Post Tensioning
Post tensioning, or PT, has become increasingly popular over the past few years, as the technology has been perfected with better materials and construction methods has made this system a preferred one. PT works are predominantly used in most projects
Post-tensioning is a concrete reinforcement method. The tendons are tensioned after the concrete has set via Post-Tensioning. Before casting, corrugated ducts are often installed into the concrete along the centroidal axis. The tendon was put into the duct, strained, then fixed against concrete once the concrete had solidified and had suitable strength. Grouting fills the remaining space in the duct with mortar. Post-tensioning may be done as a precast or cast-in-place process.
Material
Strand
A typical strand used in Post tension having tension strength of 1860 N/mm², in comparison typical non Pre – stressed rebar having 460 N/mm². Strand is manufactured from 7 individual cold – drawn wires, outer wires helically wound around one center wire (king wire) as per code, Strand coatings do not affect the anchorage’s capacity or efficiency. For improved corrosion protection we offer systems using galvanized iron ducts.
Sheathing
The material for the duct shall be high density polyethylene with more than 2% carbon black to provide resistance to ultraviolet degradation. The thickness of the wall shall be 2.3±0.3mm as manufactured. The duct shall be corrugated on both sides. The duct shall transmit full tendon strength from the tendon to the surrounding concrete over a length. The duct shall be sufficient strong to retain shape and to resist damage during construction.
Anchorage
The pre stressing force is transferred to the concrete at the anchorage. Anchorage where the stressing take places are called active anchorage, and the other are called passive anchorage. Sometimes stressing is made from both ends of a tendon to reduce friction losses. The anchorage shall be safe and secure against b oth dynamic and static loads as well as impact.
Benefits of Post Tensioning
- Strength and increased resistance to differential movement in slabs, rafts and beams
- Eliminates/Reduces deflection
- Eliminates/Reduces cracks in slabs
- Faster construction cycle reducing duration of formwork, scaffolding and manpower
- Economical as lesser reinforcement and concrete is used
- Greater spans possible due to fewer beams required
- Thinner slabs for increased height advantage
- Reduced building mass especially significant in high seismic zones
Steps in Post Tensioning
Below graph illustrates typical average savings with the use of Post-Tensioning “PT” slabs compared to RCC slabs:
Pre Tensioning
Prior to pouring the concrete, the tendons are tensioned against the abutments via pre tensioning. After the concrete has dried and reached the necessary strength to transmit the pre-stress to the concrete part, the tension force is released and disconnected. The tendon attempts to contract back to its original size, but the concrete resists due to their link, inducing compression force in the concrete. Precast is often used for pre-tensioning. When tendons or pre-tensioning wires are drawn out to create tensile force, wedge and barrel are the two components needed to hold them firmly. The wedge will get locked into the barrel’s altered finish as the force is applied.
Differences Between Pre Tensioning and Post Tensioning
| Pre Tensioning | Post Tensioning |
| Pre tensioning is done in factories, making it appropriate for precast building or structures. | Post tensioning may be done both in factories and on-site. |
| Pre tensioning is less expensive since sheathing is not required. | Post tensioning is more expensive due to the usage of sheathing. |
| Prior to casting the concrete, the pre-stressing wires, also known as the strands, are tensioned, and then concrete is cast around the cables. | Concrete is poured once the strands are contained within a conduit. The tensioning of the strands is done once the concrete has achieved the required strength. |
| Pre tensioning is Ideal for smaller and lighter sections which are easy to be transported. | Post Tensioning is not restricted to size of members. System can be used for longer and heavier sections like bridges etc. |
| Moulds are used to produced pre tensioning members | In lieu of wires, cables are utilized, and jacks are used for stretching. |
| The pre tensioning process evolved as a result of the bonding between the concrete and steel tendons. | The concrete post-tensioning technique is designed for bearing. |
| Loss of strength is more than 17%. | Loss of strength is not more than 15%. |
| It is more durable and robust. | The durability depends upon the anchorage system. |
| Similar prestressed elements are produced using this technique. | Products are modified using this technique in accordance with a structure. |