Post-tensioning systems

In the early fifties, post-tensioning systems for the prestressing of concrete were developed by specialists, such as VSL in Switzerland.

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Even though post-tensioning is a mature technology, it is still a fantastic tool for the design engineer as it enables the active definition of the internal load path in concrete structures by superposing a favorable state for the internal stresses. This enables deformations to be minimized, helps reduce the thickness of members, reduces reinforcement congestion, facilitates segmental construction without the need for wet joints and allows the use of high-strength steel.

Post-tension concrete may be a mythical method for some contractors.

However, some projects can greatly benefit from this prestressing method.

Additionally, concrete contractors should implement post-tensioning in a variety of situations.

This article will cover post-tension concrete&#;s what, when, how, and why.

What Is Post-Tension Concrete?

Post-tensioning is a method of prestressing concrete. Prestressing concrete is when concrete has added compression internally. Doing so counteracts the external loads that will be placed on it.

Post-tensioning adds reinforcement and strength to the concrete with tensioning steel rods.

As the name implies, this prestressing happens on-site after the concrete has fully dried.

Now, many people understandably mistake it for pre-tensioning.

Pre-tensioning is when the steel strands are tensioned before placing them into the concrete. This step usually occurs during precast concrete construction.

History of the Post-Tension Method

A French man, Eugene Freyssinet, often receives credit for being the first to use post-tension concrete in for a marine terminal.

It wasn&#;t until that the construction of the Walnut Lane Bridge in Philadelphia relied on post-tensioning.

These days, this method is so popular there&#;s an entire institute dedicated to advancing the industry &#; The Post-Tensioning Institute.

How Does Post-Tensioning Work?

To understand how post-tensioning works, you need to learn about the behaviors of the two materials involved:

Concrete and steel.

Concrete is strongest when it is under compression. Meanwhile, steel is strongest under tension.

Post-tensioning combines both materials in their strongest states.

The result?

A concrete slab that can resist much higher loads than traditional concrete structures.

Who would have thought reinforcing steel was the best way to create reinforced concrete?

Adding rebar alone can improve the durability of concrete under tensile stress. Yet, post-tensioning improves that while adding strength to the concrete through compression.

How To Install Post-Tensioning

To install a post-tensioning system, you need specific tools while following a specific series of steps.

Equipment Needed

A hydraulic jack is the only piece of working equipment you&#;ll need on your job site to implement post-tensioning.

The size and strength of the concrete members are being prestressed. Post-tensioning involves the elongation of very high-strength steel.

A powerful hydraulic stressing jack will pull on the prestressing steel without causing a malfunction.

Materials Needed

There are several materials involved in post-tensioning structural concrete:

1. Tendons

Post-tension cables &#; also known as tendons &#; are made from a seven-wire braided steel cable. These tendons are very strong and can yield up to 243,000 psi.

There are two different types of tendons.

A bonded tendon uses grout to permanently bond the tendon to the sheathing.

With an unbonded tendon, grease is used, and the tendon is free to move within the sheathing.

2. Tubes or Ducts

To protect the tendons from corrosion from the water in the poured concrete, it must be placed inside a tube.

These can be made from thin sheet metal pipes, plastic ducts, or tubing. The seams should overlap to prevent any seepage from occurring.

3. Anchors

Anchors are vital in applying tensile forces to the tendons while keeping the tensile forces in place.

These devices attach to the tendons and anchor them into concrete on one end while the anchor on the other side attaches to the jack.

Steps to Take

There are a few steps to follow to create a post-tensioned concrete slab.

Here&#;s a look at the process:

1. After the concrete formwork is built but before the concrete is poured, the tubing filled with tendons is placed flush alongside the rebar.
2. The concrete can then be poured, being extra careful not to allow any to get into the tubing. It&#;s also important to keep the tubing in the correct spacing during the pour.
3. The concrete is allowed to cure to about 75% of its drying time, which is about 23 days.
4. The post-tensioning tendons are stressed with the hydraulic jack to 80% of the strand&#;s tensile strength.

For example, a ½-inch 270 strand should be stressed to 33,000 pounds, according to PTI.

Lastly, the steel tendons are anchored into place, the ends are trimmed, and grout is placed into the anchor pocket to secure them.

The Benefits of Using Post-Tension Concrete

You may wonder if all this extra work is worth the effort. Let&#;s discuss the benefits you can reap using this method on your concrete floors.

Design Freedom

Post-tensioning gives architects more design freedom. The method allows for fewer columns and thinner slabs to support the rest of the structure. After all, the concrete slabs are now beast-mode strong.

Post-tensioning also allows architects and engineers to create structures with dynamic designs.

Reduced Floor-to-Floor Height

Post-tensioning also saves floor-to-floor height in commercial buildings. This allows for more floors to rent out without changing the building height. Not only does this save money, but it also provides the opportunity to make more money as well.

This reduced floor-to-floor height also translates into cost savings for:

• Electrical
• Plumbing
• Other construction costs

Think about all the savings that could occur when you cut the height of a building. Elevator shafts are shorter, requiring less material. The building facade will also require less material.

