Print/Save as PDF
Link to Ruiyi
The early stages of the architectural process are a time to make large-scale decisions related to the building&#;s form and structure. During this time, your design team may present different structural system options, discuss benefits and drawbacks, and make recommendations.
One recommendation may be post-tensioned concrete. This method introduces tension to the concrete after curing to strengthen the structure, reduce the likelihood of cracks, and allow for thinner slabs.
While this structural system offers many advantages, it can limit flexibility and complicate future modifications. We recommend considering your long-term goals and the potential for renovations and retrofits.
To help you with your decision, this article will define post-tensioned concrete and outline its pros and cons.
What is Post-Tensioned Concrete?
Post-tensioning is a method of reinforcing concrete by introducing tension after the concrete has achieved a specified strength.
Tension is introduced through high-strength steel stands or bars, typically called tendons. During construction, these tendons are placed in forms before the concrete is poured. The tendons are then tensioned with hydraulic jacks after the concrete cures.
While concrete works best under compression, steel works best under tension. Post-tensioning maximizes the use of each material for efficiency. It can be thought of as a &#;sweet spot&#; that provides the strength of concrete and steel without overusing either one.
Often, post-tensioned concrete works well for parking structures and high-rise commercial or residential developments. We have also seen this method used for tennis courts.
In this situation, the tennis courts sat over an unstable site once occupied by a city dump. Post-tensioned concrete&#;s strength prevented cracking or vibrating that would likely occur with a more traditional concrete system.
Pros of Post-Tensioned Concrete
The most significant benefit of post-tensioned concrete is its strength. The combined performance of concrete and steel allows for thinner slabs and longer spans.
Achieving the same strength with another structural system would require more columns and deeper beams. For this reason, post-tensioned concrete best aligns with structures where longer spans are desired, like parking structures.
Post-tensioning's ability to provide longer spans works well for parking garages.
Post-tensioning also allows designers to decrease a building&#;s floor-to-floor height&#;the vertical distance between the finished floors of two consecutive levels. Other systems require deeper beams and thicker slabs, increasing the distance between floors.
As such, post-tensioning can reduce a building&#;s overall height and lower costs for components like façade treatments. In short, post-tensioned concrete improves material efficiency, minimizes waste, and allows for more open interiors. It can help solve multiple challenges&#;from cost concerns to aesthetics and performance.
Cons of Post-Tensioned Concrete
Although post-tensioned concrete offers many benefits, it can complicate future modifications to the building. Vertical penetrations must be planned during the design process due to the tendons, and after construction, you can only cut through these pre-planned areas. If you cut through the tendons, the structure loses its strength.
With these limitations, post-tensioned concrete works best for buildings with stacked layouts, like residential construction and parking garages. Buildings requiring more flexibility, like healthcare facilities, may run into problems in the future. For this reason, some organizations&#;like the General Services Administration (GSA)&#;have rules against using post-tensioned concrete.
Although post-tensioned concrete can be limiting, other structural systems present a degree of inflexibility. A more traditional concrete system requires more columns, which can also complicate modifications. You and your design team should discuss the potential for future changes to determine the right approach.
If you are looking for more details, kindly visit post tension system solutions.
At the same time, post-tensioned concrete can add complexity to construction. It requires precision and specialized knowledge, potentially limiting your options to a smaller set of contractors.
Is Post-Tensioned Concrete Right for Your Project?
Post-tensioned concrete is a popular structural system for large-scale developments. Depending on your project type and goals, your architect may recommend it in the early stages of the design process.
Combining concrete and steel&#;s strength, post-tensioning can reduce cracking and help achieve longer spans and lower floor-to-floor heights. It can also lower initial costs by improving efficiency and reducing building height.
Despite the benefits, post-tensioned concrete is not ideal for every project. If you desire more flexibility, a more traditional concrete or steel structural system may be a better fit.
For buildings with stacked layouts&#;like parking structures and residential developments&#;post-tensioned concrete may be the right solution. Learn more about these building types by reading about the rules of thumb for parking structure design and considerations for high-rise construction.
For post-tensioned (PT) concrete structures with single strand unbonded tendons, protection of the exposed stressing end of the tendons can become a significant maintenance item, sometimes within a few years after original construction. The anchorages at the stressing end of the tendons are located in recessed pockets, typically at exposed slab edges or beam ends. Encapsulated PT systems include grease caps that fit over the tendon tails at the anchorages to form a tight seal. During new construction, the pockets are typically filled with a trowel-applied non-shrink grout that is nonmetallic and chloride-free, or precast PT plugs are used. The trowel-applied patches are prone to cracking at their perimeter and debonding from the pocket.
The contributing factors to a patch failure: pocket surface contamination and lack of surface preparation; poor mixing and application techniques of patch materials; incomplete filling of pockets; and excessive shrinkage due to inappropriate curing techniques and/or use of inferior shrinkage-prone repair materials. Incomplete filling of pockets can also occur if the tendon tail is not cut sufficiently short in the pocket to properly seat the grease cap onto the tendon. As a result of these failures, the de-bonded patches can loosen and become a potential falling debris hazard. Further, there is an increased risk for tendon corrosion due to moisture leaching into the pocket, particularly if the grease cap was not fully seated or was neglected to be installed at all.
