What is Post-Tensioning (PT)?

Post-tensioning is a method of prestressing concrete in which steel tendons are tensioned after the concrete has hardened.

How prestressing works

The tensile strength of concrete is only about 10% of its compressive strength, so plain concrete cracks under load. Prestressing puts the concrete in compression in the regions where load causes tension. The tension from the load must first cancel this compression before it can crack the concrete.

How post-tensioning works

The tendons are strands or bars that lie in ducts embedded in the concrete. After the concrete has hardened to a set compressive strength, a hydraulic jack pulls each tendon to a set force. Then the force passes from the jack to an anchorage at the end of the tendon. For strand tendons, wedges grip each strand in a wedge plate that bears on the anchorage.

The usual strand is seven-wire strand to ASTM A416, Grade 1860 MPa (270 ksi).

Bonded and unbonded tendons

In a bonded system, cementitious grout is pumped into the duct after stressing. The grout is chemically basic, so it gives the steel a passive environment. It also bonds the internal tendon to the structure. An unbonded tendon is not bonded along its length. Greased, plastic-sheathed mono-strand is one example.

Grout voids

Excess water in a duct can cause bleed and grout voids. Voids can make corrosion of the tendon more likely. The FHWA manual describes inspection of grouted tendons by drilling and use of an endoscope or probe. It also names infrared thermography, echo impact and sounding as other methods to detect voids.

Prestressed pavement on airports

FAA AC 150/5320-6G says that prestressed pavements have been used on airports to a limited extent. Prestressing tendons keep the whole section in compression. This allows thinner panels and greater distances between joints than jointed plain concrete pavement. Repair and retrofit of in-pavement fixtures can be challenging. FAARFIELD rigid pavement thickness design does not apply to these pavements.