Imagine you are making pancakes. You separate the wet ingredients from the dry ingredients, then carefully mix. Somehow lumps still manage to form in your batter. Lucky for you, a lumpy mixture should not ruin your pancakes. But if, instead of pancakes, you were mixing grout to fortify a bridge tendon, following the recipe and mixing thoroughly would be crucial. Grout generally consists of Portland cement, potable water, and other ingredients that are added to improve the performance of concrete. When these ingredients are mixed and applied properly, the grout hardens and dries, protecting the tendons from corrosion. Most of the time, this process is accomplished successfully and the bridge is well-protected. However, if materials or methods are defective, then the grout can stay soft and moist, which can itself cause corrosion.
Proper grouting ensures the long-term health of bridge structures, which has made finding and repairing defective grout an industry-wide effort. For an agency like FDOT, maintenance efforts focus on first identifying where there is defective grout and then figuring out how to remove or repair it. Previous FDOT research developed a Tendon Imaging Sensor that helps locate defective grout. This latest research project, Evaluation of Techniques to Remove Defective Grout from Post-Tensioning Tendons, helps FDOT figure out how to address it.
A remediation team of researchers and FDOT staff was put together to tackle this issue. Hydrodemolition, a process that uses high-pressure water to remove concrete, asphalt, or grout, was proposed. Before spending the time and money in the field, Will Potter and Christina Freeman in FDOT’s Structures Design Office teamed up with Dr. Trey Hamilton at the University of Florida to test the approach in the lab. And, as sometimes happens in research projects, they learned what not to do. FDOT Professional Research Engineer Christina Freeman explains, “The major finding from our research was that the [hydrodemolition] approach wouldn’t work.”
The research team set out to investigate viable methods and found that instead of inundating the tendon with water, they needed to dry it.
Limitations of Hydrodemolition in Tendon Grout Removal
Tendons comprise a critical structural component that links precast concrete segments together to form the bridge. Once the concrete segments are in place, tendons are used to join them into the bridge span. 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 and to structurally bond the tendon to the surrounding concrete.
Grouting tendons can be a difficult process and soft grout (areas with free moisture) can lead to corrosion of the steel cables over time. Removing the soft grout is important to maintaining the health of the tendons, and therefore the overall bridge. Bridges have both external tendons located outside the concrete and internal tendons embedded in the concrete. Removing soft grout from internal tendons is difficult because they are encased in hardened concrete. Manual methods for grout removal would require a cost-prohibitive demolition of the concrete surrounding the internal tendon. Even with external tendons, manual grout removal methods are expensive.
Hoping to find a feasible solution for internal tendons, the team thought they could wash out the soft grout using hydrodemolition. Picture it like power-washing a sidewalk, only the highly pressurized stream is strong enough to break down grout or concrete. Given how water can permeate concrete, it is understandable why hydrodemolition may have seemed a good way to get at the internal tendons. In the lab, the research team tried a variety of configurations of water injection and debris discharge locations. Regardless of method, large quantities of grout were still left in the test strand or residual grout was found trapped between strands.
Scaling Up Drying Methods for Tendons
The research team then explored going in the exact opposite direction and investigated a tendon drying method. While this method typically is not applied to bridge tendons, the research team already knew drying could work on a smaller scale. Dr. Trey Hamilton explains, “This method had been used pretty successfully on single strand tendons, but this was the first time we’d done it on a bundled tendon.”
To scale up the drying method, the research team needed to set up several pieces of equipment to pump dry air through the test tendons. The lab setup included a compressor with dry air that was pumped through a desiccant dryer and filters, pressure regulators and reading locations, and a vacuum to help pull the air through the system.
Source: Evaluation of Techniques to Remove Defective Grout from Post-Tensioning Tendons Final Report
The research team found that maintaining this equipment in the lab was critical for success. Will Potter of FDOT’s Structures Design Office says, “It’s not necessarily an easy process to dry the tendon, but as long as you follow protocol and give it time, it will work.” To deliver air to the test tendons at a constant pressure, the research team used a pressure regulator to ensure air was pumped into the system at no more than 20 psi. Since the team was pumping very dry air through the system, the next factor for success was simple: time. The team found that test tendons with multiple layers of soft grout and slightly denser grout needed 117 days to dry, and test tendons with soft grout trapped between layers of normal grout needed 167 days.
The research team measured corrosion potential during and after drying, and found some moderate corrosion. To prevent this, they suggested adding a step to the drying process or an alternative approach to applying it. They recommended following the drying process with the immediate injection of a corrosion inhibitor or performing the drying process with nitrogen instead of air.
With valuable lessons learned about hydrodemolition, drying equipment, and corrosion potential, the FDOT research team has already seen their work put to use on a bridge project in Jacksonville. Successful remediation of soft grout saves FDOT time and money and improves longevity.
Further Reading
BDV31-977-58 Evaluation of Techniques to Remove Defective Grout from Post-Tensioning Tendons
Final Report | Summary
BDV25-977-24 Development of Tendon Imaging Sensor
Final Report | Summary
Florida Department of Transportation Research Showcase 