Concrete has been a building material since before Roman times. It worked well enough but it often cracked and broke under heavy stress. It wasn’t until 1867 when Joseph Monier used steel reinforcement bars to make a better flower pot that concrete truly came into the modern age of building.
Steel reinforcement is used in nearly every facet of concrete construction today. Rebar (reinforced bars), as it is commonly know, is added to the form work of slabs, precaste and tilt wall construction on a residential, commercial and industrial scale around the world. Without Rebar, we wouldn’t have any of the structures we have today.
Now that’s all beginning to change again with the advent of post tensioning concrete. Post tensioning is a method of strengthening concrete reinforcement bars after the concrete has been poured, creating a compression effect that literally tightens the concrete molecules together.
Here’s how it works. Unlike conventional inactive rebar that simply holds the slab intact after pouring layers thicker than 3 ½”, post tensioning construction involves setting a grid of steel cables vs. steel rods. Similar to big guitar strings, post tensioning concrete is tightened both before and after concrete.
Just like conventional rebar, post tension cables are set into footers and slabs where it is required by the engineer. Solid lengths are cut as needed at the factory and supplied to the site. They are set as one continues piece from one side of the slab to the other and stick out each side of the form work. They set on top of conventional rebar chairs to maintain there depth in the slab.
Next concrete is poured into the forms and finished. When the concrete is about 70% of its finished strength, the tension cables are pulled tight by a hydraulic jack. This in effect “squeezes” the concrete together, creating a superstructure of concrete. Since concrete only strengthens as it is compressed, it forms a much stronger slab or wall than a conventional pour.
While much of the technology for post tensioning is used in commercial applications it has trickled down to the residential field. For more information on post tensioning concrete and if its right for your next project, check out The Post Tensioning Institute Website for more details.