Maintenance and Repairs

November 26, 2024 Published by Toronto and Area Chapter - By Sally Thompson, Michael New

Post-Tensioning

From the Fall 2024 issue of CCI Toronto Condovoice Magazine.

What is it and how might it impact your building?

Occasionally, we encounter a condominium corporation who are not aware that their building structural system is posttensioned concrete. This is a problem, because post-tensioned buildings need some extra care to manage risk. This article explores post-tensioning and what condominium boards and managers need to know.

What is Post-Tensioning?

Most concrete-framed high-rise buildings in Ontario are constructed using concrete reinforced by steel rebar (see Figure 1, below). In a small percentage of buildings, high-strength steel posttensioning cables are used as the primary reinforcement, supplemented by rebar. Post-tensioned cables consist of 7 individual wires, twisted together to form a single cable. The cables are placed in the formwork before pouring concrete. After the concrete cures, these cables are stretched to induce tension using a high-pressure jack. The stress in the cables is transferred to anchors, primarily at the cables’ ends, which lock the cables in place. This creates compression in the slab and enhances its structural properties. Post-tensioning allows for thinner slabs, longer spans, and a lighter overall structure. However, the tension needs to be maintained throughout the building lifespan to preserve the structural integrity of the building.

Historically, the cables were coated ingrease and wrapped in paper, but modern buildings use a plastic sheathing over the cables with grease inside (See Figure 1, below).

Figure 1: Conventional Rebar (above) and Post-Tensioning Cables (below)

Figure 2: Cables replacement underway in a parking garage

What are the Long-term Risks?

If the cables rust, they can break and release their tension. Cable rusting can be caused by water which accesses the cables either from the time of construction, causing failure many years, even decades after construction, or by water which accesses the cables during the life of the building, perhaps due to leakage.

Conventionally-reinforced structures typically only suffer rebar corrosion if salt and water are present. Otherwise, the rebar is protected by a “passive-layer” which forms on the steel when it is placed in concrete. Unbonded post-tensioned structures only require water to rust, because the cables sit inside a sheath, rather than being cast into the concrete directly. They have no passive layer and are therefore subject to atmospheric corrosion. Of course, while salt is not necessary for the corrosion of post-tensioning cables, if it is present, the corrosion will be much worse.

When a conventionally-reinforced structure deteriorates, the deterioration is slow, with a gradual transfer of load from a deteriorating section of the structure to surrounding areas. The concrete around the rusting rebar is typically pushed away from the rebar by the ever-increasing volume of the rust, resulting in characteristic delamination of the concrete, and lots of warning of an impending problem. When a posttensioning cable fails, there is no visible warning because the concrete around the rusting cable is not impacted by the corrosion. Failure results in a very rapid transfer of load onto adjacent cables. If these adjacent cables are also corroded, then the sudden imposition of load can also cause them to subsequently fail. In this way, post-tensioned structures are prone to more sudden failure.

Is my Building Post-Tensioned?

It is very diffcult to visually identify a post-tensioned building. Most are identified via review of the structural drawings. Without structural drawings, there are a few tell-tale signs that may indicate a higher probability that a structure is post-tensioned. These include:

• Slab Thickness: Slabs in conventionally reinforced structures are generally 200mm thick. In post-tensioned structures, slabs can be thinner than this, down to about 150mm thick.
• Column to column spans: Most conventionally reinforced buildings have column-to-column spans of about 6m. Post-tensioned buildings often have spans of 6.5m or more.
• Slab Edge Joints: Some post-tensioned buildings have visible joints around the perimeter of garage slabs, between the slab and the wall, with the slab resting on a ledge built into the foundation wall. Conventionally reinforced slabs will generally be continuous with the foundation wall. Absence of these joints is not conclusive though, because some post-tensioned slabs run directly through the walls (if the tendons were stressed from the outside face of the foundation wall). The presence of joints is also not conclusive, because some buildings isolate the slabs from the walls to avoid vibration transmission (like next to a subway).
• Builder: In Toronto, many, but not all, Tridel-built condominiums constructed in the 1980s were post-tensioned.
• Grout plugs: Small circular openings, about 75mm in diameter that were used to access the cables for stressing may be visible at some slab edges. After the cables are stressed, they are filled with grout. These grout plugs can sometimes be seen at slab edges at balconies, in stairwells, or behind wall cladding (such as behind brick).

