The Bay Area’s Post-Tension Repair Specialists
Maintain the Structural Integrity of Your Building With Post-Tension Repair Services
What Are Post-Tension Slabs?
Post-tension slabs are a special kind of concrete slab, often used on commercial construction projects in areas where there is a need for:
- Thinner slabs
- More space between columns
- A column-free space
Similarly, they also allow for greater building height and more efficient space usage.
Post-tension concrete slabs contain high-strength steel cables. These post-tension cables in concrete are called tendons. They’re installed before the concrete is poured and then tensioned as it cures. When the concrete is squeezed by the tension load, the tensile strength is increased.
While concrete has incredible compressive strength, steel has high tensile strength, making the combination optimal for load-bearing areas and allowing for thinner and longer unsupported spans of concrete. As a result, post-tension slab foundations generally experience less cracking and deformation.
If the post-tension slab construction process sounds complicated, the repair process can be even more so, and most contractors in the Bay Area do not have the experience or qualifications to repair post-tension repairs. Airtight Construction has become the contractor of choice when a building in the area requires post-tension repairs.
Do You Have an Existing Post-Tension Slab?
It’s important to establish whether a slab is post-tensioned before you start work on it. The steel cables are under enough pressure that the tensile forces can rip the steel tendons out of the concrete and injure someone if cut. The slab may fail once its post-tensioning strength is lost, leading to significant structural issues. If you’re installing a new post-tensioned concrete slab, you already know what you’re working with, but how would you know if one already exists in your building?
Post-tension slabs are supposed to be clearly identified as they are dangerous to cut into. Often, a warning is stamped into the concrete, often with the instructions, “Do not cut or core.” If you see this on the concrete, you know it is a post-tension slab.
Some older buildings, however, use signs. Unfortunately, these signs can be damaged, lost, or removed over time, making it imperative to check for other signs of post-tension slabs. The most common visual clue is the presence of small, circular patches in the concrete where the holes for the cables have been filled after curing. If available, you can also consult your building’s plans.
Can Post-Tension Slabs Be Repaired?
Yes, post-tensioning repairs are possible. However, not just any concrete contractor can handle them as they require specialized qualifications and expertise.
Turn to a reliable contractor like ATC with the experience to do it correctly. While post-tension slabs are less prone to cracks, they may need repairs or periodic maintenance over time.
Do You Need Post-Tension Repairs at Your Building?
Before repairing load-bearing concrete structures at your building, it’s critical to identify whether post-tension slabs were used during construction. If you don’t know, Airtight Construction can help.
Key Components of a Post-Tensioning System
The post-tensioning process results in a concrete slab that’s tougher than the sum of its parts, but what exactly are those parts?
The Ducts
The ducts of a post-tensioning system are thin sheet metal or plastic pipes running through the concrete that are connected by screw couplings and specially sealed. They encase the steel tendons.
The Post-Tensioning Tendons
A post-tensioning system’s reinforcing steel wires are called “tendons” in the industry. Housed inside the ducts, made of prestressed steel (generally with high carbon content), and sealed with a protective coating, tendons are the key element that brings extra strength to a concrete slab. Typical strand sizes are 0.50 and 0.60 inches. They yield about 243,000 PSI as the average strand’s tensile strength. Compare that to the 60,000 PSI standard for a piece of rebar, and you can see why so many commercial applications utilize post-tension slabs on larger projects.
Anchors
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 Installed
Those three components may sound deceptively simple, but together they form 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 floor height far greater than average. But how are they installed?
Just as the conventional rebar is installed in reinforced concrete, so are the ducts and steel strands. The site engineer decides on the pattern, and they’re installed in the ducts to prevent moisture in the concrete from damaging them.
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 is then left to dry to a point where it has achieved about 75 percent of its overall strength, which typically takes 20 to 23 days in typical Northern California weather.
Applying the Proper Post-Tensioning Force
When the foundation is ready, the post tension cables are stressed by using stressing jacks. Typically they are post-tensioned to 80% of the strand’s tensile strength. When using a typical 1/2-inch grade 270 strand, that creates about 33,000 pounds of force. The steel stretches slightly as tensioning occurs while the concrete compresses.
Once the tensioning is complete, the prestressed steel is anchored on the other side. This gives a permanent mechanical connection that keeps the tension and compression in play. Any extra cables are trimmed and the anchor pocket is sealed closed.
The Advantages of Post-Tensioned Slabs
While it sounds like a complex process, post-tensioned slabs can be an immensely beneficial option for buildings when it comes to:
Architecture and Design
As mentioned above, post-tension slabs allow for a thinner slab and the absence of columns. This gives greater freedom to the designer, allowing for more open spaces without sacrificing safety.
Commercial Applications
As a side effect of a thinner slab, you can increase floor height, allowing for additional levels and extra commercial space with the same building height. Post-tension slabs are especially useful because the weight is distributed over the whole slab, not just the piers and footings. This is why they are often used in parking structures as the reduced need for columns allows for better space usage.
Lower Dead Load
Another benefit of these lighter slabs is the reduction in overall material weight without the loss of reinforcement. A post-tension slab can be up to 30 percent thinner than a conventional concrete slab, thus reducing a structure’s overall weight.
Enhanced Reliability
Post-tension slabs offer improved durability, reduce cracking, and generally take less effort and cost to maintain over time. Deflection can also be controlled, as the post-tensioning can be balanced as needed immediately after stressing.
Use in Non-Ideal Conditions
Post-tension slabs can also help you make the most of adverse soil conditions, wetter soils, or even help you make better use of below-grade areas on your property. Expansive soils and soils with poor load-bearing capacity often require post-tension slabs.
