Post Tensioning – High Tech for Concrete Slabs on Ground

For as long as concrete has been used for residential slab on grade, industrial, commercial and warehouse flooring, mat foundations, recreational surfaces and tilt wall panels, cracking has been a problem. Reinforcing concrete with rebar has improved resistance to cracking and fracturing, but with limited success. If soil problems (lift, shrinkage or swelling) are sufficiently severe, rebar doesn’t provide the strength needed to prevent damage.

As opposed to the static reinforcement offered by rebar, post tensioning applies a dynamic compression load through a set of tendons (steel strand cables) that pass through a slab and are anchored at the ends. Slabs are typically poured with 3,000 psi concrete and range from 4 to 8 inches in thickness, depending on the project and loading conditions. After a slab has cured to the prescribed hardness, the tendons are stressed to 33,000 pounds ( 33 Kips) to apply the compressive load to the slab. When the tendons are stressed, they will stretch (elongate) approximately 4 inches per 50 feet of length.

Although the basic premise of unbonded post tensioning hasn’t changed since its inception 50 years ago, implementation technology has improved dramatically. Steel tendons are now encapsulated in plastic to alleviate corrosion, and in highly corrosive environments, watertight anchorages with end caps can be utilized.

Use of post tensioning has increased in recent years because it provides a number of structural benefits. It reduces cracking that results from slab shrinkage and reduces the number of joints are required- or may be eliminated completely. If cracks do occur, they remain small because the tendons exert a tight hold, keeping the sections on either side of the crack together. Post tensioning also makes longer spans possible for elevated slabs.

In addition to its strength enhancing characteristics, post tensioning also offers cost advantages. A post tensioned slab typically requires substantially less rebar than a conventional slab. Slab thickness may also be reduced, providing another cost savings.  Deflections are reduced as well.

Post-Tensioning is extensively incorporated into multi-story and high rise concrete construction as well.  There are numerous benefits for commercial applications such as apartments, condominiums, parking garages, hotels and office buildings. The specifics of those applications will be covered in the future.

Use of post tensioning isn’t limited to new construction, it can also be used to strengthen existing structures. External post-tensioning has been utilized to increase structural capacity and reduce deflections.  This can be incorporated with FRP carbon fiber strengthening systems.  In either case, proper engineering is a critical ingredient for a successful project. Most concrete slabs don’t exist as islands, but as integral parts of complex structures. As such, the inter-relationship of other structural components influence post tensioning engineering. Tendon Systems provides both the post-tension engineering expertise and the on-site field service personnel to support the project consultants, owners and general contractors to ensure successful on time, on budget project completion.

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