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A Post Tension Primer

January 8th, 2018   Author: Doug Thaler

A Post Tension Primer

 

Post-tensioning is a technique for reinforcing concrete structures. Post tensioned tendonsThe prestressing steel cables inside the sleeves or plastic ducts are positioned in the forms before placing the concrete. As the concrete gains strength, the cables are stressed to design forces before the application of the service load and are anchored att the outer edge region of the concrete.

 

Post-tensioning is a type of prestressing which means that before the concrete has to support the service load, the steel is pulled or tensioned. The post-tensioned concrete means that the tension is applied after pouring the concrete, however before applying service loads, it is still stressed and hence it is called prestressed.[1]

 

The basic element of a post-tensioning system is called a tendon. One or more pieces of post tensioning strand are coated with a protective corrosion inhibiting coating and a sheathing or bare strands with a duct is used to house them. In order to transmit the forces into the structure, there are anchors on each end of the tendon. The term post-tensioned tendon consists of a complete assembly which contains the strand or bar, anchorage assembly, the duct and any corrosion-inhibiting coating. [2]

Strands today consist of seven high-strength steel wires are wound together to form a 270ksi strand.  One or more strands would comprise a tendon.[3]

 

Types of post-tensioning

 Two main types of post-tensioning exist :

 

  1. Unbonded
  2. Bonded (grouted)

 

Unbonded tendons

In unbonded tendon, the strands are sheathed and not actually bonded to the concrete surrounding it. One of the most common example are monostrand tendons. They contain corrosion inhibiting grease coated seven wire strands. The anchorage has a two-piece conical wedge for gripping the strand and a high strength iron casting.

 

Bonded tendons

Bonded systems consist of a metal or plastic duct embedded in the concrete in which two or more strands are inserted. A large multi-strand jack is used to apply stress on these strands and is anchored in a common anchorage device. The most common use of the Bonded system is used  in bridges, in cable-stayed bridges, in the superstructure (the roadway) and the cable-stays.

 

The heavily loaded beams in buildings make use of bonded systems.[4]

 

Uses of post-tensioned tendons

 Our bridges, residential foundations, parking infrastructure, walls, columns and commercial or residential buildings make use of Post-tensioning.

 

The post-tensioned reinforcement technique, when used in floor construction, can result in longer spans between supports or thinner concrete sections. This method proves quite advantageous for the construction of buildings and structures with open spaces. The reduced thickness of the structural floors in the building can decrease the ceiling to floor height of each level and also results in the reduction of the total weight without sacrificing the potential loads. [5]

 

The use of post-tensioned tendons in bridge construction provides design flexibility allowing quite demanding geometry requirements which include significant grade changes, complex curves, and variable super elevation. In stadiums, this technique allows highly creative architectural approach and long clear spans. [6] 

 

The technique also helps in eliminating the cracks and shrinkage in concrete. Furthermore, post-tensioning can also help in resisting earthquakes.[5] These advantages have allowed for the construction of many or our nations bridges, dams and buildings with modern architectural design and increased functionality and larger open spaces for the users or occupants.

 

 

Inspection of post-tensioned tendons

In order to ensure economical and safe construction, detailed inspection of sophisticated structural systems like post-tensioned concrete has become a necessity. When post-tensioning is properly used it can save money in construction costs by a considerable amount. The proper quality control and inspection at the proper time can help avoid completion delays, tendon elongation discrepancies, post tensioned tendons and bridge inspectionsconcrete distress and repair costs associated with improper construction sequence. The use of a third party firm that conducts independent construction engineering and inspections during the construction process can help insure contractor integrity and help avoid very expensive issues down the road.  Though inspection is not a cure to all of the construction problems, it plays a significant role in the elimination of job-site problems and also in the assurance of quality construction. Eliminating the structural inspection just for saving a few bucks is asking for trouble.

A number of manual inspection solutions existed which involved opening the anchorage caps to inspect the visible parts, sheathes and connections visually. To inspect the cement grout filled sheathes some manual procedure which included Hammer sounding opening of windows were used.

 

However, these methods were manual, subjective in nature and have not provided the DOT (Department Of Transportation) or asset owners with precise actionable information to help them maintain their structures in good form. The Infrastructure Preservation Corporation has changed that utilizing nondestructive testing and robotics to develop post tensioned tendon inspection services that save time and money, while dramatically improving defect detection thresehold.

 

IPC’s (Infrastructure Preservation Corporation’s) contribution int the inspection of post-tensioned tendons with the help of nondestructive testing and robotics

Owners  of large concrete structures prefer to utilize  nondestructive inspection technology   for detecting and quantifying loss of metallic area due to  corrosion and other damage including fractured wires inside  tendons. IPC has brought together a number of nondestructive testing methodologies and combined them with robotics and advanced signal analysis to deliver quantitative data that far exceeds todays manual methods. IPC’s TendonScan peers through the external wrapping and grout to locate section loss within a post tensioned tendon. 

 

Another device can locate incipient damage  within a tendon and still another, ground penetrating radar or “GPR” can map tendons and rebar through concrete.  Asset owners given the choice, would prefer to have quantitative data about their assets so they can best allocate their budgets and schedule maintenance and repairs. The benefits of helping to extend the service life of these structures safely is incalculable.  IPC is disrupting the infrastructure inspection industry by utilizing our patent pending technology to  conduct a detailed condition assessment for critical Infrastructure owned and maintained by various State and Municipal  DOTs for asset owners and the department of transportation.

 

Due to the history of manual subjective inspections and testing and the current condition of our infrastructure, there have been increasing demands for nondestructive testing (NDT) and nondestructive evaluations (NDE) in the field.  Our company, IPC is transforming 50 year old manual inspections to bring modern technology and robotics to infrastructure inspections nor just for post tensioned tendons but for infrastructure inspections worldwide.

 

Our company IPC proposes an NDE technique which uses an automated cable monitoring system and a cable-climbing robot which can reach the damaged point. [8]

 

The magnetic flux leakage technique is one of the most cost-effective pre stressed concrete inspectionand reliable methods for inspecting different structures like tank floor, pipelines, slime wire ropes, aerial tramways and coral mine wire ropes. Furthermore, this technique allows us to provide quantitative data in a nondestructive manner. Our company also has robotic inspection devices for suspension and suspender cables.

 

 

 

 

For more information or pricing:

 

References

[1]         “Web page 1.” [Online]. Available: https://www.concretenetwork.com/post-tension/.

[2]         “Web page 2.” [Online]. Available: http://www.concreteconstruction.net/how-to/construction/post-tensioned-slabs_o.

[3]         “Web page 3.” [Online]. Available: https://www.concretenetwork.com/post-tension/basics.html.

[4]         Post-Tensioning Institute, “What Is Post-Tensioning ?” no. December 2000.

[5]         “Web page 4.” [Online]. Available: http://www.concreteconstruction.net/how-to/construction/post-tensioned-slabs_o.

[6]         “Web page 5.” [Online]. Available: http://www.bbrnetwork.com/technologies/what-is-pt.html.

[7]         “Web page 6.” [Online]. Available: https://www.researchgate.net/publication/229045600_Why_Not_Enlarge_the_Non-Destructive_Testing_NDT_Experience_in_Construction_Maintenance-Problems_Practice_Ideas_etc.

[8]         “Web page 7.” [Online]. Available: https://www.hindawi.com/journals/sv/2014/929341/.

[9] “Web page 8” [Online] Available: https://www.infrastructurepc.com.

 

 

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