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In decades past, post-tension concrete was not a common term for most contractors.
Nowadays, this method is so popular that there’s an entire institute dedicated to advancing the industry—the Post-Tensioning Institute. In fact, some projects can significantly benefit from this prestressing method.
What is this advanced technique, and how does it work? Read on to find out.
Table of Contents
1. What is Post-Tension Concrete?
2. How Does Post-Tensioning in Concrete Work?
3. Post-Tension vs. Pre-Tension
4. Two Types of Post-Tensioned Concrete
5. What Are the Benefits of Post-Tension Concrete?
6. What Are the Disadvantages of Post-Tension Concrete?
7. Applications for Post-Tensioned Concrete
Post-tension concrete is prestressed concrete with added reinforcement and strength through tensioning steel rods.
These rods add internal compression to counteract the external loads placed on them.
In post-tensioning, high-strength steel cables are installed in ducts or sleeves next to the rebar after the concrete has been poured and cured on-site during concrete construction.
After the concrete hardens, the cables are tensioned using hydraulic jacks that pull them tight, applying stress to the concrete.
The cables are anchored at the ends of the concrete slab, and in the case of bonded post-tensioning, grout is injected into the ducts to protect the tendons and bond them to the concrete.
Pre-tensioning involves stretching the steel tendons (usually cables or wires) before the concrete is poured, usually in a factory setting where the tendons can be tensioned across a mold or form.
Pre-tensioning is often seen in precast beams, panels, bridge girders, and slabs.
On the other hand, post-tensioning is typically done on-site, making it more flexible for custom or large cast-in-place projects, such as bridges, parking garages, large commercial buildings, and complex shapes. (Keep reading: Cast-in-Place Concrete: The Process, Benefits, and More)
Pre-tensioning uses a bonded system because no ducts or sheathing are used. However, post-tension systems can be either bonded (with grout) or unbonded (using plastic sheathing and grease to protect tendons).
Both post-tensioning and pre-tensioning increase the strength and durability of concrete by applying pre-stress forces to counteract the tensile stresses that concrete would experience under load.
Post-tension concrete can be applied in two different ways: bonded or unbonded.
In a bonded system, the steel tendons are placed inside ducts — usually made of plastic or metal — before the concrete is poured.
After the concrete is poured and cured, the tendons are tensioned using hydraulic jacks.
Once tensioned, a cement-based slurry is injected into the ducts, filling the space around the post-tensioned cables. This creates a bond between the tendon, grout, and surrounding concrete member.
Bonded post-tension concrete is often used in large-scale infrastructure projects, such as bridges, dams, and large buildings, where long spans and high load capacity are essential. It is ideal for situations requiring greater durability and structural integrity.
In an unbonded system, each tendon is individually coated with grease and encased in a plastic sheathing before being laid into the formwork.
After the concrete is poured and cured, the tendons are tensioned. No grout is applied, allowing the cables to move slightly within their sheathing.
Unbonded post-tension concrete is standard in residential buildings, slabs, and parking garages because it is simpler and faster to construct. It is more suited to structures that experience frequent movement or temperature changes.
Since the concrete construction process is already complex and expensive, why add the step of post-tensioning?
These benefits explain why:
A post-tensioning system allows for thinner slabs and longer spans between supports, reducing the need for additional columns or beams. It increases concrete design flexibility and results in a more open layout, ideal for large commercial or residential spaces.
The tensioning process compresses the concrete, minimizing the formation of cracks caused by shrinkage or environmental stresses. It also reduces deflections in the concrete. Post-tension concrete slabs are more durable and have more aesthetically pleasing surfaces.
By pre-stressing the concrete, post-tension systems can handle higher service loads, making them suitable for bridges, parking structures, and other concrete structures requiring substantial strength.
Related: What’s the Best Concrete Strength Testing Method for Your Project?
Post-tensioning uses less concrete and steel reinforcement than conventional methods. The reduced slab thickness lowers material costs, and the need for fewer columns and support structures can lead to overall construction savings.
Post-tension concrete allows architects more creative freedom to make curved and irregular shapes, so it’s often used in structural design requiring complex geometries.
Even though post-tensioning is advantageous, it has some drawbacks that make it less suitable for certain projects.
The materials, equipment, and specialized labor required for post-tensioning can lead to higher upfront costs. These expenses may outweigh savings for smaller or less complex projects.
Post-tension systems are more complicated to install than traditional reinforced concrete. They require skilled workers and precise execution — including tensioning cables with hydraulic jacks — which increases the risk of mistakes during construction.
