At Post Tensioning Solutions we specialize in the repairs of all post-tensioning (PT) systems. We have years of experience, specialty equipment, and products to get the job done right. You can rest assured it will be done by experienced Post-Tensioning Institute (PTI) -certified professionals with the highest standards for quality control and safety. We never replace a tendon with a smaller diameter cable. A smaller cable diminishes the original forces that were required and adds potential safety hazards.
We are here to help you find the best solution for any PT repair you need help with whether it is a broken tendon, failing grout pockets, special openings in an existing PT structure, or a concrete failure.
Testing in Our Shop
We perform tests in our shop prior to going out to do a specialty repair.
This adds to the safety of all personnel and assures success in the field.
Unbonded Greased Monostrand
Unbonded post-tensioning is typically used in new construction for elevated slabs, slabs-on-grade, beams and transfer girders, joists, shear walls, and mat foundations. Light and flexible unbonded monostrand can be easily and rapidly installed. It allows for thinner and longer slabs, uses less concrete and reinforcing steel, and reduces cracking.
The epoxy resin coating gives the strand high corrosion protection and outstanding resistibility against fretting fatigue because the coating requires specialty wedges to bite into it. You must also take into consideration the extra seating loss when stressing epoxy-coated strand.
Button Head Systems
Though no longer used, the button head system revolutionized the construction industry. It was first used in building construction in the mid 50s to control deflection and reduce slab thickness on lift slab construction.
There were two major issues with this system. First, the elongation had to be figured out exactly. Any deviation between the tendon length and the length between edge forms required either a new tendon or moving the edge forms before pouring the concrete. Second, since the shims and the stressing washer ended up on the outside edges of the constructed slab, they had to be covered with a second concrete pour. By the late 1960’s it was replaced by the strand system.
Bonded post-tensioning comprises of tendons from one to multiple strands (multistrand) or bars. For bonded systems, the prestressing steel is encased in a corrugated metal or plastic duct. After the tendon is stressed, cementitious grout is injected into the duct to bond it to the surrounding concrete. The grout creates an alkaline environment which provides corrosion protection for the prestressing steel. Bonded multistrand systems, while used extensively in new construction of bridges and transportation structures, can be and have been successfully applied to commercial building structures. When these multistrand systems are used for large structural elements, such as beams and transfer girders, design advantages include increased span lengths and load carrying capacity, and reduced deflection.
What Causes an Unbonded Monostrand Tendon to Be Damaged or Broken?
The main cause of post-tensioning tendon deterioration is the presence of moisture, which over time can cause tendons to corrode and eventually break. Unlike conventionally reinforced concrete, contaminants such as salt are not required to cause tendon corrosion, although they will increase corrosion rates. There are several ways in which moisture typically enters a PT system:
- Entering the sheathing prior to or during construction (i.e. strands are left unprotected at the plant, during shipping, or on a construction site during precipitation);
- Penetrating through cracks, expansion joints, or leaking cladding systems and into unprotected portions of the PT system during the building’s service life;
- Condensation on cold surfaces, such as glass or window frames, leaking into poorly protected tendon ends;
- Entering the system during repairs (through the use of hydro demolition or exposed PT components that are subjected to precipitation).
Other causes of PT tendon failure include:
- Mechanical Damage: Physical damage to the tendons caused by cutting, drilling, or coring into a concrete member (severing tendons while coring for new drains or installing bolts for framing, window installation, new roof anchors, or any other type of concrete penetration).
- Anchor Slippage: Strand slippage from the anchors, which can occur naturally over time if the tendon was not anchored securely during construction.
- Under Stressing: Tendons not stressed to the levels specified by the design documents may be considered as failures if the reduced stress results in the slab not meeting loading requirements prescribed by applicable Codes.
- Slab Failure: Bursting reinforcement not installed per plans or poor concrete which may have been cause by using too much water in it, rain, or any other weather-related item.
- Bad Pocket Coverage: Water intrusion at the stressing end can result from not cutting the tail of the tendon short enough. PTI specifications say the tail should be from 1/2” – 3/4” from the wedges. If the tail is too long, the grease cap cannot be seated properly, allowing water to seep in around the tendon and cause corrosion. Poor grout mixture will also allow water to intrude into the pocket over time, allowing the tendon to rust and corrode.
How to Prevent Damage to Post-Tension Tendons
Exterior tendons experience more direct contact with aggressive environmental conditions, such as parking garage slabs and roof slabs that are exposed to deicing salts and precipitation.
The source of moisture ingress in interior PT slabs is typically the slab edges, as they may be exposed to exterior moisture such as rain and snow if the building enclosure’s water control layer is not performing adequately. Regular inspection and maintenance of the building enclosure will help maintain a watertight condition and will mitigate tendon failure due to moisture penetration at the slab edge.
Mechanical damage to PT tendons generally results from careless coring or drilling through a concrete slab or other types of concrete penetration (and subsequently through PT tendons). A small investment in structural scanning services prior to creating any openings in a slab will significantly reduce the risk of damaging tendons and the associated repair costs.
Exterior slabs are exposed to more aggressive environments than interior slabs. The direct exposure to heavy precipitation (ex: building and parking garage roof slabs) and salt/water runoff from cars (ex: parking garage slabs) greatly increases the likelihood of PT tendons getting wet during the service life of these structures. According to a study done by PTI: 95% of tendon failures in exterior slabs are the result of moisture entering the system during the structure’s service life. Frequent points of moisture ingress include cracks and construction joints in the concrete slab and at slab edges. To protect the strands from moisture penetration, a proactive approach should be taken. This may be accomplished through slab and expansion joint waterproofing with periodic maintenance.
The majority of tendon failures are due to factors occurring during the service life of the structure, which can be mitigated through preventative maintenance. Through the implementation of a sound capital planning strategy, regular monitoring, a diligent maintenance program, and thorough record-keeping, unbonded PT structures can be managed effectively and affordably while preserving real estate asset value.