Detours around bridges in a critical freight transportation route create costs to the trucking industry, taxpayers and state economy. New load rating factors for the slab-span bridges across Shingle Creek will give MnDOT more flexibility in managing truck traffic and keeping freight moving efficiently.Continue reading Updating Load Ratings for Shingle Creek Slab-Span Bridges
In recent years, MnDOT has seen an increase in cracking of low slump overlays. Cracking of overlays allows chlorides to get into the bridge deck which leads to deterioration of the reinforcement and eventual delamination or spalling. This means that the bridge deck needs to be repaired or even replaced before the service life has been reached.
What is a low slump overlay? A low slump overlay is a technique used by DOTs to extend the life of bridge decks. They are typically 2″ thick and designed to provide low permeability.Continue reading New Project: Bridge Low Slump Concrete Overlay Mix Design for Mobile Mixers
Reinforced concrete bridges are built to handle heavy loads and routine traffic for 75 years or more. But bridges in climates like Minnesota’s are exposed to moisture and chlorides from road salts that may penetrate these structures and corrode the steel.
In a recently completed research project, funded by MnDOT and the Local Road Research Board, researchers studied a rural bridge built in 2017 near Elgin, MN, that used glass fiber–reinforced polymer (GFRP) rebar in the bridge deck. They found that GFRP performed well, proving sufficiently strong for use as an alternative to corrosion-susceptible steel rebar.Continue reading GFRP Rebar Shows Promise for Use in Bridge Decks
Well-documented efforts undertaken two decades ago to mitigate corrosion of a Highway 394 reinforced concrete bridge have given researchers the perfect scenario for evaluating the treatments’ long-term effectiveness. The test results are mixed: State-of-the-art methods for electrochemical chloride extraction and fiber-reinforced polymer wrapping of bridge elements performed well in combination, but poorly in isolation.Continue reading Methods to Mitigate Bridge Corrosion Show Mixed Results 20 Years Later
A new spreadsheet tool developed by the Minnesota Department of Transportation draws on historical data to help project engineers better estimate bridge construction time. The method allows users to project time-frames based on bridge design elements, work scheduling and other inputs, utilizing estimates from comparable projects in a 10-year database of bridge-building data.Continue reading Putting Research Into Practice: New Tool Estimates Bridge Construction Time
An evaluation of five maintenance paint coating systems on Minnesota steel bridges with localized corrosion found that each maintenance coating performed well, and, if corrosion is identified early and maintenance painting occurs, the service life of the paint coating system can be extended five years before repainting is required. Based on the test data, researchers recommended an update to MnDOT’s Bridge Maintenance Painting Manual that includes guidance on when to apply maintenance paint coatings and when to remove paint and recoat bridges.Continue reading Detecting Corrosion Early Extends Service Life of Bridge Paint Coating
A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.Continue reading Using Debonded Strands to Reduce End Stress in Bridge Beams
MnDOT has funded a study to evaluate the use of non-lethal ultrasonic acoustic devices to temporarily deter bats from bridges before and during construction projects.Continue reading New Project: Use of Innovative Technology to temporarily Deter Bat-Bridge Use Prior to and During Construction
The Minnesota Department of Transportation (MnDOT) had suspended the use of post-installed epoxy-coated rebar for concrete barrier repairs as a precautionary measure because chemical adhesives used in the process are not designed for use with coated bars. But laboratory testing (conducted in a recent MnDOT-sponsored research study) has now shown that using these adhesives with coated rebar for post-installation works well and provides a safety level 200 to 300 times that predicted by manufacturer specifications. MnDOT is considering research recommendations to modify the installation process in order to resume using coated rebar in post-installed concrete crash barriers.Continue reading Using Chemical Adhesives to Post-Install Epoxy-Coated Rebar in Concrete
Researchers provided recommendations and general guidance to assist MnDOT in developing cost-effective strategies for future investigation and repair contracts of post-tensioned bridges built in Minnesota before 2003. To develop these recommendations, researchers identified grout voids in post-tensioning ducts on two representative bridges, documented strand corrosion and repaired voids by filling them with grout.
“With the guidelines developed in this project, we have a good basis for cost-effectively and efficiently inspecting post-tensioned bridges built before 2003,” said Dustin Thomas, South Region Bridge Construction Engineer, MnDOT Bridge Office.
