Contact us at 661-733-7009 or request a consultation to discuss your post-tensioning concerns.