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Bridges Supported By Post Tension Tendons Are Made Safer With Robotic Inspections

July 7th, 2017   Author: Doug Thaler
What is a post tension tendon?

“Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are pre-stressing steel cables inside plastic ducts or sleeves. Post tension cables in effect allow us to safely build large concrete structures. They allow larger spans, thinner slabs and greater distances between support commons. Post tension tendons are used in bridges and box girder structures worldwide

This construction method has made it possible to build
structures that might not otherwise be possible.

Because concrete is not flexible it can crack under loads. Although rebar is used to reinforce the concrete to prevent cracks from growing the concrete can still sag under increased loads. PT or post tension cables are load bearing and can stand large increases in loads, bearing up the concrete.

There are many benefits that are gained by using a post-tensioned approach. It allows for flexibility of design, faster construction and lower material costs. Over the long term, it can reduce maintenance costs and allow for increased loads.

Bridges that utilize post tension tendons are able to contain long and clear spans, have few beams, slender members and typically thinner slabs which is not possible with other construction methods. This process reinforces or strengthens the concrete or other material with high strength steel bar or strands known as “tendons”. These tendons also significantly reduce the structures weight and foundation load which is very useful in seismic areas. They are responsible for some of our most beautiful and critical infrastructure.

Post tension tendons help in construction of complex bridges which has specific geometry requirements such as complex curves and super elevation. Extremely long span bridges can be constructed with the help of post tension tendons.

Post tension tendons are made of pre-stressing steel strands, anchoring them in the concrete and adding grouting into the ducts. Post tensioning was developed in the 1930s and became a popular building method after world war II. Recent cases of corrosion of post tension tendons in complex bridges have become a significant problem.

After getting information about tendon failures, department of transportation’s started to inspect their post tension structure inventory to assess and quantify the rate of tendon corrosion and to estimate the remaining lifespan of the affected member. Realizing issues were mounting the bridge owners looked for ways to inspect these tendons that were more accurate than just a visual inspections. Over the years a number of methods have been utilized to inspect tendons including sounding, gpr, borescope and electro-magnetic. All of these worked to some degree however they were slow, time consuming and not commercially practical to properly conduct field inspections. One company Infrastructure Preservation Corporation recently developed a portable robotic means to inspect external post tension tendons and appropriately named TendonScan®.

TendonScan® conducts a comprehensive post tension

tendon inspection service that utilizes non destructive testing and evaluation methods that can find corrosion and section loss within a PT tendon.

What it is?

As we have mentioned above that it is a comprehensive tendon scanning system which works on non destructive testing principals to find out voids, water intrusion and bleeding grout in external tendons as well as section loss and corrosion.

Why was it developed?

During construction of complex bridges, tendons are filled in grout to prevent corrosion. But during grouting process, the grout tends to separate depending on grout’s quality, admixtures, pumping equipment and procedures. This separation result into low pH grout at tendon points and anchorage which made these locations susceptible to corrosion. Locating issues early in the tendons lifecycle and making repairs allows us to extend the service life of these tendons. Critical to the health and safety of the structures they support and the public that utilize these structures, it is important to conduct regular inspections.

Complex bridge inspections are incomplete without tendon testing and the visual subjective testing currently employed is just not adequate enough to ensure a structures safety. TendonScan® peers through the concrete and steel tendon and conducts the most accurate assessment to date of post tension ed tendons.

How it is used?

It is a battery operated robotic system that uses the latest in non destructive testing and modern technology. This system couples itself with the tendon and travel along it either manually or robotically to perform a MRI like inspection of the material inside it. The inspector stands by a remote mobile control unit which monitors the sensor. The sensor works through wireless connectivity to transmit the collected data

back to the base station. The information is transferred to control station for real time assessment of the tendon condition. The image of internal section of tendon is then illustrated in 3D color graphics so that any discontinuity like water intrusion, void or bleeding grout can be identified and quantified easily.

 
What problem does it solve?

This tendon assessment system helps in locating corrosion, voids, bleeding grout, section loss and other problems inside and outside the tendon. Moreover, it also helps in monitoring the discontinuities over a period of time which helps in deciding that when to repair or replace a post tension tendon.

To learn more about post tension tendon inspection services and the latest in transportation infrastructure inspection services, you can visit our website https://www.infrastructurepc.com/ or contact us at [email protected]

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