A Post Tension Primer

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Post tension tendon inspection

Post-tensioning is a technique for reinforcing concrete structures.

Post tensioned tendons

The 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 at 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 that contains the strand or bar, anchorage assembly, the duct, and any corrosion-inhibiting coating. [2]

Strands today consist of seven high-strength steel wires that 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 an unbonded tendon, the strands are sheathed and not actually bonded to the concrete surrounding it. One of the most common examples is mono-strand 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, cable-stayed bridges, superstructures (the roadway), and 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 superelevation. In stadiums, this technique allows a 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 of our nation’s 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 on construction costs by a considerable amount. The proper quality control and inspection at the proper time can help avoid completion delays, tendon elongation discrepancies, concrete 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 ensure contractor integrity and help avoid very expensive issues down the road.

Post tension tendon inspection

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 procedures 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 Infraspect has changed that by utilizing nondestructive testing and robotics to develop post-tensioned tendon inspection services that save time and money, while dramatically improving the defect detection threshold.

Infraspect’s contribution to 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. Infraspect 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 today’s manual methods. Infraspect’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 are incalculable.  Infraspect 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 non-destructive evaluations (NDE) in the field.  Our company, Infraspect is transforming 50-year-old manual inspections to bring modern technology and robotics to infrastructure inspections not just for post-tensioned tendons but for infrastructure inspections worldwide.

Our company Infraspect proposes an NDE technique that uses an automated cable monitoring system and a cable-climbing robot that 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 floors, pipelines, slime wire ropes, aerial tramways, and coral mine wire ropes. Furthermore, this technique allows us to provide quantitative data in a non-destructive manner.

Our company also has robotic inspection devices for suspension and suspender cables.

Today the best way to insure the safety of our bridges and post-tension tendons is Infraspect’s TendonScan®

TendonScan® can be used manually or robotically. TendonScan peers through the post-tension tendon to magnetize the steel using proven magnetic flux leakage technology. Combining magnetic flux leakage (MFL) with specialized software and robotics, Infraspect can locate loss of metallic area/corrosion long before it becomes a danger to the public.  TendonScan provides real-time quantitative information to asset owners on the condition of their post-tension tendons. 

Magnetic flux leakage

TendonScan results locate issues long before they become a danger to the public.

Magnetic flux leakage


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

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

[6] “Web page 5.” [Online]. Available: https://www.bbrnetwork.com/technologies/post-tensioning/.

[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://infraspect.com.

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Infraspect is a complex bridge inspection, non-destructive testing (NDT), and robotic engineering company. Infraspect utilizes advanced non-destructive testing and robotic technology to produce condition assessment reports that determine the condition and deterioration of today’s infrastructure.

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