Chances are you’re familiar with rebar, the most common material used to reinforce concrete. Rebar is what’s known as “passive” reinforcement, but to make certain concrete slabs extra versatile and strong, contractors can use a form of “active” reinforcement: post-tensioning.
Post-tensioning is a way to reinforce concrete after we pour it but before any pressure is allowed on the surface. It’s a way to “prestress” the material so that it can stand up to common tensile forces which it will encounter.
How Does Post-Tensioning Work?
Post-tensioning uses high-strength steel strands to strengthen the concrete. These strands, known as “tendons,” are cast into a concrete slab or beam through the center of the slab, then tensioned to a high degree of stress after the concrete has been cast and cured. Contractors then pull the ends of the tendon through the anchorages while pressing against the concrete. This compresses the concrete between the anchor points so that the material is ready to hold heavy loads.
By adding these tendons, contractors can reduce the material’s inherent weakness when in tension. This allows for the creation of thinner slabs, reduced deflections, and lighter formations when compared to other forms of reinforced concrete. At either end of the tendon, the builders will fix an anchorage assembly into the surrounding concrete.
There are two main types of post-tensioning: bonded and unbonded.
Bonded Post-Tensioned Concrete
Bonded post-tensioned concrete is when the tendons bond to the surrounding concrete because of special grouting. It also protects the tendons from corrosion, keeps them in pre-tension so they aren’t reliant on end-anchorage systems, and prevents some typical structural problems in the final concrete structure.
Bonded post-tensioning is often found in large slabs in structures built in soil that is prone to swelling, such as clay. This swelling creates problems for the perimeter foundation, but the tendons take all these stresses and support the building without significant flexure.
Unbonded Post-Tensioned Concrete
Unbonded post-tensioning is different because the tendons can move more relative to the concrete. They are usually a series of high strength steel strands formed by braided wires packed in the grease-filled plastic sheathing, which forms a tendon. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking, allowing the tendon to take on tension to the concrete.
This method of post-tensioning has several advantages, including the ability to adjust the cables to poor field conditions. There’s also no need for post-stress grouting, and the contractors can de-stress the tendons before attempting any necessary repair work.
Are Post-Tensioned Concrete Slabs Used At Home?
In North America, builders use unbonded post-tensioned concrete because it’s more versatile. As such, it requires fewer support beams and columns, offering greater design flexibility. It also uses thinner slabs with less concrete so that the weight of the building is lower.
The steel tendons used for residential projects are usually 48 inches at the center, with commercial foundations having much more steel. A post-tensioned concrete slab used for residential projects will be roughly 8 inches of 3000 psi concrete. Once the concrete has reached 2000 psi, the contractor will stress the tendons for up to 10 days. If you’re worried about the proximity to obstacles, don’t worry: the steel can be routed around any obstructions.
Post-tension reinforcements have to be installed with a high degree of skill. The final product is a more complex structural system, and it can be quite susceptible to breakdowns by moisture. It’s also vulnerable to tendon breakage and sudden ruptures, so frequent inspections can be necessary. Whether we use this form of prestressing depends on your project!