Post-tension tendons can be vulnerable to corrosion if not properly sealed or maintained. Over time, repairs or inspections may be necessary, especially if the steel is exposed to moisture or aggressive environments.
Not all construction contractors are equipped or trained to handle post-tension projects, limiting availability in certain regions. Specialized expertise is necessary, which can result in higher labor costs or difficulty finding qualified professionals.
Improper tensioning or installation of the tendons can lead to structural issues or failures. Safety protocols during construction must be followed rigorously. The sudden release of the tensioned cables can cause tragic accidents.
As mentioned, certain projects might not benefit from or need the extra post-tensioning process.
On the other hand, some structures wouldn’t be able to handle the heavy loads they will face without the extra tensile strength post-tensioning brings.
The following are typical applications of post-tensioned concrete:
Post-tensioned concrete allows for thinner slabs and longer spans between columns, reducing the number of support columns and freeing up space for larger floor areas. This leads to more usable interior space and lighter overall structures.
These are commercial buildings like skyscrapers, multi-story residential buildings, and commercial office towers that make money with each square foot. Their main challenge is to build light enough up top with a strong enough foundation.
Like high-rise buildings, bridges benefit from longer spans with fewer support piers.
Post-tension concrete allows bridges to handle heavy loads and resist the environmental stresses they typically face, such as temperature fluctuations and traffic vibrations.
Highway bridges, pedestrian bridges, overpasses, and flyovers are all post-tension concrete structures.
The long spans between supports made possible by post-tensioning reduce the need for columns, which also maximizes parking space and improves the layout of garages. Additionally, post-tensioned concrete’s crack resistance and load-bearing capacity make it durable under heavy use and vehicle weight.
Post-tension concrete is a must for multi-level parking garages and underground parking structures.
Post-tensioned slabs are ideal in regions with expansive soils or high water tables because they are resistant to ground movement. The pre-stressed tendons help minimize cracks caused by soil expansion or contraction, providing a stable foundation.
Examples of how post-tension concrete is used in slab-on-grade foundations include single-family homes, apartment buildings, and light commercial structures in areas with challenging soil conditions.
Post-tensioned concrete can support the large open spaces required for stadiums and arenas without the need for numerous internal columns. The fewer internal columns needed, the more clear sightlines and flexible seating arrangements can be achieved while maintaining structural integrity.
Football stadiums, basketball arenas, and multipurpose sports complexes are all good examples.
Since post-tensioning provides superior crack resistance and load-bearing capacity, it is an ideal choice for circular structures like water tanks, where internal pressure needs to be resisted.
The method also ensures that the tank remains watertight over its lifespan, which is critical for water reservoirs, wastewater treatment plants, and oil storage tanks.
Tunnels require high strength and durability to resist ground pressure and environmental factors like moisture. Post-tensioned concrete provides a robust structural system that can withstand the stresses of tunneling and long-term use.
Subway tunnels, vehicular tunnels, and pedestrian underpasses require post-tension concrete members to be safe for the public to use.
The compressive forces in post-tensioned concrete help resist the high water pressures exerted on dams and other marine structures. The durability and crack resistance are essential in preventing water infiltration and ensuring long-term stability.
Other example applications in this group include seawalls, piers, and coastal defense structures.
Post-tensioned concrete floors can support heavy loads while minimizing floor thickness, making them suitable for industrial applications where large, open floor spaces are required for machinery or storage. It also provides excellent resistance to cracking under heavy usage.
This type of flooring benefits warehouses, manufacturing plants, distribution centers, and large commercial buildings.
Post-tensioned concrete provides the flexibility to create curved or irregular shapes, cantilevered sections, and other architecturally challenging structures. The discovery of increasing concrete strength and flexibility has allowed architects to push the limits of design while ensuring structural safety and efficiency.
Iconic buildings with curved facades, cantilevered structures, and complex roof designs often use post-tension concrete mixed with shotcrete. A few examples include the Sydney Opera House, Burj Khalifa, and Marina Bay Sands in Singapore.
Retaining walls are often subject to high earth pressures and environmental stresses. Post-tensioning improves their strength and stability, helping them resist shifting, settling, and other forces over time.
Retaining walls are often used to build highway embankments, landscaping walls, and retaining structures for large construction projects.
Post-tension concrete is one of the best methods of prestressing concrete for projects requiring higher durability and strength. However, it does require knowledge, skill, and experience to employ this method correctly and safely.
Our team at FMP Construction are experts in post-tension concrete projects and have completed many successful structures using this method.
Contact us today to discuss post-tension concrete for your next project.