Maintenance costs will be much more efficient, too. Heating and cooling a smaller building is cheaper. Insurance will be cheaper.

In fact, there are many often forgotten savings that come with a reduced floor-to-floor height.

Lower Risk of Concrete Repairs

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Post-tensioning also lowers the chances of cracking due to shrinkage and improves the durability of the concrete. So you&#;ll see far fewer deflections and experience increased service load capability.

Lessen Dead Load and Reinforcement Cost

There&#;s also another way that post-tension concrete saves money:

It reduces the cost of reinforcement by lowering the amount of rebar needed. Post-tension steel tendons are cheaper than rebar. The reduced concrete and reinforcement weight also reduces the dead load on subsequent levels.

Faster Project Completion

Lastly, you&#;ll see a much quicker construction process. This is because post-tensioning increases the strength of the concrete prematurely. So the formwork removal happens earlier.

This benefit also results in the follow-on trades and project completion happening faster.

Pros and Cons of Post-Tension Concrete

Most projects would benefit from the use of post-tensioning. Some would especially benefit. But, of course, there are also some rare occasions when post-tensioning may not be the best course of action.

Let&#;s talk about the pros and cons of this method now.

When Post-Tensioning Is a Pro

There are times when post-tensioning can bolster your concrete structure and is a no-brainer, and times when it&#;s a must.

Below is a list of scenarios that should implement post-tensioning in their concrete construction process to ensure a successful project, as mentioned in this publication from PTI.

Wind and Seismic Resistance

We know that concrete can withstand an extreme load through direct compression. But it&#;s also very susceptible when undergoing lateral forces.

Wind and seismic forces can be catastrophic for some concrete structures.

Using post-tensioned slabs provides the reinforcement necessary to resist these powerful lateral forces.

Skyscrapers

The taller a building gets, the greater the need for lighter construction.

Post-tensioning makes taller skyscrapers possible by allowing for less material needed for each level. This reduces the dead load on each of the lower levels.

Conventional reinforced concrete is much heavier. That said, it also limits the building&#;s height before it&#;s too heavy to support its weight.

Curvilinear Geometries

At times, the vision inside an architect&#;s creative brain won&#;t translate inside an engineer&#;s logical brain.

Curvilinear geometries create stunning structures that were once a severely complicated process. In the past, these required many engineering solutions with high costs and extensive time investment.

The ability to use post-tensioned concrete slabs made this process much more feasible. To this day, it&#;s the most widely accepted method to create these architectural works of art.

Longer Spans

Creating a long span inside a concrete building once called for many pillars, columns, and thicker concrete slabs.

Post-tensioned concrete slabs are stronger and lighter in weight. As a result, they can create longer spans without the need for pillars and columns for support.

Elevated Slabs

Post-tensioning distributes the weight of the concrete to help prevent any sagging in elevated slabs.

When Post-Tensioning Is a Con

Post-tensioning will always increase the strength of a concrete slab. Yet, the methodology behind this innovation can make it disadvantageous on rare occasions.

These include:

Future Renovations

Once post-tensioned slabs are in place, they&#;re permanent. There&#;s no cutting or rearranging these slabs after this point.

Therefore, if you&#;re constructing a commercial building with hopes of a redesign later on, prepare for disappointment.

Of course, you can work around this if you plan for this redesign in the original design, leaving room for future knockouts or openings.

When You Have No Access to a Professional

Post-tensioning concrete is a complicated and precise process that requires skilled labor.

If you don&#;t have professionals with experience using this method, it&#;s better not to use it at all.

For post-tensioning to work, it must meet very precise specifications. The forces applied and the machinery used can also be very dangerous in the hands of an amateur.

Mistakes to Avoid When Post-Tensioning

When done properly, post-tensioning is a fool-proof engineering technique. But some mistakes can occur along the way.

To ensure that you get the most out of your post-tensioning efforts, avoid these common mistakes:

1. Don&#;t Forget To Account for Restraints

In many structures, you&#;ll have restraints to the compression of the concrete. These include walls, columns, and other structures in place.

You&#;ll need to consider these when positioning your post-tension tendons.

2. Not Prioritizing Your Finishing

After you&#;ve pulled the tendons and anchored them, it&#;ll leave behind some strand tails that require cutting.

However, these ends require a bit more work to finish the process. The end caps must be installed, cleaned from debris, and filled with mortar to seal the tendons.

3. Overbalancing Your Dead Loads

While load balancing can extend 100% dead load, pushing this too far can result in a defective slab.

Using post-tensioning in place of proper design will cause overbalancing. It&#;s not like rebar, where you can just add more and more. It requires precise amounts.

4. Relying on 3D Software for Design

Finite element software is helpful in the design process with post-tensioning. Yet, a PT professional must check all designs for correct calculations before construction begins.

As this article pointed out, post-tensioning is an engineering innovation that can and is applicable in various projects to improve the integrity of the construction further.

This method can improve the quality of your project. But it can also save you money, time, and maintenance and provide more freedom in your design process.

Here at FMP, we&#;ve successfully implemented post-tensioning in many of our client&#;s projects, and we&#;ve seen first-hand the benefits of that choice.

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