It is very important to seal the PT stressing pockets as soon as possible after the stressing records have been approved to minimize the risk of any possible corrosion. The most effective way to manage deterioration of a structure is to keep up with its maintenance. This includes practices to limit moisture and ensure adequate drainage and waterproofing by protecting expansion joints and seals. To add longevity to the stressing pockets, you could apply a waterproofing coating over and around the stressing pockets after the grout has cured to better protect the system. This would need to be done after the grout has fully cured.
Replacing deteriorated grout or PT plugs and implementing protocols to prevent mechanical damage to tendons prolongs the durability of the structure. Just like your car needs regular maintenance and oil changes, your building will need to have regular inspections and maintenance as well.
New Construction: How to Properly Cut the Tail of a Post-Tension Tendon and Finish the Pocket to Prevent Damage and Corrosion
The Post-Tensioning Institute (PTI) specifies three steps in this process:
1.) Cutting the Tendon Tail
-
- Cut the tendon tails within 1 working day after acceptance of stressing records by the Engineer of Record (EOR). The strand tails are to project beyond the wedges no less than 0.50 in., no more than 0.75 in., or as specified by the PT supplier to accommodate proper seating of the encapsulation caps.
- Ways to cut the cable:
- Oxyacetylene Torch
- Abrasive Cutting Wheel
- Pocket Shear
- Plasma Cutter
- A qualified inspector shall verify that tendon tail cutting was completed within the specified time frame and that tendon tail lengths are within specified tolerances.
2.) Installing the Encapsulation Cap
-
- Install the encapsulation cap as soon as practical but not to exceed 8 hours after cutting off tendon tails. (Do not install grease cap when the strand end is still hot.)
- The encapsulation cap shall be filled with an approved PT coating material. Installation of the encapsulation cap within the prescribed period of time shall be inspected by a qualified inspector.
3.1) Grouting Wedge Pockets
-
- Fill the stressing pocket with nonmetallic, nonshrink grout within 1 day after tendon tail cutting. The patch material shall be a high-quality premixed, cementitious, chloride-free nonmetallic, nonshrink grout, mixed with limited amount of water per manufacturers&#; recommendation for use as repair material. The material shall be submitted to the EOR for review and approval prior to use.
- The patch material shall not be installed until the stressing pocket has been cleaned with a wire brush or similar to ensure the proper bond of the patch material; a bonding agent may be applied.
- The patch material shall be suitably packed into the stressing pocket and struck-off flush with the outer face of the member.
- Installation of the nonmetallic, nonshrink grout within the prescribed period of time shall be inspected by a qualified inspector.
- The inspector shall submit a report to the EOR containing data on the three steps of the finishing operation: 1) cutting the tendon tail; 2) installing the encapsulation cap; and 3) filling the stressing pocket.
3.2) If Using PT Plugs Instead of Grout
-
- Use a reaming tool to clean the inside pocket. Drill out the hole until reamer is flush with the slab edge. Do not use a high-speed drill. Start drill before entering the pocket to prevent drill from jamming.
- Blow out hole to remove all debris.
- Install grease cap.
- Use PT plug checker to ensure the plug will clear the grease cap and sit flush with the slab edge.
- Using a high-strength, non-sag, 2-part epoxy to dispense enough epoxy to cover several holes. (You will use about .5 oz per plug.) Mix together with a brush and apply a liberal amount to the side of the PT plug.
- Insert PT plug into pocket.
- Thoroughly coat the face of PT plug and surrounding area with the epoxy to finish the seal.
Repairing a Post-Tension Stressing Pocket
Once you have identified a pocket that needs repair, typically by a visual inspection or surface sounding (tapping a hammer on the concrete and checking for voids. A void will have a hollower sound to it) you can proceed with the guidelines for repairing the stressing pockets.
- Chip out and remove existing grout and grease cap from the stress pocket. Concrete inside pocket will need to be roughened up in order for the new grout to form a bond to the old grout. You can achieve this by using a Dremel tool or stiff wire brush.
- Clean stress pocket. Stressing pockets must be free from any grease, form release agents, dirt, loose concrete, or any deleterious material. Blow or vacuum out the concrete dust and debris. A bonding agent may be applied for better adhesion.
- Measure strand tail and cut if too long. The tail should be no longer than 3/4&#; from the end of the wedge so the grease cap will fit securely on and not be in the way of a good seal with the grout.
- Install new grease cap to seal the strand end. The grease cap protects the strand from water intrusion.
- Grout stress pocket with the appropriate grout and smooth flush with surface edge ensuring there are no air pockets and voids.
- After grout has fully cured you can apply a waterproofing over the area to improve the durability of the repair.
Repair grouts are designed specifically for filling concrete voids in vertical and overhead applications and are suitable for filling single-strand stressing pockets of PT tendons. It is very important to follow the manufacturer&#;s mixing recommendations; adding too much or too little water will weaken the mixture, and it will not be able to withstand the elements as it was intended.
In conclusion, regular inspections and maintenance are essential to manage the deterioration of a structure. This includes practices to limit moisture and ensure adequate drainage and waterproofing by protecting expansion joints and seals. By following these guidelines, PT stressing pocket problems can be minimized.
If you want to learn more, please visit our website post tension anchor.