If you are not sure if your building is post-tensioned, then it is well worthwhile to make the effort to obtain a set of the original structural drawings and have the building engineer review them. Structural drawings are often registered at the registry office or may be retained by the municipality.

What Testing is Required?

If you have a post-tensioned building, you need to have a specialist complete periodic inspection of the cable system. These inspections consist of making openings into the slab to expose sample cables and evaluate their condition. There will be a visual evaluation for rust and water within the sheath. The engineer will also complete a penetration test, which is done by hammering a heavy flat-head screwdriver into the cable, between wires. For a properly stressed cable, it would be nearly impossible to get the screwdriver to penetrate. For a de-stressed wire or cable, penetration is easier. An experienced tester can also identify cases of inadequate stress levels.

To access the cables, concrete is chipped out, usually from the underside of the slab. After testing, the opening should be covered by a steel plate with insulation in the cavity, rather than replacing the concrete, so that the same cables can be accessed again in the future at lower cost. The steel plate needs to be robust to protect from eruption should the tested cable fail in the future, and the insulation and steel are key to maintaining the fi re protection for the cables. At each cycle of testing, the engineer will revisit the openings made during prior investigations and will expand the sample to include additional locations. Over time, this allows the sample size to become more robust, which is good, because the risk of deterioration also increases with age.

Steel plates pose no problem in parking garages but are not ideal on the ceilings of suites. Regardless, it is advised to keep the cables accessible for future review.

What Costs Should We Plan For?

Periodic testing of the post-tensioning cables in your building should be planned in your reserve fund study. Typically, cables located in a parking garage will be tested every five years starting after about ten years. Cables in the tower floors, if any, will probably be tested every 10 years, starting after about 20 years. If problems are observed during testing, then the testing frequency may be increased to allow closer supervision of the structure.

If serious problems arise, then repairs may become necessary and will need to be planned in your reserve fund study. Most commonly, problems are worse in parking garage slabs due to water and salt exposure. Only on rare occasions will we find failed cables in the occupied floors of a building.

Replacement costs of failed cables can vary greatly depending on access and the impact on the surrounding cable system. However, one cable can seldom be replaced in isolation because the cables tend to run in bundles of 2 or more cables. It is dangerous to work adjacent to stressed cables. Therefore, to replace one failed cable, the contractor often needs to de-stress the remaining cables in the bundle or even closely intersecting cables. De-stressing of these cables cannot be completed without shoring the entire section of the slab. Figure 2 shows an example of cables being replaced.

What Else do I Need to Know about Post-Tensioning?

It’s crucial to avoid drilling or coring a post-tensioned slab without precise knowledge of the cable locations. Accidentally cutting or nicking a cable necessitates a structural analysis to assess if the structure remains sufficiently strong or if repair is necessary. As discussed, replacing a broken cable usually requires de-stressing and replacing several nearby cables leading to significant shoring and a very costly repair.

The condominium board in a post-tensioned building should periodically remind owners of this risk and mandate an x-ray or groundpenetrating radar (GPR) scan before any significant drilling/coring activities. The scan should be overseen by an experienced engineer. This scanning can be quite expensive and disruptive, particularly x-rays, which require evacuation of multiple floors. Some nominal drilling may be permitted, but the corporation will need to have a structural engineer define the maximum depth that can be drilled without risking damaging the cables.

Key Takeaways:

• Post-tensioned structures need to be monitored and maintained very differently than typical conventional-reinforced structures.
• These structures are more susceptible to localized damage related to leakage, drilling, coring, etc.
• Procedures should be in place to ensure that coring or drilling into the post-tensioned structural elements is done safely to prevent costly damage.
• Failures can be sudden, so routine monitoring is necessary.
• Periodic monitoring will let you allocate appropriate budgets in your reserve fund study.
• It is important to have a professional engineer with posttensioning repair qualifications from the Post-Tensioning Institute oversee the monitoring and repair of these special buildings.

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Sally Thompson, P.Eng. LCCI, Managing Principal, Synergy Partners

Michael New, P.Eng., Senior Project Manager, Synergy Partners