When a post-tension cable breaks, the steel strand releases thousands of pounds of stored energy almost instantly. The cable can snap back or whip through the surrounding concrete, and the slab or beam it was supporting loses a portion of its compressive strength. Depending on how many cables are affected and where the break occurs, the consequences range from localized cracking to, in rare cases, progressive structural collapse.
What Happens Physically
Post-tension cables are steel strands stretched to extremely high tension (typically 33,000 pounds of force per strand or more) and locked into place with anchors at each end of a concrete slab, beam, or bridge. That tension is what gives the concrete its load-carrying ability. When a cable breaks, all of that stored energy releases in a fraction of a second.
The broken strand can recoil violently inside its plastic sheathing or duct, sometimes punching through the concrete near the anchor point. In slabs, this often blows out a cone-shaped chunk of concrete at the anchor end, a failure known as a “blowout.” The surrounding concrete, now carrying the same load with less internal support, may crack, sag, or deflect visibly. In a slab with many cables, losing one strand may cause only minor redistribution of forces. Losing several can compromise the structural integrity of the entire member.
Why Cables Fail
Corrosion is the leading cause of post-tension cable failure in existing structures. The steel strands are protected by grout or grease inside a plastic or metal duct, but that protection can break down over time. Chloride contamination is the most common culprit. Chlorides from deicing salts, coastal air, or even defective grout materials attack the thin oxide film that shields the steel from rusting.
Federal Highway Administration research found that corrosion begins when chloride levels in the surrounding grout reach about 0.4 percent by weight of cement. At 0.8 percent, corrosion intensifies significantly, with deeper and more numerous pits forming on the steel surface. For context, the American Concrete Institute sets limits at just 0.06 to 0.08 percent for prestressed concrete. In one notable case in Corpus Christi, Texas, investigators found chloride levels as high as 5.27 percent in the grout of a post-tensioned bridge cap, roughly 65 times the allowed limit.
The situation worsens if the grout has voids, cracks, or has carbonated (lost its alkalinity). In carbonated grout, chloride levels as low as 0.04 percent can trigger active corrosion. Water infiltration through cracks in the concrete or failed seals at the anchors accelerates the process. Other causes of cable failure include construction damage (a cable nicked during installation), overloading, and fatigue from repeated stress cycles in bridges.
The Danger to People
A breaking post-tension cable is genuinely dangerous. The energy release can turn the strand, anchor hardware, or concrete fragments into high-speed projectiles. OSHA has documented fatalities from post-tensioning equipment failures where workers were struck and killed despite wearing hard hats. The forces involved are simply too great for standard personal protective equipment to absorb.
The risk is highest during active tensioning on construction sites, when workers are near the jacking equipment and the strand is at peak stress. But cables can also fail years or decades later due to corrosion, and those failures can be just as violent. In parking garages, residential buildings, and bridges, a blowout at an anchor can send concrete chunks across a room or onto traffic below. Anyone standing near the anchor end of a failing cable is in the most dangerous position.
Signs a Cable May Be Failing
Broken or deteriorating post-tension cables often leave visible clues on the concrete surface before a dramatic failure occurs. The most common signs include:
- Rust stains or orange streaks on the concrete surface, often running along the path of a cable or appearing near anchor pockets
- Grease or oil seeping through cracks or at anchor locations, indicating the cable’s protective sheathing has been breached
- Concrete spalling (flaking or popping off in chunks) along the cable path or at the slab edge near anchors
- Sagging or visible deflection in a slab or beam that wasn’t there before, suggesting one or more cables have lost tension
- Cracking patterns that follow the layout of the tendons, particularly longitudinal cracks running parallel to the cable
- Exposed or protruding anchor hardware at slab edges, sometimes with corrosion visible on the metal
In parking garages and coastal buildings, these signs tend to appear first at the lowest levels, where water exposure and chloride concentration are highest. If you notice any of these in a post-tensioned structure, it warrants a professional engineering inspection. By the time rust stains are visible on the surface, the corrosion inside may already be advanced.
How One Break Can Spread
The biggest structural concern with a cable break is whether it triggers a chain reaction. When one cable fails, the load it was carrying transfers to the remaining cables and the concrete itself. If those elements are already near their capacity, or if corrosion has weakened neighboring cables too, the added stress can cause additional failures. This cascading process is called progressive collapse.
Research on prestressed structures shows that the location of a cable failure matters as much as the number of cables lost. A cable breaking near the middle of a span produces different failure patterns than one breaking near a support. Mid-span failures tend to create localized buckling and hinge points in the structure, while failures near supports can actually be easier for the structure to redistribute, depending on the design. Studies have found that structures with redundant cable arrangements can improve their resistance to progressive collapse by 44 percent for mid-span failures and over 120 percent for failures near supports.
In well-designed structures with many cables, losing a single strand rarely causes collapse. The concrete and remaining cables absorb the redistributed load with some cracking and deflection but no catastrophic failure. The real danger is in older structures where multiple cables have corroded simultaneously, or in designs with minimal redundancy where every cable is critical.
How Broken Cables Are Repaired
Replacing a post-tension cable inside an existing concrete structure is difficult and often impractical, since the cables are embedded within the concrete. Instead, engineers typically restore the lost capacity using external post-tensioning. This involves attaching new steel strands or bars to the outside of the structural member and tensioning them with jacks to apply the same compressive force the original cable provided.
External post-tensioning has several practical advantages. According to the Post-Tensioning Institute, it can restore a deteriorated structure to its original strength or even exceed it. The new tendons add minimal weight and take up little headroom, which matters in parking garages and buildings with tight clearances. The work can usually be done without relocating utilities or shutting down the entire structure.
One key difference between external post-tensioning and other repair methods (like bonding steel plates or wrapping with fiber-reinforced polymer) is that external tendons actively push on the structure. They apply real compressive force the moment they’re tensioned. Other reinforcement methods are passive, meaning they only engage when the structure deflects further under load. For a member that has already lost significant capacity from cable breaks, the active loading of external post-tensioning is often the only way to fully restore performance.
In less severe cases where cables have lost some tension but haven’t fully broken, engineers may re-grout voids, seal cracks, and apply corrosion inhibitors to slow further deterioration. The structure is then monitored with periodic inspections and sometimes with embedded sensors that track changes in cable tension over time.