Post-Tension Concrete [Guide]

In decades past, post-tension concrete was not a common term for most contractors.

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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.

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. 

Contact us to discuss your requirements of post tensioning system services. Our experienced sales team can help you identify the options that best suit your needs.

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.

It&#;s no secret why builders use post-tensioning systems in their construction; these systems are critical to strengthening concrete used in modern large-scale building projects. The same argument applies to post-tension slabs&#;a method used when pouring the slab foundation in commercial or residential construction. This article further explores post-tension slabs and some of the advantages and challenges of their usage in construction projects. 

What are Post-Tension Slabs?

The concrete industry started experimenting with strategies and techniques to strengthen and reinforce concrete in the s. The prestressing concept, now a standard in residential and commercial building construction, involves using a steel cable or tendon to squeeze and compress concrete before the concrete endures a structural load. As the construction industry continued refining its techniques to standardize prestressing, most builders eventually began using post-tensioning techniques for reinforced concrete slab foundations. The Federal Housing Administration officially endorsed the use of post-tensioned slabs in , and the building method is now embraced and co-opted as the preferred method for builders.

Post-tensioned slabs contain a tendon, or steel cable, that runs crisscrossed throughout the center of the slab after the concrete has already hardened. As the poured concrete dries, the post-tensioned cables stretch and tighten, which applies a significant force to the concrete slab. This technique strengthens and compresses the concrete, which helps reduce cracking and structural instability caused by unstable soil conditions in marshes, swamps, and lake areas. The process helps residential homes and commercial buildings achieve greater load-bearing strength. Post-tension slabs are more durable and stable than traditional slabs that lack the reinforcement of steel cables.

Why Use Post-Tension Slabs?

Concrete slabs have inherent structural weaknesses; they are especially susceptible to expansion and contraction due to seasonal fluctuations and changing temperatures. Anyone that has experienced winter knows that thawing temperatures usually lead to potholes, broken water mains, and cracks in the pavement. Concrete slabs are no exception. The swelling and contraction of soils based on their contact with water is another element that adversely affects the integrity of concrete slabs. Poor and unstable soils found in Southwestern states and Mexico lead to issues with soil expansion, making it difficult for builders to lay concrete slabs.

Builders depend on post-tension slabs to reduce the risks of cracking, contraction, and soil expansion and strengthen the foundation&#;s structural integrity.

Post-Tension Slabs: Potential Challenges

Post-tension slabs are stronger than traditional concrete slabs; however, that does not necessarily mean a post-tension slab is without challenges. Read on to learn more about builders&#; challenges when using post-tensioned slabs.

Long-Term Planning 

Post-tensioned slabs are an excellent foundation for a new structure, but builders must plan accordingly for proper drainage. Gutters, downspouts, roof pitches, and balconies redirect rainfall. Builders need to be aware of the location of drainage as it can seep into the foundation and lead to cracks.

Good Things Aren&#;t Cheap & Cheap Things Aren&#;t Good

Laying the foundation of a post-tension slab requires professional workers, quality materials, and expert knowledge to understand the job&#;s complexity. In the building and construction industry, it&#;s essential to research the service provider and contractors you partner with before signing a contract.

Check the Blueprints, Again

Since post-tension slabs require the addition of steel tendons to strengthen and reinforce the concrete, the tendons must precisely align as shown on the blueprints. Improper tendon locations can cause uplifting, as the tendon applies more force than the weight of the concrete. As a result, the concrete might be physically lifting the slab.

The Advantages of Post-Tension Slabs

Aside from producing crack-free tennis courts, smooth parking garages, and a secure foundation for someone&#;s new home, post-tension slabs offer builders an array of advantages compared to traditional concrete slabs. Here are a few of the major benefits.

Cost Savings

Post-tensioned slabs require less concrete than traditional ones, saving builders money. Post-tensioned slabs are an investment that helps homeowners reduce the risks of concrete contraction and cracks, which inevitably costs money to repair. That&#;s why post-tensioned slabs are an initial investment worth it in the long run.

Shed Some Weight

A lighter yet stronger slab of concrete allows builders to design their structure with less material and overall surface space, providing more space to build supporting pillars, walls, columns, and beams. Post-tensioned slabs are also thinner, giving builders more creative freedom when designing floors.

Minimize Risks

No matter how much planning goes into a construction project, things happen, and the idea for builders is to minimize risks. Post-tensioned slabs may help prevent cracks from forming, but concrete is not immune to the elements. The good news is that if cracks form on post-tensioned slabs, they are held together and do not spread as quickly as those on traditional foundation slabs. Therefore, post-tensioned slabs are a more durable solution.

Look to the Experts when it comes to Building Repair Services

Commercial and residential structures require rational planning and maintenance. Whether your building has a traditional concrete slab or a reinforced post-tensioned slab, building owners need to maintain the structure&#;s integrity. That&#;s why across the United States, owners and property managers rely on Valcourt as their trusted partner to protect their investments. Whether it&#;s waterproofing, restoration, or other commercial building services, Valcourt offers over 35 years of experience protecting properties from the elements. Contact Valcourt Building Services today!

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