Post-Tension Concrete Explained: Techniques, Benefits, ...

Post-tension concrete is a mainstay technique of modern engineering and building projects. These post-tensioning methods are popular because:

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They offer exponential improvements to the strength of concrete structures.

• Post-tensioned structures, while improving strength, can also reduce concrete and steel material costs.
• By placing concrete under compression, post-tension structures are less likely to crack.
• Thinner, concrete post-tensioned, slabs allow builders to accommodate more floors in a high-rise construction, maximizing return on investment.
• By profiling the strands, designers can control how the load is distributed throughout the structure.
• Stronger post-tensioned concrete structures typically require fewer supports, ideal for projects that require longer spans, including bridges.

When building with post-tensioned concrete, projects consume fewer resources without compromising strength or quality. By using fewer materials, project costs fall, profit margins strengthen and the carbon footprint of the project shrinks &#; all important goals of any construction program.

How does post-tension concrete work? A brief overview

Post-tension concrete combines the relative merits of concrete and steel to deliver a final product that is much stronger than either used alone. Concrete offers superb strength when under compression. However, when that slab structure is placed under tension, it can be weakened and is prone to cracking.

Rebar

Adding steel reinforcement (rebar), a passive reinforcement system, to the pour offers some tensile strength improvement &#; but it still has structural capacity limitations. Rebar-reinforced concrete may still be liable to cracking for long, unsupported slabs for instance.

The solution? Post-tension concrete, which is an active reinforcement system. There are two main types of Post Tension Systems; Bonded and Un-bonded. This article focuses primarily on the bonded post-tension method. A post-tension concrete structure will probably contain rebar, typically around the post-tension anchorage areas.

Post-tensioning strands

The main element in the post-tensioned structure is the addition of the post-tensioning strands. These steel strands run through ducts embedded in the structure. The strands are secured at one end of the structure, with an anchorage device, and then passed out of the opposite edge of the structure. The installation process is generally relatively straightforward.

Pouring the concrete

Pouring then continues as normal, with special care taken not to allow concrete to leak inside the tendon duct (doing so risks creating rust and other long-term structural problems).

Stressing the strands

Once the concrete has sufficiently cured, the exposed strands are stressed with specialist tools to create internal forces that place the concrete in compression to counteract external loads. The exposed end of the strand, typically referred to as the &#;LIVE&#; end, is secured using an anchorage device embedded in the structure to maintain the desired tension for the lifespan of the structure. In many cases, after final tensioning, the duct may be filled with a cementitious filler to further protect the strands from corrosion and to bond the tendons to the ducting.

Adding post-tensioning in this way allows for the creation of a far stronger concrete structure with superior wind and seismic resistance qualities.

Where is post-tension concrete used?

Stronger, longer, thinner and more crack-resistant than traditional structures &#; there are many places where post tension concrete is used, including:

• Bridges &#; Long, load-bearing bridge spans are perfect for crossing gaps at height.
• Skyscrapers &#; By using post tension slabs to reduce deflections and cracking, allowing for thinner slabs and larger spans between supporting columns.
• Parking structures &#; Again, large spans between columns are essential for maximizing available space for car parking.
• Industrial floors &#; Fewer floor joints and less cracking make post-tension concrete an extremely durable surface for factories and other industrial structures.
• Sports stadiums &#; Heavy load-bearing capabilities and cantilevered architecture require high-tensile concrete solutions.

Indeed, any application where there must be long, unsupported stretches of load-bearing concrete is a candidate for post-tension slabs.

What equipment do you need for post-tensioning concrete?

The steel strands running through post tension concrete are typically placed under 33,000 pounds-force (lbf) tension. To support this high level of tension, tendons are made from very strong, 7-wire, spiral-wound steel. Delivering a precise level of tension requires specialist tools, specifically stressing jacks and tensioning pumps.

Introducing the Enerpac ZU5 Post-Tensioning Pump

The Enerpac ZU5 Pump series is particularly popular with post-tension slab contractors because of its increased flow rates. The ZU5 family boosts productivity by stressing tendons 30% faster than traditional hydraulic pumps. There are two ZU5 pump variants available according to the specific needs of your project:

The ZURB-P tensioning pump for single-acting mono-strand stressing jacks. This is ideal when working on slab-on-grade projects.

The ZUPB-P tensioning pump for double-acting mono-strand stressing jacks. This is ideal when working with elevated slabs.

Extremely efficient and effective, both pumps improve productivity, allowing your team to stress more tendons each day, reducing labor and associated costs.

You can find the full specifications of Enerpac ZU5 Post Tensioning Pumps here. Single-acting Double-acting

Anything else?

Pouring post-tension concrete in place may need additional equipment, particularly where the project calls for structures to be raised above ground level. If heavy lifting is required, equipment like strand jacks or synchronous hoist systems may be an excellent addition to your on-site assets.

To discuss a project or to obtain a quote please get in touch.

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. 

What is Post-Tension 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.

How Does Post-Tensioning in Concrete Work?

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.

Post-Tension vs. Pre-Tension

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.

Two Types of Post-Tensioned Concrete

Post-tension concrete can be applied in two different ways: bonded or unbonded. 

Bonded

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.

Advantages

• Stronger load distribution: The bonding between the tendon, grout, and concrete creates better stress distribution throughout the structure.
• Durability: The grout acts as protection for the steel strands, making bonded systems more resistant to corrosion.
• Crack control: The bond provides better control over the development and widening of cracks, improving the concrete&#;s overall integrity.

Disadvantages

• More complex installation: The grouting process adds a step requiring careful application to avoid voids or gaps.
• Higher cost: The materials (grout, ducts) and labor needed for grouting increase the project cost.

Applications

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.

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Unbonded

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.

Advantages

• Simpler installation: The absence of the grouting process makes installing unbonded systems faster and less complex.
• Flexibility: Since the tendons can move independently within the concrete, unbonded systems can accommodate more movement. 
• Easier repair: Tendons can be individually replaced or repaired without disrupting the surrounding structure.

Disadvantages

• Less crack control: Since the tendons are not bonded to the concrete, the stress distribution is not as even, which can lead to less effective crack control.
• Vulnerability to corrosion: Without grout to protect the tendons, they rely solely on plastic sheathing and grease for protection. If the sheathing is damaged or penetrated by moisture, corrosion can occur.
• More maintenance: Unbonded systems typically require frequent inspections and maintenance due to the lack of corrosion protection from grouting.

Applications

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.

What Are the Benefits of Post-Tension Concrete?

Since the concrete construction process is already complex and expensive, why add the step of post-tensioning?

These benefits explain why:

Greater Structural Efficiency

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.

Improved Crack Resistance

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.

Higher Load-Bearing Capacity

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?

Material and Cost Savings

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.

Design Flexibility

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.

What Are the Disadvantages of Post-Tension Concrete?

Even though post-tensioning is advantageous, it has some drawbacks that make it less suitable for certain projects. 

Higher Initial Costs

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.

Complex Installation Process

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.

Maintenance Requirements

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.

Limited Contractor Availability

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.

At FMP Construction , we have years of experience applying the post-tension method to projects that would benefit from it, but we are among the few.

Potential Safety Risks

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. 

Applications for Post-Tensioned Concrete

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:

High-Rise Buildings

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. 

Bridges

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. 

Parking 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.

Residential Foundations (Slab-on-Grade)

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.

Sports Stadiums and Arenas

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. 

Water Tanks and Storage Structures

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

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. 

Dams and Marine Structures

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.

Floors in Industrial and Commercial Buildings

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. 

Complex Architectural Designs

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

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.

Recap

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.

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