Post Tension Slabs
Are you looking for a post-tension slab contractor? Or do you need your existing post tension slab maintained? Post-tensioned slabs are a special kind of concrete slab, often used in areas where there is a need for thinner slabs and/or with greater space between columns, or even in a column-free space. This is why they are used in commercial foundations so frequently. Similarly, they also allow for greater building height, and more efficient space usage.
The Airtight Construction team takes a look at how to determine if you need a post-tension slab, what installing one entails, and how to repair it once in place.
What is a Post-Tension Slab?
Post-tension concrete slabs contain high-strength steel tendons, which are high-strength steel wires wound together (like cables), which are placed under high tension inside sleeves after the concrete has set. It’s a type of hybrid slab, and as we mentioned, allows for a thinner, longer unsupported span that still delivers great strength. Concrete has amazing compressive strength. Steel has high tensile strength. So the combination is hard to beat for load-bearing areas.
Also, post-tension slab foundations generally experience reduced cracking and deformation. They’re installed as the concrete is wet, and tensioned through rebars after the concreting. When the concrete is ‘squeezed’ through a heavy load, the compressive strength is ‘activated’. The post-tensioning tendons are also pulled, bringing great tensile strength into play, and creating a stronger, tighter support.
Key Components of a Post-Tensioning System
So the post-tensioning makes a concrete slab that’s tougher than the sum of its parts. What exactly are those parts?
1) The Ducts
The ducts of a post-tensioning system are thin sheet metal or plastic ‘pipes’ through the concrete that are connected by screw couplings and specially sealed. They encase the typical steel strand tendons.
2) The Post-Tensioning Tendons
A post-tensioning system’s reinforcing steel wires are called ‘tendons’ in the industry. Housed inside the ducts, made of pre-stressing steel (generally with high carbon content), and sealed with a protective coating, tendons are the key element that brings extra strength a concrete slab. Typical strand sizes are 0.50 and 0.60 inches. This should yield about 243,000 PSI as the average strand’s tensile strength. Compare that with the 60,000 PSI standard for a piece of rebar, and you can see why so many commercial applications utilize post-tension slabs.
Anchors hold the post tensioning tendons within the concrete, either at the ends or to join two lengths. They help to distribute forces to the concrete after the stressing process is complete.
How Post-Tensioned Slabs are Made
Those three components seem deceptively simple, but together they make a powerful post-tensioned system that can support anything from a home’s foundation in adverse soil conditions to parking lots and commercial buildings with a building height or floor height far greater than average. How are they installed? Let’s take a look.
This is not a simple job. You will need an experienced post-tensioning contractor who knows exactly what they are doing. The labor must be skilled, and the personnel involved correctly certified in tensioning to do a proper of concrete slab tensioning. Steel strands under tension and high tensile forces can be dangerous if not handled correctly.
Installation of the Concrete Slab
As the conventional rebar is installed, so are the ducts and steel strands. The site engineer will decide the pattern. They’re installed in the ducts to prevent the water in the concrete from doing damage.
One end of the post-tension cable is anchored, while the other is left open and held with a plastic pocket former. If necessary, joining is used. Concrete is then poured. It will be left to dry to a point where it has achieved about 75% of its overall strength. This is usually 20-23 days in typical Northern California weather.
Applying the Proper Post-Tensioning Force
When the foundation is ready, the post tension cables will be stressed by using stressing jacks. Typically they are post-tensioned to 80% of the strand’s tensile strength. When using typical ½-inch grade 270 strand, that’s about 33,000 pounds of force. The steel will stretch slightly as tensioning occurs, while the concrete compresses.
Once the tensioning is correct, the pre-stressing steel will be anchored on the other side. This gives a permanent mechanical connection that keeps the tension and compression in play. Any superfluous cables will be trimmed, and the anchor pocket grouted closed.
The Advantages of Post-Tensioned Slabs
While it sounds like an arduous process, post-tensioned slabs can be an immensely beneficial addition to a building.
What makes post-tension slabs so useful?
1) Architecture and Design
A post-tension slab allows for a thinner slab and the absence of columns, as mentioned above. This gives greater freedom to the designer, and often allows for a better look without sacrificing safety.
2) Commercial Applications
As a side effect of that thinner slab, you can get greater concrete floor height, which can also allow for additional floors (and extra commercial space) within the same building height. They are especially useful because the weight is distributed over the whole slab, not just the piers and footings. Similarly, they are often used in parking structures because of the reduced need for columns, allowing better space usage.
3) Lower ‘Dead Load’
Another benefit of these lighter slabs is the reduction in overall material weight without loss of reinforcement. A post-tension slab can be up to 30% thinner than a conventional concrete slab, thus reducing a structure’s overall weight.
4) Enhanced Durability
These slabs show improved durability, reduced cracking, and typically take less effort and cost to maintain. Deflection can also be controlled, as the post-tensioning can be balanced as needed immediately after stressing.
5) Use in Non-Ideal Conditions
This style of foundation can also help you make the most of adverse soil conditions, wetter soils, or even help you make better use of below-grade areas of the property. Expansive soils, or soils with bad load-bearing capacity, are typical candidates.
As a result, they create sturdy, reliable construction elements that are in ever greater demand globally.
Do I Have an Existing Post-Tension Slab?
If you are doing a new installation of a post-tensioned concrete slab, that’s one thing. But how would you know if one already exists in your building?
Post-tension slabs are supposed to be clearly identified, as it is highly dangerous to just cut into one. Mostly, this is done through stamping into the concrete, including the warning ‘do not cut or core’.
Some older buildings, however, used signs. These can, of course, be removed or lost. So it’s imperative to check for other signs of such slabs. The most typical visual clue is the area where the cables were tensioned and then patched, which leaves evidence of small, circular areas that have been filled in. You can also consult your building’s plans, if available.
It’s important to establish whether a slab is post-tensioned before you start work on it. The steel cables are under such pressure that their tensile forces can, quite literally, rip out of the concrete and injure someone if cut. Or the slab may fail once its post-tensioning strength is lost.
Can Post-Tension Slabs be Repaired?
Yes, post-tensioning repair is possible! You simply need a reliable contractor like ATC with the experience to do it correctly. While post-tension slabs are less prone to cracks, they can need a helping hand over time in the form of periodic maintenance.
Whether you need assistance with existing post-tension slab maintenance, or need a new installation, Airtight Construction can help. We have had decades of experience in installing and repairing post-tension work throughout the San Francisco Bay Area. If you need to know more about post-tension slabs, have any questions, or want to start an installation or repair, feel free to get in touch today.