Bridging The Gap: New Device May be FDOT’s Missing Link in Assessing Bridges

It’s early morning and a group of bridge inspectors are running through their checklist of field equipment they need for the day ahead. Reflective vest and hardhat? Check. Car keys? Check. Coffee? Check. After pulling up to the site and getting situated, they head to the bridge to get started.

Bridges on Florida’s State Highway System are regularly inspected using a combination of techniques, ranging from simple visual inspection to use of advanced technologies. These inspections programmatically assure FDOT is monitoring the health of the state’s bridges and promoting their longevity. There are several components to a complete bridge inspection, but an important one for FDOT is evaluating the condition of bridge tendons, which utilize high strength cables that keep bridge segments together. However, inspecting tendons is invasive, costly, and time-consuming, and there are not effective nondestructive methods to evaluate their condition. There had to be a better way.

In a series of research projects, Drs. Alberto Sagüés, Chris Alexander and their team of graduate students at the University of South Florida have been working with FDOT to develop a tendon imaging sensor. Dr. Sagüés, the initiating Principal Investigator on the project, says, “We wanted to create a device that could be implemented at very moderate cost and would be simple to operate.”

Binding Bridges with Tendons

Tendons are a critical structural component that link pre-cast concrete segments together to form the bridge. Once the concrete segments are in place, tendons are used to join them into the bridge span. External tendons are located outside the concrete and easily accessible, while internal tendons are embedded in the concrete.

Tendons themselves are made up of a bundle of steel cables running through a duct that has been backfilled with grout to keep the steel from corroding. Grouting tendons can be a difficult process and the presence of voids or water that has not completely mixed in the grout can lead to corrosion of the steel cables. Checking tendons for these deficiencies is important to identify any potential for corrosion. However, as the grout-embedded strands are not visible in the duct, there is no way to directly inspect the tendons without disturbing them in the process.

View of the inside of a bridge tendon
Source: Adapted from Development of Tendon Imaging Sensor Final Report

FDOT has explored multiple technology options to get a better picture of what is going on inside external tendons. FDOT Project Manager Adrian Steele in the State Materials Office says, “We have had industry representatives come in and demonstrate their units to us, but they weren’t quite what we were looking for.”

In some cases, commercial units were too big to fit into tight spaces. In others, a high-powered generator and a crew of three was needed to operate the machinery. What FDOT really needed was something small, easy to operate, and capable of detecting deficiencies in grout. With these notes in mind, Drs. Sagüés, Alexander and his students went to the drawing board.  

Beg, Borrow, or Steel: Leveraging Metallurgical Principles for Better Testing

First, the research team developed a tendon imaging sensor that detects the primary material that makes up tendons: steel. Their method uses magnetic data to locate the steel cables within the duct and then uses impedance measurements to detect voids lying between the cable bundle and the duct. Higher impedance typically corresponds with grout deficiencies like voids, normally low impedance indicates the grout is in good condition, and unusually low impedance suggests water accumulation. Through lab tests and further development of the sensor, the project team has been able to produce cross-sectional images in real time.

Sample outputs produced by the Tendon Imaging Sensor. The left image was taken where a void was cast into the tendon. The right image was taken where no void was cast. Patent Pending.   
Source: Development of Tendon Imaging Sensor Final Report

The device is a few inches long and is hooked up to a computer to produce a color-coded image. To create an image, the device is lightly clamped around a tendon. The user then rotates the device 360 degrees around the tendon, which immediately produces an image of the cross-section. In all, it takes less than a minute to get an image at one location along the tendon.

FDOT was eager to see the imaging sensor in action. Field trials of the device were conducted at the Sunshine Skyway Bridge in Tampa and the Ringling Bridge in Sarasota. FDOT Project Manager Ron Simmons in the Materials Office saw plenty of reasons to get excited about deploying the device. He explains, “It fits in a briefcase and doesn’t require three people or a high-powered generator to work.” Unlike other technology FDOT has explored, the tendon imaging sensor meets their key criteria: small, easy to operate, and capable of detecting deficiencies in grout.

Prototype of the Tendon Imaging Sensor. Top image shows the device closed, as it would be around a tendon. Bottom image shows the device open. Patent Pending.   
Source: Development of Tendon Imaging Sensor Final Report

Concrete Benefits

Conducting tests using the tendon imaging sensor has given Dr. Alexander, the lead Principal Investigator for continuation of the project, important insight into upgrading the device. He says, “We are looking at possible improvements to detect smaller voids and water.” The research team is also exploring ways for the device to rotate around the tendon automatically, instead of requiring the operator to do it manually.

Moving forward, FDOT and the research team plan to continue field tests and make enhancements to the device. Further down the road, the tendon imaging sensor could be a valuable tool for FDOT at initial construction to verify tendons have been grouted properly, avoiding corrosion before it happens. There are also major benefits to using this device when conducting biannual bridge inspections. One day, instead of relying on limited methods, FDOT could have this device on its regular checklist for field inspections. That would certainly be a powerful tool in FDOT’s toolbox.

Further Reading

BDV25-977-52 Field Demonstration of Tendon Imaging Methods
Final Report | Summary

BDV25-977-24 Development of Tendon Imaging Sensor
Final Report | Summary