The distinguishing characteristic of an unbonded tendon is that, by design, it does not form a bond along its length with the concrete.
The capability of undonded tendon to withstand the load ensures additional design flexibility. The combination of high-strength prestressed steel and concrete ensures the resistance to compression and tensil forces following the different loads application.
First step is to place tendons in necessary profile. Concrete is then being poured. After it reaches the required strength, (usually 3-4 days after concrete pouring), tendons are stressed with required force using stressing jacks.
For installation of such system we use high-strength steel coated monostrands wrapped in plastic sheathing. The force in the stressed tendon is transferred to the concrete primarily by the anchors provided at its ends.
• to act as a bond breaker;
• to provide protection against damage by mechanical handling; • to form a barrier against intrusion of moisture and chemicals.
• to reduce friction between the strand and the plastic sheathing • to provide added protection against corrosion.
Undonded system is extensively used in different construction fields, such as construction of new buildings, repair works, reconstruction and conservation. It can be effectively used for slabs-on-the-ground, construction of different residential and office buildings, shopping malls, schools, spotrs-ground, parkings, warehouses, stadiums etc.
For projects held in aggressive, corrosion environment, it is recommended to install so-called encapsulated system. This system is designed specially to avoid the moisture accumulation in anchors. This is important because it is anchor that transfers the forces from tendon to the concrete. It is also one of the main functional part of the system, thus it must be properly protected. Encapsulated system protects structure against moisture penetration during whole life time of construction. As opposed to standard post-tensioning unbonded system, encapsulated tendon is entirely wrapped in watertight sheathing, including anchors, strand, grease, sleeves and caps, which are installed over strand overlength at both ends.
Encapsulated system entered the market at the end of 1980.
However the first inventions were very expensive, and their installation was quite time consuming. As consequence such system wasn’t very popular. As technology developed and upgreaded, the demand of encapsulated unbonded system increased as well. Such systems are mainly installed in open structures, subject to corrosion. For example, encapsulated system is installed in 90% of parkings. But now many construction market players tend to install encapsulated system for for their projects less influenced by the moisture.
• Encapsulated anchor wrapped in plastic sheathing
• Transparent plastic tube about 15 cm long with protrusions to connect with encapsulated anchor, creating sealed space from anchor till strand sheathing.
• Caps are tightly installed in anchor. It protects “strand-wedge-anchor” connection.
Cap contains with grease, which will fill all space around strand and eliminate air voids.