“From a fiscal perspective, it makes sense to do limited inspections on these bridges before committing additional resources to more comprehensive inspection and repair,” said Mark Chauvin, Associate Principal and Unit Manager at Wiss, Janney, Elstner Associates, Inc.
What Was the Need?
Some concrete bridges in the United States are strengthened using post-tensioning—a method of reinforcing concrete by running steel strands through a hollow plastic or metal duct placed within the concrete element. Tension is then applied to these strands with a hydraulic jack, com-pressing the concrete and creating internal stresses that resist external traffic loads. Post-tensioning improves the durability of concrete and virtually eliminates cracking.
Post-tensioning ducts are filled with grout—a mixture of cement, sand and water that hardens around the steel strands. This practice prevents the strands from corroding if they are exposed to air, water and deicing chemicals.
Grouting materials used in bridges built before 2003 frequently produced voids where grout did not fully fill the post-tensioning ducts or cover the strands. These post-tensioning strands were vulnerable to corrosion, which can lead to deterioration in bridge elements over time. Once transportation agencies and the industry became aware of these issues, they improved their construction practices and began using prepackaged grout materials with additives so that the post-tensioning ducts would be completely filled.
About 40 post-tensioned bridges were built in Minnesota before 2003 that might still require repair. MnDOT commissioned a two-phase project to develop techniques for evaluating these structures. In the first phase of the project, completed in 2012, researchers inspected a representative sample of these bridges and developed a general inspection protocol to guide future investigations. In the second phase, described here, researchers developed additional guidance about grout repairs, as well as the most cost-effective contracting methods for such repairs.
What Was Our Goal?
The goal of the second phase of this project was to provide recommendations and general guidance that MnDOT could use to develop cost-effective strategies for future investigation and repair of post-tensioned bridges built in Minnesota before 2003. As part of this project, researchers identified grout voids in post-tensioning ducts on two representative bridges, documented strand corrosion and repaired voids by filling them with grout.
What Did We Do?
In 2013, researchers inspected three spans on two Minnesota bridges for voids around post-tensioning strands. They began the project by using ground penetrating radar to map the location of the ducts. Once the ducts were located and mapped, researchers used a borescope to visually inspect the duct interiors at locations where voids were likely to be present. When they found voids, they documented the percentage of the duct filled by grout and the extent of corrosion in the post-tensioning strands within the ducts, if any. Following inspection, researchers filled the voids with grout and installed sensors within the voids at two locations to monitor the long-term corrosion of post-tensioning strands.
Using their experience with these repairs, researchers then created guidelines that would help MnDOT develop cost-effective strategies that can be implemented in future post-tensioning duct investigation and repair contracts.
What Did We Learn?
Researchers found voids in 32 percent of inspected ducts. These voids were typically at least 10 feet long and about one-half the diameter of the duct. Although prestressing steel strands were exposed at approximately half of the grout voids, no significant corrosion of the strands was observed at any location. Light to moderate corrosion was usually observed on the inside surfaces of the galvanized metal ducts at grout voids.
The guidelines developed by researchers address:
- Typical work plans for investigation and repair, including considerations for bridge access and traffic maintenance during inspection and repair.
- Document review, including bridge design drawings and post-tensioning shop drawings.
- Visual surveys to identify signs of distress near post-tensioning ducts.
- Procedures for borescope inspection and remedial grouting repair.
- Various contract and project approaches for developing specialized inspection and re-medial repair contracts, with a discussion of the advantages and disadvantages of each approach. Using multiple contracts with graduated levels of inspection and repair will most likely provide MnDOT with the best value.
- Planning-level cost information for seven representative pre-2003 post-tensioning bridges identified by MnDOT to assist in future budget calculations.
The guidelines developed in this project will provide MnDOT with a framework to solicit and procure similar engineering and construction services contracts for post-tensioning bridges in Minnesota. Researchers recommend exploring additional techniques to more rapidly assess and inspect post-tensioned bridges, including noninvasive investigative methods that do not require drilling holes.
This post pertains to Report 2017-04, “Considerations for Development of Inspection and Remedial Grouting Contracts for Post-tensioned Bridges,” published January 2017.