A post tension slab is built to last. Each unbonded tendon is a steel strand coated in a corrosion-inhibiting PT coating and sealed in a plastic sheath, and in good conditions that system protects the steel for decades. The trouble is that field conditions are rarely good conditions.
Chlorides do not announce themselves. They move into concrete with water, reach the steel, and start corrosion long before a crack appears. In an unbonded tendon, the damage can stay hidden inside the sheath until the strand has lost a large part of its section.
This article explains how chlorides reach a tendon, how to assess the risk on an existing slab, which repairs exist, and how to design and build against it from the start.
How Chlorides Attack a Post Tension Slab
Chloride-induced corrosion is not a surface problem. Chlorides from deicing salts, seawater spray or contaminated materials are carried into the concrete's pore network by water, especially under wet-dry cycling, and then move deeper by diffusion. How fast depends on the concrete's water-cementitious ratio, its curing and its cracks.
Prestressing steel is less tolerant of chlorides than ordinary rebar. That is reflected in the mix limits: ACI 318-19 caps water-soluble chloride in prestressed concrete at 0.06 percent by mass of cementitious materials, far below what it allows in most nonprestressed concrete, as the NRMCA guide to ACI 318 exposure classes summarizes.
Why the post tension slab sheath does not always protect
For an unbonded tendon, the sheath and coating are the barrier. ACI 423.4R-14, ACI's report on corrosion of unbonded single-strand tendons, makes the key point: once water gets inside the sheath, the coating is rendered ineffective, and repairing the surrounding concrete does not necessarily protect the steel inside. Common breaches:
- Sheath torn during placement, vibration or foot traffic
- Damage where the tendon bears on chairs or changes direction
- Gaps in coating along the strand
- Poorly finished anchorages: missing caps, poorly filled pockets
Today, PTI Technical Note 18 (2014) requires all unbonded single-strand tendons in ACI 318 structures to be encapsulated, anchors included, regardless of exposure. Older slabs may not have that protection. How a hidden failure shows itself is covered in the visible signs of a post-tension cable failure.
Assessing Chloride Risk on an Existing Post Tension Slab
The question for any post tension slab is whether chlorides at tendon depth are high enough to have started, or to soon start, corrosion. It takes sampling, targeted testing and direct inspection together.
Step 1: Chloride profile from cores or powder samples
Samples taken at increasing depths and tested for acid-soluble chloride under ASTM C1152 give the most direct evidence. Sample where water and chlorides concentrate: slab edges, joints and around stressing pockets. Plotting chloride against depth shows how close the front is to the tendons. If it is approaching tendon depth, intervention is justified even without visible cracking.
Step 2: Half-cell potential mapping
ASTM C876 half-cell testing indicates the probability of active corrosion of uncoated reinforcing steel. It screens large areas quickly. On unbonded tendons it is indirect, because the strand sits in its own sheath, so treat it as a way to target samples rather than a diagnosis.
Step 3: Anchor pockets and exploratory openings
Stressing pockets are the most accessible part of a post tension slab. Opened by a qualified PT repair contractor, they show the cap, coating, sheath and strand condition directly. Where results point to a problem along the span, the tendon is exposed at that location. The full inspection sequence is in our pillar, the dangers of corroded tendons: post tension slab inspection and preservation.
What worked on site
- Chloride profiling at 0.5 in increments gave a clear picture of penetration depth.
- Anchor pocket inspection caught active corrosion where no surface cracking was visible.
- Half-cell mapping screened large slab areas efficiently and reduced the number of samples.
- Photographing sheath breaches at deviation points showed a correlation with the deepest chloride readings.
What to watch
- Relying on surface cracking alone: tendons with 20 percent section loss showed no cracks.
- Single-depth samples misrepresented the profile; gradient sampling is essential.
- Half-cell mapping alone was not diagnostic; it needed chloride data to confirm.
- GPR located tendons but said nothing about their corrosion state.
Post Tension Slab Repair Options
The right repair depends on how far corrosion has gone, how accessible the tendon is, and whether the slab can carry its loads while the work is done. ACI 423.4R-14 lists the main options: splicing, strand replacement, tendon replacement, external post-tensioning, external reinforcement, and acoustic monitoring to track further wire breaks.
Post tension slab strand or tendon replacement
Where the strand has lost significant section, replacement is the most reliable repair: remove the damaged strand, install new strand of the same specification, re-anchor, stress, and check elongation against the calculated value. ACI 318-19 Section 26.10.2 requires any difference above 7 percent between gauge force and elongation-based force to be investigated and corrected.
Anchorage repair
Where corrosion is limited to the anchor zone and the strand beyond it is sound, replacing corroded hardware, restoring coating at the tail, installing a new cap and refilling the pocket with nonshrink grout may be enough. That decision belongs to an engineer, based on what the inspection found.
Repair cost depends on access, the number of tendons, finishes to remove and replace, and engineering. The Post-Tensioning Institute and ICRI publish a joint guide for the evaluation and repair of unbonded post-tensioned structures, with a 2024 edition available from the Post-Tensioning Institute. Ask any repair contractor for PTI certification in unbonded PT repair.
Designing a Post Tension Slab Against Chlorides
- Choose the right exposure class: where deicing salts or seawater can reach the slab, ACI 318-19 exposure class C2 applies: a maximum water-cementitious ratio of 0.40, a minimum strength of 5,000 psi, and increased cover, per the NRMCA guide. Decide this at design, not after the first winter.
- Lower permeability: supplementary cementitious materials such as fly ash and slag cement reduce chloride penetration.
- Protect the sheath: inspect tendons at delivery and before the pour, repair any sheath damage as the tendon specification requires, and keep vibrators off the tendons.
- Finish the anchors properly: caps on every anchor and pockets filled with nonshrink grout.
- Cure and seal: good curing lowers early permeability; joints and cracks need sealants suited to the exposure, maintained over the slab's life.
Corrosion also changes how a slab performs under extreme loads, see post tension slab earthquake resistance vs conventional RC. And every new hole is a new path for water: read is it safe to drill into a post tension slab before any coring.
This article describes mechanisms and methods; it does not assess any specific slab. The governing code editions and an on-site evaluation by a qualified engineer take precedence. Our sourcing rules are in our editorial policy. If you have chloride test results, an inspection report or a repair proposal and want a second opinion, a technical consultation on tendon corrosion and chloride exposure reviews it with you.
